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Edge Connectors, ENIG Plating, and Galvanic Corrosion

Yellowstone is inching slowly towards the start of manufacturing. One question that’s arisen is the type of surface plating to use on the PCB. The default / cheap plating is HASL or Hot Air Solder Leveling, which is just a thin layer of solder, consisting mostly of tin. This is what BMOW’s other products use. But engineering wisdom says that for an edge connector, or any PCB surface that will send electrical signals across a mechanical contact surface, you should use ENIG plating. ENIG is Electroless Nickel Immersion Gold, and it’s a layer of nickel covered in a second layer of gold. It’s more expensive, but more durable.

So ENIG then? Well, maybe not. I recently learned about a problem called galvanic corrosion that occurs when two dissimilar metals are in contact for a long time. The peripheral card slots in an Apple II computer have tin fingers, I believe – at least they’re not obviously gold-colored. Does that mean an ENIG plated board inserted into an edge connector slot with tin fingers is doomed to contact corrosion and premature failure? A quick check of other Apple II peripheral cards in my office showed they all have gold-colored connectors. I’m uncertain if they’re ENIG or something else, but probably ENIG.

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8 Comments so far

  1. Ferdinand - December 22nd, 2021 1:01 pm

    You’ll probably want to have hard gold on that edge connector. It’s much thicker than the gold layer from ENIG and will resist the wear for a much longer time. The chemical process used to deposit is different as well. If memory serves correctly, all contacts will have to be shorted for the process. This short is then milled off after the fact.
    Some card edge connector also require the edge to be beveled, but I don’t know if that’s the case for Apple II cards.

  2. Scott - December 22nd, 2021 1:34 pm

    Hard gold is still gold, which is not recommended for contact with tin. Though I suspect that concern is often ignored and the combination frequently used anyway.

    Before making a decision based on galvanic corrosion, you need to know for sure what the metal plating on the slot contacts is. I wouldn’t be surprised if the contacts are tin; however nickel would have been a better choice and is galvanically compatible with gold.

  3. Steve - December 22nd, 2021 1:35 pm

    Hard gold sounds expensive. 🙂

    The prototype cards haven’t used beveled edges, and it doesn’t seem to be an issue. But when I carefully examined some original Apple peripheral cards I could see they do have beveled edges.

  4. Steve - December 22nd, 2021 2:46 pm

    I researched this a bit more, and some kind of gold looks like the way to go. Every Apple II peripheral card I can find uses a gold finish. But is it ENIG, or hard gold, or something else?

    I’ve asked for some price quotes to compare hard gold fingers with beveled PCB edges against ENIG with square PCB edges. From what I’ve read, I fear the hard gold may be substantially more expensive. The current Yellowstone prototypes are ENIG with square edges. Maybe we’ve just been lucky, but they’ve survived a large number of insertion and removal cycles during testing, many more cycles than I would expect a typical real-world customer to ever see. Maybe I need to quantify the number of expected insertion and removal cycles over the card’s lifetime.

  5. Scott - December 22nd, 2021 3:34 pm

    Yeah, it seems most sensible to use whatever material contemporary cards used. Worst case is that users may occasionally have to re-seat a card, but no worse than any other Apple II card.

  6. Steve - December 22nd, 2021 6:17 pm

    I’m unsure what exactly happens if a board is ENIG plated on its edge connector and goes through a high number of insertion and removal cycles. What problems might appear? Would it be enough to reseat the card or wipe the contacts clean? Or would the contacts be permanently degraded somehow (higher resistive loss?) or break entirely? And what’s the approximate threshold for a “high” number of cycles? I’ll see if I can get some answers from the PCB vendor.

  7. Ferdinand - December 23rd, 2021 2:21 am

    The gold layer of the ENIG surface is extremely thin, the standards specify 0.05 um as the minimum. It is likely that it would just wear off, which could in turn lead to corrosion of the contacts. Hard gold is much, much thicker and generally more durable. However, if the additional cost is way beyond what you budgeted for I would just get the ENIG plating and add a note to the user manual.
    I wouldn’t worry too much about galvanic corrosion, as it needs humidity to happen. Planning for that kind of situation is IMHO asking too much for a consumer product like this.

  8. Nick - December 23rd, 2021 12:05 pm

    Lack of a beveled edge on something that gets inserted once and never again, probably not the end of the world. But if it’s going to be removed and inserted multiple times over a long period could have issues. Better safe than sorry and add the bevel.

    Also, I’ve seen several companies that produce cartridges for retro gaming consoles, and any time there is no bevel, people let the world know on social media. Another reason to add the bevel, bad PR.

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