Electronics Assembly Knowledge, Vision & Wisdom
Superior Thermal Cycling Reliability of PB-Free Solder Alloy
Superior Thermal Cycling Reliability of PB-Free Solder Alloy
Alternative solders meet stringent environmental regulations, requirements for greater mechanical reliability, and high temperature service environments.
Materials Tech

Authored By:
Takehiro Wada and Kimiaki Mori Koki Company Ltd.
Hiki-gun, Saitama, Japan

Shantanu Joshi and Roberto Garcia Koki Solder America Inc.
Lake Elsinore, CA, USA
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Summary
The consumer electronics industry has widely adopted tinsilver-copper (SAC) solder alloys for lead-free reflow soldering applications. The automotive electronics and power device industry demand high thermal fatigue resistance as compared to consumer electronics. Though SAC solder alloys fulfill most soldering requirements alternative solders are needed to meet more stringent environmental regulations, requirements for greater mechanical reliability, and more demanding high temperature service environments such as under the-hood in automobiles and in avionics systems. The alternative Pbfree solder alloy must satisfy both, the process as well as the reliability requirements. The new solders must form joints with acceptable strength, and, at the same time, must withstand thermal fatigue over the projected operational life of the assembly. Mechanical properties and microstructure are, therefore, among the main parameters to be controlled in the candidate solders in an industrial process.

Development was done on alternative lead-free alloys with varying percentages of tin-bismuth-indium-silver in comparison with a number of conventional lead-free alloys in soldering evaluations. By addition of small amounts of Indium, tin base solder alloys are strengthened without lowering ductility and improves the thermal fatigue resistance. However, adding large amounts of In causes phase transition from beta-phase to gamma-phase. beta-phase consists of a tetragonal crystal structure with many slip planes and directions and it is therefore very easy to deform. On the other hand, gamma-phase consists of a hexagonal crystal structure which consists of lesser number of slip planes and directions than beta-phase. Therefore, choosing an accurate amount of Indium to maintain a hexagonal crystal lattice was paramount for this study.

As-reflowed and aged samples (1000 hrs. at 150 Celsius in air) were prepared for mechanical and reliability tests. Tests included, tensile tests of solder alloy with micro dumbbell using as cast and after aging, x-ray diffraction was performed on solder powder, cross sections were prepared and microstructure was examined using scanning electron microscopy and energy-dispersive x-ray diffraction.

Accelerated thermal cycling test was performed on the boards assembled with various different alternative alloys and the results were compared with conventional Pb-free alloys. It was seen that indium and bismuth containing alloys lasted as much as 40% more than the conventional SAC alloys. The results of the tests are reported.
Conclusions
By evaluating Sn-based SABI alloy which contains 4, 6 and 8wt% of In for mechanical properties and thermal cycle resistance after reflowing the following conclusions can be drawn.

Greater than 6wt% In addition causes high presence of InSn4 which has low ductility, reduced Ag3Sn microprecipitates and increased In4Ag9 compound phase. In consequence, ductility in pull test and crack resistance in thermal cycle test will be impaired. If the In addition is less than 6wt%, there is not enough improvement by solid solution strengthening of In in Sn, and delivers relatively low strength, thus providing relatively low thermal cycle resistance than 6wt% In addition. Based on these results, it is concluded that the proper In addition to Tin-Silver-
Bismuth-Indium solder alloy is 6wt%.
Initially Published in the SMTA Proceedings
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