How to Solder SMD PCBs (VESC Board)

The electric bike (check out my YouTube channel and Instructables page for more info) that I’m building will need a 3 phase brushless motor controller, and I’m going with the open source project VESC (links at the end). I’ve used theversion 4.12 of this controller for a few years on my electric skateboard and it still works flawlessly, so I decided to stick with it. Also, I already had the PCBs, since I got the from OSHpark, which sends you three copies minimum.

I’m going for a hot air reflow approach, so I decided to make this Instructable and show you how I usually assemble this kind of boards. I already have the hot air gun and some time ago I got some solder paste. As you can see in the photos, it’s pretty old and dry, but kind of usable.

• Hot air gun
• Solder paste
• Some kind of siringe to apply the solder paste

If you want to get the board, just download the files linked at the end of the Instructable and upload the gerber folder to OSHPark. On that link you’ll also find the list of components necessary, which you can order online from any reseller. I got mine from Mouser.

The board is not really cheap because it's a 4 layer board, but if you plan on making the three copies it's not expensive.

First of all I printed a sheet with the name and position of all the components, together with a list that associates the names to the values. Following this list, I picked all the components from their bags and placed them on the first sheet. I’m placing twice the number of components because I’m soldering two boards at the same time.
Just a note here, I’m doing the side without the logic ICs first, because I prefer to heat them up just once.

If the boards are not new, you should clean them with alcohol or isopropil alcohol (best option). If you have neither, acetone is fine, just make sure to not put too much and remove it quickly.

Once the components were ready, I put a small amount of solder paste on every solder pad. Be careful! It’s easy to put too much. Fortunately you don’t need to place it perfectly, since the surface tension of the tin will arrange it properly later, when reflowing. If you check the photos you'll se that my end result is not even close to perfect.

Then comes the fastest part: pick and place all the components from the sheet to the board. So nice to watch it come together so fast, with so (apparently) little effort!
Make sure they are reasonably aligned, but don't go crazy over alignement. The surface tension of the molten metal will take care of half the job later.

In the end, blast it with the hot air. Mine is set to around 300°C. There’s a lot of debate around the temperature values. In a perfect world you solder at just above the melting point of tin and it takes 1 second. In the real world that doesn’t work, you need to raise the temperature to reduce soldering time, but if the temperature is too high you’ll still ruin the components. THANKS REAL WORLD.
A very nice thing you can do is preheat the components before soldering. Just keep the hot air gun at 20/30cm for a while and then get gradually closer. When the soldering is done you should do the same thing in reverse. The exact same process goes for the other side.

Components that have been kept in humid environments for long may need to be baked before soldering, since the absorbed humidity may crack them with the high temperature.

Before giving any power to the board, you should very carefully visually inspect the board, especially the fine pitch components, since they are the most prone to shorted pins. Use a lens or something. I often use this lens on the phone camera. It’s not the best, but it’s cheap and good enough for inspection. The perfect tool for the job is a microscope, but I guess if you have one you probably don't need me explaining how to reflow

Then, when powering up, don’t connect it to a battery! Battery can usually provide very high bursts of current and if there is a short, the whole board might blow up. Instead, I recommend connecting to a current limited power supply, set to maybe 100mA or less, so shorted components may overheat but not explode.

Another essential thing that remains to do is to flash the MCU and test the board. I’m gonna do a quick YouTube video on that too, as well as an Instructables article!

Also, one board is gonna end up on an electric bike, while the other... who knows. Suggestions are welcome!

You should definitely follow me on both channels!