What is Electroless Nickel Plating?
Electroless Nickel Phosphorous (ENP) is a nickel-phosphorous alloy (NiP) deposited by a chemical process, unlike the galvanic process of nickel electroplating that requires an electric current.
Phosphorous content (%P) ranges from 2%P to 14%P, and specific plating bath formulations produce deposits with low phosphorous (2% to 5%), medium phosphorous (6% to 9%), and high phosphorous (10% to 14%) content.
Thickness ranges from a few microns for electronic applications to more than 75 microns for heavy duty corrosion resistance on rough surfaces.
What is Electroless Nickel Plating good for?
Low Phosphorous content (<6% P) deposits exhibit a high hardness range as deposited and can be heat treated to further increase hardness and wear resistance. At maximum hardness it approaches the hardness of hard chrome, but with better corrosion resistance especially in alkaline environments such as chemical processing plants.
Medium Phosphorous content (6% – 8% P) deposits are used for general corrosion resistance on steel, copper, or aluminium alloys, and provide a bright uniform hardwearing finish. It has a high plating rate and is used in decorative, industrial, and electronic applications.
High Phosphorous content (>10% P) deposits are used where the greatest corrosion resistance is required. At > 10%P the deposit is amorphous with no grain or phase boundaries that would otherwise provide initial corrosion sites. Hardness is relatively low as deposited but can be significantly increased by heat treatment. Typical applications are in oil and gas exploration, where the environment can be corrosively acidic
In all cases the deposit is of uniform thickness, even in recesses and internal surfaces. However, agitation of the plating solution is needed to ensure a flow of solution over complex shapes and through internal bores.
How can we measure and verify that an ENP coating meets specification?
Key deposit parameters are thickness, phosphorous content (%P) and hardness
Primarily a function of immersion time, although the plating rate can vary with changes in plating solution chemistry, temperature, and pH, so regular checks and adjustments are needed.
A handheld DELTASCOPE® FMP30 can be used for non-destructive thickness measurement of high phosphorous deposits (non-magnetic when P >10%) on steel substrates.
A portable FISCHERSCOPE® X-RAY XAN® 500 XRF system can be used for non-destructive thickness measurement for low and medium phosphorous content ENP on steel, and all deposits on other substrates, especially for components that are too large for conventional chambers.
Thickness measurement range using XRF is ~0.1 microns to 40 microns, depending on phosphorous content.
Conventional XRF ENP thickness applications use a fixed phosphorous content(%P) or deposit density in the thickness calculation. This can introduce significant errors for deposits of >20 microns if the %P varies from nominal.
Accurate thickness measurements can only be made if %P is simultaneously measured
Phosphorous content (%P)
%P is a function of the plating solution chemistry by design, and is influenced by day to day and longer term chemistry, temperature, and pH changes
A FISCHERSCOPE® X-RAY XDV®-SDD XRF system can be used to accurately measure deposit thickness and % P, providing full verification of the deposit to specifications, such as IPC 4552 and 4556 for ENIG and ENEPIG in the PCB industry.
Simultaneous measurement of both thickness and %P removes the possibility of significant thickness errors that can occur when using a conventional fixed %P or density. This is particularly important for high phosphorous ENP where the deposit corrosion resistance is directly influenced by %P content.
It’s recommended that both thickness and %P of ENP deposits should be measured at incoming goods inspection (IGI), as this approach provides verification of the two key parameters that determine ENP performance.
Typical measurement performance of the FISCHERSCOPE® X-RAY XDV®-SDD is shown below.
NiP/Fe thickness and %P composition measurement
|NiP / Fe Measurement with XDV-SDD|
|Mean||12.5 μm||10.1 %||89.9 %|
|Error||0.27 %||0.91 %||0.1 %|
|Number of Readings||30|
Hardness and wear resistance can be specified and achieved by using the appropriate plating solution chemistry and plating parameters.
Conventional Vickers micro-hardness is determined in a cross section of an ENP deposit of >50 microns, but cross section preparation is a costly time consuming process.
A FISCHERSCOPE® HM2000 instrumented indentation hardness (IIH) system takes 60 seconds to test the hardness of ENP coatings perpendicular to the surface, even if the coating is only 2 microns thick. Penetration of the coating is limited to 10% of its thickness, which makes it a non-destructive test.