SMTA International Conference Proceedings

The Impact of Alloy Composition on Shear Strength for Low Temperature Lead Free Alloys

Authors: Traian C. Cucu, Ph.D., Carl Bilgrien, Ph.D., Anna Lifton, Irina Lazovskaya
Company: Alpha Assembly Solutions, a MacDermid Performance Solutions Business
Date Published: 10/14/2018   Conference: SMTA International

Abstract: 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).

Key Words: 

Lead-free, low temperature soldering, shear force, thermal-stress, thermal-cycling, CTE mismatch, reflow process window.

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