Have you ever looked at a chip that's smaller than your fingertip, and has no pins, and wondered how you could ever possibly hand-solder it? another instructable by Colin has a nice explanation of doing your own reflow soldering, but if the your chip is not BGA, and you want a technique that's quicker and won't put as many poisonous fumes into the air, read on...
p.s. here's what you need:
- soldering iron (fine tip)
- microscope (or very, very good eyesight)
- some flux will help (flux pen)
Step 1: Check out the chip
Make sure you know which orientation the chip is supposed to go on the PCB. In this picture, you can see the little dot to the left of 'CYG'. The convention for chips is that that the little dot indicates the top-left corner of the chip, and you can have a look at the PCB layout diagram to figure out how the chip is meant to be oriented on the board.
Step 2: Tin the pins (and maybe the pads)
Turn the chip upside-down, and melt a little dab of solder onto each of the pins. You can do the same for the board too, if you want. Make sure you heat the metal of the pad enough to melt the solder itself, rather than melting the solder with the tip of the iron directly. After you tin all of the pads, use a flux pen to put some flux onto the board where the chip will attach.
Step 3: Put the chip in place
Turn the chip right-side up, and gently nudge it into place with a pair of tweezers until it's centered on the spot where it belongs.
Step 4: Connect top to bottom
Now comes the fun part. One by one, you need to heat the balls of solder that you've created, so that they become connected to the chip *and* the board. You can do this by touching the pads/pins from the side with the soldering iron tip, and sometimes wiggling it up and down to encourage a connection to form. A nice trick for the first pin that you solder (can be any pin, it doesn't really matter which) is to hold the chip firmly in place with a pair of tweezers (pin it down to the board), and touch the hot iron to the pin/pad in one corner until the solder bridges the gap. With any pin, you may need to wiggle it up and down, or add a little more solder (see the picture) to get it connected. But don't add too much, or you risk bridging pins underneath that shouldn't be bridged. With even just one connected pin, the chip will be stable enough that you can do the rest without having to keep it pinned down. You can work your way around the chip, connecting each pin to the board until you've got them all. See the next step for how to make sure you've connected everything successfully.
Step 5: Check your work
Now you can tilt the chip up on and look at the connection points, to see if you've made all of the connections successfully. Zoom in enough that you can see if the solder is going all the way from pin to pad or not. For the ones that are not, add a little more solder to the pad and wiggle the iron up and down until you get it to bridge, like a stalagtite meeting a stalagmite.
Step 6: Go for it!
Once it all looks good, give it a try! With a microcontroller, the first thing to do is to try programming it and see if it responds. From there, you can test if it can interact with the things that it's connected to (LED's, sensors, actuators, etc). Happy soldering!