Study on adhesion of underfill materials for flip chip packaging

One of the key materials used in flip chip technology is the underfill, which enhances the reliability of the flip chip device by orders of magnitude. The fundamental adhesion issues regarding underfill materials were studied.; The surface property of passivation materials and solder mask materials were characterized. Combination of both wet cleaning and dry cleaning is very effective in removing surface contaminant. The base component of surface tension increased dramatically after UV/O₃ treatment due to increase of oxygen concentration on the surface.; Underfill materials absorb moisture before and after curing. High moisture absorption in uncured underfill with acid anhydride as curing agent can lead to significant low adhesion strength of the underfill after curing. Low moisture absorption in non-acid anhydride cured system has no significant effect on adhesion. Aging in a high temperature and high humidity environment after curing results in degradation of adhesion. The adhesion retention strongly depends on underfill formulations and the surface property. Hydrophobic surface has much better adhesion retention than hydrophilic surface after high temperature and high humidity aging. The adhesion hydrolytic stability of hydrophilic surface can be improved by using silane coupling agent. The orientation of coupling agent at the interface plays an important in property of adhesion retention during high temperature and high humidity aging.; The mobility of absorbed water and polymer chain was studied with solid state nuclear magnetic resonance technique. Water within epoxies below its glass transition temperature is significantly less mobile than free water. The mobility of water in an epoxy in its rubbery state is close to that of free water. The higher mobility of polymer chains and water contributes to the faster adhesion degradation during hot-wet aging. Various approaches were explored to improve the adhesion between epoxy and pyromellitic dianhydride-4,4 ′-oxydianiline polyimide (PMDA-ODA PI) passivation. Sodium hydroxide treatment is more effective to improve the adhesion between PMDA-ODA PI and epoxy than amine treatment. The hydrolytic stability of adhesion between epoxy and sodium hydroxide treated PMDA-ODA PI depends on the curing condition of the epoxy formulation. Epoxy cured with imidazole or its derivatives shows excellent adhesion to PMDA-ODA PI.