hey BLF
i have this aluminum pill that is hollow. i made a copper disc out of a coin. but it does not fit perfectly. i wanna will the voids with some solder. but i cant get the solder to flow on the aluminum.
i use 60/40 solder with rosin core. but all the rosin goes onto the aluminum. i placed the solder iron until the solder melts on both surfaces.
do you you guys have any tips on how i can do this?
grtz
DenTilloZie
JB weld maybe? 
It doesnt work, there is a product specifically for soldering aluminum but that wont stick to copper.
Your best bet it so remake the copper disk, make it just slightly oversized and press fit it in or do something like I did on one of my many SK68’s-
Drill tiny hole(s) around the rim of where the copper will contact, fill the holes with copper wire and expand that wire by hammering it with a punch so it fits tight (again sorry this is all one step)
you can do this at several points around the diameter and then solder to the individual points
This is how I made a good ground and also made it so I could solder the driver in (solder connection is made on bottom of PCB)
thanks!! ill try it this way.
There are a couple of products out there, many DO solder alu and copper.
I have some here, and it works:
http://www.ebay.com/itm/280651340670?\_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT
I didn’t try much yet with it, but just soldered a piece of copper wire to a piece of alu sheet, with a soldering iron (80 Watts).
Because with a flame you get much oxidation, which you do not need at all, i figured.
It just ate away some ferric coating of the soldering tip though… 
Must be the flux core… has to eat through alu oxide, which apparently is tough stuff…
Anyway, i got it in last week, and reading what you just wrote, i thought: Hey, that’s not what i bought, is it…?
I think it solders many metals, bought it for alu on alu and alu on copper / brass.
let us know how it works, I have a light that would really benefit from adding on some copper fins to the Al head.
For now i can say it’s a bit messy and doesn’t flow as nice as regular solder on regular metals.
It smokes nasty too…
If you are simply putting a copper disc inside a pill, just use AA or some other thermal glue to hold it in. Soldering aluminum is a real pain in the arse.
The problem is that the aluminum oxidizes super fast.
The stuff i got lets you pre-solder the parts though, so you can put together the parts just by heating and pressing.
The ebay stuff sounds like “Lumiweld” or “Technoweld” or “Uniweld” — all similar as far as I know— key to using all such solders is using a stainless steel brush or scraper right through the puddle of melted solder, to break through the oxide layer on the aluminum. Once the elemental aluminum is exposed (to the solder) it can stick; the melted solder keeps the oxygen away (otherwise the aluminum oxidizes as fast as you scrape it).
Aluminum oxide is also called corundum - extremely hard stuff.
maybe if you mig weld that pill. Be curious how the alum-copper solder and clean up goes…