Surface Mount Technology, May 1995.

Optimum Processing Prevents PQFP Popcorning

P. Yalamanchiii, R. Gannamani, R. Munamarty, P. McCluskey, and A. Christou


Package Delmaination and cracking during reflow soldering is a critical issue for the reliability of plastic encapsulated semiconductor devices. It is caused by the phase change and expansion of internally condensed moisture in the plastic during exposure to the solder reflow temperatures. A critical crossover point occurs when the strength of the plastic can no longer sustain the rising stresses generated by the expanding vapor, resulting in the worst case, the propagation of a package crack (commonly referred to as "popcorning"). Relaibilty concerns arise from the development of cracks, which can cause catastrophic failures due to shearing of the wedge wirebonds. Or long-term reliability may ne jeopardized due to entry of ionic (corrosive) contaminants. Plastic molding compounds are inherently hygroscopic and will absord moisture to a level dependent on the ambient humidity and temperature. Moisture-induced sensitivity to surface mount solder-reflow conditions can produce internal package damage via delamination of the molding compound either at the die surface, lead-frame fingers or die-paddle interface. All such damage can occur at much lower levels of ingressed moisture, but high moisture level generally cause popcorn cracks.

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