Interaction between micro Cu bumps and substrate with thin ni/thick au SF in TCNCP FC packaging

Abstract

For high demand of large bump density, fine pitch micro Cu bumps are usually connected to trace on the substrate with flip chip technology. In this study, TCNCP (Thermal Compression with Non-Conductive Paste) method was used to attach die onto substrate trace with surface finish (SF) of thin Ni (0.1um) and thick Au (0.4um) in a flip chip (FC) package. The micro Cu bump used in the device under test has a dome shaped Sn-based solder cap of 13um in height. The ratio of Au layer plated on the substrate trace to Sn cap on the Cu bump is about 10 wt%. However, it has been proved that the micro joint obtained with TCNCP can pass TCT 1000 cycles, HTS 1000hours and uHAST 192hrs without failure. SEM/EDX study on HTS samples (0hr, 500hr, 1000hr) shows that the joint has a large amount of IMC mainly consisting of (CuxAu1-x)6Sn5 phase after reflow but after 1000hr independent Cu3Sn phase appeared near to die bumps. Phase segregation appears near to the die bump when HTS time goes up to 1000hrs. From this study, we found that Cu element in the IMC formation may come from Cu bumps on the die at initial stages and then come from Cu trace on the substrate at later stages. Though Ni could not be detected, its presence is displayed by retarding Cu3Sn formation near to the substrate side. Cu-rich IMC (Intermetallic Compound) phase formed at this location after HTS 1000hr suggests Ni disappearance after long time aging. The phenomenon that voided microstructure after HTS 500hr appeared dense after HTS 1000hr can be explained by Cu diffusion from the trace on the substrate.