Best way to trace existing PCB (not for piracy!)

[daxliniere]

Hey all,
The membrane keypad on my garage sale Mitutoyo DRO has some breaks. Discontinued decades ago and replacement part (if available) will likely be 5-7x more than I paid for the whole DRO (which otherwise works).
I attempted several different micro-surgery methods to no avail - the tracks are held to the plastic film with wishes and unicorn tears.

So, I figured I would make a PCB with physical switches (low-profile tactile pushbuttons) to replace it. I've scanned the back of the membrane keypad and would love some advice as to what is the best way to clean it up and get it into a format for JLCPCB/PCBway/etc, please.

I've attached what I've done so far. (The PNG is sourced from am SVG file, which I had to 7-zip up to attach to this forum.)


Thank you in advance,

Dax Liniere.

[ataradov]

The easiest would be to measure the locations of all the buttons and recreate the board in a CAD. KiCad is free and would do the trick. There is no good way to get manufacturing files from a picture.

Don't bother with the film, make a regular PCB with a connector and use a regular ribbon cable.

[daxliniere]

Thanks, ataradov, but where there is a will, there is a way.

Yep, I'm not evening thinking of another membrane/flexible 'board'. 

[ataradov]

If you are familiar with any CAD tool, it would take a couple of hours at most to recreate this.

If you are not familiar - this is a good opportunity to learn. All you would need is create one button footprint. You won't even need to make the schematic, you can just layout the board.

[daxliniere]

Yes, this is exactly what I'm asking: What is the best tool to use to trace over this? I have no desire to draw up the schematic as it's irrelevant to manufacture.
Should I do this in CAD/F360 or should I be using a dedicated PCB design tool? I will eventually need Gerber files, I'm sure.

[ataradov]

And you can also attempt to add a real connector to this board. Details would depend on the mechanical design of the case. You can cut the existing cable, mount a connector (most likely using epoxy), and use thin wires to restore the traces. It might work.

[ataradov]

The best way is to just recreate traces by eye. Once you have locations of the buttons, routing is trivial to copy by simply looking at it. And locations of the buttons can just be entered as numeric values by hand. There is no need to do overlays or anything like this.

If you are not already familiar with any CAD tools, then use KiCad.

[daxliniere]

Hey Alex, thanks for your replies.

I am familiar with CAD, but KiCAD seems like as good a place as any to start with this process.

The original ribbon connector is unsolderable, even with very careful surgery, flux and a low soldering iron. I tried connecting in an IDC kinda way, but connection was sketchy at best. This is why I concluded the traces couldn't be repaired.
My plan was to see what connectors JLCPCB has available and source one, plus a double-ended ribbon cable to match.

As for fitting it into the case, the tact switches will be mounted directly behind the existing membrane switches, so it's effectively invisible from the outside. The only consideration is that the rows of 7-seg LEDs will sit a bit further back to accommodate the switch PCB.

I'm going to give KiCAD a try and see how far I get with it.

Thanks again and all the best,
Dax.

[ataradov]

It is not really clear from the pictures. It looks like the PCB just has the standard exposed pads for the rubber buttons and the actual buttons have a conductive rubber pad that shorts the terminals. Why not recreate the same? Why use buttons?

If the keyboard is actually membrane and contains the conductive part too, then I would still go with the membrane buttons, just the metal ones. They are readily available in different sizes https://www.aliexpress.com/item/3256801263748081.html They would have comparable thickness, so should not affect the mechanical fit.

I also would not bother with JLCPCB assembly. There is nothing to "assemble" here.

My idea with the connector was to mechanically fix it to the board, then  this part would be obviously solderable. Then run thin wires parallel to the existing tracks and use some conductive glue, no heat is necessary on that side. It is very hard to tell if this would work without looking at the PCB closely.

[andy3055]

Can you use conductive touch up paint and repair the broken traces?

[ataradov]

There is too much damage for the paint alone. Using thin wires to duplicate long sections of damage may help. But that paint in general is pretty hit or miss even on much less damaged sections.

[daxliniere]

Hey Alex. Actually, you've given me an idea. I tried and was successful at removing the flexible PCB from the front 'sticker' that has the conductive pads and metal dome 'snap' switches.

I wonder if I can use the flex PCB as a UV mask to etch my own board?
One potential issue is that the flex PCB has a kind of conductive paint applied to the contact fingers.

[ataradov]

There is no way to use that as a mask. Just design a PCB with the same routing, order it in 0.8 mm without solder mask and you will get the same functionality with a minor increase of thickness. But the domes would make contact with the PCB at the same level, so functionality would not be affected.

[ataradov]

Another option I would consider is hand wiring a PCB using low profile buttons (like this https://www.aliexpress.com/item/3256801553660538.html) and either a prototype PCB or just a plain copper clad board. This would require some effort, so I'm not sure it is worth it.

For the board of this size with gold finish PCBWay quotes  $100 (5 pcs). and with a HASL finish it is around $60.

[daxliniere]

There is no way to use that as a mask.

Hold my beer...
Seriously though, let me teach you something really cool:

Another option I would consider is hand wiring a PCB using low profile buttons (like this https://www.aliexpress.com/item/3256801553660538.html)...

Dude, that's what I said I was going to do in my very first post.

So, I figured I would make a PCB with physical switches (low-profile tactile pushbuttons) to replace it.

[ataradov]

Seriously though, let me teach you something really cool

I've started making PCBs when ordering them for $5 was not a thing. I'm aware how PCBs are made. If you think you can do - go ahead. I'm telling you, it is not as easy as it seems. Also, you would spend more money on chemicals and equipment than just ordering a professionally made PCB.

