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72 PCB007 MAGAZINE I JUNE 2018 PTH Failure Mechanisms and Using IST as a Tool, Part 1 Trouble in Your Tank Feature Column by Michael Carano, RBP CHEMICAL TECHNOLOGY Introduction Getting to the root cause of a defect is not a simple task. As stated numerous times in these columns over the last several years, a defect may have its genesis in a process or process- es several steps removed from the operation where the defect manifests itself. This edition of "Trouble in Your Tank" will elucidate the difficulty of root cause defect analysis when incorrect assumptions (and thus conclusions) are made. Further, I will illustrate that when proper tools are used to study the issue, addi- tional data is gathered that supports the actual cause of the defect. The Issue In this instance, a fabricator was in the pro- cess of evaluating a different electroless cop- per process. While the initial start-up and sub- sequent production was satisfactory, there was a defect noted after several weeks of op- eration. At first, in-circuit test (ICT) found an open. Cross- sectioning revealed a fracture in the electroplated copper af- ter solder float. The fabricator then decided to perform additional testing us- ing interconnection stress test (IST) from PWB Interconnect Solutions. The internal brain- storming team strongly felt this issue must be related to the electroless copper process. Yet there was no concrete evidence to support this theory. What follows is a systematic procedure to get to the root cause of the problem. But first, a bit of background on IST. IST Testing and Mechanism IST has been a standard failure analysis method in the industry for over two decades. IST is a tool that provides insight into PWB failure mechanisms and allows fabricators and

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