At first, I always thought that using SMD components is something that requires heavy duty industrial processes. The components are incredibly small, and I thought that a computer controlled robot is mandatory in order to place them correctly. I also had no idea how the actual soldering happens. But when I started looking at it, I quickly realized that these assumptions were simply incorrect.
When I made the first iteration of the watch winder, I decided to use two SMD mosfets as a trial. It turned out to be not as hard as I had first thought. As I moved to my next projects, I decided to start preferring SMD over through hole. SMD components have a lot going for them:
- Much wider variety of components
- Easier to manage lots of different components
- I can make smaller PCBs with SMD components.
- Turned out not to be as difficult as I had first thought, there are tons of Youtube videos to explain different techniques.
- Components are generally cheaper
- I always like to learn new skills
At first, I thought that I would simply use a soldering iron with a fine tip to do this, and I did manage to make some boards on my next project that were sort of OK. The solders weren't pretty, but they worked. The problem with that though is, that if you want to remove a component, particularly a SOT-8 or SOT-16 package, it just can't be done. So after some research I decided to buy a hot air station, I bought an Atten ST-862D as it had good reviews and wasn't too expensive. The one advice I would give to anyone considering buying one, is to choose a station with the hot air unit in the main box, not in the handle. It is a bit more expensive, but it is capable of producing much better airflow.
So now soldering was a breeze, but I thought that I could still improve my workflow. I was afraid that I would damage the components if I needed to heat them too long. So then I decided to add a hot plate to my arsenal. That made a big difference ! Instead of trying to heat the entire PCB with hot air, the board was already close to the reflow temperature. The hot air station only needed to provide the final bit of heat, making the whole process much easier to control.
Workflow
After a lot of trial and mostly error, I found my preferred workflow
- I also order a stencil with new PCBs (there are exceptions). It makes applying the solder paste so much easier. I printed this jig to help the process.
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| The black part is the same for all PCB:s, the green part is specific to each PCB design. The holes have magnets to keep the stencil in place. |
- After the paste is applied, I lay the components with tweezers. I found that angled tweezers (of good quality) are best for the job.
- Then I put the PCB on the hot plate and heat it up to 170 °C. I let it stay there for one or two minutes and then apply the hot air from the heat gun.
- I apply 350 °C with 10% airflow from the heat gun until the solder paste reflows.
- After the PCB has cooled down a bit, I inspect it visually with a digital microscope for obvious faults or shorts. I check all ICs with a multimeter for shorts. If there are any, I try fix them with the soldering iron.
Now there are exceptions. If I have a PCB that has QFN packages, I will not use a stencil. The main reason for that is that the hot air station easily blows small QFN packages out of alignment. For similar reasons, I have for now limited myself to 0805 or larger packages when it comes to resistors or capacitors. I did try to put a 0603 in place, but it just kept flying away.
QFN packages require special attention. Those can be a real pain to get done right, but the best method that I came up with is this. I first apply regular leaded tin to all the pads on the PCB where the QFN package is to be installed. After I am happy how the pads are tinned, I put a drop of sticky flux on the footprint, and then position the package in place. The flux keeps the component from flying away while it is being heated. After that I do the reflow as I did with the stenciled PCB, but now I just solder the QFN in place and once it is soldered, I can inspect it visually. Now since I didn't put any other components on the PCB yet and I no longer can use a stencil, I need to apply solder paste with a syringe to each pad of the remaining components. That usually takes some time, and I usually also put too much paste on the pads. The paste likes to stick to the syringe, so the best way to handle that I came up with was to use a wooden cocktail stick to fine position it.
Did I mention I always put too much paste on the pads ? So much so, that more than once while the PCB is on the hot plate the paste simply becomes more fluid and becomes one big puddle of paste on both ends of a resistor. That is normally not a problem though, because once the solder reflows, surface tension usually pulls all the solder into the right place.
If I have a short between two pins of a SOT-16 package, I fix that with the soldering iron (and a hoof tip) and some flux. Normally that is enough to make the solder flow to the right place due to surface tension.
Conclusion
I found that SMD is actually easier to do than THT, particularly if you need to remove a component for whatever reason. Cleaning a hole of a THT component is a colossal pain, and you risk damaging the pads doing it. But you do need more tools to achieve that. With THT you only really need a soldering iron, with SMD you need also a hot air station, preferably a hot plate, a microscope (or at least a good magnifying glass or both) and you need to play with more chemicals (solder paste, flux and IPA to clean up the mess).
I still do use THT where it makes sense (for example connectors), but SMD is now my default choice for all new designs. Soldering SMDs turned out to be a lot easier and actually more fun than I had originally thought.
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