Underfill Cure Induced Micro-Anomaly (CIMA) and its mechanism and reliability implications

Cure Induced Micro-Anomaly (CIMA) are worm like hollow microstructures found within resin rich region of underfill after curing within a BGA package with combination of copper die bumps and Sn-Ag substrate bumps. CIMA leads to solder extrusion during secondary reflow. Root cause of CIMA has been identified to be from thermal mechanical stress induced by the stiffness of the joints formed between the die and substrate, dissipation of stress causes the underfill to crack. Addition of stress induction from package warpage, high Prebake temperature has shown to worsen effects of CIMA. Step-cure and flip curing has shown to reduce the effects.