New Generation Pre-deposited (No-Flow) Underfill for Low-Cost Flip chip Assembl?

Authors : MG Firmstone, PM Bartholomew and DJJ Lowrie.

Multicore Solders Ltd, Advanced Products Division, Kelsey House.Wood lane End Hemel Hempstead, HP2 4RQ, Herts. UK

Mfirmstone@aol.com, MartinBartholomew@compuserve.com. DavidLowiegcompuserve.com

Abstract The achievement of an acceptable balance between

Flip Chip reliability, process throughputicomplexity, final yield and cost is an increasingly difficult task, especially in the competitive hand-held electronics market. A simple process, compatible and integrated with normal SMT processing, is a desirable goal which will be enabled by the substitution of a self-fluxing, pre-deposited underfill for the conventional post-deposited materials currently in use. The pre-deposition process also has the potential to move Flip chip assembly into mainstream SMT, especially if the ability to rework can be built-in.

The benefits of a ‘no flow’ process have been well documented in the recent past. The limitations of the previously reported materials in current use have been overcome via a unique chemistry which can be tailored to the application. Room temperature storage, effective fluxing, coupled with no outgassing, and a choice of reworkability after reflow or non-reworldfall cure, can now be achieved within a single materials technology.

This paper describes the properties of the new family of materials compared to conventional post-deposited underfills. The development sequence and the procedure for characterisation of material properties, including the evaluation of the effectiveness of the fluxing action on a range of solder alloys, is documented.

A typical application is described, outlining how a minimum of 2 process steps can be eliminated, improvements in materials handling, process robustness, and ultimate yield, have been realised. A simple rework regime is proposed, and the almost ‘drop in replacement’ aspect of the new material is discussed.

Introduction The benefits of underfilling a flip chip assembly, in a

non-hermetic environment, with an organic encapsulant are well documented. In particular, the reliability of the component, in terms of the resistance of the bump metallisation to fatigue effects during thermal cycling, is considerably enhanced [ 1,2].

Until recently, the vast majority of underfilled assemblies have been produced using a system that involves the capillary flow of underfill encapsulant into the gap between a pre-assembled die and substrate. Very sophisticated dispensing equipment has had to be developed in order to control the dispensingiflow process, ensuring a minimum of incomplete fills and voids. After the underfill flow process, it is normal practice to dispense fillets around the perimeter of the die. This ensures an even distribution of mechanical stresses around the die, and provides additional protection against the ingress of moisture from the environment.

Simpler process to control - equipment can be less sophisticated (and cheaper) than with capillary flow

Lower material and processing costs

Higher throughput - no delay due to capillary flow process

Much reduced tendency for incomplete filling and voids to form

Additional filleting process not required (assuming optimised dispense pattern)

Self-fluxing action - self-alignment feature of solder bump reflow retained

Process sequence more akin to conventional SMT than capillary flow process - easier for SMT engineers to adopt

typical process sequence for the self-fluxing underfill,

The consequence of incomplete fill and voids can be a major reduction in reliability due to incomplete ‘clamping’ of the flip chip bumps, especially if the void is adjacent to a bump. Also, Al track corrosion on the die is possible due to incomplete encapsulation of passivation pinholes. Another potential problem is the effect of non-uniform electrical properties in the encapsulant across the die surface. This will become more of an issue as the operating frequencies of computing and telecom products continue to increase. Despite the levels of control that can be exerted over the capillary flow process, incomplete fills and voids can still be a real problem. This, if anything, will tend to get worse with filled underfills as die to substrate gaps, and bump pitches continue to reduce.

A new method of dispensing underfill, the so called ‘no-flow’ process has been developed in recent years [3]. The underfill is dispensed onto the substrate prior to flip chip assembly, necessitating a self-fluxing action on the solder bump reflow process. The chemistry of the underfill is designed to convert fiom a flux to a cured epoxy encapsulant during the solder reflow process and subsequent post baking procedures.

