Electronics Assembly Knowledge, Vision & Wisdom
Impact of Alloy Composition on Shear Strength for Low Temperature Lead Free Alloys
Impact of Alloy Composition on Shear Strength for Low Temperature Lead Free Alloys
As assemblies, undergo temperature/power changes during use CTE mismatch causes added stress and strain on the solder joint.
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Authored By:
Traian C. Cucu, Ph.D., Carl Bilgrien, Ph.D., Anna Lifton, Irina Lazovskaya
Alpha Assembly Solutions, a MacDermid Performance Solutions Business
South Plainfield, NJ, USA

Summary
The low temperature assembly processes are continuing to rise in popularity throughout the assembly industry and the Sn-Bi based alloys act as an enabler for this transition. The use of a lower assembly temperature (lower peak, lower thermal energy used during the assembly process) brings a sum of benefits: Lower expansion/shrinkage due to CTE mismatch, reduced dynamic warpage, lower thermal stress during the assembly process and the use of environmental friendly processes. There is a plethora of different materials that are used to manufacture the boards and the parts that get assembled on the boards and they all have very different coefficients of thermal expansion (CTE). As these assemblies, undergo temperature/power changes during use (equipment switching on/off, day and night temperature changes, etc.) CTE mismatch causes added stress and strain on the solder joint. Increase in strain could result in solder joint degradation that ultimately will lead to joint failure (crack initiation and propagation through the solder joint fillet.

The next generation of low temperature (LT) solder pastes recently introduced to the market makes use of new low temperature alloys (LTA) and new chemistry platforms These innovations have brought the LTA joint mechanical performance nearly to the performance of a Sn-Ag-Cu (SAC) joint. The new alloy considered for this paper (X46) is a Sn-Bi system with micro-additives that further improve the base alloy properties, bringing its performance within SAC305 performance.

In this paper the mechanical performance of the joint is characterized using the shear test for passive components. This measurement is looking at the joints formed with the alloy by itself. In order to simulate the induced stress caused by the CTE mismatch occurring during the normal operation in the field, the test boards were put through two thermal stress regimes: thermal cycling (TC) and thermal stress (TS). The shear test was performed after predefined number of cycles, for both TC and TS in order to generate the degradation curve for the given joint. Different reflow profiles have been employed for the LTA, to further define the process window for a given alloy (shear resistance degradation).

Conclusions
The mechanical degradation of the joints formed with a set of LTA has been studied using SAC305 as a benchmark. The best overall performer was an alloy with micro-additives, with the alloy composition optimized for higher performance. The results suggest that selective addition of micro-additives can yield higher thermal-mechanical performance of the joint performance.

For the LTA in the test, as demonstrated for the LTA with micro-additives (X46) the mechanical degradation of a joint subjected to a thermal cycling type of stress, can be improved by both alloy design and by fine-tuning the assembly process (the reflow profile used to form the joint). The X46 alloy exhibited a high flexibility, showing good results (better than or equal to the benchmark) for all three thermal envelopes used in the assembly process. This suggests that it has a wide process window during the assembly process that allows its use in both joints formed with the alloy by itself and mixed joints.

The shear strength degradation for the LTA (high Bi alloy) is lower for the alloy optimized through the addition of micro-additives, and it shows that the optimized composition has an influence on the alloy performance. The reasons and mechanisms of this are out of the scope of this paper and it is a theme that is proposed for further studies.

Initially Published in the SMTA Proceedings

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