Lap Timer


Electronics Production Project

Board Design

This week is a continuation of lap timer project from last week, moving from design to production. I started by refining the board schematic and layout (cleaning up traces and adding mounting holes). I also corrected the input logic on the ESP32, which had been reversed.


Corrected Code


The final board design, shown below, was used for milling and assembly.



Layout


Milling

I milled the board using the Roland SRM-20 in the Architecture shop. The goal was to cut the copper traces cleanly, drill through holes and mounting holes, then outline the board for removal. I followed the protocol discussed in our required lab session and the Architecture shop milling tutorial. I also found this Aalto Fablab Youtube video helpful for milling the through holes.

Steps:

  1. Prepare files
    • Export the .png traces, holes and outline from Altium
    • Use modsproject.org → Programs → Open Server Program → PCB → Roland SRM-20 → mill traces (1/64).
    • Set the cut depth to 0.004 in and tool diameter to 0.0156 in (1/64 in) for traces.
    • For the outline, switch to mill outline (1/32) and set depth to 0.012 in.


  2. Set up the machine
    • Take the FR-1 board down firmly on the sacrificial bed using double-sided tape, pressing evenly so it's flat.
    • Mount the 1.64 in end mill in the collet.
    • In VPanel, set the XY origin at the lower-left corner of the board.
    • Lower the Z until the bit barely touches the copper surface. Loosen the screw slightly, press it slightly into the surface, and then tighten again.


  3. Mill the traces
    • Load the traces file in Mods and send the job to the mill.
    • Watch the first few passes to make sure the copper is being cut away cleanly (not just scratched) and that the bit isn't cutting too deep.
    • Use Pause and Resume in VPanel (DO NOT UNPLUG THE USB).

  4. Change tools for the drilling
    • After milling the traces, raise the Z and switch to the 0.1 in. end mill.
    • Re-zeo the Z in the same way.
    • Switch to the through hole file in Mods and send the job.


  5. Change tools for the outline
    • After milling the through holes, raise the Z and switch to the 1/32 in. end mill.
    • Re-zero the Z in the same way.
    • Switch to the outline file in Mods and send the job.


  6. Remove and Clean the Board
    • Carefully pry the board off with a spatula.
    • Clean of the tape residue with isopropyl alcohol.


  7. After Milling
    • Check each pad under the microscope to make sure there's no leftover debris from milling.
    • Check continuity between all connected pads using multimeter.
    • Use deburring tool to smooth board edges.

Soldering

After milling and cleaning the PCB, the next step was soldering all of the components onto the board. I used the soldering iron in the Architecture shop with Pb-free solder following the instructions discussed in our required lab session and this Architecture shop soldering tutorial.

Steps:

  1. Organize components
    • Stick all components onto a strip of double-sided take so they don't roll away or get lost.
    • Label each part with its reference (e.g., R1, U1) so that it is easy to find while soldering


  2. Set soldering station temperature
    • For most parts, I set the temperature to 350°C
    • A few components, like the IR LED and photosensor, included solder temperature profiles in their datasheets. I adjusted the soldering temperature when soldering those components to ensure that I don't go above their reflow peak (point where splder melts and creates the necessary connection without exceeding the maximum temperature ratings of the component).


  3. Apply flux to pads
    • Flux helps the solder to "wet" the pad and component leads by cleaning oxidation and improving surface tension.

  4. Plack and tack one side of the surface mounted component using tweezers
    • Apply a tiny bit of solder to one pad on your PCB.
    • Hold the surface mounted component with tweezers and position it over that pad
    • Rehead the same pad with soldering iron while gently pressing the component into place. The solder will melt and hold that one side down.
    • Verify that all polarity sensitive parts (LEDS, phototransistors) are oriented correctly.


  5. Reflow the other side by heating the opposite pad and applying a small amount of solder.
    • Keep the soldering tip tinned and clean to avoid dull joints.
    • If a joint looks frosty, reflow it with flux because it's likely oxidized.
    • Avoid reheading the same pad multiple times

  6. Inspect each connection under the microscope after soldering. The solder joint should resemble the one in the image below:


  7. After Soldering
    • Clean the board using isopropyl alcohol and soft brush to remove leftover flux reside.
    • Check for bridges used multimeter.

Final Product