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There are two major driving forces in the electronics industry. First, from a design perspective, is the continual shift towards higher circuit density and I/O counts per semiconductor package. This results in high I/O devices and increased levels of component density at the board interconnect level. The second major driver, from a business standpoint, is the exponential transition of business to Electronics Manufacturing Service (EMS) providers. This outsourcing of assembly is spurred by a significant cost benefit to the Original Equipment Manufacturers (OEMs). Also, outsourcing allows the OEMs to focus on their core competencies of system design, while the EMS providers focus on and take the responsibility for the manufacturing systems. Consequently, OEMs (especially small startups) were spared the huge investments, time, research and development required for complex Surface Mount Technology (SMT) assembly. The objective of the electronics assembly process is to provide an electrical contact path between the terminations of the electronic/electrical devices and the board level interconnects with desired mechanical strength and reliability. Within the surface mount electronics assembly process, the solder paste deposition step is the key to a low defect, high yield assembly process. It is the first step in the surface mount assembly sequence. By design, downstream assembly operations do not rectify the upstream operation errors. Historically, approximately sixty percent of the SMT defects are ascribed to the solder paste deposition step. Increased I/O count coupled with component miniaturization further constrain the process limits for the paste deposition process. Process control is a necessity for high SMT yield. Today, automated inspection equipment can measure the critical process variables while overcoming the limitations of visual inspection and time constraints. These pieces of automated inspection equipment can quantify the process and consequently facilitate effective process control. In this research endeavor, studies targeted towards process improvement were conducted under the Six-Sigma process improvement structure. Considering an EMS environment, the improvement efforts focused on process development and control to ensure repeatable production results. Historical defect data analysis was conducted to identify the principal defects that occur during assembly. Based on the understanding of the processes, the defects were then assigned to their root cause and the relevant process. The analysis indicated that the stencil printing process is the principal source of soldering defects in SMT assembly. The issues considered include the appropriateness of the process measurement system, process parameters and process control issues, equipment set-up, equipment variability, tool selection and storage, solder paste handling and storage, and operational issues. Based on the studies that were conducted, process parameters such as the snap off distance, separation speed, print pressure, and print speed were standardized for a combination of component complexities and stencil thicknesses. The performance of the measurement system was validated prior to the study. Of the two systems that were validated, the more precise system was selected for further study. A 'design of experiments' based approach was used to arrive at the preferred process parameters for different process requirements. The effect of product transfer between a set of equipment was studied and recommendations were suggested. Equipment and material handling issues were also addressed. The process improvement effort was successful in standardizing a large number of factors from an engineering perspective. With operator training and associated system changes, the goals of Six-Sigma quality increasingly seem to be feasible.


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