Improved process control can help the ENIG process shine.

There are two keys to a successful black pad/nickel corrosion-free ENIG process. The nickel deposit should be tight and uniform with minimum crevices between the nickel domains. Crevices between the domains, as seen in a 5000X SEM, occur where all nickel corrosion is initiated. The gold thickness should not exceed the range of 2 micro inches to 4 micro inches as stated in the IPC-4552 ENIG Specification, preferably deposited from a “non aggressive” gold bath.

The ENIG process is designed with the initial steps of pretreatment, followed by a palladium catalyst before the nickel deposition and the subsequent immersion gold. The pretreatment ensures the right copper surface, cleanliness and morphology for the uniform deposition of palladium. A uniform palladium layer will allow the uniform initiation of the nickel deposit on the copper surface.

There are certain requirements that must be met before the parts are introduced to the ENIG line. If the whole circuit is to be plated without solder mask, the laminate must be fully cured and completely cross-linked. Residual monomers can be released in the electroless Ni bath with adverse effects on the deposit and bath performance. The same logic goes for soldermask, it must be properly exposed and developed to minimize the occurrence of a positive or a negative foot. Undeveloped solder mask residues wind up being cured and then harden on the copper surface and may not be removed in the pretreatment cycle. Equally important is soldermask curing to eliminate monomers released in the Ni bath. If tin is used as an etch resist, tin stripping prior to the ENIG line must be completed with no residual tin left on the copper surface.

The process starts with a cleaner. If the cleaner is not optimum, the subsequent etching will not produce the desired surface for catalyzation.

The microetch bath is next. It removes and etches the copper to expose a fresh layer for palladium deposition. It also determines the morphology of the surface. In general, a smoother, more even morphology is preferred.

Nickel deposition is initiated over a the palladium catalyst. Nickel will not deposit on the copper surface without the catalytic palladium layer. The palladium is deposited by an immersion reaction that displaces copper from the surface. The copper goes into solution, giving up the electrons that will in turn reduce the palladium ions in solution to Pd metal on the surface.

The nickel bath is a dynamic bath that undergoes continuous change from make-up to dump. The obvious components of the bath are nickel sulfate and sodium hypophosphite, as the reducing agent that is the source of electrons for the deposition of the nickel, as well as the source of phosphorous in the deposit. Temperature (175o F to 190o F) and pH are critical parameters in maintaining a consistent rate of deposition throughout the life of the bath.

Other components of equal importance are proprietary stabilizers, chelating agents and buffering agents. They are also responsible for the uniformity of the deposit thickness and phosphorous content on the different features being plated on the board. There is also the byproduct of the plating reaction, namely sodium orthophosphate. Its build up is the primary determinant of bath life.

Because of the dynamic nature of the electroless nickel solution, it must be well controlled if a consistent deposit is to be achieved. An automatic controller is a must for proper operation. Most controllers rely on measuring the nickel concentration using spectrophotometric techniques in the visible frequency range. In addition, the controllers are also equipped to measure and maintain pH within a very tight (0.01) range. As the nickel drops, the controller automatically triggers a pump to replenish the addition of nickel sulfate. At the same time, it triggers another pump to add the corresponding amount of the reducing agent. The proprietary ingredients, in most cases, are premixed with one or more components to be replenished. Some suppliers offer these components in separate containers with their own pumps for better control. The more sophisticated controllers modify the addition or end points in response to bath ageing, maintaining very tight control over the balance of chemicals and the deposition rate of nickel through the life of the bath.

The immersion gold bath is the last deposition step in the process, and not all are created equal. An ideal gold bath is one that only displaces the nickel needed to supply the necessary electrons required for the immersion gold to deposit. The more aggressive baths allow side reactions that displace/corrode nickel without the deposition of the appropriate amount of gold. If the parts are left for an extended period of time in these types of bath, the probability of nickel corrosion increases and could be very problematic.

Understanding and managing the ENIG line will produce the right ENIG deposit. The same basic principles may be common to different suppliers, but different formulations result in changes in the operating window for parameters like temperature, pH and bath life. Having a clear understanding of the chemical solutions and the help of a professional technical support team can ensure that the bath performance is not compromised. PCD&F

George Milad is the national accounts manager for technology with Uyemura, International Corp; This email address is being protected from spambots. You need JavaScript enabled to view it..