FPC contacts not tin plated, problem?


Today I was debugging a product with a membrane/keypad, I noticed that the contacts of the flat cable were black, ie not plated, I never saw this before, I am used to the FFC cables only

Is there any problem related with this kind of termination? I suppose is for lowering the price and, as it has a considerable resistance, it is possible to “carbonize” the contact and have a bigger resistance? if a high voltage is applied, like an ESD, because is what I see, high resistance if measured form the contact, and low if measured form the track itself (just before the contact)

Thank you,

Edit, when I said black, it was not shinny or bare metal look, cleaning with alcohol did not remove the black, but scrapping did it, so the ESD test did create that layer, I though it was a different kind (lower cost) termination.

Do you have a photo? While I’m not sure about it upon first reading, a picture can really help. Plus, I’m very curious to see this sort of thing.

Was it like this? Carbon looking. This is from an LCD for a child’s cheap calculator.

I did not know that there are 2 options for traces on membrane panels, copper and silver that are actually printed.

As you can see, contacts are not clear and shine, it is not copper, so I think silver should be, light and the angle of the photo does not help, but is the looks exactly as a pencils tip.

In any case, is bare metal that, somehow*, has a thin layer of something.

*Many ESD test were applied to the membrane.


This appears normal, It’s likely a carbon ink based FPC. These are cheaper to produce than copper tracks, but have a naturally higher resistance.

I’ve most often seen these types of FPC in laptop keyboards, and membrane keypads.

The only sympton was a set of keys that shared one pin of the matrix, due to higher resitance, but the other pins also had tens of ohms, just not enough to fail.

Traces are metal