Silver Solder for HV Insulator

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Luke Harrill
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Silver Solder for HV Insulator

Post by Luke Harrill » Tue Jan 08, 2019 10:44 pm

A little while back, I purchased a HV insulator from Richard. I now have a 6" CF flange with a hole to seat the silver-plated brass skirt. I went to the local welding store and found a "STAY-BRITE Silver Solder Kit" which included some flux and 3.5/96.5 Ag to Sn solder. I read the FAQ about repurposing a HV insulator, but here is my dilemma: this solder has a melting point of 430F! It is nice in the respect that I will not be killing the ceramic to metal joints with heat, but will that be able to handle the temperature of a fusor in operation?

-Luke Harrill

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Dennis P Brown
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Re: Silver Solder for HV Insulator

Post by Dennis P Brown » Tue Jan 08, 2019 11:59 pm

The ceramic/solder/flange section should never get to 300 F much less that M.P. Mine was a ceramic/epoxy(!)/glass seal (my flange was a glass blank), and though I had to water cool my fusor main body, that HV ceramic/epoxy section never got hot enough to even harm the epoxy. I did (later) add a small fan and radiator fins on the ceramic section but the item never got hot enough to burn the epoxy.

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Richard Hull
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Re: Silver Solder for HV Insulator

Post by Richard Hull » Wed Jan 09, 2019 3:37 am

Most manufactured glazed ceramic to metal seals use a sodium silicate "fired" seal and this can easily handle that soldering temperature. No problem.

The BeO skirted insulator on my fusor was honest to God silver soldered at 900 + deg F and the skirt in contact with the seal glowed a very dull red as the "easy flo" jewelers silver solder flowed into the machined well of the 2.75 conflat which was at full red heat!! That particular seal on that BeO insulator held just fine at that temperature. It remained vacuum tight.

Just remember, soldering, much more than in welding, requires plus ultra preparation of the surfaces to be joined. I would pre-"tin" all mating surfaces this will help warrant a full and completely sealed joint.


Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Rex Allers
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Re: Silver Solder for HV Insulator

Post by Rex Allers » Wed Jan 09, 2019 6:42 am

Richard, I'm curious, what method did you use when you did the silver soldering? Torch, or something else?
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Luke Harrill
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Re: Silver Solder for HV Insulator

Post by Luke Harrill » Wed Jan 09, 2019 4:48 pm

I will certainly pre-tin the surface, but do you also agree that I will be able to safely solder the stalk into the flange with this low-temp stuff? My fear is that the fusor will overheat the joints during operation. I wouldn't want to lose vacuum and kill my turbo or xrt.

Also, Rex, here is the FAQ that detailed his procedure:
viewtopic.php?f=29&t=11513

-Luke Harrill
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Last edited by Luke Harrill on Thu Jan 10, 2019 4:02 am, edited 1 time in total.

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Richard Hull
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Re: Silver Solder for HV Insulator

Post by Richard Hull » Wed Jan 09, 2019 8:51 pm

That insulator is an NOS, never installed 70kv insulator to be placed on a steel capacitor case in manufacture. It was designed from the start to be low temperature 60/40 or even 50:50 (wiping metal) soldered to the steel case. Thus it is supposed to survive standard tin/lead soldering temperatures. I think the skirt is pre-tinned brass. Use a magnet to check this.

I would see if the solder you have will truly pre-tin the SS Conflat first! Make sure it adheres. If it will, then you are probably OK. As for the skirt on the insulator flange, lightly sand the bottom and clean it with alcohol. Apply liquid rosin flux and then tin it. The solder should race all over the prepared area.

Yes, due to the super high temperature of true silver soldering (60% silver) I was absolutely forced to use my oxy-acetylene torch on the BeO insulator on fusor IV. You will almost certainly have to use a simple propane torch to heat that big CF flange to the required tinning and soldering temperature.

The rest is up to you. If seemingly successful do not use a running tubo to test it for vacuum tightness!!! Just use only your forepump through the not running turbo to check this out. If good enough to hold a roughing vacuum (5-25 microns), it will certainly be good for the turbo.

Finally, you must orient you insulator absolutely vertical. It cannot or should never be horizontal or upside down in you final running position.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Luke Harrill
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Re: Silver Solder for HV Insulator

Post by Luke Harrill » Fri Jan 11, 2019 3:56 pm

Well, impatience got to me last night when I decided to tin the SS flange. I began by placing it on two fire bricks and heating it with a butane pencil torch. That was taking forever, so I stepped it up to a large, rose-tipped propane torch. After it got pretty hot, I lightly fluxed a ring around the flange. When I applied the solder, it didn't really spread out as much as I hoped. But when I added more flux, it would run pretty well wherever I applied it.

I got a nice bead pooled up around the hole and then I realized I was out of solder. Oops. It was now or never, so I heated the skirt and generously fluxed it. I sat it down on top and pressed it just a little. After it cooled, I noticed that a lot of solder ran into the floor when I sat it down. I hope I got a full seal around it, but I won't know for sure untill I put a stalk in and draw a vacuum.

I tried to clean it with acetone, but here are a few photos.
Note: acetone works better than alcohol to find cuts on your hands.

-Luke Harrill
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Dennis P Brown
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Re: Silver Solder for HV Insulator

Post by Dennis P Brown » Fri Jan 11, 2019 5:39 pm

Seriously, if you are worried about the ceramic insulator heating up enough to damage/compromise your joint, do as I did. Install a small inexpensive fan to keep a constant air flow. The shape of the insulator is ideal for that type of cooling method.

Luke Harrill
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Re: Silver Solder for HV Insulator

Post by Luke Harrill » Fri Jan 11, 2019 5:58 pm

I think I will definitely implement that idea, Dennis! Thank you so much for your input!

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Richard Hull
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Re: Silver Solder for HV Insulator

Post by Richard Hull » Fri Jan 11, 2019 9:04 pm

From what I see, it looks as if you did a fine job. I would not worry about the blobs inside as long as you dissolve and clean up all the internal flux that might have got inside and acetone and methyl ethyl ketone should take care of that but do that on a table outside, never indoors and wear gloves resistant to both solvents.

There is never too much cleaning of joints in soldering and after that never too much fluxing. All warrant ready adhesion and flow of solder. all such joints in vacuum demand tin/silver solder One of the best is 96.5tin:3.0silver:0.5copper. Never use antimony or cadmium bearing solders in vacuum service. Lead is OK, but not desirable for any number of reasons. (high heat and health to name a few.)

The best solder for most anything vacuum related in 50:50 tin-indium, but all components should remain cold at room temperature and never exceed 125 deg F, ideally. You can even solder glass to SS with this solder that is vacuum tight.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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