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PCB-Sept2017

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36 The PCB Magazine • September 2017 PROCESS ENGINEERING & DEFECT PREVENTION important is the double impact of the wasted process time and effort invested to the point of detection, and the necessity of repeating that processing. The combined schedule disruption will cascade through the producer, the immedi- ate customer, all the way to end-customer's de- livery/implementation commitments. 3. Third, the cost impact of the lost labor and materials, possibly compounded by expe- diting costs (overtime and raw material rush re- placement premiums). 4. Fourth, lost opportunities for other valu- able or urgent work that cannot be accom- plished while resources and materials are di- verted to the recovery effort. More complex printed circuit board struc- tures may require multiple "loops" through the process from multilayer layup through tin strip, and selective plating increases the num- ber of loops through the imaging and plating sequences. These have the obvious effect of in- creasing the required process time, but also the less-obvious effect of increased risk of yield loss. The message here: Process engineering adjust- ments and close attention to detail are needed. Various simulation models of cumulative ef- fect of process complexity on yield have been put forward (including some which pessimisti- cally predict that no printed board of sufficient complexity can (mathematically) ever be suc- cessfully produced). While the picture is not quite that bleak, it is clear that complexity adds to cost, build time, and risk of loss (with the resultant schedule impact). To the extent pos- sible, additional early effort expended in sim- plifying a design, or its build process will yield benefits in the long run. Process Design and Control With increasing circuit board complexi- ty comes the greater possibility of defects. A surprisingly common underlying cause of de- fects, particularly in high-mix/low-volume op- erations (characteristically, quick-turn, proto- type, specialty production, etc.), where frequent changeovers between products produced is ex- perienced, is incorrect (or sub-optimal) conver- sion of the incoming data and requirements into the working process sequence and/or test and measurement requirements. The wide vari- ety of work types and configurations processed in such operations may delay recognition that an error has, in fact, occurred until it is too late to salvage the product. Common errors include (in no order of crit- icality): • Omission or transposition of selective plating or etching mask areas from data package to production working phototools • Skipped, or mis-ordered process steps • Data collection instructions omitted for a specific process step not readily retrieved later, or necessary test vehicles (coupons) not added to the initial panelization scheme • Premature removal of electroplating buss-bars (electrical tie-in), in-process test points, or of etch-resist metallization • Transposed (or just incorrect) dimensions incorporated into mechanical operation programming (drilling, milling), especially when manual or override is required by the data supplied. Process control excursions (or process break- downs) that are undetected until after an opera- tion remain as hurdles to high-yield, flexible-re- sponse manufacturing. Scrap (or suspect prod- uct) containment is a major focus at high-speed, high-volume processes, particularly continuous (reel-to-reel, automotive, or cellular production, for example) processes. Automatic monitoring and shutdown (or at least alarm) of key pro- cess parameters is becoming the norm. Process control and process monitoring are key levers that one can pull to insure process repeatabili- ty and performance consistency. However, pro- cess engineering improvements often overlook process conditions such as rinse water tempera- ture and cleanliness, pH controls, rectifier issues that reduce electroplating quality, malfunction- ing temperature controllers, etc. These are just a few of the potential process engineering issues that if not corrected, will contribute to print- ed board defects and loss of revenue. Learning that some very expensive printed circuit boards are found to fail in the field due to lack of pro- cess engineering or lack of simple controls is

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