develop a 'PCB design guidelines' doc. to encourage consistency and manufacturability

[gbulmer]

Urgency: Revision 2.
Artefacts impacted: ukmarsbot-brd-YYYYMMDDx.pdf and line_sensor_1p1_brd.pdf and their underlying PCB ECAD

It is unclear from the renders that both PCBs make the same assumptions about PCB manufacturing tolerances and sizes. These tolerances and sizes help identify candidate PCB manufacturers.

Further, some people may have the facilities to manufacture their own PCBs using etching or CNC displacement routing.

Making the tolerances generous, allows for a large set of manufacturers at lowest cost, and also helps those folks who might want to try DIY.

Hence it would be helpful to publish the guidelines for, eg:

• minimum track and space, and recommended track and space for power
• via hole and annulus sizes
• pad hole and annulus sizes
• drill sizes, especially mounting holes where a minimum number of different size fastener helps
• clearance between copper and board edge
• designators for Part Name, eg 'Rnn' for resistors, 'LEDnn' for LEDs, etc.
• orientation of Part Name, eg. all 'South to North', to make Part Names easier to read during assembly

These are guidelines, so if something needs to vary, just document it, and move on.

As a 'strawman proposal':

1. track and space: signals minimum 0.3mm, power minimum 0.5mm,
2. clearance copper to board edge 1mm,
3. smallest drill 0.5mm.

Using the fewest different drill sizes makes it easier for someone to DIY the PCB.

Guidelines don't need to be completed before making further progress. Whatever is written would help ensure some consistency, so it might be worth writing something 'quick and dirty' soon to assist in getting some consistency among PCBs, and add to it as needed.

(Side Note: Once the PCB guidelines exist at all, it might be worth adding it's identifier to the PCB so future generations can understand why the PCB is the way it is.)

Not essential, but desirable: try to make the PCBs look consistent. So if corners are chamfered on the ukmarsBot PCB, all PCBs should follow that, where it makes sense.

Side Comment: I prefer bends in tracks to be made from multiple 45º angles, and not curves. This is both practical and aesthetic. Practically, straight lines and 45º bends take least room. In my experience straight line tracks are easiest to edit and change, but that may be a function of my ECAD program. Also AFAIK, straight lines create the smallest aperture in the ground plane which is 'a good thing'. (Others members likely know more than me about this, so I will defer to their judgement.)

[gbulmer]

Having raised the issue, I want to add, there are arguments for ignoring 'DIY PCBs' and focusing only on PCB manufacturers. Especially now. Probably beyond Revision 2. This is even more so because PCBs are very cheap, and there will be some available from UKMARS.

Hence tolerances and sizes don't need to allow for 'DIY PCBs'. If this becomes a requirement, it can be dealt with at that time when the constraints would be concrete.

However, 'PCB design guidelines' would help achieve consistency, manufacturability and use.