Damage mechanics of Electromigration and Thermomigration in electronics packaging solder joints under time varying current loading
The insatiable demand for miniaturization of consumer electronics has brought continuing challenges in the electronic packaging field. As a consequence, immense information processing duties, high current density and large joule heating are exerted on the package, which makes Electromigration (EM) and Thermomigration (TM) serious reliability issues. EM/TM failure results from many factors such as high current density, chemical potential, stress concentration and joule heating and etc. As a result, vacancy forms at the cathode side of conductors while mass accumulates at the anode side. Increased impedance, material fatigue, open and short circuit failures are related to this mass diffusion degradation mechanism. In this work, a multiphysical numerical model as well as experimental techniques have been developed to investigate failure mechanisms of flip-chip (FC) Ball Grid Array (BGA) package under time varying current loading. In the experimental part, high frequency Pulse Direct Current (PDC) and Alternating Current (AC) electromigration degradation tests are carried out on Sn96.5%Ag3.0%Cu0.5 (SAC305- by weight) solder alloys. During the test, frequency, current density and duty factors are used as controlling parameters. The nominal current density is in 10 5 A/cm 2 range, frequency in MHz range, and duty factor varied between 30% and 80% with a controlled ambient temperature at 70 °C. Skin effect due to time varying electromagnetic field is studied in this part. Failure of test vehicle follows Weibull's distribution. A new Mean Time to Failure (MTTF) model is developed to describe the lifetime of solder joints versus current density, frequency, duty cycle and temperature. In addition to the experimental work, a set of thermo-electrical-mechanical coupled partial differential equations governing material degradation process during EM and TM is discretized numerically for Finite Element Analysis (FEA) purpose. The FEA model is formulated in FORTRAN and implemented to the commercial FEA package ABAQUS through UEL and UMAT user interface. Extensive simulations are performed to reveal the vacancy accumulation, stress and strain development, and damage evolution process of FC BGA package under time varying current loading: both AC and PDC. A few techniques are presented to alleviate EM and TM damage of FC BGA package.