This week we had our circuit boards delivered, and began pasting accelerometer chips to them so we can start testing and refining the prototype. To do so, we had to learn a few new tricks, including the proper use of solder paste, stencils, and hot air guns. When we're done, we'll have 1`4 boards with three different types of accelerometer chip, from which we will choose an accelerometer for the final product.
Here's a brief overview of the process:
1. File down any projections on the sides of the boards.
2. Align the stencil sheet to the circuit board, and tape the sheet in place.
3. Turn on the hot air gun and set the temperature to the melting point of the solder paste, as indicated on its data sheet.
4. Place a small dab of solder paste just above the stencil on the sheet.
5. Use the solder paste spreader (any card works just as well) to scrape the solder paste across the stencil. Keep pressure applied to the stencil to ensure no paste is pushed anywhere except the stencil area. Spread the paste until the stencil is completely filled in, and you can no longer see the gold of the circuit board.
6. Gently remove the board so as not to disturb the solder paste. It should not look like this:
7. Use tweezers to delicately place the chip in the correct orientation on solder paste on the board.
8. Using the hot air gun, heat the board using circular motions until the solder paste turns matte gray.
9. Then, aim the gun directly at the chip until the solder paste melts and becomes silver, about 30 seconds.
10. Remove the hot air gun, and use the tweezers (board will be very hot!) to inspect the board for bridging.
11. Use a multimeter to test each pin against the rest, looking for bridging. Often chips will have two or more pins acting as a common ground, these will show a connection. Read the chip's data sheet to confirm that these pins are indeed common grounds, and not just bridged.
12. Bask in the satisfaction of pasting your own circuit boards!