Ranjit Gannamani & Ramesh Munamarty
Because epoxy molding compounds are hygroscopic ingressed moisture accumulating at the interfaces during reflow soldering is subjected, together with the entire device, to temperatures as high as 230o C - well above the glass-transition level of most molding compounds. Stresses developed in such cases include rapid vaporization of the absorbed moisture resulting in a build-up of pressure, thermal expansion mismatches at the interfaces and decreases in the adhesive strength and fracture toughness of the molding compound. The latter defects then become the driving forces behind the delamination and propagation of package cracks or popcorning.
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