9. Electronics Production - Week 8

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9.1. Assignments

3D Scanning and Printing

group assignment

• characterize the design rules for your in-house PCB production process individual assignment

• make and test a microcontroller development board that you designed

• extra credit: make it with another process

9.2. Group Assignment

9.2.1. General PCB-milling

We used the Marcera Carvera for milling the PCB, which is very convenient because of the automatic tool changer.

9.2.1.1. Preparation

For preparation, we did the following steps: (-1. HOME THE CARVERA)

0. We milled our bed flat so that the PCB is exactly flat on the surface

1. We first check, if the board is straight (with a straight object, here a caliper):

   • if not, we just bend it, until its straight

2. Next, we stuck double-sided tape onto our PCB and the PCB on our milling bed. Important here is:

   • The tape should have: thin adhesive, not to strong, no strings/structure strengthener in it and on the side

   • let edges of tape over PCB

   • look for dirt etc.

   • don’t need to tape everywhere

   • press down!

   • should not move

3. Another very important step is the exact Z-Zero point, which should be on top of the PCB with a high accuracy.

   • to deep -> mill might break (especially normal, thin mills, v-bit is fine)

   • to shallow -> path with not wide enough

   Approaches:

   1. Use the auto-z-probe of the carvera, but this is not very precise und thus unsuitable

   

   2. Drop the tool, drive down and grab it again, but this approach also does not work that nice with de Carvera, because of the rubber stopper

   3. Use a multi-meter, spin the mill and touch mill and board with the multi-meter and drive down, until it beeps

   

   We have chosen the 3. Method, which resulted in a good z-zeroing.

4. Now just setting the x and y work origin and milling could start.

9.2.1.2. GCODE-Generation

For generating the GCODE, we use the Fabmodules(Source-Code) with the already prepared Carvera-Program.

The Fabmodules use a black and white PNG file and look for edges from white to black, meaning that the path to carve out should be white. Also in the prepared program, an exterior can be chosen simultaneously to do the path-carving and cutout in one go. The ‘mill 2D PCB all’ program is used for removing all of the copper from the PCB.

So the workflow breaks down to those steps, when the pcb is designed in :

1. Generate the black and white PNG of the traces and exterior

   • more notes on those step is below, because its not trivial/easy (as I hoped)

2. Go to the Fabmodules and open the Carvera-Program (right-click->open program->carvera->mill 2D PCB)

3. On the left modules, ‘Select PNG file’ and upload on the top the pathes and on the bottom the exterior cutout.

4. Next set your mill and PCB parameters on the middle ‘mill dual tool’-module (especially the tool diameter and cut-depth and max-depth) and machine-speeds on the ‘multi_toolpath to gcode’-module (maybe cut- and plunge-speed and spindel RPM).

   • we just changed:

     • first tool diameter: 0.2mm

     • cut depth: 0.3mm

     • cut max: 1.6mm

     • and reduced cut speed to 6mm/s and plunge-speed to 3mm/s

     • also in later setting we used for ‘offset-number’ 4, which resulted in better separated paths (from the rest-copper)

5. Then click ‘Calculate’ on the middle module to generate the gcode, you can then save the .nc file and look at the GCODE on a little preview (if it does not pop up, use ‘view multitool_path ‘ module)

9.2.1.3. Milling

In Preparation we already setup the Carvera.

So we just need to upload the GCODE tho the machine (bottom-left ‘page’-icon -> upload), select the file (in the view, click select), last time check the g-code in the preview and then start the milling (bottom left play-button).

Then use a scraper or knife to cut the taps and carefully break out the board from the rest. To clean up the traces, use the knife/scraper or steel wool to remove the ridge.

9.2.2. Design-Rule

To determine the design-rules, we used Neil’s Traces-Test.

9.2.2.1. Outcomes

0.2mm vbit

• track width ~ 0.254mm

• clearance ~ 0.178mm

9.3. Individual Assignments

9.3.1. From KiCAD to PNG

For exporting the paths routed in KiCAD, there are many options (GERBER, SVG, PDF,…) but did it following steps

1. ‘Plot’ (the plot icon next to the printer) the traces and in the popup select ‘SVG’ in the top left dropdown and then select the layers to generate the paths (in my case only ‘Edge.Cuts’ and ‘F.CU’) and then ‘Plot’ in the bottom right.

