Assembly Process and Application Studies of Pre-Applied Underfill Non-Conductive Film (NCF) and Non-Conductive Paste (NCP) for Advanced Packages

Semiconductor package sizes are getting smaller and bump pitches narrower while the number of input/output (I/O) continues to increase. The traditional flip-chip process, which assembles the chip with copper (Cu) pillar interconnects to the substrate using a mass reflow technique faces several hurdles as package dimensions become more challenging. These include high stress between low-k die and substrate, die shift, and die cracking when the pitch is narrower than 100 μm. The thermocompression bonding (TCB) process can help overcome these challenges. Pre-applied underfills like non-conductive paste (NCP) and non-conductive film (NCF) enable the TCB process. NCP is applied directly on the substrate, while NCF is applied on the wafer prior to TCB. This paper discusses the key material properties and TCB parameters that are critical for good solder joint formation and void-free gap-filling. During the NCP TCB process, void issues are common and several factors (e.g. dispensing volume and pattern, bonding parameters and material properties) contribute to void formation. In this study, different dispensing patterns were evaluated to better understand this issue. Based on the experimental results, specific patterns appear to help reduce voiding. In terms of TCB process, focus was on the effect of bond speed on void performance. The speed of pushing out the NCP clearly affected the flow of NCP & void performance. Similarly, TCB process for NCF also require setting the right bond parameters to ensure good solder joints and zero void performance. Three key parameters were studied for bond optimization: bond force, contact temperature, and ramp rate. A wide range of bond forces, contact temperatures, and ramp rates were chosen for the experiments. After performing the TCB process, joint shape and void performance via CSAM were evaluated. Based on the study, an optimal condition was identified for better application performance. Subsequently, reliability tests such as MSL (85° C/85% RH) and uHAST were conducted.