Soldering on keyboard controller circuit board
I had the same question a few weeks ago. I wanted to build a presentation footswitch controller - up / down / PgUp / PgDn - using an old Lenovo keyboard.

I found that the coating can be scraped off easily using a flat blade screwdriver taking care not to damage the copper. The material is like a powder coating and is hard but brittle. Careful tinning with a fine solder left me with a neat tinned edge connector.





Figure 1. (1) The Lenovo keyboard controller PCB. (2) The 'rows' membrane (18 lines). (3) The columns membrane (8 lines).

Examining the keyboard matrix I was able to determine that there were 26 'pins' split into 18 rows and 8 columns. Plugging the USB connector into my computer and running KeyboardTester.com's online app I was able, by carefully connecting rows and columns, to find the matrix points required for my application.



Figure 2. Partially filled out keyboard matrix. I had what I needed. ;^)

Be aware that since both rows and columns are scanned you won't be able to do a simple pull-down or pull-up arrangement from your micro to simulate a keypress.