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From: Boris Mohar
Subject: Re: PCB puzzle
X-Newsreader: Forte Agent 1.92/32.572
Date: Sat, 28 Sep 2002 09:07:17 -0400
NNTP-Posting-Date: Sat, 28 Sep 2002 09:07:25 EDT
Organization: Bell Sympatico
On Sat, 28 Sep 2002 05:50:30 -0700, Kevin Kilzer
>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.
Have you tested a bare board? It could be the problem with the device
itself failing. Is the temperature cycle done with power on? Is this a
multilayered board? I assume the proper cleaning procedures were in
place. Some water soluble fluxes are highly conductive and if they were
not completely removed the residuals may creep out.
Viatrack Printed Circuit Designs
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