Process capability evaluation of fine-pitch SMC placement in SMT for PCB production

Abstract

Traditionally, design is first developed for the product functionality and handed it over to the manufacturing for implementation. There is little communication between them unless a serious problem occurs. This approach results in long lead time and high cost. Actually, the decision made during product design stage dictates material, fabrication methods, assembly methods, inspection techniques, and production system. Decision made during design stage accounts for 70% of life cycle cost. The requirements of equipment, materials, and method for SMT assembly should be identified together with the functionality of product during its design stage. In other words, the specification of SMC and PCB, inspection technique, assembly process, and the capability of pick-and-place equipment and of machine vision system must be simultaneously decided to meet the product functionality. Therefore, prior to putting them in the assembly line, not only the capability of the required equipment, but the dimensional compatibility of PCB and SMC must be properly determined to ensure the high production yield and process reliability. To comprehend the capability of the equipment, the simplest method is to refer the published specification of equipment supplied by vendors. Unfortunately, the specifications provided by vendors are always exaggerated more or less due to commercial competition. Besides, the lack of standardization of terms and evaluating procedures makes the measurement formidable. How to evaluate the actual process capability becomes a critical issue. The objective of this research is to develop a model for evaluating the process capability of SMT fine-pitch component placement in coverage and in width direction. The key factors are the variation in inspection, geometric deviation of SMC leads and PCB pads, mechanical variability of pick-and-place equipment, and the capability of machine vision system.