I have an 20mm XM-L2 T5 5B1 emitter from FastTech (I've gone through quite a few of these by the way) which has the "+" contacts shorted to the back of the mpcb, which in turn is shorting to my heatsink it is mounted to (this is for my lawnmower, not a flashlight). Has anyone run across this? My 2nd emitter is not shorted at all. I'm thinking this won't be too big of a deal so long as I am careful to keep my heatsink isolated from my mower's metal hood I am mounting too (this is currently the case because I found in the past I had inadvertently connected the two XM-L's together by way of contact to the hood).
Hmm . . . I don't think so, but I can check. I do remember the top layer of the pcb was peeling up in some locations, so possibly it was torn up near that joint and my solder caught hold.
I've seen it happen before with a direct thermal path MCPCB, where LED+ was bridged to the center thermal pad during reflow. I would try and reflow it on your stove and try again. I know you can open up a ticket with Fasttech, but who wants to wait another week for parts?
Now that RMM mentioned it, I recall having the same problem in the past but it was my fault. Before I started using solder paste, I would sometimes get too much solder on a pad and it would bridge over to the thermal pad. My normal routine for checking for shorts would catch it and I would just pull the emitter and old solder and reflow to fix. Funny how you can do things automatically without conscientiously thinking about it.
Yes it also has happened to me. Is an absolute PITA if you ask me. I've especially had issues with the copper DTP sinkpads. Those silkscreens are sometimes off. Less so with the noctigons. But a pita none the less.
All you need is a stray filament to solder itself to the edge.
Every time I solder a wire, I touch one probe of my DMM to the pad, and the other to the edge of the star. It’s saved me a lot of grief.
I unsoldered the "+" lead and removed the screws and it's still shorted, so I guess I'll try my hand at reflowing. I watched Old Lumens video and I think I'll practice with some junk LEDs first. Probably not as hard as it looks.
Reflow the emitter off the board, use a DMM on diode check, touch the pads, if it beeps…then the pad is shorted
When I reflow with my hot air gun from the bottom…and the solder melts, I gently nudge the emitter, the surface tension of the liquid solder pulls the emitter back on the pad, then I give it a light tap on the dome to seat it (sometimes the solder squishes out in little balls, then I quickly take a paper towel soaked in rubbing alcohol and on the bottom of the star press it against the star and let the evaporative cooling sizzle off the alcohol and cool the star quickly)
Well my first practice with a reflow was a fail. I used an old dead XM-L on a 16mm star which I did a gas dedome on. I put it in a vise and used my 25watt soldering iron on the bottom. I was able to get the emitter off, but for the life of me I couldn't get it reattached. I'm guessing I need more heat (will probably try my soldering station). I could also try a heat gun, but all I have is a cheap one.
That cheap heat gun should do great. I've been using my el cheapo from Harbor Freight for years. I use the low setting. Best to use helping hands as they don't suck as much heat from the base.
Hot air is by far best way for me. You have so much control over the heat. Do you have any solder paste? That and a hot air gun will make reflowing so easy it's almost silly. You just put a little solder paste on the pads (just enough to coat the pads if you spread it out very thin). Then place the emitter on the pads and apply heat from underneath. The emitter will dance a little as the flux boils off and then suction down onto the pads. No need to push down unless you have too much solder. Generally takes less than a minute.
If you already have tinned pads and emitter, use flux (contains no solder) on the pads instead of solder paste.
Thanks ImA, that's the same heat gun I have. And no, I don't have any soldering paste yet (it's on the slowtech boat from China). I do have flux though. I too wondered about using helping hands instead of the vise, but then I thought "well it did come off on the 1st try". I guess a heat gun will be much easier when reflowing 4 LEDs at once on a multi-LED board?
