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OT- Curious assembly defect in board

The person doing board repair this morning asked me if I'd like to look at a Logicflex board that he was ready to scrap. He'd spent about an hour on it and couldn't fix it. I did the usual checks and concluded that A15 was shorted to something but couldn't find where. I gave the board back to him, told him about A15 and said to go ahead and scrap the board.

He eyeballed the board for about 10 minutes and found a 0805 .1ufd capacitor that had gotten under the SMT DIP socket and had been reflowed/waved between A15 and A16.

The chip was completely covered by the socket and could only be seen at an oblique angle and only over a few degrees.

I've worked in electronics for over 30 years and this is the most perverse assembly error I've ever seen.

Reply to
Jim Stewart
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That is interesting. It reminds me of similar problems we found on some ECL memory boards we were debugging. They used a fly cutter to cut down the through hole pins after the board was assembled. Often we would find that these tiny pin shards had gotten back into the solder flow and would end up in very unusual places on the boards. Like in your case, sometimes they would be nearly impossible to see.

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Reply to
rickman

We used to have boards that had inexplicable failures, due to invisible (to the casual naked eye) paths left between circuit runs. We could routinely run a scratchawl or Xacto knife between the runs, and often that would remove the problem. This was a long time ago, and I haven't heard of such problems for a long time.

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Reply to
CBFalconer

I think it was a problem with double-sided board processing. I fixed many a board like that. They looked like little hairs under the microscope or magnifier. Nasty, nasty board fault. 100% electrical test avoids that, but is expensive for some products- it may be cheaper to throw away a few percent of the stuffed boards if that happens. It took from a couple minutes to 30 minutes to find the faults, and often required parts to be desoldered. The problem is that you can't be sure it's a board fault and not something else that requires attention in production until you actually find the problem.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

I had a similar batch when the layout had a flood fill of ground and about half of the boards failed. This was despite the boards supposed to have been "bare board tested". Although the link was invisible I only had to run a knife between the offending pair of traces to fix the board.

Peter

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Reply to
moocowmoo

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