Causes of cracks in SMD and type-specific remedies

Packages used with surface mount devices (SMDs) have the unique reliability problem of package cracks produced by soldering stress. Package cracks are classified into three distinct modes, depending on the location of origin and direction of spread. In type I, the crack originates on one edge of the die pad and spreads to the bottom side of the package. In type II, the crack originates on one edge of the die pad and spreads to the top surface of the package. In type III, the crack spreads from an edge of the chip to the top surface of the package. To remedy this cracking problem, it is necessary to determine the type of cracking that may occur, and take a countermeasure specific to each type. The authors investigated possible causes for each type of package crack, and techniques for crackproof package design optimized for individual crack types were developed. The following three techniques for anticrack package design, or a combination of two or more, have proved effective: (1) molding compound with improved resistance to soldering heat, (2) polyimide coating on the back side of the die pad, and (3) improved lead frame design