EN Types


Nickel Boron

Nickel boron alloys deposited from formulations utilizing dimethylamine borane (DMAB) as the reducing agent usually containing from 0.2 to 3.0% boron in the deposits.

These deposits exhibit many of the same characteristics as nickel phosphorus deposits, but they are also especially suited for use in the electronics industry because of their electrical and physical qualities. The lower concentration boron deposits provide high electrical conductivity, exhibit low contact resistance and show good as plated hardness and wear resistance. These types of deposits are easily soldered or brazed and exhibit good shelf life solderability. EN boron deposits have found many applications were they can reduce or eliminate the usage of gold in the electronics industry. Higher concentration boron containing alloys are very hard and wear resistant coatings although the cost of producing these deposits is many times greater compared to newer, lower phosphorus deposit formulations capable of similar hardness and wear properties.

Composite Coatings

EN deposits with dispersions of polytetrafluoroethane (PTFE), silicon carbide, diamonds or other insolubles that are codeposited to improve wear resistance. PTFE deposits provide low frictional coatings in specialized wear situations.

Ternary Alloy Coatings

Also called poly alloys, these are electroless nickel deposits which contain more than two elements. An example is nickel boron thallium (5% boron/1% thallium) which is very hard as deposited and provides excellent wear resistance in specialized wear environments.

Nickel Phosphorus

Electroless nickel phosphorus deposits exhibit many desirable characteristics. They have a low coefficient of friction and are anti-galling. They have a good as plated hardness compared to other types of plated coatings, and the hardness can be further increased by subsequent heat treatments. These deposits also provide good corrosion resistance in many environments. Various systems are available formulated to codeposit from 1 to 13% phosphorus in the deposit. Selecting the process that will produce the optimum phosphorus content for a given application is sometimes difficult. The effects the phosphorus content has on the service properties of the nickel phosphorus alloy needs to be understood. Generally, the higher phosphorus (above 9%) alloy deposits are often softer as plated, can be heat treated to improve hardness and tend to be non-magnetic as plated. They do not wear quite as well in the as plated condition but are more corrosion resistant especially in acid environments. The higher phosphorus alloys plate at slower deposition rates and their deposits do not tend to be full bright compared to the lower phosphorus deposits/systems. Conversely, the lower phosphorus alloy deposits tend to be harder as plated and after heat treatment. These deposits are more wear resistant and tend to exhibit better corrosion resistance in alkaline environments. The phosphorus content in the alloy is the most significant parameter to control because changing phosphorus contents also changes the properties.

CLASSIFICATIONS:

1 to 3% = Low phosphorus
2 to 4% = Medium-low phosphorus
5 to 9% = Medium phosphorus
10 to 13% = High phosphorus

The suitability of any of the types of systems is driven by the end use application or desired property. It is unlikely that one type will meet all requirements since, for certain applications, specific properties may be compromised. For any EN bath, variations in operating conditions will produce some variations in phosphorus content. As an example, plating with air agitation produces a 4% phosphorus deposit versus a 6% deposit being produced with no agitation from the same system. Changes in bath formulations are used to produce deposits with larger differences in phosphorus contents, thus different bath formulations are used for each of the four classifications even though there can be some overlap in phosphorus content. While the properties of an EN deposit do not exhibit radical changes with slight changes in phosphorus content, there can be significant differences in deposits from each end of a given class. Many of the properties of EN coatings and the EN coating process are interrelated. Changing one property often results in changing others whether desired or not. While a general purpose EN process, such as the 7 to 9% alloy system may be suitable for day-to-day job shop use for a high percentage of jobs, the development and use of the medium-low and high phosphorus processes are designed to optimize deposit properties for a wide range of engineering applications.

 

 


EN Equipment

 

Given the nature of electroless nickel plating, proper equipment is critical in obtaining optimum performance as well as trouble free operation of the plating solutions. The twin tank design most commonly used today throughout the plating industry was pioneered and patented by Elnic, now a part of MacDermid Incorporated. The design allows one tank to be cleaned and passivated, while the second is in operation, ensuring that a tank is ready to plate in when needed. Stress relieved, virgin polypropylene is the preferred material of construction for electroless nickel plating tanks. Anodically protected 316 alloy stainless steel tanks are also utilized today. The combination of cost, ease of fabrication and resistance to plate out make polypropylene the material most widely used. While stainless steel can be, and frequently is used, its tendency to plate must be conrolled by careful use of anodic passivation. Anodic passivation can be used very effectively today. The two most common methods of heating an electroless nickel solution are steam and electricity. Although the capital investment for steam is usually higher than electricity, operating costs for steam are generally much less.

Steam is normally supplied to the solution heating by way of immersion coils or external heat exchangers. The proper materials of construction must be employed in either method to avoid solution plateout or solution contamination.

