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PCB-Sept2017

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34 The PCB Magazine • September 2017 FEATURE As of this writing, I have explored many topics related to specific defects that plague printed circuit board fabricators. A key under- lying theme of these writings underscores the critical need of the skilled troubleshooter to get to the underlying cause or causes of the defects. Defects may "manifest" or be detected in or af- ter a specific operation within the printed cir- cuit board manufacturing process, but the un- derlying root cause may have occurred earlier (perhaps much earlier) in the process. As I have written in prior columns, I chose to present the anomaly or defect where it is most likely to be detected, then subsequently presented the most likely root causes wherever 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 troubleshooter's task is complicated by the fact that there are many possible ways to combine or sequence the individual pro- cess steps available to achieve the desired end structure. As an example, a simple, single-lamination multilayer printed wiring board may involve 30 to 50 process steps, while a complex, multiple lami- nation (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 verified 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 make la- tent errors visible. Much effort is and has been expended in attempts to improve this, with limited success. Thus, it remains a troublesome issue. The concern then is the defect may not manifest itself until several downstream process steps. One may refer to this as the latent defect. The impact of these latent defects is four-fold: 1. First, the detection and verification of the defect may require real time and the diversion of skilled resources—already in short supply in most lean-running modern operations. 2. Second, there is a schedule impact in today's just-in-time operating mode, both of the process flow disruption in- herent in the de- tection and in the verification of the defect. Far more PROCESS ENGINEERING & DEFECT PREVENTION by Michael Carano RBP TECHNOLOGY

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