[sdiy] Soldering copper to aluminum using indium

cheater cheater cheater00social at gmail.com
Wed Aug 18 09:10:57 CEST 2021

Just one question - sorry for the flood of replies - why didn't you
pre-tin the surfaces first and then join them like I outline in my
previous email? I assume that is because the indium wouldn't bond to
the glass/crystal anyways, and you just wanted a mechanical fit rather
than a metallurgic joint, would that be correct?

On Wed, Aug 18, 2021 at 8:41 AM cheater cheater
<cheater00social at gmail.com> wrote:
> Not a rambling reply at all and very helpful, thanks. I was planning
> on using indium wire or even bar (honestly whatever I can get the
> cheapest), pre-tinning the mating surfaces with a thin layer, and then
> putting them together and heating up the copper - which is a thin and
> small surface. That should heat the indium enough to become fluid, and
> that would take the indium on the heatsink with it, and a bond would
> be formed. Any extra indium would have to be squished out. The
> heatsink only has a finite capacity of removing heat and getting it
> hot locally is possible even with a single good soldering iron.
> I might cut the required 5cm strips down to shorter bits, not sure
> yet. A 5cm strip will service 4 chips lined up in a row which have no
> spaces between them.
> On Wed, Aug 18, 2021 at 1:28 AM peter foti <11hertz at gmail.com> wrote:
> >
> > I’ve done quite a bit of Indium soldering at work over the years. I think using a hot plate would be a big help to you, if you can. What form is your Indium? If it’s a sheet, I would recommend assembling everything and letting it come up to temperature on the hot plate (start cold). First, wipe both sides of the Indium with a lint free paper towel, moistened with alcohol or methanol, several times to remove the oxide layer. The Indium will get really shiny. I’ve had good luck using plumbers flux. It’s very aggressive, but it worked better than the fluxes from Indium Corp.
> >
> > When the Indium melts, things might shift around, so you may have to nudge them back into place, or use something to apply force to your assembly without increasing its thermal mass too much.
> >
> > We usually did Indium reflows in an Argon or Nitrogen environment, but I’ve also been successful doing open air reflows on a hot plate. The copper we were using had been passivated and gold coated, a luxury you won’t have at home, so that’s were the plumbers flux will be a big help. The flux will liquify and get everywhere, so plan on cleaning it up after. We used lab grade kerosine, followed by acetone and then IPA to clean the assemblies. We were soldering laser crystals to copper heatsinks that were going to operating in vacuum, so we were pretty intolerant of any residue. You may be able to use common flux cleaners.
> >
> > Sorry for the rambling reply. Feel free to ask any questions if anything needs clarification.
> >
> > On Tue, Aug 17, 2021 at 10:06 AM cheater cheater via Synth-diy <synth-diy at synth-diy.org> wrote:
> >>
> >> Hi all,
> >> I am modifying a cooler and I was wondering if anyone had experience
> >> soldering copper to aluminum using pure indium or indium-silver solder
> >> In97Ag3.
> >>
> >> I have been able to deposit a good bond of solder to the heatsink in
> >> question using normal Sn60Pb39Cu1 (the Cu is for not dissolving solder
> >> tip plating) using AmTech NC-599-v2-TF flux.
> >>
> >> However, 60/40 will only have a thermal conductivity of 50 W/mK as
> >> opposed to 73 W/mK of the Indium-Silver solder or 83 W/mK of pure
> >> indium, plus indium and the indium solder melt at 156C and 143C
> >> respectively as opposed to 183-190C for normal 60/40. This is
> >> important because the heatsink itself has its fins soldered to the
> >> base using a solder which I think is at around 200-220C melting point.
> >>
> >> However, I am not aware of the properties of indium when soldering,
> >> and I am curious if anyone has any experience, or can suggest any good
> >> fluxes.
> >>
> >> I want to solder copper sheet metal (roughly 40x20x1.5mm) to a part
> >> copper, part aluminum heatsink base (the border between those two
> >> happens where I have to put down the sheet metal).
> >>
> >> I am hoping for a sturdy bond that will withstand some pressure and
> >> which will transfer heat well.
> >>
> >> BTW, I know this will require a lot of power from the soldering iron,
> >> I have six very powerful soldering irons available, and if need be I
> >> can use them all at once.
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