Agitation of either the parts or the solution is vital to successful electroless nickel plating. Without constant renewal of the plating solution coming in contact with the parts, localized depletion can occur resulting in poor coating uniformity. In addition, hydrogen bubbles must be driven from the part surface in order to avoid pitting or "fish-eyes". Finally, adequate agitation prevents localized overheating which may result in solution decomposition. Constant filtration of electroless nickel solutions is essential to remove bath particles introduced into, or generated in the bath. Both cartridge filters types and bag filters are commonly employed. Given the high cost of installation and maintenance of wound cartridge filters and the down time associated with changing out filter cartridges, woven polypropylene bags have become the preferred filtration method. The bags are relatively inexpensive and the minimum amount of back pressure means pump life is prolonged. Also, the bags can be changed quickly. Racks for plating ferrous and copper alloys should be capable of carrying 30 to 50 A/ft of part surface during electrocleaning and striking without overheating or excessive voltage loss. Suitable materials are steel, stainless steel, copper and titanium. Of these, steel or plastic coated steel is most often used. Because electrolytic steps are not required to process aluminum alloys, plastic as well as metals can be used. Barrels for EN plating should be fabricated from natural polypropylene. If added strength is required, glass filled polypropylene is preferred. Polypropylene gears should be used to turn the barrel.

MacDermid's experience in the manufacture of electroless nickel plating equipment is unequalled. Through our equipment division and extensive equipment network, we offer an entire spectrum of products, ranging from basic startup plating materials to the EN plating industry's most sophisticated automatic production lines. Each has been meticulously designed and engineered to the highest standards of quality and performance.



Physical Properties of Electroless Nickel -Phosphorous Deposits

 

PROPERTY

HIGH PHOS

MID PHOS

LOW-MID PHOS

LOW PHOS

% PHOSPHOROUS

10-13

5-9

2-4

1-3

ASTM B733-97 CLASSIFICATION

TYPE V

TYPE IV

TYPE III

TYPE II

 

 

 

 

 

DEPOSIT DENSITY (g/cm3)

7.6 - 7.9

8.1 - 8.4

8.5 - 8.7

8.6 - 8.8

PLATING RATE (MIL/HR)

0.3 - 0.5

0.6 - 1.0

0.7 - 1.2

0.4 - 0.8

(mm/hr)

7 -13

15 -25

18 -31

10 -20

HARDNESS AS PLATED (HK100)

480 - 550

500 - 650

625 - 800

750 - 850

HARDNESS AFTER HEAT TREATMENT (HK100)

850 - 950

850 - 1000

850 - 1100

900 - 1100

TABER WEAR INDEX AS PLATED (mg/1000 CYCLES -CS-10 WHEEL, 100 g LOAD)

22 - 24

16 - 20

10 - 14

7 - 13

COEFFICIENT OF THERMAL EXPANSION (mm/m/°C)

8 -10

10 -16

16 -20

18 -22

ELECTRICIAL RESISTIVITY (mOHM-CM)

>110

70 - 110

30 - 50

20 -40

THERMAL CONDUCTIVITY (CAL/CM/SEC/°C)

0.010

0.012

0.015

0.015

TENSILE STRENGTH (MPa)

650 - 900

420 - 1000

350 - 600

200 -400

DEPOSIT STRESS AS PLATED

NEUTRAL TO

SLIGHTLY

NEUTRAL

SLIGHTLY

 

COMPRESSIVE

TENSILE

 

TENSILE

ELONGATION (%)

1 - 2.5

0.5 - 1

0.5 -1

0.5 - 1.5

MODULUS OF ELASTICITY (GPa)

55 - 70

50 - 65

45 - 65

55 - 65

MELTING RANGE (°C)

880 - 900

880 -980

1100 -1300

1250 -1360

COERCIVITY (Oe)

0

1 -8

10 -15

15 -25

MAGNETIC PROPERTIES AS PLATED

NONMAGNETIC

SLIGHTLY MAGNETIC

MAGNETIC

MAGNETIC

 

 

TO MAGNETIC 

 

 

 

 

 

 

 

Engineering Properties

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Deposit Stucture

Hypophosphite reduced electroless nickel is one of the very few metallic glasses used as an engineering material. Depending upon the bath formulation, deposits may contain from 1 to 13% phosphorus dissolved in nickel. The structure of these coatings depends upon their composition. Deposits containing more than 8.5% phosphorus have no crystalline structure or separate phases and are normally amorphous to x-rays. Deposits from 5 – 8.5% phosphorus contain different phases of nickel and are partly crystalline. Those deposits below 5% in phosphorus content and crystalline are typically laminar in structure. Bath formulations can have a dramatic effect on deposit structure through changes in plating rate, deposit content and deposit stress levels.


