Studies on parameters for popcorn cracking

Theoretical analysis of the popcorn phenomenon requires the thermo-mechanical and moisture diffusion properties of the polymers in the packages under investigation. Some of these properties, including fracture toughness, are given for the four commercially available epoxy molding compounds (EMCs) used. Fracture toughness measurements with precracked beams as well as analyses based on a simple method to estimate the toughness from bending experiments are used. For the latter investigations, the basic assumption is that the filler particles act as initial flaws. The problem is analytically treated as a beam with a surface crack, and an estimate of the critical fracture toughness can be calculated by a simple formula. By means of 3D-FE analyses the moisture diffusion into a thin quad flat pack (TQFP) package is studied for various standard moisture preconditioning levels. It is shown that the different popcorn failure types correspond to different moisture distributions within the die attach layer depending on the different preconditioning levels.