If I want to assemble a set of prototype boards by hand, I need an assembly diagram, a bill of materials, bare printed circuit boards, a Kapton or stainless steel stencil, and a pile of parts.

Assembly Diagram

One way to make an assembly diagram is to use the F.Fab layer for one-sided boards or to use the F.Fab and B.Fab layers for two-sided boards. To do this, draw a box around the board on the F.Fab layer and add the board name and the date.

KiCad Board View

Make sure to click ‘F.Fab’ and ‘B.Fab’ on the list of layers in the Plot menu. This is the F.Fab gerber file in Gerbv:

Gerber View

This is a rough draft showing how I assemble a typical board with surface mount components using a reflow oven. I bought the boards from OSH Park, the stencil and white assembly jig from OSH Stencils, and the board components from Digikey.

OSH Stencils sells leaded solder paste. Lead-free paste needs to be brought up to 250C (482F) but leaded solder only has to be brought up to 220C (428F). I find leaded solder to be easier to rework afterward and I don’t have to be as worried about applying high heat to the components.

The Steps

  1. Gather the surface mount components.
  2. Gather all the tools and materials, and make sure the oven is ready to be turned on.
  3. Apply paste and place components.
  4. Bake the boards.
  5. After the boards are cool, use a soldering iron or hot air gun to rework the board.
  6. Slowly and carefully test resistance to make sure there are no shorts, then carefully apply power to one section of the circuit at a time.

Step 1: Gather Parts

Gather the parts and set them out on the table in well-labeled containers or in the orientation that matches the board. You don’t want to have to open packages and try to remember where parts go when the paste is drying and there are lots of tiny parts already placed on the board.

I generate an assembly diagram in KiCad based on extra information in my part footprints, and I reference a bill of materials printout to place the parts based on that assembly diagram.

You’ll also want the bill of materials to match reference designator (C1, R1, etc) to part number.

In the past, I’ve printed out this diagram or drawn a crude outline of the board on a piece of paper, and then placed the parts there. If you aren’t worried about electro-static discharge damaging the parts, that’s an easy place to start.

Step 2: Gather Tools and Materials

This is a good time to check the oven or hot plate. I use a toaster oven set to about 220C (428F) but a hot plate will work just fine. Make sure whatever you’re using is empty, clean, and ready to use. You don’t want to be scrambling to fix things up with tiny parts sitting on top of drying paste on a board.

You can just put the board in and watch for the solder to turn into a molten liquid, so you don’t need a thermometer, but I wanted to have a temperature display and a buzzer to tell me when the board was at 220C so I created a reflow monitor hat for the Teensy 3.2. It’s open hardware and available on Github.

Now, it’s time to gather the materials. First up is the general stuff:

  • Isopropyl alcohol
  • Paper towels

And the most important stuff:

  • The boards!
  • The parts!
  • Hand file to smooth the board edges

If the boards are greasy or grimy, wipe them down with the isopropyl alcohol. You’ll also need this stuff at the end to remove the solder paste from all over everything.

  • Gaffer’s or painter’s tape that’s easy to remove
  • Test jig that’s the same height as the PCB
  • OSH Stencils paste spreader card
  • Stencil

Stuff to place and tidy up the paste:

  • Solder paste
  • Q-tips
  • Toothpicks

Stuff to place the parts:

  • Tweezers

Stuff for reworking after reflow:

  • Flux pen or flux in a bottle
  • Solder wick to remove solder
  • Soldering iron

Step 3: Apply Paste and Place Components

The boards from OSH Park come with mousebites left over after the support tabs are snapped off. Smooth these with the hand file so the boards fit firmly in the assembly jig.

Place the assembly jig on the clean, flat table and secure it around the board with painter’s tape. You don’t want tape that will be too sticky, so use something that comes up easily.

Next, place the stencil on top of the board and carefully align the holes in the stencil with the pads underneath. I place the tape on the top and bottom of the stencil before I align the stencil, and then I use my thumb to press and slide from the center of the top piece of tape out to the left and right to secure the stencil to the upper half of the test jig. I repeat that with the bottom side to secure it to the lower half, and then I check to make sure the stencil didn’t move.

