Seventeenth Annual Conference of POMS. April 28 - May 1, 2006, Boston, MA.“OM in the New World Uncertainties"

Abstract code: 004-0423

LEAD-FREE SOLDERING IMPLEMENTATION IN THESEMI-AUTOMATIC IMMERSION TINNING PROCESS FOR

THE TO-92 PACKAGE INTEGRATED CIRCUITS.

Elsa M. Benavides & Rafael A. Delgadodraelsa@benavidesphd.com, rafael.delgado@us.bosch.com

División de Estudios de Posgrado e InvestigaciónInstituto Tecnológico de Ciudad Juárez,

Ave Tecnológico 1340, Cd. Juárez, Chih. 32500 México. http//www.itcj.edu.mx

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ABSTRACTThis paper describes the work developed in the lead-free semiautomatic immersion tinning through a wave soldering process of electronic terminals for Hall-Effect technology components. Currently, new pending environmental legislation of the European Union Waste Electrical and Electronic Equipment and the Restriction of Hazardous Substances will require electronics manufacturers to eliminate lead from soldering. Lead is the major constituent in tin-lead solders alloys currently being used in the electronics industry. A process DOE was developed with five input variables detailed as solder bath temperature, conveyor speed, percentage of copper content, pre-heating temperature and base material pre-cleaners concentration, and nitrogen pressure. Although the type of flux and its pre-heating temperature are considered critical in this immersion tinning process, they were clearly defined at the initial stages of the research transforming the initial flux type variable into a constant to reduce the DOE scope. The validation tests depend of the selected alloys compatibility with the different lead frame underplatings. The results indicate that the output variables of wetting and brightness acceptance criteria, plating thickness, peeling and solderability tests were all critical. These high impacted variables in conjunction with some others can be used to predict the process behavior.

Keywords: tin-lead solders, lead-free alloys,

INTRODUCTION

The semiautomatic tinning process where this research work was developed has some particularities that make it

unique in some way. It’s an immersion tinning performed basically through a wave soldering process. This special

characteristic forced the researcher to combine the information, experiences and technical skills from two well-

studied processes into a solely different application where qualitative and quantitative variables had to be

simultaneously considered and finally analyzed during this research.

Empirical investigation in conjunction with Six Sigma Methodology scrutiny concluded that the high impact

input factors were mainly the bath soldering temperature, percentage of copper contained in the lead-free alloy

(directly related to the solder temperature variable) the conveyor speed and the base metal pre-cleaners

composition. Although the flux type and its pre-heating temperature are considered critical in this immersion

tinning process, they were clearly defined at the initial stages of the research transforming the flux type initial

variables into a constant in order to reduce the DOE scope. The output key variables that supported the success of

the work implementation were the wetting and brightness acceptance criteria, plating thickness and peeling test.

Solderability is also clearly a critical output for this process application. This last test mostly depends on the

selected alloys compatibility with the different lead frame underplatings.

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During the 2nd Annual Convention for Lead-Free that took place in November 2002, an initiative was

established which finally defined that the maximum percentage of lead allowed in any electronic device would be

0.1%. Several associations were involved during the discussions like the European SOLDERTEC and Japanese

JEITA (Japan Electronics and Information Technology Association). It was initially created as a recommendation

for the electronics industry, but later it became an international legislation when the EU WEEE (European Union

Waste Electrical and Electronic Equipment) and RoHS (Restriction of Hazardous Substances) Directives decided

to adopt July 1st, 2006 as the deadline to implement this requirement. This determination made obligatory that

every supplier that would want to continue or start trading of his electronic products within the European and

Japanese markets, should prove that his soldering and similar processes would be lead-free among other substance

restrictions. Besides lead, the European RoHS Directive also restricted the use of another five substances, which

were considered of high risk for the human health and the environment. Those substances were the hexavalent

chromium, cadmium, mercury and some flame retardants utilized in molding better known as polybrominated

biphenyls (PBB) y polyibrominated diphenyls-eters-oxides (PBDO). Lead was defined as a substance which

nature will have carcinogen repercussions and genetic affectation. Because of these reasons, its use was restricted

exclusively in those applications where there is not a similar reliable substitute yet (i.e. very high temperature

processes).

This change entails an immediate impact for those markets’ vendors specifically concerning the electronics

industry and more recently, in the automotive supply chain. China (the future world greatest market) also adopted

equivalent legislations to the EU WEEE/RoHS which target date has been set in some cases, also July 1st. The

obligation is to be ready, the soonest possible, to allow every vendor in the supply chain to perform its required

products re-qualifications.

This research by itself represents a high impact project and strategic planning for all of us who worked in the

electronics industry. This industry is probably the one that has experimented the fastest evolution in the last 50

years since the invention of the transistor.. It represents for us the engineers a true challenge in technology,

processes and materials science applications because of the high numbers of lead-free alloys available in the

market. Since the end of 2001, there has been a constant development of lead-free components by the big

manufacturers, however every process is in some way unique and there aren’t two processes entirely equal. The

definition of the critical variables in each case depends of many factors. Some of these factors have to do with the

product specific application, the equipment costs associated, the specific process under study and the cost-benefit

analysis usually requested by the organization. All this should be managed very carefully to maintain the same

level of product quality and reliability when using the conventional tin-lead alloy.

The projects of the evaluation of new products and processes within the organization, specifically in the Hall

Effect IC manufacturing area, represented the kick-off for the qualification of the following soldering and tinning

processes: immersion tinning and manual dipping; wave, reflow, immersion and hand soldering. Also and in

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accordance to the global initiative to convert all soldering and tinning processes to lead-free within the

organization, the starting point was to be focused in the identification and implementation of this initiative in the

key high volume process of semiautomatic immersion tinning of the Hall Effect IC sensors terminals.

The compilation and organization of the information generated during the project development at

Honeywell® was put together as an initiative to establish this research as an exploratory study since it covered a

topic that had not been widely studied even after looking for supporting data from different sources (seminars,

webinars, lead-free forums, etc.). However as the topic where became deeply researched, and data was generated

trail after trail, the study became more correlation-explicative. This, because one of the main objectives is to

establish the relationships among the different processes input variables and their impact or effect on the output

key variables. Even that this would suggest a mere correlative study, it does not end at this point since it pretends

to decode the causality dilemma. This means that the main purpose is to obtain the explanation about the impact of

the critical input variables in order to predict the results of the output variables through a mathematical model. This

was only possible through a series of systematic observations that allowed the repeatability of the process under

analysis.

