Analysis of Solder Paste Misalignment Defects in the SMT Printing Process

Abstract

Surface Mount Technology (SMT) printing is a critical process in Printed Circuit Board (PCB) assembly, where the quality solder paste deposition significantly affects product reliability and manufacturing yield. This study aims to analyze solder paste misalignment defects in the SMT printing process and evaluate the effectiveness of process improvements using Statistical Process Control (SPC). Weekly production data collected over a seven-month observation period were analyzed using a Fishbone Diagram to identify the root causes of defects, while a Control Chart, specifically a U-Chart, was employed to monitor process stability and evaluate defect performance before and after corrective actions. The analysis identified uncentered machine X/Y offset parameters and instability of the pneumatic clamping system as the primary causes of solder paste misalignment. Corrective actions were implemented by recalibrating the machine coordinates and stabilizing the PCB clamping mechanism. The SPC analysis using the U-Chart demonstrated a significant reduction in the average Defect Per Unit (DPU) from 0.0384 during the baseline stage (Weeks 1–12) to 0.0033 after the implementation of corrective actions (Weeks 13–28). Furthermore, the average production yield increased from 96.16% to 99.67%, indicating improved process capability and consistency. These results confirm that applying SPC via U-Chart monitoring is effective for evaluating process performance, identifying process improvements, and reducing solder paste misalignment defects in SMT printing.