1.2 Background

The vapor pressure of moisture inside a nonhermetic package increases greatly when the package isexposed to the high temperature of solder reflow. Under certain conditions, this pressure can cause internal delamination of the packaging materials from the die and/or leadframe/substrate, internal cracks that do not extend to the outside of the package, bond damage, wire necking, bond lifting, die lifting, thin film cracking, or cratering beneath the bonds. In the most severe case, the stress can result in external package cracks. This is commonly referred to as the ‘‘popcorn’’ phenomenon because the internal stress causes the package to bulge and then crack with an audible ‘‘pop.’’ SMDs are more susceptible to this problem than through-hole parts because they are exposed to higher temperatures during reflow soldering. The reason for this is that the soldering operation must occur on the same side of the board as the SMD device. For wave-soldered through-hole devices, the soldering operation occurs under the board that shields the devices from the hot solder. Throughhole devices that are soldered using intrusive soldering or ‘‘pin in paste’’ processes may experience the same type of moisture-induced failures as SMT devices.