8. Fabricate Development Boards

This week I fabricated my own Printed Circuit Board (PCB), by milling copper-laminated sheets and then soldering components onto it. Then, to test that it works, I wrote a code and uploaded it onto the PCB.

Assignments for this week (Mar 12 - Mar 18):

Group assignment:
- Characterize the design rules for my in-house PCB production process:
document feeds, speeds, plunge rate, depth of cut (traces and outline) and tooling.
- Document the workflow for sending a PCB to a boardhouse

Individual assignment:
- Make and test a microcontroller development board that I designed

Group Assignment:

Design rules for PCB production
At FabLab Kamakura, the mods’ default values for Roland SRM-20 worked perfectly.
But from our first trial which didn’t mill properly due to slanted surface, we learnt the importance of fixing the board securely onto the bed.

Documentation link here

How to fabricate a Development Board:

A development board is a specialized type of Printed Circuit Board (PCB) designed for prototyping and developing electronic systems, while a PCB is a general-purpose board used for connecting electronic components.

For this assignment I will be milling the board that I designed in Week 6.
This is the machine and tools I used;

Machine Model: Roland SRM-20
Machine Specifications: Max. operation area: 232.2(X)×156.6(Y)mm, Mechanical Resolution 0.000998594mm/step, Milling speed 6～1800mm/min, Max rotation 7,000 rpm
Toolpath creation software: mods
Endmill: Milling bit 0,4 mm (1/64”) SE 2FL for traces, and Milling bit 0,8 mm (1/32”) SE 2FL for outline
Material: FR-1 Copper-clad laminated substrate (cut substrate) / 紙フェノール（FR-1）の銅板, with thickness = 35um.

1. Prepare Circuit Board Design

I exported my KiCad design as 2D vector (.svg) files.

2. Generate the Toolpath (.rml file)

To generate the toolpath for trace and outline, I used mods.

Toolpath refers to the precise trajectory followed by a cutting tool during a machining operation.

mods is a modular cross platform tool intended for use with commonly found machinery in fab labs. It is based on independent but interrelated modules and can be used in various tasks, including CAD, CAM, machine control, automation, UI development, input device integration, and responsiveness to physical models.

1. Visit the mods site.
   

2. Start by right clicking anywhere on screen > Program > Open Program > Roland > Mill 2D PCB. A node program will appear. Select the milling machine (This time, Roland SD-20 mill 2D PCB)
   
   

3. First, we read the 2D vector data (png or svg). This time, I read svg, and for svg we need to invert it.
   

4. Then I set PCB defaults to adjust the milling tool settings. After setting the parameters (this time I used the default values below).
   → Tool Diameter: .0156 inch (1/64)
   → Speed: 4 mm/s
   → Offset number: 4
   → Offset stepover: 0.5
   → Plunge rate: On Roland SRM-20, there was no option to adjust this
   → Depth of cut (traces): 0.004 inch (max: 0.004 inch)
   → Depth of cut (outline): 0.024 inch (max: 0.072 inch)
   select Mill traces or Mill outliine (we need to create both, so we will repeat this process).
   

Key Parameters for setting the Toolpath:
Tool diameter: Diameter of the drill bit that we are using
Cut depth: How deep the endmill will mill in a single pass. It is usually around 0.1mm, but it can be as deep as around 0.15mm if you want to remove more. Be careful to not cut too deeply as it will cause more wear-and-tear on your endmill. As it was for the CNC machine, general Rule of thumb is 1/2 the Diameter of the dndmill.
Max depth: How deep the material will be cut in total. Divide by the cut depth to get number of stepdowns.
JOG: the height that endmill will jump to every time it jumps from one point to the next. Keeping this value low will speed up the process.
Number of offsets: How many times that machine will repeat the same path. The default of 4 is usually a good number.
Offset overlap/stepover: In percentages, how much overlap there is between 2 offsets. Usually we use 50 to overlap exactly in the middle of the next path.
5. I also need to change the XYZ origin to 0, as for some reason the defaults are set to 10.
6. I also toggled the Save file option as ON.
7. Then I went to to Mill Rastor 2D, and selected Calculate to generate the Toolpath.

