Effect of joint sizes of low-temperature sintered nano-silver on thermal residual curvature of sandwiched assembly

Residual bending could be formed in sandwiched assembly as a result of coefficients of thermal expansion (CTEs) mismatch and temperature difference. An optical measurement system was developed to study the residual curvature of sandwiched assembly bonded by three different kinds of die-attachment materials, i.e., nano-silver paste, Pb/Sn solder, and SAC 305, in this paper. We found that all the sandwiched assemblies at ambient temperature concave toward the alumina side and the residual bending was relieved as the temperature rose. It also turned out that the residual curvature in the assembly grew as the joint size increased. The residual bending in nano-silver assembly fell in between those of Pb/Sn solder and lead-free solder, SAC 305 (Sn96.5Ag3.0Cu0.5). Finite element analysis (FEA) was also introduced to simulate the bending behavior of the assemblies. It was shown that the FEA results agree well with the experimental one. In addition, a one-dimension theoretical model was developed and could characterize the relation between the residual curvature and the joint size.