Bond At The End: A Comprehensive Study of a New High-Throughput Bonding Process

Since its inception, the flip-chip bonding process has allowed for an ever-increasing density of interconnections in microelectronic packages. While traditional mass reflow (MR) soldering continues to be useful, alternative techniques such as thermal compression bonding (TCB) have evolved to support denser interconnection technologies. However, the TCB process requires relatively long cycle times and is sensitive to chip site warpage, as well as solder volume, bonding force or alignment variations. We report on this study a new solid-state thermocompression bonding process. It relies on temporary mechanical joints that are formed at the beginning of the packaging process, using pressure at a temperature below the solder melting point. The electrical interconnections are formed when the solder joints are completely remelted at the end of the packaging process, when the BGA solder balls are soldered to the substrate. We call this the Bond At The End (BATE) process. We have investigated the critical bonding parameters needed to achieve the initial temporary bond. The optimal bonding parameters were found to be 200°C in temperature, 0.25 N/bump in force, with a 4.5 s dwell time and 6 s total bonding duration. Applying an Ar/H2 plasma treatment before the assembly process also improved the bonding strength. A thorough investigation of the mechanical anchoring mechanism was carried out. Strong mechanical joining appears to be the result of the migration of interfacial grains boundaries and of the resulting interlocking of the solder bump surfaces. The quality of the final joints after BGA reflow was characterized with optical micrographs, x-ray imaging, and C-mode scanning acoustic microscopy. Under the optimal bonding conditions, we observed a high electrical yield and good reliability in thermal cycling, consistent with finite element modeling indicating a normal thermal stress in the final solder joints.