IN-PROCESS STRESS ANALYSIS OF FLIP CHIP ASSEMBLIES DURING UNDERFILL CURE
Author: Prema Palaniappan Company: Georgia Institute of Tech. Date Published: 8/23/1998
Surface Mount International
Abstract: The electronics industry is currently evaluating flip chip technology for high performance, miniaturized assembly applications. This is primarily because of the high I/O density, small form factor, and superior electrical performance provided by flip chip on board technology. Flip chip on low cost circuit boards (FCOB) furnishes a reliable interconnection provided underfill materials are used. Underfills overcome the thermomechanical reliability issues associated with the thermal expansion coefficient mismatch between the board and die. The selection of underfill material is critical to achieving the desired performance and reliability. Processing of underfills during assembly can result in large residual stresses within the silicon die. In some instances these stresses can be large enough to cause die fracture. In this work, low cost flip chip on board assemblies are analyzed during the underfill cure process. In-situ stress measurements are performed over the active face of the die during processing and relative m-plane stresses are measured. Four different commercial undertill materials have been evaluated and a relative comparison is presented. This paper also studies the effect of the cure parameters on the properties of a selected commercial underfill and correlates these properties with the stresses induced in flip chip assemblies during processing. The material properties of a selected underfill processed under varying assembly conditions were determined. In-situ stress measurements in the flip chip assemblies processed under the same conditions described the stress distribution and maximum stress at the die/underfill interface and material properties with the residual stresses were correlated as a function of assembly process parameters. Experimental measurements are made using flip chip test vehicles based on Sandia National Laboratories’ ATC04 Assembly Test Chips.