I'm hoping that someone may have some experience with soldering the heat sink of a MOSFET directly to a sheet of aluminium in an oven (either infrared or vapour phase)?
Can it be done? Is it reliable? What to watch out for? What flux to use? What solder paste to use?
To echo what others have said...
No. Not even no, Hell No... I'm wanting to solder TO-220 / TO-3P / TO-247 through hole MOSFETs onto a 2 mm (or 3mm) thick aluminium plate. The MOSFETs would need to be clamped in place before soldering & the legs would hang over one edge of the plate so they can be soldered onto a regular PCB adjacent to the plate.
0. Those aren't SMT packages, for a reason. (There are TO-263 and TO-268's for that!) Many of those come with tin plating which is just fine and solderable, but many also come with nickel plating that's a nightmare to bother with. I've SMT'd TO-220's here and there for little projects, I know.
1. Use copper. Copper is solderable, also more conductive so you need less of it, perhaps even to the extent that you need one or two fewer transistors.
2. Use a PCB. Heavy copper PCBs are available most anywhere. There's even the kind with machined (I guess) inner "layers" of considerable thickness. Though you may end up paying as much for the PCBs as for trying to solder aluminum...
3. Use more transistors. You're looking at five or ten bucks of transistors, versus sky's-the-limit inert gas, breathe-the-fumes-and-you-die soldering. Transistors are
fucking cheap. I haven't seen a single design where the transistors were anywhere near a significant fraction of the total product cost. The labor of inserting them and screwing them down easily overwhelms their cost (except maybe in China, but then, locally sourced, classic type, transistors are quite cheap, too).
They're not vacuum tubes. Pile them on!
3a. While you're at it, give up on all that annoying mounting hardware! Spring clips are fantastic: they don't bend the device, they spread the load evenly, and you can use much bigger devices (MAX247 and such) that aren't hampered by pesky trivia like
screw holes.
There are even heatsink-and-clip duos that are cam-lock or clip-in-place. No screwdriver at all!
We require 4 MOSFETS to be soldered to the plate, so there would be 12 component legs to give some strength between the boards. Both boards would be mounted with 4 screws into our plastic enclosure.
Plastic?
So, no heatsink?
God man, just stretch the PCB a bit and use D2PAKs on 2oz copper! You won't notice a difference!
Tim