Reliability performance of 30μm-pitch solder micro bump fluxless bonding interconnections

Three dimensional integrated circuit technology has been recently received much interests because it could meet the requirements such as small form factor, high performance, low cost and heterogeneous integration in system-in-package technologies for next generation electronics. For high performance 3D chip stacking, high density interconnections are essential. In the current types of interconnects, solder micro bumps have received much attention due to its low cost in material and process. In this study, two methods of fluxless chip-on-chip bonding process with 30μm pitch lead free solder micro bumps interconnection were demonstrated and the reliability of micro joints was also evaluated. The plasma treatment and the formic acid treatment were used to remove the tin oxidation layer upon the surface of solder micro bumps. The assembly of the chip-on-chip test vehicle with a micro bumps diameter of 18μm and a pitch of 30μm was conducted. There were more than 3000 micro bumps with Sn2.5Ag solder material on both the silicon chip and the silicon carrier. A solder micro bump structure of Cu/SnAg having a thickness of 7μm/15μm was fabricated in silicon chip and the silicon carrier consisted of Cu under bump metallization (UBM) with a thickness of 5μm. During bonding process, the micro joints were formed at pre-bonding and reflowed in reflow oven with a peak temperature of 250 °C. Then, the die shear test was used to check the solder micro joints bonding strength between the silicon chip and carrier by two methods of fluxless process. After bonding process, the microgaps were then filled by a capillary underfill and cured at 165 °C for 2 hours. Reliability tests of temperature cycling test (TCT) and high temperature storage (HTS) were selected to assess the effect of the fluxless bonding process on the reliability of solder micro joints interconnections.