2. Open the SVGs in Inkscape, open ‘Document Properties’ and click ‘Resize to content’

   • IMPORTANT, add a little margin (e.g. 1.5mm, also change from pixel to mm format, if not already) so that lines on the edges are properly recognizes in fab-modules

3. Select all and center it again (you need to group everything and ‘CTRL+SHIFT+A’, the ‘Align and Distribute’-Tab).

4. Maybe invert the colors for lines and filled pads (can be annoying, because traces are stroked paths and pads are not stroked but filled shapes -.-), but can also be done in the fab-modules

   • IMPORTANT for the exterior, the howl board and exterior need to be distinct shapes, NOT PATHs, so it should look like my examples below

5. Then, if the image and the paths should look right, open the ‘Export’-Tab (CTRL+SHIFT+E), and Export it as a PNG

   • IMPORTANT set the DPI to ~1000, so the PNG has a high enough resolution for the Fabmodules!!

Then for the milling process, see above

9.3.2. Test Bridge for USB-to-Serial

For testing out above PNG generating method, I used the USB-to-Serial Bridge from the lecture and adjusted the design-rules in KiCAD (in ‘Board Settings’ under ‘Design-Rules’). Also tried out tracing a bitmap to paths which flowered good.

9.3.2.1. PNG

9.3.2.2. GCODE

9.3.3. Milling

9.3.4. Soldered

9.3.5. Own PCB

I improved my PCB from the Electronics Design Week, so I fixed the DRC and shortened the board a bit. The tricky part here was milling the hols, which did not work out unfortunately..

9.3.5.1. PNG

First Images where not inverted, which resulted in way to big holes.

Then I inverted it but there was a weird line in the gcode preview, which was because the inverted version on the right side was not completely black:

So I fixed it in GIMP and then the path was better.

9.3.5.2. GCODE

Here the weird line from the wrong png.

And here with the fixed version:

9.3.6. Milling

Here the version where I forgot the Inversion:

Here the holes are way to big…

And here the version with inversion:

Bettet (I know, I didn’t clean it right)

9.3.7. Design Fail

After it to me was pointed out, that:

• traces are most of the time way to big, I used 0.3mm but 0.4-0.6mm would be better

• I used way to small 0-ohm resistors (and also I didn’t needed them at all -.-)

• I should place SMD and through hole components on different sides

9.4. Notes

9.4.1. KiCAD

• U .. select entire tracks (may need to be pressed multiple times)

• under/near chip small track width

• good base track width:

  • 0.4mm

  • 0.6mm

  • thicker for power lanes

9.4.2. Fab modules

• Fab Modules

• use chrome

• offset-number -> rounds around traces

  • 0 == fill/mill all away

• think about tool thickness when milling

• DRC Check in KiCAD!! (Design-Rule Check)

• png dpi >= 500

9.4.3. Carvera

-> make folder for yourself on it (for gcode)

• nc .. Numerical Control

• PCB-milling z == 0 on top of the PCB

• for centering/finding exect x,y zero

  • ‘Edge-Finder’

• PCB mill-shank is smaller then wood milling shank

  • don’t forget ring, if not on it already

• check, if tool changer tool is deep in the holes (the plastic ring should be almost in)

• adapter for probe if used another shank

• always home!!

9.4.4. PCB Milling

• RC1 vs RC4 important for design-rules

• before milling the PCB, use a hair-ruler to check, it it is straight

• most error is the PCB is not fixed enough or it it is not straight enough

• PCB glewing very important (double-sided tapes)

  • thin adhesive, not to strong, no strings/structure strengthener in it

  • let edges of tape over PCB

  • look for dirt etc.

  • don’t need to tape everywhere

  • press down!

  • should not move

• important -> make sure, baseboard is flat! (just mill it flat)

• z-zero-ing needs to be perfect (easyiely brake mill)