Black pad

Although rare, the infamous and irreparable PCB defect can have disastrous consequences.

Feb. 16, 2009

by John Buchanan

Few topics in the printed circuit board industry are as controversial as black pad, a failure associated with a poorly formed joint at the solder/nickel interface. Although it is a rare phenomenon, appearing on only 1 to 2 percent of boards or less, when it happens it is expensive and frustrating. There are numerous theories as to the causes of black pad, but no definitive reason.

Black pad only occurs during the electroless nickel phosphorus and immersion gold (ENIG) process that has established itself as a preferred solderable surface finish for high-reliability applications involving complex circuit designs. When a flawed joint is stressed, the connection is easily broken, leaving an open circuit exposing dark corroded nickel, which gives “black” pad its moniker.

The latest research shows excessive nickel corrosion during the immersion gold deposition causes the condition, says George Milad, national accounts manager for technology at Southington, Connecticut, USA-based Uyemura International Corp., a chemical supplier that offers ENIG chemistry to the electronics industry.

The threat is minimal for a nickel deposit that shows an even topography at 5000X, says Milad, who chaired the IPC committee that drafted the ENIG specification IPC-4552, addressing black pad more than five years ago. But an irregular topography, with distinct crevices between domains, is where corrosion causes black pad. Irregular topography can be caused by a nickel bath running outside its control limits, contaminated incoming copper surface or inadequate pre-treatment in the front end of the ENIG line.

A Bigger Problem?
Werner Engelmaier, president of Ormond Beach, Florida, USA-based consulting firm Engelmaier Associates LC, believes black pad affects significantly enough to be a continuous talking point of the industry. “Some manufacturers have had massive problems,” Engelmaier says. “It happens to a high degree that should be of concern to the industry.”

The “classical definition” of black pad is too much phosphorus, which is left behind when nickel is dissolved, says Engelmaier, who acknowledges that not everyone accepts it. “The more phosphorus, the weaker the interface,” he says. “You might start out at 7 percent phosphorus, but after one reflow you might end up at 9 percent or higher. If you have numerous reflows and repair procedures, the phosphorus content of the interface increases every time. I have seen levels as high as 14 to 15 percent.” The higher the phosphorus content, the greater the risk of black pad, Engelmaier says.

Further, Engelmaier believes that if you are new to the ENIG process, you should avoid it, in favor of immersion silver, because there is much more assurance that everything will go smoothly. This will help avoid the devastating effects of black pad, he says.

George Wenger, senior principal FMA and reliability engineer at Warren, New Jersey, USA-based Andrew Wireless Solutions, a provider of wireless infrastructure, has wrestled with black pad going back to his days at Lucent Technologies.

“The reason it’s serious is that it happens at the wrong time,” Wenger says. “It happens when you least expect it, when you’ve got a new product you’re trying to sell to a new customer. If your new customer experiences the problem, it is devastating for you. If we do not know exactly why the failure is occurring, then we have a very difficult time trying to prevent it.”

As a supplier, Milad has witnessed the damage that black pad can do, beyond the cost of the defective boards produced. The problem, Milad says, is that black pad is almost never detected at the board manufacturer site, but is detected after assembly when the board is fully populated. Once the assembler discovers the problem, a blame game over who’s at fault ensues between PCB manufacturer, assembler, ENIG supplier and consultants. The resolution, whatever it is, turns out to be expensive for everyone, as they attempt to determine the root cause and assign responsibility.

Finding a Solution
As an expert and supplier, Milad believes a big part of the solution is an immersion gold bath where the nickel exchange with gold is closest to ideal, based on the chemical stoichiometry of the exchange reaction equation: Ni + 2Au+ → 2Au + Ni++.

“A well-controlled nickel bath is one of the keys to the elimination of black pad,” Milad says.

Engelmaier agrees with research done by industry experts that says the pH level during the nickel bath is the key factor because it determines how much phosphorus is co-deposited.

Wenger touts immersion silver as a strong alternative. “We have been using immersion silver for most of our boards since 1996,” he says. “I tell people that once you try immersion silver, you’ll want to do it forever, because it works.”

For more information on IPC-4522, attend IPC APEX EXPO session 4-14 “Plating Processes Subcommittee” Wed., April 1.