The surface preparation prior to applying electroless nickel over any type of substrate determines the ultimate success or failure of the plating…
Many papers have been written about the importance of proper surface preparation prior to depositing any electroplated coatings, including electroless nickel. The significance and importance of surface preparation and its effect on the resulting deposit quality is too often overlooked as a primary cause of problems in the plating line. It is always easier to look at the plating tank first when trying to identify a problem. One of the significant purposes of this paper will be to identify other primary areas to focus one's efforts when problems are encountered on the plating line.
To illustrate a point, the importance of providing proper surface preparation prior to electroless nickel has many similarities to situations we encounter everyday outside of work. For example, the problems we face at home when we want to paint an item are similar to problems faced by the finisher today. The choice of color is something we can be indulgent about. Other factors, such as type of paint, are much more critical. There are many types of paints, including, epoxy, enamel and polyester to name a few. Choosing the type of paint depends on what functionality or type of service condition the part will see. An example would be the summer I painted 4 hours to earn extra money for college. I did not spend the necessary time practicing good surface preparation techniques. What I thought had been saved in time cost me many additional hours repainting that fall and into the next spring. What a valuable lesson to be learned and never lost or forgotten.
Plated coatings, especially electroless nickel coatings, are also chosen in a similar fashion. After much experience, it is soon realized and confirmed that the selection of a suitable prepaint or preplate process is as important as the selection of the finish itself.
Cleaning 101. Today as a technical service manager, I continue to see many mistakes or shortcuts taken on the plating line. If I had been rewarded with a nickel for the times I heard, "We have production to run…We don't have enough time to spend additional time in the cleaners and rinses…We don't have enough room in the line for another tank…We will let the chemistry work for us." All actions produce some consequence that will effect the resulting quality of finished parts. Unfortunately, many of the negative consequences are only realized when preparation cycle shortcuts have been taken. It is important that the proper pretreatment guidelines or best practices are followed to insure success. There are also many basics or theorems that reflect or impact the final quality of plating. The following theorems contained in the Mini-Metallurgy Course 101 provide a good basis for review.1
Mini Metallurgy Course 101
These theorems are often misused, misunderstood, disagreed with or overlooked and ignored in the haste of the production quota. The plating solution is often the primary focus when defects are found on the finished work. Our inability to realize the significance of the Mini-Metallurgy Course101 causes us to expend our energies unnecessarily. I have made many unnecessary service calls to reinforce the statement, "It has been proven many times over that 90-95% of plating problems are cleaning or preparation related." This statement is often referenced and can be found in many written articles on the subject of plating. The theorems appear straight forward, but there are many times when they are forgotten or ignored. For example, attempting to process steel, copper and brass on the same line sharing the same preparation steps will normally cause higher than acceptable rejects and be the root cause for many other problems due to substrate cross contamination.
There are a lot of good examples of proper process cycles, but there is always the need for some trial and error testing when selecting the optimal process cycle. If we could assume that any one type of part is always the same, such as having the same heat treatment scale, amount of lubricant or consistent surface finish, then the cleaning and plating process would be more predictable. Although the chemistry we use is based on science, we still require a lot of fine tuning in selecting the proper pretreatment for a particular substrate.
ASTM specifications can be a good source of information for finishing operations. These specifications are an important source of information regarding strategies for preparation cycles. The following table of ASTM B08 specification documents covering "Recommended Practices for Preparing Metals for Plating" is a good reference on surface preparation.2
Cleaning Various Metals
Nickel and Allo
ys Stainless Steels
Significance of acronyms and plating. I am a graduate from the John (Jack) Horner school of plating. I have taken the liberty to oversimplify and restate his Mini-Metallurgy Course 101 approach to surface preparation. For those unfamiliar, Jack is recognized in the finishing industry as one who, over 30 plus years, solved many plating problems through practical, yet simple approaches. He taught me that coining phrases and spouting acronyms help to categorize and file the information we learn daily. When problem solving "a person is more likely to be successful if you look for the horses first and zebras last." Simply restated, look at the basics, the obvious, before jumping to the unusual or obscure when attempting to solve problems. Another of his phrases that impacted my approach to problem solving the preparation cycle is, "Whatever one puts on, must be taken off." This statement simply implies that successful plating is a series of steps that require the removal of the previous pretreatment process from the surface of the part. For example, removing the oil from the part in the cleaner and leaving cleaner residue on the surface of the part is normal and expected. Plating can be broken down into simple steps where something is removed from the surface in one step and displaced with something else in the next step. For successful or trouble free plating to occur, the concept implies that the final film from one process step needs to be compatible with, nor cause a problem, with subsequent process steps. The cleaner, acid activate, plate and post treatment are all simple stages in the electroplating process. Recognizing this fact makes the pretreatment for any alloy a simple process. The primary difficulty comes with choosing the proper activation for the particular substrate processed.
