FWIW, during a build a couple of days ago I tried using strips of 0.188" thick Sorbothane between the plate and PCB, and found this to still be too much contact; it felt like too much pressure to bring the switch bottoms into contact with the PCB. I ended up thinning some of the strips with scissors to reduce the thickness by about a third, which, as you might imagine, was not exactly a precise operation. But it was close enough to get meaningful sound dampening. This experience got me wondering what the ‘ideal’ thickness might be for this application.
Working from the Cherry MX specs, there should be at most 3.5mm (0.138") of clearance between the bottom of a 1.5mm plate and the PCB. There are 0.125" sheets that appear on Amazon, but I’m not sure that’s quite thick enough to make good contact, and while I don’t have a good set of calipers, a quick straightedge measurement also suggests 0.125" isn’t quite enough.
Going deeper into the rabbit hole, this engineering guide suggests that a deflection of 10-20% is good for dampening with shape factors between 0.3-1.0. The shape factor is the ratio of loaded (dampened) surface area to unloaded surface area. As an approximation to the total surface area of a 60% plate, the dimensions of a GH60 PCB are 285mm x 94.6mm, which gives us a surface area of 41.8 sq in. As an approximation to unloaded surface area, we can use the combined footprint of 65 switches, which is 0.61" x 0.61" x 65 = 24.2 sq in. Loaded = total - unloaded = 41.8 - 24.2 = 17.6. This yields a shape factor of 17.6 / 24.2 = 0.72. So, 10-20% deflection on a thickness of 0.138" suggests we want the Sorbothane filling for our plate/PCB sandwich to be somewhere in the 0.151"-0.166" range.
All of which is to say - at the moment, I’m less excited than I originally was about 0.18" precut plate/PCB sheets, but more excited about something like 0.155", assuming sheets of that thickness are feasible from both manufacturing and pricing perspectives.