“Heat and Slide” (hand soldering) Technique
Let’s start with the hand soldering method. You really need three hands for this method! I suggest some kind of PanaVise or similar. You just need something to hold the board steady and level while you solder on parts. You also need a soldering iron with a small tip. I found a screwdriver or chisel tip worked better than a conical tip. Of course you’ll need some solder, liquid flux and solder-wick. Finally, you’ll need a toothpick or two and a set of tweezers. Once you’ve got all that you’re ready to start. Then since the LED’s are really, really, small, you may want a loupe or some other means to magnify your view. A bench light like this works pretty well. You’ll find that those of us who are really near-sighted (like me) will have a definite advantage. Take your glasses off and you may not need the magnifying glass / loupe. At last, one benefit of being extremely near-sighted!
In brief, here’s an outline of the “heat and slide” technique to solder the LED’s. Check out the video below which also demonstrates the technique.
- Prepare by taking 7 LED’s out of the tape reel and place them on the table.
- Put a small puddle of flux (diameter of a dime, no more) on a piece of wax-paper or plastic. We’ll dip the bottom the LED’s in it before we solder.
- Use a toothpick to make sure the LED’s on the table are upright and oriented properly with the rest of the board. There’s marking on the bottom of the LED, but once they are placed you can’t see that mark so I go off the fact that the LED itself (the dark dot within the plastic) is not centered in the plastic. The ‘dark spot’ goes toward the top of the board. ie the end with the two large holes.
- Place a small dab of solder on the first (#1) pad where you are ultimately going to place the LED. Either pad will do, but I recommend the pad is closest to the board edge.
- Make sure this is a small dab of solder. You should barely be able to see it pillow up on the pad. If it creates a ball of solder, that’s too much! Take some off. Use the solder-wick if you need to. The resulting pillow should be less than 0.5mm high.
- Now, pick up the first LED with the tweezers and just touch the bottom of the LED to the dime-size puddle of flux. Don’t drop it in, just enough to wet the bottom. Then while still holding the LED with the tweezers…
- Place the LED slightly below the (#1) pad you just soldered (ie the center of the LED is over the other (#2) pad. Use the toothpick again to hold the LED down on the board when you release the tweezers. Otherwise it’ll stick to the tweezers!
- Now pick up the soldering iron and with the tip just barely on the (#1) pad, not in the middle of the pad, but on the edge, melt the pillow of solder.
- Once the solder has melted, keep the soldering iron in place and use the toothpick to gently slide the LED to it’s final position on the pads. This should put one end of the LED on the still molten (#1) pad and the other end on the un-soldered #2 pad. You might need to use the toothpick to juggle things around a bit to get good alignment, then remove the soldering iron and let the pad solidify. If you’ve placed the LED correctly, you should just be able to see a sliver of the other (#2) pad sticking out from underneath the other end of the LED.
- One problem with the very small lightweight SMD’s like these LEDs is that they tend to stick to the tip of the soldering iron. You’ll seethis in the video. Use the tip of the toothpick to hold the LED in place if necessary when you remove the soldering iron.
- Another problem with these very small parts is that they tend to ‘float’ on top of the solder. What I’ve found helpful is after the initial placement put the toothpick vertically on the top of the device and apply a very light pressure to the top of the device and re-heat the soldered (#1) pad. This ensures that the device is flat against the surface of the circuit board.
- To solder the remaining pad/side of the LED, hold the end of the solder in the right angle made by the LED and the circuit board. Touch the tip of your soldering iron to the groove where the solder is. The solder melts and should be pulled under the LED.
- Now, verify you’ve got a good solder joint by lightly brushing your finger across the LED to make sure it doesn’t pop off. Do this carefully because if the LED flys off onto the floor, you’ll never find it again! Also use your multi-meter to verify the electrical connection.
- Now you’ve only got 48 more to go! Once you’ve got the hang of the technique, I suggest you do the LED’s a row at a time. Run down the row placing your solder dab on the 7 #1 pads, the position 7 LED’s on the board, then heat and slide them one at a time into position. Then do all 7 #2 pads.
Here’s the outline of how to solder the SOIC Tiny26 onto the other side of the board.
- For multi-pin devices, the technique is similar. Start with a dab of solder on one of the corner pads.
- Make sure you’ve got some flux on all the IC legs.
- Heat the corner pad and using a toothpick, gently slide the chip into place.
- Remove the heat and verify the placement is correct.
- Now using a similar technique to the (#2) pad above, hold the solder where the pin meets the chip and apply heat. The solder should melt and get sucked underneath the pin.
- Repeat the procedure for each pin. Don’t worry at this point if you happen to form any solder bridges.
- Once all the pads are soldered, use solder wick to remove any solder bridges that may have formed. This should still leave solder under the pin holding the pin and pad together.
- Visually inspect and verify with your meter to make sure all the connections are good and you’re done!
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.