To answer the question posed as the title of this paper, we do not need to look far. At one time, it was only the plater or finisher who had responsibility for the end quality. Today, ISO, QS and other quality programs are teaching us that the responsibility for producing quality electroless nickel plated work is shared by the specifier, fabricator, machine shop, heat treater, plater and end user. The primary focus for the finisher still involves how well the SCRAP process works. This acronym refers to the Substrate, Cleaning, Rinsing, Activation and Plating process. I am encouraging finishers to embrace the SCRAP approach to plating electroless nickel. It is our nature to look at problems, specifically on the plating line, as complex situations that cause ulcers, headaches and sometimes a great loss of money. It is very easy to spend many dollars and energy producing plated scrap.
It is easy to be overwhelmed when problems occur. Our approaches to identifying and solving problems when distressed are not usually successful. In the past, it was very easy to take the shotgun approach to problem solving: try as many "fixes" as you can in the shortest amount of time and hope one of them eliminates the problem. If successful, then try to figure out which "fix" actually worked so the problem can be avoided in the future. I am advocating that finishers take the SCRAP approach for improved effectiveness when problems occur. If one looks at each element as a separate component, the likelihood of being overwhelmed during the process is lessened while the effectiveness will be measured with less scrap.
The substrate in SCRAP. Electroless nickel is applied to many types of substrates to take advantage of the coating's properties. Today, we have a better understanding that the final electroless nickel deposit quality is only as good as the quality of the base substrate metal since the coating's ability to level or hide imperfections in the base material is poor. In fact, any defect in the substrate will be more visible after the part is plated with electroless nickel. Very few, if any, electroless nickel systems will cover up base metal problems such as porosity, slivers left by machining or polishing patterns. The manner in which a part is stamped, cast, drilled or heat treated can have a significant impact on the final plated product. Difficult-to-remove oils or compounds can be imbedded into the surface of the part causing dull or nonadherent coatings. The heat treatment or surface hardening processes carried out on the substrate should be known before attempting to plate with a normal process cycle. Modifications in the preparation cycle will likely be required. It is strongly suggested that the incoming work surface condition be examined before processing begins so the plater alone does not assume the responsibility for a poorly machined or manufactured part. It is important for the machine shop or metal working department, specifier and finisher to make a strong effort to communicate and work together to obtain a high, consistent level of quality for the finished part.
For electroless nickel deposition, not all substrates are catalytic and initiate plating without requiring additional activation steps. Nickel, steel and aluminum all can initiate electroless deposition with standard preparation cycles. Copper and brass are non-catalytic and require additional activation steps to insure plate initiation. It is important to understand the limitations of some substrate types regarding the ability to plate.
The cleaning in SCRAP. When we refer to cleaning, we think of alkaline soak cleaning as the traditional first step in the preparation cycle. The mechanisms of cleaning should be reviewed and help to explain how cleaners are designed to function. For reference, the appendix in this document contains information regarding the function of components in cleaners and mechanisms of cleaning. The type of soil to be cleaned and the type of substrate determines the strategy or approach to solving a potential cleaning problem.
