Metallurgy Electroless Plating.pdf

CHAPTER 21

ELECTROLESS PIATING

The electroless plating process, also called

autocatalytic deposition, deposits a uniform coat-

ing onto catalytic surfaces, regardless of the shape

of the part. Once a primary layer of metal has

formed on the substrate, that layer, as well as each

subsequent layer, becomes the catalyst that causes

the reaction to continue. Electroless plating, in

contrast to conventional plating, does not use

external electric current to produce a deposit.

Deposition occurs in an aqueous solution contain-

ing metal ions, a reducing agent, and a catalyst

(part). Chemical reactions on the surface of the

catalytic part being plated cause deposition of the

metal or alloy. Table 21- I lists the different com-

ponents in the plating solution, along with their

respective functions.

Nickel is the most common metal deposited

autocatalytically. The chemical reactions that occur

when using sodium hypophosphite as the reducing

agent are as follows:

(HZP02)- + H20 + (HZP03)- + H2

Both nickel and phosphorus are reduced simul-

taneously, and the coatings produced are metastable

solutions of phosphorus in nickel. When the

coatings are properly heat treated, nickel-phosphide

particles, which increase hardness and improve

wear resistance, form within the coating. The

coating thickness is not limited, except by practical

considerations.

Electroless

CHAPTER

CONTENTS:

Metals

Deposited

21-1

Plating

Process

21-3

Precleaning

21-3

Cleaning

21-4

plating provides several unique

Initiating

21-4

Plating

21-4

characteristics

superior

to electrode position

processes:

Process

Control

21-4

The process produces a uniformly thick

coating on both simple and complexly

shaped workplaces.

The process may be applicable to a variety of

substrates ranging from metals and semi-

conductors to nonconductors.

The use of an auxiliary power supply and the

need for electrical contact is eliminated,

except for precleaning and surface activa-

tion purposes.

Some electroless deposits have unique and

controlled chemical, mechanical, and mag-

netic properties.

Deposit

Properties

21-9

Equipment

21-9

Appearance

and Adhesion

21-9

Porosity

21-10

(1)

Hardneas

21-10

catalyst

Ni++ + H2P020 + H20 -

NiO + (H2P03)- + 2H+

Thickness

21-10

(2)

Composition

21-11

(HjPOz)- + H+ - P + OH-+ H20

(3)

Corrosion

Resistance

21-11

METALS DEPOSITED

Wear

Resistance

21-11

Safaty and

Environment

21-11

Electroless plating processes are used by indus-

try to alter the surface of a part, providing a

uniform, conductive metallic coating. Nickel,

copper, cobalt, and gold are the most commonly

deposited metals. The commercially produced

electroless alloys are listed in Table 21-2.

Composite coatings have also been successfully

deposited. These coatings consist of particles of

such materials as synthetic diamonds, silicon car-

bide, aluminum oxide, and polytetrafluorethy lene

(PTFE), codeposited with nickel or cobalt. Com-

posite coatings enhance the wear and friction

characteristics of the metal deposit.

Producing specific properties in the deposit

from a particular plating process requires careful

control. For example, the hardness of the base

metal determines which heat treatments are

required to prevent hydrogen embrittlcmcnt caused

by the pretreatment processes. It maybe necessary

to control the surface finish, elongation, structure

of the deposit, reflectivity, conductivity, melting

point, composition, coefficient of thermal expan-

sion, density, internal stress, and elasticity to

achieve specified properties. The producer must

therefore have a working knowledge of how the

process parameters affect the deposit, Some of the

properties of electroless metal deposits are sum-

marized in Table 21-3.

PROPERTIES

The properties of electroless coatings are varied

and are determined by the specific metal and

process employed. In the case of electroless nickel,

the corrosion and wear resistance, conductivity,

solderability,

and magnetic

properties

of the

APPLICATIONS

The applications of electroless metal deposits

are many and diverse. Each metal has specific applica-

tions for which it can be used. Typical metal deposits

include copper, gold, nickel, and nickel alloys.

deposit find useful application.

Contributors of sections of this chepter are: Dr. George D. DiBari, Product Manager, Technical Services,

International Nickel, Inc.; Phil Stapleton, Technical Director, Stapleton Company & Associates.

Reviawers of sections of this chapter are: Teri L. Arney, Senior Rasearch Chemist, Elnic, Inc.; Edward G.

Buckley, Technical Service Manager, Technic, Inc.; Barry R. Chubs, Technical Marketing Specialist, Enthone,

Inc.; Dr. George D. DiBari, Product Manager, Technical Services, International Nickel, Inc.; Ronald Duncan,

Director of Research and Engineering, Ehric, Inc.; Dr. Nathan Faldstein, Surface Technology, Inc.; Gerald A.

