After a long and involved conversation with colleagues regarding attaching heat pipes using a low temperature solder, such as Indalloy 290 (97In 3Ag; 143 C) or Ind281 (58Bi 42Sn; 139 C) , here are a few ideas that we've come up with to make your decision easier.
You can use either Ind290 (97In 3Ag) or Ind281 (58Bi 42Sn) in your process for bonding the copper heat pipes. But, if you are going to solder to bare copper, we do not recommend the use of an In-containing alloy.
When soldering to hard-to-wet materials like aluminum, you can use the Indalloy Flux #3 to bond directly to an aluminum heat-sink. The very strong Flux #3 would have to be removed after soldering, but would provide the best solder joint.
If you use bare Ni on the surfaces you can use either alloy. More good news, you can also use a less aggressive flux. In this case, consider using a flux such as 4-OA, which is easier to use than the Flux #3, but would still have to be cleaned after soldering.
If you plate a solderable surface over the Ni, such as Ag, Sn, or Au, then you can use an even less aggressive flux than that, such as a No-Clean type, like 5-RMA.
In the end, Indalloy 281 (58Bi 42Sn) was chosen for the bare copper heat pipe.
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