ENIG vs HAL

[gaminn]

Hi,
I have exposed pads on a PCB that will serve as button. These pads will be touched by a piece of conductive rubber to make a contact.

What surface finish should I select? The environment will be indoor (e.g. standard living room).

Thanks

[wraper]

The best would be carbon coating. Is it for frequent or occasional button presses? The issue with ENIG is that it's a very thin soft gold layer which is not mechanically durable. HASL is simply uneven, especially in lead free variant.

[gaminn]

Uneven surface is not a problem at all. The conductive rubber is soft and the pad on the pcb has "advanced" shape. I think even with very uneven surface the contact with the rubber will be good.

My concern is mainly about long term stability of the surface. How quickly will lead free HAL oxidize (in normal room)?

[EEEnthusiast]

Yes, carbon coating is best suited for this. Please open up any remote controller. The high chances are that the pads are carbon coated.

[gaminn]

I suppose carbon ink is printed on the pcb using screen printing. What surface can it be printed on? If it needs pure copper then the production of the pcb will be a little complicated as there are other components on the PCB (so HAL or ENIG is needed).

[EEEnthusiast]

Please check with the FAB house. Not all of them can do this and everyone would have a separate requirement.

[wraper]

Carbon is also good, but carbon do wear off, list like gold.

Carbon is thick so even if it does wear somewhat, does not really matter. Also I've never seen worn off carbon switch pads.