Failure Analysis
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Destructive Evaluation: Popcorn Assessment
All plastic encapsulated microcircuits (PEM's) will absorb moisture because the epoxy molding compounds used in packaging are hygroscopic. During the solder reflow process, surface mount PEM's are exposed to temperatures of 210 to 260°C for periods of 30 seconds to 5 minutes. These high temperatures and rapid ramp rate will prevent the absorbed moisture from escaping. As the moisture begins to vaporize, the local pressure will increase rapidly, leading to delamination at the die/package interface and cracking of plastic packaging. This phenomenon is known as "popcorning". The extent of popcorning is influenced by package materials, package geometry, and assembly process parameters.
When the cracking and delamination during popcorning is not severe, electrical characteristics of the PEM can remain stable for a period of time after the manufacturing assembly process. Nevertheless, reliability risks are introduced, since the plastic is free to move relative to the die surface during temperature excursions and moisture can be trapped at the die surface, creating conditions for accelerated corrosion. Studies have shown that delamination at the die surface can reduce expected lifetime operation by causing wire bond degradation, metallization shifting, passivation cracks and corrosion
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