Even that a process like this process implies the detailed description of the different variables under study; it

does not pretend to be independent. The goal is to achieve an analysis by establishing relationships between the

variables in order to predict the behavior of this process, considering that it is very specific. In terms of knowledge,

the objective is to reach an explicative investigation because of the nature and properties of the new materials

utilized. Due to the types of variables to relate, and to the initial goal is to determine which ones are critical as

process inputs based in their impact in the following key outputs: quality of the solder bath of the components

terminals (variables correlation). The new alloys characteristics and the critical of some process parameters (i.e.

temperature window, conveyor speed, and some other will have a high impact in the outputs soldering wetting

solder composition and acceptance criteria. This in addition to the flux self-properties that are required to

guarantee the correct cleaning and contamination removal from the terminals base material. Bixenman et al.

(2003) provided evidence that cleaning will be more difficult for lead-free assemblies. All this involves knowledge

in deep about the different materials behavior in order to meet the electronics industry product standards. In its

correlative nature, a series of key input and output variables are identified for being DOE analyzed once the

process is in control. By this means, there will be a big opportunity to establish a high confidence prediction level.

Using the explicative approach, we’ll be able to set the bases for similar lead-free processes (i.e. wave and

immersion soldering, re-flow, hand soldering and manual tinning) even if the they differentiate quite a bit from the

one studied here. This is a great advantage of this research, the methodology and knowledge that will be obtained

from it will allow its application to processes where similar materials will be utilized even if the variables may be

substantially different. This will only be possible through a clear and systematic structure of the techniques of data

management.

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The greatest advantage of this investigation is based in the methodology and knowledge that will be obtained

from it and its application to processes where similar materials will be utilized even if the variables may be

substantially different. This will only be possible through a clear and systematic structure of the techniques of data

management. In the same manner, the planning and problem definition have been determined through their

respective investigation questionings generating the correspondent hypothesis and variables.

The theoretical framework is presented and supported by studies mainly performed for wave soldering

developed by researchers, since that type of soldering is the most approximate and contribute to this study. This

research work is considered a mixed approach because of the involvement of qualitative and quantitative variables.

A process DOE was developed with five input variables detailed as solder bath temperature, conveyor speed,

percentage of copper content, pre-heating temperature and base material pre-cleaners concentration, and nitrogen

pressure. The validation tests depend of the selected alloys compatibility with the different lead frame

underplatings. The results indicate that the output variables of wetting and brightness acceptance criteria, plating

thickness, peeling and solderability tests were all critical. The goal was to achieve the response to the existing

dilemma about the best method to evaluate, qualify and implement a lead-free solution for the semiautomatic

immersion tinning process for the Hall Effect sensors in TO-92 package.

THEORETICAL FRAMEWORK

Ecologic Awareness in the Electronics Assembly Industry and Environmental philosophies and policies

Recent terms as “Green Manufacturing” or “Industrial Ecology” (Munie et al. 2002) have become, more and

more often, part of the industry standards especially for the electronics assembly operations, where the overall

environmental impact of any decision made has been taken into consideration. The intent is to optimize the use of

limited resources such as energy and materials, which should yield in the most environmental benefit possible.

Environmental issues have affected the methods used in electronics manufacturing. Take the 1990 Clean Air Act

and the Montreal Protocol for the elimination of the chlorinated fluorocarbons (CFC solvents) as two of the most

recent examples of these policies (CFCs were traditionally used to clean electronics hardware). New cleaning

(aqueous and semi-aqueous cleaning) and soldering technologies (low residue/no clean) were developed and

implemented as a direct result of this legislation. The impact of lead-free solders is much wider than the effects

caused by the CFCs elimination since lead is a major constituent in tin-lead solder alloys used everywhere in

electronics manufacturing from individual components finishes and boards (through hand, wave and reflow

soldering) (Whiteman, 2000)

With the WEEE Directive in Europe outlawing lead from some electronics devices produced and imported in

the EU by 2006 and foreign competition driving the implementation of lead-free electronics assembly around the

world, additional questions concerning the integrity and reliability of the various alloy compositions continue to

arise. A tremendous amount of interest exists in lead-free soldering. Most of this is derived from a fear of

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legislation and marketing activities. This has spurred a great deal of committee and consortia activity, some of

which has been very valuable to the industry. The total impact of lead-free implementation depends not just on the

characteristics of lead and the alternative alloys but also on such factors as energy use, recyclability, reliability of

products and support from an infrastructure for the chosen materials.

The environmental metrics required to evaluate the alternative options to lead alloys should rest on:

Materials used in the manufacturing of the product (including the environmental consequences of

obtaining these materials).

Materials consumption during the operation of the product.

Energy used in manufacturing the lead-free product.

Energy used in product operation.

Recyclability and/or re-usability at end of product life (including the new End of Life of Vehicle –ELV-

currently under implementation definition).

Emissions through the life cycle, i.e. in materials extraction, manufacturing, use and disposal/recycle.

Recyclability of manufacturing waste streams.

It is this approach, treating the manufacturing process as part of the total environment and not in isolation that

characterizes today’s industrial ecology. “The concept of industrial ecology is one in which economic systems are

viewed not in isolation from their surrounding systems but in concert with them. As applied to industrial

operations, it requires a system view in which one seeks to optimize the total industry materials cycle from virgin

material to finished good, to component, to product, to waste product and to ultimate disposal. Factors to be

optimized include resources, energy and capital” [Gilbert, 2004]. Once this concept is accepted, the question

becomes one of application. The tool-set putting these concepts into practice is called “Design for the

Environment” which is the next step.

The move toward lead-free electronics now underway in Europe and Japan is in addition motivated by the

requirements of end-of-life management and marketing strategy. Most of the low cost special tailored alloys,

mainly tin-copper (both metals with enough reserves, known to support the conversion to lead-free) are patented

by different vendors (Cookson, Nihon, Sony, etc.). These alloys can present manufacturing problems because it

will be difficult to maintain the exact composition in wave soldering as an example. Also most of the proposed

alloys require higher processing temperatures giving rise to a significant increase in energy usage. Even so, the

main concern regarding lead-solder has been the leaching of this heavy metal from electronics in landfill. Although

at present, several lead-free alternatives have been studied even for years in hope of finding a drop-in replacement

for Sn-Pb solder, none have been found. Each one depends of the specific process application and requires a

particular study in order to guarantee the same level of quality used to have with eutectic tin-lead. In conjunction to

the elimination of lead approach, it has been the improvement of the infrastructure and methods for metal recovery

and reuse in electronics.

