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I took apart a broken calculator to show my daughter how it functions. It had a small "traditional" FR-4 PCB mounted underneath the LCD glass, but the pushbutton area was printed on a paper-like substrate:

enter image description here

The paper is plasticized and shiny, but there are fibers present if it is torn. It has the equivalent of plated-through-holes. The conductors are thin, matte-back material that look similar to toner.

Where the paper meets the FR-4, the traces connect together with some sort of adhesive (thin glue or tape). In other words, the copper traces are covered by matching "toner" traces, and affixed somehow...

The calculator was purchased at a Dollar Store for, you guessed it, one dollar :)

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up vote 1 down vote accepted

This is a kind flexible PCB, and as Samuel mentioned, it probably uses PET as the substrate but many other options are available.

Flex PCBs can be manufactured with a range of materials, including Kapton and Epoxy composites

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It's called a membrane switch.

As for the material, Wikipedia says:

... a membrane switch is a circuit printed on PET or ITO. The ink used for screen printing is usually copper / silver / graphite filled and therefore conductive.

In this case it's a membrane keypad. Very commonly used in electronics that need to be cleaned (microwaves and medical devices) or made very cheaply (keyboards and calculators).

Though your dollar store calculator probably didn't buy it from them, 3M (at least) makes a version with paper in it. I don't doubt it's a method that has been used by a lot of other manufactures.

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I'd still go as far as to call it a PCB - in fact it's quite literally a printed circuit (board). It also does have components - switches :) – Tom Carpenter Feb 16 at 17:54
    
@TomCarpenter Yeah, it's only slightly different. But there are no components, no more than landing pads are a component, it requires a membrane overlay to make contact with the exposed traces. – Samuel Feb 16 at 17:56
    
@Samuel Given that you can manufacture resistors, capacitors, and inductors using nothing more than traces themselves for filters and such (en.wikipedia.org/wiki/Distributed_element_filter), I would say that there very well could be circuit components on such a printed sheet. (Of course, dealing with the effects of flexibility would probably be a bit of an issue, as well as the frequency limitations.) – JAB Feb 16 at 20:07
    
@JAB As I said, "can't have any components mounted to it". A trace itself counts as a component in your definition, which makes it all a bit meaningless to point out. – Samuel Feb 16 at 20:11
    
It can have components on it, I once had a keyboard and the controller chip was sitting on it, pins bend and hold onto the membrane with a plastic bracket – PlasmaHH Feb 16 at 21:50

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