Designated solderability preservative: the judgment on what defines an ideal PCB surface finish varies greatly depending on the vantage point of the judge.

HAPPY SPRING EVERYONE, from a new member of the Final Finish Forum. I'm stepping in to replace my friend and colleague, Don Cullen, who is focusing his attention these days on solar energy.

It seems like only a blink ago that I was walking out of college with my chemical engineering degree in hand--ready to tackle some pressure vessel somewhere. Now twenty years later, I find that I have been "dunked" into the world of plat ing and surface finishing for a majority of the time. I've gone from blast furnaces to nanoparticles along the way. A few years ago, fate brought me into sharp focus with the world of printed circuit board final finishes. My charge was to help corral the ideas and innovations of a group of smart chemists into some valuable chemical processes for PCB surface finishing.

Naturally, I set myself to the task of understanding what the industry stakeholders needed from a "valuable" PCB surface finish. I distinctly remember listening to a well-respected industry veteran hammer home his point that a PCB surface finish should be a "solderability preservative," and nothing more. Our simple task as process providers was then to slow or to stop Mother Nature from oxidizing the copper on the PCB surface. Extending the discussion into more elaborate areas, such as enhancing signal integrity or enabling greater board design flexibility, was out of line. Based on my experience, this sounded easy. We could get it done, coffee breaks and golf outings included.

It turns out it's not that simple. The fact is that a whole host of evolving technologies, environmental trends and irritating laws of chemistry and manufacturing demands dictates which finish is the best choice for a given application or board builder. There simply is not one finish that fits all wants and needs, and that will likely remain the case until printed circuit boards go the way of the slide rule.

Take for example a problem getting a lot of press lately: the "premature" corrosion of PCB pads in working, end-use environments. It turns out that, in some pretty nasty environments, surface finishes can corrode to the point of causing device failure. In this case, the finish needs to shield that reactive copper through the many years of its life in a real-world application. So much for only needing to protect a board in a nicely sealed package up until the time of assembly, that would be too easy; it appears that we also need to assure 10 years to 20 years of trouble-flee harsh environment operation. Seems fair, right?

It did seem fair to me, until the reality of how harsh this harsh environment really is. To demonstrate how corrosion resistant any given finish is, one has to test it in a controlled, standardized test. Upon doing this against long accepted standards such as the Batelle mixed flowing gas (a nasty mix of chlorine, sulfur-bearing, and other noxious gases belched by the most offending factories or manufacturing processes), what is found? Most people are finding that most finishes hold up quite well, with no system failures. Well, that's nice, but we haven't demonstrated that we can create the problem being seen on PCB pads. So, we have to create a test that does the job. When we enlisted the help of a corrosion testing lab to create a "corrosion test on steroids," what happened? The conventional equipment could not support the test because the nasty environment chewed up the test chambers.

Earlier on, I dropped a comment about manufacturing challenges and "irritating laws of chemistry." That can be the case for a surface finish that may be great on the board surface but a real "bear" to work with. Electroless nickel immersion gold (ENIG) fits that description pretty well. It's a good bet you'll never find a plating line operator singing the praises of this finish.

It's the nickel portion of the process that can make life difficult. This is an electroless plating process, one of the more nuanced and demanding of proper process controls. Instead of plugging in a power supply or relying on the self-limiting nature of immersion plating, electroless processes make use of some fancy interplay between electron donors and acceptors within the plating solution. In the case of electroless nickel, chemical reducing agents, stabilizers, nickel ions and other materials are blended together. When this brew sees a properly activated copper surface, the nickel magically plates, and you have the beginnings of that solderable surface you are looking for. The problem is that these electroless systems sometimes seem to have a mind of their own. They are notorious for plating on places that we don't want them to, like plating tanks and board racks. When this happens, it's usually done in grand style. An innocent control mistake leads to a small seed of nickel metal plated somewhere that it shouldn't be. From there, nickel ions run like a herd of lemmings toward the walls, and a tank becomes nickel plated.

The point is that final finishes always end up needing to do a bit more than simply preserving solderability. The judgment on what defines an ideal PCB surface finish varies greatly depending on the vantage point of the judges. From this perspective, we do have something resembling a democracy. If all parties are not pleased, we'll likely hear some relatively vocal dissent. It keeps us busy (with less time for golf) and moving forward.

JOHN SWANSON is director of Final Finishes and Interconnect Technologies at MacDermid Inc; [e-mail].