Recently read an article about reflowing and soldering paste, it was extremely hypercritical about things so had to order stencils for the pcb’s… but with LED’s im probably stuck with hand application of solder paste as i will need gerber files for noctigons to send to the stencil maker, I can’t seem to apply a thinner layer but here’s some pictures i took…
So, How do you generally apply solder paste? Can it be improved.
A little bump on the dome removes the excess solder to the sides, but I’m not very satisfied.
That's exactly what I do but probably a little messier. If I get any excess out the side I remove it with a sharp blade as some isolaters wont sit properly with the lumps there.
I use a wooden toothpick to paint the solder paste onto each contact and then run the toothpick sideways to thin the layer of paste to the desired thickness.
I use the same process but before applying solder paste I heat up MCPCB a bit so that when I get really close with solder paste it liquifies (does not melt into solder) and I can apply small quantity (although I always seem to apply too much). After reflow, while the led is hot I press it down so it would be as close to MCPCB as possible.
But I am also not satisfied with this process…
Now I obtained some semi transparent tracing paper that is 0.11mm thick (I hope it will be thick enough) and I will try to make stencil out of it…
I bought a stencil from Texaspyro a year or so ago. These are made from kapton, and work quite well. Put a small amount of paste on the stencil and spread with an old credit card, or razor blade, for the correct thickness layer. These stencils are available elsewhere, just can’t remember where I saw them.
I mixed my paste with extra flux, so I have to use a good deal more than most people. I have found I use about 30% more than the pic in the OP. Even then, sometimes I’m afraid i didn’t use enough because I rarely get the bumps on the sides.
But, if you apply too much, because central pad does not have “overflow” part it will cause emitter to be raised a bit. I inspect every led after reflow with my 25x goggles and I can see that, if you had a hair thin needle you could run it under emitter, where those 2 split lines are that divide central pad and + / - pads. That’s why I now press every led while solder is still in liquid state but this can cause emitter to move a bit from its self-centering position…
Rule for applying solder paste is: applied as little as it is needed, more is not better. That’s why they make stencils if PCB is populated by pick and place machine.
I’d still say that a little too much is better than too little. I have my amounts dialed in fairly well, I have removed a few led’s after reflowing to check coverage before i settled into my curent process.
What i do is i heat my frying pan up a just a little and lay my copper board down and add my paste, it then spreads across the pads to make a even layer and then i lay my led's down
then i use my SS Tweezers and tap the top of them, then if any extra solder comes out the side i use my soldering gun to suck up the little blobs
After the paste is applied, I use the edge of the barrel on my soldering iron. With tweezers, I balance the PCB on the edge. I'll sometimes add a bit of extra paste onto the soldering pads for the driver wires. This has the benefit of tinning them in the same step, but gives me an extra visual indicator of when the solder has melted sufficiently. I watch closely while heating, and when I see the emitter shift and settle into place, I use the tweezers to lift the PCB and place on a brick or other flat surface to cool. It's that easy. My first reflows were a pain, as I was trying to use a torch and helping hands to hold the PCB, but that overly complicates the issue. As long as you can lay your soldering iron sideways and balance the PCB, this technique works very well (for me). No failures yet!
That's looks like too much paste. Also, I don't run paste across all three pads crossway like shown. What I do is use a hot air reflow unit and place the mcpcb in my small vise, so I can heat the mcpcb from below. I heat it up first (Not a lot of heat, but about 10-15 seconds from underneath), so when I put the paste on, it flows onto the pads. That makes me use less, because it doesn't pile up. I just use a small pick to apply the paste. When I put the led on, I apply flux to the underside of the led, (on the pads), then I put it in place and reheat from the underside of the mcpcb till the paste melts. I always put paste on the wire pads too, so I can see when it melts. Once it's melted, I use something flat, (usually the handle end of my tweezers) and I press on the top of the dome, to squeeze out the excess solder. I am not all that gentle, I press enough so I know that I have as much solder squeezed out as I can and I hold, till it hardens so it won't pop back up again. Then, when it's cold, I remove all the solder blobs around the edges.
On something like a 3-up, I use a brass disc with a brass rod on it, so I can hold and press down on all three leds at the same time.
Thanks, exactly what i needed… Some great suggestions here, I’ll try pre-heating the board before applying solder paste to see any difference in results. No matter how much less solder paste i try there are always solder blobs , some bigger than the other, which make me think i put too much and the blob size should be minimal.
Thanks OL for the input , i’ll go with the stencil as i’ll be making a bunch of them on regular basis, but you’re right it might not be the best way to put solder paste on all three pads. It has worked for me in the past (albeit not optimal) and the solder paste flows towards the pads / heat , its quite spectacular to see when the solder layer distributes itself to the three pads via surface tension.
I have also noticed that the bump on the led dome works well to evenly attach the led but if there is a bit more solder paste the led is raised a little , not too noticeable but it is a lot when we are dealing with only a couple of hundred microns as our optimal solder thickness, while the end product may be 300 microns and you’ve put 150% of the solder paste and might not help with longevity it higher currents we’re used to push the led’s with… How do you hold all three led’s on the noctigons in that case i am always a bit worried as higher temperatures cause the dome to be softened already…
I would say there is no need to order the triple stencil, a single pad stencil is ok.
(You can order multiple pads on one stencil and cut it to a desired shape and size)
The triple stencil is hard to make it flat when to paste the solder onto the board, and still end up inappropriate solder,
but you can control it very well with a single pad stencil. (the one in the left).