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Several proactive actions have to be taken by both, the industry and governments to address the environmental

issues from the holistic perspective that represents the conversion to lead-free soldering and similar processes.

These can be mentioned as:

Develop an industry wide educational program to spread the word of Industrial Ecology and Design for

the Environment.

Add these attributes (of Industrial Ecology and Design for the Environment) to industry roadmaps.

Work to showcase examples of recycling and environmental excellence and improvement of many of

them now in place.

Foster environmentally “Green Design” by the government, tax incentives, foster education at the state

level to aid industry in making improvements in process (efficiency, materials and power), encourage

economically viable recycling for an infrastructure for returned electronic equipment (efficient recycling

centers), encourage research in the area of recycling and green design.

Raise the level of public awareness and education in the areas of conservation and recycling. Stress the

challenges of energy and materials consumption. And finally foster the environmental and business

improvement through cooperation

Existing Soldering Processes and Technologies

The American Competitiveness Institute’s Electronics Manufacturing Productivity Facility (EMPF) initiated a

lead-free soldering program. The program’s objective was to become familiar with lead-free soldering by

determining the process variables associated with lead-free soldering concentrating on surface mount technology

(SMT) applications. The program then will determine the differences between lead-free soldering, tin-lead and tin-

lead-silver soldering; with respect to solder joint appearances, board finishes, solderability and process residues.

The plan was to compare results from using lead-free solders with tin-lead and SnPbAg solders serving as a

baseline. The issues covered in the program consisted of:

Screen-printing / components placement.

Reflow soldering thermal profiling / equipment.

Boards finish solderability.

Components finish availability.

Lead-free solder joints.

Lead-free solder residues.

a) Wave Soldering

From prior investigations, it’s understood that lead-free solders do not wet as well as their Sn-Pb counterpart.

Solder paste vendors have indicated that their lead-free solders are not affected by board surface finishes.

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Consequently, this asseveration can be also applied to our case of study since the surface is bare copper just like

the boards mentioned here. By experimentation, this claim will be verified. For the different processes discussed in

this paper, we should consider that almost every component is already available in a lead-free finish. Vendors

started introducing lead-free finishes almost from the beginning to the market although it’s being mainly

determined by market demand and specific customer requirements.

In addition, there is not enough product reliability data with respect to material compatibility, solder joint (which

in our case we will associate with the term of wetting in order to avoid confusions) and electrical reliability (see.

Clech, 2004; Seeling and Suraski, 2005). Lead-free finishes under consideration have been Sn, Sn-Cu (and its

family series), Pd-Ni, Sn-Ag and Ag although this last one in much less percentage. While it has been possible to

manufacture modules with lead-free solders with current soldering equipment, the lead-free solder joints will look

different than their Sn-Pb counterparts. The lead-free solder joints will not be as bright and shinny as Sn-Pb and

will have a grainy appearance. Havia et al. (2005) reported the solder quality achieved in a lead-free wave

soldering process using Sn-Ag-Cu (SAC) alloy. On the basis of their results, it cannot be shown that SAC process

could be optimized as well as tin lead process with respect to solder defects. Arra et al. (2002) found in their

research that with optimum flux and process parameters, it is possible to achieve acceptable process performance

with the SAC alloy using a dual wave system.

b) Hand Soldering: iron and pot.

EMPF performed brief trials with lead-free hand soldering with the objective to determine the difference

between lead-free and Sn-Pb solders. It was determined that the solder tip temperatures were higher. Generally

speaking, the solder temperature required was above 650ºF (343ºC) for lead-free versus 600ºF (315ºC) for tin lead.

The soldering iron must remain on the solder joint longer, prolonging the dwell time to promote adequate heat

transfer to the hardware. However the soldering iron must be removed quickly to avoid causing “icicles” on the

solder joint. The phenomenon is dependent upon de alloy utilized. Operators must be more diligent in assuring that

their soldering irons are clean when using lead-free solders as opposed to Sn-Pb solder. Lead-free solders are more

sensitive to dirty solder tips than their Sn-Pb counterparts, probably due to the higher solder temperature

employed, which accelerates the tips oxidation process. The lead-free solder joints will appear grainy, with a duller

finish than Sn-Pb’s. It is felt that in order to achieve the same quality of solder joints; a more active flux may be

required to overcome any solderability issues.

c) Reflow Soldering

There is no drop-in replacement for Sn-Pb pastes used in surface mount technology soldering; but when

considering melt point, toxicology, cost, availability and chemical resistance, the Sn-Ag-Cu or SAC series has

emerged as the most acceptable compromise, both for solderability and reliability. For SMT, the most serious issue

is backward compatibility related to insufficient reflow temperatures or time above liquidus for SAC balls- (217ºC

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eutectic temperature) with Sn-Pb paste combined solder to melt. For the combined alloy in the joint to completely

melt, the joint temperature must be greater than 207-210ºC and held long enough for mixing to occur. Low peak

reflow temperatures (208ºC) with short time above Sn-Pb liquidus (30-60 sec) resulted in unacceptable solder

joints. The Sn-Pb solder paste melted, but large portions of the SAC solder balls were intact. This lack of melting

in the SAC alloy originated reliability issues. A solution to this issue is based on longer times above liquidus (90-

120 sec) at a higher temperature (222 ºC).

Analysis, identification and justification of the new materials

This paper shall take an interesting depth view of lead-free Sn-Ag-Cu and Sn-Cu-Ni alloys and compare the

reliability testing results and process considerations. Although the Sn99/Ag0.3/Cu0.7 and Sn99/Cu0.7/Ni0.3 alloys

have a relatively short history in the hybrid circuit and electronics assembly industry, the fact that they’re low cost

alloys (since they derive from the Sn-Cu family) has made them very attractive for manufacturing operations.

