(The) influence of reflow profile on the thermal fatigue behaviors of BGA solder joint

Abstract

In service, an electronic package is subjected to thermal cycling fatigue corresponding to power on/off and cyclic environmental temperature changes during transportation or storage. Solder joints, which provide mechanical support as well as electrical connections, have long been used in electronic packaging applications. The microstructure evolution of solder alloy has become known as a major factor influencing the mechanical behavior of solder joints. Thus, it is important to study the effect of microstructure changes on the thermal fatigue of a solder joint induced by temperature changes. In this study, the thermal fatigue reliability of two solder alloys, eutectic 63Sn37Pb solder and lead-free 95.5Sn4.0Ag0.5Cu solder, was examined. In order to attain different microstructures, the cooling rate during the reflow process was controlled obtaining a fast cool and a slow cool. For the thermal fatigue test, four test specimens were prepared, 63Sn37Pb fast cooled and slow cooled and 95.5Sn4.0Ag0.5Cu fast cooled and slow cooled. The results of the thermal cycling test from -40°C to 125°C show that 63Sn37Pb specimens reflowed with a slow cooling rate have a longer life than the specimens reflowed with a fast cooling rate. In contrast, for lead-free 95.5Sn4.0Ag0.5Cu solder, a fast cooled specimen has longer life than a slow cooled specimen. To verify the relationship between microstructure development and thermal fatigue reliability, the cross sections of specimens that underwent 0, 500 and 1000 thermal cycles, as well as failed specimens, were inspected with SEM. The composition of IMC (intermetallic compound) and phases in the solder alloy were analyzed with EDX. Before the microstructure inspection, finite element analysis was performed to determine the primary failure mechanism and most vulnerable solder joint to be investigated with SEM.