A first automated reel-to-reel fluidic self-assembly process for macroelectronic applications is reported. This system enables high speed assembly of semiconductor dies (15,000 chips per hour using a 2.5 cm wide web) over large area substrates. The optimization of the system (hour 99% assembly yield) is based on identification, calculation, and optimization of the relevant forces. As an application the production of a solid state lighting panel is discussed involving a novel approach to apply a conductive layer through lamination.
This communication reports on recent progress towards a first implementation of a self-assembly machine that is based on surface-tension-directed-self-assembly. The reported assembly process is no longer a discontinuous small-batch hand-operated process but resembles an automated machine like process involving a conveyer belt and a reel-to-reel (RTR) type assembly approach with automated agitation. As a comparison, the assembly rate of conventional chip level pick-and-place machines depends on the cost of the system and number of assembly heads that are used. For example, a high-end FCM 10000 (Muehlbauer AG) flip chip assembly system can assemble approximately 8000 chips per hour achieving a placement accuracy of 30 μm. Our current design achieves 15,000 chips per hour using a 2.5 cm wide assembly region which is only a factor of 2 better than one of the faster pick-and-place machines; scaling to 150,000 chips per hour, however, would be possible using a 25 cm wide web, which would be a factor of 20 faster. In principle, scaling to any throughput should be possible considering the parallel nature of self-assembly. In terms of placement accuracy our precision increase with a reduction of chip and solder bump size. Generally, it exceeds the 30 μm limits for the components that have been used. Under optimized operational conditions, we achieved an assembly yield of 99.8% using the self-assembly process. As an application the assembly machine is applied to the realization of area lighting panels incorporating distributed inorganic light emitting diodes(LEDs)
They are convinced that the key to the high yield and high alignment accuracy is inherently related with the high restoring force of liquid solder bumps that they continue to use in our experiments. Over many years we have experimented with other forces including Coulomb forces, hydrophobic and hydrophilic forces, magnetic (not published), gravity and shape recognition and they have yet to find another approach that achieves a similar yield and alignment accuracy. The approach can be extended using shape recognition concepts to enable unique angular alignment and contact pad registration or using sequential batch assembly processes to assemble more than one component type on the substrate if desired. Moreover, it is possible to transfer the chip onto other flexible or stretchable substrates. It should also be possible to extend this scheme towards smaller chips sizes in the future.
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