Mechanisms of die and underfill cracking in flip chip PBGA package

This paper focuses on understanding the mechanisms of die and underfill cracking during MRT (Moisture Resistance Test, JEDEC level 3) and TCT (Thermal Cycling Test, -55-125/spl deg/C). A parametric study has been performed to understand the influence of die and substrate thickness, and metal attachment on die cracking. It is found that a combination of thinner die and thicker substrate leads to good results. In the case of metal stiffener attachment on die backside using low modulus adhesive, die cracking is eliminated. Underfill cracking can be categorized into three groups, popcorn cracking, corner cracking, and edge cracking. The popcorn and corner cracking originate from interfacial delamination between underfill and die passivation. Such cracking is improved by baking the organic substrate before the underfill process and by using high adhesion strength underfill. Since the mechanism of underfill edge cracking is very complicated, mechanical simulations and experiments are conducted to understand it. The authors conclude that underfill edge cracking is closely related to the local CTE mismatch between underfill material and silicon die, and can be eliminated by using low CTE underfill material and by control of underfill fillet size.