Some in the industry feel comfortable utilizing these alloys as lead-free alternatives. Several potential issues have

to be considered when working with these alloys. First, the high melting temperature of the alloys, 221-228ºC for

Sn99/Ag0.3/Cu0.7 better known as SACX0307; and 227ºC for Sn99/Cu0.7/Ni0.3 known as SN100C in the

industry. Second their peak reflow temperatures of 240-260ºC, which in some cases is too high for many surface-

mount parts and processes. However, their low silver content condition has made them accessible or the some of

the best choices for high volume low cost operations and consequently for the general consumer electronics

applications.

More important, is that these alloys have shown good reliability tests very similar to the standard tin lead and

better results if compared with the original Sn-Cu (Sn99.3/Cu0.7). This has been an advantage over the high-silver

content alloys. In addition to cost; the high silver alloys have experimented fatigue-testing fails which further

investigation has revealed the silver phase change as the root cause. This is provoked by the various cooling rates

at the different areas of the high-silver content alloy. A structural weakness observed during this test could occur in

a solder interconnect and potentially lead to a field failure.

Despite the concern regarding patent legislation, in general most of the world is setting on the Sn-Ag-Cu (SAC)

family of alloys. But which exact alloy formulation should one select? The paper will focus on the low-silver

content alloys and their product, process and equipment implications of the selection. Before we go any further,

it’s useful to compare these two alloys empirically. In general both alloys are very similar since both are

considered from the Sn-Cu family. Both offer very good fatigue characteristics and good overall joint strength and

sufficient supply of base materials (Nihon Superior (2003). However some minor differences do exist which worth

further discussion.

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Melting points

The melting points of these two alloys are very similar (218-228 ºC for SACX0307 and its balance SACX0300 –

Sn99.7/Ag0.3; and 227 ºC for SN100C and its balance SN100Ce–Sn99.7/Ni0.3). Although it is debatable as to

whether this will have an impact in real world applications. However if one can control the wave-immersion

process strictly, the temperature reduction will have a positive effect initially in terms of reducing the components

exposure to high temperatures avoiding reliability risks; secondly in extending the life of the solder alloy because

of the reduction of the copper dissolution rate in the deposit, and finally in prolonging also the life of the

equipment since even 5-10 ºC reduction when continuous use can contribute to this purpose.

Wetting

When comparing these two alloys, it is necessary to question why one would select an alloy with silver content

instead of nickel, as this will increase costs (silver and nickel better known as stabilizers in these alloys). Some

have theorized that the silver content, even if it’s low, will aid in wetting. However, as the wetting tests have

demonstrated, the alloy with low nickel content actually wets stronger and faster than the one with the silver

content, although this better condition is obtained at a higher temperature because of the nickel material intrinsic

properties (typically 10ºC higher). While it is logical to contain costs, there are some issues with Sn-Cu series

(where the SnCuNi –SN100C belongs to) alloys that must be considered. First, the high melting point of the lead-

free alloys selected (227°C typically) makes them in some cases, prohibited in many temperature sensitive

applications.

In addition, and as widely proven, these SACX0307 and SN100C lead-free are poorer wetting alloy compared to

the tin-lead solder. However the cost tendency drove the efforts of the experimentation through the low cost alloys

mentioned above. Initially and during the preliminary evaluation runs, the process itself forced us basically to

increase the immersion contact time at the wave in order to try to reach above 2 sec if possible. Scimeca et al.

(2003) point out this is the typical value for lead-free alloys. However it has a direct impact in the process output,

which by the way, was very closely monitored by production management. The contact time increase was

accomplished by reducing the conveyor speed from the typical tin-lead 1.1 ft/sec to 0.6 ft/sec. We kept in mind

that insufficient contact time would result in incompletely tinned leads. We noticed also that wetting time could be

shortened by raising solder temperature, although this will have a direct impact in the solder life time and also

might affect the critical internal gold wire bonding of the IC since raised temperature would be too close to the

absolute maximum operational.

An important difference between the SACX0307/SACX0300 and SN100C/SN100Ce is their good fatigue

characteristics (similar to tin-lead) if compared with the pure Sn-Cu alloy. This poorer wetting results in the

mandatory use of a nitrogen atmosphere to guarantee the optimum cleaning of the solder. More active fluxes are

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also required to minimize the wetting related defects. An important consideration when deciding about the flux

selection has to be with the intrinsic nature of the friendly-environment lead-free process itself. In our case of

study, the corporate implemented an environmental policy, which restricted the use of alcohol-based fluxes. Tests

information available (supported by the market main flux vendors) has indicated that the best options to start are

the volatile organic compounds free or VOC-free fluxes.

After working very close with the flux manufacturer, the Superior 30DS (double-strength) VOC-free water-

soluble became the right choice because of its environmental-health safe and lead-free specific application design.

This flux was defined after several short trials where specific fluxes from different suppliers were evaluated and

once reviewing the technical specifications of the samples received.

The conclusion was that in order to meet both the environmental and lead-free requirements, the initial selection

had to be a VOC-free water soluble type because the immersion tinning requires a post cleaning operation to

remove all undesired residues from the process The Superior 30DS which met above two requirements and with a

very good pre-heating activation temperature range (90-135°C) instantly became the number one selection for our

evaluation. The fact that this flux will have a low activation temperature will avoid the necessity of changing this

section of the equipment. In addition, the flux reaches is maximum activation at 260°C, which is the temperature

the new lead-free solder alloy suppose to be working at.

The selected flux was entirely compatible with both lead-free alloys and as a background, it had been

successfully utilized by a couple of tinning or immersion soldering manufacturers although both at a much higher

soldering temperature (330 ºC typ). This was going to be the first time the Superior 30DS flux would be used at

275 ºC. Although the soldering quality achieved with fluxes designed for traditional tin-lead soldering was also

reasonable; the quality could not be sustained for more than 2-3 days which agreed to what vendor had indicated

before initiating the trial runs. This difference may originate from the fact that lead-free fluxes are usually higher

solids content and higher acid values. In order to compensate the residues left by these high acid fluxes, a water-

soluble type was chosen to match the post washing section of the immersion tinning process. Fig. 1 shows the

Semi-automatic Immersion Tinning Process for TO-92 IC.

Similar to a traditional wave soldering process, this lead-free immersion tinning-wave soldering was

recommended to have a slightly higher temperature in order to mitigate the thermal shock. When raising the

preheating temperature, caution was taken against deactivating flux prematurely. Since a rise in temperature

subject components to greater thermal stress, it was weighted that for critical characteristics purposes, the more

adverse effect to the ICs was a higher preheating temperature since this would result in a poorer wetting condition.