Iron and Iron Alloys
Copper and Brass
Hot, strong alkaline cleaners and strong mineral acids
Hot, lower alkalinity cleaners and acid salts
Hot, mild and inhibited cleaners and specially developed acids
Lower temperature, mild and inhibited cleaners and nitric acid mixtures with fluorides
It is important to remember that a pretreatment cycle that works well on one substrate with one type of soil may be completely ineffective on the same substrate with other types of soil. This important realization is often overlooked until many rejects have been produced on the process line. Soils not only vary in their basic nature, but the same soil may be more difficult to clean depending upon the original method of application. Was the soil deposited on a part that was stamped, machined or heat treated? A variety of soils commonly found on work in a plating facility include machining oils, drawing lubricants, buffing compounds, sulfurized oils, chlorinated oils and waxes. This first group of soils are handled in the cleaning portion of the process cycle. There are many more types of materials that can be categorized as a soil, including dirt, smut, oxides, rust and heat scale. These types of soils are primarily handled in the activation process.
Once a specific type of cleaning strategy has been chosen, the factors of operating temperature, concentration and agitation are important considerations. Temperature usually has the most impact on the cleaner's ability to "clean." Every 10F increase in operating temperature provides a significant improvement in the effectiveness of soil removal. In the removal of some buffing compounds and chlorinated waxes, elevated temperatures are a necessary requirement to soften and completely dissolve the soil. Soak cleaners of this type are specially formulated for high-temperature operation. Not all cleaners are formulated to operate at high temperatures. Running outside the operating temperature range can cause cleaning problems. If a cleaner is not properly designed for high-temperature operation, the surfactant system will separate (oil out) in the solution because the cloud point is exceeded. The cloud point represents a measure of a surfactant's ability to stay soluble at a given concentration and temperature. Simply stated, cloud point is the measure of the solubility (or insolubility) of a cleaner's surfactant ingredients once the solubility is exceeded. Once this occurs, not only will the cleaning efficiency be reduced, but the surfactant oil residue will contribute to additional contamination and will be dragged down the line into other process tanks. The surfactant system for any cleaner is the heart of its formulation. Surfactants function to solubilize, dissolve, emulsify or undercut soils or their components. Electrocleaners are specially designed to allow current to be passed through the solution to improve cleaning. Because electrocleaners are not designed to remove oils, they will be discussed in the activation section. This is where they more appropriately fit.
Cleaning: Important Points to Remember
The rinsing in SCRAP. The concept that rinsing is the most critical portion of the plating process (next to the finished plated component) is often lost. Our industry's emphasis on waste reduction or water management has great influence on our de-emphasis of its significance and importance on the process line. In some areas of the country, the plating is secondary to the waste treatment concerns. However, buying into the idea that the "plating cycle is a series of steps designed to remove something" requires that the rinsing must be viewed as the primary process step to accomplish this task. After exiting the soak cleaner, what is present on the parts surface interface? Normally, we would find caustic or sodium hydroxide residue, soil residue, soakwetter or solubilized soil-surfactant and unused wetter or surfactant. Increasing the rinsing time effectively removes these materials at the surface interface, although the film remaining does contain minimal wetting agent or surfactant. In most cases this condition is not harmful to the next process stage (usually an electrocleaner). It is also easy to visualize that a simple rinsing scheme, moving the work up and down for one or two swishes, is not likely to be very effective. When we wash our hands, how effective are we at removing the soap residue and film with one quick rinse? Increased time (up to 2 min) in rinses following alkaline cleaning processes are recommended to ensure that the films remaining on the surface interface are adequately removed. This technique of rinsing for 2 min has been demonstrated many times. The success rate is very high. Rinsing efficiency can be improved with the use of air agitation, use of a counter and dam tank overflows3. Fresh process water should enter the rinse tank from a standpipe at the bottom of the tank. This preferred design will cause the fresh water to flow from the bottom of the tank to the top of the tank where it exits the overflow dam.
The installation of a rinse between the soak cleaner and the electrocleaner is an effective approach at reducing the soil load in the plating line. Many process lines are setup to exit the soak tank and proceed directly into the electrocleaner tank without a rinse. This saves process steps, however, testing and experience have shown that the emulsified oils and other byproducts from the soak cleaner do not contaminate the subsequent processes as readily simply by installing this rinsing scheme. It has also been shown that installing the rinse tank in this manner maintains and extends the life of the electrocleaner. It is easy to see the positive impact from the incorporation of this small process cycle modification.