The pre-deposited (no-flow) route has the following major benefits compared to conventional capillary flow underfilling :

1

compared the standard process for conventional capillary flow materials is illustrated below :

256 0-7803-4934-2/98 $10.00 01998 EEE

Pre-deposited1 Self-fluxing

Dispense dfill

Place die

Reflow solder bumpslcreate edge fillets

Post-cure dfill

Inspecthf necessary

Electrical test

Capillary flow

Apply flux to bumps

Place die

Reflow solder bumps

Clean (if req.)

Inspect

Dispense dfill, allow to flow

Dispense edge Fillets Cure dfill

InspectiX-raylSAM

Electrical test

Reworkability is a desirable characteristic in an underfill. Conventional hlly cross-linked thermoset materials (e.g. unmodified epoxies) remain relatively hard and difficult to rework when heated. Epoxies that have been specially modified to make them soluble in certain mixtures of solvents [4] have been developed but are not yet in widespread use. During the development of the new underfill materials it became apparent that it would be feasible to rework at the point when the structure was partially cross-linked, i.e. alter the solder reflow process, but before the post reflow bake.

Multicore Solders Ltd has been active in the development of self-fluxing underfills. The work documented in this paper was directed at the achievement of a self-flux ng underfill that did not suffer fiom the problems associated with a poor (peven) fluxing action, and the necessity for low temperature storage.

Development of Enhanced Self-Fluxing Pre-

Multicore Solders, due to it’s extensive expertise in solders and flux chemistry, and strong links with the polymer chemistry industry, is in a excellent position to further enhance the concept of self-fluxing pre-deposited underfill technology. The potential problem of premature cross-link h g during storage (normally necessitating storing the material in a fieezer) was designed out at an early stage by the use of special activators. In fact, the new encapsulants can be stored for long periods (several weeks) at room temperature.

Some of the ‘no-flow’ underfills have been found to be unpredictable in their fluxingiunderfill transformat ion behaviour. Metal salts are formed during the fluxing action due to the dissolution of oxides ftom the die buinp metallisation (especially eutectic Sn-Pb alloys). The mctal salts have the effect of catalysing the epoxy crosslinking process, thus leading to the possibility of reducing the

Deposited Underfills

effectiveness of the fluxing action and preventing the self- alignment process during solder reflow. In extreme cases it has been found to be possible for a thin insulating layer of polymer to form in between the die bump and substrate/board pad, leading to open circuits. This is particurlarly observed in CSPs which have an “overhang” outside of the bump area as shown in Fig.1. It is thought that the mechanism involves a tilting action as the first solder balls melt which creates a capillary action which in t u n creates a deficiency of underfill at the opposite edge. The ratio of metal salts to epoxy is thus increased and catalyses the curing reaction. A thin film of (part) polymerised epoxy forms either on the ball, in the solder mask aperture, or both. When the solder balls in these vicinities melt they are unable to make contact with the board metallisation. It has been observed that the bumps can be robustly glued down to the substrate metallisation, but with no electrical connection.

I I

U

I Chip Placed in Underfill I

Underfill Fills Gap

Reflow Cycle D l I

Chip ‘Tips’ as Solder Bump(s) Melt

Underfill flows - Capillary Action

Fig. 1 Potential Failure Mechanism in Underfilled Component

A similar problem can occur where the solder mask aperture is too small for the unreflowed solder bump

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The epoxy is trapped in a thin film and does not become mobile with the bulk of the material as the viscosity reduces with increasing temperature. Again, the ratio of metal salts to epoxy is high and premature curing occurs.

Of course these problems can be overcome via careful design and control (perhaps stand-offs on such CSPs) . Careful balancing of the profile may also alleviate the effect. However, in practice, it is mush better to put the design expertise into the material. In this way the potential failure mechanism is eliminated and a robust, tolerant process results which is not constantly balanced on a knife- edge. A modification of the fluxing I curing chemistry has thus been introduced whereby the metal salts produced during fluxing are not catalytic to the epoxy reaction and the fluxing activity is retained and actually enhanced.

It was found, during assembly trials, that C4 bumps (Pb rich), interfaced to HASL, did not give rise to appreciable amounts of catalysing metal salts, due to the much lower levels of Sn oxide present and that trouble free flip-chip assembly could be achieved with the originally developed material without the modification to the fluxing I curing chemistry. However, this may well depend on the oxidation state of the HASL, benigness of design, andor profile, or other factors as yet unknown. It is vital, therefore, that the fluxing action is not compromised as defective solder joints must not be allowed to exceed the low parts per million level, in line with current microelectronics assembly practice.

