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72 SMT007 MAGAZINE I APRIL 2021 getting worse. With widespread use of lead- free, the yield problems are compounded. When yield problems persist, most people blame manufacturing. is is unfair and pre- vents companies from implementing the nec- essary corrective actions. Who in the company is responsible for the defect? If you really think about it, no matter what your job title is or which department you work in—engineering, manufacturing, qual- ity, purchasing, or management—what you have in common is the responsibility for qual- ity and cost. You affect quality and reliability in different ways. No one group can ever solve this problem alone. If you take a 50,000-foot view of the problem, there are three major areas that affect quality: design for manufac- turing, quality of incoming materials, and manufacturing. How the board is designed, and what com- ponents are selected, impact cost and qual- ity. is is mostly the responsibility of the designer. Just about everything you need for manufacturing is purchased from outside— components, boards, and materials such as sol- der paste and flux. e person responsible for vendor selection is generally the purchasing manager. Yes, defects are caused in manufac- turing but there is nothing they can do about poor design or bad quality of incoming materi- als. So, let us look in more detail at these three areas: design for manufacturing, incoming material quality, and manufacturing. Design for Manufacturability Keep in mind that DFM is a key driver, if not the most important, of manufacturing yield. However, few circuit and board designers understand manufacturing processes. A DFM document must be company specific. Using an industry standard such as IPC-7531 (formerly known as IPC-SM-782 when I initially chaired this committee in the mid-'80s) is a good place to start. Some major items that should be included in a DFM for SMT products are: • Establish design rules and guidelines while emphasizing the importance of differences between them • Component selection criteria, including consolidation of parts lists to reduce redundancy and eliminate obsolete parts • Paneling considerations • Fiducial requirements • Land-pattern design • Solder-mask consideration • Via-hole location • Design for test • Anything unique to your design With widespread use of high-pin-count BGAs that cannot be inspected visually, suf- ficient test coverage for in-circuit test (ICT) should be seriously considered. Keep in mind that no inspection method is perfect. e only way to prevent defects from escaping to the field is to rely on overlapping test and inspec- tion methods. Once a DFM document devel- oped by a well-trained team is finalized and released, the possibility of DFM violation gen- erally does not arise. Creating a DFM document is not easy; it will, however, correct problems at the source and prevent their recurrence. is is critical in an environment where essentially all manufac- turing is being outsourced or sent offshore. Incoming Materials Quality No matter what components, boards, solder paste, flux, etc., the designer and manufactur- ing people selected, the quality of incoming How the board is designed, and what components are selected, impact cost and quality. This is mostly the responsi- bility of the designer.

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