Proper mounting of LEDs onto the MCPCB solder pads

SMT newbie here, so I need a little help please!

I'm ready to mount a Nichia 219 to new XP-G MCPCB as part of my first ever P60 drop-in build. Other than reading, watching YouTube videos, and asking countless questions of some very generous members over the past several months, I have no other education, and certainly no practical experience in electronics, micro-circuitry, or SMT/SMD technology. However, a very particular question came to mind about the application of the solder paste.

I plan on applying a strip of solder paste inline with the anode, cathode, & thermal pads on the MCPCB. But I just saw a YouTube demonstration by Cutter of reflowing an LED, after applying a single, perpendicular strip of paste across all three pads. Can I assume the reason for that is that they know the solder mask on the PCB is sufficient to draw the paste into each pad, thus avoiding "bridging?

Curiously, if two or all pads were inadvertently bridged, what would happen to the emitter and PCB?

Thanks all!
John

Edit: Btw, I came across an interesting article about multi-LED PCB solder pads. Click here.

As long as you don't put too much.
Nothing would happen they'd just be bridged.
If you tried to use it you'll just get a short circuit.

You don't have to put past horizontally though.
http://www.youtube.com/user/digikey?v=QRW4gBcsRkY&feature=pyv&ad=5942790449&kw=soldering

hellokitty[hk] said:

As long as you don't put too much.
Nothing would happen they'd just be bridged.
If you tried to use it you'll just get a short circuit.

You don't have to put past horizontally though.
http://www.youtube.com/user/digikey?v=QRW4gBcsRkY&feature=pyv&ad=5942790449&kw=soldering

Click to expand...

Thank you hk! It's good to know that if an error is made with too much paste, then the emitter & board will still be fine.

In order help me control the amount dispensed, when I bought the paste, I picked up a few of these 25 gauge tips for the syringe. I also picked up a bunch of small cc syringes from the local Rite-Aid pharmacy and put small amounts of paste in a few of them to keep on the bench, while putting the remainder in the fridge until needed.

Coincidentally, that Digikey video is the one I referred to when I started the learning process. :thumbsup: Thanks again!

Hi John,

I've never had the solder paste bridge the pads yet. I have applied to much and that causes the emitter to "float" on the solder. If you do this, heat it back up, remove the emitter and use a desolder wick to remove the excess from the board and the solder on the emitter bottom will be enough to reflow it back on the board.

If you are doubtful of how to apply the paste just lookup the datasheet for the solder stencil pattern. Preheating is important humidity can do damage in the long term.

think2x said:

Hi John,

I've never had the solder paste bridge the pads yet. I have applied to much and that causes the emitter to "float" on the solder. If you do this, heat it back up, remove the emitter and use a desolder wick to remove the excess from the board and the solder on the emitter bottom will be enough to reflow it back on the board.

Click to expand...

Thanks Jamie, that's another great tip! In addition to watching how solder paste was applied in a single line across all three pads on that Cutter video, the possibility of applying too much paste, due to my arthritis, was a concern. However, using the fine gauge tip on the syringe made application of the paste much easier to control the amount.

Incidentally, on another project where I needed to remove solder to change the mode sequence on the driver, I noticed a huge difference in the quality of desolder wicks. After using Zephertronics desolder wick with Type R flux for the first time this past week, I now realize how effective a high quality wick can be. It was night and day compared to the first roll I bought at RS.

koala said:

If you are doubtful of how to apply the paste just lookup the datasheet for the solder stencil pattern. Preheating is important humidity can do damage in the long term.

Click to expand...

Thanks Koala! Preheating in accordance to the emitter's spec sheet is a very good point. Due in part to the low tech, but very effective method I use for reflow, that's why I bought an IR thermometer.

For now, although application of solder paste for me will only be occasionally adding a new emitter to a new PCB's solder pads, as a SMT newbie and in the interest of learning, I'm very curious what you are referring to about "how to apply the paste". I would appreciate if you would elaborate. Thanks.

jcalvert said:

Thanks Koala! Preheating in accordance to the emitter's spec sheet is a very good point. Due in part to the low tech, but very effective method I use for reflow, that's why I bought an IR thermometer.

For now, although application of solder paste for me will only be occasionally adding a new emitter to a new PCB's solder pads, as a SMT newbie and in the interest of learning, I'm very curious what you are referring to about "how to apply the paste". I would appreciate if you would elaborate. Thanks.

Click to expand...

Your fine gauge tips and syringe method is a good way to apply the paste.