Magnetic Self-Assembly of Ultra-Thin Chips to Polymer Foils

A self-assembly process is developed for the placement and alignment of Ultra-Thin Chips (UTCs) to polymer foils. The chips are presented within the working range of a magnetic force field, and subsequently driven to and aligned at a target location. A low-viscosity die attach adhesive layer supports chip mobility during alignment, and is UV-cured after assembly to generate a mechanical bond. An adaptive electrical interconnection scheme compensates the position errors present after assembly. Standard Ni + Au bumps provide sufficient magnetization to generate the required alignment force. Numerical modeling confirms that over a long range magnetic forces operate on a chip and drive it to a target location. Also, an asymmetric bump arrangement supports achieving a unique in-plane orientation. Experimentally, chips with a thickness of 20 mu m were successfully trapped and aligned with a repeatability of +/- 100 mu m in x and y-direction, and the best achieved cycle time is below 1.0 s. The cycle time depends considerably on the viscosity of the die attach adhesive. The presence of unique in-plane orientations, depending on the bump arrangement, is demonstrated.