Chapter 3 Troubleshooting Electroless Nickel Plating Solutions
Contents
1. By noticing imperfections early problems can be avoided later in the plating process itself Pretreatment of various substrates is covered extensively in another section of this book EQUIPMENT MECHANICAL NEEDS When bath parameters appear to be satisfactory and pretreatment processes are in order the next area of concern is the type of equipment and the mechanical 104 ELECTROLESS PLATING techniques that are being used Electroless nickel solutions should have constant filtration to eliminate any particulate bath impurities such as dust sand or loose maskant material With proper filtration 210 turnovers hr at 5 um or less roughness of the final deposit can be greatly reduced or even eliminated Replacement of these filters on a regular basis is recommended so that retention of pore size of these filters is maintained Since electroless plating is an autocatalytic process it is vital to remove all foreign particulates so that plate out will be virtually eliminated Plate out if allowed to occur can lead to roughness of the deposit bath decomposition and excessive plating costs The type of heating that is used in the plating process is also of great concern Electroless nickel plating tanks can be heated internally or externally Excessive localized overheating can cause plate out roughness or even bath decomposition Lack of agitation of the plating solution can also cause problems Solution stratification can occur resulting in2. alloys poorly cleaned or exposed plating racks or the water itself Heavy metals that are of special concern include but are not limited to lead cadmium copper bismuth arsenic and palladium Lead can be built up in an electroless bath by dissolution of leaded alloy substrates and by the improper use of lead as a masking material Lead concentrations of greater than 5 ppm can cause dark deposits skip plating pitting short bath life and even cessation of plating Copper contamination of electroless nickel baths can be equally detrimental Copper concentrations of 2100 ppm will cause immersion deposit on ferrous alloy parts which in turn causes adhesion problems of the electroless nickel plate Poor pretreatment which leads to poor initiation on copper may allow excessive amounts of copper to dissolve in electroless nickel baths Checking preplate cycles where acid pickles leave an immersion copper deposit on ferrous substrates may also be the source of copper ions Cadmium is usually introduced by the use of plating racks that had been previously used in cadmium plating Contaminated cleaners may also contain cadmium from stripping operations Cadmium and lead can also build up in the bath when either or both are used as brightener and stabilizer respectively Cadmium concentrations of gt 3 ppm can cause dark deposits feathering around the holes and skip plating Caicium and magnesium are introduced from the water supply system
3. gas pitting patterns and or streaking of the deposit Proper agitation allows uniform distribution of plating chemicals and helps to eliminate localized overheating Work rod agitation or clean filtered air is suitable for most electroless nickel solutions Agitation of the solution or parts is necessary to provide a fresh supply of solution to the parts and to enhance the removal of hydrogen gas produced during deposition in most cases high temperature stress relieved polypropylene is the material of tank construction Etched tanks can become more active towards electroless processes and should be replaced to minimize plate out on the tank walls Stainless steel may also be used as an alternative for tank construction A small anodic charge will minimize plate out on these tank walls Caution must be used when plating in stainless steel tanks even anodically charged tanks because the walls may become catalyzed and plate accompanied by an excessive usage of metal Plastic drop in liners are convenient to use provided all sizing materials are leached out prior to use This is usually accomplished by soaking the liners in hot deionized water or dilute sulfuric acid and then neutralizing them with acid or alkali Retained sizing materials in the liner can cause pitting and roughness besides creating a foam on the surface of the solution CONTAMINATION OF THE SOLUTION When all other parameters appear to be in order extraneous solution conta
4. Chapter 3 Troubleshooting Electroless Nickel Plating Solutions Michael J Aleksinas The chemical reduction potential of the electroless nickel plating reaction can be affected by many factors hence troubleshooting these solutions can be difficult and time consuming A working knowledge of the chemistry of the electroless process coupled with the selection of the proper pretreatment techniques is necessary to insure reliable consistent results In order quickly and effectively to solve the technical problems that may arise it is first necessary to outline the typical problems that can arise These problems can be categorized into four basic groups Bath chemistry imbalance Improper substrate preparation activation Equipment mechanical problems Solution contamination By taking a logical approach to these four categories troubleshooting can be accomplished effectively and efficiently BATH CHEMISTRY Compohents of the electroless bath include an aqueous solution of nickel ions reducing agent s NaH PO DMAB etc complexing agent s buffer s and stabilizer s These components work in concert with each other and operate in specific concentration temperature and pH ranges Optimizing these parameters can be different from solution to solution Maintaining the optimum metal content reducer concentration pH and temperature of the solution can minimize many of the technical problems that can arise Analytical techniques to deter
