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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