Whiskers in germanium transistors etc
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I have been in email correspondence with a collector (Chris Walter) about the whiskers that develop in AF117 etc transistors. He initially contacted me offering a couple of photos showing these in an OC170 transistor he had opened up. His photos are attached and the main text of his accompanying email is here
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Reading this website, in particular the Basic Info/FAQ section, shows that "tin whiskers" are a MUCH bigger problem than the irritating failure of transistors in a few old radios. It has caused an emergency recall of a batch of pacemakers, failure of sattelite control systems etc. It seems that the move to lead-free soldering etc could cause further problems as more tin is used in electronic assemblies. |
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wow thats pretty fascinatign! I'v herd of NiCd batteries growing wiskers too. What actually causes it in transistors?
Thanks for sharing those photos! |
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I have heard that during the 1960s impure germanium was used and thus it continues to 'grow' inside the transistor and somehow reacts with the metal(s) used for the screening can. For all I know, however, this could all be a load of codswallop.
Over the years I have kinda studied this problem and it does seem the whiskers are more likely to appear when a radio is left disused for long periods and much less so when in regular use. |
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The photos seem to suggest that the whiskers grow from the case rather than from the germanium.
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Hmmm.....that NASA site is very interesting and gives examples of whiskers growing from both tin and zinc, so I'm left wondering what other metals can also suffer this effect.
There is also an example of whiskers growing after 3 years of 'ambient storage' which does indeed imply, but not confirm, this is more likely to occur during disuse rather than use. As PS pointed out, the whiskers do indeed appear to be eminating from the transistor case rather than the germanium. Question now is this; are these whiskers a property of, say, tin - or do they occur due to some sort of reaction between the tin and some other substance in very close proximity? Must now find out if AF117 etc transistor cases are tin or zinc plated on the inside. If they are, why would they be ? |
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I am that Chris. The whiskers definitely grow from the case. As I said, there is rainbow colouring, changing colour even along one and the same fiber. That means the diameter is changing (narrowing down towards the end). Although measuring only 1/200th of a hair in diameter, the whiskers are still some tenthousand atomic layers across. There may be quite high electric fields near the tip, although the voltages used are low. I remember, however, to have read that such whiskers consist of a single monocrystal, and grow unidirectionally under the influence of the much higher internal electric field gradients alone.
I still have the opened transistor and am happy to follow suggestions what to do with it. |
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I suspect the team at NASA are interested in getting hold of any samples they can to help with their research, which is why they are so keen to investigate this for us. Otherwise why would NASA be remotely interested in helping a group of vintage radio collectors? I will also take them up on the offer of an "anecdote" on their website, if this is appropriate once we have further information on the material used. |
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Presumably the shuttle is full of 1970s metal cased transistors 8-o Best regards, Paul |
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I have just replied to their email as below:
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There's quite a bit of relevant information here:
http://www.calce.umd.edu/lead-free/t...ISKERRISKS.pdf |
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FYI Normal metals that you encounter in everyday life have very five crystal structures and are usually impure and also have been worked in some way so you may be surprised how different these metals behave when ultra-pure and single crystal. I was amazed the first time I handled a single crystal bar of pure tin... At first it seems to have no strength at all - you expect a solid bar of metal but it is actually more like Plasticine - it cannot support its own weight and sort of flows and sags where unsupported. As it does this you see slip planes appearing as chunks of metal slide along the plane directions of the crystal. But this state doesn't last long because as it moves the crystal structure fines up and it quickly turns hard and more normal. You also notice as you handle it that it crackles - this sound is caused by a process called twinning in which single crystals suddenly switch their orientation. When the crystals are really big you can actually hear the atoms snapping into their new alignment. It's dislocations in the crystal structure that prevents a material freely flowing and bending about. When in this pristine state the atoms can slip over each other so easily that you get this strange behaviour - and you can more readily appreciate how a fine whisker could be jacked out from a metal surface if some small cyclic force is applied to it. |
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Just a further thought, mention has been made whether these whiskers grow whilst equipment is powered up or not.
I notice from Chris Walters pictures of the opened up OC171 transistors that the whiskers grow from the case inwards. Could this be down to a polarity thing as well as the type of metals used? As far as I know the case (or screen) is connected to chassis which is positive in relation to the other electrodes particularly the collector which would explain why the most common failing of AF117 transistors is the collector screen shorts. It would certainly be interesting to see what our friends at NASA make of those AF114's Paul has sent them. |
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Mike ;D |
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I have just received the following interesting email from Jay Brusse at Nasa:
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Please take a look - some of the photos are fascinating! |
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Thats great Paul :D very interesting please keep me posted of any more developments 8-)
Mike ;D |
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Crikey, these guys dont mess about do they!
Either that or they have far too much spare time, and desperatly need a hobby! Very interesting though Cheers Sean |
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Interesting stuff ... and all the way from NASA too:D
I particularly like this bit ... Quote:
They probably used a laser ;) |
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Truly amazing! It's a wonder these AF??? ever work! :o
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