There is also no need for any of this UV stuff. This board is simple enough that it can be made using printer and iron method. Way easier, way cheaper.

Dude, that's what I said I was going to do in my very first post.

From your first post is was not clear how that keyboard is designed. With the most recent pictures it is much more clear how it is actually designed.

And again, I would only go for the switches if you are making it on a protoboard with hand wiring. If you are ordering a PCB, go with the original pattern and use the original buttons. If you want to get fancy, make the button mesh footprint be compatible with the physical button.

With those switches I would also worry about plastic wear. The cap on the switch is pretty small, and over time it would imprint on the plastic and potentially break through. So, I would also add some sort of hard baking to the plastic domes if you go that route.

[tooki]

There is no way to use that as a mask.

Hold my beer...
Seriously though, let me teach you something really cool:

Yyyyeah... I'm pretty sure he's WELL aware of how DIY etching works, bro.

Another option I would consider is hand wiring a PCB using low profile buttons (like this https://www.aliexpress.com/item/3256801553660538.html)...

Dude, that's what I said I was going to do in my very first post.

So, I figured I would make a PCB with physical switches (low-profile tactile pushbuttons) to replace it.

No, he doesn't mean the same thing. You're talking about having a custom PCB made, and he clearly understood that based on his many replies before that one. When he offers the alternative of "hand wiring", he means NOT using a custom PCB, but rather generic perfboard or similar, and actually manually running actual wires to the buttons.

[SiliconWizard]

If you really want to make a PCB out of an image and it's single layer, and you're not too keen on recreating from scratch, wouldn't the following tool help?

https://www.eevblog.com/forum/eda/i-made-an-svg-vector-graphics-to-gerber-converter/

Not tried nor have any opinion on this, but you can contact the author.

[tooki]

Yes, this is exactly what I'm asking: What is the best tool to use to trace over this? I have no desire to draw up the schematic as it's irrelevant to manufacture.

Unless you have zero interest in ever designing a PCB now or in the future, then you absolutely should draw the schematic first. Schematic->PCB layout->Gerber->Manufacturing is THE process for making PCBs, so a) it's the process the software is designed around, and b) doing the schematic means you CAN'T wire the PCB wrong. I cannot underscore enough how important (b) is! So if you have even the slightest interest in electronics, learn the process, you'll be doing yourself a favor.

P.S. I'm not talking hypothetically about the advantages. At work, a fellow apprentice was tasked with copying some PCB layouts (for practice boards) . I didn't see the result till much later when we needed to change something. Oh. My. God.  Turns out, he'd done it without first creating schematics, and thus had to "creatively" undermine the layout software every step of the way to get the job done. All of the mechanisms in the software that are there to prevent errors simply don't want you connecting "random" copper, which is exactly what it is without a schematic. It would have been far better for his learning, as well as for future project maintenance, to do a schematic first.

[daxliniere]

doing the schematic means you CAN'T wire the PCB wrong.

Do you know what else guarantees you can't wire it wrong? Using the original transparent flex PCB as the photo-positive mask!

[ataradov]

Do you know what else guarantees you can't wire it wrong? Using the original transparent flex PCB as the photo-positive mask!

You can't use it directly because of the bridges. You need to remove those at least. Plus you need to check that this green material is UV transparent. It is highly likely that it is not.

But if you think you can do that, try it. It would be a good learning experience.

But the first issue you would need to solve is find a copper clad material in that size with pre-applied photoresist. This alone would likely be more expensive than a manufactured PCB. And chemicals involved in this process are highly corrosive and need to be handled and disposed with care. It is a huge headache, especially if you don't plan on making more PCBs yourself.

[tooki]

doing the schematic means you CAN'T wire the PCB wrong.

Do you know what else guarantees you can't wire it wrong? Using the original transparent flex PCB as the photo-positive mask!

Are you here to get answers to your question, or to be a smartass and try to lecture people more knowledgeable than you?

Your original question was:

So, I figured I would make a PCB with physical switches (low-profile tactile pushbuttons) to replace it. I've scanned the back of the membrane keypad and would love some advice as to what is the best way to clean it up and get it into a format for JLCPCB/PCBway/etc, please.

Making a replacement PCB for tactile switches rules out reusing the old membrane layout to begin with. So even if you could make an optical copy of the membrane traces onto a blank PCB, it wouldn't get you to your stated goal.

[daxliniere]

The stated goal was to get the keypad of the DRO operational again. The solution evolved throughout this thread as all good ideas do. (The design 'journey' has been documented here.)

Alex. Where on earth did you get the idea that it would not be UV transparent? Sure, some aren't. You seem to be set on your negative train-of-thought about this.
And who cares about the bridges? If you apply a moment of thought, you would know that they could easily be removed (and replaced with a back-of-board jumper wire).
And, and, and why does it need to be pre-applied photoresist film? (as if that's hard to find anyway...  )

In the end, it would take me a few hours to redraw the flex PCB. Having said that, by the time I drive to RS to get the positive-photoresist PCB and other stuff, it might be the same time as redrawing it and cutting on my CNC mill.

See? Evolution of the solution.

[ataradov]

Sure, go ahead. I don't really care what you do.

If you have a CNC mill, then it would probably work as well.

[nctnico]

Or just buy a brand new replacement for less than $100:
https://www.ebay.com/itm/363797941713

Otherwise I'd design a regular PCB (create schematic and route the whole thing) but it will be hard to beat the price of the readily made keypad from the Ebay listing. And don't bother with making your own PCBs. Last week I got rid of all my UV exposure and related stuff. Too much hassle and poor results.