Mechanisms of die and underfill cracking in flip chip PBGA package

Recently, flip chip method becomes popular technology in electronic packaging industry because of small package size and high electrical performance. However, in order for flip chip technology to become more popular, there are several hurdles to overcome, for instance, assembly cost and reliability issues. The cost issue is the most critical parameter among them. Therefore, the trend is that substrate material for flip chip changes from ceramic to organic due to cost issue. There are a lot of reliability issues to be solved in flip chip package with organic substrate. Die cracking, solder joint cracking, underfill cracking, and delamination between the underfill and either die or substrate are typical failure modes in flip chip package using PCB (Printed Circuit Board). This paper was focused on understanding of the mechanisms of die and underfill cracking during MRT (Moisture Resistance Test, JEDEC level 3) and TCT (Thermal Cycling Test, -55 - 125 degrees C). A parametric study has been performed to understand the influence of die and substrate thickness, and metal attachment on die cracking. It was found that the combination of thinner die and thicker substrate led to good results. In case of metal stiffener attachment on die backside using low modulus adhesive, die cracking issue was eliminated. Underfill cracking can be categorized into three groups, such as, popcorn cracking, corner cracking, and edge cracking. The popcorn and corner cracking in underfill were originated from interfacial delamination between underfill and die passivation. Such a cracking was improved by baking the organic substrate before underfill process and by using high adhesion strength underfill. Since the mechanism of underfill edge cracking was very complicated, mechanical simulations and experiments were conducted to understand it. In conclusion, the underfill edge cracking was closely related to the local CTE mismatch between underfill material and silicon die. The underfill edge cracking was eliminated by using low CTE underfill material and by the control of underfill fillet size.