RoHS Options Close By

Sometimes you feel like a nut and sometimes you don't. I hope that phrase isn't copyrighted by the Peter Paul Mounds candy company. Hmm. I'll just claim "fair use." I'm pretty sure that when I started writing this, it fit in somehow but now I can't really remember where I was going with it.

Oh yeah. Sometimes you feel like Immersion Silver and sometimes you feel like ENIG. Right? Well, if that's the case, we have some good news for you. A lot of you are loyal pcb customers of Sunstone.com (or the old name PCBexpress.com) and have to build RoHS boards. Sunstone has been offering Immersion Silver for quite some time now and they have just recently added ENIG finished pcbs.

Silver is pretty good but ENIG can have a longer shelf life and better corrosion resistance. Check it out at Sunstone.com

OSP PCB Finish Problems

OSP (Organic Surface preservative, not Oregon State Patrol) isn't all that commonly seen in our shop. It has been a pretty common pcb finish for high-volume, low-cost products for quite a while and I've heard that it's starting to show up more frequently on complex boards these days too. Some of the new formulations are pretty good for RoHS applications.

We got a set of OSP pc boards in just the other day with a lot of fine pitch parts and a few big, honkin BGA lands. Click on the two thumbnails for bigger images that do a good job of illustrating one of the potential pitfalls of OSP boards. In the close-ups, you can clearly see two different colors of copper land pads. That's not normal. It indicates an unsolderable, contaminated finish. If you look at an OSP board and see a pretty multi-colored pattern like that, get on the phone with your pcb fab shop and get the boards re-done.

Take a second look at the closer close-up here. This board also does a good job of illustrating the use of NSMD (Non Solder Mask Defined) pads. Most BGA manufacturers recommend the use of NSMD pads. This allows the BGA ball to sag down a little more and grip on the side of the pad too. It also illustrates the proper way to mask the trace between the pad and the via. That's very important too. That mask will keep the solder balls from being sucked off of the BGA.

Duane Benson
Suddenly, State Patrol...

Restoring Immersion Sliver PCBS?

I've written before about immersion silver finished boards and some of the challenges associated with that technology. It's a popular choice for RoHS these days because it's inexpensive compared to some of the others, has a nice planar surface and, conveniently, is, in fact, RoHS compliant.

It has a somewhat high nuisance factor though, because of its proclivity to tarnish and susceptibility to fingerprint grease and other surface contaminants. All the board houses will say it doesn't have any of those issues, but it does. Immersion silver pcbs need to be kept clean, dry and dark. Even then they still have a shelf life. A silver board doesn't take solder well if it's been stored too long, poorly or mishandled. Nag, nag, nag. Okay - got that out of my system...

So, what do you do if your boards have surface issues? Do you toss them? Do you send them to a psychologist? Do you turn them into expensive cubicle art? Do you use them like a Frisbee®? One of my manufacturing engineers told me that some board houses will re-plate them for you. For a fee, of course.

Apparently, in some cases, they can dip the boards and etch the old silver surface off and then re-plate them. I don't know if all board fab shops can or will do this, but if you have a couple of hundred dollars worth of unusable PCBs, it would certainly be worth a call to your Board fab shop to see if they can re-plate them for you.

Has anyone already had silver boards replated? Drop me a comment - I'd like to hear how it worked out for you.

Duane Benson
No Tarn-X, please

Lead-free pcb finishes

RoHS became Euro-law more than a year ago but it seems like the Industry still hasn't agreed on what an ideal RoHS pcb finish might be. Part of the problem is that we aren't just dealing with RoHS issues these days. The advanced packaging just adds to pcb-grief.

More and more components are showing up in QFN, micro BGA and wafer-scale packages. All of these are more sensitive to surface flatness. Lead-free HASL (Hot Air Surface Leveling) is decent for solderability, but it can have a bumpy surface which those little packages don't like.

ENIG (Electroless Nickle Immersion Gold) gives a nice flat and level surface with good solderability and high resistance to tarnish. But if the processing at the board fab house isn't spot-on, it can suffer from black pad problems - especially with BGA and QFN packages. Silver, again has a flat surface and good solderability, but it can corrode in salty or high sulfur environments (like near the coast or in industrial facilities) and is  susceptible to tarnish and oxidation.

I ran across an interesting article on the subject in the online Lead-Free Magazine. The first half is interesting but mostly about pot soldering. If you don't find that interesting, scroll down to about paragraph 12 for their take on the board surface issue.

Duane Benson
Warning: Plot complication
Warning: Plot complication
Warning: Plot complication

Immersion Silver

Immersion Silver seems to be gaining popularity as a RoHS board finish. It is less expensive than Electroless-Nickel Immersion Gold (ENIG), safer then Immersion Tin, easier to use than Organic Solder Preservative (OSP) and has a more level surface than Hot Air Surface Leveling (HASL).

Early versions of the process tended to create boards with a very short shelf-life due to silver migration, microvoids and tarnish. Current processes are much better but we have found that silver boards still need better handling and closer inspection to maintain solderability.

Here is an example of simple tarnish. This board has been exposed to the air for about three months. This board could probably still be used but would need some careful cleaning. At first glance, the pattern looks a bit like the uneven surface that is common with HASL boards, but the surface is perfectly flat. The dark area is tarnish, not a shadow. Click on the image to see it in close up.

In the second example, the silver has migrated leaving a completely unusable patch-work board surface. This is more of a worst case, having been exposed to air for nearly a year. We would never attempt to assemble parts on a board like this. It also illustrates why solder mask is so important on Immersion Silver surfaced boards.

If you do choose to use Immersion Silver boards,

1. keep them sealed and avoid storing them in bright areas.
2. Use them as soon as you can.
3. Carefully inspect them before use.

As time goes by and the industry gets more experience with this surface, I'm sure it will become as reliable and robust as we all want and need it to be, but until then, use it but use caution.

Duane Benson
And silver kills salmonella bacteria too.

Silver and microvoids

Here is yet another reason to use NSMD (non solder mask defined) pads for your BGA placements. We generally recommend NSMD pads for your BGA placements because this technique tends to result in a better mechanical connection. I ran across an article in PCB007 that gives another reason. Apparently SMD (solder mask defined) pads are more susceptible to microvoid problems; especially with immersion silver boards.

If you are using immersion silver, ask your board house some difficult questions about their reliability, board shelf life and process control. If their plating process is poorly controlled, you can get uneven deposition and may end up with microvoid problems. Poorly processed silver boards can also tarnish when exposed to air and can give a very short pre-assembly shelf life if not properly sealed.

Another good resource is the website www.microvoids.org. They cover a lot of detail relating to microvoids and surface tarnish. If you need boards fabbed with immersion silver board finish, check out our partner, Sunstone Circuits. They do a pretty good job with the immersion silver process.

Duane Benson
Silver keeps the bacteria down too