Week no.6

Electronics Design

PCB Schematics and Layout

Assigments :

Redraw the echo hello-world board

Add a function like a button and LED

Check the design rules, make it, and test it

Re-Designing the Hello Board

Preparation

I´m an old user of Eagle Schematics, started to work with it at more than 10 years ago, at that time it was still developed by CadSoft Company. But I hadn’t made any schematics for the last 5 years, so I was afraid it may be very different from the eagle I used to know (Used until V4.1 I think).

So I used my Autodesk Account to download and install the new eagle 8, what I realized is that didn’t had changed much (besides installation and oppening page). The procedure of adding libraries was the same, so I downloaded and placed into Eagle LBR folder the SparkFun Eagle Library and the components Library for FabAcademy.

Create a FabAcademy Project Folder and Started with Eagle and the sample Hello Board schematics, the idea was to make a flexible (all unused PINs out to a connector) board adding elements that should be of interest to further assignments and final project like test Buttons, Leds, sensors and Display.

And so were added some (usefull) components:

3 Buttons - to test interface and user navigation
A power Connector - 2pins (VCC+, GND)
2 capacitors - 10uF at processor power pin and other 0.1uF near the ISP connector
A RGB Led - Status and debugging
Upgrade to FDTI connector - Pushed out unused pins
4 Mounting Holes - Grounded pad for screws

Routing the Board

Actually were made 3 versions until reach the final design. Considering the small size and few components, it was an obvious choice to route all components manually (None Auto-router). The design rules for the Hello Board meet the following requirements:

Clean and small design, single sided
No jumpers or wire routes outside the board
Filter capacitors near the microprocessor and another near the SPI VCC
Take benefit form remaining un-machined cooper surface for ground shielding and improved stability
All processors pins accessible through a pin header
Could be screw mounted to a case ..
.. or become an user interface along with a Display using the SPI or FDTI connector
Respect tolerances, outlines and isolation, approved by Eagle ERC/DRC check !

Routing Hints :

Double check signal names used like VCC or GND, careful not to add a second source power like VCC+ doing so you will have separated signals.
Create a polygon around your board and name it GND (or the ground signal you used), it will create a mesh of ground around your wiring. This is good, for the mill bits, for the electromagnetic shielding, for a stable GND, for testing after with a multimeter, all gain. But you need to remember clicking Hatsnest.
Put names and annotations on wiring at Schematics, is nice to remember and check signals afterwards.
Check your connections with the SHOW tool (the eye) if you open both Board and Schematics on screen it will highlight your signal in both windows.
Do a ERC check and SHOW ERRORS, check the clearances, it will ensure you get no shorts when soldering !

PCB Fabrication - first attempt !

I tried to use a new (actually an old) fabrication method to produce the board by chemical reaction with iron perchloride. But to draw the schematics layer on the board I used the laser-cut machine. Our laser-cut is not full spectrum and has low power (40W), and so it has power to only vaporize a thin layer of a protective ink previously applied. It works beautifully, incredible precision and finishing!

This fabrication method can be archived by following these steps:

Prepare the PCB to the desired size
Create and save file of circuit board traces (PNG)
Apply a thin spray ink layer to protect the cooper
Engrave the traces file with the laser-cut machine to the PCB
Prepare the iron perchloride solution and submerge the PCB
Stir the solution, if available, provide heat to accelerate the chemical reaction
Clean the PCB board and you are done !

FAIL : Choose carefully your ink ! The corrosion didn´t worked as expected, the laser cut couldn´t remove the ink leaving a very thin layer. It was probably because I used a automotive spray with metal particles. The last picture above shows a second try with red spray ink with the same effect: I tested different speeds and power for the laser, at some point the laser burned the metal layer finally exposing the cooper. I suggest look for the ink composition and choosing a non metal based for this to work !!

And here is a video of the laser "etching" for the Hello Board:

PCB Fabrication - second attempt !

Ok, after the failure I went back to the milling process! But I reused the laser test board (painted red) to take advantage of the isolation ink layer (let´s try it). So by the pictures you can see a cool red finishing in the milled board, of course I needed to scrap the ink prior soldering. Also made another one (Silver) as a backup in case I couldn’t use the painted version.

Exported the TOP Layer and the OUTLINE to PNG files and used the Fabmodules procedure (Week4) to generate the G-CODE. Opened in Roland VPanel, glued the board with double side tape, referenced the mill bit at the Board surface and executed the machining. Swipe the pictures below :