Toaster Oven Reflow Soldering
Soldering Surface Mount Devices (SMD) can be challenging work, especially with the no-lead (legs, not pb) parts that are so popular with device manufactures these days. A useful and surprisingly easy solution is "Reflow" soldering. Reflow is when you use a paste of solder to hold down parts while the board is heated in an oven. During heating, the solder melts and surface tension makes the parts align themselves in a much more uniform way than is possible with an iron. I've got more information about the toaster here, including a controller I made for it.
Preparing for reflow is a simple process. The most important part, as usual, is having a good foundation. In this case that means a good circuit board. I have made my own boards in the past, but BatchPCB is just so damn convenient. They create 2-layer boards with soldermask and slikscreen for $2.50/inch. The turn around time is pretty long, ranging from 3 weeks to as long as 6 weeks. Above is an image of the board I'm using for this example, and one I got from BatchPCB.
While it's not completely necessary to have a solder mask, I find that it really helps keep the molten solder where it is supposed to be. If you make your own board, make sure you get a complete etch and that there are no specks of copper around pads. They could prevent the solder from staying where it belongs.
I've just started to lay out all the surface parts before I begin a large project. It's kinda nice, all the stress of finding the parts is isolated to a time where you aren't in a hurry. I printed out the sheet twice as large so there would be plenty of room for the paper tape that carries passives.
Once everything is laid out, ready to go, It's time to break-out the solder paste. I keep my solder paste in the fridge, but I've heard of some people that just leave it lying around, and have for months, and say that it's just fine. I like applying it cold, because it's a bit more viscous; though that means you have to press a little harder on the plunger.
Placing the paste on the board is probably the most delicate step. If you put too much paste on, some problems can arise. While the board gets warmer the paste spreads out (just like a baking a cookie) and if there are several components close together the blobs might join. If this happens the surface tension may drag them together as the solder melts. Another reason to avoid too much solder is that you'll create solder bridges on IC parts with close leads. To be fair, these are nearly unavoidable. Finally, if you add too much the extra may squeeze out in a really strange way. However, if you put on too little, you'll need to add more solder with an iron. There is clearly a balance, but I'd err on the conservative side. The amount I put on in this picture was about right. Basically, it should look a bit like a Hershey's kiss. For ICs just put a bead along the leads. Put on a little as possible while covering all the pads evenly. I have to remove some with solder wick from time to time.
Putting everything the board is easier than with standard soldering. You don't need to make sure that things are lined up very accurately. However, for the reason outlined above, it is important to make sure that the solder paste doesn't touch between the small components.
I place the boards in a cold oven, and bake at 450°F for about 8 minutes. You should watch to see the solder melt. Don't worry about thermometers, or controllers, or even time really. Trust your eyes. If the solder melts you know that you have achieved the proper temperature. Leave it in there for about another 30seconds with the heat on to make sure that you've at least reached the critical temperature everywhere (while hopefully not exceeding it much). Turn the heat off and leave it in for about another 20 seconds. Open the door and wait for about 5 minutes and remove the part. That's all there is! Good luck!
A note from the future... I've had people reach out and say that they've burned their boards following these instructions. I can't emphasize how important it is to use your common sense!! If the board is starting to look charred, TAKE IT OUT! If the solder is molten, you're done! Don't just blindly leave it in and let it burn.