5. If allowed to build up through evaporation these contaminants will cause rough hazy pitted deposits Precipitation of insoluble compounds may even lead to spontaneous decomposition of the plating solutions The proper corrective action is to check the deionized water system or if using tap water to change to deionized water Palladium is used in the activation of non catalytic substrates If not rinsed properly palladium ions or particles may be introduced into the electroless bath which form nuclei and cause spontaneous decomposition of the plating bath Most of the kinds of contamination described above can be reduced by carbon treatment for organic contaminants or dummy plating for heavy metals The effectiveness of these techniques depends on the type and quantity of the contaminant and the age of the plating solution Carbon treatment can leave residual carbon which could lead to considerable roughness or dullness Some contamination can even be leached from the carbon itself Carbon treatment can lead to destabilization of some electroless nickel baths because of its ability to 106 ELECTROLESS PLATING remove the stabilizers and brighteners necessary to the bath chemistry itself Excessive dummying of the solution may lead to severe reduction of other bath constituents such as nickel hypophosphite and stabilizers Maintaining optimum ratios of these constituents is imperative to successful operation CONCLUSION In addition t
6. Too low temperature Too low pH Low nickel or hypo concentration Metallic contamination Organic contamination Aged bath Poor agitation Poor surface preparation Metallic contamination 107 Suggested Remedy 1 2 3 4 5 6 7 1 2 3 4 1 Improve cleaning and pickling cycle Improve rinse and transfer time Analyze and correct zincating solution Dilute or dummy plate solution Carbon treat solution Reduce transfer times Correct time and or temperature of heat treatment Filter solution and locate source of solids Improve cleaning and rinsing Lower pH Improve rinsing clean rinse tanks Leach tanks and filters prior to use Reduce drag out check replenishment cycles Use deionized or distilled water Improve filtration 2 Reduce workioad lower pH 3 4 Carbon treat solution Dummy plate 5 Improve agitation work rod 1 2 3 4 9 6 1 2 3 preferred Raise temperature Raise pH Check and correct Dummy plate Carbon treat Replace with new bath Improve agitation Improve and correct cleaning cycle Dummy plate 108 Step skip plating Low deposition rate instability No deposition Dark deposits Rapid pH changes ELECTROLESS PLATING 4 Surface residue 5 Gas patterns 1 Metallic contamination 2 Substrate effect i e leaded alloy 3 Bath overstabilized 1 Low temperature 2 Low pH 3 Low ni
7. ckel or hypo content 4 High orthophosphite content 5 Too small a workload 6 Overstabilization 1 Bath temperature too high 2 Too high pH 3 Localized overheating 4 Improper passivation of tank 5 Airborne contamination 6 Drag in of catalytic metals 7 Large additions made of replenishers 1 Overstabilization 2 improper substrate surface 3 Too low temperature or pH 4 Metallic contamination 1 Contaminated rinse after EN 2 Improper surface preparation 3 Too low pH and or temperature 4 Low bath activity 5 Organic contamination 1 Drag in of pretreatment 2 Excessive workload 4 improve rinsing minimize silicate drag in 5 Reposition work increase agitation 1 Dummy plate 2 Copper or nicke strike prior to plating 3 Dummy plate or dilute bath 1 Increase operating bath temperature 2 Raise pH 3 Analyze and correct 4 Discard all or part of bath 5 Increase workload or reduce agitation 6 Dilute or dummy bath 1 Lower temperature 2 Lower pH 3 Locate and correct 4 improve passivation solutions and times 5 Clean area of dust and loose dirt 6 Improve rinsing 7 Use more frequent additions to maintain consistent stabilizer concentration 1 Dilute bath avoid large additions of replenishers Substrate may not be autocatalytic and require a nickel or copper strike i e Stainiess steel copper 3 Analyze and correct 4 Electrolyticailly
8. dummy sotution 2 1 tmprove rinsing 2 Improve pretreatment 3 Check and correct 4 Analyze bath constituents and correct 5 Carbon treat 1 Improve rinsing 2 Reduce workload and check plate out on tank and heaters Troubleshooting Electroless Nickel Plating Solutions 109 High nickel usage Cloudy plating solution 3 Bath not at proper pH range 3 Check pH and adjust to 1 2 3 4 2 3 High drag out Bath decomposition Plating on tank and equipment High surface area pH too high Drag out losses excessive Under complexed solution optimum buffered range 1 Reduce drag out with replacement of stagnant rinse after plating tank 2 Cool and filter solution 3 Filter solution and strip tank and heaters 4 Reduce workload size 1 Lower pH with dilute Sulfuric acid or hydroxyacetic acid 2 Reduce sources of drag out losses Add more make up additive 3 Add more make up additive
