[sdiy] Soldering copper to aluminum using indium

[cheater cheater]

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.