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Large PCB for keyboard - rigidity problem

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tester43:
Hello,

Project that I do: rebuilding the keyboard for the retro computer. Keyboard is similar to normal PC.
It's a large PCB: 46cm x 15cm;
Limitation is on the thickness of the PCB. Edges are supposed to go into enclosure slots which are narrow, around 1.5mm max. Max of max thickness, abusing the plastic a bit is 2mm.
I plan to use PCB, double layer is needed.
It will bend when pressed <- Not good.

What I am thinking is to use of the reinforcement ribs (concept drawing attached) to help with the rigidity of the construction. Ribs would go from max left to max right as reinforcement. Soldered on the edge in several points.
Materials of PCB that we have are FR4 (2 layer) and aluminum (1 layer limit).

I recognized following options:
1. FR4 main PCB with ribs made of FR4 + maximum copper left on the pcb
2. FR4 main PCB with ribs made of aluminum + maximum copper left on the pcb
3. Single layer aluminum PCB (1.6mm?) with second layer in form of _very_ large flex soldered on its back to create necessary connections.

what do you guys think?
How can I create a PCB that will not bend when pressed on the key?

Ian.M:

--- Quote from: tester43 on May 16, 2021, 04:24:44 pm ---How can I create a PCB that will not bend when pressed on the key?

--- End quote ---
You cant easily for reasonable cost.  Soft solder isn't a reliable method of attachment when significant mechanical sheer or tension forces are involved, so unsupported soldered ribs are doomed to fail. 

You could epoxy together FR4 PCB and ribs with tab and slot joints left free of copper, soldermask and silk screen around them so you could get a direct epoxy to FR4 bond.

Another possibility would be a stiff backplate with multiple standoff pillars spaced so the deflection between any two is negligible, and screws into them through close-fitting holes in the PCB forming a girder structure.  The backplate could even be an identical unpopulated PCB.

Consider some sort of frame to support the PCB, possibly 3D printed or laser cut, and glued to a sheet of the same type of plastic.

fcb:
Can you arrange the ribs at right-angles to the main PCB. Use slots in the main PCB and tangs in the rib PCB's - bit like one of those plywood dinosaurs?

radar_macgyver:
Mechanical keyboards often use a steel backing plate (~1 mm) to stiffen the keys. The plate must have a bend on it to provide stiffness along the long axis, and is attached with screws (not soldered). The keyswitches also have a collar that transfers any downward force onto the plate, preventing the soldered joints from getting stressed. Aluminum PCBs are still quite bendy when made into larger areas. I have some COB LEDs strips that will bend quite easily (and also damage the wire bonds).

technix:
You can use that aluminum PCB, then instead of the large flex you can use 0-ohm SMT resistors as jumpers.

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