2017-04-16 Designing the PCB

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I'm using Fritzing by the way. I like its ability to describe physically the breadboard, so when capturing schematics, I first capture the breadboard, and then arrange the schematics. This is one more step than capturing schematics (or say half a step, since components and relations between them are captured already), but I found it less error prone than going directly from the breadboard to the schematics. Furthermore, laying out the schematics is an additional chance to catch a missing resistor or two.


General rules

I'm using a 2-layers PCB.

I'm placing the power close to the center of the board, with a "star" topology to distribute power to the various components.

The power is as much as possible on the bottom face, using 48mil traces, with everything else on the top side as much as possible. No 90º angle for signal, and as little as possible for power.

The most controversial of my rules among my fellow electronic engineers is that ground is a circular bus around the PCB (this way, you're never too far from ground). This may be seen as an antenna, but this is mostly "constant" current, or at least not pulsed.

Missing silkscreen / PCB files

I had to adapt several Fritizing files and create other ones: Adafruit GPS and Micro-SD breakout board, a chinese BMP 180 breakout board, a resistor array, the Rockblock 1 satellite modem and even a BS 270 MOSFET Transistor !

Fritzing use SVG files that I create or modified with inkscape. The process is usually quick and handy when you've done it once.

Adafruit stuff

Adafruit published Fritzing files for its breakouts, however I needed the position of the screw holes to attach them firmly to the motherboard, so I just created a PCB silkscreen part with the external footprint, screw holes positions and a line to align my footprint with the connector in Adafruit files (thus I didn't modified Adafruit files, I just created an additional silkscreen aligned with the original file in the PCB view).

The detailed dimensions are published by Adafruit, so it's very easy. Only the diameter of the screw hole in the breakout board is missing, by I took it from the GPS description, assuming (quite safely I think) they are not using many different dimensions.

The resistor array

There is none in Fritzing installation (as far as I can tell), and I was unable to find one, so I created all the views (breadboard, schematics and PCB) from scratch, the breadboard being the more difficult for me, since it's the more visual, and I'm the worst person in the world when it comes to drawing.

BS 270

The TO-92 N-MOSFET in Fritzing has a wiring different from the BS 270 I'm using, so I had to create a new one from the one provided, which only took a couple of minutes.

The Rockblock 1

Although it's connected with a ribbon cable, it's screwed on the PCB for mechanical simplicity, so again I created a silkscreen footprint by using the dimensions provided in the technical documentation.

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