PCB Home Etching

This is my first attempt at using SMD components. It is also my first attempt at using a toner transfer method , and developing a design using EagleCAD.

I am making a circuit that reads a cheap digital caliper and outputs the values as a keyboard. to be used for CAD programs. It has also been designed to be a business card that prints out a resume.

Before I continue, I must reference the group of projects that I took ideas from in order to begin this little project.

Other Projects of inspiration

USB PCB Business Card by Frank Zhao
http://www.instructables.com/id/USB-PCB-Business-Card/

Increadible POV PCB Business card with full small scale circuit fab setup.
http://www.instructables.com/id/Circuit-Board-Lab-POV-Business-Card/#intro

Nerdkits DRO the functional part
http://www.nerdkits.com/videos/digital_calipers_dro/

Benito USB to serial
http://dorkbotpdx.org/blog/feurig/build_your_own_usb_to_serial_device

PCB etching youtube tutorials by Anomus:
http://www.youtube.com/watch?v=di5WD7AhVzI

Anomus' blog:
http://aonomus.wordpress.com/

Build Log

These are the parts  that arrived from digikey

Board after rubbing off carrier paper and prior to etching

When the board was wet the paper was translucent and it was hard to see that some paper remained on the board. After the first 10 minutes of etching most of the copper was removed except for some critical areas in high deensity spots.

After the first attempt at etching - note the white areas where papaer remains inbetween busy areas. 

After the toner transfer - looks good, some patchy parts. Particularly some of the traces for the breakout did not transfer properly (center top).

Frank Zhao notes in his build that a USB connector is 2.3mm and the standard PCB is 16mm therefore some kind of packing is required on the PCB to elevate the pads and make contact with the USB connector.

Final shape cutout

The PCB had to be filed to get it thin enough to fit into the slot on the caliper. I also guestimated the pitch wrong.  I will need to fix that in the next revision.

Tinned the connectors to make them more durable.

The final board except for the ICs

The adjustment nut on the caliper conflicts with the header pins. Should have seen that one coming. I will fix that in the next revision.

Lessons learnt and improvements for next design.

Design

• Increase DRC minimum spacing from 8mil to 16mil
• Change trace widths to:
• 10mil where passing under components
• 15mil inbetween
• 20mil where space allows\
• Move quad pack back and up to make more room for programming pins
• Adjust caliper tab spacings
• Make pads bigger on ISP 6 pin stack
• Round/fillet corners on the card
• Join USB and caliper connectors
• Adjust caliper outside dimension
• improve ground via for left side

Construction

• More time/heat/pressure to transfer the toner
• more spent removing excess paper after transfer.
• Remember to touch up bad parts with sharpie.
• better control of temperature and dosing.