Workflow advice please

I am new to using KiCad and I am trying to design a simple board to get me started.
I want to mount a small, completed, pcb on KiCad designed one, along with a small transformer, fuse and input connector. Both the existing and the new boards have simple mounting holes in their corners.
So far I have created a schematic of the connector, fuse and transformer and connected it to a new component that represents the completed pcb.
Should/can I place the mounting holes in the new component before adding to the schematic?
I wasn’t able to place a 3 way screw connector on the new component, is there a way or do I add it in the schematic?

It’s best to put everything in the schematic. I place mounting holes down in the corner of the drawing. I don’t think they even really need to be inside the border of the page frame.

IMHO, these should be handled separately…

For your new board, I’d follow @hermit’s advice and put your board’s mounting holes on the schematic. There are schematic symbols for mounting holes in the provided symbol libraries. There are also PCB footprints for mounting holes in the provided footprint libraries. You will need to choose which of the footprints to use (different sizes for different mounting hardware as well as different pad sizes/shapes) when assigning the footprint to the symbol in your schematic.

For the completed board that you are treating as a component I would make a custom footprint. First, start with the connector. If it is a standard connector that already exists as a footprint start there. Add to that connector the mounting holes for the module’s board. The datasheed for the module should give good dimensional information relating the mounting holes to the connector, or both to a common feature. This will lock the module’s mounting hole positions relative to the connector so you don’t need to remember to move the mounting holes if you need to nudge the connector over by 1mm.

(There is precedent, as some components (mostly connectors) have locating studs/pins that are intended to fit into small holes in the board to help keep the part aligned before and during the soldering process. The mounting holes for your module can be thought as analogous to the index holes for locating studs.)

I’m not sure I understand what you mean here. Is your module board connected to your main board with discrete wires that go into the 3-way screw connector? If so, this would be the connector that you add your mounting holes to.
Or are you wanting to add the 3-way screw connector to the module board? If so, that is out of scope of the schematic for your board and should be added to the schematic of the module board.

On second thought, I might be assuming your workflow incorrectly, or you may be assuming a tighter integration between schematic symbols and PCB footprints. In KiCAD, the association is loose. Symbol libraries are separate entities from footprint libraries. While you can assign a footprint to a symbol in the library, this isn’t necessary. It isn’t even necessary that you use the a pre-assigned footprint when you place a symbol in your schematic. But you do need to take care, the only checking that a footprint is appropriate to a symbol that KiCAD does is by pin number. Any 3-pin symbol can be assigned to any 3-pin footprint and KiCAD won’t complain. It’s up to you as a board designer to make sure that the 3-pin footprint is physically compatible with the desired end component.

I don’t think this is correct. I could find no schematic symbol for a PCB, after pressing a in eeschema and searching hole (none), mounting (none), connectors (no holes or mounting holes).

Mounting holes are a board attribute, so you should be able to place them in pcbnew. However, I could not find them in pcbnew either ! The closest footprint for a mounting I could find was a pad. A pad is okay if you want to connect nets at a mounting hole, or not.

Now if the designer does want to use a mounting hole to make an electrical connection, it might be desireable to have schematic symbols for mounting holes. But this is a special case, not a general case. Only on occasion does a circuit designer want to do this.

In Eschema Mechanical->Mountinghole

In PCBNew->MountingHole

All of them in the standard library since at least v5 …

I like the style of Sparkfun or Adafruit, where they put all the symbols in the schematic directly, so in the screenshot below for example, you have the fiducial and the mounting holes (and also the logos) in the comments.

Another example from Adafruit


Thanks everyone, some useful information to digest.
Perhaps I was a bit too brief with my construction details. The completed board (oldboard) is a ±15V linear PSU which has two holes/pads for wiring a trafo secondary and a its own 3 way screw connector for its output. This was the connector I wasn’t able to add to the new component.
The board I am designing (newboard) will have a Mains 3 way Input connector, a fuse and the small trafo. I have been forced into designing this pcb as there wasn’t room for my Veroboard lashup!

It seems the way to go is to put everything in the Schematic and then start assigning footprints and dimensions.

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Thank you, der.ule. I just learned that the drop-down menus don’t display everything that is available, and that fiducials can be placed independently of mounting holes. I like the Adafruit style too, even though it is not apparent to me without looking at the whole drawing how helpful the artwork can be.

If I understood you well you have mounting holes at your old PCB and you want to have at the same positions the mounting holes at your new motherboard PCB. In such case I would design footprint for your old PCB having holes in that footprint and not at the schematic. That way if you change position of your old PCB at your new PCB the holes will be always moved correctly with it.
But that is only if I understood you well…

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I learned that fiducials also have schematic symbols. (round circles) but was still mystified by the washing symbol with the cross in the middle.


got me to:

And it is an eagle project. Not KiCad.
So my closest match is still that you should not take an AD7280 to the dry cleaner.

Thanks again people.
Before starting with KiCAD I roughed out what I needed in my engineering CAD program.
So no, the mounting holes are not co-incident.
A picture is worth a thousand words !

I don’t know what is really needed but if I were doing something like that I would try to have as big distance between primary and secondary as possible - just to have it as good as possible.
My first thought is to rotate trafo so the 30V would be on the left.
My second thought is to swap trafo with the terminal block and fuse so trafo would be to the right and 30V to the right of it.

Since the PSU board already has its own 3-way screw connector, why do you need to add one for the output on your board? Doesn’t it come for “free” already mounted on the PSU board?

Or is this the 3-way that you are talking about? (BTW, your picture below only shows a 2-way screw terminal for your line input. I guess your design has changed slightly since that rough drawing.) I’m not sure what you plan on for the Earth wire, but if you want it to be your chassis ground then you can either use one of your mounting holes and ground the hole, or add another grounded mounting hole to use as your dedicated CGP instead of a mounting standoff.

Ok, it took this dumb American a bit to find out that trafo is German for transformer. I hadn’t come across that before, so thanks for the vocabulary lesson. :sunglasses: (BTW, I’m trying to be genuine here, not snide. Please don’t read any snark into my comment.)

I am guessing that they are basing the fiducials symbol on the “aligment target” still used in PCBnew


I adopted the symbol for my own library (and reduced the size) because I found the mounting hole and fiducial a bit ambiguous.


Yes to both points.
The Mains In connector was changed to a 3 way with an unused centre. Gives the correct creepage distance.
The NewBoard grew in width as I toyed with the CAD layout and I think I will now put the Mains In connector top LH corner lengthwise outside the mounting hole. Then the fuse right of the mounting hole, also vertical, then the trafo. I will play with my CAD again.

Yes it does, I was just trying to make it look right!

Sorry about the shorthand. We often say in England “the UK and US are Countries separated by a common language” :smiley:

This seems a better layout.

Or, just keep the same footprint placement, and only swap the two pins of the primary transformer winding…

I believe you need a 5mm clearance for 230Vac, and 8mm between mains voltage and things that can be touched. This is just a guideline, look up real data before making the layout.

I have always worked with 3mm creepage (L to N) for UK Mains, and tried to avoid sharp corners. So the KiCad version will use curved lines. (not so easy with my CAD program).
And a minimum of 5mm between Mains and low voltage (~20V) AC or DC.

I am afraid I don’t understand your suggestion, could you elaborate please?


There are zillions of mounting holes in eeschema - some with pads, some without; some that you can ground/connect, some you can’t. I agree its best (but not required) to put them in eeschema. I would also position the mechanical bits that must line up first and then design the traces and components around them. In platform designs, such as autos or backplane/chassis conformant electronics, these are called “hard points” since they are out of your control.

Enjoy - G