Issue link: https://iconnect007.uberflip.com/i/1189040
44 SMT007 MAGAZINE I DECEMBER 2019 them as controlled documents and keep them constantly updated, at least quarterly. So, what should the content of these documents include? Design for Manufacturability DFM is a key driver, if not the most impor- tant part, of manufacturing yield. However, few PCB designers understand manufacturing pro- cesses. 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 it for almost a decade starting in the mid-1980s), is a good place to start. But you need to customize it to your application. Some major items that should be included in a DFM for SMT products are: • Established design rules and guidelines that emphasize 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 the widespread use of high-pin-count BGAs that cannot be visually inspected, suffi- cient test coverage for ICT should be seriously considered. Keep in mind that no inspection method is perfect. The only way to prevent defects from escaping to the field is to rely on overlapping test and inspection methods and ensure that the majority of your defects are shorts and not opens. Once a DFM document developed by a well-trained team is finalized and released, the possibility of DFM violation generally does not arise. Creating a DFM document is not easy, but it will correct problems at the source and prevent their recurrence. This is critical in an environ- ment where essentially all manufacturing is being outsourced or sent offshore. Manufacturing Processes How should one identify key manufacturing process issues? First, characterize each pro- cess, including printing, placement, solder- ing, inspection, test, and repair. You need to document the details of equipment- and non- equipment-dependent variables that control yield. For example, each board has a different thermal mass, so it is a good business practice to develop a unique profile for each product regardless of the type of oven used. Most companies are ISO-certified these days and think they have a documented process, but they don't. Look at your ISO document. Most likely, there are no details about designs and process. Instead, it is a generic boilerplate doc- ument that never goes out of date and ensures that you will easily pass audit and re-audit. But they are useless since they don't have the critical design and process details that a rec- ipe requires. However, it is not the fault of ISO; ISO does not prevent you from including the details, but few companies do so for the afore- mentioned reasons. In addition to having the right design, qual- ity incoming materials, good manufacturing capabilities, and trained personnel are criti- cal to achieving high yields. Failing to realize you need a good recipe, the right ingredients, and a good chef to improve yield makes the problem seem like the weather—out of our control. SMT007 Ray Prasad is the president of Ray Prasad Consultancy Group and author of the textbook Surface Mount Technology: Principles and Prac- tice. Prasad is also an inductee to the IPC Hall of Fame—the highest honor in the electronics industry—and has decades of experi- ence in all areas of SMT, including his leadership roles implementing SMT at Boeing and Intel; helping OEM and EMS clients across the globe set up strong, internal, self- sustaining SMT infrastructure; and teaching on-site, in- depth SMT classes. He can be reached at smtsolver@ rayprasasd.com and has an upcoming SMT class April 20–22, 2020. More details at www.rayprasad.com. To read past columns or contact Prasad, click here.