The increased regulatory requirements on waste reduction also have influenced the ability to provide effective rinsing. Still, rinsing is a key process variable that requires control. Water is a primary constituent in cleaning and activating solutions, thus good quality water should be used when making new process tanks. Impurities such as calcium or magnesium contribute to total water hardness. These impurities impair the performance of cleaners by precipitating active ingredients as insoluble residues that reduce the total cleaning action and increase the sludge in the tank.
The activation in SCRAP. Next to rinsing, activation is the most critical step in the preparation process. After cleaning the oils from the surface, all oxides need to be removed to allow proper atomic bonding of the plate to the substrate. Choosing the proper activation can be difficult because of the diversity of substrates available for plating and the need to properly activate each one. How much scale is present on the surface? Is there rust on the surface? These are important considerations regarding requirements for the activation cycle.
The activation process can be broken down into different types or groups. The most common types are acid activators based on acid salts, mineral acids such as sulfuric acid or hydrochloric acid and activators based on citric acid. The choice of which acid to use and its strength depends on the type and condition of metal. Is it a cast or wrought material? Is it susceptible to smut formation? However, hydrochloric acid should not be used because chlorides may be detrimental to the electroless nickel deposit if corrosion resistance of the deposit is an important requirement. Some simple tests can confirm the best choice for the particular application. Other activators in the cleaning line include electrocleaners, electrolytic acids and nickel or copper strikes. For nonconductive substrates, activators based on palladium chloride are commonly used. In either situation, matching the substrate to the best activator is the challenge for successful application of electroless nickel.
Acid activators. There can be many choices of acid activators depending upon the type of substrate. Acid activators that are based on acid salts or bisulfate are used with electrolytic current to improve the activation of many types of substrates. Many of the 300 series stainless steel alloys benefit from this type of activation. Activation is a critical part of the finishing process that involves removal of oxide, rust or smut from the substrate. It can be easy to over activate or under activate any substrate. The control of concentration, time and temperature is just as important for acid activators as it is for alkaline cleaners. Inhibitors are sometimes added to mineral acid activators, such as hydrochloric or sulfuric acid, to minimize attack on the basis metal. Unfortunately, a side effect of these inhibitors is their tendency to form tenacious films on various substrates that are difficult to rinse. This difficulty in rinsing has been shown to cause problems with plating quality, including porosity or dull deposits. If the work does not show an immediate visual defect from the presence of the residue, it is likely to cause adhesion loss later during adhesion testing. The surfactant wetting agents that lower the surface tension are more permissible to be added to acid activators because they are more easily rinsed than the inhibitors. Surfactant wetting agents for acid activators are usually of a similar type that can also be added to the electroless nickel solution. This compatibitliy does not pose a problem with contamination to the electroless nickel. It is important to keep all oils and oily residues out of the acid tanks. The oils tend to redeposit on the surface of the work when transferred from the acid tank. The occurrence of oil in the acid tank may be an indication of inadequate cleaning or rinsing of the work. Work that is heavily scaled can contain oil residue under the scale that is not easily removed in the soak cleaning. After the acid activation removes the scale, the oil residue is released into the acid. Adding the surfactant wetting agent to the acid activation step helps minimize the effect of this problem. This action should not be a substitute for resolving the problem.
Alkaline activators. Electrocleaners are traditionally thought of as part of the cleaning cycle. They are intended to remove soak cleaner film residues while also acting to desmut surfaces using DC current. Upon examining their formulations, there is not much "cleaning" ability built into the products. They are characterized by high caustics to provide solution conductivity. They have very little surfactant loading built into their formulation. It is my preference that electrocleaners be redefined as an alkaline type of activator. Why? Electrocleaners generate either oxygen or hydrogen gas at the cathode or work depending upon whether they are used anodically or cathodically. When connected cathodically (direct), hydrogen gas is generated at the surface of the work. This hydrogen gas acts as a good activator. The disadvantage of a cathodic electrocleaner is that soils and other contaminants have the potential to redeposit on the work. If cathodic electrocleaning is used, keeping the solution relatively clean (low contamination) is required. Anodic current flow is more traditionally used as the standard for electrocleaning because particulate soils are removed from the work by the anodic reaction. Because oxygen is produced at the work, metals that easily form oxides such as high nickel alloys should be cautiously electrocleaned anodically. In these situations, the significance and choice of the acid activation step following the electrocleaning becomes more critical. The use of periodic reverse-direct current is a useful combination for many substrates. The use of anodic or periodic reverse current minimizes the introduction of hydrogen into high-strength alloys. The switching between cathodic and anodic cycles produces a good activation scheme for the removal of smut and oxide. Removing the work from the electrocleaning tank on the anodic cycle is important in this scheme.
