I am designing a PCB that will have some Mill-Max pogo pins make direct contact with exposed copper pads on the PCB. This will have many connects and disconnects, think hundreds to thousands.
I know that it needs to be at least gold coated in order for this to be resistant to corrosion, but does that mean ENIG plating is enough? Or do I need selective hard gold plating?
I have been putting solder on my pogo-pin pads. I apply the solder by hand on my prototype boards and have openings in the paste mask for those boards that have solder-paste applied with a stencil.
I find that the solder bumps have been far more reliable than bare pads for repeated use.
For those situations where the pogo-pin pads are used only occasionally, I find that whatever plating the board house uses is reliable. For me, this is both HASL and ENIG.
But: the pads will not stand up to repeated use. The cure is simple - just add a tiny bit of solder to each pad. A rounded bump is all that you need.
I work in the semiconductor test industry. We've designed large boards, 40 layers, sometimes more. And typical pcbs too. We make a connection from our lager pcb to a probe card using pogo pins. These connection like yours are in the thousands.. Even 10k.. Because of the forces involved and the number of pogo pins making the connection. We use a selective hard gold plating, exactly: Minimum 30 micro inches gold over 150 micro inches nickel. With very good results over the years. Hope that helps..
That's a lot of connection cycles! In that case, in addition to the plating, it is important to choose the correct pogo pins.
A simple sharp point will eventually wear through the plating. The rounded, Mill-Max tips will last longer. But there are more available options. For example, these are a few of the different options from IDI (now Smiths Interconnect):
The H-style tip, especially, should always have a good point of contact regardless of wear. Not only will the probe find some high point on the contact, but it will rotate slightly over time and so find entirely new contact points.
If you couple that with @DwayneReid 's recommendation of putting solder over ENIG (or HASL) pads, it should last a very long time. This combination is particularly effective because the "soft" solder will provide a good landing, even as it is deformed by multiple connection cycles.
Is this just for occasional (testing), or one time (initial programming) use?
Hard gold would be great, but overkill. ENIG would be good, even the Hot Air thing (drawing a blank on the name) that iteadstudio uses as their base option would be good enough. I have iteadboards that I've had quite a while that look as good as when I received them, some are months old, some even older.
