Epoxy curing inhibition by residual flux on a flexible printed circuit in the flip-chip packaging process

Abstract

This study investigates the root cause for epoxy uncuring problem from residual flux on the flexible printed circuits (FPCs) in the flip-chip packaging process. In the FPC assembly, lead-free solder paste is printed on a copper pad. Then, epoxy is filled underneath the IC die by dispensing from the side of the die. Nevertheless, since the product size in the present day is so small that the distance between solder pad and the underfill area becomes very close. The contact between flux residue generated after the heat treatment of the solder (i.e. reflow process) and the underfill epoxy results in uncuring problem of the epoxy. In this study, epoxy curing was also studied by using Differential Scanning Calorimetry (DSC), Thermo Mechanical Analysis (TMA), Dynamic Mechanical Analysis (DMA), and Fourier Transform Infrared Spectroscopy (FT-IR). The results showed that TMA can identify epoxy curing clearer than other methods. FT-IR analysis show that incompatibility between the flux residue and epoxy inhibited the cross linking of the epoxy. Further analysis also suggested that polyethylene glycol is the composition in flux that is responsible for the uncuring problem. Then attempts were made to reduce the flux residue by adjusting the reflow profile (both temperature and time) and finding the appropriate process for the industries. Moreover, the change in reflow conditions could not completely eliminate the flux. The best way to solve this problem is the cleaning of the residual flux by de-ionized water before the underfill process.