PCB Trace spacings, and mounting holes

[Cam]

Hi all,

I'm designing a few PCBs, and there's a few fundamental things I realise I don't know/understand - hopefully someone can help.

How close to the edge of the PCB can my traces be? (in mm, or mil)? I can't seem to find a definitive answer, but I guess there must be a general rule? I've generally got between 2 and 3mm - is this enough?

Mounting holes: Firstly, can someone explain what people are doing/thinking when they have pads/vias right through the board, which are then (sometimes) used as earths? Why wouldn't I just have a hole, with no copper in sight? Secondly, how should I go about mounting a PCB in a rack (say, a 1u case)? What do people use? I've seen people using glue, and sticky plastic things, but I'd rather have it mounted properly (like a computer motherboard) - any tips?

I hope someone can help out!

Thanks

 

[JohnRoberts]

Hi all,

I'm designing a few PCBs, and there's a few fundamental things I realise I don't know/understand - hopefully someone can help.

How close to the edge of the PCB can my traces be? (in mm, or mil)? I can't seem to find a definitive answer, but I guess there must be a general rule? I've generally got between 2 and 3mm - is this enough?

It depends on the design rules supported by your board maker. The tolerance and even how the boards are cut apart. Over the decades I've lived with different rules. I've had boards that were nested together on panels with common edges, so one shear operation cut two board edges. The tolerance of the shear operation and mechanical stresses means tiny traces cannot be too close to board edges. Modern board houses often mill board edges provided better precisions and less mechanical stress.

Mounting holes: Firstly, can someone explain what people are doing/thinking when they have pads/vias right through the board, which are then (sometimes) used as earths? Why wouldn't I just have a hole, with no copper in sight? Secondly, how should I go about mounting a PCB in a rack (say, a 1u case)? What do people use? I've seen people using glue, and sticky plastic things, but I'd rather have it mounted properly (like a computer motherboard) - any tips?

I hope someone can help out!

Thanks

Double sided boards with plated through vias get plated all at the same time. Imagine a PCB sheet with copper on both sides and just holes drilled in it. Electrodes connect to top and bottom surfaces and all the holes get plated at the same time, wether they need it or not.

If you check with the PCB house that will make your PCB they should publish design specifications.

JR

 

[ruffrecords]

Here is a link to some general PCB layout guidelines. It does not answer your specifi question but it does ontain a lot of useful info.

https://server.ibfriedrich.com/wiki/ibfwikien/images/d/da/PCB_Layout_Tutorial_e.pdf

Cheers

Ian

 

[john12ax7]

There is no definitive limit,  lots of variables.  But board routing / cutout is the least accurate step.  So as a general rule keep all copper at least 10 mil from board edge, this includes traces,  copper pours,  ground planes,  etc.

Obviously more is better if you have the space for it,  and for critical traces,  high voltages, etc,  you would want even more.  Since you are already using 2-3 mm (80-120 mil) that is plenty for most applications.

 

[gyraf]

..distance from COMPONENT to board edge is determined by the needed clearance for the soldering jig/frame to hold the pcb into place while soldering.

Jakob E.