Improved underfill adhesion in flip-chip packages by means of ultraviolet light/ozone treatment

The interfacial adhesion between the epoxy-based soldermask and underfill resin is successfully improved by means of ultraviolet light/ozone (UV/O₃) treatment. There is an optimized treatment condition that can impart the highest interfacial bond quality. The underlying mechanisms are evaluated based on several techniques, including X-ray photoelectron spectroscopy (XPS), contact angle measurement and nanoindentation of the UV/O₃ treated soldermask surface. Functionally similar changes in chemical element, wettability, and elastic modulus are identified of the substrate with respect to treatment time, which are also correlated to the improvement of interfacial bond strength. In particular, there is a linear relationship between the weighted dipole moment (obtained from the XPS analysis) and the surface polarity (calculated from the contact angle measurements) of the treated surface, confirming that the same conclusion can be drawn from the two different techniques. The UV/O₃ treatment is proven to be efficient in retaining the interfacial bond quality even after hygrothermal ageing.