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I have a board that I want to remove the existing microcontroller and apply my own. I'll be swapping out one brand of microcontroller and putting in something completely different and so I intend to make a small PCB with the required micro and interface, but I need a way of breaking out the old microcontroller pins to the new board.

Basically, I want the reverse of this:

https://www.raypcb.com/things-to-make-sure-for-having-awesome-qfn-pcb-pad-design/

Here you would mount a SMD component and break out the pins you want, but now imagine you have the SMD pads available to you and you need to interface them elsewhere.

We are talking about 56 pins in a 8mm x 8mm square arrangement, 14 pins each side:

dimensions

Remove this and use the pads to connect to my PCB

My main intention is to NOT desolder the rest of the board (~50 mechanical keys), instead remove this and interface with what is already there and keep the rigidity of the original board.

Desoldering the keys I am concerned I will damage some. If it wasn't for this fact I would just design my entire board, and transpose the pieces from one to the other, however, I am not confident of not being able to remove all components safely.

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    \$\begingroup\$ That's unlikely to be very viable mechanically. What is the original product? Can you come up with a custom firmware for the original MCU that makes it report key events over some simple 2 wire channel, if it does not already function as some sort of USB keyboard that you could host on something else? \$\endgroup\$ – Chris Stratton Aug 14 at 2:23
  • \$\begingroup\$ The part in the picture is a Cypress PSoC. Reprogramming it isn't trivial (no XRES pin), but will still be easier than reworking the board to install a completely different part. \$\endgroup\$ – duskwuff Aug 14 at 3:05
  • \$\begingroup\$ @ChrisStratton that's what I fear (mechanical stability). What I was considering, was to make a PCB with an identical pad structure, inverting it and treating the PCB as a SMD component itself, ie reflow the PCB to the PCBin the same way you would a SMD to a PCB. The reverse layer would then have breakouts for the pins. I am very much a noob at this and consider this may be a far fetched or impossible dream proposal/idea. I figure, I can only ask the question. \$\endgroup\$ – Madivad Aug 14 at 3:30
  • \$\begingroup\$ @duskwuff yes it is, and unfortunately it's not the 24894 which does have a XRES pin. However it does have a power cycle mode of programming, discussed from 3.2 (pg 11) in cypress.com/file/42201/download \$\endgroup\$ – Madivad Aug 14 at 3:32
  • \$\begingroup\$ working with a toolchain I do not understand at all (and the fact that it is windows only) was the impetus for the question in the first place. I am wanting to use a teensy or AVR to install QMK, all of which are supported and well documented. If I had a windows system, I'd consider attempting to develop for it natively, and I don't explicitly rule it out, but I don't really want to reinvent the wheel either. Also, I am using this as a learning exercise. I am wanting to build my own keyboard and thought by hacking this, it would be a good place to start. \$\endgroup\$ – Madivad Aug 14 at 3:38
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The thing you're looking for is called an interposer. It's a small PCB that adapts one footprint to another.

A number of companies specialize in them. Some links below...

https://www.advanced.com/products/package-conversion-adapters/qfp-adapters

http://www.isipkg.com/wp-content/uploads/2016/09/ISI-Obsolescence-Solutions.pdf

http://www.proto-advantage.com/store/interposer.php

That said, it may be cheaper to lay out the entire board for the micro of your choice.

Another approach is to use a DIP-based assembly with your device (e.g., Arduino Nano), and connect that to the QFP using magnet wire. It's tedious, but it will get the job done. Once you've stabilized your design then you can re-layout the board with confidence.

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You might be able to mount your smaller MCU on a castellated-via PCB as shown in the picture below. This requires that the pin-pitch on the original chip is not too small, so there is room for the plated holes.

Microchip mounted on castellated via PCB

Alternatively you could make a pcb with the original chips' footprint on the bottom side and your new chip on the top side, but it will be much harder to solder correctly or even position.

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