Mounting Nano/Pro-mini to pcb (as flat/low as possible)

I am making a custom pcb that will eventually just have the Nano/Pro-Mini connected to it (instead of having to design the whole 'Arduino compatible' circuit on-board,.. so to peak)..

I know may of you have great tips and awesome 'out of the box' ideas.. so I wanted to get some feedback from you guys..

  • How can you mount the Nano/Pro-Mini (havent decided yet on which will be used in the end).. as close the main (mother) board?

Pro-Mini's have nothing on the bottoms.. and can be almost 'flat/flush' on the pcb.. but how make/secure a connection to SMD pads?

SMD headers I guess?

  • I'd prefer a non-thru-hole solution.. (as the underside of the main/mother pcb will have leds on it.

** basically a circle pcb with one side having leds.. the back side will have the needed passives and what not.. and have the 'Arduino' plug right in.. or soldered directly mostly.. and getting it as 'flat/slim' as possible if the goal here.

Thanks for any suggestion,... thoughts....etc

I wonder if it would be possible to sand away the edges of the Arduino board to convert it to castellated?

There is a Nano derivative sold on Aliexpress that only has components on the top side.

You might be interested in the new official Nano boards, which have all components on the top side and are castellated:

So you want the lowest profile mounting of a ProMini to a mother PCB as possible!

Do you want the ProMini to be removable?

You could use a castellated ProMin.

You can make your own castellated ProMini by cutting/sanding off the PCB edges.

This is what I use:

A full-size sheet of medium grid sandpaper glued (PVA) to an equally big size of MDF lives under my work bench.
Slide the edges of circuit boards over it to remove sharp edges.
Could try that to make a castellated PCB.

Here is my out of the box idea. What parts of the Pro Mini are you using? If you won’t be actually using the small on-board regulator, or the power LED or pin 13 LED, there’s really nothing left except the ATmega328P and a resonator and a couple of capacitors. So if you’re making a PCB anyway, just put the traces on it that you need for whatever pins you’re using on the ATmega328P and include it in your design. The reason I suggest this is an Arduino is really nothing more than a breakout board that makes it easy to prototype designs that will use ATmega328P. But the ATmega328P is really your Arduino. Depending on what you’re doing, you may not even need the resonator, perhaps internal 8MHz is good enough for your project. In which case all you need is ATmega328P soldered to your PCB with a couple of 0.1uF decoupling capacitors.
I will admit I think these ideas about castellated Pro Minis and Nano are really nice.