Atomic level, solders

In diffusion creep, atoms diffuse along the grain boundaries from zones with compression stress to zones with tensile stress. This creep mechanism is very slow and is activated with low stress levels and high temperatures. The creep rate also depends on the diffusion length the atoms have to travel and thus on the grain size. Diffusion creep does not play an important role in solder joints of electronic assemblies because of its slowness. 

It can also be seen from Fig. 7 that Ag and Cu diffuse rapidly in Sn. While these measurements are for single-crystal Sn at tracer diffusion levels, it is to be expected that these constituents in Sn-Ag-Cu Pb-free solder diffuse very rapidly in the solder matrix. For instance, Cu is observed to have a diffusion coefficient of approximately 3 x 10 cm /sec at 150 °C in the slow direction in single-crystal Sn [56]. If this number is used for the diffusion coefficient in polycrystalline Sn, then Cu atoms would only require approximately 100 sec to diffuse throughout a 50-p,m-diameter solder ball. In fact, as reviewed in detail below, large Cu concentrations in Sn-Ag-Cu (SAC) alloys profoundly affect IMC formation. 

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