I just went through the same problem. We have an inductor that is made with a bundle of 30-strands of Litz wire that is vaccuum coated with varnish, and have the ends of the wire bundles tinned that we have to solder to SMT pads on the board. The inductor that we got from the company that made it didn't prep the end of the wire bundles very well. After a post to the IPC Technet about my problem, I got some advice from a Indium engineer who told me that I should use a low tin content solder to prep and tin the Litz wire at a high enough temperature to burn the insulation off, yet not disolve the copper of the wire. He recommended a 70Pb/30Sn solder. I bought 10-lbs of it and filled a tinning pot up and it tins the Litz wire beautifully...
-Steve Gregory-
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