11/5/2013— Jia Huang
We sent this out to our backers more than a month ago, but somehow it never made it to the blog! For those of you who weren’t backers by 9/27: enjoy!
We announced the Ambient Sensor module as a Class A module a few weeks back (as always you can get yours here). We put it together this past week and thought you guys might be interested in the process.
We first draw out the schematic and PCB layout in DipTrace.
Then, we get a small number (usually 2) PCBs made for the prototype run of a module. The parts are sourced from Digi-Key.
Our modules use surface-mount components, which means that they are soldered onto the top of the board, with no wires poking through. Because of this, we use solder paste, place components with tweezers, and then heat the whole board to attach all the components at once.
We apply solder paste to the pads on the board using a syringe. Then, we use dental picks to scrape off excess paste. (Basically, all of our tools are repurposed medical equipment; if it’s good enough for humans, it’s good enough for our PCBs.)
Next, we place the components. This squishes down the solder paste and makes the board a little messier. But despite it looking like it’ll fuse into one metal blob, the solder paste always ends up working somehow…
In order to make the components bond to the board, we heat everything up until the solder flows. This summer, we used a $20 electric skillet and literally fried our boards. Lately we’ve been able to use a reflow oven at Olin College, which has helped tremendously.
After baking the board, we set it out to cool before putting on the headers by hand. Remember how the solder paste looked really terrible three images up? Since solder paste is made out of
unicorn blood tiny balls of solder and a liquid cleaning compound called flux, it pretties itself up magically in the oven.
All of the module header pins you’ve seen in other photos have been through-hole mounted (the pins go through holes in the board). The headers on this batch of modules are surface mounted instead; we’re experimenting with how different header types feel when attaching and detaching modules.
Now we just have to test it and make sure it actually works.