Controlled collapse reflow chip joining

Abstract

Solder reflow connection of semiconductor devices to substrates has been shown to be a reliable, effective, and readily automated technique. Rigid copper spheres, which remain rigid during solder reflow, have been used successfully for some time as a major element of the contact joint. However, to expand the capability of such joints to larger devices such as multiple-transistor chips in hybrid components, ductile metallic joining pads can be used on the devices instead of the copper spheres to reduce mechanical strains and permit multi-pad devices to make proper contact to module lands during reflow. This paper describes a technique that prevents these solder pads from collapsing and permits large scale production. Termed `controlled collapse,' the method is based on limiting the solderable area of the substrate lands and chip contact terminals so that surface tension in the molten pad and land solder supports the device until the joint solidifies. The result is a sturdy, testable connection of high reliability (bond strength 30-50 gm, pilot-production yields exceeding 97%, predicted failure rate - based on laboratory tests - considerably lower than that of copper ball contacts). The process is economically adaptable to automation and offers considerable latitude in fabrication and control tolerances.