Electromigration analysis of solder joints under ac load: A mean time to failure model

In this study, alternating current (ac) electromigration (EM) degradation simulations were carried out for Sn95.5Ag4.0Cu0.5 (SAC405- by weight) solder joints. Mass transport analysis was conducted with viscoplastic material properties for quantifying damage mechanism in solder joints. Square, sine, and triangle current wave forms ac were used as input signals. dc and pulsed dc (PDC) electromigration analysis were conducted for comparison purposes. The maximum current density ranged from 2.2 × 10 ⁶ A/cm ² to 5.0 × 10 ⁶ A/cm ², frequency ranged from 0.05 Hz to 5 Hz with ambient temperature varying from 350 K to 450 K. Because the room temperature is nearly two-thirds of SAC solder joint's melting point on absolute temperature scale (494.15 K), viscoplastic material model is essential. Entropy based damage evolution model was used to investigate mean time to failure (MTF) behavior of solder joints subjected to ac stressing. It was observed that MTF was inversely proportional to ambient temperature T ^1.1 in Celsius and also inversely proportional to current density j ^0.27 in A/cm ². Higher frequency will lead to a shorter lifetime with in the frequency range we studied, and a relationship is proposed as MTF ∞ f ^-0.41. Lifetime of a solder joint subjected to ac is longer compared with dc and PDC loading conditions. By introducing frequency, ambient temperature and current density dependency terms, a modified MTTF equation was proposed for solder joints subjected to ac current stressing.