BALL DETACHMENT EVALUATION FOR TAPE BALL GRID ARRAY PACKAGES
Author: S. H. K. Lee Company: HK Univ. of Science & Tech. Date Published: 8/23/1998
Surface Mount International
Abstract: For a ball grid array package, one crucial measure of robustness is the ball-shear strength. Quite simply this allows one to assess the quality of the cohesion between the solder spheres and the underlying bond-pads. As the ball- shear strength decreases, the chance of the solder sphere detaching increases. The objective of the present study is to closely examine the ball-detachment issue and to suggest an additional parameter which, when combined with the two existing parameters, would yield a clearer assessment. Ball detachment was induced on existing TBGA units, and the detachment mode was found to be a cleavage at the nickel and solder interface. When inspected under a microscope, the fractured surface was uncharacteristically flat. EDX analyses showed this detachment to be unrelated to the well- known tin-copper intermetallization. Initial finding suggests the supplier processes to affect the detachment. Experimentations were performed at the supplier and based on this evaluation, improvement actions were formulated and implemented. The improved units were shown to have a lower ball detachment rate as well as higher mean ball- shear and cpk values. Furthermore, an additional parameter, span/SD ratio, was identified as being a possible indication of solder sphere detachment.