Interfacial Behavior of Flux Residues and Its Impact on Copper/Underfill Adhesion in Microelectronic Packaging

In the micro-electronics industry, flux treatment on copper is an important procedure to ensure maximum adhesion to copper. In this research, sum frequency generation (SFG) vibrational spectroscopy has been applied to investigate molecular behavior of flux molecules at buried copper and underfill (UF) interfaces in situ. SFG is a second-order nonlinear optical spectroscopic technique, which can provide molecular structural information of surfaces and interfaces with a submonolayer interface specificity. Two model fluxes, adipic acid and phenylacetic acid, and a commercial no-clean flux were examined. Without washing or heating the surface of flux-treated copper, the buried interfaces between copper (treated with flux) and UF are dominated by ordered epoxy in UF. Washing or heating the surface of flux-treated copper leads to disordered copper/UF interfaces, greatly increasing the adhesion. The buried interfacial structures obtained from SFG results are well correlated to the adhesion strengths measured using the lap shear testing method. This research demonstrates the importance of the washing or heating steps of substrate surfaces to increase the interfacial adhesion.