ams - r&d - hi-rel lead-free assy_smta

8/7/2019 AMS - R&D - Hi-Rel Lead-free assy_SMTA

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Vapor PhaseVapor Phase

A Current Overview and itsA Current Overview and its

Application for Pb Free ReflowApplication for Pb Free ReflowAtul C. MehtaAtul C. Mehta

Jet Propulsion Laboratory California Institute ofJet Propulsion Laboratory California Institute ofTechnologyTechnology

AndAnd

David S. SuihkonenDavid S. Suihkonen

R&D Technical Services Inc., Burnsville MinnesotaR&D Technical Services Inc., Burnsville Minnesota

SMTA InternationalSMTA International

October 8-11, 2007October 8-11, 2007

Orlando, FloridaOrlando, Florida

AMS Materials 866-772-5108AMS Materials 866-772-5108


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Vapor Phase or Condensation HeatingVapor Phase or Condensation Heating

Invented in 1973.Invented in 1973.

Uses an inert liquid chosen for its boilingUses an inert liquid chosen for its boiling

point. With vapor temp tied to the boilingpoint. With vapor temp tied to the boilingpoint of the liquidpoint of the liquid

Creates a very stable, uniform, heatCreates a very stable, uniform, heat

transfer in the form of vapor.transfer in the form of vapor.

Transfers heat with no shadowing,Transfers heat with no shadowing,

deflection or reflection.deflection or reflection.



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Earliest Vapor Phase UnitsEarliest Vapor Phase Units

First units used vapor only.First units used vapor only.

Next generation added some form of preheat inNext generation added some form of preheat in

the form of a secondary vapor blanket thatthe form of a secondary vapor blanket that

boiled at a lower temp then the primary vapor.boiled at a lower temp then the primary vapor.



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Typical Inline Vapor Phase SystemTypical Inline Vapor Phase System

ComponentsComponents

Some form of conveyor system, Preheat Zone,Some form of conveyor system, Preheat Zone,

Reflow Zone, Cooldown Zone, Fluid Filtration,Reflow Zone, Cooldown Zone, Fluid Filtration,

Vapor Recovery, Cooling System, ExhaustVapor Recovery, Cooling System, Exhaust

System, Controls and Recipes.System, Controls and Recipes.



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Advantages for Pb Free Solder ReflowAdvantages for Pb Free Solder Reflow

Constant, uniform temperatures. No sideConstant, uniform temperatures. No sideto side or front to back temp. variations.to side or front to back temp. variations.

Absolute limit to maximum temperature,Absolute limit to maximum temperature,

no overheating of components or board.no overheating of components or board.Fast even heat transfer regardless ofFast even heat transfer regardless of

mass differentials, architecture, ormass differentials, architecture, or

package.package.No shadowing, deflection or reflection ofNo shadowing, deflection or reflection of

heat transfer.heat transfer.



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Heat Transfer ComparisonHeat Transfer Comparison



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Typical Vapor Phase Thermal ProfilesTypical Vapor Phase Thermal Profiles

Left graph using convected preheat.Left graph using convected preheat. Right Graph using vapor preheat.Right Graph using vapor preheat.



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Vapor Phase And PB Free ReflowVapor Phase And PB Free Reflow

Vapor Phase allows the user to close theVapor Phase allows the user to close thewindow between solder reflow temps. andwindow between solder reflow temps. andprocess temps.process temps.

A solder that reflows at 221 C* can beA solder that reflows at 221 C* can beeasily processed in a 230 C* vapor with noeasily processed in a 230 C* vapor with norisk of overheating.risk of overheating.

Transfers this uniform heat with littleTransfers this uniform heat with littleregard to mass of product.regard to mass of product.Uniformity through condensation.Uniformity through condensation.



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A Current Overview and its ApplicationA Current Overview and its Application

for Pb Free Reflowfor Pb Free Reflow

1.0 Introduction1.0 Introduction

Lead-containing solder has been

standardized and used for past 65 years

as the principal joining material for Level

2 packaging defined as attaching

component level packages to a suitable

substrate to produce printed wiring

assemblies (PWAs).AMS Materials 866-772-5108AMS Materials 866-772-5108


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Introduction (Contd.)Introduction (Contd.)

In past few years, there has been the ever-In past few years, there has been the ever-

growing importance of what is calledgrowing importance of what is calledDesign for the Environment (DFE) andDesign for the Environment (DFE) and

Green Manufacturing.Green Manufacturing.