Many plating defects have been traced back to the electrocleaning step. For example, a customer was experiencing a problem with a pattern showing up on the plated component. After spending many hours of trying to solve the problem in the plating tank, it was decided to look at other steps in the process line. It turned out that due to a less than optimum working concentration in the electrocleaner parts were exiting the tank with a slight flash rusted condition. How can you obtain flash rust from an alkaline electrocleaner? Actually this is not very difficult under the proper conditions. Too high a voltage with a lower conductivity resulting from the low concentration caused the slight flash rusting condition. Visually, it was difficult to see the flash rust, but this condition was removed in the subsequent acid activation. Unfortunately, the pattern that remained after the removal of flash rust by the acid activation step was the pattern showing up on the plated component.
Alkaline permanganate. This relatively old technology is used in processes where heavy heat treat scale or carburized residues appear on the surface. Mixed at a ratio of 25% w/w potassium permanganate to 75% w/w of sodium hydroxide, a working concentration of 2 lb/gal of the mix is used at 150-190F. It has proven to be an effective alkaline activator for steel substrates because of its ability to remove heavy scale without any basis metal attack. Parts or components that are unable to be successfully electroless nickel plated using conventional practices benefit from the incorporation of this process step into the preparation cycle. In many situations this has been a real problem solver.
Electrolytic strikes. These are often recommended prior to electroless nickel plating to improve the adhesion on many high-strength, high-carbon, stainless or other nickel-chromium type alloys. Most common are strikes based on nickel formulations. These include Wood's, Watts and Sulfamate formulations. Copper cyanide formulations can also be used but are not favored because copper is not catalytic in electroless nickel and additional activation steps are required. Although copper cyanide chemistries are good cleaners, the use of copper deposits as strikes may effect the corrosion resistance of the plated component in a negative manner4. ASTM B-656 references many commonly used strikes prior to electroless nickel plating5.
The plating in SCRAP. Any electroless nickel plating process can be deposited over a properly prepared surface. When surface preparation problems occur, the properties of the electroless nickel deposit are jeopardized. Deposit performance, such as corrosion resistance, is closely tied to the surface preparation success or failure6. Any step in the surface preparation that creates porosity, pits or roughness from smut formation will impact negatively on the corrosion performance. Plating over a film that is not adequately rinsed will likely be the root cause of poor adhesion or increased porosity of the final deposit.
A primary feature of electroless nickel deposits is recognizing that "What you see in or on the substrate is what you see in the plated deposit." Electroless nickel deposits are known for reproducing the topography of the substrate. They are not leveling deposits like electrolytic nickel processes. If a streak or mark appears in or on the substrate, expect the electroless nickel deposit to magnify the defect. One should not expect the electroless nickel to hide the substrate problem. In fact, it is common for an electroless nickel deposit to magnify or highlight a defect. Visual streaks and smear patterns in the deposit, previously thought to be a stabilizer imbalance in the electroless nickel, are easily eliminated by improving the pretreatment rinsing. Today, closer working relationships with suppliers and manufacturers of parts or components make it easier to resolve plating issues related to surface preparation. ISO and other quality standards contribute to a better understanding and the resulting resolution of problems. Automotive companies are also more interested and involved with quality from plating companies. In August of 1996, the Chrysler Material Engineering group sent a letter to their Tier I fastener suppliers recommending better communication and cooperation between them7. They were identifying and pointing out some specific manufacturing responsibilities for the various groups. It is important that these types of pro-active approaches continue in our industry. This is an important step to recognizing each individual's responsibility when it involves problem solving. The following section was taken from this letter:
"All Tier I suppliers must recognize they have complete responsibility for all aspects of quality of the finished part. This not only includes dimensional conformance, heat treating and plating quality, but also conformance to our hydrogen embrittlement specifications. We have found that many of the Tier I suppliers are shipping fasteners to the heat treaters with heading lubricants or thread rolling compounds still on the parts. These compounds produce several problems for the heat treaters and platers. First, most heat treaters are equipped to remove only common oils and greases. Their washing solutions are not formulated to remove the great variety of lubricants used in the fastener industry such as calcium oxide, molybdenum disulfides, graphites or stearates. Thus, subsequent heat treating operations may "burn" these lubricants onto the surface of the steel. When the parts are shipped to the platers, it requires very aggressive acid pickling to remove these burnt on coatings, which subsequently subject the parts to the possibility of hydrogen embrittlement. In our fastener standards, we currently require that either the Tier I supplier or heat treater clean the parts prior to placing them in the furnace. Since the Tier I supplier is the only one aware of what lubricants were used on the parts, it is logical that only he has the know how to remove them. Therefore, at this time, we are urging all Tier I's to review their cleaning procedures, and if necessary, implement methods to provide clean parts to their sub-suppliers."