Fig 4. An example of the uniformity of electroless nickel
coatings. Unlike the copper overplate, the 25um thick
electroless nickel reproduces the profile of the internal
threads of the part. The substrate is leaded steel.
100x magnification, etched in 1% picral.


Deposit Uniformity

One especially beneficial property of electroless nickel is its uniform coating thickness. With electroplated coatings, thickness can vary significantly depending upon the part's configuration and its proximity to the anodes. Not only can these variations effect the ultimate performance of the coating, but they can also cause additional finishing to be required after plating. With electroless nickel the coating thick- ness is the same on any section of the part exposed to fresh plating solution. Grooves, slots, blind holes, and even the inside of tubing will have the same amount of coating as the outside part. This is illustrated by Fig. 4 which shows the uniform plating thickness on the internal threads of a small spray nozzle. With electroless nickel, coating thickness can be controlled to suit the application. Coatings as thin as 2.5 micrometers (0.1 mil) are commonly applied for electronic components, while those as thick as 75 to 125 micrometers (3 to 5 mils) are typical for corrosive environments. Coatings thicker than 250 micrometers (10 mils) are used for salvage and repair of worn or mismachined parts.

 

Physical Properties

 

 

 

 

 

 


Melting Point

Electroless nickel is a eutectic alloy with a wide melting range. Unlike a pure compound, it does not have a true melting point. The melting range for electroless nickel coatings varies depending upon the phosphorus content of the deposit. All electroless nickel coatings begin to melt at approximately 880 C (1620 F), which is the eutectic temperature for nickel phosphide (NiP). The temperature at which the coating is completely liquid, however, increases with decreasing phosphorus content from about 880 C (1620 F) at 11% – the eutectic point – to approximately 1450 C (2640 F) for pure nickel. Thus, the melting range becomes wider as the phosphorus content is reduced. Practically, this means that all commercial coatings contain large quantities of liquid material at temperatures above 880 C (1620 F). For example, at 900 C (1650 F) the coatings containing 5,8 and 10.5% w/w phosphorus are 46, 74 and 100% melted.


Density

The density of electroless nickel coatings is inversely proportional to their phosphorus content. Shown in Figure 5, density varies from about 8.5 gm/cm3 for very low phosphorus deposits, to 7.75 gm/cm3 for deposits containing about 10.5% phosphorus.


Fig. 5 Effect of Phosphorus
content on density of
electroless nickel.

Electrical Resistivity

The electrical properties of these coatings also vary with composition. For high phosphorus deposits, electrical resistivity is generally about 90 micro-ohm-cm. Accordingly, these coatings are significantly less conductive than conventional conductors such as copper. For low phosphorus deposits, electrical resistivity is about 20 micro-ohm-cm.

Because of the relatively thin layers used, however, for most applications the resistance of electroless nickel is not significant.

Heat treatments precipitate phosphorus from the alloy and can increase the conductivity of electroless nickel by 2 to 4 times. The formulation of the plating solution can also effect conductivity. Tests with baths complexed with sodium acetate and with succinic acid showed electrical resistivities of 61 and 804 micro-ohm-cm, respectively.

Phosphorus content also has a strong effect on the thermal expansion of electroless nickel. This is shown in Figure 6, which is based upon deposit stress measurements on different substrates.


Fig. 6 Effect of phosphorus content
on coefficient of thermal expansion.

Mechanical Properties
The mechanical properties of electroless nickel deposits are similar to those of other amorphous deposits. They have high strength, limited ductility and a high modulus of elasticity.

 

 

 

 

 

 

 

 


Tensile Strength

The ultimate tensile strength of most coatings exceeds 700 MPa (100kpsi). That is equal to many hardened steels and allows the coating to withstand a considerable amount of abuse without damage. The effect of phosphorus content upon the strength and strain at fracture of electroless nickel deposits is shown in Figure 7.



Fig. 7 Effect of phosphorus content
on stress and strain at fracture.

Ductility

The ductility of electroless nickel coatings also varies with composition. The ductility of coatings is about 1 to 1.5% (as elongation). While that is less than that of most engineering materials, it is adequate for most coating applications. Thin films of the deposit can be bent completely around themselves without fracture, and the coating has been used successfully for springs and bellows. Electroless nickel, however, should not be applied to articles which subsequently will be bent or drawn. Severe deformation will crack the deposit, reducing corrosion and abrasion resistance. With lower phosphorus deposits, or with deposits containing metallic or sulfur impurities, ductility is greatly reduced and may approach zero. The effect of phosphorus content upon the strain fracture of electroless nickel coatings is also shown in Figure 7.

