Adhesion characteristics of underfill material with various package components after plasma and UV/ozone treatments

The role of underfill in enhancing the reliability of flip chip packages is of paramount importance, particularly for the low-cost packages made of organic printed circuit boards. Amongst many reliability issues in flip chip packages, delamination between the underfill and other package components is detrimental to the mechanical and functional performance of the package because delamination often leads to premature failure of the whole device. In the present study, the interfacial bond strengths of both conventional and no-flow underfill resins with die passivation, solder, and polyimide soldermask are measured based on the button shear test; and the surface characteristics of these substrates are analysed using several analytical tools, including atomic force microscopy, X-ray photoelectron spectroscopy and contact angle measurement. Plasma and UV/ozone treatments are applied before encapsulation to improve the underfill-package component interface bond. It is found that the interfacial bond strength of underfill with solder was the weakest, while it was highest with silicon nitride passivation layer amongst the above substrates studied. Both plasma and UV/ozone treatments improved the interfacial bond strength with polyimide soldermask along with a decrease in contact angle of the surface. Among the various thermodynamic parameters the spreading coefficient was shown to have close correlation with the underfill-solder interfacial bond strength. The significance of this finding is discussed.