These days, you just can’t get around Surface Mount Devices (SMDs). The times when you could get all your chips in DIP configurations and plug them into nice little sockets are, well, gone. Unfortunately, this means if you are inclined to release magic smoke, your board-mounted chips are a bit harder to replace. The good parts are that chips are cheaper, and occupy much less space. You can jam more neat stuff in tiny spaces, as I’ll show shortly.
The tough part is that many of today’s small outline packages (TSOP, SOIC, etc.) are nearly impossible to hand-solder, and certainly not quickly and repeatably. Ideally, a good reflow soldering technique not only allows us to use these new devices, but potentially even speeds assembly.
With the magic of the china/ebay connection a functional reflow oven can be had for $300-1000. If you’re not sure you want to pull that trigger yet (or will eventually spring for off-site assembly), you can get away with a toaster oven, a temperature controller, some calibration and patience. Your market-variety toaster oven will set you back <$50, will get to 450F (ballpark maximum reflow temp), and perform reasonably well. We’ll show you how to do it here, and you can apply more or less everything to the same process with a professional oven. (We may even re-edit when we get ours in the mail).
Step 1. Prepare your board with paste
The idea is simple: apply solder paste to the pads on your bare board. When the paste reaches a high enough temperature, the organic binders will combust/evaporate and leave nice, clean solder joints. The components will also be hot enough at this temperature to wet with solder, and due to surface tension will tend to shift around to the centers of pads. Bonus.
Depending on the pitch of your devices, this step requires various degrees of precision. If you’ve got SMD resistors, caps, fuses, etc., you can really just glob on a dot and call it good, as long as you don’t get so generous that you bridge contacts. This is fairly hard to do, as the solder does not wet non-metallic surfaces (yay soldermask). However, if you have fine-pitch devices (such as the shift registers in the board shown), you want to be careful to not overapply, or you will potentially bridge contacts. You then get the extra-fun task of going back in with a rework tool or needle-point soldering iron to disconnect them. Remember: it is always easier to add solder than remove it, especially from SMDs, and even more so for devices with hidden pads. For this step, I use either a needle tip from the tube of solder paste, or dispense a small amount on something disposable and use a toothpick. This avoids the difficult task of controlling dispense rate from the tube. When you get a PCB design you like, you can get a screen for this step and save much time.
Shown below is an LED indicator board before and after paste application. While it appears a truly ugly application of paste, the bases are covered: each pad has paste, and not too much of it. The paste will melt and coat each one of the pads. Stray paste will either migrate to the pad or can be easily wiped off after reflow, as it will not stick to the solder mask – it will ball up.
Step 2. Apply your devices
Of course making sure you get your orientation right, stick the little guys on there. Do what you can to get them as close to the board as possible by pushing them down with your tweezers.
Step 3. Determine your temperature profile
Typically, you want a profile something like the one shown here for Kester Solder Paste in Figure 1.
Although I’ve seen elsewhere that the earlier stages are not critical and that you can just cook at reflow temperature for four minutes or so, we chose to attempt to recreate this profile as closely as possible, because we like to read directions. Just kidding. In any case, we wanted to be able to replicate and tweak the process if we needed to. So we took some liberties with temperature conversion and rounding and ended up with the planned profile steps shown below. This was set up to run on one of our beta CuPID Controllers laying around the shop with an SPI thermocouple. This setup and temperature profile have served us rather well, after a bit of said tweaking.
Step 4. Place and bake
I really wanted to say Shake and Bake, but that would not have made any sense. Place your board in the oven, and set your recipe running. You can see that we ended up with a pretty reasonable actual profile, considering we’re using on/off control, a completely uninsulated chamber and, well, a $35 toaster oven.
Here below we have the board in the oven. Pretty impressive what a Target special will do for you.
Step 5. Test your results
When it’s done, let it cool! The solder paste needs a while to solidify, and the last thing you want on a well-placed board is to see them drift when they come out of the oven. Not to mention the potential for burns here. Shown below is the final product, before any cleaning of stray solder has been done.
This board, despite the ugly job I did applying the paste, came out 100% for one-to-one connections and continuity. Mission accomplished. What does this board do? Shown below is the board mounted in a basic CuPID Control unit connected to a Raspberry Pi. Below you can see rotation through all of the LEDs. Four RGBs and four single color for a total of 16 pixels. I’ll show how this is done through some Python and SPI in another post.