Guest Editorial -For Plateworld.com
Don Baudrand, Don Baudrand Consulting, e-mail:email@example.com
Failure of Electroless Nickel Coatings, Detection and Prevention
When expected results are not realized when plating electroless nickel, failure analysis should be done to determine the reasons/causes. For corrective action to be effective, knowledge of all the possible causes for failure must be considered, including the determination that a particular type of electroless nickel coating is correct for the intended use. This paper discusses many causes for failed coatings. Means of detection, measuring and corrections are suggested. Deposit characteristics such as hardness, tensile strength, electrical and electrical properties, corrosion and corrosion resistance, toxicity and environmental factors are included. Factors influencing deposit characteristics are also enumerated. The many types of electroless nickel coatings and their differences as applied to different applications are discussed. Improper selection of the type of deposits and appropriate thickness for the applications are high on the list of failure causes. Guidelines are suggested.
Too often electroless nickel deposited coatings fail to meet expected performance. Sometimes the expectations exceed the capabilities of any electroless nickel coating. More often, the wrong thickness or the wrong selection of the type of electroless nickel is specified or used. There are several types of electroless nickel processes providing different physical characteristics in the deposited coating. For example, low phosphorus, medium phosphorus, low phosphorus alloys and nickel boron processes having low or high boron content are used by the plating industry. Most Platers have knowledge of the appropriate selection of electroless nickel are best suited for the application. Many engineers who specify electroless nickel do not know the capabilities, thickness, effect of basis material, or shortcomings of electroless nickel products resulting in specifications that lead to failure of the coating for the intended purpose.
Failure can take place in many ways. For example: Adhesion failures such as peeling, blistering or flaking of the coating, insufficient thickness to adequately protect the basis material from corrosion or wear, rough deposits where roughness leads to wear failure, or increased friction, or detracts from appearance, or results in corrosion failures. Pitted deposits result in corrosion failures or poor appearance. Knowledge of the physical characteristics of the various types or electroless nickel coatings will lead to better judgment in the selection of the process that will suit the application best. Physical characteristics, failure modes, detection of failed coatings and remedies are discussed.
Poor Corrosion protection or resistance
Wrong selection of plated coating for the application. Contaminated plating solution. Poor chemical control. Rough or porous basis material. Insufficient plate thickness.
Detection is usually visual, or specific accelerated tests such as neutral salt spray (ASTM B 117), CASS (copper/ acetic acid salt spray), humidity tests or various porosity tests such as the "ferroxyl" test.
Selection of the right plating process deposit depends on the required performance. Electroless nickel processes vary thus the deposit characteristics vary. Nickel phosphorus coatings with High phosphorus content (9.5-11 wt. percent Phosphorus) exhibit the best corrosion resistance. That is they can withstand chemical attack better than other alloys. However in order to protect the basis material, it must be pore free. These deposits are not sacrificial to steel alloys. Any porosity would lead to corrosion of the basis metal if it were an iron alloy. Copper alloys have a much lower corrosion potential and require less thickness for good protection.
Mid phosphorus deposits (5-8% P) can protect basis materials also if it is pore free. Many mid Phosphorus processes produce deposits which contain sulfur compounds and other trace materials which make these deposits more vulnerable to attack by corrosive chemicals, sometimes requiring greater coating thickness than for High Phosphorus (P) coatings.
Low phosphorus (1-4%P): Most low P processes produce coatings that are corrosion resistant, but not as effective as high P coatings. Low Phosphorus processes can also contain trace materials that can cause corrosive attack. One benefit to low P deposits is low electrical resistance. Pure Nickel is about 7 micro-ohm-cm. 1- % P deposits approach that of pure nickel. 4% P deposits are about 20 micro-ohm-cm. For comparison, tin and lead are about 11 micro-ohm-cm resistivity. High P deposits can be as high as 150 micro-ohm-cm. Nickel boron deposits compare to the very low P deposits in most characteristics. Some nickel boron deposits contain trace materials, which make them more vulnerable to corrosive chemicals. The specific resistivity for low Boron (0.2-1% B) deposits is about 7.5-10 micro-ohm-cm. For high Boron deposits 2-4% B the resistivity is about 12-18 Micro-ohm-cm. Nickel boron deposits are well suited for many electronic applications. The cost per gram of nickel deposited is about 8 to 10 times that of nickel phosphorus processes.
Failure to meet thickness specifications is one of the most frequent causes for rejection.
