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From: Kevin Kilzer
Subject: PCB puzzle
Date: Sat, 28 Sep 2002 05:50:30 -0700
Organization: MindSpring Enterprises
X-Server-Date: 28 Sep 2002 12:51:15 GMT
X-Newsreader: Forte Agent 1.8/32.548
About 8 months ago we made a batch of products and put them on the
shelf. This month sales got around to selling them. Standard
production testing involves a few temperature cycles, -40C to +80C.
The process is hot first for 1 hour, then cold for 3 hours, then hot
again for 1 hour, and so on.
All units failed during the 2nd hot cycle, and most recovered at room
The problem was traced to a single pin on a connector to a device. In
our application, the pin is intentionally left open (per the device
manufacturer), but was found to be a low-impedance to an adjacent
signal trace within the PCB. Some units had higher impedance, some
lower (range about 60 ohms to 10 Mohms).
The PCB has 8 devices connected similarly. All 8 show the problem
after enough temperature cycles. We have eliminated the device itself
as the problem.
Heating that area of the PCB with a heat-gun causes the problem. Two
cycles in the oven cause the problem. Returning to room temperature
usually clears the problem.
The PCB manufacturer lapped down an un-assembled PCB and saw nothing
unusual. The artwork was checked for scratches.
The connector is a typical pin-header (0.050" spacing) from a major
manufacturer. If we heat a connector with a heat-gun, we can get the
pin-pin resistance below 10Mohms, but this is not very scientific
(although it did not melt).
I'm looking for suggestions as to the cause.
Thanks in advance.
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