Both emphasize the use ofBoth emphasize the use ofenvironmentally benign materials so thatenvironmentally benign materials so that

the environment is impacted as little asthe environment is impacted as little as

possible.possible.AMS Materials 866-772-5108AMS Materials 866-772-5108

V PhV Ph


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Introduction (Contd.)Introduction (Contd.)

Although there are a number of potentialAlthough there are a number of potentialsubstitutes for tin-lead solder, allsubstitutes for tin-lead solder, all

substitutes have properties somewhatsubstitutes have properties somewhatdivergent from that of tin-lead and willdivergent from that of tin-lead and willrequire both process development andrequire both process development and

validation.validation. In electronics the impact will be felt byIn electronics the impact will be felt bybare printed wiring boards, components,bare printed wiring boards, components,and electronic card assemblies.and electronic card assemblies.



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Application for Pb Free ReflowApplication for Pb Free Reflow

2.0 General Issues with Lead Free2.0 General Issues with Lead FreeSolderSolder

Processing temperatures (of Sn-Ag(-Cu-Sb)Processing temperatures (of Sn-Ag(-Cu-Sb)alloys) are higher than those used for 63Sn-alloys) are higher than those used for 63Sn-37Pb. Typically by 3637Pb. Typically by 3600 CCHigher reflow temperature requires use ofHigher reflow temperature requires use of

higher Tg board materials.higher Tg board materials.Performance of components.Performance of components.

Sn whiskers.Sn whiskers.High Sn content of Pb-free alloys.High Sn content of Pb-free alloys.



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3.0 Objective3.0 Objective

Assemble ten PWBs using SAC 305Assemble ten PWBs using SAC 305alloy and thermal cycle.alloy and thermal cycle.

The thermal cycle Commonly used forThe thermal cycle Commonly used for

space flight hardware is -space flight hardware is --55C to 100C.-55C to 100C.



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4.04.0 Pertinent Process InformationPertinent Process Information

Paste containingPaste containingRosin-based flux wasRosin-based flux wasused to produce all electronicused to produce all electronic

hardware.hardware.

Using the terminology of Mil-F-14256,Using the terminology of Mil-F-14256,

the classification of these products isthe classification of these products is

rosin mildly activated (RMA).rosin mildly activated (RMA).



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Pertinent Process InformationPertinent Process Information(Contd)(Contd)

The solder paste was applied using aThe solder paste was applied using a

semi-automated screen printersemi-automated screen printerensuring that the paste was depositedensuring that the paste was depositedin a uniform and consistent manner.in a uniform and consistent manner.

All boards were first visually inspectedAll boards were first visually inspectedfor proper paste deposition after thefor proper paste deposition after thestencil operation.stencil operation.



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Pertinent Process InformationPertinent Process Information(Contd.)(Contd.)

A laser-based solder paste height andA laser-based solder paste height and

width measurement system was used withwidth measurement system was used witha resolution of 0.0001 inch (2.5 m).a resolution of 0.0001 inch (2.5 m).

This system provided real timeThis system provided real time

information on the uniformity of solderinformation on the uniformity of solderpaste deposition.paste deposition.



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An automated placement machine wasAn automated placement machine wasused to place parts on the printed wiringused to place parts on the printed wiring

board (PWB).board (PWB).

A batch vapor phase reflow equipmentA batch vapor phase reflow equipmentwas used to form the solder joints.was used to form the solder joints.



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The reflow process consisted of preheatingThe reflow process consisted of preheating

the PWA with infrared heaters.the PWA with infrared heaters.

Preheat was applied to remove pastePreheat was applied to remove pastevolatiles and to initiate the activation stage ofvolatiles and to initiate the activation stage of

the paste.the paste.

The PWAs were reflowed using a constantThe PWAs were reflowed using a constantboiling perfluorinated polyether materialboiling perfluorinated polyether material

(boiling point 240C) for soldering the lead-(boiling point 240C) for soldering the lead-

free SMT PWAs.free SMT PWAs.AMS Materials 866-772-5108AMS Materials 866-772-5108


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The reflow liquid, since it boils at aThe reflow liquid, since it boils at a

constant temperature, minimizes theconstant temperature, minimizes thepossibility of overheating the PWAspossibility of overheating the PWAs

during reflow and ensures that the inertduring reflow and ensures that the inert

vapor blanket performs a uniform andvapor blanket performs a uniform andconsistent soldering operation.consistent soldering operation.



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Assembly Process (Contd)Assembly Process (Contd)

The BGAs were daisy-chained. VariousThe BGAs were daisy-chained. Various

mechanical packages were selected formechanical packages were selected forthe test and were acquired.the test and were acquired.