After some analysis, it was determined that the component could withstand the thermal shock originated by the

switching from 135 to 275 ºC in a matter of 1 sec typ. The raising should be in such a way that soldering

intermetallic will occur.

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Patent situation

It is desirable for the industry to find an alloy that is widely available. Therefore, patented alloys have been

viewed as undesirable. Unfortunately, the alloys discussed throughout this paper belong to this last classification:

SACX0307 patented by Cookson Electronics and SN100C by Nihon Superior. This is because there isn’t still in

the market similar low silver or nickel equivalents that can perform as well as the patented ones. Qualitek has

initiated with its low-silver own alloy Sn99/Ag0.2/Cu0.8 own alloy although a little late we believe. A more

circumspect view needs to be taken to understand the impact of patents and the true number of sources available

for these alloys. For example, while SN100C started being manufactured and distributed by Nihon Japan

exclusively; it’s being recently released for being manufactured and distributed by several other vendors who have

merged in this lead-free enterprise. This joint has created a wider availability of the alloy within the US where this

material was obtained for this research work.

Cost of metals

The high silver content alloys results in a costly bulk solder form alloy (typically in the range of $18+ per pound).

To fill a wave soldering pot of 250 lbs (like the one used for this paper experimentation), every lb used represents a

difference of $10-13/lb. To combat this expense, the alternative patented alloys studied here have emerged as the

best options which standard cost per lb is in the range of $9-11. The risk of this dual alloy process remains since

we should remember that lead-free technology would be in combination with tin-lead for some time until the

transition is fully implemented worldwide.

Dual Alloy Assembly

It should be noted that, in addition to the problems associated with the use of a new lead-free alloy, the

utilization of two solder alloys (i.e. Sn-Pb and Sn-Ag-Cu or Sn-Cu-Ni for wave soldering and similar applications)

could result in application incompatibility as well. Although it’s is undesirable to intermix two alloys because this

could result in non-uniformly alloyed solder joints, it’s also known by experience that this intermix is inevitable

due to the electronics world won’t switch to lead-free simultaneously. The whole process could take years before

being entirely lead-free and even then, we will end facing the issue that there will be several approved lead-free

alloys combined in the different soldering processes. The bottom line is that a dual alloy assembly process could

potentially result in reliability problems. This is another reason to be very careful when deciding about the lead-

free alloy to be implemented.

There isn’t a drop-in solution especially if there are several soldering processes that need to be converted to

lead-free in a very short period of time (i.e. wave, reflow, immersion and hand soldering and even immersion

tinning).

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Fig. 1 Fig. 1 Semi-automatic Immersion Tinning Process for TO-92 IC®

Although the materials standardization should be the main goal, it’s very possible that we will end with

different fluxes for the same alloy depending of the specific application. For example: while a water soluble flux

may be the best option for a wave soldering or immersion tinning process, a no-clean flux would be the optimum

for hand or reflow soldering where no post cleaning is possible.

As pointed out earlier, silver is the cost element in the Sn-Ag-Cu alloys. Since the low-silver content alloys

eliminate the potential for silver phased change problems and offer improved wetting and a slightly lower melting

temperature (in addition that they are available from several solder manufacturers), they have been recommended

for widespread use in Japan by the JEIDA industry organization. The low-silver alloys provide users with the

advantages of the Sn-Ag-Cu family of solders at a lesser cost, which makes them suitable for all soldering

applications contributing to eliminate the problems associated with a dual alloy process.

Equipment Considerations

Lead-free solders may cause severe corrosion to materials used in wave and similar soldering machines like our

Corfin solder-dip immersion tinning pot and pump. Different solutions to prevent material degradation are for

example titanium construction, nitrided stainless steel treatment (which is the option we selected for our new lead-

free pots), melonite QPQ coating, ceramic-coated stainless steel (option utilized for our manual dipping pots –

matter of an ulterior study) and cast gray iron. The two 250 lbs capacity solder pots used during the research

13

project were nitrided stainless steel made in order to withstand the eroding impact of the lead-free alloys. Pots

were manufactured to have a 10-year lifetime.

Solder and Dross Concentration

During the research, solder and its dross were analyzed regularly. Dross was analyzed by solder vendor

laboratory to observe if solder and dross compositions differ from each other. Any copper percentage above 0.7%

would indicate a solder degradation by copper dissolution from the IC lead-frames. It was supposed, based in other

investigations and technical information and seminars from lead-free specialists, that the intermetallics between tin

and copper like Sn6Cu5 are heavier than SAC/SN solders meaning that they will not float on the solder surface like

with tin-lead. It was assumed that intermetallics heavier than solder will sink to the bottom of solder pot and on

that way copper would not be removed with dross removal in the same proportion as there is copper in the solder.

On that way, copper concentration will increase. Copper will dissolute from the component lead frames and this

process characteristic is even more critical in lead-free alloys than in tin-lead even after a few hundreds of lots. If

by any cause, the lead-free tinning process (pot) has been contaminated by lead over solder substitution or solder

transition, lead dissolved into the solder pot from that contamination (even if it’s just the components) might entail

problems.

If the process has been contaminated and the applied solder contains 0.1% of lead, the lead concentration will

not be considerably lessen in the solder pot even if new solder is introduced into it. Almost the whole solder

volume ought to be changed, which would be a remarkable expensive action. An important note to mention at this

point is that during the different trials of the research, it was noticed that “needles formations” tended to appear

when utilizing the SACX0307 alloy every time the solder was reaching those few hundreds of lots just mentioned

in above paragraph. This needles formation behavior did not happen with SN100C alloy.

At the beginning the “needles” were associated with iron or another heavy metal contamination, however they

seemed to appear almost every time wetting problems started even on the surface of the solder.. This was

contradictory with the expectations of this lead-free alloy. Laboratory analysis identified a high copper

concentration as the root cause of those needles determining that copper was dissolving at a much higher rate than

initially expected. The copper concentration raising was in direct relation with the fact that every certain dozens of

lots, the tinning process forced itself to increase its solder temperature in 5-7 ºC because of the wetting problems

experienced each time. When reaching 310ºC (which is the absolute maximum operational for the Hall devices) a

high concentration of tin-copper whiskers on the surface is noticed and they start to stick to the IC leads. A

cleaning process basically consisting in removing the dross and whiskers then is performed.

