5. Electronics production¶

In this week, I made an in-circuit programmer, milled a PCB and programmed it. In the group task, we characterized the design rules for the PCB production process by trying different milling bit diameter sizes and cutting depths.

Group assignment¶

Characterize the design rules for your PCB production process

The group work was done with Michael and Arash.

mods¶

The following procedure was done at mods to generate the required files. As shown in subfigure 1, right click and select programs. Then select “open server program”, then choose Roland/mill/SRM-20/PCB. After this the default program will open which wants to communicate with the Roland over sockets, but since in our case Roland is not connected to the same PC, so instead of sending the output to a socket we send it to a file. So as shown in subfigure4, delete the websockets module, right click to add a new server module called file/save. Connect the input port of this module to the output port of “Roland SRM-20 milling machine” output port.

Steps taken to get the milling program in the mods

In the png file, the mill runs on the edge of the white, a bit on the black side and it takes away the black part of the png files. The recommended dpi for the png file is 1500. The offset number tells the number of passes the tool makes and the offset step-over tells the overlap percentage.

We used the mods software to generate the rml files needed by the roland cnc mill. In the picture below we have marked with red all the places where some change was done. In the top left corner, we had to put in the png file name. In the right corner we have the tool settings. In the middle “mill traces” is selected as we are milling the traces and not cutting the outline. In the bottom left the origin is set to 0,0,0. These are the rml files for milling traces and cutting the outline.

Steps taken to get the files needed for milling from the mods program

Milling Process¶

For the milling, we used a Roland SRM-20 milling machine. In order to operate the machine, you need to turn it on from the top right corner. Below are screenshots of some of the important pages of the SRM-20 manual. The manual can be downloaded from here.

Main parts of Roland SRM-20.	Vpanel software.
Vpanel software.	Cut screen options.

To start the milling, the work piece is put on the sacrificial surface using double sided sticky tape. For milling, you have to put the desired milling bit into the machine. When the PCB is in place, you open the front cover and using your fingers, put the milling bit into the chuck and tighten it using a hex key. The bits are fragile and should be carefully inserted as they easily break if they fall. The process is shown below.

Prepare the work piece.

Insert the milling bit.

The main process for milling involves positioning the milling bit along the X, Y and Z axes. This moves the bit along the length and width of the board and vertically (defining the starting point for milling). Save the X/Y origin settings when ready by using the X/Y button on the VPanel software (No. 9). Then position it vertically and when the bit is near the board, you should loosen the collet with the hex key and carefully lower the bit to the surface of the board. Save the Z origin settings using the Z button on the VPanel software (No. 9). Finally upload the file to be milled by first pressing Cut button on the VPanel software (No. 12) which will open a dialogue box where you will ‘Add’ the file and the press ‘Output’ which will start the milling process.

Set the origin.

Start the milling.

We used 3 different parameters (shown in the table below) for the milling bits. We first milled the PCB with a flat bit of diameter 0.4 mm with a cutting depth of 0.1 mmm. Then for the other two we defined the diameter settings in the software of the program.

Desired output/Input traces that should be milled.

Tool	Cutting Depth Setting	Tool Diameter Setting	Output (Actual)	Design Rules
Flat - 0.4mm	0.1mm	0.4mm	Cannot make very small spacing between two traces (left side), however can remove large amount of copper and can create thin traces.	Thinnest trace: 1 mil, Spacing b/w traces : 16 mil
V Shape	0.1mm	0.2mm	Can make small spacing between two traces (middle), however can not remove large amount of copper and create very thin traces.	Thinnest trace: 5 mil, Spacing b/w traces : 8 mil
V Shape	0.06mm	0.1mm	Can make finer spacing between two traces, however can not remove large amount of copper and create very thin traces.	Thinnest trace: 14 mil, Spacing b/w traces : 5 mil

Individual assignment¶

Make an in-circuit programmer by milling the PCB
Program the PCB

PCB Fabrication¶

I used Brian’s version for the design.Below are the pictures for creating the rml files for creating the traces and outline. I used 0.4mm flat bit for milling and 1mm bit for cutting the pcb. These are the rml files for milling traces and cutting the outline. The finished pcb is shown below.

Creating tool path for the traces.

Creating tool path for the outline.

Finished PCB.

Assembling the PCB¶

Below are the schematic, PCB layout and component list, as taken directly from Brian’s page.

Schematic.	Layout.
1x ATtiny45 or ATtiny85 2x 1k resistors 2x 499 resistors 2x 49 resistors 2x 3.3v zener diodes 1x red LED 1x green LED 1x 100nF capacitor 1x 2x3 pin header

I used the soldering iron in my lab instead of Fablab as it is easily accessible to me after work hours and it is of much better quality. Unfortunately as I don’t have more than two hands, I didn’t take any photographs of myself while soldering the board. I started the assembly by first putting all the components on a paper. I cleaned the milled PCB with alcohol. Then I secured the PCB on a chuck and made sure that the fume extractor is pointed to it. The soldering iron was set to 380 C. I used solder paste and copious amount of liquid flux while soldering. Given the small pad size, I used a small toothpick to put a very little amount of the solder paste on the pads. Given its viscosity, it helps in keeping the components stable before soldering. After soldering all the components, I used a flux cleaner to clear up all the residual flux.

Starting the assembly.	Cleaning the PCB with alcohol before soldering.
Always make sure that fume extractor is working and placed nearby.	Microcontroller in all its glory.
Cleaning the residual flux.	Finished assembly. Also in the picture state of the art, solder paste applicator 5000.

After soldering every component, I made sure that there was no short between the power pins and the relevant signal pins.

Programming the ATtiny45¶

Again, I took the source code from Brian’s page. Since I am using windows linux subsystem, I used the apt to download the required tools.

sudo apt install avrdude gcc-avr avr-libc make

I compiled the code but soon found out that WSL does not support USB sub system, which meant I could not use WSL for programming the board. So, I got the windows binary of the avrdude and used that for programming. Furthermore, I was using usbasp programmer, so I had to change the commands for flashing the microcontroller. zadig was used to provide a generic signed driver for the windows. After successfully programming the microcontroller and verifying that it appears as a USB device, the reset fuse was blown. After that VCC was disconnected from the VPROG pin on the ISP header by removing the bridge on the solder jumper.

Programming the board using avrdude and USBasp programmer.	Programming all the fuses except the reset one.
Adding the drivers using zadig.	Newly made programmer being recognized as a USB device by the PC.
Blowing the reset fuse, now this programmer cannot be reprogrammed using the normal low voltage programmers.
Finished programmer with the solder jumper removed.

Files¶

Problems¶

Emergency stop by the Roland mill. While going through the third pass of making the outline, I got the following message on the PC and it didn’t go away till I pressed the power button. I was worried that I would lose the origin but after turning the machine on again, the software was fine and it did remember the origin. What actually caused the problem, that I don’t know.

Emergency stop by the Roland mill.
Windows linux subsystem does not handle the USB sub system, so could not use avrdude in WLS to program the microcontroller.