A new criterion for package integrity under solder reflow conditions

Moisture-induced package cracking during the solder reflow process is a critical reliability problem with plastic integrated circuit (IC) packaging. Such cracking, typically referred to as popcorning, occurs from the evaporation and expansion of moisture absorbed by the hygroscopic epoxy molding compound (EMC). Although the device may still test functionally good after assembly, the cracks introduce a path for ionic contaminants to infiltrate into the package and cause corrosion-induced failures. The following mechanism is proposed. Moisture in the outer and hotter portion of the EMC vaporizes first, while most of the moisture in the inner and cooler portion of the EMC remains in the liquid phase. Some localized hydrostatic pressure can become positive, generating subsequently high pressure that can initiate and propagate cracks. This paper reports the effects of the localized vaporization, which is a major albeit still neglected loading mode contributing to popcorning failures. For simplicity, only two-dimensional modeling problems are investigated in this phase of the study. Numerous moisture-induced cracking experimental data are examined to provide correlation with the proposed failure criterion. Among the parameters studied are package form factors, die-to-die pad ratio, molding compounds, moisture content, and heating profiles.