Capillary-Based Fluidic Self-Assembly for 3D High Interconnect Density Electronics
Shaghayegh Abbasi, Rajashree Baskaran, Karl F. Böhringer
Interest in the assembly of wafer-scale 3-dimensional microelectronics with high interconnect density has significantly increased in recent years due to the need for smaller sizes and higher speeds. The electronic parts are usually very thin to optimize the form factor, hence they are hard to handle with pick and place assembly robots. In addition, a very high alignment accuracy (<1μm) is needed due to the small size and high density of interconnects, which requires a very expensive pick and place assembly setup. The serial nature of pick and place assembly decreases the assembly speed significantly. In this project capillary-based self-assembly is used as an alternative method for assembly, mainly because of its parallel nature and self-alignment capability. The assembly is performed in two steps: electronic parts are first arranged on an assembly template using capillary forces acting on their back sides, and then they are transferred and bonded to the final substrate (Fig.1). The first step of the assembly, which is performed in aqueous environment, includes introduction of electronic parts to the substrate with lubricant-coated binding sites, and agitation of the system. After assembly is completed due to capillary forces, the lubricant is heat-cured, which provides mechanical binding of the parts to the substrate (Fig. 2). Alignment accuracy, yield, and other assembly outcome characteristics are being investigated as a function of assembly parameters (Including material system, agitation methods, and scaling).
Figure 1: Wafer-scale assembly of electronic parts: First, parts with circuitry and interconnect pads are assembled on the assembly template. Then they are flipped on the final substrate, which has complementary circuitry and same-size interconnect pads. The interconnect pads on parts and substrate are connected using solder or metal-metal bonding.
Figure 2: (a) Selective coating of sites with heat curable hydrophobic adhesive. (b) Delivery of parts in water medium that results in formation of a capillary bridge between the hydrophobic sites of the parts and the substrate. The adhesive is then heat cured for mechanical bonding.