Copper Migration in Flip-Chip Substrates Under Biased-HAST Conditions

Biased-humidity testing is a critical reliability qualification requirement for integrated circuit packages. The benchmark of THB (Temperature Humidity Bias) at 85C/85%RH/bias for 1000 hours is a long duration test, so biased-HAST (biased Highly Accelerated Stress Test) conditions are adopted. Accelerated biased-HAST durations are based on aluminum corrosion failure mechanisms and may not be applicable to other materials such as in flip-chip packages. One such failure mechanism is discussed here. Flip-chip packages with build-up substrates were exposed to biased-humidity at 85C, 110C, or 130C and 85%RH. After 96 hours of 130C/85%RH/bias, failures occurred with signatures ranging from bake-recoverable leakage to shorts. Physical failure analysis revealed copper migration in the substrate build-up dielectric rather than classical CAF (Conductive Anodic Filament) within the substrate core. No copper migration occurred with conditions of 110C/85%RH/bias, 85C/85%RH/bias, or 130C/85%RH/no-bias. Given that the J...