Hardening type heat treatments reduce both the strength and the ductility of electroless nickel deposits. Exposure to temperatures above 220 C (420 F) cause an 80 to 90% reduction in strength and can destroy ductility, especially in lower phosphorus coatings. The ductility of high phosphorus coatings is not significantly reduced until heated to above 260 C (500 F).

The modulus of elasticity of non-heat treated electroless nickel coatings containing 10 to 11% phosphorus is about 200 GPa (28x10 psi) and is very similar to that of steel.

The modulus of elasticity of deposits containing 7 to 8% phosphorus is only about 120 GPa (18x10 psi) and is more similar to that of copper alloys. Heat treating electroless nickel coatings at temperatures above 200 C (400 F) causes their modulus of elasticity to increase significantly.

Deposit Appearance

Deposit appearance varies considerably depending upon bath formulation and substrate geography. Baths can be formulated to produce deposits that vary from matte to extremely bright.

Since electroless nickel deposits have virtually no leveling capabilities, these coatings mirror the finish of the surface to which they are applied. As a result, even a very bright deposit may appear dramatically less bright on a casting or blasted surface if compared to a similar deposit on a polished surface.

If maximum corrosion protection, good deposit elongation, and low stress of high thicknesses with minimum pitting are desired brightened deposits are normally not recommended.

Adhesion

The adhesion of electroless nickel coatings to most metals is excellent. The initial replacement reaction, which occurs with catalytic metals, together with the associated ability of the baths to remove submicroscopic soils, allows the deposit to establish metallic as well as mechanical bonds with the substrate. The bond strength of MacDermid EN coatings to properly cleaned steel has been found to be 400 MPa (60 kpsi) or more. The adhesion to aluminum and aluminum alloys is less, but usually exceeds 300 MPa (40 kpsi).

With non-catalytic or passive metals, such as stainless steel, an initial replacement reaction does not occur and adhesion is reduced. With proper pretreatment and activation, however, the bond strength of the coating normally is at least 140 MPa (20 kpsi). The adhesion to copper alloys is usually between 300 and 350 MPa (40 and 50 kpsi).

With metals such as aluminum it is common practice to bake parts after plating for 1 to 4 hours at 130 to 200 C (270 to 400 F) to increase the adhesion of the coating. These treatments stress relieve the part and the deposit and provide a very minor amount of codiffusion between the coating and substrate. They are most useful where pretreatment has been less than adequate and adhesion is marginal. With properly applied coatings, baking will have only a minimal effect upon bond strength. Electroless nickel coatings also have excellent hot hardness. Up to about 400 C (750 F) the hardness of heat treated electroless nickel is equal to or better than that of hard chromium coatings. As deposited coatings also retain their hardness to this temperature, although at a lower level.

Hardness & Wear Resistance

One of the most important properties for many applications is hardness. As deposited, The micro-hardness of electroless nickel coatings is about 500 to 700 HK100. That is approximately equal to 45 to 58 HRC and equivalent to many hardened alloy steels. Heat treatment causes these alloys to age harden and can produce hardness values as high as 1100 HK100. That is equal to most commercial hard chromium coatings.

For some applications, high temperature treatments cannot be tolerated because of part warpage or because of their effect on the substrate. For these, it is sometimes possible to use longer times and lower temperatures to obtain the desired hardness.
Treatment at 340 C (650 F) for 4 to 6 hours and at 290 C (550 F) for 10 to 12 hours are commonly used for electroless nickel deposits. Those treatments can produce hardness values of 950 to 1000 HK100.Treatments at 260 C(500 F)are also occasionally used, although the resulting hardness is lower. At temperatures of 230 C (450 F) and below, only a minimal increase in hardness is obtained. Accordingly such treatments are only rarely used, except for hydrogen relief or adhesion improvement.



Hardness & Wear Resistance

Electroless nickel coatings have excellent resistance to wear and abrasion, both in the as-deposited and hardened conditions. Laboratory tests have shown fully hardened coatings to have wear resistance equal to hard chromium under both dry and lubricated conditions. This is illustrated by Table 1, which shows the results of typical Taber Abraser Wear tests of electroless nickel coatings, and compares them to electroplated nickel and chromium. The excellent wear resistance of electroless nickel often allows it to replace high alloy materials and hard chromium.

Tests with electroless nickel coated V-blocks in a Falex Wear Tester have confirmed a similar relation between heat treatment and wear, and have shown the coating to be more resistant than hard chrome under lubricated wear conditions. This is illustrated by Table 2 for an electroless nickel deposit containing approximately 9% phosphorus.

The effect of phosphorus content upon the wear experienced by electroless nickel coatings under lubricated conditions is summarized in Figure 8. These rotating ball tests showed that after heat treatment, high phosphorus deposits provide the best resistance to adhesive wear.