If you’re going to be doing this a bunch, it’s probably worth figuring out the reflow technique. It’s a bit more involved first getting set up, but it’s faster if you have a bunch of boards to do all at once.
Start by reviewing the SparkFun article here, but we’re going to it modify a bit. Most of the web related material around DIY reflow soldering talks about using solder paste. Well, that might be ok, but try to find good solder paste and you’ll find that it’s expensive, has to be refrigerated, and has a short shelf life. Plus, you’ve really got to have a paste stencil to really make this work. Overall, not something that goes well with a quick and dirty reflow technique. So here, we’re just going to use plain old ordinary solder and no stencil. Tools you’ll need are: regular solder, small soldering iron, tweezers, toothpick, a cheap electric skillet that says it goes to at least 400°F, some solder flux, and optionally solder wick if you intend to make mistakes. Also you may want a fume extractor of some sort in case you’re worried about breathing too many flux or solder fumes.
Note that even though the skillet says it maxes at 400°F, that’s not true. It will actually overshoot to around 470°F or 480°F. How far it overshoots will depend on your starting temperature. You’ll get the maximum overshoot and thus the maximum temperature starting at room temperature. If you start with a warm skillet, the maximum temperature will probably be less. Get a hand-held IR temperature ‘gun’ if you want to be more exact with your temperature control.
Don’t use your breakfast skillet! After you’ve soldered with the skillet, never use it for food again!
Here’s video of the the whole thing done with Reflow skillet
- Rather than solder paste, we want to just place a small dab of solder on every LED pad n the LED side of the board.
- The size of the dab is much more critical here than using the “Heat and Slide” method. The dab needs to be a smooth little pillow. No pointy tips, no bulging mounds. You’re going to have to place the LED on top of the solid solder dab’s and not have the LED roll off. Try just wiping the tip of your soldering iron across the pads to eliminate any pointy tops. If you get to much solder, try cleaning your soldering iron tip and wiping across the pad again. Still to much? Then use the solder wick to remove it and start again.
- Optional step – if you’re still having trouble getting smooth little pillows of solder, try this. Throw the board (solder side up!) on the cold skillet and crank the temp to 400°F. The indicator light on the skillet should turn on for a few minutes and then turn off. When the light turns off, crank the temp back down to 0 (off). This should peak your board temp around 420°F, hot enough that it should have reflowed the solder pillows and hopefully smoothed them out some.
- Now similar to the “heat and slide” method, grab the LED with your tweezers and dip the bottom of each LED in flux, then place the LED directly over the pads. The solder flux will not only help the solder to flow properly, it will be a bit sticky and thus keep the LED’s positioned. Just don’t flood the board with flux. Too much and it will start bubbling when you reflow and that will move your nicely placed parts around! It’s easiest to use the toothpick to properly align the LEDs on their pads after the initial tweezer placement.
- Once everything is placed, double check your placements. For the LEDs on this board, make sure the dark dots are toward the top of the board. Triple check if you must because once you reflow, it will be much harder to correct things!
- Starting with a room temperature skillet, carefully transfer your board to the skillet. Place the board right over the skillet’s temperature sensor (the pointy thing on the temperature dial).
- Recheck your parts placement just to make sure nothing has moved in the transfer.
- Now the tricky bit. Turn the skillet temp all the way up. Wait until the temp peaks and the indicator light turns off and then turn off the skillet.
- Watch closely and you should see the solder melt and the LED’s settle into position.
- Once things cool, check for bad solder joints. Run your finger lightly over the LEDs to make sure none come off. Then check with your multi-meter to make sure you’ve got good electrical connections.
- Now’s the time to fix any bad connections. Reposition the bad LEDs and go through the reflow heating cycle again. Or, if you only have one or two duds, use the “heat and slide” technique to correct the few problem children. Yes, it’s possible to remove the LEDs and replace them at this point, just heat’em up on the skillet and use your tweezers to pick them off the hot board.
- Note, for components on the other side of the board, you CANNOT use this technique, you have to hand solder those.
Additional notes on attaching components
LED Orientation – All LEDs are oriented on the board the same way. There is marking (a triangle) on the bottom of the LED itself. The point goes up. (‘Up’ is toward the two mounting holes, ‘Down’ toward the programming pins) However, once you place the LED’s on the board, you can’t see the triangle marking any more to determine correct orientation. Instead, use the location of the dark spot inside the LED. The dark spot goes ‘Up’, see the picture on the right for correct orientation.
Attaching the battery clip – The mounting ears on the battery clip have been trimmed a bit to align with the pads on the board (original design was for a smaller battery clip, not the current CR2032 clip). When soldering the battery clip, make sure it remains level and does not short out any traces. Also put a small dab of solder on the center (-) pad. Not a lot, but just enough to raise the bottom of the battery off the circuit board. This also helps to prevent the bottom of the battery from shorting any traces. See photo on the below.