Metallurgical analysis and hot storage testing of lead-free solder interconnects: SAC versus SACC

Lead free solder alloys have been developed which contain small concentrations of rare earth additives such as cerium and lanthanum. It is believed that rare earth element additives refine the microstructure of the solder and improve the mechanical durability and thermomechanical integrity of lead free solder joints. This paper presents the results of a comparative analysis of solder joints with the commonly used SAC 305 alloy (Sn 3.0% Ag 0.5% Cu) and a SACC alloy (Sn 3.0% Ag 0.5% Cu 0.019% Ce), which contains a small concentration of a Cerium additive. The influence of cerium on the microstructural refinement of the bulk solder; the metallurgy of the intermetallic compounds; and the damage evolution of the solder during thermal aging tests are investigated. Mechanical die shear tests and lead pull strength tests have been conducted to compare the mechanical strength and the morphology of the fracture surfaces of SAC and SACC solder joints. Lead pull testing of samples with the cerium doped, SACC alloy exhibit more ductile fracture surfaces and PCB pad lifts, rather than in SAC alloy samples, which exhibit brittle interfacial failures. This change in the fracture surface morphology suggests that cerium additives improve the mechanical integrity of the bulk solder, and correlates with previous materials testing on bulk solder samples of SACC and SAC, which show that SACC solder has a higher Young’s modulus (higher stiffness), higher yield stress, and higher strength over a wide range of strain rates and temperature conditions. On the other hand, in the hot storage tests, the SACC samples exhibit thicker intermetallic formations at the bulk solder to component interface than in SAC samples, which suggests that SACC alloy solder joints could be less mechanically robust than SAC solder joints under dynamic loading conditions such as drop testing.