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AF117 Tin whiskers.

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AF117 Tin whiskers.

Post by slidertogrid » Fri Apr 22, 2016 10:01 am

I realise this has been discussed to death but I have a question, could lack of use be a factor in the tin whisker problem?
I have a couple of Radio mobile car radios they are fitted in my Rover P5Bs one 1969 and one 1972. Both of the radios worked fine right into the early 2000s while the cars were in regular use at weekends etc. then both cars were laid up for around 10 years.
After that both radios had stopped working and the fault in both cases was shorts in the AF11x transistors.
Since zapping the transistors both sets have continued to work.
Meanwhile my aunt has a Roberts R707 which had been used almost every day from new (on a mains adaptor) until about three years ago when a kind family member bought her a DAB radio.
The Roberts was stored in a cupboard until the DAB radio packed up. (The aerial suffered fatigue and fell off! :aaj ) When tried again the Roberts wouldn't work on any waveband until I gave the module a clout.
I could make the set cut on and off by tapping the module then suddenly I couldn't. The set stayed working even when I gave the module quite a sharp tap.
The surprising thing is that it has continued to do so since.
So I just wondered could using the transistor in any way 'keep the whiskers at bay' in some way? I'm not suggesting that the transistor is "zapping" its self but is it possible that the whiskers are repelled from the transistor junction in some way when it is in use?
One it is unused they manage to penetrate the junction and cause the usual shorts?
I wondered if damp could be a factor but one of the Rovers was stored in a heated garage and in the case of my auntie's Roberts it was stored in a sideboard next to a radiator in the dining room of a modern bungalow. Damp wouldn't have affected that.
I have zapped a variety of Af1xxs in various sets and have never had any re fail however I have had some that died when Zapping.. :elc:
I am tempted to do an experiment. I have two working TR82s which have AF series transistors which as far as I can see have never been disturbed. I also have a third with Transistors that have been zapped by me.
I am going to put the zapped one and one original in a cupboard and forget about them, the other original set will be kept on a shelf it the workshop and used from time to time.
I want to see if the unused sets have a problem in the future and particularly if the zapped set has a relapse..
:bba
Rich.

 
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Re: AF117 Tin whiskers.

Post by CTV » Fri Apr 22, 2016 10:44 am

Interesting thread and questions raised. I have a Roberts R200 set used frequently which has AF117's and it works flawlessly. However I have an EverReady I picked up at an estate sale, I believe it might have the AF117 whisker problem I've read about and you're discussing here.

 
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Re: AF117 Tin whiskers.

Post by Terrykc » Fri Apr 22, 2016 11:13 am

That's an interesting point you raise, Rich.

Not all AF11x transistors were not born equal, for a start. Someone said recently - I don't know if it was here or not - that those made by Philips in the Netherlands never, or rarely, caused problems, whereas those made here by Mullard did. This might suggest that the thickness of the tin plating in the Dutch process was much thinner than the home grown variety.

In the Trade Tales section I referred to a Ford car radio model that frequently suffered from IF transistor failure - always the same one and, of course, it was an AF117! This might infer that there was a design fault causing premature failure, though exactly how you could manage this in a small signal stage eludes me completely!

Of course, in the mid 60s, nobody had ever heard of the tin whisker problem but now, with my marvellous powers of hindsight (!) I wonder if this was a Mullard batch with a particularly thick tin coating that all ended up in the same parts bin at Plessey, where the sets were made. I certainly never had one of these radios return, which seems to rule out all other possibilities.

Looking into your theory, I wonder if it might work something like this?

Tin whiskers grow at the same rate, whether the transistor is in use or not but, if under power, when the whisker first contacts part of the transistor, it is probably only one or two molecules thick and, unless it is a very sensitive part of the assembly, enough current flows to melt the offending part of the whisker. This might happen several times during the growth of the whisker until it has got enough muscle to survive!

Another possibility to consider is that there is an electrical field surrounding the transistor proper, so could the collector lead-out, having the highest potential relative to the earthed can, attract whiskers to their destruction whereas, in an unpowered device, they would straight for the vital organs and, as I've suggested, continue to grow in current carrying capacity? This would also apply, to a lesser extent, to the base and emitter lead-outs.

