Screaming Circuits: January 2009

Oxidized Silver? What do you do???

I've written before about the shelf life of immersion silver PCBs. In that post, I alluded to the possibility of carefully cleaning an oxidized immersion silver pcb but I didn't say how to do it. Sometimes an oxidized board can be cleaned. Sometimes it can't.

Their are really only two industry recommended practices:

  1. Send the boards back to the fab house to have them re-plated
  2. Get new boards if re-plating is too expensive

That being said, you can sometimes use a large rubber eraser to clean them. It's easy to mess the boards up though so it won't always work. I would not do that in a production environment or with an expensive board. And it won't do anything for plated through holes for thru-hole parts. For that, you'll need to go back to your board house.

Here's what one of our engineers said about the process:

"The problem with immersion finishes, silver in this case, is that the oxidation or tarnish is the remaining oxide/sulfide/sulfate/chloride compound (depending on what's in the area to react with) conversion of original silver content. The immersion coating is so thin to begin with, cleaning away the tarnish would basically leave you with little to no silver and the undercoat (usually nickel) exposed. Also, the type of tarnish/contamination is a big factor. The sulfur compounds (sulfide/sulfate) are worse - i.e. cause more solderability problems, then the oxide/chloride compounds. I'm not a chemist, so don't ask why.... :) Of course, the only way to find out what particular type of contamination is most prevalent on a give board is to have it analyzed.... Probably cheaper to get new boards!"

I've heard about some studies at Sandia National Labs regarding the life and solderability of tarnished silver boards. I'll try to look that up and if I can find it, I'll post some notes about it.

Duane Benson
sulfide, sulfate
sulfide, sulfate
Swiftly fly the years
One season following another
Laden with happiness and tears

Speaking of Common QFN Issues...

Classic via in qfn pad Here is the classic QFN via in pad. It simply isn't possible to solder the center pad properly with that much open via real estate.

The best way to deal with this is to fill the holes with something that will still do a good job of conducting heat away and then plate over the holes.

Barring that, you could put solder mask caps over the holes on the component side shown here. Most manufacturers recommend that the soldermask cap diameter be 100 - 125 um wider than the via to minimize voiding and thermal insulation.

Some people will put soldermask caps on the back side of the board. That may work, but it still tends to be problematic. Solder can still wick down in the vias leading to excess voiding. The via caps can pop open resulting in an open via that sucks solder off of the pad.

Duane Benson
Look closely and you might see Paul Lynde in the center via

Novel QFN Land Pattern

Novel QFN groundingBecause the center land is so much bigger then the side pads, QFN solder paste can be a problem if not handled properly. I've written about options before, but here's another approach. I ran across this QFN land pattern the other day.

This one's a bit easier to start with because either there aren't any vias needed or they and filled and plated over. It also looks like there isn't much of a thermal issue here. In some cases the center pad is needed for heat sinking, but in other it's just there as a ground. My guess is the latter is the case here.

By creating a bunch of openings that are the same size as the side contacts, this QFN will get good grounding and there won't be any issues with too much or too little solder paste getting in the way. It pretty much bypasses the standard QFN problem.

Note that the designer must have the same pattern in the solder mask layer as in the paste layer. If only the paste layer was like this, the solder would spread out and there would likely be too much voiding. It might not connect in the center at all. If only the mask layer was like this and the paste layer was fully open, there would be so much solder with nothing to stick too. It would just be a big mess. Yuck ☠.

Duane Benson
QFN Tetris anyone?

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?"

« December 2008 | Main | February 2009 »