9. mination can bea reality The electroless nickel plating reaction is affected by many impurities Trace impurities can be organic in nature such as oils or solvents or inorganic such as silicates or nitrates Metallic contaminants such as lead copper cadmium bismuth etc can cause severe problems if they are introduced into the bath in excessive quantities Most metallic ions will plate out and have little effect on the electroless nickel process if they are introduced into the plating solution in small quantities The organic contaminants can come from degreasing solvents oii residues mold releases drag in of cleaners or acid inhibitors and unleached equipment Troubleshooting Electroless Nickel Plating Solutions 105 or filters These contaminants will manifest themselves in cloudy streaked deposits along with poor adhesion Inorganic ions such as nitrates can be introduced from improperly neutralizing tanks after stripping with nitric acid High levels of nitrates can reduce the plating rate or even stop deposition entirely Silicates are equally detrimental Drag in of these ions usually from preplate cleaners can form gelatinous films on the work which is manifested by cloudy deposits or pitting Improved rinsing will decrease this occurrence Metallic impurities can have a profound effect on electroless nickel plating and can be introduced by drag in from previous tanks dissolution of base metal of the substrate i e leaded
10. mine these parameters can be obtained from text books or from suppliers One of the first and easiest bath parameters to be checked is pH The pH is determined by pH meter or pH paper Caution should be used here because some pH papers may differ from the electrometric readings by as much as 0 5 pH units In some electroless processes pH papers can give erroneous values because of the salt ion effect of the solution As an electroless bath ages there 101 102 ELECTROLESS PLATING is a buildup of ions such as NH Na K SO Cl etc and all can contribute to the change in ionic strength of the solution Consequently pH paper may act differently in new solutions than in old solutions Electrometric pH readings are preferred Calibration of the pH meter is mandatory for reliable information Electrometric pH meters require standardization between two units of the pH scale e g 4and 7 One buffer solution should be above the expected pH of the sample to be measured while the second buffer should be below Once the meter is standardized to both of these standards the solution pH can be measured A common error in reading pH accurately is temperature variations between the bath sample and the two buffers The proper procedure is to have identical temperatures for both the buffers and the sample Thus a hot bath sample should be cooled to room temperature or the buffer solutions should be heated to the same temperature as the sa
11. mple A high pH value of the plating solution can cause abnormally high plating rates that may lead to roughness pitting and or cloudy deposits Too low of a pH will cause slow deposition rates and matte dull finishes The temperature of the solution should also be closely monitored for consistent high quality deposits Accurate temperature controllers are necessary Checking temperature controllers with certified thermometers provides further assurance that temperatures are maintained at proper levels Unchecked temperatures can lead to decomposition of the plating bath temperature too high or no plating at all temperature too low Finally solution imbalance as a result of poor maintenance of the nickel and or reducing agent concentrations can give slow plating rates poor coverage and dull deposits Maintaining the nickel and reducing agent concentrations within 10 to 15 percent will usually provide consistency in terms of rate color and stability Periodic analysis of these constituents is required with the frequency of analysis proportional to the workload being processed Excessively large replenishments can lead to overstabilization or suppressed rates as many of the components used for replenishment often contain high concentrations of stabilizers and or brighteners which may act as catalytic poisons In home brew situations where commodities such as pH adjusters metal salts and reducing agents are added to the plating solu