Laitinen, Product Manager. Allied Kelite Products, Witco Chemical Corp.; Jovona L. McDowell. Manager.

Laboratory Services, Elnic, Inc.; Gary Sha whan, Electroless

Nickel Product Managar,

Enthone,

Inc.; Phil

Stapleton,

Technical Director,

Stapleton Company & Associates.

21-1

CHAPTER 21

METALS DEPOSITED

Copper

Electroless copper deposits are used in engineering applica-

tions to provide conductivity, such as in the through-hole

plating of printed circuit boards and in the preparation of

plastics for decorative plating. The deposits are uniform,

conductive, and can be applied over many substrates that have

been properly activated. Electroless copper deposits are also

being used, in combination with electroless nickel, to solve

electromagnetic shielding problems.

Gold

Electroless gold deposits are used for improving solder shelf

life by preventing oxidation of the surfaces. Gold deposits are

normally thin, under 0.1 mil (1 mil = 0.001”) (2.5 ~m), and are

applied over electroless nickel, which prevents diffusion of the

gold into copper. Electroless gold is used in electronic applica-

TABLE 21-2

Commercially Produced Electroless Alloys

TABLE 21-1

Typical Components in Electroless Plating Solutions

Electroless

Process

Ni Co P

B T1 Trace

Copper

99.9

0.1

Component

Function

Gold

99.98

0.02

Metal salt

Provide the metal ions to be

Nickel

phosphorus low

(e.g., nickel sulfate)

plated.

0.3

Reducing agent

Provide the reducing power at

Nickel

phosphorus

medium

(e.g., sodium

the catalytic interface.

hypophosphite)

0,3

Completing

agent

Completing of the metal ions

Nickel

phosphorus high

(chelators)

and preventing bulk

0.1

decomposition.

(e.g., lactic acid)

Nickel cobalt

phosphorus

Buffering agent

To resist the pH changes

75 20 5

0.3

(e.g., acetic acid)

caused by the hydrogen

released during deposition.

Nickel cobalt

phosphorus

Accelerators (Exultants)

To help increase the speed of

the reaction.

16 78 6

0.3

Nickel boron

99.5

0.5

0.1

Inhibitors (Stabilizers)

To control reduction reaction

in the plating solution.

Nickel thallium

boron

3.5 4.5 0.3

(Stapleton Company)

TABLE 21-3

Summary of Deposit Properties

Property

NiP9

NiCoP

NiB

NiTIB

Hardness,* Vickers

(100 g load)

---

500-550

570

650-700

750

Modulus of elasticity,

106 psi (GPa)

---

17-28.4

(118-196)

---

( ::8)

(f’:8)

Tensile strength,

ksi (M Pa)

---

35.5-128

(245-883)

42.7-99.5

---

15.6

(108)

(294-686)

---

Internal stress,

ksi (M Pa)

Density, g/ cm3

---

___

-4.3 to +42.7

(-29 to +294)

+42.7

(+294)

+15.6

(+108)

8.9

19.3

7.95

8.1

8.3

8.25

Thermal expansion,

pm/m” °C

12.1

Melting point,

OF~C)

1980

1940

1635

1740

2640

1980

1080)

(1060)

(890)

(950)

(1450)

(1080)

Magnetic, oersteads

up to 400

Resistivity, K Q” cm

1.6

2.1

20-110

<lo 30-70

(Stapleton Company)

* Hardness values given are before heat treatment.

21-2

CHAPTER 21

METALS DEPOSITED

tions to provide coverage on isolated circuits where conventional

electroplating is not possible. Electroless gold deposits are also

being used on printed circuit boards to improve wear.

about 0.5-5 .0% boron, the balance being nickel. When they

contain between 3 and 4% boron, these deposits possess good

wear and erosion characteristics for engineering applications.

Nickel Phosphorus

Electroless nickel-phosphorus deposits are widely used to

reduce corrosion and wear. Corrosion protection is provided by

isolating the base material from the environment and by the

natural oxide layers that form on the coating surface, Wear

resistance is provided bythenatural lubricity and hardness of

the coating. Electroless nickel can be deposited onto many

materials in thicknesses ranging from 0.05 to 100 roils (0.0013 to

2.5 mm). A typical engineering coating would be approximately

1 mil (25 ~m) thick. The amount of phosphorus is from 1 to 13%

and can generally be controlled to ~0.5Y0. The properties of

these coatings depend on the phosphorus content, which should

be specified when developing applications (see Table 21 4).

