In this tutorial I will describe a technique for soldering to fine pitch SMD pads. Using this approach I was able to solder a wire to a 0.3mm (~ 12mil ) pad.
1) Start by tinning the pads:
- place liquid flux on the pads
- take a "ball" of solder on the soldering iron tip and gently touch the pads, if you get to much solder connecting the pads, just "wipe it off" with a clean soldering iron
- the pads that have fresh flux will attract the solder
- the tinned pads should have small "pillows" of solder on top of them.
2) The wire should be thinner than the pad. You can get a thin wire by separating a thicker stranded wire. Tin the wire the same way as you tinned the pads
- first place liquid flux on the wire
- next touch the wire with a soldering tip that has some solder on it
3) This is the main step, the main idea is to mask the pad being soldered with 3 pieces of temperature resistant tape (Kapton tape).
The setup looks "dirty" when magnified, because of the sticky flux, you clean it up usually at the end with alcohol.
Notice that the vertical piece of tape (on the picture) is holding the wire in place. The right part of the wire goes under the right piece of tape.
4) Now we'll perform the actual soldering, which takes 3-4 seconds, all the hard work was done in previous steps.
- Apply flux to the place where wire touches the pad.
- Touch that place with an iron that has some solder on it.
- The flux should attract the solder and you should end up with a wire attached to the pad.
Here is the final result, after the tape was removed and chip was cleaned with alcohol:
- solder likes hot metal (the tip of the iron)
- but even more, it likes hot metal with flux (when you touch the pad with a hot iron some solder will stick to the pad since it gets hot and in addition it has lots of fresh flux).
On the picture above notice the difference from the tinned pads and untinned pads (top ones are untinned , left ones are tinned, others are partially tinned).
You may want to cut the piece of wire sticking to the right.
After you soldered all wires you can insulate them with liquid electrical tape.