8. The trace toolpath (.rml file) is generated and automatically downloaded (If it’s not displayed automatically, press View). It is a good idea to examine the toolpath to identify potential issues in the machining process before the actual fabrication.

It’s also useful to note the toolpath dimensions as it will help you when milling, in determining whether a particular area of copper is enough for your board. 9. I then repeated this procedure for the outline edge-cut toolpath. We select PCB defaults with below values, and select “mill outline” in step 4 above.
→ Tool Diameter: .0312 inch (1/32)
→ Speed: 4 mm/s
→ No. of offsets: 1 → Plunge rate: On Roland SRM-20, there was no option to adjust this
→ Depth of cut (outline): 0.024 inch (max: 0.072 inch)
select Mill traces or Mill outliine (we need to create both, so we will repeat this process).

Once the toolpaths are completed, I can proceed to milling the PCB. I exported the 2 .rml files onto a USB drive.

3. Mill

At FabLAb Kamakura, we normally use the Roland SRM-20

1. First, the milling machine needs to be connected to the PC, and the machine turned on. It’s also a good idea to turn on a lamp for good visibility.
   
2. We then open the software that can control the milling machine, which for SRM-20 is VPanel.
   
3. Then the copper board needs to be placed on the bed. Make sure it’s fixed securely to the bed with adhesive tape, and pressed down for good measure, to get a level surface.
   
4. Next, the endmill needs to be set to the machine, (we start with the trace first, so I set the 1/64 inch). We don’t need to secure completely tightly this time, as we will be setting the Z origin soon (but make sure it won’t drop to the board).
   
5. We then set the endmill’s XYZ origin. For XY origin, we use the arrow buttons on Vpanel and assign it as XY origin on the software.
   
6. For the Z-axis the origin needs to be set manually (for SRM-20). I move the bit down, so that it’s nearly touching the surface. Then, lightly supporting the mill with my finger so it won’t drop, I unscrew to lower the mill to the surface. Then screw it again tightly and assign this as the Z origin on the VPanel software.
   

7. When ready, I start milling. First lift the Z by a little bit, then select Cut, delete all files, and then open the .rml file, and finally select output.
   

8. After cutting, remove the dust with brush, remove the endmill carefully, and and repeat the above for outline cut. Remember, we must replace the bit to 1/32 inch, and without changing the XY origin, we need to reset the Z origin.
   

Once milling is finished, remove the board (using the acetone solution and spatula if necessary), and wash the board with soap, but make sure to dry it well (electricity and water don’t go well).

Troubleshooting documentation;
For some reason that we couldn't figure out, we couldn't get the machine to mill properly for the first few trials. The milling wasn't producing dust, and the milled surface was still shiny, and when I held the milled board against the light, you could barely see the traces.

We decided the problem was was the endmill (although it was brand new, and there seem to be nothing wrong with it), and once we changed the mill, it milled properly.

4. Assembly and Soldering

Once the board is milled out, we can solder all the components onto it. This is the workflow:

1. Set up the environment
   A well-organized workspace is key to efficient and accurate soldering.
   
   → Ensure the space is well-lit. It is recommended to use a portable lamp.
   → Magnifying glass (with lamp attached) is also recommended, especially for beginners and if you don’t have good eye-sight
   → Ensure the workspace is well-ventilated, to avoid inhaling harmful fumes. It is recommended to use a portable fan. → Use an anti-static, heat-resistant mat. → Arrange all your tools within easy reach, to streamline your workflow. 　→ Sharp tweezers: To keep components in place
   　→ Flux: for making the solder flow well.
   　→ Solder wire: T used 0.3mm 60/40% metal/lead mix.
   　→ Soldering iron: (I set it to 370C degrees), set on soldering iron stand.
   　→ Desoldering wick: to remove excess of solders
   　→ Wrabbing arms - They are arms that help keep your hands stable. They are optional (I didn’t use it this time) but they can help with smaller components
   　→ Easy access to the KiCad designs for reference.
   
2. Set out the components
   To keep track of all the different components you need, and to avoid losing the especially small components, it’s a smart idea to write out your components on a notebook, and use masking tape to secure them to the notebook
   
3. Also consider fixing the board in place using double-sided tape, though it has the disadvantage of making it harder to rotate it.
   
