Advancements in Flip Chip Assembly Equipment and Processes

Abstract

High I/O devices such as microprocessors, applications processors and field programmable gate arrays have transitioned from wire bonding to flip chip interconnect as the I/O densities have increased above 2000. As the bump pitch shrinks the standard process flow for production flip chip processes is challenged. As the bump pitch continues to shrink the accuracy of standard flip chip bonders is not adequate for the fine pitch packages of tomorrow. The options to resolve this issue are extending the accuracy for standard flip chip bonders or moving the assembly of these packages to the inherently more accurate thermo-compression bonders. This paper will discuss the pros and cons of each approach along with showing data which indicates what accuracies are actually required. Although substrate manufacturers have developed low CTE designs which mitigate the warpage caused by the mismatch between the Si die and the substrate as the assembled package travels through the reflow oven, warpage at finer pitches is becoming more and more difficult to control in flip chip processes. Thermocompression (TC) bonding is seen as the next-generation packaging technology that will resolve this issue through local reflow of the solder and elimination of the reflow oven. Despite the tremendous technical and quality advantages of TC bonding, adoption has been limited by the relatively low throughput of the first generation thermocompression bonders. In this paper we describe bonding results obtained with an innovative flip chip bonding method to optimize the process to dramatically improve the throughput by applying flux directly to the substrate rather than dipping the pillars in a bath. A study of this process and comparison of various methods of accomplishing it along with their related costs are discussed in the paper. A second large productivity improvement that is promising by eliminating the need for cooling the die before transferring die that has pre-applied underfill film laminated to it is also studied with productivity models developed. Finally a unique equipment concept for managing the transition from mass reflow to thermo-compression bonding will be presented.