The test methods used to evaluate fluxing action and catalysing potential are described in the next section. The reflowiunderfill (pre)curing profile used throughout the development programme is shown in fig.2

50

04

0 32 64 %U81601!32224256288320

Time (seconds)

Fig 2 Solder reflow/underfill pre-cure profile

Characterisation of the Properties of Newly Developed Underfill Materials

Fluxing action Fig 3. illustrates one of the test specimens that was

used to verify the fluxing action. The method used in this case was to place a small solder pellet onto an alumina tile which was then subjected to a thermal profile as indicated in Fig.2. Good wetting was indicated by the solder forming a sphere

with a bright, shiny surface. High melting point solders were assessed solely on surface appearance. T w \-ersions of the underfill were evaluated. the original deyelopment ix-ersion 11 and version 2 incorporating the modified fluxing curing chemistp. Several lead-fiee solders were included in &e 12s:.

The detailed results of this work are given in Table 1 (set Appendix). Fluxing was good , i - ep good in all cases with version 2 demonnating enhanced fluxing action on \-e? high tin metallurgies

Underfill J-1 -.J \rA I

Fig.3 Wetting Test Sample Configuration

Catalysis / Premature Curing Potential As previously discussed, the tendency for thin layers

of the material to @art) cure prematurely is undesirable for both self-alignment and joint formation A simple but extremely effective method was developed for observing and quantifying the effect based on the force required to penetrate the part polymerised underfill. Test pieces were prepared from copper clad FR4 with one third of it’s surface coated with the solder alloy under investigation. The board was then coated with a contolled 100 micron thickness of underfill (Fig 4.) and reflowed using the profile given in Fig 2. Following reflow, probe C (Fig 4.) was placed in contact with the bare Cu surface on the test piece. Probe A, or B, was then slowly pushed into the underfill until a short circuit was indicated by the resistance meter. The degree of force on the probe to accomplish this was recorded in each case. The probe B measurement, which is fairly constant as it always monitors the underfill on Cu, acts as a control and baseline measurement for comparison to the various solder alloys tested.

The results of this work are given in Table 2 (Appendix). The degree of cure after one reflow cycle is related to the hardness and thus the comparative penetrating force required. It is clear @om the figures that the effect is mainly associated with the tin bearing alloys and that a dramatic improvement is effected with the modified chemistry of version 2. (especially with the 63/37 eutectic). This has been confirmed by the live assembly trials.

anticipated and will be investigated further. Again, version 2 appears to solve the problem.

The catalytic effects of the In and Bi alloys were not

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Resistance Meter r=

Solder Coated With Underfill

Copper - Probe Point

Copper Coated With Underfill

Fig. 4. Test set-up for resistance probe measurements

Results of Microsectioning Samples that were microsectioned have shown

extremely good wetting of the Sn 63 eutectic solder to the Cu surface of FR4 boards (not specially cleaned ) This is illustrated in Fig. 5. below.

Impurity levels For a self-fluxing underfill system to be reliable,

great emphasis must be placed on the elimination of any potentially corrosive, or conductive, acid radicals and ionic species. During the transformation f7om flux to epoxy resin, the chemistry should ensure that all acid and hydrolysable ionic contaminants are minimised and 'locked-up' in the structure of the epoxy. It was found, during the development programme, that it was necessary to apply a post reflow bake (15OC for 30 minutes) after the solder bump reflow process. This had the effect of completing the cure that was initiated during solder reflow, neutralising the fluxing chemistry via the stoichiometry of the cross-linking process. Typical formulations have shown Acid Residue values of less than 5 mgigm, and ionic C1 less than 15 ppm. It is anticipated that the final version(s) will also be in this range. Optimum component reliability and electrical performance is thus achieved via the post reflow bake, and the potential problems of catalysing the system to achieve compete cure during the reflow cycle have been well expounded here. Whilst a post bake is not entirely process efficient,the benefit is that the fluxing action and joint formation is not compromised, and due to the partially cured state of the underfill, rework of the component can be achieved prior to post bake