Tools to solve problems. The waterbreak test has traditionally been used to evaluate how clean a surface may be. The test is based on the ability of a properly cleaned surface to retain an unbroken film or sheet of water. Unfortunately, the test is subject to misinterpretation under certain conditions. Problems occur when retained alkali or surfactants remain on the surface from inadequate rinsing or from a hydrophilic smut being deposited on the surface. Normally, dirty surfaces show a water break. When this smut remains on the work it is not sufficiently cleaned but does not show any signs of water break. Immersing the work in a dilute acid solution before inspection will usually produce a water break situation. Some process acid solutions contain wetting agents that will not produce a water break under these conditions. Instruments that use a technology called photo electron emission (PEE) can be used to troubleshoot cleaning or rinsing problems8. Based on the principle known as optically stimulated electron emission (OSEE), they are also useful in the development a specific cleaning scheme for a part or substrate. The instrument functions by illuminating the surface with UV light which interacts with the surface to emit electrons that are collected and converted into a voltage and displayed as a value. A clean surface as recognized by the monitor emits the maximum amount of electrons while a contaminated surface attenuates the electron flow from the surface and results in a lower value. Basically, the clean surface produces a high reading and a soiled or contaminated surface produces a low reading. The values are used to determine "relative" differences in the surface quality.
Using OSEE. As reviewed previously, the presence of thin film contaminants on surfaces can result from inadequate or incomplete cleaning or rinsing methods. They also form due to oxide growth during the time between cleaning and activation or from failure to properly protect cleaned surfaces from oils, greases, fingerprints, release agents or from the deposition of airborne molecules generated by adjacent manufacturing or processing operations. In one instance, a customer was having problems with inconsistent adhesion of its electroless nickel deposit over carbon steel. Of course, the initial focus of the problem was the electroless nickel solution. After verifying the electroless nickel solution was in optimum condition and that the percentage of defects was only 15% of the total production for the particular part, an investigation into the process cycle was begun. Using the OSEE instrument and a matrix of evaluations, parts were removed from the line and tested after the first rinse after soak cleaning, electrocleaning and acid activation. As the production day progressed, the OSEE values from the rinse tanks steadily decreased. After reaching a value one half of where they were when the day started, a much higher percentage of poor adhesion defects was realized. It was determined, through instrument testing, that the water flow in the rinses after the electrocleaner was of insufficient quantity and quality to remove the film before the parts were reaching the acid activation step. A surface film was being set up on the work and the resulting higher percentage of poor adhesion was noted. Using a garden hose with fresh water to rinse the parts coming out of the rinse tank station produced a dramatic increase with the OSEE readings. Today, increasing the total water flow for the line and more frequent dumping of these rinses between shifts has continued to keep the reject rate low.
The interactions between numerous types of soils with the various types of cleaners can present overwhelming possibilities regarding the outcome of any cleaning situation. Is an emulsifying type cleaner better than a cleaner that splits the soil? What is the best approach? For example, certain types of water soluble cutting fluids or their residues can be easily "set up" or precipitate on the surface of work if the wrong type of alkaline cleaner is chosen. Another effective use of the OSEE instrument is during development of a cleaner or a cleaning cycle for a particular part. It is effectively used in determining the best approach to the removal of a specific type soil. One can determine how "free rinsing" a cleaner will be based on the type of soil being removed. How effective the surfactant is rinsed is determined by using the OSEE instrument as well. In situations where the best products for any prep application need to be determined, comparative cleaning tests are conducted using the "best practice" or present cycle with OSEE used as a guide.
