Laser Based Assembly of Ultra Fine Pitch Bumped ICs For Chip-on-Chip Proximity Coupled Applications

This paper describes a process flow for creating ultra-fine-pitch solder interconnects between stacked die. The application for this die stack is Proximity Communication (PxC). Tight geometric control of both the bumping and assembly processes are required in all three dimensions, x-y-z, to enable efficient capacitance communication between the die. The test vehicle for this program consists of a bridged-stack design, where one chip is used as communication link (bridge) between two other die (island). The solder bumps perform several critical functions in this application: 1) provide for electrical transmission of power, ground, and low frequency signals between chips, 2) create precise x-y alignment between the stacked die which allows for good overlap of the non-bumped pads which are used for proximity coupled data transmission between die, and 3) provide for a narrow and tightly controlled standoff distance between stacked die to enable efficient capacitive coupling. The bumping process consisted of depositing an e-Ni/Au layer as the UBM (3 μm) on both the bridge and island dies, creating micro bumps of Sn (3 μm) on the bridge die, and creating a second set of flip chip sized bumps (100 μm SnAgCu) on the island die for assembly to a ceramic substrate. The bumped wafers were thinned to 150 μm, singulated, laser assembled together using the micro bumps to create a three chip unit, and then laser assembled to a ceramic substrate using the flip chip bumps. Assembly is accomplished by aligning the bumped bridge die to the corresponding unbumped island die and laser heating the micro bump area to reflow the Sn solder to form the fine pitch interconnect. The second assembly to the ceramic substrate is also accomplished using the laser heating process. This laser technology allows for highly localized heating, which allows for independent reflow and assembly of the two different solder bump regions, i.e. Sn micro-bumps and SnAgCu flip chip bumps.