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for Pb Free Reflowfor Pb Free ReflowFigure 1 - Photo Of PWBAFigure 1 - Photo Of PWBA



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5.15.1 PWB and Component PreparationPWB and Component Preparation

All PWBs were cleaned in an AccelAll PWBs were cleaned in an Accel

centrifugal cleaner using Vigon A200centrifugal cleaner using Vigon A200

chemistry.chemistry.

Viagon chemistry consists of a 20%Viagon chemistry consists of a 20%

solution of a proprietary blend ofsolution of a proprietary blend of

alcoxypropanols and aminealcoxypropanols and amine

compounds in DI water with 1%compounds in DI water with 1%

corrosion inhibitor and 0.1% defoamercorrosion inhibitor and 0.1% defoamerAMS Materials 866-772-5108AMS Materials 866-772-5108


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5.2 Screen printing5.2 Screen printing

Printing parameters were as follows:Printing parameters were as follows:

Stencil Type - Stainless steel with foilStencil Type - Stainless steel with foilthickness of 7 mils;thickness of 7 mils;

Squeegee Type - Metal Blade;Squeegee Type - Metal Blade;

Squeegee pressure setting - 4.5 kg;Squeegee pressure setting - 4.5 kg; Squeegee speed - 15 mm per second.Squeegee speed - 15 mm per second.



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5.3 Component Placement5.3 Component Placement

Components were placed usingComponents were placed using

automated placement machine withautomated placement machine withvision alignment system.vision alignment system.

Flux paste was applied at CSP andFlux paste was applied at CSP and

PB18 devices.PB18 devices.



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Application for Pb Free ReflowApplication for Pb Free Reflow5.4 Solder Paste Reflow5.4 Solder Paste Reflow

A thermal profile was generated for theA thermal profile was generated for the

assembly prior to reflow. Figure 2 Shows theassembly prior to reflow. Figure 2 Shows theprofile. Figures 3 and 4 shows the boardprofile. Figures 3 and 4 shows the board

being reflowed.being reflowed.

Average preheat temperature was 163Average preheat temperature was 16300 CC

Average maximum reflow temperature wasAverage maximum reflow temperature was

23923900 CC

Average time above liquidus was 74 secondsAverage time above liquidus was 74 secondsAMS Materials 866-772-5108AMS Materials 866-772-5108


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Figure 2Figure 2



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Application for Pb Free ReflowApplication for Pb Free ReflowSolder Paste Reflow (Contd)Solder Paste Reflow (Contd)

Vapor phase reflow system was used toVapor phase reflow system was used to

reflow the solder paste.reflow the solder paste. It consisted of an infrared preheatingIt consisted of an infrared preheating

zone followed by a constantzone followed by a constanttemperature boiling vapor zone at 240temperature boiling vapor zone at 240C.C.

Reflow fluid used was GaldenReflow fluid used was Galdenperfluorinated polyether material with aperfluorinated polyether material with a

boiling point of 240C.boiling point of 240C.AMS Materials 866-772-5108AMS Materials 866-772-5108

V PhVapor Phase


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Figure 3Figure 3

PWA readyPWA ready

for reflowfor reflow


V PhV Ph


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PWA inPWA in Figure 4Figure 4

Reflow stageReflow stage

BoilingBoiling

FluidFluid



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Solder Paste ReflowSolder Paste Reflow(Contd.)(Contd.)

The assembly was first preheated toThe assembly was first preheated to

approximately 163C at the rate ofapproximately 163C at the rate of0.81C/second.0.81C/second.

It was reflowed in vapor phase system.It was reflowed in vapor phase system.

The dwell time above liquidus was 74The dwell time above liquidus was 74seconds.seconds.


V PhV Ph


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6.0 Post Reflow Cleaning6.0 Post Reflow CleaningAll PWAs were cleaned in an AccelAll PWAs were cleaned in an Accel

centrifugal cleaner using Vigon A200centrifugal cleaner using Vigon A200chemistry.chemistry.

