Six Sigma has received much attention in PCB industries recently. Six Sigma management is the pursuit of the reduction of variation in the critical process to achieve continuous and breakthrough improvements that impact the bottom-line of the organization and increase customer satisfaction. This research studied the optimization of an electrostatic spray coating, a critical operation in solder masking process, in a PCB company using Six Sigma DMAIC methodology. The thickness and appearance were chosen as the critical-to-quality characteristics. The DMAIC methodology is presented step-by-step in this thesis. The results indicate that the project has significantly improved the productivity of the solder masking process after improving the capability of the spray coater.