If silver is the best conductor of electricity, why is gold used in electronics instead?
In: Engineering
So silver is used in electronics, just not as common as gold.
The underlying material is almost always copper. It’s a great conductor, reasonably plentiful, manufactured in volume using easy to control electrochemical deposition processes. It’s also very highly reactive to oxygen. So, it needs to be protected by something.
For most of your circuit board, the exposed copper is protected by the soldermask, the (usually) green stuff.
For the parts that need to be exposed, there are a variety of surface finishes.
– HASL, or hot air solder level, is a thicker coating of solder applied to the board by dipping it into a vat of molten solder, then using airjets to blow off the excess. It’s messy and uneven, and was the industry standard in the old days. Today, you can still get it at most suppliers, either with traditional lead solder or lead-free solder.
– White Tin is used as a solder finish, but it fell out of favor because of tin whisker growth. Metals can move on their own, which I find fascinating, but they are generally all bad for your electronics.
– OSP, or organic solderable preservative, is a thin organic coating that merely prevents oxidation. It’s good if you will assemble onto your circuit board very quickly after its fabrication. It’s not good if your boards need to sit longer on a shelf, because it wears off relatively fast.
– IAG or Immersion Silver is an ion replacement process that swaps a thin layer of your copper ions with silver ions. Silver is less reactive, but will still tarnish. Silver, and copper, are unfortunately both also highly susceptible to sulfur corrosion. In the presence of sulfur, the right amount of humidity, and bit of a catalyst like chlorine, both silver and copper will degrade in a way called creep corrosion. Again, the metals move on your board which is neat but bad for performance. There are organic coatings now available that stop this process. See MacDermid Enthone’s AlphaStar or MacDermid’s CM Plus. They also stop tarnish, so an immersion silver finish today really isn’t any worse than gold.
– ENIG, or electroless nickel immersion gold, uses two chemical deposition processes. Electroless nickel uses a redox reaction to plate nickel as a barrier between the copper and gold, as otherwise copper ions will migrate through the gold to the surface. (Have I mentioned that metals moving on their own is cool?) Then, gold is used to provide the tarnish-free surface. Nickel unfortunately is a terrible conductor, and gold, when pulled up into solder joints, can make them susceptible to fracture under shock. This process also relies upon sulfur, and a bad mix can result in a sulfur layer appearing between the nickel and gold, which causes your parts to fall off, a phenomenon known as black pad. ENIG is by far the most common surface finish, but I don’t think it’s the best.
– ENEPIG, or electroless nickel electroless palladium immersion gold, tries to fix some of ENIG’s flaws by adding a layer of palladium as well. This lets you use even less gold, so there’s less that gets pulled up into solder joints, and provides a layer of palladium for the high frequency signals to run in instead of nickel.
– EPIG or EPAG are newer finishes that remove the nickel and attempt to plate palladium directly to copper (or perhaps with an organic intermediate). There’s a company out there (Lilotree) that advertises a finish that is gold directly on copper with just an organic intermediate, but they treat their finish as a trade secret so it’s rarely used and a niche product.
– When you have a connector, like an edge mount connector, or pads where you will impact pogo pins, you can plate with hard gold on nickel. This is much much thicker than ENIG but provides a durable surface that won’t wear off with repeated use. You really can’t solder onto hard gold without risk to your joints due to gold embrittlement, but you can do this selectively where needed and use a different finish for the rest of the board.
I have, in my career, designed products that used about all of the above. All things considered, I think immersion silver with a corrosion inhibitor applied is the best surface finish on the market. But, I acknowledge that ENIG, despite its flaws, is more prevalent.
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