Using conductivity. Rinse water quality can be monitored and flow controlled with conductivity meters. The principle of conductivity works and is related to the level of contamination in the rinse water. It is important to realize that using conductivity by itself will not always ensure good quality rinsing. Effective rinsing is related to the time in the rinse, the temperature of the rinse water and the concentration of contaminants on the work and in the rinse. There have been many times when plating problems were solved by simply using what is commonly called "the two minute rule." During pretreatment, if one keeps the work in the appropriate rinses for 2 min, there is a noticeable reduction in most types of rejects. Remember the analogy about washing your hands with soap and rinsing with water? How long does it take to completely rinse the soap residue with cold water? Is 30 sec, 1 min or 2 min necessary to provide adequate rinsing? Rinse water temperatures tend to be 50F especially in winter months in many parts of the country. One can shorten the time needed to rinse the residue by heating the rinse water although this may not be cost effective or practical on a plating line. Immersion time is a critical variable when concerned about rinsing schemes.
Conductivity meters are designed to control the amount of water used in continuous flow rinse tanks. The conductivity sensor is set at a tolerable contamination level and submerged in the tank. Water cannot flow into the tank until dragin from rinsed parts causes the contamination level in the tank to rise above the set point. The sensor signals the solenoid valve to open, adding fresh water. Using these devices will help to maintain optimum water purity, insuring effective rinsing. However, there are concerns when using this equipment. The problem and most often asked question is, "What is the best conductivity for a process rinse?" This is a difficult question. There is no value to look up in a book. Because plating has many variables, each situation has to be evaluated individually. Water temperature as well as many other variables also affect conductivity values. Keeping all variables as consistent as possible will assist with conductivity control.
In another instance, a customer asked for assistance in setting up the conductivity meter controls for its process. These controls were installed after the electrocleaner, after the acid activator and after the plating tank. It used counter-flow rinses in the process line. In the lab, percent v/v standards of each major process solution were made up. The electrocleaner, the acid activator and a new and used electroless nickel solution were added to separate rinse water samples in the following increments: 0.5%, 1.0%, 3%, 5%, 8%, 10%, 15% and 20%. The conductivity for each sample was measured and the data graphed. This information assisted with the rinsing scheme setup for the line. For this particular installation, it was determined that a 5% v/v contamination level with the counter-flow rinsing scheme was acceptable. The conductivity controls were set up and adjusted to add fresh process water at his point.
Additional control for cleaners. One challenge that every finisher faces is determining when a cleaning process solution is no longer viable to produce quality work. At what point is the performance going to change without causing rejects? To help with this dilemma, a nonstandard analytical approach can be examined. Using three titration indicators has shown to be useful in tracking cleaner life9. Azo violet (pH 11-13), phenolphthalein (pH 8.5-9.7) and methyl red brom cresol green (pH 4.2-5.4) can all be used to gain greater process control with alkaline cleaners. The azo violet is used to measure free alkalinity as sodium hydroxide while phenolphthalein is used to measure the free alkalinity of all other builders in the cleaner formulation. The methyl red brom cresol green is a measure of total alkalinity. Most standard titrations for cleaners are based upon the phenolphthalein endpoint to determine the working concentration. In many situations this has been shown to be a misleading titration.
