Here's a very explicit description of what I did with such a feedthrough.
You need:
-low-temperature ("soft") silver solder, 3-4% Ag in tin
-aggressive acid-based stainless steel flux (e.g. LaCo N-3)
-2.75" 304SS ConFlat flange, with 1.625" bore (largest sold by Lesker)
-lathe. A hacksaw would work, at the expense of time and cleanliness
-very small gas torch
The skirt on the feedthough is apparently stainless steel that has been plated, along with the flange, with some other metal. Chuck up the feedthough by its flange and slowly, gently cut off the flange with a parting-off tool, leaving 5 mm of the skirt behind. Be ready to catch the liberated feedthrough. The remaining skirt should fit somewhat loosely into the back of the 2.75" CF. Use sandpaper to roughen the outside of the skirt piece and the inside of the CF; heat with a small torch and tin the pieces. Tinning seems easiest if the part is turned while heating and applying flux and solder--let the solder flow by gravity around the surface. File or sand if necessary to make them fit together properly again. Mount the feedthrough "upside-down" by its terminal in a vise and put on the CF (knife edge up). Heat the flange by its underside while applying flux on the underside and inside of the joint with a syringe w / very fine needle to protect the solder from oxidation and help it flow. When the solder is hot and fully molten in a smooth bright bead around the entirety of the joint, it's done. The metal will look very ugly on account of the flux residues. Gently cool the part SLOWLY by spraying with a water mister bottle. When the water starts to stay on the surface without immediately boiling, transfer the hot part to a pot of boiling water and get rid of as much of the flux residue as is feasible. A toothbrush or Scotch-Brite pad helps. That's it. Not hard to do. A hard silver braze such as Stay-Silv 15 would entail the same steps, but you'd use the fluoride-based paste flux and necessarily an enormous amount of heat to get the flange hot enough. A somewhat tighter fit between skirt and CF would be advisable as well. I did not go this route because I was concerned about overheating the ceramic-to-metal joints on the feedthrough.
(Caution when using acid-based flux. This is made from concentrated HCl and zinc chloride. It will soften your fingernails and flat-out eat your clothes, and it will certainly splatter and boil when being added to a hot part. Wear goggles by all means. Don't do this job near anything metallic that you value, as the vapors will eat it alive. Such jobs are best done outside. Wash hands and all other tools involved with the job very thoroughly. This stuff initiates corrosion like nothing else on earth with perhaps the exception of aqua regia, so take cleanup seriously!)
-Carl
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2006-05-24 17:43 (Non-vacuum?) BeO feedthrough 
(Greg Courville) [Latest: 2006-07-02 16:46]
2006-07-02 12:34 Re: (Non-vacuum?) BeO feedthrough 
(Carl Willis)