Abnormal Failure Behavior of Sn-3.5Ag Solder Bumps Under Excessive Electric Current Stressing Conditions

Abnormal failure behavior of flip chip Sn-3.5Ag solder bumps with a Cu underbump metallurgy under excessive electric current stressing conditions is investigated with regard to electromigration lifetime characteristics and damage evolution morphologies. Abnormal behavior such as abrupt changes in the slope of the resistance versus stressing time curve correlate well with the changes in mean time to failure and the standard deviation with respect to␣the resistance increase ratio, which seems to be strongly related to highly␣accelerated electromigration test conditions of 120°C to 160°C and 3 × 104 A/cm2 to 4.6 × 104 A/cm2. This is closely related to changes in the damage evolution mechanism with time, even though the activation energy for electrical failure is primarily controlled by Cu diffusion through Cu-Sn intermetallic compound layers.