Assignment #4
electronics production

The fourth Fab Academy assignment is electronics production.
We have to use a CNC milling machine to produce an ISP programmer.
The first thing to do is to mill some FR1 sheets to test the design rules of the circuit we’ll design.

1 – export the files for milling

After looking at work by previous Fab Academy students, I chose a PCB designed by Axel Cornu . The design improves on aspects of Brian’s FabISP and comes with a great introduction featuring Professor Farnsworth, which was enough to win me over! I downloaded the Eagle version from Axel’s page and opened it in KiCad. I exported the top copper layer and the Edge.Cuts outline as Gerber files for milling.

The circuit will be made of the following parts
1x – ATtiny45 microcontroller
1x – 100 nF capacitor
2x – 47 Ohm resistors
2x – 150 Ohm resistors
1x – 1.5 kΩ resistor
2x – 3,3 V Zener diodes
2x – LEDs
1x – 3x2 pin connector
Turning the geometry into a milling path

2 – generating milling paths and G-code

To generate the G-code I decided to use software called FlatCAM. FlatCAM is an open-source G-code generator designed by Juan Pablo Caram. It imports Gerber and SVG files and creates toolpaths around them. I started with the circuit traces. The software needs the tool diameter, 0.4 in my case, and the feed rate, for which I chose 30 mm per minute. There are three main steps:
1 – import the gerber file
2 – Create a toolpath around it
Turning the geometry into a milling path
After preparing the traces, I moved to the outline. However, FlatCAM’s “Board cut” function generated a rectangle around the Gerber geometry rather than following the PCB’s USB-shaped outline.

I had to find my way around it, so I did it by following this tutorial.
I returned to KiCad and created a fill on the bottom layer with the desired board shape. Back in FlatCAM, I treated this geometry like a trace, using a larger 0.8 mm bit and a 1.5 mm cutting depth. This produced a path around the outline I needed. I also added tabs manually in “Edit geometry” to keep the board in place.

Once this was done, I exported the two G-code files and moved to the machine.

3 – Milling the PCB

With the G-code ready, it was time to use the machine: a Stepcraft 2/600 Black Edition. This version has more precise stepper motors for work such as PCB milling. I used a Proxxon spindle, with its rotation speed adjusted manually using the knob on the spindle.

First, I milled a square on a polycarbonate sheet to make sure the working surface was parallel to the machine’s motion. I then attached the FR material using double-sided tape. In UCCNC, I positioned the tool at the lower-right corner of the material and clicked “Zero all”. Setting the Z origin required more care: I placed the spindle just above the surface, loosened the bit, lowered it gently until it touched the material, then secured it and set the Z origin.

The first attempts did not go well. In CAM, I had set the cut depth to 1.5 rather than 0.15, cutting a trace-shaped hole through the PCB. Surprisingly, the bit did not break. I corrected the G-code and tried again, but the bit was in poor condition and the cut was still bad. After replacing the bit, the traces finally milled well. I switched to a 0.8 mm bit for the outline, which worked on the first attempt. At last, I had my PCB!

4 – Solder on the PCB

I was super happy with my PCB milling session, and I finally moved to the soldering phase, where I could finally put all the components together and start to make my PCB work. I was very excited, but as soon as I touched the surface of the PCB with my soldering iron, the track charred immediately and my PCB was no longer usable… After this traumatic experience I just started to put tin on the other parts to get a little more used to the practice, but I think I made quite a mess. I’m going to make another board, and then I’ll try to solder the components avoiding direct contact between the iron and the PCB

5 – Solder on the PCB 2 – the copper battlefield

After milling another PCB, I was nervous about soldering it. I used the old board and other failed attempts to practise first.

I followed several tutorials for precise instructions. The one I recommend most is this one by the mighty Collin Cunningham. The tutorial is very well made, the only thing that I feel like adding to it is that since we are not putting solder mask on our PCBs we’ll need to be extra careful not to ruin our board (like I did before…), and the use of solder paste really helped a lot in the process. Besides this, I milled two more boards and placed almost all the parts on them, but as soon as I touched the pin connectors… ...BAM! The connectors went away, bringing a substantial part of the traces with them. This was truly frustrating, and it left me no choice but to throw away the board and start over again (what the hell...).

I did my best to keep my morale up and soldered everything again. Fortunately, this time it went well. Here is the result:

6 – flashing the ISP programmer

To flash the ISP programmer I used an Arduino, because I don’t have an already working ISP programmer. I used this tutorial to do it

Once the Arduino was set up, I edited the firmware’s Makefile to select its USB port and the ATtiny45 controller.

After that, I followed this tutorial. I opened the folder in my terminal and ran “make hex” to generate the HEX file, although the terminal reported “nothing to be done for hex”. Then I ran “make flash” to flash the ISP programmer.

Source files

Here are all the source files for the Electronics production assignment:
1 – KiCad file for the FabISP 45 Key
2 – GBR + SVG files for the FabISP 45 Key