Cleaning cycle consisted of 5 minuteCleaning cycle consisted of 5 minute

wash at 50 C and 10 minute DI waterwash at 50 C and 10 minute DI waterrinse at 55 C followed by 10 minuterinse at 55 C followed by 10 minutedrying at 80 Cdrying at 80 C


V PhVapor Phase


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7.0 Visual Inspection and X-Ray7.0 Visual Inspection and X-Ray

All PWAs were inspected under aAll PWAs were inspected under a

microscope at 12 X magnification. Themicroscope at 12 X magnification. Theobservations made are listed below.observations made are listed below. The solder flow generally appearedThe solder flow generally appeared

good except that the solder appearedgood except that the solder appearedgrainier compared to Sn/Pb soldergrainier compared to Sn/Pb solder

joints.joints. See Figures 5 through18See Figures 5 through18



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Figure 5Figure 5

BGA 256BGA 256

SolderSolder

JointsJoints






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for Pb Free Reflowfor Pb Free ReflowFigure 6Figure 6BGA 256BGA 256

SolderSolder

JointsJoints

Close upClose up



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for Pb Free Reflowfor Pb Free ReflowFigure 7Figure 7A-MLF72A-MLF72

Solder JointSolder Joint



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for Pb Free Reflowfor Pb Free ReflowFigure 8Figure 8A-MLF72A-MLF72


Close upClose up



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for Pb Free Reflowfor Pb Free ReflowFigure 10Figure 10PBGA 1156PBGA 1156


Close upClose up



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for Pb Free Reflowfor Pb Free ReflowFigure 11Figure 11X-RAY IMAGEX-RAY IMAGE

OFOF

ULTRACSP 192ULTRACSP 192



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for Pb Free Reflowfor Pb Free ReflowFigure 12Figure 12X-RAYX-RAY

IMAGEIMAGE

OF PB18OF PB18




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Figure 13Figure 13

X-RAY

IMAGE OF

PBGA256




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Figure 14Figure 14

X-RAYX-RAY

IMAGE OFIMAGE OF

PBGA256PBGA256

CLOSE UPCLOSE UP




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for Pb Free Reflowfor Pb Free ReflowFigure 15Figure 15

X-RAYX-RAY

IMAGEIMAGE

OF PBGA 1156OF PBGA 1156




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X-RAYX-RAY

IMAGEIMAGE

OF PBGA 1156OF PBGA 1156CLOSE UPCLOSE UP




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X-RAYX-RAY

IMAGE OFIMAGE OF

PBGA 1156PBGA 1156CLOSE UPCLOSE UP




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X-RAYX-RAY

IMAGE OFIMAGE OF

A-MLF36A-MLF36




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apo asep



6.06.0 ConclusionsConclusions

The following conclusions could be drawn.The following conclusions could be drawn.

No problems were encountered during theNo problems were encountered during the

printing process with lead free paste. Theprinting process with lead free paste. The

printing was uniform for all PWBs.printing was uniform for all PWBs.

A higher preheat dwell was required duringA higher preheat dwell was required during

the reflow process. This was due to the higherthe reflow process. This was due to the highermelting temperature of the solder.melting temperature of the solder.




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pp



Conclusions (Contd.)Conclusions (Contd.)

Although the solder fillets appeared toAlthough the solder fillets appeared to

be generally good, the solder jointbe generally good, the solder jointappeared grainier than those formed byappeared grainier than those formed by

Sn63/Pb37 solder.Sn63/Pb37 solder.

In the upcoming thermal cycling tests,In the upcoming thermal cycling tests,

several PWB assemblies will beseveral PWB assemblies will be

subjected to thermal cycling tests.subjected to thermal cycling tests.




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pp



7.07.0AcknowledgementsAcknowledgements The research described in this paper wasThe research described in this paper was

carried out at Jet Propulsion Laboratory,carried out at Jet Propulsion Laboratory,California Institute of Technology, under aCalifornia Institute of Technology, under a

contract with the National Aeronautics andcontract with the National Aeronautics andSpace Administration.Space Administration.

Authors wish to thank Reza Ghaffarian fromAuthors wish to thank Reza Ghaffarian fromJPL and and S. Manian Ramkumar fromJPL and and S. Manian Ramkumar from

Rochester Institute of Technology forRochester Institute of Technology forproviding sample boards and componentsproviding sample boards and componentsfunded under NASA Electronic Part andfunded under NASA Electronic Part andPackaging Program (NEPP).Packaging Program (NEPP).




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pp



8.08.0 ReferencesReferences

((1) Getting the Vapors John Sprovieri, SMT1) Getting the Vapors John Sprovieri, SMTMagazine, February 2002Magazine, February 2002

(2) 3M Product Information, 1990(2) 3M Product Information, 1990

(3) Single Vapor Batch Reflow to Eliminate CFC(3) Single Vapor Batch Reflow to Eliminate CFC

Usage Dale Linman, Centech Corp. 1999Usage Dale Linman, Centech Corp. 1999

(4) Proven Vapor Phase Accommodates Higher(4) Proven Vapor Phase Accommodates HigherTemperatures of Lead Free Solder, SolvayTemperatures of Lead Free Solder, Solvay

Solexis 2004Solexis 2004


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