Plastic Package Moisture-Induced Cracking

With the older “through-hole” technology packages such as the Dual In-line Package (DIP), soldering to printed circuit boards was accomplished using a wave solder. The DIP leads went through holes in the printed circuit board, where they were exposed to molten solder during the mounting process. The DIP’s body was never subjected directly to the solder melt, because it was protected from the heat of the molten solder by the printed circuit board. However, with the advent of "surface mount" technology (SMT), i.e., PLCC, PQFP, and SOP, the problem of plastic package moisture-induced cracking arises. The higher lead count surface mount packages have reduced lead pitch compared to through-hole packages. As a result, SMT packages have to be mounted directly to the board surface rather than through it. The advantage of surface mounting allows both package body size and thickness to decrease, while lead count increases. Essentially, a higher density of integrated circuits can be mounted on a printed circuit board using surface mount packages instead of through-hole packages. Soldering surface mount packages directly to printed circuit boards has to be performed by directly heating the entire surface mount package and the PC board together. Direct heating of the surface mount package, combined with its ability to absorb moisture vapor, increases the likelihood of moisture-induced cracking during reflow conditions.