Modeling thermally induced viscoplastic deformation and low cycle fatigue of CBGA solder joints in a surface mount package

A nonlinear finite element model was used to analyze the thermally induced viscoplastic deformation and low cycle fatigue behavior of the lead-tin (Pb-Sn) solder joints in a 32mm ceramic ball grid array (CBGA) surface mount package. The effects of the cyclic frequency, hold (dwell) time, and temperature ramp rate on the response of the the viscoplastic (creep and plastic) deformation for the CBGA solder joints were studied by applying four different low cycle thermal fatigue loads to the package. The modeling results show that the maximum viscoplastic strains occur in and around the edge CBGA solder joint. The cyclic creep strain (ratchetting) is very significant and dominates the constituent of the accumulated viscoplastic strain. The equivalent plastic strain increases with the increase of cyclic frequency and ramp rate, and decreases as the hold time increases. However, the equivalent creep strain decreases with the increase of cyclic frequency and ramp rate, but increases as the hold time increases. In the solder joint, the Pb37-Sn63 solder paste has approximately 2/spl times/ larger equivalent plastic strains, and 10/spl times/ larger equivalent creep strains than that in the Pb90-Sn10 solder ball during thermal cycling.