Effects of temperature and moisture on durability of low cost flip chip on board (FCOB) assemblies

The durability of flip chip on board (FCOB) assemblies under cyclic thermomechanical and constant hygromechanical loading is assessed in this dissertation using experimentation and analysis.; In the thermomechanical study, the effect of underfill material, chip size, joint geometry and solder bump volume (stand-off height) on FCOB durability is investigated by factorial parametric studies under thermal cycling conditions. Flip chip boards without underfill were tested until failures occurred in all FCOB assemblies. FCOB assemblies with underfill were tested until a significant number of failures was observed, in order to perform statistical analysis. The underfill is found to increase durability by two orders of magnitude. Pseudo-3D FEM models were developed to analyze all flip chip configurations represented in the test matrix. The analysis consists of viscoplastic stress analysis followed by fatigue damage modeling. An energy partitioning (EP) fatigue damage model is used to predict the durability. A hypothesis is proposed to explain the improvement in FCOB durability due to the presence of the underfill. The EP fatigue model is modified in accordance with this hypothesis. The modified model is then used to estimate the acceleration factor between the accelerated test conditions and life cycle conditions for all FCOBs in the test matrix. Design guidelines are developed using parametric sensitivity studies for selecting underfill materials for FCOB assemblies.; In the hygromechanical study, failure of FCOB assemblies subjected to several conditions of constant temperature and humidity were analyzed. The failure mechanism is hypothesized to be fracture of the intermetallic layers between the copper bond pad on the die and the eutectic solder joint. The cracking is induced by stresses in the solder joint caused by hygro-swelling of the underfill. In order to understand the influence of the intermetallic layer on the durability of solder joints, specially designed and fabricated specimens were subjected to high temperature aging. The strength of the intermetallic layer was then measured for different amounts of aging. Stress in the solder joint induced by moisture ingress and swelling in the underfill is estimated through simulation. This analysis is divided into two parts: (a) mass diffusion analysis to determine the history of moisture concentration distribution at different ambient temperature and humidity conditions; (b) hygromechanical swelling analysis to determine the stress history in the solder joint using the result of the mass diffusion analysis. The damage model used to predict failure is a simple stress-based criterion for intermetallic fracture, which states that when the increasing hygromechanical stress exceeds the decreasing strength of the aging intermetallic, failures occurs. This model is found to explain the experimental trends reasonably well, and is therefore used to estimate the acceleration factor between accelerated life test and life cycle conditions for FCOB with two types of underfill.; Clearly, the presence of underfill increases the thermomechanical durability, but decreases hygromechanical durability. The results of this study are therefore important for optimizing the fatigue endurance of the FCOB solder joints under thermomechanical and hygromechanical life cycle environments.