Guest Editorial -For Plateworld.com                                                 

 Don Baudrand, Don Baudrand Consulting,   e-mail:donwb@tscnet.com

 

Doís & Doníts of Electroless Nickel Plating

Solutions using sodium hypophosphite

Electroless nickel plating (EN) has application in almost every industry. The family of characteristics of EN deposits is defined by variations in the composition of the plating solutions. High phosphorus, (P) 10% or more, produces a family of deposit characteristics such as highest corrosion resistance, most ductile, high electrical resistance, Mid range P deposits can be made brighter, have somewhat less corrosion resistance, but the MidĖP EN baths plate faster at a given set of plating conditions. Low P EN baths, 4% P deposits, are the hardest of the family and 1-3% P have low electrical resistance 8-10 micro Ohm cm. (lower than most solders)

Common characteristics include:

Diffusion barrier

Uniform thickness over all surfaces that can be reached by the solution.

Hard deposits

Magnetic properties can be varied. (Non-magnetic, high P to magnetic, mid and low P)

Solderable using special fluxes

Salvage worn or miss-machined parts

Composite deposits are possible such as PTFE, diamonds, Silicon carbide, nano-particles etc.

Alloys are possible such as tungsten, molybdenum, boron, etc.

Plating Solution Control

Electroless nickel plating solutions are more sensitive to solution composition and operating conditions than electroplating solutions. More care and attention is required to maintain the proper concentration of each component in the plating bath and operating parameters such as temperature, time, agitation, pH. Additions of maintenance chemicals must be made frequently or preferably continuously using chemical feed pumps.

Stabilizers require close monitoring because they are easily lowered or lost with disastrous results such as complete plate-out of all the nickel in the bath. Particles introduced into the solution consume the stabilizer quickly. The concentration of stabilizers in the bath are a few parts per million. They are attracted to surfaces, so when particles are in the bath they have a large surface area onto which stabilizers can attach.

Copper and brass are not catalytic to the plating reaction and require starting either by cathodic electrical current or by pre-treating in a catalytic solution, rinsed and introduced into the plating bath. Copper and brass left in the solution without starting to plate will deplete the stabilizers quickly. (Putting copper sheet into the bath is sometimes used to lower stabilizer content when accidental high concentration occurs.) Sometimes leaded brass, steel, or copper parts may require plating. Plating directly onto these leaded metals may introduce lead into the solution. Lead is a very good stabilizer and can over stabilize the bath causing slow plating, pitting or stop plating completely. It is best to pre-treat these leaded materials using Fluoboric acid or sulfamic acid to minimize the amount in the surface If possible copper strike and nickel strike these leaded metals to provide the very best finish and protect the EN bath. Recently lead has been regulated such that lead free EN plating products have been developed. Replacing lead in the formulas was not easy. Lead is one of the best stabilizers and is effective as low as 0.3 ppm in the bath.

EN plating baths have a terminal point requiring the discarding of the bath and replacing it with a new solution is required. Bi-products accumulate such as sulfate and orthophosphite. As the concentration of these bi-product increase, the bath characteristic change somewhat, such as lower plating, rate change and changing the P content in the deposit. If the bath is used for too long a time, precipitation of the lease soluble materials will salt out causing large particulate area and complete plate-out of the nickel may occur. One easy way to monitor the solution is by measuring the specific gravity (sp.gr.) of the solution. The sp.gr. will increase with use of the bath. After some experience using this method one can predict the time for changing the bath. Another advantage in following the sp.gr. is that early detection of accidental over adds of water, particularly when there is an overflow out of the plating tank. This is not a substitute for analytical testing the components of the bath according to the suppliers recommendations.

Temperature and pH control is necessary to allow predictable plating time to achieve a desired thickness. Although plating can take place at 150 F (66 C) the deposit rate would be very slow. Typical temperatures are 85C to 90C. (185F to 194F) As the temperature is raised the deposit rate increases. The pH should be held constant during plating. The typical range of pH is 4.5 to 5.0. The lower pH, the slower the deposit rate, and the phosphorus content in the deposit will be higher.

The sodium hypophosphite in the bath is consumed in the plating reaction forming orthophosphite. Continuous additions or very frequent additions are recommended to maintain the plating rate and provide constant deposit properties. All the constituents work together to produce the desired result. Most proprietary EN systems provide make up and maintenance components that will keep the EN bath working well if added in the recommended amounts. Note, that because you have some control over the plating rate and bath loading, these additions and controls should be monitored and controlled within narrow limits for best results. EN baths have a terminal point and should be removed from service, waste treated and replaced with a new solution.

A word about bath loading. This refers to the surface area being plated as related to the number of gallons of plating solution in the tank that is accessible to the part being plated. It is measured in sq. ft per gallon. There are low and high limits for good results. Generally the lower limit is about ľ sq. ft. of surface area per gallon of solution. The higher limit is about 1 sq. ft./gallon of EN solution. The maintenance additions for most proprietary solutions are based on 0.5 to 0.6 sq.ft/gallon. The most important information to know is that the efficiency of use of hypophosphite varies with bath loading. To achieve the best efficiency and lowest maintenance cost use the highest bath loading that is practical for the system you are using. A real life illustration: A plating shop in England working the bath 24 hours a day at 1 sq.ft./gal bath loading resulted in using 35% less sodium hypophosphite. Because of the efficient use of hypophosphite due to high bath loading, the cost of replenishment went down considerably and the number of turn-overs before the terminal point greatly improved. A turn-over is the number of times that all of the nickel in the plating bath has been consumed. The average number of turn-overs is from 4-9 with the typical about 6 times. Thermal decomposition of hypophosphite takes place during plating whether or not the bath is plating. Low bath loading can result in pitting, streaks, or no deposit at all. Low bath loading combined with high agitation can stop the deposition.

Doís

* Use high bath loading to improve efficiency and save costs.

* Use nickel acetate or nickel hypophosphite for part or all nickel replenishments of the nickel component. These are slightly more expensive, but may extend the life of the bath.

* Use the bath and shut down as soon as possible.

* Accumulate items to be plated so the actual plating time is short.

* Add maintenance materials continuously or make frequent additions

* Remove anything that falls into the tank as soon as possible

* Pump the plating bath through a suitable filter into a storage tank soon after use

* Use a covered tank for plating and for storage

* Strip the nickel from the walls and bottom of the plating tank frequently. I prefer sulfuric acid and hydrogen peroxide instead of nitric acid. Any nitric acid left after rinsing is harmful to the plating bath

* Use high density polypropylene tanks or equivalent

* Filter the plating solution continuously using a filter and filter media recommended for EN Baths

* Follow the instruction of the chemical suppliers

* Use mild agitation for most formulas. High agitation can cause streaks, step plating or no plating at all. (There are a few baths that depend on mild air agitation for help in stabilizing the bath.) Most recommend away from air as the source of agitation.

Doníts

* Let particles of any kind get into the plating bath.

* Let nickel plated onto the insides or bottom of the plating or storage tank accumulate

* Under-load or over-load the bath, (sq,ft./gal)

* Let nitric acid stay in the tank or filter system or storage tank

*Use flexible tank liners. They contain waxes , oils, and sometimes cadmium and most use thallates. All of these are bad for the plating bath. They cause pitting, over stabilization dark streaks, etc.

* Let the EN bath stand idle for long periods of time. Hypophosphite will be consumed and reduce thus shorten the bath life. (fewer turn-overs)

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