Flex PCB

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I have a 1oz flex PCB (FPC) that has 2 0.4mm pitch connectors that need to be SMT and 9 4.5×1.0mm pads that have conductive rubber contacts that are press-fitted onto the flex PCB. The flex PCB has a one-time bend radius of 15.5mm. The image is attached that shows the rubber Contact over the flex PCB Pad.

Right now, the flex PCB finish is ENIG: Nickel thickness is 2-4um (78-156uin) and gold thickness is 0.03-0.05um (1.18-2uin). There is potential for saltwater to enter the device and create a ‘pool’ of saltwater around the rubber contact/pad which causes corrosion of the pad. There is potential for 36V to be on one of the contacts with a very low current (a couple of milliamps).

I am looking to improve on corrosion and am wondering what type of finish would be best. Would go from a 2uin thick layer with ENIG Finish to an 8uin thick ENIG Finish, will that give us a large improvement and help against our corrosion issue? Or, do we move to a Hard Gold finish that is a little thicker (i.e. 12uin) and not has issues with soldering the 0.40mm pitch connector? If I move to a Hard Gold finish, are there any layout things I need to consider?

Sorry. I’m not connecting. What are you talking about: * Bare ENIG pads * ENIG pads that have SMT and other components solder connected to the pads

So, you’ve got this ENIG board in an enclosure that takes-on water … What is getting corroded? Talk more about that …

Sorry if I was not clear. We have a flex PCB Circuit Board that is ENIG Finished. There are two connectors that are SMT to either side of the flex PCB to connect to other PCBs within the system. Then, we have conductive rubber contacts that are press fit on top of the bare contacts (No solder/SMT) on the flex PCB (Which are ENIG finished).

The flex PCB board takes in salt water (not immersed, sprayed onto the enclosure) and the pads that the conductive rubber contacts sit on getting corroded. This is the area where the saltwater gets into when the user sprays the outside enclosure. There is an external object with gold contacts that get placed in these contacts and the user then sprays saltwater onto the object. There is potential for the saltwater to get through the contacts and onto the flex PCB. Through these contacts, there could be up to 36V (2mA).

Gold should not be corroding in saltwater. I have swum hundreds of miles in saltwater with my wedding ring on my finger. My ring is shiny and corrosion-free as the day it was cast over 120 years ago.

It’s possible that your gold thickness of 0.03-0.05um (1.18-2uin) is porous. In fact, it probably IS porous. But even if the gold is porous, the electroless nickel is corrosion resistant

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