Wow. That's a lot of acronyms in just one title! But here's a quick description of what we've found to be the best way to solder the 4-pin surface mount LEDs for our laser shades project.
To begin with, we're working on a homemade PCB with exposed copper tracks - none of what follows would probably be required, if the board were "professionally" made, with solder mask and so on. But we've lots of exposed copper. And that means lots of opportunities for bridging (and lots of potential points of failure!)
Here's a close-up of the board we're working on:
There's not much room between the top-left pin of one LED and the top-right of the succeeding one (we're placing our LEDs from right to left). Also, it's possible that while soldering the top-right pin of the LED, there's a chance of bridging onto the power rail running along the top of each row.
(Note that the holes drilled into the board serve no mechanical purpose - but the do allow the wearer some kind of vision while wearing the shades!)
Our preferred method of hand-soldering SMT parts is to coat the pads with flux pen and place the solder paste directly onto the pads. Except, in this case, the tiniest little movement while placing the LED could result in solder paste being smeared between the pads and the tracks above. And should this happen, there's no guarantee we'll be able to get our soldering iron in there, to burn away any excess.
Instead of placing the solder paste onto the pads, we're using a bit of wire to apply the paste onto the sides of the pads on each RGB LED.
There's no solder being applied to the underside of the LEDs (which probably means the connection is not as "true" as it might be) but when we apply heat via the iron, the solder that's been applied to the sides of the pads makes a nice, neat connection to the pad underneath.
It's a slightly peculiar way of applying paste (and may well be frowned upon by a few people) but it does make soldering rows of these in close proximity a little easier!
After applying each LED, we test for continuity between the data out of the preceding LED and the data in on the LED just placed. We also test for continuity between the Vdd pin and the power rail, and the Vss pin and the ground rail. Lastly, we test for continuity between the Vdd and Vss pins (to make sure that there is none, otherwise we've shorted something during soldering!)
After about 40 minutes, we had our first row soldered up:
We've six rows in total, but some of these are not full rows - so without counting, we reckon we're probably about a fifth of the way there!