An improvement in reflow performance of plastic packages

The influence of die pad design and die attach adhesive on the resistance of a thin small out line package (TSOP) to reflow cracking has been investigated. Mechanisms of reflow cracking were studied using scanning acoustic tomography (SAT) and scanning electron micrography (SEM). For more precise analysis, computational calculations of stress were conducted. In the case of dimpled die pad, an experimental die adhesive with lower moisture absorption and higher adhesion strength showed excellent resistance to reflow cracking, resulting in crack-free performance in level I preconditioning tests. However,packages with the slot lead frame failed in level I preconditioning tests. The failure was due to the interfacial delamination between the bottom surface of die and the epoxy molding compound (EMC). SAT showed that the delamination initiated at the periphery of the slot during temperature cycling in preconditioning. During the subsequent soaking and IR reflow, moisture condensed at the delaminated interface generated the high vapour pressure that exceeded the fracture strength of the EMC. In other words, the interfacial integrity between the bottom surface of die and the molding compound is critical to the reflow cracking in packages with slot lead frame.