The tin-copper whiskers are found at the bottom of the pot just below where the wave is produced. Even after

this cleaning process, the wetting problems did not go away and it was necessary to start fresh with new solder.

The root cause was determined to be an accelerated copper dissolution rate due to the continuous increasing of

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solder temperature, which was necessary to correct the wetting problems. Previous studies had reported less large

primary Ag3Sn precipitates in SAC alloys depending of the cooling rate of the specific alloy composition. Large

primary Ag3Sn would have an influence on the ductility of the alloy. However in our particular case of study the

Sn6Cu5 intermetallics were the ones primarily found as whiskers when composition was analyzed at the laboratory.

Evaluation, Analysis and Qualification Procedures (Product Reliability And Process Repeatability)

Since copper dissolution can not be eliminated but just reduced in acceleration, we can only hope it can be

controlled to maximize the number of processed lots and equalize it to tin-lead behavior. This is achievable by

following the next steps:

Copper dissolution rate is much higher in this immersion-tinning-by- wave process than in any standard

PCB soldering process. This is because the lead frames of the ICs are bare copper C19400 without any

under coating. Then copper frame immediately starts to dissolve as soon as it makes contact with the

wave.

As the copper concentration increases, a higher temperature is required to maintain the same initial

wetting characteristics. This makes a no-return cycle until reaching the maximum allowed temperature of

the device.

The start point was the replacement of the SACX0307 (Sn99Ag0.3Cu0.7) alloy for the SACX0300

(Sn99.7Ag0.3) as the preferred alloy. By eliminating the initial copper concentration, the life of the solder

in the deposit can be prolonged.

Although solder manufacturers theory had indicated that the less copper in the alloy, the higher the rate of

dissolution until reaching its natural stabilization. This means that the initial 0.7% copper concentration also

represents the rate dissolution stabilizer. Not having copper in the starting alloy should increase the dissolution rate

until reaching stabilization from which both alloys will start behaving the same. This characteristic would

represent that both alloys could end having the same lifetime.

However the only alternative was the SACX0300 since the SN100C required a higher operational temperature

(290-295ºC), which was close to the device allowed maximum absolute (310ºC). Any process variation could raise

the solder temperatures and in order to avoid this potential situation, the SN100C and its SN100Ce balance were

placed as second alternatives in case the SACX0300 would have not worked. The initial temperature of the fresh

solder was maintained as low as possible at 262ºC instead of going to 275ºC or higher from the beginning. The

initial optimum was set by visual examination of the wetting at the process output until it was considered

completely acceptable and repeatable. This 262ºC starting temperature was very close to the 260°C used for tin-

lead.

Cleaning by dross elimination was performed once a week minimum, or even more often depending of the

wetting inspection feedback. Although more solder was consumed (probably an additional 15-20%), this process

15

helped in achieving the 1000 lots goal set by engineering management. This amount represented the same quantity

of lots regularly processed when using tin-lead.

By the time this research was considered complete, a total of +1300 lots had been processed with the same lead-

free solder change showing no signs of remarkable copper dissolution (corroborated by the weekly laboratory

analysis performed to the solder sample showing less than 0.4% copper) where solder temperature had just been

increased to 272 ºC leaving still a 30ºC gap before reaching the maximum allowed. The dross removal was

possible by reducing the solder temperature to 240°C.

At that specific temperature, the dross emerges to the surface and it can be easily removed assuring the periodic

elimination of tin-copper whiskers The refill or balance was done by using only the SACX0300 which became the

sole approved alloy for this immersion tinning process. Conveyor speed could be increased although not as fast as

with tin-lead. End conveyor speed was 0.85 ft/sec with opportunity of continue being improved.

Acceptance Criteria and Specifications

Both tin-lead and lead-free alloys are going to be used in the same factory and even in the same process during

the transition period. This is because automotive and some other customers will continue ordering tin-lead

products while the rest has already requested them in the lead-free versions. Over this time, caution must be taken

to prevent solders from mixing with one another. Each of the two processes requires separate and well-marked

tools so that no tin-lead solder will be transferred to the pot for lead-free and vice versa. On adding solder, the

operator must make sure that no tin-lead bars are accidentally put in the lead-free pot and vice versa. Every solder

bar has its composition marked over it. Solder bars of different materials should therefore be stored well apart or

have their composition clearly marked over the packaging (ProTechnik, 2000).

Regarding wetting in general, solder tinning of both lead-free alloys fulfilled the IPC-A-610 workmanship

standard specification. However, the wetting result could fall closer to the acceptance limit depending of the frame

under-plating, the direction of immersion wave solder tinning (parallel preferred to perpendicular) and the frame

design itself. Another major difference was the rougher and duller surface appearance exhibited by the lead-free

alloys (especially the SACX0307), which is only cosmetic if voids and other porosity imperfections are not

present. Galarza et al. (2002) reported the impact of soldering atmosphere, solder bath temperature and conveyor

speed concerning the soldering defects needs to be analyzed. For our study, tinning is the soldering equivalent and

the average bridging and wetting results were considered. As could be seen, nitrogen clearly reduced bridging due

to better conservation of flux activity and improved wetability in a non-oxidizing atmosphere.

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Fig 2 Copper Frames before and after the Immersion Tinning.

METHODOLOGY

The correct utilization of all the resources during this research allowed us to obtain the answers for the key

objective of this research. Which match alloy-flux should be selected based in the product and process principles?

And, which technique should be applied to allow us the establishment of a reliable prediction method for the

process behavior? The specific piece of equipment under study is the semiautomatic immersion tinning solder-dip

machine where the Hall Effect sensors TO-92 package are processed. The Standards from the Association

Connecting Electronics Industry (2005) were used.

