Tin "whiskers" growing inside potentiometer...

[nuke]

Here's one I hadn't seen before. Working a '67 Super Reverb and the Vibrato channel just didn't sound "right", thin, lacking gain. One of those things if a guitarist says and the tech kinda wants to roll his eyes a little.

But indeed, it was subtle, but true and the Normal channel had more gain, and deeper, richer tone.

So went to trouble shooting and eliminating things and I found that reading ohms around points in the tone stack and volume control were very different between the two channels, with the vibrato channel reading much lower to ground. The weird one was disconnecting the middle pot, which is the only path to ground in the tone stack, and I still had low readings. After disconnecting a few more things, and lifting the 0.1uf cap on the board, I found that the treble control pot was reading about 9k to ground on the bottom leg, with nothing connected to it. Hmm. That means it was the pot itself.

I popped it open and looked at it under magnification. What I saw was the formation of tin crystals from the plating of the back cover of the pot. These are also called "tin whiskers". They form super fine and long crystals of tin over time and look like baby hairs. Some of these had reached the edge of the carbon track near the low-leg of the pot.

Attached is a photo of the inside of the Stackpole brand potentiometer cover. I removed it to see what was causing the short. I've seen banged and dented pots short to the resistance track, but this is the first time I've seen this on the plating of an amp pot.

Very fine little wisps of hair-like crystals.

IMG_1715.jpeg

Link to a NASA paper on the phenomenon.


https://nepp.nasa.gov/WHISKER/background/index.htm

[Stevem]

That’s crazy!

Thanks for putting the time into tracking this down and to sharing it also because my own 1966 Revetb Rocket might be suffering from this same issue .

[R.G.]

Dang! Tin whiskers are a big concern in SMD parts because the mandate of lead-free made the IC makers go to pure tin plating on leads. But this is the first time I've ever heard of it being a problem in something as big as a pot.
Wow! That's a new one on me.

[pdf64]

Tin whiskers were a big problem in the UK rail network, they'd grow between relay contacts etc when they're in trackside signalling equipment cases in cold damp locations.
As I recall, an eventual mitigation strategy of heating the trackside cases, and a periodic removal and close inspection (dismantling) schedule of every relay (!) was eventually settled on.
An absolutely massive, onerous workload!

I recall a few (several) reports over the years (decades!) on various amp forums of pot issues caused by tin whiskers. Includes 60s to more recent (RI) Fenders.
Here's an example https://www.thegearpage.net/board/index ... t-27846002

[nuke]

Yeah, I was working in the computer industry when we transitioned over to fully lead free solder and all the ROHS stuff. We had some experience with whisker growth, but were on the lookout for it and it was a known problem by then.

This particular Super Reverb was factory built with a mish-mash of pots from different vendors. All but the middle control are factory original. But the volume pots are from different vendors, one has a plastic shaft, the other has a metal shaft. About half the pots are Stackpole, some are CTS and maybe one or two might even be something else.

I got to looking at the Vibrato channel tone stack closely, since the middle tone pot had been changed to an Alpha by some prior tech.

The Stackpole pots are showing crystalline growth on the outside. They were probably plated with pure tin to make them easily solderable. They look at first glance like "white rust" which is usually benign. But it is kind of sparkly looking instead of snowy looking.

If you encounter this, no amount of spraying will remove it. You'll either need to replace the pot or disassemble it to physically remove the whiskers. I opted to open it up, partly to see what was up and partly because I don't have another 250K 2-35 taper pot around to replace it with.

Not sure what can be done to stop it happening again. I think maybe painting the inside of the pot cover with Glyptol, or conformal coating like the NASA article says. Tin wants to crystalize, and I think it will just keep doing it. Hopefully, it will take another 60 years for it to become an issue again.

[pdf64]

Caig claim that DeoxIT prevents dendrite https://acct113328.app.netsuite.com/cor ... =overwrite

[R.G.]

Good idea - if it's real. Worth a sphlotz/sphritz to try it.

[nuke]

NASA says they ain't dendrites.

According to their research, only a relatively thick Uralane conformal coating over susceptible pure tin plating will buckle the tin crystals per Euler's buckling formula.

https://nepp.nasa.gov/whisker//referenc ... sld005.htm

• What Can Prevent Tin Whisker From Being a Problem?
- Do not use electroplated pure tin
- Reflow tin and quench in a hot oil bath (according to the literature)
- Mix with at least 2% of Pb (according to the literature) - Cover with conformal coating (frequently proposed but rarely
demonstrated)

(interesting slide deck!)

Besides, there was plenty of DeOxit or some such in all these pots, and all over the surrounding chassis for that matter.

It's not oxidation, it's literally that the element tin, really, really, really, really wants to grow into crystalline form. It will force its way through thin conformal coatings.

Crazy, ain't it?

The cure is not to accept parts with pure tin plating. Especially not in aerospace.

[soundmasterg]

That is crazy! But I guess we shouldn't be surprised....lots of tin plating with no lead added makes for lots of tin whiskers in modern electronics. 60 years is quite a good life before the problem manifested. Here's a NASA webpage that documents satellite and other failures due to tin whiskers. There was a nuclear power plant that would have had a meltdown due to tin whiskers on a primary control board if a secondary control board hadn't kicked in when the primary one failed. Pretty scary!

https://nepp.nasa.gov/whisker/failures/

I was working at Rodgers Instruments/Roland when they were doing the ROHS conversion of all their new products. They had to maintain a separate line because their warranty was 50 years and had to sometimes make parts for their old organs, and those would use leaded solder and parts. The new line had to be lead free. They took a look at the issues and changed their warranty to 15 years and then later 10 years with the lead free parts because they didn't think they would last much longer without issue. Can you imagine a church spending $60k for an organ and only getting 10 years before they might have to replace their organ? I'm glad the US still allows leaded solder because why should I punish myself with trying to make everything lead free? Of course if I was going to make a product and sell it worldwide it would have to be leadfree. I remember reading about some efforts to go lead free with solder in the 1920's-1930's, and they eventually gave up. Sometimes I think they were right to do so!

Greg

[GAStan]

• What Can Prevent Tin Whisker From Being a Problem?
-Mix with at least 2% of Pb (according to the literature)

It seems re-tinning with leaded solder would work. Not something one would want to do everywhere, but could be worth the effort to save an original component.

[nuke]

• What Can Prevent Tin Whisker From Being a Problem?
-Mix with at least 2% of Pb (according to the literature)

It seems re-tinning with leaded solder would work. Not something one would want to do everywhere, but could be worth the effort to save an original component.

I think it would be difficult to tin the inside of the potentiometer cover with solder. I thought about maybe spreading 63/37 paste and using hot air. I think it might run all over the place though and render the cover unusable.

The other thought I had was perhaps to bake the cover hot enough to remelt the tin electroplating and quench it.

In the end, I scrubbed it clean, wiped a trace of Deoxit 100 on it with a swab and put it back together.