This study develops process improvement steps which are suitable for solder mask process in printed circuit board industry. The improvement steps are based on structures, Define, Measure, Analyze, Improve, and Control from the concept of lean six sigma. To develop these steps, the study addresses the natures of solder mask process in printed circuit board industry and then applies the concepts of process improvement of lean six sigma. The crucial parts, tools and methods of improvement steps are summarized in this study to enhance the efficiency of process improvement. Furthermore, experienced experts are in-depth interviewed for validation of the model. The unanimously recognition of experts proves that the concepts, effectiveness, and practicability of the model can be implemented effectively in process improvement of solder mask process in printed circuit board industry.