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Subject: Re: PCB puzzle
X-Newsreader: Forte Agent 1.91/32.564
Date: Sat, 28 Sep 2002 14:55:09 GMT
NNTP-Posting-Date: Sat, 28 Sep 2002 15:55:09 BST
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.
Can the cold cycle produce trapped condensation?
Is it standard FR4 material?
>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).
Measured with an electronic device (DVM) or a moving coil meter?
60 ohms is a fault but 10M with a DVM /may/ not be.
>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.
Why does a single application of the heat gun produce the fault when the first
oven heating does not?
>signal trace within the PCB
Is this trace really "within" i.e. on an inner layer of a multilayer board?
Sorry about the questions but more information needed for remote servicing than
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