4. Once ready, start soldering → The tip is to solder from small components to bigger components, and from center outword, and from low components to higher components.
   → Check if you put enough soldering, sometimes when it’s too little there might be connection errors.
   → For through-hole type connections, like Xiao’s pins, check that the soldering went through the components “legs”. It has to flow between the pin and the copper pad. → For diodes, such as LED, you need to place it in the right direction.
   
5. After finishing soldering, clean the PCB very well with alcohol.
   

Soldering Workflow Now that we have all the components ready, follow these soldering workflow steps.

Heat up your soldering iron. The ideal temperature is between 300-350 Celcius.

Apply flux to the area that you want to solder and you can also dip you soldering iron in the flux as well. You can use as much flux as you want, because flux is your friend! Here, we’re using toothpick to apply the flux.

Start soldering! The order to do it is to first stick the soldering iron in your right hand to the copper surface. Then, put the solder wire right in and take it out as soon as it melts. If all is right, this step should take less than a second.

In the above picture, you can see my soldering iron tip is touching the MCU😱 This is not a good idea! Avoid touching the components with ~300 celcius heat! It might destroy the MCU!

Examine your soldering result. Ideally, we want to make a skateboard ramp-shape in between two perpendicular surface. The ideal result is to have a shiny fish, not too blob-y, and in this case, looks like Mount Fuji!

🖍️ If you find yourself put too much solder, you can retract some of the excess solder by using the soldering wick. Don’t forget to always apply flux!!

🖍️ You can also distribute some of the excess solder to other surface area by melting the solder again so that it sticks to the iron and transfer to other pads.

If your soldering looks well clean up the excess flux by swiping Isopropyl Alcohol with tissue or cotton bud. Cleaning up the excess flux is important since the flux can also interfere with the electrical continuity

Troubleshooting documentation;
As you know, the LED needs to be positioned in the correct direction for it to work. According to my instructor Tsuchiya san, the standard in Japan is a green line on the Anode side. But after soldering it down that way, we tested with the multimeter, and in fact it was the other direction for this particular LED. So I had to remove it and resolder it. Hence the messy solders around the LED :( This is why it's a good idea to test the LED before fixing it down!

5. Testing

Before using the fabricated board it’s important to test the connections and the various components with a multimeter.
First make sure the board is fully disconnected, and the multimeter is set to the right settings, then probe two areas where they should be connected, to test continuity and open circuits, or where they shouldn’t be connected, to test for short circuits. If a beep occurs, it means that the 2 probed areas are connected.

Systematically examining the connections and components in this manner can identify potential issues before they lead to device failure. This saves time and resources down the line, and also prevents more serious problems that could occur as result of faulty electronics.

6. Programming

Finally, I programmed the board to execute a blinking LED operation.

// Xiao ESP32C3 Program to blink the LED

 #define led_pin D6

  void setup() {
    Serial.begin(115200);// initialize serial communication at 115200 bits per second
    pinMode(led_pin, OUTPUT);
  }

  void loop() {
    Serial.println("Light!");
    digitalWrite(led_pin, HIGH);
    delay(500);
    digitalWrite(led_pin, LOW);
    delay(500);
  }

No Debugging was needed.

Reflections:

- On PCB Design:
I had imagined the sockets to lay down sideways, like the Fab Xiao on Adrian’s page, however, physically this was too challenging because of the traces nearby creating risks of short circuit. I would like to keep these considerations in mind in my future designs.
Also, when soldering the LED, I thought it would have been nice to have it be a RGB LED, for testing different resistor values etc. I would like to try this during the output device week.

- On Milling:
Like with CNC machines, having a good endmill makes a big difference. I would like to treat mine as carefully as possible!

- On Assembly:
It’s always smart to test the correct direction of LED component before soldering it down.

- On Soldering:
Rico suggested I practice soldering before the real thing, and this made a huge difference on the actual soldering of my board (simply comparing the speed with my other classmates).

Useful links:

• Board Testing tips

Assignment Checklist:

• [ x ] Linked to the group assignment page
• [ x ] Documented how I made the toolpath
• [ x ] Documented how I made (milled, stuffed, soldered) the board
• [ x ] Documented that my board is functional
• [ x ] Explained any problems and how I fixed them
• [ x ] Uploaded my source code
• [ x ] Included a ‘hero shot’ of my board