Process development of a flip chip in package with anisotropic conductive film (ACF) for lead-free soldering

In our previous work, we have established a baseline anisotropic conductive adhesive (ACA) assembly process with positive temperature cycling and temperature humidity reliability results. With the experience gained, further work was carried out to assess the compatibility of anisotropic conductive film (ACF) technology for high I/O applications in lead-free soldering conditions. Here, we report the assembly process development of a 13mm × 13mm flip chip BGA package using ACF to meet the stringent reflow temperature of 260°C, required for lead-free soldering. The effects of bond pressure distribution and alignment accuracy was found to be more critical in this 8mm × 8mm test die and 800 flip chip bumps. A three-factor design of experiment (DOE) was carried out to investigate the effects of assembly parameters such as bonding pressure, temperature and time on moisture sensitivity level (MSL) performance reflowed at 260°C. Results showed that higher bond force is undesirable and contributes to delamination at critical interfaces. With process optimization, the flip chip BGA with large die and high I/O was able to meet lead-free soldering requirement.