A primary problem with the phenolphthalein endpoint is that alkaline cleaners formulated with sodium hydroxide have an affinity to absorb carbon dioxide from the air. The older the cleaner, the more carbon dioxide that has been absorbed. This absorption forms sodium carbonate that contributes to this titration. The problem is the fact that as sodium hydroxide is used up or converted to carbonate, the phenolphthalein titration value stays the same or increases. This action signifies a loss in cleaning efficiency. An azo violet titration of zero indicates that the free sodium hydroxide formulated into the cleaner has been consumed. Since many soils are more easily removed when free sodium hydroxide is present, a decrease in detergency and a resulting decrease in cleaning can be expected. The high methyl red brom cresol greem mixed indicator titration is indicative of high levels of contamination from ingredients of soils that can also contribute to this titration. Examples of these types of soils are soaps, fatty acids, fatty oils and amines. Even though the insoluble soils are removed from the work, some soluble portions of these soils are accumulating in the cleaner and making it difficult for the cleaner to perform its desired function. A reduction in detergency is usually the result. This titration strategy, when employed, can be an effective tool to assist with the determination of cleaning ability or assisting in the troubleshooting of problems.
When all else fails. When problems occur, our solutions are only limited by our innovative approaches or imagination. Some situations present themselves where normal approaches do not produce the necessary results. What do you do? I have been faced with many of these situations and have found the following approaches successful.
Pre-dips. Although not mentioned as part of the pretreatment cycle, pre-dips have been important life preservers on the plating line. When adhesion problems are encountered or there are smears or streaks in the nickel plate, an ammonium hydroxide pre-dip is an effective weapon. Placed in the line as a "dead" rinse station preceding the electroless nickel tank, it produces many advantages. Used at 0.25 to 0.5% v/v, the ammonia is a good film remover and can be harmlessly dragged into most electroless chemistries. Its ability to remove films may be the reason why we find it in most glass cleaners. It also functions to assist with the initiation of the electroless nickel on the work surface. Initiation of the nickel phosphorus is especially critical on large heavy mass components where heat up time of the part is important for deposition to occur. Also benefiting from an ammonium hydroxide pre-dip would be parts with higher levels of porosity because the ammonia quickly initiates plating inside the pores. Providing a slightly alkaline film prior to the acid electroless nickel has proven to minimize potential problems on many process lines. Sodium bicarbonate can also be used as an effective pre-dip prior to electroless nickel. Don't be afraid to give either of these options a try.
Flexibility. Often times when problems occur on the plating line and there appears to be few options for change in the process cycle, what is the best approach? Allowing flexibility in the preparation cycle will solve many problems. Even if one is unable to adopt the approach on a process line due to limitations, don't discount trying the approach. It will identify the problem. Maybe there is another way to resolve the problem. An example of one such time occurred with a long standing customer. It had for many years successfully plated a medium (8%) nickel phosphorus deposit on high-carbon steel automotive parts. The customer maintained its preparation cycle at optimum levels. Its documentation was always updated, and it generally followed accepted plating practices. The customer is a quality shop in the true sense. One day it was told to change from the 8% phosphorus electroless nickel to one that produced a deposit of 4% phosphorus. The idea for the change was to take advantage of the improved hardness and wear properties of the lower phosphorus electroless nickel. It installed the process and immediately saw a higher incidence of chipping. The reject rates were unacceptable at 30%. Due to variations with the deposit properties and the difference with the nickel initiation caused by a different type of electroless formulation, there was an adverse reaction. These were the exact same substrates that had been successfully plated at only a 2.5% reject rate previously. The plating process cycle now required a change to allow for successful plating. The old cycle of soak, anodic electroclean, hydrochloric acid, anodic electroclean required a modification to be successful in this situation. The new cycle changed the order and added some additional steps to produce successful results. The new cycle of soak, hydrochloric acid, periodic reverse electroclean and ammonium hydroxide pre-dip produced excellent results. Except for adding a pre-dip, all the same products continued to be used in the process. The resulting reject rate now was only 0.8% for the particular part with the lower phosphorus electroless nickel. This story is significant in that a change with the pretreatment solved a potentially dangerous problem. Many times I have seen similar situations create a change of chemistry suppliers on the process line. This is a case where communication and a good working relationship averted serious results.
Many reported electroless nickel plating problems can be traced back to a relationship between the substrate condition and how well it reacts or acts in the preparation cycle. The importance of preplating operations can not be over emphasized when attempting to produce quality electroless nickel deposits. Having the best electroless nickel solution or system will not change the fact that a poor surface preparation will cost the finisher more time, more money and more headaches. It is hoped that this information reviewed with other written papers on the subject will provide guidance and insight in solving problems on the electroless nickel plating line.