Fluxes Design for Suppressing Non-Wet-Opens at BGA Assembly
Authors: Fengying Zhou, Fen Chen, and Ning-Cheng Lee Company: Indium Corporation Date Published: 9/22/2019
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 the coefficient of the thermal expansion mismatch between the die and the molding compound, often resulting in non-wet-open (NWO) defects at the BGA assembly. NWO defects have 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 to suppress NWO defects. At the BGA assembly, after printing solder paste onto the PCB, the BGA bumps are dipped into creamy flux or solder paste prior to being placed onto the paste printed. This flux pickup effectively suppresses NWO defects by serving as a cold-welding barrier. The effect of solid-flux coating is more effective in suppressing NWO defects. A low-activity flux is considered to have a wider reflow profile window than a medium-activity flux. Alternatively, the solder bump of a BGA can be pre-coated with solid flux at the packaging house. The ranking of efficiency in suppressing a NWO is listed below: solid flux coating > creamy flux > creamy solder paste; low-activity flux > medium-activity flux. As a cold-welding barrier, both no-clean flux and water-soluble flux are considered effective. However, a water-soluble flux should not be used with a no-clean solder paste due to compatibility and reliability concerns. Furthermore, only no-clean solid flux is recommended for solder bump coating purposes, mainly due to moisture pickup issues associated with water-soluble flux.