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Tin whisker on a large scale?

Phones- Digital, Analogue, PABX, Receivers, transmitters & military equipment.
 
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Tin whisker on a large scale?

Post by Niall » Tue Dec 17, 2013 6:53 pm

Forum members may remember I was looking for a diagram for a Mutek transverter. Information on this gear seems to be basically unobtainable, however a helpful amateur with a working one gave me some voltage readings which enabled me to track down the fault.

A TO220 cased switching transistor had shorted its heatsink plate to the tinned copper groundplane it was bolted to despite the intervening mica washer. When I lifted the transistor there was greyish goo in the hole in the mica washer. There is no sign of liquid spillage and I wondered if this could be "Tin whisker"?

There is 12 volts between the tab and groundplane.

 
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Re: Tin whisker on a large scale?

Post by Michael Watterson » Tue Dec 17, 2013 7:53 pm

Usually thermal paste!

 
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Re: Tin whisker on a large scale?

Post by Niall » Tue Dec 17, 2013 8:35 pm

I don't think it was, thermal paste is normally white and there was no evidence of any on the washer or the transistor.

It was dark grey and sort of fibrous until I wiped it off when it turned into a smear. I was surprised at the absence of thermal compound on a traditional mica washer, but the transistor is quite generously rated and may well be bolted down more for rigidity than heatsinking.

 
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Re: Tin whisker on a large scale?

Post by Michael Watterson » Tue Dec 17, 2013 8:46 pm

There was grey stuff too! But it could be white paste with tin growth / contamination.

 
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Re: Tin whisker on a large scale?

Post by Lloyd » Wed Dec 18, 2013 7:30 pm

We use some grey coloured thermal paste at work called 'Arctic silver'.

Horrible stuff...

Regards,
Lloyd.

 
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Re: Tin whisker on a large scale?

Post by Michael Watterson » Wed Dec 18, 2013 8:04 pm

Some thermal pastes are electrically conductive, esp ones intended for ICs.

 
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Re: Tin whisker on a large scale?

Post by Niall » Wed Dec 18, 2013 9:19 pm

I really don't think it was thermal paste, the deposit was only in and around the mounting hole in the mica washer. There is an identical transistor nearby on the board so I unbolted it and checked underneath in case there was an incipient problem, it was completely clean with no sign of any paste ever having been used.

Whatever it was was less substantial than heatsink compound, if you know what I mean. It more or less disappeared when wiped, whereas my experience of heatsink compound is that it is incredibly persistent and the smallest amount gets everywhere. This is a particularly unpleasant characteristic of the original heatsink paste, silicon grease, as it is bioaccumulative.


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