Large PCB for keyboard - rigidity problem - Page 1
Forum section: electronics / projects

author: tester43
author_rank: Regular Contributor
timestamp: May 16, 2021, 04:24:44 pm
content: 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 pcb2. FR4 main PCB with ribs made of aluminum + maximum copper left on the pcb3. 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?

author: Ian.M
author_rank: Super Contributor
timestamp: May 16, 2021, 04:48:23 pm
content: Quote from: tester43 on May 16, 2021, 04:24:44 pmYou 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.

author: fcb
author_rank: Super Contributor
timestamp: May 16, 2021, 04:56:40 pm
content: 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?

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: May 16, 2021, 04:59:03 pm
content: 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).

author: technix
author_rank: Super Contributor
timestamp: May 16, 2021, 05:42:00 pm
content: You can use that aluminum PCB, then instead of the large flex you can use 0-ohm SMT resistors as jumpers.

author: tester43
author_rank: Regular Contributor
timestamp: May 16, 2021, 05:46:56 pm
content: Hi fcb, IanM, radar_macgyver,thank you for the support here.Following the responses:@FCB: I think I could.... It would be maximum several mm. i.e. 5mm. But in general it takes much space. It's similar to the first simple idea to screw the U shaped aluminum profile on the back. Still considering this. Probably will have to make an experiment.@IanM: Alternative would be to do the same with _a_lot_of_solder_points. I could use the FR4 based rib oriented at 90 degree, soldered at ... 20 points. I understand your point with solder being point of failure, but epoxy is really a point where I dont feel competent or "i know what I'm doing" area.@Radar_macgyver - problem with backplate is that it goes on the back of PCB directly. I will have leads soldered and sticking out on the reverse. BUT: I could make a very thick pcb made from aluminum. That's because I could design a backing plate as PCB and have all leads safely placed in the holes of the aluminum. Simply saying: aluminum plate with holes drilled. Holes would be large enough to safely avoid any contact with solder points.I guess we come to the guestion: Main working PCB from 1.6 PCB to be accepted by the enclosure and aluminum backplate from for example 2mm aluminum. 2mm is the moment where cost starts to become a main factor.Would you say there is a chance?

author: tester43
author_rank: Regular Contributor
timestamp: May 16, 2021, 05:48:42 pm
content: Hello Technix,I was thinking about it. Aluminum and something to go "over the traces". That's correct and should work. I am limited with 1.6mm layer. Not sure if it's rigid. I simply didn't have it in hand ever before... From experience I would guess that 1.6mm alone is quite flexible...edit: I could actually even use THT resistors soldered to pads. Then I would have large freedem in length of connection.... Interesting.edit2: stupid me, as I wrote in the first post, in order to be "elegant" I could use the flex pcb soldered to the back of the aluminum plate. It would deliver necessary traces and look good

author: technix
author_rank: Super Contributor
timestamp: May 16, 2021, 06:05:02 pm
content: Can you have a thick frame that is not going to the edges of the PCB? If so, just put a steel frame in there and screw the PCB in there.

author: tester43
author_rank: Regular Contributor
timestamp: May 16, 2021, 06:26:42 pm
content: steel frame would be ideal, but.... I dont have it and manufacturing would explode the cost.I am thinking about getting somewhere the U shaped aluminum profile and putting that on the edge of pcb.I think it comes down to the question what's more rigid:1. 1.6mm FR4 + 2mm Aluminum backing, or2. 1.6mm Aluminum + 1.6mm (or 2mm) FR4 backing.3. 1.6mm FR4 + 1.6mm FR4Can't combine Alu with Alu as it would short cicuit everything.

author: phil from seattle
author_rank: Super Contributor
timestamp: May 16, 2021, 09:02:54 pm
content: I have looked in many keyboards - from that era to modern day. None of them relied on a truly rigid PCB. Most had the rear of the keyboard case provide support for the PCB. An old IBM PC keyboard I took apart, iirc, had rubber shock mounts between the PCB and the case. It was a fantastic keyboard. A more modern one had a ribbing pattern in the back case that the PCB lay on. I am guessing from memory but I think a span between 75-125mm was common. I would redesign your case to meet the requirements rather than try to design the PCB to fit something you have. Design from the outside in, not the inside out.

author: Someone
author_rank: Super Contributor
timestamp: May 16, 2021, 09:48:22 pm
content: Quote from: phil from seattle on May 16, 2021, 09:02:54 pmAlso:Quote from: tester43 on May 16, 2021, 04:24:44 pmHow did the original design resolve this?

author: tester43
author_rank: Regular Contributor
timestamp: May 16, 2021, 10:16:58 pm
content: hi,original design is based on membrane buttons. Membrane is sandwitched between steel plate (back) and plastic chassis (facing up). Buttons are on top of plastic chassis.So in short words: original design has it based on steel plate (with folded edge).

