Thought you guys might enjoy seeing how these boards are actually made. I hand assemble every board, but it's really not as hard as it sounds. Takes a little time, but with a little practice and the right tools anyone can do surface mount stuff and have it come out looking good! You didn't think these were being made in China or something, right?
Usually I make them in batches of 2 or 3, but this time I was just making 1 because this is a test board. This is one of the new B13/S13 Open RT boards, actually destined to go in an Altima ECU.
To start with though you've got to have something to listen too because this work gets a little tedious. Tonight's podcast is FLOSS Weekly...
First the bare board gets cleaned with rubbing alcohol to remove any dirt or dust. Then it gets placed in this jig I made. All the surface mount stuff is done with solder paste. Basically it's like screen printing a t-shirt, but I'm screening solder paste onto the pads on the board. So the jig holds the board in place...
...and this metal stencil is taped over the top. This is a desktop stencil for really small production runs, but it's pretty much the same processes as a large volume production run. Those use framed stencils and of course have machines doing this (usually, you'd be surprised what gets hand done in China).
With the stencil taped in place I can raise and lower it to swap the boards in and out when doing a batch.
Here's the paste. A small jar like this will do hundreds of boards, and if you keep it refrigerated (not in your food fridge!) will last a really long time too. You've got to stir it up and get it nice and smooth. It feels kind of like peanut butter (not the natural type, more like JIF). Oh, your not supposed to drink or eat around it either, so, um, ignore the Diet Mtn Dew can in the background...
Lay down a nice glob in a line...
...and then just squeegee it forward and then back again across the board. This takes a little practice to get right. The trick is in the angle and the force you apply to the squeegee. Do it just right and it won't smear on the smallest pads. When I'm making more than one board I gently clean the find pitch pads between doing each board, otherwise you get a little smudging. You can still build a board with smudging, just you almost always end up having to go back and do a little clean-up by hand with a soldering iron when your done.
So here's what the board looks like after applying the paste. No smudging, paste perfectly applied on each pad.
Next I place the chips. The little machine on the left is a chip plucker, and the wand tips are for large or small chips. When your first drop a chip into place it will usually slip slightly off the pads, just a little nudging with fine tweezers centers them up.
Hard to see in this picture, but the big chip is ever so slightly twisted and needs a little nudge. They can be slightly out of alignment and the surface tension will pull them straight when you reflow, but its really best to spend a little time and get them right on the pads. Tedious...
Perfectly aligned now!
This is my build box for this board. Each different board gets one, and each envelope is for each type of part that needs to get placed. The envelopes are in the order in which the part needs to be placed.
All the passives (resistors, capacitors, etc) get placed by hand with a fine set of tweezers. This is the most tedious part, there's 25 of just one type of capacitor!
This is my coloring book. Each type of board gets it's own book. Each page in the book tells me where each particular type of part is located on the board, and how many of each part I need to pull. Even though I was the one that designed the board danged if I can remember where each resistor goes!
Almost done with the surface mount stuff, now just to reflow. If you notice all the surface mount stuff is on the top- that's because I use a hot plate to do the reflow. It seems funny at first, but trust me it works great. I've used expensive ovens before and the $10 hot plate beats them dead- with a little practice moving the board around you can get the heating really uniform across the board.
The coolest part is watching the solder paste just suddenly reflow and turn from gray peanut butter into a nice shiny solder joint. It's really hard to get a good photo of it, sorry! This is it going in...
...and now about half the board has reflowed. The four high current switches on the left always relfow last because they're connected to a nice large heat sink.
...and that's the board cooling down. Hot board! Notice the perfect joints, they come out looking really good. I usually let them sit on the hot plate at like 100F for a minute or two to pre-heat them, then ramp it up to 350F until it reflows, then gently pull them and let them sit and cool for a few minutes.
All that's left is soldering the through hole stuff and this board is built. Now just to flash the MCU on it and it's ready to be installed in an ECU.
Hope you enjoyed the tour!
If you're interested in doing projects yourself that involve soldering small stuff like this SparkFun Electronics has a really good set of tutorials aimed at novices-