Magnetic force driven self-assembly of ultra-thin chips

This paper introduces a novel self-assembly method for integration of Ultra-Thin Chips (UTCs) on flexible polymer foils. Chip assembly is done by using a patterned magnetic force field instead of a direct physical contact. The chosen approach uses the standard Ni-Au bumps present on the UTCs without addition of extra magnetic material. The chip is driven by the magnetic field gradient produced by a magnet unit underneath the foil and is positioned at the desired assembly point. At the assembly location the foil is provided with a low viscous die-attach adhesive layer which is a few millimeters bigger than the chip size. The adhesive promotes the mobility of the chip during alignment and creates a mechanical bond when cured after the chip reaches its final position. Chips with a variety of sizes in the millimeter range and thickness of 20μm were presented to different locations within a 4×4mm area on the die-attach adhesive. With a homemade acrylate mix used as die-attach adhesive, a lateral alignment precision of around ±100 μm was achieved and the best cycle time was 0.9 seconds.