I found out that soldering a 0.4mm Quad Flat No-leads package requires good eyes. I did the soldering wrong at first, and probably toasted the chip when testing it - at least it did not work after resoldering it under a microscope. I *knew* that I had two extra chips somewhere - but could not find them.
Thus I decided to go with a ready-made breakout board instead. Etched a new circuit board - and discovered that placing the electrodes behind a 6mm fibre cement board did not work even though I had tested it before doing the first printed circuit board.
New strategy: Create 12 "buttons" from a aluminum rod on a lathe. Use a long time figuring out how to solder leads to the aluminum. Drill holes in fiber board and mount buttons. Decide it looks ugly and scratch the concept.
Finally I gave up and bought a water proof stainless steel matrix keypad from Alibaba.com. It cost me 30 USD for the keypad, 30 USD shipping, and ~15USD in VAT, a total of 75 USD. It is more than I like, but it works! ;-) So now I am back to using a PCF8574 8-bit I/O expander for the keypad.