That was a crazy idea, but finally works.
The patient was an old xm-l T5, which was on a poor alu star. The brightness sag was quite big at 3A and even bigger at 4A. The brightness increase stopped about at 3.8-4A.
so the method:
I desoldered the emitter, and scraped out the dielectric layer on the star.
Yes, aluminium cannot be soldered, but what about a tight metal to metal contact?
Because the alu is unsolderable, I had to presolder the emitter directly. See the picture (this is too much tin on the emitter) :
I added a little more tin to the electrical contact point on the star, and finally re-soldered the emitter to the star.
before re-soldering:
There was a little trick, because I had to push the emitter to the star until the tin solidified.
And the result: much lower sag at the same current, at 4A the sag was about 70-80lumen with the original star within 1-2sec, and only 20-30 with the modified led. Brightness increases up to 5.5-6A instead of 3.8A.
At least 100-150lumen difference at 4A.
I have not exact datas yet, but as I can estimate, cooling efficiency of this method is between the copper star and the alu star+dielectric layer.
Thanks for posting. I suspect the aluminum SinkPad boards will deliver similar performance. I think Relic has some free samples headed his way for comparative testing.
My aluminum SinkPads are here. I think these will perform OK, not as good as a copper one, but OK. The center pad has been plated to accept solder, so there is a direct bond of sorts. I’ll get these fired up on the weekend, maybe.
I didn’t expect to get MT-G2 ones though. And notice the familiar looking XM-L 16mm copper one? (XP-G one is hiding).
Its actually possible to solder to aluminium. You just have to destroy the aluminium oxide layer without letting the bare aluminium get in contact with air again. So it should work if you put a drop of oil on, scrape the stuff off and then solder. I made it work with an XRE PCB, but it looked like complete crap afterwards. Its the ugliest way.. better drill through. But I wouldnt bother with drilling 2 holes like comfychair but just drill one with a diameter of 2.6mm in the center, thats where the heat comes from. ;)
