What to call the PCB that a daughterboard is plugged into?

Question

I'm looking for a concise term to describe something.

Suppose I'm making a gizmo which has application-specific circuitry but also requires substantial computing power. Not wanting to reinvent the wheel, I decide to incorporate a pre-made board, call it PCB "B", which is a single-board computer like an Udoo x86 or Raspberry Pi, for said computing power.

I design a larger PCB, call it PCB "A", which has "B" attach as a mezzanine board via pin headers, stand-offs, etc. PCB "A" has the application-specific circuits and maybe other stuff such as a power supply, connectors to the outside world, etc.

What exactly do you call PCB "A"?

Once upon a time, the physical relationship of "A" and "B" would have qualified "A" as a motherboard. Problem is, since the ubiquity of PCs, that term now carries specific connotations; in particular, it would tend to imply the CPU and computer chipset are on "A" when really they're on "B".

Mainboard has the same problem, as it's generally understood as a synonym of motherboard.

Backplane is not really applicable either, because "A" is not just a bus interconnection board.

Various SBCs have their own terms like shield or cape to describe daughter cards, but while "A" may use some of the same pin headers, using these terms to describe "A" seems to misrepresent it as being diminutive to "B".

Is there some generally understood term for PCB "A" that doesn't carry the wrong connotation?

(For lack of anything better I might call it the "application board", but I prefer to use standard terminology where possible.)

Answer 1

What exactly do you call PCB "A"?

A motherboard and the PCB that mounts on it is called a daughterboard.

I hear what you say about the implication of it being mistaken for a PC motherboard but it still gets called a motherboard in my book. PC motherboards have not cornered the exclusive use of the term.

Answer 2

In the case of computing you have a motherboard which as you say contain the main brains of the computer, and then daughterboards which tend to contain peripherals or other interfaces.

In your case however you have things the other way around. You have the brains on a small module ("B"), and then the interfaces and peripherals on the larger board ("A"). In this setup I would actually not go for daughterboard/motherboard, but rather "Compute Module" (or SoM/"System on Module"), and "Baseboard" or "Carrier Board". Your "B" board would be the module, and your "A" board would be the baseboard.

This is actually quite a common arrangement for things like FPGA/SoC modules where you have a baseboard that has various interfaces and connections, and then interchangeable modules which simply break out the FPGA pins to useful connectors and have the various power supply gubbins on them.

Answer 3

These are the terms I use or used. Motherboard: mainboard, carrier board, primary board, or by function/application like processor board, FPGA board, memory board. Daughter board: secondary board, or module 1 or 2 of more than one, subcircuit, application board like transmitter/receiver board.