Fatigue life prediction of solder interconnects in area-array microelectronic packages and the effects of underfill

High cycle vibration fatigue tests are performed on Ball Grid Array (BGA) packages employing a specially designed Cyclic Controlled Curvature Cantilever Device (C4D) and an imaging system in which a stroboscope is used with an optical sensor to freeze the vibration of the critical solder interconnect in BGA specimens. The failure mode is identified as crack initiation and propagation on the component side along the nickel/solder interface. A primary crack starts from the inner edge, progressing stably until the secondary crack begins on the opposite edge. The crack growth is then accelerated till the complete crack has been formed. Averagely, the time spent in crack initiation, stable propagation and accelerated crack propagation are about 15%, 60% and 25%, respectively. Vibration tests at various frequencies were also performed. The cycles to failure is found to be frequency-independent from 50 Hz to 100 Hz.; Several commonly used fatigue life-prediction models, such as Solomon's model and Paris' law, are examined based on failure parameters computed from nonlinear finite element analysis. It is found that while the damage models usually show large discrepancy, the fracture model can correlate with the test data within a factor of 1.5.; The High Sensitivity Moire Interferometry method is also used to capture the mechanical response in the BGA three-point bending and Flip Chip four-point bending tests. Both packages come in two forms: with and without underfill. The influence of underfill is shown to be obvious.; Underfill curing-induced shrinkage problem is investigated using a unit cell finite element model comprised of solder and underfill in cylindrical shape. A two-phase solution scheme is utilized: The shrinkage is modeled in the first phase by introducing a thermal contraction to the underfill; the solder is allowed to creep based on a hyperbolic creep law in the second phase for 1000 hours. A compressive residual steady state stress in the solder interconnects can be reached within several weeks time. This compressive stress can extend the fatigue life of solder interconnects undergoing Mode II cyclic shear according to a relative life model proposed by Larson and Verges (2003).