I disassembled a cheap digital tire pressure gauge that had quit working (leaking silver oxide cell, as I discovered). After deciding it was unrepairable I removed the circuit board, which had the LCD display on the back. It fell off and seemed to be completely unattached. There was a ribbed rubber strip between a plastic "step" on the the display and the row of contacts.
Here are some images of the LCD:
Edge view showing the "step" ↑
The rubber spacer ↑
I happened to view the display under bright light with a specular reflection from the plastic, and saw this:
Since the ribbing on the spacer runs "vertically" from the contacts on the PCB is that how power and signal are delivered to the faint, almost invisible traces on the display? I don't see any embedded wires in the rubber… is each rib designed to conduct along its length but be insulated from its neighbors? The spacer looks and behaves just like a simple piece of rubber.
What is this technology (both the "spacer" and the invisible traces on the LCD) called?
It's called "zebra strip", and yes, the black areas are conductive while the white areas are insulating. The strips do not need to line up one-for-one with the contacts on the board and the glass, as long as there is at least one for each connection (usually several) and there's enough space between the contacts so that they don't get bridged by a black zone.