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The evolution of low-temperature solder alloys requires thoughtful consideration of the thermal and mechanical reliability requirements specific to each application. Thermal cycling and drop (mechanical) shock tests have long been used as board-level proxy tests for evaluating and quantifying the ability of electronics assemblies to sustain abrupt, short-term mechanical stresses, as well as continuous cyclic stresses. For example, higher or lower thermal cycling and drop shock performance of Sn-Ag-Cu alloys have been associated with very specific microstructure features of the resulting alloy.
Sn-Ag-Cu alloys with 3-4 wt.% Ag, such as SAC305, have a higher amount of Ag3Sn, which results in higher thermal cycling performance, as shown in Figure 2.1. Lowering the silver content to <1 wt.%, such as in SAC0307, increases the tin content and reduces Ag3Sn in the microstructure, resulting in relatively lower thermal cycling and higher drop shock performance. Further improvement of drop shock performance can be achieved with minor alloying additions, which are generically referred to here as X, such as observed in SACX 0307.
A similar model has not yet been identified for low-temperature solders, but it is reasonable to conclude that the performance of LTS alloys depends on both the individual and combined effect of its various alloy constituents, including micro-additives. However, constructing such a model is not as straightforward, given the variety of additives and varying bismuth content of available solders (Figure 2.2). With over a decade of developing and testing SnBi solder alloys, it became quite evident that bridging Sn-Bi drop shock and thermal cycling performance gaps was not a trivial solution.
