Initial development work on a high throughput low cost flip chip on board assembly process

As a concept to achieve high throughput low cost flip chip on board assembly (FCOB), a process development activity is underway to implement area based underfill printing, integrated chip placement and underfill flow, and simultaneous solder interconnect reflow and underfill cure. This paper presents the initial process development work. The initial goals are to provide a relative comparison to typical FCOB processing, demonstrate feasibility of the process, and identity the critical process variables in the proposed process. Initially, FCOB assemblies were assembled using a typical flip chip on board assembly process to provide a comparison benchmark. The assembly process involved placing the chip, dispensing flux, reflowing the solder interconnects, cleaning the flux residues, dispensing the underfill, and curing the underfill. To demonstrate initial feasibility and determine critical process parameters, a second group of assemblies were made using the proposed process. The assemblies built with the low cost process involving dispensing a controlled volume of underfill material on the chip site and aligning and placing the chip onto the substrate pads providing underfill flow via compression flow. Next, the assemblies were reflowed in a commercial reflow oven in an air atmosphere to simultaneously form the solder interconnects and cure the underfill. The sets of assemblies were compared to determine the advantages and disadvantages of the new proposed low cost process over the conventional process in terms of assembly time and yield. Critical process variables identified in the low cost process include underfill volume control, underfill position control, and bump height planarity.