This weekend the final resistor I was waiting for arrived. As a result I was able to solder the button pad board together and mount all the buttons. I placed the board on my infrared preheater, applied some solder paste using the syringe dispenser on each of the pads, and then using a pair of Weitus tweezers started placing 0603 resistors from the piece cut from a reel. I then turned the heater on and let it sit for a bit before turning it up a few more times until I got close to the temperatures I needed.
RoastGeek Button Pad v1.0 on Infrared Pre-Heater with SMD resistors placed.
Without a stencil it’s pretty hard not to goop too much solder paste onto the pads. In most cases I had a suitable amount but a few it got a bit too much. For most of the cases I had too much I removed some while it was still liquid-like from the preheater softening the paste. I used the tweezers with a tiny bit of paper towel to wipe some away on the worst spots. Solder paste is essentially a mixture of a type of flux with a fine dust of the solder. When the temperature gets high enough the flux material starts to liquify and the solder particles flow with the wet flux-like material towards the meeting point of the pads and the item being soldered. As you apply a final amount of heat the solder particles melt and the flux-like material starts to evaporate/burn up. To do this I used a hot air rework station to heat up the area near the resistors. It has a pretty gentle flow that I can adjust with a knob so I didn’t have any issue of the resistors blowing away. This tends to be a common problem using a hot air rework tool to solder SMD parts. Once they were all in place I turned down and then a bit later fully off the heaters and allowed it too cool down.
0603 Surface Mount Resistor soldered to the board with an infrared preheater and a hot air rework tool.
Once the surface mount resistors were in place I tested each pad on each side of every resistor and confirmed that the correct resistance was measured. This was to confirm that I didn’t have any “over gooping” resulting in the resistor being jumped by solder underneath of it. Everything checked out ok so I continued on to mounting the buttons and header pin connections by hand with a standard iron.
RoastGeek Button Pad v1.0 with buttons, caps, and headers mounted
At this stage I tested the board with an Arduino sketch and was able to measure each button as expected. This board has a much more accurate number compared to the last board. Each button previously would measure a number that ranged 3-4 from the highest to lowest reading. This one is dead on for almost every single button except for 2 of them that vary by 1.
I then mounted the board to the back side of the control panel. I discovered that the standoffs I am using are about 1-1.5mm too tall so I will need to either grind them down or find a different set of stand offs. I’m thinking of getting some Nylon or ABS ones that have a self-retaining clip for the circuit board side and a threaded end for accepting a screw from the front panel. I also need to begin wiring up some wiring harnesses to go between the LEDs and potentiometer over to the new board.
I’ll add a picture of the mounted board here once that takes place.