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I'm hacking a pair of active-shutter 3D glasses. Leading to each lens is a flex PCB with 2 traces. I'd like to connect into those traces about 5 mm away from the lenses. How can I go about making a connection into them?
I suggest neither melting nor crimping because the copper is way to thin and fragile (unlike magnet wire, where you melt away the insulation - and unlike a comparatively massive stranded or solid wire used for crimping, the 17, 35 or 70 µm Cu layer on flex "boards" is just way too thin for either method!).
Flex boards are usually composed of a polyimide (PI) substrate below and a coverlayer above the etched copper traces:
xxxxxxxxxxxx coverlayer or flexible soldermask
------------ Cu traces (thin!)
============ PI substrate
PI is heat resistant and as a base material for copper traces to be soldered on, is about as good as good ol' FR4.
Try finding out on which side you have the coverlayer, use something sharp to scratch it off, and try to solder onto the copper that remains on the PI substrate. The best way would be to clean away the coverlayer (or flex soldermask) and put the thing into a ZIF socket, because solder blobs on a flex "board" might not last long under mechanical stress - but I guess for a hobbyist's approach, soldering might do.
I've never done this so this is a idea only. I would probably start by carefully scraping or abrading away the plastic over the conducting copper. Once you have bare copper, soldering to it should be easy enough.
I have a number of times scraped away the soldermask over a copper trace to eventually break the trace or just connect to it. Soldermask is more ridgid and brittle than the plastic of a flex PCB, so I expect scraping the flex PCB material will be harder, which will make it easier to accidentally go too far and scrape right thru the copper too. Still this is what I'd probably try first.
Experimenting and practicing on a large conductive area or on a spare flex PCB sounds like a good idea. I expect there's a technique to this that won't be apparent until a mistake or two.
