Removed organic solderability preservative (OSPs) by Ar/O2 microwave plasma to improve solder joint in thermal compression flip chip bonding

Organic soderability preservative (OSP) is the cheapest metal surface finish to apply for corrosion inhibition of bare copper. In conventional flip chip process, the various OSPs could be dissolved by rosin pre-fluxes before the chip placement to enable solder bonding. The thermal compression flip chip bonding with non-conductive paste (TCNCP) process is one type of no-flow underfill process. In a TCNCP process, the OSP layer must dissolve in self-fluxing agent in NCP, but the process does not have adequate time for allowing complete dissolution to occur. In order to assist in OSP removal, this work attempted to Ar/O₂ microwave plasma to remove the OSP layer on Cu trace of a substrate. The SEM-FIB investigation revealed that almost all of OSP layer could be removed with plasma, while only few OSPs remained in the relatively deep concavity on the surface of Cu trace. The analysis results by SEM-EDX showed that the carbon content of the treated plasma residue decreased from 44.69% to 11.5%. The Raman spectrum also showed that the bands of vibration mode in benzimidaozle almost disappeared, indicating the successful removal of the OSP layer by Ar/O₂ microwave plasma. The cross-sectional SEM image of the bump on trace produced with the TCNCP process indicated that thin intermetallic compound (IMC) layer formed at the solder/Cu trace interface when incorporated with Ar/O₂ microwave plasma pre-treatment. The SEM-DEX composition analysis delineated that the IMC layer consists of Cu₃Sn and Cu₆Sn₅. These results indicate the successful removal of OSP.