If the stencil is large enough that it’s lifting a bit at the left and right side of the board, I’ll place tape there as well to make sure it’s taut. I don’t want paste getting pushed under the stencil.

Using an exceedingly non-scientific process, I squeeze some paste out around the stencil and then use the spreader card at a very low angle to smear the paste around until all of the pads are covered.

The first couple of times I did this, I used way too much solder paste and squeezed a bunch of it under the stencil. Less is definitely more, and going light prevents solder bridging so you’ll have a lot less rework after the board is baked.

Carefully pull the stencil up from one side and remove it, but I usually leave the rest of the jig in place. This example has way too much paste. I usually go back over the board with a toothpick and remove some paste, making sure there’s not paste bridging between pads on the tiny components.

Paste will wick to the metal of the pin and pad when it liquifies in the oven, but I’ve found that a little more attention at this step saves a lot of pain and suffering later on. One thing to keep in mind is that the more rework you have to do, the more heat you’re applying to the components, and ideally you’ll only ever have to expose the components to one pass of the oven.

I use tweezers to place components. The assembly diagram has the polarity and ‘pin 1’ markings to show how the parts should be aligned. I start with the bigger components first, since they’re the ones that can be unwieldy and they’ll knock the other smaller components out of position. Things can get messy really fast.

I’m right-handed so I have a tendency to rest the heel of my right hand on the table, so I start placing components from left to right so I don’t knock any of the already-placed components out of alignment.

I also keep a paper towel handy with some iso alcohol so I can wipe off the tweezers when they get paste on the tips and start sticking to the components.

Step 4: Bake the Boards

When all the components are placed, I carefully remove one of the jig pieces and walk the board over to the toaster oven. I place the board in the oven, shut the door, and turn on the heat. If I don’t have a temperature reading, I just watch as the oven ramps up heat until about three minutes in when the solder paste liquifies and turns into molten metal. The flux inside burns off, and the metal that’s left over wicks up to connect the metal component pins to the copper pads.

I count to twenty slowly from when I see that first pad liquify, or I try to wait until all the pads have liquified, but sometimes it can be hard to tell. The components are likely to shiver and move slightly in response to the surface tension.

The tiny resistors and capacitors may tombstone, which means the surface tension on one of the pads was so much greater than the other that it pulled the component into a vertical position. The best way to fix this later is to use a hot air gun. Resist the temptation to reach into the oven and adjust the components! All of the components can be knocked off the board if you bump it by mistake. Save the rework for afterwards until you’ve made a few boards and get comfortable.

Once all the solder has melted, I turn off the oven and open the door, and walk out of the room for a few minutes to resist the temptation to adjust things or move the board too soon. This is a good time to collect the extra parts, make sure everything’s bagged and tagged, throw away the used tape, and wipe the paste off of the stencil, jig, table, and my hands.

After the board has cooled so it can be physically handled, I pick it up and take it over to the rework station.

Step 5: Inspection and Rework

Tombstoning is best fixed by a hot air gun, but you can use a soldering iron to at least pluck the part off the board by melting the one pad it’s still connected to. Apply solder to each pad so they’re even, then add some flux and solder the component in place while hold it in place with tweezers.

Bridged pins occur where solder has connected two or more pins that shouldn’t be connected. Place some flux on the solder wick and hold the wick across the pins, then apply heat with the iron to the wick until the extra solder move onto the wick away from the pins.

Touch up any other joints that look unfinished or suspicious.

Step 6: Test, Test, Test, and Test

This is the most dangerous step, since the impulse is to apply power and fire up the board. First, do a resistance check with the digital multimeter across the voltage and ground pins. If these short out, they can damage your computer if you’re applying power from the USB connector (protip: use a cheap disposable hub!) and the short can blow up your components if you don’t have any fuse protection onboard.

If those voltage rails look isolated and there aren’t any other shorts, go ahead and apply power. Carefully test all the voltage rails you have, whether it’s 9V and 5V and 3.3V, and make sure everything is working as expected.

After that, you can go ahead to program the board and start testing your peripherals. I like to start with blinking LEDs and making buzzer noises, and then move on to serial connections and whatever other more complicated functionality I’m going for.