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Flexible Circuit Board Finish ENIG or Hard Gold?

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#76212

Flexible Circuit Board Finish ENIG or Hard Gold? | 24 August, 2016

I have a 1oz Flex PCB (FPC) that has 2 0.4mm pitch connectors that need to be SMTd and 9 4.5x1.0mm pads that have conductive rubber contacts that are press fitted onto the FPC. The FPC has a one time bend radius of 15.5mm. Image is attached that shows the rubber Contact over the FPC Pad.

Right now, the 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 salt water to enter the device and create a 'pool' of salt water around the rubber contact / pad which causes corrosion of the pad. There is potential for 36V to be on one of the contact 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 going from a 2uin thick layer with ENIG Finish to a 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 have 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?

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#76213

Flexible Circuit Board Finish ENIG or Hard Gold? | 24 August, 2016

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 ...

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#76222

Flexible Circuit Board Finish ENIG or Hard Gold? | 25 August, 2016

Sorry if I was not clear. We have a Flex Circuit Board that is ENIG Finished. There are two connectors that are SMTd to either side of the Flex 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 (Which are ENIG finished).

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

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#76226

Flexible Circuit Board Finish ENIG or Hard Gold? | 25 August, 2016

Gold should not be corroding in salt water. I have swum hundreds of miles in salt water with my wedding ring on my finger. My ring is a 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 also [http://www.techmetals.com/wp-content/uploads/2015/03/Corrosion-Resistance-of-Electroless-Nickel-Coatings.pdf]

Something isn't correct. I'd go to an independent or college analysis lab for answers.

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#76227

Flexible Circuit Board Finish ENIG or Hard Gold? | 25 August, 2016

Right, but these contacts have up to 36V running through it 20 minutes at a time unlike your wedding ring when you are swimming. If you take a look at the picture I posted in the last comment, that is what we are seeing over the course of use.

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#76228

Flexible Circuit Board Finish ENIG or Hard Gold? | 25 August, 2016

Shawn ... I believe you. I believe what you're saying.

I don't believe that you received the metals that you ordered. From the way it looks I'd guess that you have no nickel and that you're seeing copper corrosion.

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