Potting Vacuum Feed-Throughs

Since we’ve used this procedure on non-ITAR detectors, I feel fine sharing it:

A vacuum feed-through consists of three parts:

First is the thing you’re feeding through the vessel wall. In most cases this is an electrical signal. In others it’s optical. I’ve built feed-throughs for both. I’ve only built one optical feed-through, which contained multiple fiber optics. We decided not to connectorize them at the feed-through. With electrical feed-throughs, it’s typical to have a connector on one or both sides of the feed-through, though this isn’t always the case. If a connector is used, the easiest way to do this is by using a hermetic vacuum-rated connector. Most of the time these contain the provision for o-rings, so this may be the extent of the feed through since it supplies the other two components I’m about to go into.

The second part of a feed-through is some sort of mechanical shell that will actually connect to the vacuum vessel. In the case of the fiber optic feed-through, this was a custom machined aluminum part. In the case of a hermetic connector, this can be the connector itself. In the case of a non-hermetic connector, or a connector that doesn’t readily mount to the outside of the vacuum vessel, it’s typical to make a custom machined part as well.

The third part of a feed-through is to have some means of making a gas-tight seal between all the bits. In the case of a hermetic connector, that’s the o-ring. In the case of a custom feed-through, typically it’ll involve an o-ring in a groove that will bear on a flat surface on the cryovessel, or vice-versa. It will also involve some means of sealing the wires or fiber optics that pass through it.

The easiest way to accomplish that last part is to use some sort of resin. This won’t work with extremely high vacuum situations, but it’s good for most instrumentation purposes. The resin we use is Stycast 2850FT, which is black, optically opaque, and a good electrical insulator. We use a Stycast 24LV catalyst, mixed at 7% to the 2850FT. 24LV has good curing characteristics, and is a low-volatile catalyst, meaning it won’t continue to ooze organic vapors into your vacuum system once the potting is complete.

Stycast is neat stuff to work with, but in order for it to make a good potting resin there are some procedural details that need to be followed pretty closely:

• Surface prep is king. Let me repeat that: Surface prep is KING! We degrease everything, and give every part a final alcohol clean just prior to assembly. Dirty parts make crappy feed-throughs.
• We store our Stycast in a dorm fridge. This gives it a longer shelf life, but it also makes it hard as tar. Stycast needs to be heated to 60C in order for it to mix with the catalyst.
• Once the Stycast is heated and mixed with 7% 24LV catalyst, it must be mixed for five full minutes in order to ensure a homogeneous solution. The pot life on this stuff is close to an hour, so you have the time. Take it.
• After mixing the solution must be degassed. This is a vacuum application, after all, and virtual leaks are the bane of a vacuum system. We use a small dessicator hooked up to a two-stage roughing pump. The Stycast will foam like nuts when you pump on it, so you need to keep a steady hand on both the vacuum and the bleed valves. The trick is to foam it up and slump it a couple of times to get the bubbles on the top, then foam it up and hold it there until the bulk of the bubbles pop and it slumps on its own while under vacuum. At this point you’re clean. This takes roughly another five minutes.
• There are tricks you can do with pouring Stycast to minimize trapped gas as well, but I won’t go into those. Look for a good manual on casting plastic parts using resin.
• If the geometry of the feed-through cavity is complex, it’s best to fill the small voids first using a syringe loaded with the warm Stycast. This avoids trapped volumes of air.
• Finally, fill the cavity until you’re satisfied with the level inside.
• Pop it back in the oven, and heat to 60C for at least an hour.

I like to make two witness samples, one that goes inside the oven and one that stays outside. These are just dabs of Stycast on something flat like a chunk of metal or a razor blade. I also toss the remaining pot of epoxy into the oven along with the feed-through. Here’s my rationale: Stycast takes 24 hours to cure at room temp. It takes an hour at 60C. If the inside witness sample is rock hard, things are starting to kick in the larger volumes of the pot and the feed-through. You’re getting close! If the outside witness sample is rock hard, the stuff in the oven is rock hard as well. Don’t rush the process, if you can help it. A feed-through should last essentially forever. Rushing it and flexing semi-hardened resin so that you create a trapped volume or worse yet create an air path through the thing ruins a lot of hard work. Be patient!

That’s it. I’ve used this procedure to make a whole slew of feed-throughs, both on science instruments we use on a daily basis as well as lab instruments we use for our own testing. If you’re careful, you’re clean, and you don’t rush things, you’ll have a good solid feed-through that’ll pass signals in and out of vacuum for years to come.

– Tom

P.S. Sorry, no pictures. As soon as I pot one that isn’t for an ITAR controlled device, I’ll take pictures and post them.