author: technix
author_rank: Super Contributor
timestamp: May 17, 2021, 02:09:32 am
content: You can sandwich a reduced-thickness regular PCB with a reduced-thickness metal backplate, for example a 1mm steel backplate + 0.6mm FR4 PCB. Then bolt them together using screws and nuts. (You can do AFAIK up to 4-layers with 0.6mm PCB.) This banks on modern material science giving us more rigidity in the steel alloy with reduced thickness came from AFAIK military technology, and modern electronics manufacturing giving us multilayer super-thin precision PCB came from smartphones.

author: tester43
author_rank: Regular Contributor
timestamp: May 17, 2021, 07:31:48 am
content: Not sure if you are serious. Anyway, I think I found a way to make it solid 2mm.With additional reinforcements and new supporting point, I think we should be able to make it usable.thread can be closed;

author: tooki
author_rank: Super Contributor
timestamp: May 18, 2021, 11:26:01 am
content: Of course he was serious, that’s a perfectly valid and affordable solution.FYI, threads here aren’t normally closed when the answer has been found.

author: tester43
author_rank: Regular Contributor
timestamp: May 18, 2021, 03:57:50 pm
content: hi - thanks - I was not sure, and in case the previous answer was "ironic" I wanted to stop the mocking.So you think that usage of several thiner layers will work better than single one?

author: tooki
author_rank: Super Contributor
timestamp: May 18, 2021, 10:47:21 pm
content: The point is that a steel plate is much stiffer than FR-4, so if you can take the standard 1.6mm PCB thickness and replace 1mm of that with steel, it will end up stiffer.

author: tooki
author_rank: Super Contributor
timestamp: May 18, 2021, 10:55:33 pm
content: Quote from: technix on May 17, 2021, 02:09:32 amThat’s simply untrue. Super thin multilayer boardsenabledthe modern smartphone, because they existed long before. Bear in mind that a lot of the surface mount stuff we think of as “modern” (because it became dominant in the last 20 years) first came out in the 60s, and could be found in consumer portable electronics by the early 80s (like in even the earliest Sony Handicam). Cameras have used insanely thin boards and flat flex for aaaaages.As for the thinness of layers, well before we had smartphones, we were making insane computer boards with up to 26 layers. They had to use thin layers since you didn’t want to end up with a PCB 5mm thick…

author: technix
author_rank: Super Contributor
timestamp: May 19, 2021, 03:14:03 am
content: Quote from: tooki on May 18, 2021, 10:55:33 pmI stand corrected. However AFAIK it is smartphones that drove the prices of those super-precise super-thin boards down to the point of being casually affordable.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 20, 2021, 03:17:39 am
content: TL:DR.A possible solution is to order multiple PCB's (batches are cheap) and then simply bolt two PCB's together with some distance holders between them. To make the total stiff, you need a lot of connections between the two PCB's. Use about 10 bolts or more.For distance holders, you can use strips of wood or plastic, or just use M4 nuts if your bolts are M3. You can also use M3 bolts and then screw on 2 nuts, then add the other PCB and a 3rd nut to make it complete.

author: Nominal Animal
author_rank: Super Contributor
timestamp: May 20, 2021, 06:25:32 am
content: As Doctorandus_P mentioned, atorsion boxdesign would work here.Essentially, the PCB would be the top skin of the torsion box. (Note that the rest of the box does not need to fill the volume under the PCB, it can be smaller).The bottom skin is the plastic enclosure in many mechanical keybords, but it can be just a structural sheet of whatever.Aeroplane wings, doors, and such use a honeycomb structure of something very lightweight (inside doors and even lightweight tabletops often have paper-based structures inside); and doors and such have skins on all sides, but to get the rigidity in this particular case, all you need is a fixed connection between the two skins. For example, a dozen screws through studs.The Mechanical Engineering subsection would be a better place to discuss exactly why such torsion box structures – which look very flimsy – are in fact very, very sturdy, but suffice it to say that for a human typing, even someone like me with sausage fingers who often uses excess pressure when excited or angry, would be well supported by an ordinary PCB backed by 5-8mm standoffs, and another PCB material. Such a structure is massively more rigid than just two PCBs on top of each others; the standoffs – the lightweight material between the skins – really do make the whole stronger/more rigid.If you want to experiment with this, take three sheets of paper, and use ordinary paper glue (stick or spray) to make a sheet-sized box. Inside, put a ribbon of paper, edge on, with just small tabs glued to the top and bottom surface. You'll be surprised how rigid and how much weight that structure can hold.

author: tszaboo
author_rank: Super Contributor
timestamp: May 20, 2021, 07:37:49 am
content: No need to reinvent the wheel.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: May 20, 2021, 08:37:31 am
content: Quote from: tooki on May 18, 2021, 10:55:33 pmAt the risks of going off topic, you can go like <50um dielectric thickness per layer, combined with 10 or 20um copper, that is <70 um per layer. So a 10-layer board can be <1 mm thick. If you go with polyimide or so dielectrics, you can go thinner still. (<20um?). Past that, you go to more chip-style manufacturing where you grow dielectrics, and then obviously you have different order-of-magnitude in terms of sizes.