Strain Measurements and Thermo-Mechanical Simulation of SnAgCu vs. low melting point alloy (LMPA-Q) solder joints

Thermal cycling tests showed much higher thermal cycling lifetime for components soldered with a low melting point alloy (LMPA-Q), compared to the standard SnAgCu solder. This is not expected as due to its lower melting point, the LMPA-Q solder is deforming more and having higher creep strains during thermal cycling, in particular if the maximum temperature of the cycling is close to the melting temperature. These higher creep strains are not reflected in the time to fatigue failure.To better understand the physics of failure behaviour of both solders (LMPA-Q vs SnAgCu), the microstructural behaviour of both solders is analysed with scanning acoustic measurements (SEM) and the deformations of the solder joints during thermal cycling are measured using digital image correlation (DIC) method.Finally, the deformation and strain measurements are compared to thermo-mechanical simulations. The simulations also include the grain structures in the solder joints. The outcome reveals the importance of having grains with different orientation into the modelling to explain the outcome of the thermal cycling tests.