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PCBs work flawlessly and then they donít. Sound familiar?

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7/14/2010 08:11 AM EDT

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Bellhop
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re: PCBs work flawlessly and then they donít. Sound familiar?
Bellhop   11/2/2011 7:13:00 PM
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I remember a case where new PCBs exhibited increased audio noise. One trace was inexplicably thicker on the new boards, placing it closer to an adjacent trace and causing crosstalk. Trimming the trace fixed the problem. We never got an explanation for the discrepancy, though.

sharps_eng
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re: PCBs work flawlessly and then they donít. Sound familiar?
sharps_eng   1/18/2011 7:57:11 PM
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A star-shaped fragment? Was this an isolated piece of copper area fill? These sometimes are so small they float off during a later process and get trapped under the soldermask. The CAD system might be configured to tidy up fills, otherwise areas should be manually reviewed to remove undersized islands. Bare board testing would not necessarily pick up intermittents, nor provide 100% coverage. Furthermore BBT may be carried out before soldermasking, because rework and inspection is so much easier. Hence 'grown' or contamination defects can appear afterwards.

GaryS76
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re: PCBs work flawlessly and then they donít. Sound familiar?
GaryS76   7/16/2010 7:45:05 PM
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I agree with ttt3. A bare board electrical test should certainly be done on all but the simplest PCBAs. I suspect what happened here is that the PCB laminate had a high level of moisture trapped inside. During fabrication, when the board was reflowed to adhere the tin-lead to the pads, this moisture cooked off and solder splattered on the boards. The moisture could have also been trapped in the holes/circuit vias. Definitely poor quality control at the PCB fabricator.

ttt3
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re: PCBs work flawlessly and then they donít. Sound familiar?
ttt3   7/14/2010 5:32:26 PM
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Often when ordering PCBs such as these an electrical connectivity test is performed on each individual, bare PCB at the PCB fabricator. I'm not sure if the above author's customer paid for this testing or not (it is usually an optional item), but presumably, an electrical test at the PCB fabricator's site should have caught the shorted nets on the "bad" PCBsbefore they were delivered to the customer. That being said, details of this electrical test (presumably a "bed of nails" clamshell around the bare board) are usually very guarded by the PCB manufacturer, and I've found in my experience that even these sophisticated tests can miss circuit opens/shorts on the board. In addition, sometimes the PCB itself needs to have appropriate test points designed in for ease of test (DFT).

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