From: firstname.lastname@example.org (Spam Hater)
Subject: Re: PCB puzzle
X-Newsreader: Forte Free Agent 1.21/32.243
NNTP-Posting-Date: Sat, 28 Sep 2002 11:27:46 EDT
Organization: Prodigy Internet http://www.prodigy.com
Date: Sat, 28 Sep 2002 15:27:46 GMT
I have seen "whiskers" grow inside a PCB. There is a contaminant in
the PCB, and your heating cycle is causing it to move around inside a
tiny crack in the substrate.
Lapping down an un-assembled board would show nothing. The
contaminant was probably introduced in the assembly process, and the
lapping process closes the cracks.
Look at the bright side: You found it before your customers did.
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.