Package Rupture
A destructive package failure mechanism occurs when entrapped moisture inside a plastic semiconductor package rapidly vaporizes during high-temperature reflow soldering. In surface mount assembly, popcorning creates internal steam pressure that cracks epoxy molding compounds and delaminates die attach interfaces. Moisture-induced thermal cracking ceases to be a failure risk when plastic components undergo proper baking procedures and remain dry prior to reflow exposure.
Moisture Expansion
Plastic encapsulated microcircuits absorb ambient atmospheric moisture during storage and handling before board assembly. Absorbed water molecules accumulate along internal material interfaces, such as leadframe surfaces and die attach adhesive zones. When the printed circuit board enters the reflow soldering oven, temperatures exceed two hundred degrees Celsius, rapidly converting trapped liquid moisture into high-pressure steam.
Steam pressure forces internal polymer interfaces apart and exerts intense mechanical stress on the surrounding plastic package body. The expanding vapor creates internal cracks that propagate outward, often emitting an audible popping sound during reflow. Cracking damages wire bonds and severs internal traces, leaving open paths for external environmental contaminants to enter the active silicon region.
Moisture sensitivity level ratings dictate allowable factory floor exposure times to prevent vapor-induced cracking during assembly.
Failure Prevention
Baking plastic components at elevated temperatures removes absorbed moisture before reflow processing. Storing moisture-sensitive devices in sealed dry bags maintains low humidity levels until line mounting.