In short, I think there are a couple of variables to be considered here: one is variation according to factory of or individual batches from each factory and the other is a natural whisker destroying mechanisms within an active device itself.

It certainly looks as if this latter theory of mine matches your findings, Rich!

 
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Re: AF117 Tin whiskers.

Post by Terrykc » Fri Apr 22, 2016 11:38 am

slidertogrid wrote:I am tempted to do an experiment. I have two working TR82s which have AF series transistors which as far as I can see have never been disturbed. I also have a third with Transistors that have been zapped by me.

I am going to put the zapped one and one original in a cupboard and forget about them, the other original set will be kept on a shelf it the workshop and used from time to time.

I want to see if the unused sets have a problem in the future and particularly if the zapped set has a relapse.

I don't think that will prove any thing, Rich, as you are not comparing like with like.

If we are both right, the zapped transistors will have whiskers ending in relatively large current carrying capacity balls, where the tin has been melted. If my 'self cleaning' theory about active devices is right, the whiskers will still be relatively pointed with just the tiniest of balls on the end.

But, the high zapping current you used will mean that the big balls whiskers will have ended further away from danger than the naturally zapped ones. As both types of whiskers will have continued to grow, presumably at the same pace, it will depend how long ago you undertook your zapping exercise as the natural cleansing action of the other devices will have been trimming back the whiskers continually as they grew whilst the zapped ones will have a lot of catching up to do to reach the same length again. In other words, your test won't start off with a level playing field.

 
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Re: AF117 Tin whiskers.

Post by slidertogrid » Sun Apr 24, 2016 9:50 am

Ah I see, I didn't realise there were some transistors that didn't suffer.
I take your point about my planned experiment, the idea was to find out if a previously original working set would suffer the problems if left un used and if the whisker problem would re occur on a Zapped set of transistors if again left unused for a while.
But as you say it probably won't prove much as the problem is sporadic and depends on other variables as well.
I didn't know what the problem was that caused the failure until I read about it on the internet. For years I just realised they went short to the screen I had no idea why.
In those days I would often just snip the screen wire off most times that would result in a working transistor. However as time passed and certainly by the time I had a problem with the radios in the Rovers I found that more often that wouldn't always provide a cure.
I think the transistors in those radios were the first I tried zapping. Some of the transistors survived this and worked but a couple died I used replacements robbed from a scrap Fidelity radio. They from new had never had the screen wire connected to earth and were fine.
I took a cue from that and replaced them without connecting the screen thinking at the time (wrongly) that leaving the screen wire off would possibly prevent the problem occurring.
But it seems maybe they were just good transistors from an un affected production run?
The radios both work fine with screen wire left disconnected with no signs of instability but that may be due to the fact that they are both built in metal cases and thereby the whole lot is screened?
Rich.

 
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Re: AF117 Tin whiskers.

Post by Terrykc » Thu Apr 28, 2016 11:01 am

It would probably be impossible to do but it could be intuitive if we could identify what proportion of these transistors in surviving sets are not the originals by looking for evidence of re-soldering, cut off screen wires, etc.

Then, of the original devices, checking the country of origin printed on the can, although it may be that the original numbers of Dutch manufactured devices that were sold in this country were too small to produce a reasonable result.

The point being that we all know about the numbers that failed and assume it is a very high proportion of production but nobody gives the working ones a second thought.

 
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Re: AF117 Tin whiskers.

Post by Michael Watterson » Mon May 02, 2016 12:55 pm

As Terry suggests, it's almost certainly a process / manufacturing issue.
It's much rarer with AF114, AF115 than AF117
Not all AF117 have it
It's much rarer with similar can OC170 / OC171.
Nearly non-existent with earlier metal top hat UK & USA.
Not on Russian Germanium in metal can.
Very rare or non-existent with AF12x series which is largely just a different package of the AF11x series.

Combination of too much tin plate and the composition of the "gunge" / "gel" as a medium for them to spread in, though they can grow on a surface or even in damp air.

I don't believe use or storage comes into it at all.


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