Week 8

Assignment - Electronics Production

Group assignment:
- Characterize the design rules for your in-house PCB production process: document the settings for your machine.
- Document the workflow for sending a PCB to a boardhouse
- Document your work to the group work page and reflect on your individual page what you learned
Individual assignment:
- Make and test a microcontroller development board that you designed

Group Assignment

Group Assignment Page

Design Rules for PCB Production Process

Bantam Tools PCB Mill

Based on the Bantam PCB Mill and the 1/32” flat endmill and the 0.003” engraving bit.

property	units	1/32” endmill	0.003” engraving bit	0.005” engraving bit
diameter	mm	0.794	N/a	N/a
point	mm	0.794	0.076	0.127
taper angle (total angle)	°	0	30.475	29.998
feeds	mm/min	1100	350	350
speeds	rpm	27500	16500	16500
plunge rate	mm/min	381	200	200
pass depth	mm	0.8	0.25	0.25

Test for track/trace width

I decided to try Neil’s test.

Bantam’s software takes .SVG but not .PNG. So I traced the trace test in Inkscape.

This is the way to prepare the .SVG (Fill (Red?) = Engrave; Stroke = Cutout). Free version cannot use the “Advanced SVG Workflow”. One missing feature is internal cutouts, you have to save multiple .SVG files and set them up together. Each file can take a Cutout Placement setting of Outside/Center/Inside. You have to use a stroke thickness, so for the 1/32” endmill, I made the strokewidth 0.82 mm. You can select Invert: Yes, within Cutout, which would be a nice way to remove the outer copper if you were worried about shorting to it.

Note: Units have to be in Inches in Document Properties. (File > Document Properties).

Bantam Mill SVG Setup

Which I applied as:

Bantam Mill SVG Setup

Download: linetest_inkscape_traced.svg

Bantam Mill Preview

Bantam Mill Test Job

Results

Test Result

A was 1/32” flat endmill only
B was 0.005” engraving bit and 1/32” flat endmill
C was adjusted by -0.2mm i.e., (board thickness back to 1.6mm), using 0.005” engraving bit and 1/32” flat endmill.

Test Result C

Definitely needs some deburring, but shows the capabilities of our milling machine

A result of SVG milling was that the machine never tried to mill tracks that were deemed to narrow in the CAM software. Eg., with the 1/32” flat endmill we’re just seeing ignored tracks. But that may be ok.

Board uneven-ness. To mount my FR-1, I used double-sided tape and a mounting bracket. From the result, you can see engraving didn’t reach the corner of the second board. And I watch it go through all the toolpaths, just fly over that corner. I had done the touch-off on the metal bed. I assume that I measured the material thickness/tape thickness incorrectly. To remedy, I will change the Z-offset by 0.1 - 0.2mm.

My measured design rules are as follows:

property	unit
technology	-	pcb milling
trace (width)	mm	0.100
	inch	0.004
track (spacing)	mm	0.175
	inch	0.007
layers	-	≤ 2 (with bracket etc.)
min text height	mm	2.0

Sending a Design to PCB BoardHouse

I tried:

The price from JLCPCB was $5 + $23 delivery (1-2 days build time). NextPCB wanted $43. But allows you to specify Manufacturer components and position them to have it populated. I think to do that with JLCPCB, you may have to prepare it in EasyEDA and use parts from the official components’ library.

Centroid Files (NextPCB)

NextPCB accepts .Zip / .RAR / .xlsx / .xls. You can export this from your KiCAD PCB file, File > Fabrication Outputs > Component Placement, and specify the format as .CSV (This is what I did at least, but maybe .POS or .Gerberx3 ) You need to convert your .CSV as .XLSX or .XLS.

*Manufacturer Part Number	*Quantity	Manufacturer(Optional)	Package(Optional)
etc.	etc.	etc.	etc.
.	.	.	.

You need a BOM file that corresponds to your “Centroid File”, and this would take a little more time to make sure everything matches up and matches the KiCAD library.

Exporting from KiCAD

Go to File > Fabrication Outputs > Gerbers (.gbr)

And on the Gerber export window, click Generate Drill Files....

Running Software Tests

Running DMC

These errors are:

Silkscreen runs over the edge of the board (has to in some cases - the ESP32 antenna and the right angle connector I designed for),
Text to small (0.5 too small, minimum 0.8),
Footprint does not match the linked file (I changed the footprint myself, do not need populated).

Individual Assignment

This week I made a QPad, by Quentin: https://quentinbolsee.pages.cba.mit.edu/qpad-page/ and updated from the one I designed in KiCAD in Week 6, that I hadn’t tested yet.

Before you start

Check the type of OLED screen you have! Order was GND - VCC - SCL - SDA, so no change needed fortunately! Thanks Carl!

Equipment:

Bantam Mill
Solder Station (with Hot Air Gun)
Tweezers
Various Components
FR4 (two sided copper)
Flux
Laptop (using Bantam Mill software)
PPE, and extraction (Festool vacuum)

Attempt 1

Bantam Mill, Setup

Measured to be 126 x 101 x 1.6 mm. This is not including the sticky tape: 1.7mm.

Measure The Material

Stuck down the material with double-sided tape.

Material Sticky Tape

Bantam Mill, Toolsetup

Tool Change

Bantam Mill, Homing

Go to File Setup, and New CAM to load your Gerber files.

Bantam Mill, Gerber Files

Bantam Mill, Offset

Offset by 2mm in X and 2mm in Y, but tool was 1/8” so the outer profile still cut to the edge.

Remember to Generate All Toolpaths again.

Bantam Mill, Render

Result of attempt 1

To cut, I selected Mill All, and activated the extraction. The tool lowered and spun up. Initially it only cut the toolpaths for the 1/32” flat endmill, and paused. This was less than I expected. I stopped the Job, and changed the tool to the 0.003” engraving bit, which fit into the traces now. I changed to the 1/8” flat endmill for holes and cutout.

I didn’t notice that the render didn’t include throughholes for the OLED screen. To make the most of this test, I will try to make the holes with a dremmel and a 1mm bit. The pads are probably tight for this, so it’s not a good solution.

Attempt 2

I decided to try make a generic breakout board à la Adrian’s Input Device Designs. This is the design I use also in Week 9, Input Devices and Week 10, Output Devices and Week 11, Networking and Communications.

KiCAD

Skip to KiCAD files…

I had to patch my wiring, because my i2c connectors on the BME280 board were +/-/C/D on the OLED were SCL/SDA/VCC/GND and my board were GND/VCC/SCL/SDA!

I wrote a little output for the display:

Little Screen