FAILURE MECHANISMS OF PEMs AND HSMs

Thin Film Metal Deformation/Cracked Passivation:

Under temperature cycle conditions, thermomechanical shear stresses are set up between the molding compound and the die surface passivation/metallization, due to the differences in thermal coefficient of expansion (TCE) of the materials. The stress is negligible at the center of the die and increases exponentially from the center to the outer corners and edges [11,12]. Therefore, larger die experience greater stress in these areas than smaller die. Also the stress exerted on the passivation increases in relation to the increase in width and expansion of the underlying metallization lines [13,14]. Under severe temperature cycle extremes, the mold compound can exert enough stress on the die surface to fracture the surface passivation, after which the force is transmitted to the underlying metallization, causing it to deform or shear. Also, in multilevel metal systems the interlevel metal dielectric film can crack, causing a short between metal layers. Solutions include improved metal layout rules to reduce stress on the passivation at the corners and edges of the die, more planarized die surfaces to minimize the scrubbing action of the mold compound, die coatings in ultra sensitive devices, and low stress mold compounds. Reduced temperature cycle ranges may be necessary in some cases.