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How would you connect a million+ analog pins? (Seriously)

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When I first started in electronics  (I'm still learning), I had high hopes of being able to fix stuff. People like Louis Rossmann would pull up PCB schematics, and fix stuff while Dave would look at a board and practically be able to tell you what everything is without a datasheet.

Then I found out that Louis purchased schematics, and Dave knows enough to identify most things, but doesn't necessarily how how they're wired up... sadness.

So, I went to look at the PCB factories to see how they tested their PCBs. They have big machines with lots of little wires that move and probe the boards. Now you can imagine that I thought that this was a really great idea, if only I could implement it. Which brings us to this post.

For some time I've been trying to figure out the basic electro-mechanical design. Recently, I finished working on it. And theoretically, it would work.

Now I'm faced with a new problem, how would I connect several million analog pins. They have to be analog, so that I can sense the resistance of the places that they are touching. I wish to probe both new and preexisting/populated PCBs. Even if I used 16pin-input muxes, I'm looking at an amount in the tens of millions of units for a PCB tester able to probe server size motherboards -- and I'd like to be able to do eATX at least.

Is my goal an impossibility? Never!

So, how would you connect a million+ analog pins?


The quick answer is not at the same time.  That may not be impossible, but it is impractical and un-necessary.  But there may be answers which meet your needs.

Think of crossbar arrays.  1000 rows and 1000 columns gives you your million points and while a much larger size than most crossbar arrays at least is a credibly accessible number.  Now the question is how long it takes to interrogate the fraction of those million connections that you need to do.  There will be a time constant driven by the capacitance and resistance of the wires, but it should be possible to do hundreds or more configurations per second, and each configuration can access up to 1000 unique points.  Non unique points may also be of use.  General purpose measurement instruments will struggle to make meaningful measurements at these speeds, but you can do things to get the data you need quickly.

Now comes the hard part.  1000 points per axis is at best marginally enough to sample a motherboard size PWB with modern trace sizes and spacing.  The high volume parts get a custom bed of nails made with pins placed appropriately to do the required tests.  Having to limit yourself to a configurable test configuration puts uncomfortable limits on some applications, but might be good for many things.

I don't really understand which problem you are trying to solve. Why do you need a million inputs?

Test fixures do exist. Usually you have a bunch of pogo pins connecting to dedicated test points on the PCB, those are of course part of the layout and considered during construction. That's good enough.

There really is no need to test every single via etc.

If you are trying to build some "auto testing fixture", it is probably best to use a 3D printer as a basis, and just use ground and one single pogo pin and compare measurements against reference to detect faults.

Each of those pogo pins need a certain amount of force to make a contact. The smaller ones need something like 100g for full actuation, they make contact at let's say 30g. For a million pogo pins, you need 30 million grams, or 30000KG placed on the PCB to make a contact.  That's 66138,679 pounds in freedom units, or 10 Tesla cybertrucks.
You design a board for each DUT, and use the minimal amount of pogo pins to make it work. Practical limit is a few hundred. Or drill holes in plexiglass or similar sheets to place the pins, and wire them by hand.


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