Screaming Circuits: Immersion Silver and Other Lead Free PCB Surfaces


Immersion Silver and Other Lead Free PCB Surfaces

I just had a comment on an old post requesting a little more information on silver surfaced lead free boards.

"Hi, im currently doing a project regarding Immersion Silver PCB.
Pls share me more info for this kind of finishing PCB such as why must use immersion Ag for LF PCB, how to handle, and also how to prevent it from yellowish?"

Unfortunately, nearly two years after RoHS came into effect, there's still a lot of confusion and conflicting information about board surfaces. I don't think the Industry has yet agreed to a "universal" or "near universal" answer to the question either.

Back in the old days, it was fairly easy. You used SnPb HASL (Tin-lead hot air surface leveled) pcbs for 180px-SilverUSGOV most things and OSP for the lowest cost, largest volume stuff. Anything else was specialized or exotic and not mainstream. Now, though, we have quite a few choices. Still, if we stay away from the exotic and odd, we can limit our choices to a small set:

  • Immersion Silver is a good reasonable cost surface.

It delivers a very flat precision surface which is important for small components. And it's not too expensive. The biggest downside is that it needs better care during handling and storage before use. Keep it in a dark, low moisture place. Don't touch the solder surfaces with your bare hands. Be careful of the air quality. It's extra susceptible to tarnish in areas with high levels of ozone and hydrogen sulfide.

  • ENIG (Electroless nickle immersion gold) is another good surface, but more expensive

ENIG also delivers a very flat precision surface, perfect for small components. It does cost more though. It's not as susceptible to tarnish. If cost isn't an option, gold is probably your best bet for a RoHS board. However, you should still avoid touching the solder surfaces. If the gold layer is too thin, your finger oils can cause real problems on it. We do still see some ENIG boards with black pad problems. That's caused by poor process control at the PCB fab house. I've also heard that some low-cost ENIG boards will have a gold layer that is way too thin. That can cause problems with corrosion and solderability.

  • Lead-free HASL is a good low-cost option

It doesn't provide the flat surface and precision of Immersion silver and ENIG, but it is lower cost and very robust. If cost is important and you don't have really small parts, a lead-free HASL board might be your best choice for lead-free.

There are other lead-free (and leaded) board surfaces, but the three listed above are the most common and one of the three will cover just about all needs. I hope this helps.

Duane Benson
Jered says "What?"


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