Effect of joint size and pad metallization on solder mechanical properties

Metallurgical analysis, mechanical testing, and finite element analysis were conducted to understand the effect of joint size and pad metallization on solder behavior. A wide range of design and material variables was evaluated. The pad metallization affected both the intermetallic compounds at the joint interfaces and those dispersed in the bulk solder. NiAu pad metallization resulted in more creep resistant joints than Cu. These effects were more pronounced at lower test temperatures. Solder joint creep resistance increased with joint size. Larger joints were also more prone to brittle interface failure than smaller joints. This was true in both package level tests and board level tests. Finite element analysis indicated that the predicted fatigue life in cyclic drop or temperature cycle testing can be significantly affected by the test vehicle used to generate data for constitutive constants. More creep resistant behavior resulted in lower strain and work for both temperature cycle and drop test conditions. When comparing strain vs. work as a damage indicator, it is seen that work is less sensitive to variations in the constitutive constants.