Reflow XP-E2s onto a Triple PCB?

Hi,

I have one of these triple PCBs coming:

https://illuminationsupply.com/led-mcpcbs-c-52_43/20mm-triple-xpg-mcpcb-parallel-p-250.html#.UhGn35K1Frs

and I want to reflow 3 XP-E2 emitters to it, and I’m wondering how to go about doing that?

The equipment that I have available is that I can try to use to do this:

- Soldering iron (60W, adjustable)

- A small butane soldering iron/torch

- Regular rosin core solder

- Flux

  • Frying pan and gas stove/range

What I’m thinking is to:

  • Tin the pads on the emitter PCB using the soldering iron (i.e., melt some solder onto the emitter pads) first, then
  • Place the PCB in the pan
  • Position the 3 emitters on the PCB
  • Turn the stove on and wait until the solder melts and the emitters move into position
  • Turn the stove off

Does that sound about right?

FYI, I’ve not reflowed any emitters before, in case you were wondering :(…

Jim

I’d practice with a 20 mm star and spare emitter. Several methods work and you can pick one and get familiar with it.

I’ve been doing that, with a star with a dead XM-L emitter.

One question I have is: That star already had the emitter on it, so the pads are already tinned from the original mfr reflow. I notice that the layer of solder on the pads is very thin, but when I tin contact pads on a star with a soldering iron, it’s not that thin… it’s usually more like a small blob of solder melted on the pad.

Does that matter?

I’m thinking that I should tin the emitter pads, and then go over it with some solder wick after tinning the emitter pads?

EDIT: Would using low-temp solder paste be easier? If so, which one/where to get it (U.S. preferably)? I’ve looked and it seems like they sometimes call flux something like “solder paste”, whereas it’s just flux with no solder in it, and vice-versa, so I haven’t ordered any.

Yeah, it matters. You want that baby to be as thin as possible. Even a little bit of excess solder can lead to reduced heat transfer and lumen loss. Also, it increases the likelihood of an uneven mount. An unevenly mounted LED, if it’s bad enough, can make it impossible to get a clean looking beam.

The solder wick method works if you can’t get a thin enough layer on pads with your iron. I don’t think solder paste is absolutely necessary.

What about this solder paste (saw references here on BLF):

http://www.ebay.com/itm/Kester-EP256-Lead-Solder-Paste-63-37-Syringe-183-C-NO-CLEAN-10-CC-35g-1-year-USA-/151001610828?pt=US_Cell_Phone_Replacement_Parts_Tools&hash=item232865ba4c

Would that work ok?

Also, on an older thread I asked about whether low-temp (183C) is ok for emitter reflowing?

Thanks,
Jim

You all might find this funny:

I was practicing with an XM-L on a 20mm star that was originally not working, and after I removed the emitter, I accidentally dedomed it.

Then I re-reflowed it, just for practice. I tested it afterwards, and the dedomed XM-L emitter is now working :)!

BTW, when I reflowed, I didn’t see the emiiter “move to center”… I had to try to center it “manually”. I thought that when the solder melts, the emitter is suppose to automatically center itself?

I often find the melting is in in two stages (I use solder paste): first the solder melts and the flux evaporates (bubbles out), you see the shiny melted solder, then nothing for a while (I got tricked a few times that the reflowing was done), but then second (unexpectedly) the emitter is sucked into place, the layer of solder under the led gets very thin, and excess solder is pushed out forming a blob next to the led. I believe the second stage can be helped by pushing the led down a bit.

Maybe you need to add more flux? The LED won’t “float” without enough flux. Excess flux can be cleaned up with alcohol.

If using regular solder (not solder paste), when do I add flux? Are you saying to solder onto the pad, then add flux, then put the emitter on top of the solder layer (plus the flux) before heating for the reflow?

Also, re. solder paste, is the one I linked earlier:

http://www.ebay.com/itm/Kester-EP256-Lead-Solder-Paste-63-37-Syringe-183-C-NO-CLEAN-10-CC-35g-1-year-USA-/151001610828?pt=US_Cell_Phone_Replacement_Parts_Tools&hash=item232865ba4c

all right for reflowing emitters?

