I have two charging pads in the middle of a PCB that mate repeatedly with pogo pins. Worried about ENIG wearing out. Can fabs do selective hard gold on interior pads, or is that generally done on edge connectors?
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Generally pads that require hard gold are processed using an electrolytic process which nomally uses an external shorting bar. The question is how will that work if the pads ar not at the edge of the board?
I have not had to do hard gold in many years now so maybe something has changed or another surface finish might be preferred?
From a cost standpoint, it is preferred to use a single surface finish for the entire board so you do not incur increased cost for a couple of contact pads.
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Most fabs can only do electrolytic hard gold when the pads can be tied into a plating bus. That’s why it’s normally done on edge fingers. If the pads are buried in the interior of the board, the shop has to create a temporary connection to those pads for the plating step, which becomes a custom operation and adds cost. Your best bet is to check capabilities directly with your manufacturers, as this varies a lot between shops, and some will simply direct you toward a different finish if you only need durability for pogo-pin contact.
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If you want to avoid the cost and complexity of selective hard gold, you can also use surface mount contact pads (Mill-Max, Harwin, etc.) instead of bare plated pads. They’re designed for high-cycle pogo interfaces and use much harder contact materials than ENIG.
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I hadn’t considered using surface mount contact pads. That seems like a clean way to avoid wear issues entirely. I check with my fab house if this is possible
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A practical alternative, if you want to avoid hard-gold complexity is to widen the pad and request a thicker ENIG specification (or if possible thicker copper under ENIG). A larger contact area reduces local wear from the pogo tip. It extends ENIG’s life with no special fab steps or added cost. Hope it helps.
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Will,
Thicker copper…I am not sure how that would help when ENIG is the surface finish. Or did you mean thicker nickel layer?
As far as ENIG, that is self limiting as to how thick the gold can become. I have never heard of being able to specify a thicker amount.
Yes, you’re right. I meant thicker nickel in the ENIG stack, not thicker gold. Standard ENIG per IPC-4552 has nickel ranging from 100-250 microinches (about 2.5-6 μm) and gold from 2-8 microinches (0.05-0.23 μm). Some fabs may be able to go toward the higher end of that nickel range on request, which provides more wear protection before the copper is exposed.
The thicker copper comment was unclear, my bad, I meant heavier base copper (2 oz vs. 1 oz) provides mechanical support under the plating, though wear protection comes from the nickel itself.
Hard gold on interior pads is normally limited by the need for electrical connectivity to a plating bus, which is why most shops only offer it on edge connectors. However, some manufacturers can apply hard gold to interior pads using selective plating techniques with masking. This involves applying a resist or mask to protect areas that don’t need plating, then electroplating nickel + hard gold on the exposed pads. This is done before solder mask application and avoids the need for shorting tabs or edge connections. It works fine for vertical pogo-pin contact. However, this capability varies significantly between shops, so checking with your fab is essential.