Accurate thickness measurements are important. Specifications are needed that identify critical areas where it is important to maintain thickness and areas that are not critical to performance or corrosion protection. Thickness variations are a fact of life for electroplated deposits due to the laws of current distribution. Shields, auxiliary anodes and "robbers" can be used to minimize thickness distribution variations, but at considerably higher cost. The possible exception is for gold electroplating. Electroless nickel deposits are uniform in thickness where ever the solution can contact a catalytic surface. If hydrogen bubbles are entrapped during plating, little or no deposition can occur. Thickness measurements should be adjusted for density differences, particularly for Beta back scattering measurements that are compared to pure gold standards. Also for electroless nickel phosphorus or nickel-boron alloys when pure nickel is used for the standard. For example, pure gold has a density of 19.3 gm/cm3 while electrodeposited hard gold (containing cobalt or nickel) has a density of 17.5-19 gm/cm3. Since only gold is measured, a low thickness reading may result if density is not considered.
Gold thickness = Beta back scattering reading x gold density-
gold deposit density
Methods and specifications: ASTM B-659 lists more than 21 methods, referencing each ASTM method. The best method so far for testing plated deposits is the use of X-ray fluorescence. (XRF) There are numerous other methods, including microsectioning and microscope measurements; Magnetic thickness testers (limited to magnetic substrates plated with a non magnetic deposit); Micro balance for on line testing; Beta back scattering; Coulometric dissolution, surface profiler using laser scanning, Scanning Electron Microscope (SEM), Field-emission Scanning Electron Microscope (FESM), etc.
Detection is visual, peeling, blisters or Poor cleaning or poor activation.
Failure of specific adhesion tests. Hydrogen entrapment.
Testing is done by various means; --ASTM bend tests, Saw test, smash, etc. Quantitative tests are "Ring-Shear test" and "conical pull test."
Roughness Plating solution in need of filtering.
Detection is visual, feel, or Microscope. Rough basis metal. Magnetic particles on basis material. Poor cleaning. Low stabilizer. Bath decomposing.
Pitted deposits Porous surface onto which plating takes place. Inadequate cleaning prior to plating
Contaminated plating solution. High Stabilizer or high agitation.
Skip plating Contaminated surface. Contaminated plating solution.
Poor rinsing. Oil on rinse tank. Too much agitation, or stabilizer content too high.
Solution out of balance.
Poor coverage Item to be plated is not designed for plating. For
electroless nickel plating, too high stabilizer or too much agitation,
or both. Gas entrapment.
Poor rinsing, contaminated rinse water.Bleed out from porous basis material
Dull deposits that are expected To be bright. Dark deposits
Plating solution out of balance chemically.
Impurities in the solution, usually organic, sometimes metallic. Low stabilizer.
Sources of organic impurities are: Tank lining: Plasticizers, oils, colorants, fillers in the surface, biocides, impact modifiers, mold release aids and stearates must be leached from new linings prior to filling with plating solutions. All these are detrimental to plating solutions. Masking materials contain solvents and plasticizers, which can contaminate plating solutions. Proper curing can help prevent contamination. Drag-in from rinse waters, or from cracks in the rack coatings that entrap preparation solutions, introduce impurities. Removal of impurities is usually by carbon treatments, or dummy plating.
DEPOSIT FUNCTIONAL FAILURES Detection
Hardness Tested by the Knoop method, ASTM procedure. 100 gm load for hard deposits
Lower loads for soft deposits. Vickers was
formerly used. Vickers and Rockwell methods are not recommended by ASTM
for plated deposits. Correction for electroless nickel, heat treat or alloy plating.
Strength Tensile test on a dog bone one mil thick (0.001" or 25 micro meters) Measure using a tensile tester.
Ductility Elongation % using tensile test procedures.
Bend test on a 1-mil thick deposit removedFrom the basis material as in tensile testing. Bend over a radius, note first cracking.
Wear resistance Tabor Abrader, Phalanx Wear tests.
Selection of the deposit or adjustment of alloy and/or heat-treating at the correct temperature for the alloy will provide correction.
The actual test results will depend on the application. For example, electroless nickel will perform better than hard chrome on blocks of the Phaelix tester. Hard chrome will perform better than electroless nickel on a Tabor wear tester. There are many types of wear, including fretting, sliding friction, rolling friction, abrasive wear, chemical attack and erosion. All are tested differently depending on the desired results for the application.
Nickel deposits of all types are barriers to diffusion and corrosion. To protect a basis material the deposits must be pore free. Porosity if most often caused by rough or porous basis materials. Careful attention to selection of basis materials for plating is very important if desired results are to be obtained. Electroless nickel covers all areas the solution can reach. Blind holes, which entrap air or the hydrogen gas, generated when plating will not be plated. Care must be taken to be sure such holes face upward in the plating solution where gas can escape.
Nickel deposits are also good diffusion barriers to prevent migration of copper or gold into other coatings. Electroless nickel is a superior diffusion barrier for most applications.
Zinc and cadmium are sacrificial to iron alloys, that is, they corrode preferentially thus protecting the basis metal. Tin, solder alloys, palladium, gold, copper and nickel are not sacrificial coatings, and like nickel must be pore free to protect.
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