Has anyone done a survey of HP calculator keyboard technology from the HP-35 to the present? What lasts, what doesn't, and why?

Good suggestion. Interesting topic. Keyboards are the key to calculator success. From my reverse engineering:

Early Classics: gold galvanized metal strips (built as in the patent)

Late Classics: no gold, rest is the same

All Classics: keyboard can be disassembled with a screwdriver

HP21,...,27 metal strips. Keyboard can't be disassembled except
when heat-formed plastic domes are cut away

HP29C ? possibly (I guess !) the same sort of plastic snap
action contact foil as in early spices. My HP29C
feels like an early HP31E (stiffer keys, need more
force, longer key travel) and not like any of the
other HP2x. Its keyboard PCB also looks much
different.

Early Spices: Metal strips replaced by plastic snap-action foil
(one piece for all keys). Contacts probably some
sort of metal-filled paint that is printed on the
plastic.

Late Spices: The proven metal strips again.

All keyboard keys in all those models are hinged. They can't wobble,
greatly increasing life of the contact elements. A typical failure mode is a hairline crack in the metal strips just where they are
bent most when they snap. Those keys feel mushy but usually still work. The plastic key snap-action foil never lost the snap effect on any of my HPs, but some of those keys don't give any contact at all.
If the metal strip keyboards have bad contacts they usually can be cleaned and brought back to reliable operation.

TI calculators of the time had much thinner metal strips with a row of round discs. They were held in position by a self-adhesive plastic sheet. TI keys were not hinged and had a wobbly feel. Worse yet, the counter-contact was a round wire that ran down the keyboard column and was welded into a plastic keyboard backbone, giving a very tiny and unreliable contact area.

So far what I have found out. HP keyboards were the best. All other calculators were worse and their keyboards failed often, except for those who had tiny reed-switches, like the Olympia CD80, too expensive and without the tactile snap-feedback, but truly lasting forever - which HP keyboards didn't. It seems however that HP keyboards had just the right price, performance, including decades of daily use lifespan, and the right look and feel.

Regards,
Bernhard

Does any of You have information about the Voyager series keyboards? I have hoticed that the keys feels quite different on older and newer calculators. I don't think it's because of wear as all the keys feels the same except for the Enter key that is less "snappy" on both calculators.

On the voyagers, it is normal that the ENTER key feels different - it occupies twice the space of the other keys and the contact metal strip
is on the upper end - so the lever effect is lower. All of my voyagers
feel the same - and all of them are in daily service since more than
20 years and never had a keyboard contact failure (they use the reliable metal strip technology). But one of my HP15C suffered from hairline cracks in the thin foil which carries the chips, leading to dead keys - it wasn't the keyboard however, just the "printed circuit plastic foil" carrying the electronics. Later Voyagers had true printed circuit boards and should last longer. I don't blame HP to make calculators that fail after 20 years of daily service. For me, this is OK. But I'd like to be able to buy another one, with the same quality, for the next 20 years, the same machine, as I am used to it, and they don't give me that opportunity. So I will work myself through my HP calculator collection and use them, and wear them out, one after another, until the worms will eat me in, say, 40 or 50 years. And BTW, LED display HPs really are cool to work with.

Regards,
Bernhard

I have an 11c where two of the keys klicks not only when pressing, but also when releasing. Might that be a sign of a crack in the metal dome or is it normal?