12. nts Buffing compounds Oxide films replaced on Al Weld scale Fluxes oe 0 Depending on the type of soils present different pretreatments are needed Pretreatment choice should be the best available for the specific substrate and should be closely monitored Cleaners and pickling solutions should be changed at predetermined intervals to eliminate the possibility of ineffective cleaners and descalers which will cause poor adhesion streaky deposits or blistering The quality of the substrate itself also must be checked carefully as a potential problem source Often plating problems resulting from inferior substrates are wrongfully diagnosed as pretreatment or bath chemistry problems for example intermetallic compounds at aluminum substrate surfaces can manifest themselves as nodules or pits in the final plate In other cases porous castings or powdered metal substrates can entrap solutions which result in bleedout or voids The way in which a part is stamped cast drilled or machined can have a great impact on the final plated product Improperly stamped parts can imbed difficult to remove oils or compounds into the surface of the part This will lead to dull and often non adherent coatings Improper temperature control while casting aluminum or zinc die cast parts can cause lamination of the substrate which will lead to uneven appearance in the end plate Obviously then much attention must be focused upon the raw part itself
13. o the aforementioned observations a list of the most common problems found in electroless nickel plating is included at the end of this section Sources of these problems and their remedies are also listed When electroless nickel coated parts are rejected because of roughness lack of adhesion poor coverage or lack of uniformity stripping and replating is feasible The stripping should be done with as much care and planning as required in the original plating process To choose wisely from the many strippers available it is important to take into consideration the nature of the substrate the deposit is to be stripped from time thickness of the final plate and overall cost Stripping from steel usually can be accomplished with nitric acid solutions cyanide nitroaromatic caustic solutions or amine based strippers In situations where concentrated nitric acid solutions are used parts to be stripped should be dry prior to stripping and rinsed immediately after stripping Close attention to this process is required to minimize water drag in Severe etching can occur if the stripping solution does become diluted with water Copper and its alloys can also be stripped with inhibited strippers which tend to contain some type of sulfur inhibitor These strippers may also contain amines or other complexors along with oxidizing nitroaromatic compounds Aluminum alloys are easily stripped in concentrated nitric acid solutions Minimum immersion times are p
14. referred here to minimize any possible attack of the aluminum surface itself What is evident throughout this chapter is that troubleshooting is not an easy task First classifying the problem into one of the four categories then identifying the cause of the problem are the two most important steps to a viable troubleshooting strategy When the problem is identified the economics of remedying the solution should be addressed Depending on the nature of the problem the difficulty or cost to remedy the solution and the downtime required to cure the problem wiil determine whether the bath should be discarded or if it should be troubleshooted Troubleshooting Electroless Nickel Plating Solutions Troubleshooting Guide Problem Poor adhesion and or blistering Roughness Pitting Dullness Patterns streaking Electroless Nickel Probable Causes 1 2 3 4 5 6 7 1 2 3 4 5 6 e 7 1 2 3 4 5 1 2 3 4 5 6 1 2 3 Improper surface conditioning Poor rinsing On aluminum poor zincating Metallic contamination Organic contamination Reoxidation Improper heat treatment Suspended solids improper cleaning Too high pH Drag in of solids Contaminated liner or filter cartridges Inadequate nickel chelation Contaminated water supply Suspended solids Excess loading Organic contamination Metallic contamination Poor agitation
15. tions only high purity chemicals from an approved source should be used Low cost chemicals have often been found to contain impurities that may lead to pitting dullness or suppressed plating rates Insoluble materials can also be present which will cause roughness or act as nuclei for the spontaneous decomposition of the bath Keeping records of replenishment additions pH adjustments and temperature readings provides valuable information for maintaining the chemistry balance of the solution Automatic controllers have proven useful if they themselves are closely monitored The use of controllers avoids large additions to the plating bath and eliminates the possibility of bath over Stabilization When bath parameters are found to fall within the given ranges and Troubleshooting Electroless Nickel Plating Solutions 103 probiems still exist then such circumstances as inadequate surface preparation and or contamination of the bath should be examined SUBSTRATE ACTIVATION Proper preparation of the substrate to be plated is vital for quality results Poor surface preparation can cause lack of adhesion deposit porosity roughness non uniform coatings and or dark deposits A properly prepared substrate is one whereby surface contamination is removed which leaves a clean nominally oxide free surface Typical surface contaminants that must be removed or replaced prior to plating usually include one or more of the following Oils lubrica
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