I like to use sil-fos if thermal conductivity is a criterion. It is inexpensive (the low silver content ones) and flows well without flux on copper. I use it to make custom interior inserts to use as backers for TIG welding. They will not melt, and hold the shape of internal features really well.
I expect I will be trying out all of these on some practice runs.
What is tricky here is about
electrical conductivity, for which we need the best we can get.
Unfortunately, most good electrical conductors are also good heat conductors, which is what makes these operations difficult.
Sil-Fos, AKA "Easy-Flo" is interesting. It is a mix of copper and silver and phosphorus, patented in 1931, but the patent thrown open to all for wartime production. Joints brazed with SilFos can be stronger than the parent materials. While it was directly used in joints for airplanes, ships, etc. the biggest use was in fix-ups repairs. Busted gears, shafts, struts, almost anything could be put back to working state.
The way it works without flux, is the phosphorus reacts with the copper oxides to form phosphorus pentoxide which strips the surface clean. Of course, this effect only works on copper. There are several Sil-Fos mixes with a number ranging from 2 to about 15, this meaning the percentage of silver. One has to carefully select which, depending on the application. e.g Sil-Fos 15 has the lowest percentage of phosphorus at 5%, making it the most ductile, and much liked for refrigeration and air conditioning work.
Sil-Fos 18 has the highest silver content, so has the lowest melting point. I can bet that, like all stuff containing silver, it will be expensive. Small quantity use may well be best sought from jewelry suppliers.
I have to thank everybody for the sheer amount of good information.
