(I swear I wrote this post a couple of weeks ago but it seems to have gotten lost in the ether.)
The first board using Plated Through-Hole (PTH) technology worked ok. One of the down sides to PTH is the amount of real estate all of the components occupy. I'm trying to keep costs down but PTH makes the boards bigger than they really have to be. The alternative is using Surface Mount Technology (SMT or SMD for "Devices".)
Just about any electronic device you find today was built using surface mount components. There's lots of commercial equipment for doing this, so-called Pick and Place machines among them. Sadly, the cost is prohibitive for the amateur. We're stuck trying to solder tiny parts. And by tiny, I mean 0.8mm x 0.5mm tiny. Or even 0.6mm x 0.4mm and smaller. Some ICs have leads 0.8 or even 0.5mm apart. The thought of working with tolerances that small is daunting.
Scary as it is, I decided to give it a try. In order to learn how to do this, I turned to the folks at Spark Fun. They've got a surface mount version of their Simon Says kit. I'd gotten the PTH version of the kit for my nephew and we had fun building it together, so I knew that the quality of their kits was good. Realizing that my ancient Radio Shack soldering iron wasn't going to work, I also invested in Spark Fun's variable temperature soldering iron. I couldn't quite justify the cost of the Hakko.
The Simon Says kit has a good selection of parts, from a large buzzer, with big tabs that have to be soldered to some 0805 resistors and capacitors and an ATmega328P in the TFQP package (there are those 0.8mm lead spacings!) As with the PTH version, the instructions are very clear, taking you step by step through the process.
Despite the good instructions, I went to the 'net to look up SMD soldering techniques. It was very interesting to see the number of "correct" approaches to dealing with ICs in particular. Some people use a very fine tip and solder each lead. Others will run a bead of solder across the leads. Still others use some solder wick to wipe solder across the leads. Since building this kit, I've soldered a dozen or so ICs and still don't have a technique that I like.
No matter what you do, though, you're going to end up with solder bridges all over the place. This means it's time for solder wick. This is some copper webbing coated with flux -- you place it over the bridge, heat it and capilary action lets the wick soak up the excess solder. This gave me no end of grief the first time out. Either it wouldn't absorb the solder, or the wick ended up soldered to the board! It didn't help much that copper conducts heat very nicely so holding the wick close to the work area becomes uncomfortable very quickly. I suspect that part of my problem was some old wick that had become corroded and the flux evaporated. I'm still using that same stuff but I've leard to coat it with some more flux before using.
Flux. A liquid or semi-solid that helps the solder flow. When I built the kit, I didn't have any flux and that made life much more difficult. Since then I've gotten a flux pen and some "tacky flux" from Chip Quik. They advertise it as being used for rework (de-soldering) but it works pretty well for the inital stuff as well. One of the down sides of using flux is that the stuff is sticky and challenging to remove.
Back to the kit: The instructions start simply with some of the larger components and then move on to the ATmega328P. Everything went well except for the 328P, due to the bridging/solder wick problems. I must have reheated the leads on that chip 20 times before everything looked clean. I was certain that I'd baked the chip into oblivion.
Testing proved me wrong! All of the lights flashed when I switched it on and it happily played Simon Says. The only thing wrong was that it was silent. Looking at the various connections (there's a mute switch), I found that one of the first things I soldered, the buzzer, had a bad joint. The biggest, most obvious solder pad on the whole board is the one I got wrong! Resoldering that one and everything works like a charm.
There is a nother surface mount technique that involves using solder paste and a mask. You squeegee paste onto the pads and then stick the parts down and bake the whole thing. The only problem with this technique is getting the mask. Spark Fun sell a version of Simon Says that uses this technology, but when I looked into it for my own boards I found that the masks can be very, very expensive ($100/per.) There are cutting machines that are somewhat reasonably priced that you can use to make a vinyl mask, but I can't justify $800-$1600 for one of those right now. I'd rather spend the money on a 3D printer. (Hmmmm... I wonder if I could print a mask?)
All in all, I think I like the surface mount approach. The kit provided a good introduction and I've since done several boards, each one a little better than the last.