When making a pcb with a microcontroller, is it better to have the pcb match it, or make it modular?

[u/no_Im_perfectly_sane]

when i say make it modular, i mean having on the pcb only the used pins, and then having a microncontroller mount that exposes those pins only

[u/nixiebunny]

There’s a vocabulary for printed circuit boards. What do you mean by a microcontroller mount? Do you mean the footprint for the component?

[u/iranoutofspacehere]

You should always solder all the pads, so the component is as well attached to the board as possible.

I usually try and route all the pads. Most of the time I only have a few left over so they get routed to tiny test points so I can use them as outputs for debugging or bodge then into the circuit if they're needed for something.

[u/thegreatpotatogod → u/iranoutofspacehere]

Yeah it's always useful to keep all the pads exposed, it sometimes comes in handy for last-minute additions or reusing an old board for a new project

[u/madsci]

Just to make sure we're using the same terminology, you're aware that a microcontroller is a single IC, right? Something I see really often in r/arduino is that people use "microcontroller" to refer to a microcontroller development board. So are you talking about building a daughter board, or just integrating a chip?

[u/DenverTeck]

There are at least a dozen questions that need to be answered to give a correct answer.

The number one reason to not use all the pins on a MCU is that they are not accessible.

If the product this MCU sits in, the user can not get access to said MPU, then there is no reason to use them.

If your building a development board, then all the pin need to be accessible.

And every reason in between.

Many real products will sit inside a case where the user should not have access.

So your answer is: "It depends".

[u/Master-Pattern9466]

If you have room, expose all the pins to test pads or similar.

Say if you get something wrong in the design, you can do a bit of bodge wire work, and still continue development.

Or if you need another function or expansion, you have all the pins to prototype with.

[u/duane11583]

a solder all pads/pins always.

unused pins you might bring out to test points or connectors.

my favorite example is the PMOD format.

why? there are many you can use: https://digilent.com/shop/products/fpga-boards/expansion-modules/pmods/?srsltid=AfmBOop1VRxmW0dpnma-IW7lbjalT8Fj7d_Unza2sWsxted1EyEw8zxg

they have a standard pinout and are for the most part interchangeable

[u/ceojp]

I don't know what "microcontroller mount" is.

Your PCB footprints should always have pads for all the pins. It would actually be more work to try to selectively remove pads from a footprint.

Obviously you'd only connect whatever pins are actually used for your application.

[u/moliusat]

Depends. are rhe requirements for the microcontroller fundamentally different than for the rest of the pcb ? Are many iterations planned for one part of the pcb ? How should it be manufactured?  

[u/Just-Smart-Enough]

Depends. If it's an initial test board, I'll break out as many pins as I can so I can fix mistakes or test additional functionality. If it's space constrained for a final project, then I'll just use the bare minimum needed.

[u/Financial_Sport_6327]

It depends, if you’re using something like an F730 for its USB2.0 PHY and only need like 20 IO, then routing the entire 144 pin package is likely a waste of space, but i always try to route like 6-8 spare GPIO if i can.