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PCB007-June2020

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68 PCB007 MAGAZINE I JUNE 2020 The Case of Failed Boards at Assembly Defects may "manifest" or be detected in or after a specific operation within the PCB manufacturing process, but the underlying root cause may have occurred earlier (perhaps much earlier) in the process. As has been writ- ten in many of my columns published thus far, I chose to present the anomaly or defect where it is most likely to be detected and subsequent- ly present the most likely root causes wherev- er they might have been introduced. It should be noted that these examples are presented as likely starting places for the investigation of anomalies or defects presented or the kinds of causes that might be investigated. The process engineer's task (aka "the trou- bleshooter") is complicated by the fact that there are many possible ways to combine or sequence the individual process steps available to achieve the desired end structure. As an ex- ample, a "simple" single-lamination multilayer A Process Engineer's Guide to Advanced Troubleshooting, Part 1 printed wiring board may involve 30–50 pro- cess steps, while a complex, multiple lamina- tion ("sequential lamination") printed wiring board, with pre- and post-machining and other mechanical operations and selective plating processes, could involve several hundred pro- cess steps. In an ideal world, each step could be veri- fied to be correct immediately during or after the process, but in practice, the effect of many processes cannot be readily evaluated until the completion of many subsequent steps makes latent errors visible. Much effort is expended and has been expended in attempts to improve this with limited success. Thus, it remains a troublesome issue. Process Control Before presenting the main subject of this month's feature, a word with respect to pro- cess control is in order. What gets measured Trouble in Your Tank by Michael Carano, RBP CHEMICAL TECHNOLOGY

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