Nondestructive Analysis of Buried Interfacial Behaviors of Flux Residue and Their Impact on Interfacial Mechanical Property

Abstract

Flux materials are ubiquitously utilized in the microelectronics industry during back-end processing. The Cu oxides or organic solderability preservatives present on Cu posts used in flip-chip packages must be removed by flux before solder reflow and die attachment to ensure a quality connection between the substrate and the die. However, flux residues can cause solder bridging, which renders the device useless. These residues must be studied in situ at the buried interface to determine fundamental interactions and how the residues can be eliminated. In this paper, model flux residues were investigated at the surface and the buried epoxy interface with sum frequency generation vibrational spectroscopy. Glutaric acid. served as the model flux, simulating fluxes that use dicarboxylic acids as one of the main components of the fluxing agent. It was found that the flux residues greatly change the buried interface on both Cu and silica surfaces, which affects properties such as adhesion. It was also observed that the fluxes require long washing times before they are completely removed. This paper is a step forward in flux residue analysis and will help the industry better understand molecular-level details of commonly used processes.