In this tutorial, I will show you how to solder SMD components on a PCB using the reflow technique and a simple frying pan.
What you need:
- A PCB and the SMD components to be mounted
- Solder paste (I used this one from Sparkfun)
- A stencil (optional, see below)
- A putty knife (if you go with the stenciling technique)
- A frying pan or skillet
- A hob (no induction, that might fry your components)
The frying pan I used is a 2.5€ one I found at Ikea. Very cheap, but that’s exactly what we need. Don’t plan to do any cooking (other than PCBs of course) with the pan you choose for this task. Just make sure it has a flat surface, so the PCB is heated up homogeneously.
To apply the solder paste on the pads, you can use a stencil. It can be made of acrylic plastic (cheap) or metal (expensive, but lasts longer). I got mine on the following website: http://www.smtstencil.co.uk/
The guy who runs this website is very nice, and the price is not so expensive if you live in the EU (even better for UK residents). It took about a week to arrive, as I chose the cheapest shipping option. Using a stencil is better if you plan to do a few prototypes or even a production run. It can also be useful for a single board if you have a lot of SMD components (as the paste might dry if the application takes too long).
You can find a very good video tutorial (which I followed myself for this board) on how to apply the solder paste to the board with a stencil at Sparkfun:
If you don’t want to use a stencil, you can still use a sharp object (like a needle) to apply some paste on each pad. Put a little drop of paste on the pad (enough to cover 2/3 of the surface) but don’t spread it or flatten it, let the component do the job).
Whatever method you choose, you will want to keep your hands clean after applying the paste, especially if it contains lead or other toxic elements. Same for your stencil, wash it right after using it, so the paste does not dry in the holes.
Laying out the components:
Once the paste has been properly applied, begin with the smaller components (resistors, capacitors, transitors and small ICs) and move on to the big ones last.
It’s okay if they are not totally aligned with the pad, when the paste melts down, it will suck the components into place. This is actually quite fun to watch!
Once everything is ready, put the pan on the hob, the PCB on the pan, and turn the heat all the way up. (oh, and keep the room ventilated, you probably don’t want to breathe lead fumes, do you?)
The paste is going to start melting at around 180-190°C, which can happen quite fast depending on the type of hob you use. As you can see, mine is a diecast iron hob, so it took about 8 minutes to reach that point.
The melt will start in the center of the board, so even for small boards, what you want to do is to move it around to let the edges of the board melt as well without overheating/frying the center.
And that’s it! When all the pads are shiny, remove the pan from the heat, let it cool slowly, and inspect your solder connections.
Note: this tutorial applies only for soldering SMD components on one side of the board. Usually it’s better to design your board so that there are both SMD and thru hole components on the same side. Use the technique described here to solder the SMD first, then the thru hole. These can be on the other side of the board, as long as you have enough space for soldering their leads among the SMD components.
The full gallery of the Mobius Modular Motherboard (the board used for this tutorial) is available on Flickr.