When electroless nickel deposits are heated to above their

transition temperature, 390-660° F (200-350° C), they begin to

precipitation harden and crystallize, Coatings have been pro-

duced that have harnesses greater than 950 Vickers using a

100 g load. During this process, the coating changes from an

amorphous structure to a mixture of nickel phosphide and

crystalline nickel. When mid-phosphorus deposits are heat

treated at 1380° F (750° C), they increase in strength; at the same

time, high-phosphorus deposits decrease in strength due to

greater shrinkage and the microcracking that may occur. Heat

treating high-phosphorus deposits at lower temperatures, 230-

400° F (1 10-200° C), and for a longer period of time helps to

increase hardness while minimizing microcracking.

Strength, elasticity, elongation, and ductility are improved

in deposits with high phosphorus, providing superior perform-

ance in many engineering applications including electrolcss-

formed bellows, hydraulic cylinders, and pressure vessels. Electro-

less nickel-phosphorus

Nickel Thallium Boron

Electroless nickel-thallium-boron deposits are used in high-

wear applications, providing excellent lubricity and high hard-

ness at high temperatures and high loads. Deposits containing

5T0thallium and 5% boron are applied over a wide range of base

materials in thicknesses of 0.5 to 2.0 roils (13 to 50 pm).

TABLE 21-4

Types of Electroless Nickel-Phosphorus Deposits

Deposit

Phosphorus

Type

Content, %

Comments

Low

1.5 2.5

These processes provide low-

phosphorus

temperature operations for

plating on plastics. The

coatings are high in stress and

produced from alkaline

solutions.

Medium

3.5 8.5

These processes provide

phosphorus

conventional electroless nickel

coatings. The deposits are

used for preventing wear and

corrosion in engineering

applications. Coatings may be

bright depending on the

chemistry, Solutions are

operated in a pH range of 4

to 5.

deposits are also being used to provide

High

9.3 13

These processes provide

electromagnetic

shielding for plastic cabinets and enclosures.

phosphorus

increased protection in

aggressive environments over

medium-phosphorus coatings.

The amount of trace metals

plated into the deposit are

reduced, producing a more

pure deposit. These processes

operate in a pH range of 4 to

5. Coatings have higher

elongation and strength than

medium-phosphorus coatings

in the as-plated condition.

(Stapleton Company)

Nickel Cobalt Phosphorus

Electroless nickel-cobalt coatings are used in magnetic

applications where uniform, thin deposits are required. These

coatings are in the thickness range of 0.003 to 0,02 roils (0.07 to

0.5 pm) and contain between 20 and 80~ cobalt, depending on

the application.

Nickel Boron

Electroless nickel-boron deposits are used in place of

electroless gold, providing good wire bonding and solderability.

These coatings are hard, uniform, and conductive and contain

ELECTROLESS

PLATING PROCESS

Electroless plating processes depend on chemical reduction

PRECLEANING

reactions to deposit uniform metallic coatings on parts, elimi-

Precleaning removes heavy soils and scales on the surface

nating the need for external current. The coating is uniform on

and exposes the bare base material. The precleaning can be

all wetted surfaces. Thickness is determined by the length of

accomplished

by wet or dry blasting, vapor decreasing,

time the article or part is kept immersed in the sohttion. The

mechanical cleaning, and descaling, among other methods. In

entire process involves several different steps as discussed in

almost all cases, the base material will not be clean enough after

this section.

precleaning to achieve adequate adhesion. The surface will still

21-3

CHAPTER 21

ELECTROLESS

PLATING PROCESS

have oxides and oils present. For additional information, refer

to the various cleaning methods discussed in Chapter 16,

“Mechanical and Abrasive Deburring and Finishing,” and

Chapter 18, ’’Cleaning,’’ ofthis volume.

may be used to initiate electroless deposition. Special heat

treatments may be required on aluminum, beryllium, titanium,

and other metals to obtain maximum adhesion.2

PLATING

When a properly prepared material is placed into the

electroless plating solution, the surface potential reaches a value

where metal deposition is possible. The deposition potential is

dependent on the pH, temperature,

CLEANING

Cleaning removes the oils and organic material on the

surface of the parts and is first accomplished by soaking in a

solubilizing solution at an elevated temperature. Some base

materials require electrocleaning as a second step to remove

carbon or grinding material. The last step is to completely

remove the oxides inan acidic solution.

