Fluxes Effective in Suppressing Non-Wet-Open at BGA Assembly

Abstract

Abstract With the advancement in miniaturization, the die is getting thinner and the solder bump is getting smaller for BGAs. Consequently, the thermal warpage is getting more severe due to coefficient of thermal expansion mismatch between die and molding compound, and often resulted in non-wet-open (NWO) at BGA assembly. NWO has been ailing the industry for a long time, and costly rework is required to remove the problem. In this study, a “cold-welding barrier” method has been developed in suppressing NWO. At BGA assembly, after printing solder paste onto PCB, the BGA bumps are dipped into creamy flux prior to placing onto paste printed. This flux pickup will effectively suppress NWO by serving as cold-welding barrier. A low activity flux is considered having as wider reflow profile window than a medium activity flux. Alternatively, the solder bump of BGA can be pre-coated with solid flux at packaging house. The effect of solid-flux coating is equally effective in suppressing NWO. As cold welding barrier, both no-clean flux and water soluble flux are considered physically effective. However, a water soluble flux should not be used together with a no-clean solder paste based on compatibility and reliability consideration. Furthermore, only no-clean solid flux is recommended for solder bump coating purpose, mainly based on moisture pickup issue associated with water soluble flux.