A couple of years ago, I posted a Beginner's Guide to Solder Paste. I've now learned how to attach surface-mount components with a soldering iron, so here's an update!
Check out the video above for the general technique. (We're using a Pinoccio capacitive touch sensor in the demo, and here are the full details on that.)
A few extra tips:
• When you're first taking your resistors out, take out exactly the number you'll need for the current board. This builds in an error-check, since it can be tough to tell if something's missing just by looking at the circuit board.
• Keep them on something with a contrasting, solid-color background so they're easy to spot. These things are tiny!
• Flip them all right-side-up before you start the process. I do this by poking them with my fingertip and lifting them off of the surface. Since they're so tiny, they stick to my finger for a moment and then fall off -- after a few tries, you should get them facing the right way.
• After you've soldered on the headers, you can trim off the top sockets to save space, since you may only need the pins on the bottom. Use strong tweezers (or even your wire cutters) to grab the black plastic on one end and lift it away from the PCB. Then, pull the other end off. You may have to do a little at a time. Then, use the wire cutters to trim off the shiny silver "claws" that were inside the plastic headers.
• Why a 0-ohm resistor, you ask? These are sometimes used in cases where multiple versions of a board are available. This makes them easy to assemble, using the same CNC pick-and-place process every time.
• As mentioned in the video, watch out for tombstoning. This happens with solder paste as well, and refers to the case when one side of a component lifts up off of the solder pad. Just tap it back into place with your tweezers.
• In the video, you'll see that I place the resistors using only one side of the tweezers. How? Why? Well, the tweezers aren't magnetic, nor am I really a wizard (or am I) -- they're just sticky from the flux. If you grasp the component normally, it will stick to both sides of the tweezers, and you might drop it while trying to put it down. Just poke the resistor with one prong, and it'll stick well enough for you to move it to the PCB.
• Finally, you may want to get some defluxing spray and a wire brush to clean off the board after using the heat gun. Let the PCB cool, and set it on a folded-up paper towel. Then, spray the board and the tip of the wire brush, and briskly brush the flux to mix it with the defluxer. (Try saying that three times fast...) Afterwards, wipe the board down with the towel until it's clean enough.
That's it! Your own beautiful, new, populated circuit board. Now, to think of a project...