The cleaning of metals prior to electroplating is discussed in

American Society for Testing and Materials (ASTM) Standard

B 322. Most of the information in that standard is applicable to

the preparation of metals for electroless deposition. Additional

information can be obtained in those standards that deal with

specific metals and alloys. 1

ionic concentration,

and

chemical composition of the solution.

As the potential changes, a charge between the solution and

the base material is created. The charge causes the cations in the

solution to organize themselves and produce what is called the

matrix. This matrix is like a blanket that covers the surface and

controls the deposition process. The reducing agents in the

solution provide electrons at the interface. Through several

sequential reactions within the matrix, the metal ions in

solution are reduced, producing the coating.

Each electroless plating process uses unique chemistry to

achieve this reduction reaction. Powerful reducing agents like

sodium borohydride and titanium trichloride are used to plate

out thallium and gold, while sodium hypophosphite and

formaldehyde will reduce nickel and copper, respectively. The

concentrations of these reducing agents and the metal salts

change as the plating process proceeds and by-products are

produced. To sustain the reduction reactions, additions of fresh

chemicals must be made. In addition, the special additives used

to provide brightness, stability, wetting, controlled cloud

points, and controlled transition temperatures must be moni-

tored and kept at optimum levels.

Proprietary electroless processes are available, and suppliers

of these processes provide instructions for maintaining and

controlling individual processes. Electroless processes can be

maintained by performing simple chemical analyses from which

chemicals required to replenish the solution can be determined.

INITIATING DEPOSITION

Autocatalytic nickel will usually deposit on clean, wetted

surfaces. Aluminum, beryllium, platinum metals, iron, cobalt,

nickel, titanium, and their alloys can be plated directly. Certain

base metals cannot be plated directly; these include zinc, lead,

cadmium, tin, bismuth, arsenic, antimony, and alloys containing

large proportions of these metals. The metals that cannot be plated

directly should be electroplated with a thin copper or nickel

strike prior to being immersed in the electroless plating sohrtion.

Preliminary electrolysis or contact with a catalytic metal like

iron or nickel is required for deposition on copper, silver, gold,

carbon, vanadium, molybdenum, tungsten, chromium, selenium,

and uranium. In some cases, immersion deposition processes

CONTROLLING THE PLATING PROCESS

Impurities in the solution can affect the ductility, corrosion

resistance, and appearance of electroless deposits while con-

taminants in the solution can cause pitting, adhesion, and

roughness problems. Hence, the solution must be kept relatively

free of impurities to control the properties of the deposit and

avoid producing defectively coated parts that must be scrapped

or reprocessed at great expense to the metal finisher.

A deliberate, sustained campaign to keep impurities out of

solution is better than having to resort to time-consuming and

expensive treatments to remove contaminants. For this effort to

be successful, everyone involved in processing the parts should

become familiar with the sources of impurities, which are one or

l Defective exhaust ducts.

l Tank Ieachates and defective rack coatings.

. Grease and oil from pumps, hoists, and overhead

equipment.

. Dissolution of metals or parts that accidentally fall into

the solution.

l Hard water (calcium sulfate).

Assuming that all potential sources of contamination can be

identified, it should be possible to compile a list of actions to

maintain the purity of the solution and, consequently, avoid

generating rejects. Some recommended actions are as follows:

more of the following:

Remove metals or parts that have fallen into the solution

as quickly as possible.

Use adequate rinses and electrocleaning

. Contamination from alkaline cleaners, acid pickling

solutions, and other solutions by direct transfer (drag-in),

in the prepa-

or by airborne spray.

l Airborne particulate matter; for example, from grinding

ration process.

Monitor the conductivity of rinse tanks to check cleanli-

ness of the water and monitor excessive drag-out levels.

Exhaust grinding and polishing equipment properly, or

isolate these processes from the plating area.

Use de-misting agents to prevent airborne spray.

and polishing operations.

l Improper removal of grease, oil, and buffing and polish-

ing compounds from the work.

. Defective filters, pumps, and other equipment.

21-4

CHAPTER 2

CONTROLLING

THE PLATING PROCESS

. Inspect and maintain filters, pumps, and other auxiliary

l Follow the instructions of the supplier of the process for

equipment on a regular schedule.

batb makeup, replenishment, and operation.

. Inspect and repair racks and rack coatings,

Despite all precautions, problems may occur. Table 21-5 lists

l Maintain solution at near optimum levels, maintain

common problems that occur in electroless nickel plating, gives

temperature, and check pH on a regular basis.

the possible causes, and suggests solutions to these problems.~

TABLE 21-5

Troubleshooting Electroless Nickel Deposits3

Problem

Possible Causes

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