Chip-to-Chip and Chip-to-Wafer Thermocompression Flip Chip Bonding

Adoption of Chip-to-Chip (C2C) and Chip-to-Wafer (C2W) thermocompression (TC) flip-chip (FC) bonding in high volume manufacturing of advanced memory modules is accelerating. Advanced memory modules in the form of Wide I/O2, High Bandwidth Memory (HBM) and Hybrid Memory Cubes (HMC) use Through-Silicon Vias (TSVs) and copper pillars to achieve ultra-short chip-to-chip interconnects and the associated power and performance improvements. TC-FC bonding is the only interconnection method capable of building these stacks of typically 50μm thick die with thousands of interconnections between them. Today many of these memory stacks are being manufactured by a dip-flux TC process in which the die stacks are then capillary underfilled in a subsequent process step. This underfill method is challenging for high volume manufacturing especially when die are thinned below 50 um. The logical replacement for this method is to replace the capillary underfill with wafer applied non-conductive underfill films. The use of these films during the TC bonding is seen as having many advantages over capillary underfills in stacked die applications. Among these advantages are 1) stress is relieved from the electrical interconnects immediately as they are formed, 2) bondline thickness control is improved by avoiding the potential loss of bondline thickness associated with re-melting lower layers during stacking and 3) complexities of flux-cleaning and capillary underfilling with narrow die-to-die gaps are avoided. In this paper we review the advantages of TC-NCF and how improvements in TC bonder technology and TC bonding processes are used to overcome some of the current hurdles that have held back full adoption of TC-NCF in high volume manufacturing. Chief among these hurdles has been the relatively low throughput on available TC bonders. The built-in process monitoring capabilities of the advanced thermocompression bonder are used to characterize the flow and curing properties of the NCF on the time-scale of the thermocompression bonding process. This information is then used to develop the process parameters for achieving the fastest possible cycle time. Furthermore, we have demonstrated ultra-high heating and cooling rates (350°C heating/150°C cooling) with tight temperature uniformity control to shave valuable time off the process while maintaining high quality. Finally, a novel picker-to-bondhead transfer methodology enables transfer at bondhead temperatures well above 100°C, where the NCF is already soft. In traditional processes, the transfer temperature is limited to about 80°C. These higher transfer temperatures improve throughput by approximately 40% and throughputs of 1500 to 2000 units per hour can be achieved. This high throughput substantially lowers the cost of stacked die assemblies, making them competitive for a larger share of the memory market. This high throughput capability also creates a situation where the cost of thermocompression bonding compares favorably with mass reflow in non-stacked die applications.