Data collecting and processing:

Using Design of Experiments (DOE) the approach for the different evaluation runs will be presented. From here,

the results of the critical factors will be analyzed. The DOE has been planned to be a 2 levels and central points,

complete factorial and with at least one replica, since the tinning process it allows us to develop the whole

experimentation. To approach the DOE and also to analyze the data generated from it, we were assisted by the

statistical software Minitab®

a) Critical factors: To define the factors that should be considered for the DOE (screening phase) we used the

Methodology Six Sigma in conjunction with the existing studies about wave soldering. Also we considered

the process experience from the key personnel and the new alloy-flux suppliers expertise about these new

materials, (all together) The formats were focused in collecting the most valuable information for the

research.

b) Acceptance criteria: KAPPA studies supported by the new annexes for lead-free soldering contained in the

IPC Standard IPC-A-610, will assure the product reliability and best available training of the key people

that will eventually be inspecting the product and monitoring the process. This is in order to standardize the

17

acceptance criteria within the organization. We should not forget that qualitative (proportions) and

quantitative (averages) data would be handled simultaneously.

c) Sample size: A preliminary sample size of 6000 peaces has been set which equals one production lot to

guarantee the repeatability of the process. This is based in the current experience and knowledge we have

about the immersion tinning process. The final sample size will be according to the results obtained from

the evaluation runs and in accordance to the MIL-STD sampling tables for equivalent populations

contained in a production shift of 24 -26 lots.

d) Supporting tooling: Data analysis tooling like: Pareto analysis (solder bath defects), cause-effect diagram

(critical factors screening), SPC (for the critical factors identified through the PFMEA and Control Plan), t-

tests for difference of means and possibly correlation-regression analysis would be used during this study.

e) Primary and secondary sources: The data collection was taken by direct observation when the runs were

performed. . The equipment was available so most of the information came from the primary source, which

is the immersion tinning process. The rest of the primary sources were the alloy supplier technical support

and the materials analysis laboratories and also from the specifications of the new materials. The secondary

sources have been mainly seminars, webinars, scientific magazines, studies and papers from other

organizations that utilized similar processes and specialized web sites and forums.

As the deadline comes closer, day-to-day, there are more and more research papers published mainly through

scientific bulletins in the web showing the reliability results about the different processes, materials and

applications web showing especially the latest reliability results about the different processes, materials and

applications. The whole research was performed as a direct field study since it was performed at the moment the

process is running with the modified equipment dedicated to it once it is been converted for lead-free.

RESULTS

Solder joint reliability testing

In order to analyze the reliability of SACX0307 and SN100C alloys, both were subjected to various thermal and

mechanical fatigue tests. The description and results of these tests are detailed in below paragraphs. Thermal

cycling: a 30 pc TO-92 Hall Effect sensors sample from three different lots were tinned using SACX0307 and

another three samples from same lots were tinned using SN100C lead-free alloys. All these samples were thermal

shocked from -40oC to +125oC for 1000 cycles @ 30 min cycle. Tinned terminals were then examined under

magnification and cross-sectioned inspecting for cracks.

Post-test inspection showed that neither of SACX0307 nor SN100C exhibited any solder crack issue as a result

of poor wetting (peeling). In addition, well-formed solder layer between base metal (copper C19400) and the lead-

18

free alloys (as a result of the good wetting) was observed and concluded. We evaluated the tinning (equivalent to

soldering in typical wave application) by a simple method of calculating the incompletely tinned leads.

We obtained a high quality of tinned leads by using the following set values:

Flux: VOC-free water-soluble Superior 30DS recommended for high temperature lead-free applications.

Solder pot temperature: 275ºC for SACX0307 (and SACX0300) and 295 ºC for SN100C (and SN100Ce).

Contact time: 1.5 sec approx.

Soldering atmosphere: nitrogen (at tinning wave only).

Conveyor speed: 0.6 ft/sec.

All above set values had a major influence in the tinning quality of the IC leads. No splashing or pop-corn

effects in tinning were observed as a consequence of the thermal shock experienced by the IC leads when passing

from preheating to soldering temperature using water based flux. This was an indication of an accurate activation

of the flux when entering the immersion tinning section. Flux preheating temperatures from 95 to 135 ºC had no

remarkable influence on quality of tinning indicating that as long as we maintained the preheating within the

manufacturer recommended set values, we’ll be in pretty good shape. It was seen by visual examination that the

high lead-free temperature of both SACX0307 and SN100C processes did not inflict damage to the plastic package

of the components evaluated (Hall Effect and IR) which had been molded in Plaskon 3300SH and Nitto plastic

respectively. Components were subjected to electrical testing and full laboratory validation through the course of

this research.

Process Design of Experiments

The objective of this study was to investigate the characteristics of the SACX0307 (Sn99/Ag0.3/Cu0.7) and the

SN100C (Sn99/Cu0.7/Ni0.3) lead-free alloy. The equipment used was the semiautomatic immersion tinning for

TO-92 ICs using a solder wave, in order to define the right materials and combination of process parameters that

would enable it to be adopted for high volume production. The solder pot was made with a special surface

treatment which is resistant to the attack from the lead-free solders, so that the dissolution of the pot material to the

lead-free solder bath could be minimized: The equipment had a nitrogen atmosphere. The alloy selection criteria

were based in the desire to use the same alloy on all soldering processes: immersion tinning by wave, wave

soldering, immersion soldering, reflow soldering, hand soldering and manual dipping. This made the SACX0307

(and eventually also the SACX0300) to be the most attractive choice for the development study. Several foam

fluxes specially formulated for lead-free (alcohol and water based types) were evaluated in the study of these lead-

free alloys where the VOC-free water-soluble Superior 30DS was the one that best matched the process

application. The performance of the selected flux under several process conditions was tested to determine the

process window of the foam flux system. A process pre-heating temperature window of 95 to 135 ºC was

established for this Superior 30DS flux in conjunction with its dependence concerning the different tinning defects.

19

From previous studies we already know that parallel orientation is always better when focusing in wetting and

yield goals. We also realized that thermal cycling have no significant impact when monitoring lead-free tinning

defects. The minimum solder temperature observed during this experimentation, which can guarantee a good

wetting on the tinned components was 265 ºC.

The recommended solder temperature used in this immersion tinning lead-free process was determined to be

275ºC. The improved wetability can be explained by the fact that the time for the full wetting of surfaces by solder

is known to be faster at higher temperatures. In addition, the solubility of components metal into the molten lead-

free solders increases with temperature. In the same way, wetting was enhanced with a slower conveyor speed (or

longer dwell time) due also that solubility of metals into solder is time–dependent. No delaminating of any kind

was noticed neither in the package-terminal joints nor internally in the die-frame wire bonding.

