Maximum entropy fracture model and fatigue fracture of mixed SnPb/Sn3.0Ag0.5Cu solder alloys

During the transition from Pb-containing solders to Pb-free solders, joints composed of a mixture of SnPb and SnAgCu often result from either mixed assemblies or rework. Comprehensive characterization of the constitutive and fatigue fracture behavior of these mixed solder alloys is necessary to predict the life of mixed alloy solder joints. Three alloys of 1, 5 and 20 weight percent Pb were selected so as to represent reasonable ranges of Pb contamination expected from different 63Sn37Pb components mixed with Sn3.0Ag0.5Cu alloy. In recent, related, work we developed constitutive relations for these alloys; this work focuses on the fatigue failure behavior. One recent approach to modeling fatigue fracture in ductile solids is the maximum entropy fracture model. The maximum entropy fracture model is a thermodynamically consistent and information theory inspired (non-empirical) damage accumulation theory for ductile solids, validated on both area array and peripheral array packages. The model uses a single damage accumulation parameter to relate the probability of fracture to accumulated entropic dissipation. A custom-built microscale mechanical tester capable of submicron displacement resolution was utilized to carryout isothermal cyclic fatigue tests on specially designed assemblies using the three mixed alloys. The resultant relationship between load drop and accumulated entropic dissipation was used to extract the temperature and geometry-independent damage accumulation parameter of the information theoretic model for each alloy. Combining our knowledge of the constitutive behavior with the damage accumulation behavior, life predictions can be made for a wide variety of package types and mixed metallurgical conditions.