Plating Q&A: Reducing Pitting in Thick EN Deposits
Is there anything we can do to reduce or eliminate pitting in thick EN deposits?
Q. Some of our customers’ parts require thick electroless nickel deposits in the 1.0–3.0 mil range. Unfortunately, we often notice pitting of these deposits. Is there anything we can do to reduce or eliminate this problem?
A. Pitting of EN deposits has always plagued platers, especially for thick deposits. Fortunately, there are solutions to your problem. Thanks to a great deal of research in the memory disk and oil and gas industries, pitting has been closely monitored and studied and significant strides have been made in solving this issue.
Understand that pitting can be classified into five different categories. Each category will be discussed below with suggestions on how to eliminate them:
Particle pitting. This type of pitting is one of the most common sources of pitting of EN deposits. Because long immersion times are required to plate thick EN deposits, these coatings are very susceptible to co-deposition of fine particles. Shelf areas are typically prone to this.
There are two fundamental sources of particles: 1) particles formed in solution and 2) particles of extraneous origin. The first particles can be created by the creation of either nickel phosphide or nickel hydroxide in solution. The age of the EN chemistry, the type of chelators present and the manner of pH adjustments can create issues. Extraneous particles can be introduced by poor rinsing, poor cleaning and/or poor housekeeping. Good filtration is the best line of defense. Filtration with 1 micron filters, or smaller, at a rate of 8–10 solution turns an hour is best. Dispersants, additionally, have been found to be helpful.
Gas pitting. Pits due to gas bubbles are also a common occurrence. These defects are usually circular in appearance and can vary in size. Depending on the surface tension of the solution and bath age, this problem can be significant. Some suggestions to retard this problem are: 1) Improve solution movement, 2) use work-rod agitation and 3) add surfactants to lower surface tension.
Heavy metal pitting. Pits due to heavy metal poisoning are also a relatively common occurrence. Certain heavy metals are sometimes referred to as “catalytic poisons.” The presence of these ions can be deleterious to the initiation of the EN deposit and, therefore, promote pitting. Heavy metal ions such as lead, cadmium, bismuth, antimony and copper should be closely monitored. Sources of heavy metal contamination include drag-in of pretreatment chemicals, base metal impurities and the EN chemistry itself.
Organic pitting. This type of pitting is usually created by the presence of oil or pretreatment chemicals. This type usually occurs in clusters and is flat-bottomed in appearance. The most common sources are drag-in of pretreatment chemistry, unfiltered air, lubricants used in plating equipment, masking agents and/or plasticizers from hoses or liners. Good housekeeping is paramount. By replacing cleaners regularly, monitoring equipment and air supplies and providing excellent rinsing techniques, organic pitting can be minimized.
Substrate pitting. Always inspect incoming parts for defects before plating. Defects will generally be magnified by the EN deposit, especially in thick deposits. Generally, substrate pits are irregular in appearance and will vary in size. Overaggressive pretreatment of the substrate can also create imperfections of the substrate surface. Hence, overetching the substrate should be avoided. Substrate pitting can be identified by simply stripping the EN coating and observing if the imperfection is still present on the part.
More and more new applications that use thicker EN coatings are being developed every day. With a better understanding of the mechanism of pitting, less rejects and more profit will be a direct result to platers everywhere.
Mike Aleksinas is the president of MetalChem. He can be reached at firstname.lastname@example.org, or at 864-877-6175.
Originally published in the February 2016 issue.
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