Thanks,
Jim

That solderpaste looks like regular solder paste, so it should do the job well.

If you reflow the way you mentioned, by pretinning the board with solder wire and then placing the emitter, I think it is a good idea to apply some fresh flux to the emitter pads first to help the attachment of the solder, because most of the flux from the wire is evaporated during the pretinning.

Hi,

Ok, thanks for the clarification. Nowadays, I always flux the original /- contact pads on the PCB before trying to add solder when wiring the PCB to the driver. Otherwise, a lot of times, the solder just blobs on the/- contact pads.

Learned that from BLF :).

What I didn’t realize, until recently, is that after adding flux to the pads, that I should heat the pad with the iron which evaporates the flux and cleans the pad, and then touch the solder wire to the pad.

Earlier, I thought that it was something wrong that all the flux was being evaporated before I added the solder. There’s a Youtube video by PACE that shows that.

definitely use solder paste. Reflowing by tinning with solder is a pain with large single emitters, so it’d be really unpleasant with a 20mm triple. I did a couple of those parallel triples (one XP-G2/ -E2, one red XP-E) and it was pretty straightforward on my stove top, after I found one of the LEDs I dropped on the floor that is :slight_smile:

I’ve been practicing with only XM-L emitters, and I figured that doing the XP-E2 will be a bit more scary, because the emitters will be a lot smaller :(…, so yes, I’ll go with solder paste!

I went ahead and ordered that Kester solder paste, so now, more questions :).

Assuming the PCB comes with bare, untinned contacts, do I put a dab of the solder paste on each emitter pad?

Do I need to spread it around?

Do I need to use flux either before or after putting the solder paste on the contacts?

The reason for the questions is that auction has a video where its used to solder an IC, and it looked like they squeezed a bunch of the paste across all the contacts, and then just waved a heatgun over if for awhile, and then all the solder paste just flowed onto the pins, including disappearing between the pins (except for the last pin).

Thanks,
Jim

Yup. That’s the method that works for me.

If you don’t mind paying for solder paste, use solder paste. It’s definitely simpler.

I use regular solder because I don’t reflow enough to justify the $15+ cost for solder paste. If it was like $5 or so, I’d probably use solder paste. But as it is, regular solder plus flux works alright for me.

Cutter has a video demo that shows how to apply solder paste for reflow.

I just smooth a very small amount over each pad with a toothpick. Again, practice with some old LEDs and stars first as you’ll be thoroughly surprised at how little you need. Less paste also makes it a little easier too, although you obviously don’t want to take that too far - you still need some!

The paste will be thick. You will have a syringe to do it so adjust for what you have…

Mine came in a little bucket, so I put a little onto a paperclip tip. With just the smallest dip in, it sticks to the tip pulls extra with it like a drip. I just touch one end of the contact and draw the drip across. Do the same for the thermal pad and other polarity pad, being careful not to bridge any of them. Also, be VERY sparing with the paste as theres only enough surface area for a tiny amount. You should get the feeling one you have done it a couple times :wink:

Tip, as stated here I usually use some very small tweezers to “smash” the led onto the pcb to lower the solder height for better thermal path. Be careful doing so because it can obviously cause a short if the solder jumps contacts. I always check with a meter first and while in the flahlight (in the case of a sinkpad with no dielectric over thermal pad) since a positive bridged to the thermal pad could also cause a negative short.

Don’t worry about bridging the pads. The solder mask prevents that. It’s almost impossible to bridge the pads unless you use a beyond excessive amount of solder.

Yes, be very careful. I actually do not recommend doing this as 1) you shouldn’t need to “lower” the solder height if you use the right amount, and 2) it is very easy to damage the LED by pressing on any part of it while it is heated.

Try to keep the solder height low by managing the amount of solder used, and don’t do it by manually pressing it if you can at all help it. Do it only if you absolutely have to.

I use that exact solder paste and it works perfect. With the supplied tip I just squeeze out enough to cover the traces on the PCB, then drop the LED in place and use my heat gun to heat everything up until the LED floats then settles. I tried with my stove but those boards from illumination supply burn real easy. I hope you ordered a few…

Yes, I got one extra, just in case :)…