Thermo-mechanical and Electrical Properties (TCE, Tg, modulus, bulk resistivity)

Information deemed to be of a non-proprietory nature will be presented at the Conference

Viscosity stability/pot life The viscosity stability of the newly developed

materials was determined by monitoring the viscosity using a Haake model no. PK 100 cone and plate viscometer over a period of 140 days. Measurements, carried out at 25' C, at a shear rate of 300s-' , determined that there was no significant rise in viscosity until a period of 120 days storage at room temperature had elapsed (see Fig. 6)

- 1 I

Fig. 6 Viscosity of underfill during RT storage

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Rework properties Acknowledgements In order to verify the rework potential of the partially The authors viish to thank Multicore Solders Ltd for

cured underfill, a commonly available benign solvent the opportunity and facilities to c a r p out the de\.elopmenr (isopropyl alcohol, IPA) was used to remove the underfill work, and permission to publish this paps. Dr S XIannan and from the chip site following component removal. The site was Dr D Hutt of Loughborough University are thanked for their easily cleaned and prepared, kesh underfill deposited and a continuing support, preparation of samples. and micro-section new component successfully reflowed. This simple, effective photographs. regime was demonstrated following either one or two reflow cycles.

1. Initial thermal shock tests

Initial thermal shock testing has been carried out on underfilled components ranging fiom 5mm x 5mm chips to 13” sq. CSPs, mounted onto FR4 boards. Using daisy- chained continuity circuits, no failures were encountered through 100 cycles of temperature range 0°C to 100OC. Another series of tests on 7mm x 7mm flip-chipped die on FR4 also gave good results in air to air thermal shock tests (-40C to +8OC, 12 cycles). Further thermal shock tests will be

carried out to characterise the performance of the underfill over a wide range of die sizes and temperature ranges. Similarly, long-term thermal cycling tests will be performed in order to verify the thermo-mechanical behaviour of the underfills.

2 .

3.

4.

Summary and Conclusions A self-fluxing, no-flow underfill has been developed

that is free from the disadvantage of requiring dry ice shipping and freezer storage, having a room temperature storage life (pot-life) in excess of 4 months. The development phase of this material placed special emphasis on consistent fluxing action with a variety of solder alloys, including Pb-free types.

A catalytic mechanism has been identified, mainly associated with the tin salts produced during the fluxing action. This can induce localised premature curing of the epoxy, and in turn, open circuit joints. Off-line methods have been developed to examine and quantify this failure mechanism, and a fluxinglcuring chemistry developed to eliminate it. Formulations incorporating this improvement have been evaluated via the off-line tests and subsequently proven in live tests with both flip-chip and CSP formats.

Initial thermal shock test results indicate that a reliable assembly can be manufactured using the simplified pre-deposited underfill process, saving several process steps, and having the capability of being ‘dropped in’ at the stage where a tacky flux would normally be dispensed. The identification and elimination of a hitherto unreported failure mechanism has resulted in the development of a robust and tolerant materials technology, focussed on successhl fluxing and joint formation.

Room temperature shipment and storage, and the ability to rework defective components, are logistical bonuses, which, alongside the fundamental properties, represent a forward step in the propagation of flip-chip technology to the mainstream ShTI engineer.

References Schubert Dudek R, et al, ‘Thermo-Mechanical Reliability of Flip-Chip Structures Used in DCA and CSP’ Int. Symp. On Advanced Packaging materials, Braselton, Georgia, USA, 1998 Gamota D, Melton C, ‘Advanced Encapsulant Systems for Flip-Chip’ 3rd Intl. Symp. on Advanced packaging Materials, Braselton, Georgia, USA, 1997 Shi S, Jefferson G and Wong CP, ‘High Performance Underfills for Low-Cost Flip-Chip Applications’ Intl. Symp. On Advanced Packaging Materials, Braselton, Georgia, USA, 1997 Buchwaiter SL, Call AJ et. al, ‘Reworkable Epoxy Underfill for Flip-Chip Packaging’ 1’‘. Intl. Symp. on Advanced Packaging Materials, Process, Properties, and Interfaces, Atlanta, 7 Feb. 1995

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