Ross W. Keesler

Identification of process variables affecting the quality of fine pitch high performance ASIC substrates

The HyperBGA/spl reg/ semiconductor package is a PTFE based organic flip chip designed to meet the needs of high performance applications. It combines outstanding overall reliability with higher electrical performance to distinguish itself from wirebond applications, build-up technology, and ceramic alternatives. Signal layers are personalized with high wiring density and wire width ranging from 20 to 33 microns. At this time only a liquid photolithographic process can be used to resolve these ultradense fine lines and spaces while also achieving very tight impedance control. Yields thru automated optical test (AOI) test are mostly impacted by non-repairable defects as circuit opens, neckdowns and dishdowns type defects. To improve on existing yield targets, the entire photolithographic process was considered, examined and variables investigated. Higher yielding processes are absolutely critical for next generation design points. In this article, the photo process and operative variables are discussed with regard to their impact on fine line quality, yields and high reliability of the product in the field.

The effect of copper foil properties on low cycle fatigue of a multi-layer flex cable

A multi-layer high performance flex cable was used as a test vehicle for studying the effect of copper foil properties on low cycle fatigue properties of fine line conductors. We fabricated the cable using copper foils for the three conductor layers, and used advanced organic composite dielectric materials for the internal dielectric layers. We fabricated the conductors using subtractive etching as opposed to using an additive plating process. The cable had to survive multiple cycles of fatigue bending around a sharp radius and we found that selection of copper foil was critical in determining fatigue life. We identified three distinct failure mechanisms in the cables that depended strongly on the surface roughness of the foil. We then studied the mechanical properties of free-standing copper foils of varying roughness and thickness to explain the behavior of the cables. We found that the surface roughness of the foils, and the ratio of roughness to thickness, strongly effected their fatigue performance.