Solder composition was analyzed regularly. Special attention was paid to possible increase in copper or lead

content. Analysis revealed a significant increase in copper content during the test series when using SACX0307

because of the initial set point of the soldering temperature (285-290ºC) and the starting stabilization copper

amount (typically 0.7%) contained in this lead-free alloy. It was noticed that once reaching 2.5% copper

concentration or higher, the process basically became out of control presenting serious wetting problems in

tinning. The only correction when reaching this copper concentration was to change solder and start fresh. The

amount of lots that could be processed with this alloy was 40% approximately of the amount regularly processed

when using tin-lead. This situation was fortunately corrected by changing to the no-copper alloy SACX0300 and

by setting the solder temperature to the minimum allowed by the quality of the wetting (which was 270 ºC

typically).

CONCLUSIONS

A process DOE was developed with five input variables detailed as solder bath temperature, conveyor speed,

percentage of copper content, pre-heating temperature and base material pre-cleaners concentration. Although the

type of flux and its pre-heating temperature are considered critical in this immersion tinning process, they were

clearly defined at the initial stages of the research transforming the initial flux type variable into a constant to

reduce the DOE scope. The validation tests depend of the selected alloys compatibility with the different lead

frame underplatings. The results indicate that the output variables of wetting and brightness acceptance criteria,

plating thickness, peeling and solderability tests were all critical.

The different soldering parameters were investigated to find out, which ones have the most remarkable influence

on the tinning wetting. Evaluated process parameters were flux type (alcohol vs. water based), preheating

temperature (95-135ºC), solder temperature (260-305ºC), solder contact time (through conveyor speed from 0.6

ft/sec to 1.1 ft/sec averaging 2 sec contact time during immersion) and soldering atmosphere (nitrogen/air). It was

20

observed that the solder temperature and contact time have the greatest impact in the wetting of the tinned

components although the alloy type played a key role at the end of the research. Once the basic parameters were

determined, product-dependent fine-tuning of the process can be carried out. Although the plastic packages of the

Hall Effect and IR devices had been initially designed for tin-lead soldering applications, they both withstood the

higher lead-free temperatures without any major issues as could be corroborated when completing the full product

qualification at corporate laboratories.

The main problems reported with lead-free solder in this semiautomatic immersion tinning process were lower

yields by poor wetting of the lead frames, increase frequency of dross cleaning and gradual change of alloy

composition which basically became a contamination of the solder bath during continuous manufacturing. This

last issue due to copper dissolution rate which created the necessity of continuous copper monitoring. Test series

were run on a Corfin solder-dip machine equipment especially adapted for lead-free applications; and utilizing a

foam flux also especially designed for this process. The alloys used were the SACX0307 and SN100C as the initial

ones although at the end, the recommendation was their replacement by their no-copper balances SACX0300 and

SN100Ce.

DISCUSSION

Preliminary discussions take us to the following statements: Lead enters the land through the industrial wastes

and residues even with the existing confinement and recycling programs. This action contaminates the aquifers

entering the lead by this manner, into the food chain. This will generate health problems since it’s a carcinogen,

neuro-toxic and hema-toxic substance. Actions that have been taken to eliminate the lead come from the 1973

legislations, which prohibited the use of lead in gasoline, paints in 1978 and now the electronics industry to start in

2006. Leadership countries on this subject have been Japan and the European Union.

Then the elimination of lead from this industry is not a matter of if, but until when it’ll be allowed. There are a

great number of lead-free alloys available in the market with very similar characteristics than the eutectic tin-lead.

These alloys need to be evaluated with the different types of fluxes developed for this purpose. This materials

evaluation process will determine the best option for the semiautomatic lead-free immersion tinning process under

study and its implementation using the facility equipment.

Tin-lead and SACX0307/SACX0300, SN100C, SN100Ce lead-free processes are subjected to many similar

solder defects. Moreover, many of the defects found on lead-free solder can be remedied with the same methods

used for tin-lead. Tin bridges can be reduced by a change in solder wave and conveyor travel rates for both types

of processes. Solder bridges however are often observed in lead-free likely because of the higher tension of the

lead-free alloys. Lead-free process can be optimized in the same way as tin-lead. The two main differing

parameters between tin-lead and lead-free are the solder temperature and solder contact time. Incompletely tinned

components will be found if contact time is too short and/or solder temperature is too low. Using longer contact

21

time compensates the lower heating over the solder melting point compared to tin-lead solder. The process

window for lead-free SACX and SN is narrower and because of the solders melting points, they both operate

(especially the SN100C) much closer to component failure limit, giving a smaller maneuvering space. There have

been reported the use of some dopants (like phosphorous and germanium) to prevent dross formation in order to

give a drier, more powdery and voluminous dross. Although the studies with dopants are out of this project scope,

it shall become the following step for process optimization considering that dross will need to be removed more

often in the lead-free alloys. The frequent dross removal is necessary to delay the tin-copper Sn6Cu5 whiskers or

intermetallics formations, which long-term production consequences result in a change in the fluidity of the solder.

As could be seen during the research, copper levels can go as high as 10% in a relatively short period of time

(less than 2 months typically) depending of how high the solder bath temperature is set (once reaching 300ºC

copper dissolution rate raises exponentially from less than 1-2% to 10% or higher in a matter of days). Since lead-

free alloys leach copper from the frames faster than tin-lead solder especially when using SACX alloys (compared

with SN100’s), a significant risk to lead contamination is posed also if lead contaminated tools are used in solder

pot maintenance, like in removing dross.

The results presented in this study showed a significant number of lead frames that were immersion tinned

through a process very similar to a wave soldering process. On that way, a large number of copper frames could be

tinned and a more comprehensive view on lead-free tinning was achieved. On the basis of our results, the

SACX0300 can be optimized as well as tin-lead with respect to normal tinned defects keeping in mind that process

window for this lead-free process is narrower than for tin-lead solder.

Acknowledgements

We want to acknowledge the support of Honeywell® Optolectronics especially Sabino Castañeda for the support

provided to perform this research. We also acknowledge the helpful comments from Robert Wallace (Cookson

Electronics), Alan Roberts (Corfin Inc.) and Phil Baskins (Superior Flux & Manufacturing) for their contribution

to this work. Although the work performed during this research was a team effort within Honeywell

Optoelectronics, Rafael Delgado wants to specially acknowledge Dr. Elsa M. Benavides Ph.D. who supported this

research first by identifying the importance of this work and secondly, by encouraging me in publishing this paper

where the information developed throughout the research can contribute for future applications as a method for

process predictions.

References

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