AF114 germanium transistors and related ones like the AF115 through AF117 were quite popular during the 1960s, but they quickly developed a reputation for failure. This is due to what should have made them more reliable, namely the can shielding the germanium transistor inside that is connected with a fourth ‘screen’ pin. This failure mode is demonstrated in a video by [Anthony Francis-Jones] in which he tests a number of new-old-stock AF-series transistors only for them all to test faulty and show clear whisker growth on the can’s exterior.
Naturally, the next step was to cut one of these defective transistors open to see whether the whiskers could be caught in the act. For this a pipe cutter was used on the fairly beefy can, which turned out to rather effective and gave great access to the inside of these 1960s-era components. The insides of the cans were as expected bristling with whiskers.
The AF11x family of transistors are high-frequency PNP transistors that saw frequent use in everything from consumer radios to just about anything else that did RF or audio. It’s worth noting that the material of the can is likely to be zinc and not tin, so these would be zinc whiskers. Many metals like to grow such whiskers, including lead, so the end effect is often a thin conductive strand bridging things that shouldn’t be. Apparently the can itself wasn’t the only source of these whiskers, which adds to the fun.
In the rest of the video [Anthony] shows off the fascinating construction of these germanium transistors, as well as potential repairs to remove the whisker-induced shorts through melting them. This is done by jolting them with a fairly high current from a capacitor. The good news is that this made the component tester see the AF114 as a transistor again, except as a rather confused NPN one. Clearly this isn’t an easy fix, and it would be temporary at best anyway, as the whiskers will never stop growing.
Hi, reminds me of “The (solved) mystery of OC171”.
https://www.elektormagazine.de/magazine/elektor-200807/3073
The mean thing about the slowly growing “hairs” was that the transistor damage wasn’t always being obvious.
The transistor would still work in the transistor tester and in AF and ordinary DC applications, but would fail in RF applications.
A workaround used to “shock” the metal chassis electrically, which burnt the hairs.
However, the hairs would eventually grow back..
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Ah, I see you’re a man of culture as well.
Why does this read as if it was generated by ChatGPT?
With unlimited labor available, I’d be inclined to cut open the can and remove the silicone. (was it really a goo, or was it hard ?) The whiskers are on the surface, so rather than risking damage to the transistor with electrical discharge, I’d remove them mechanically. Pot the transistor with semiconductor-grade epoxy, put the can back together, smear some epoxy around where the leads enter the case, and let the epoxy cure. My guess is that tin whiskers won’t grow through the epoxy, or if they do, they will grow very slowly.
That sounnds awfully hard work and rather expensive to get ‘semiconductor grade epoxy’.
You’d be better off coating the metal producing the whiskers, the usual way is to alloy tin with lead and it does a really good job of preventing whisker growth.
Even more conveniently there’s a material with a relatively low melting point that could be used by hobbyists with a soldering iron to coat other metals…
Couldn’t you be even lazier, open the can, remove the goo, brush gently with isopropanol, and leave it like this? Why wouldn’t it work as long as you don’t stick your fingers in? Or maybe as an extra step cover with a drop of hot glue. Would the transistor not work without the can due to interferences?
Sounds reasonable if it’s kept in the dark. Germanium junctions, like silicon junctions, respond to light.
This has been a problem for several decades now, one of my jobs as a trainee in the late 80s/early 90s was to repair old portable and cabinet radios that were fitted with valves (tubes) or PNP Germanium transistors.
Fault reports of ‘crackling’ and ‘drains batteries’ were common problems caused by tin whiskers inside the cans of the AF and other metal cased transistors, the first task was to power the radio from a bench supply, check the current draw and then just listen to the radio, if it crackled without any of the potentiometers being adjusted it was usually tin whiskers in transistors or, in some cases, on the tuning capacitor, ditto for high current draw which could indicate a shorted transistor.
I don’t remember the earlier, more expensive to produce glass encapsulated transistors suffering the same problems which I guess just points to enshittification having been a problem for a lot longer than people might expect.
I wondered about more inert / less reactive coatings than metals. Would silicone work? Or acrylic. I’m not an electronics person, but found this fascinating!
Hard silicone plastic was used (is it still?) to encapsulate some ICs in the 1970s. You could visually distinguish silicone from epoxy because the silicone was glossy and the epoxy matte.
I may be mistaken but i’ve heard that the UK had a somewhat similar problem with their AGR series of reactors where Carbon whiskers were forming, it was something to do with a Nickel impurity causing the deposition of Carbon and the Nickel then migrating to the top.