Electroless Copper and Nickel-Phosphorus Plating

Processing, Characterisation and Modelling

W Sha, Xiaomin Wu, K G Keong

• Publisher: Woodhead Publishing
• Year: 2011
• Print ISBN: 9781845698089
• eBook ISBN: 9780857090966
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

Unlike electroplating, electroless plating allows uniform deposits of coating materials over all surfaces, regardless of size, shape and electrical conductivity. Electroless copper and nickel-phosphorus deposits provide protective and functional coatings in industries as diverse as electronics, automotive, aerospace and chemical engineering. This book discusses the latest research in electroless depositions.After an introductory chapter, part one focuses on electroless copper depositions reviewing such areas as surface morphology and residual stress, modelling surface structure, adhesion strength of electroless copper deposit, electrical resistivity and applications of electroless copper deposits. Part two goes on to look at electroless nickel-phosphorus depositions with chapters on the crystallisation of nickel-phosphorus deposits, modelling the thermodynamics and kinetics of crystallisation of nickel-phosphorus deposits, artificial neural network (ANN) modelling of crystallisation temperatures, hardness evolution of nickel-phosphorus deposits and applications of electroless nickel-phosphorus plating.Written by leading experts in the field Electroless copper and nickel-phosphorus plating: Processing, characterisation and modelling is an invaluable guide for researchers studying electroless deposits or materials science as well as for those working in the chemical, oil and gas, automotive, electronics and aerospace industries. - Written by leading experts in the field, this important book reviews the deposition process and the key properties of electroless copper and nickel-phosphorus deposits as well as their practical applications - Chapters review areas such as surface morphology and residual stress, modelling surface structure, crystallisation of nickel-phosphorus deposits and hardness evolution - An invaluable guide for researchers studying electroless deposits or materials science as well as for those working in the chemical, oil and gas, automotive, electronics and aerospace industries

1

Introduction to electroless copper and nickel–phosphorus (Ni–P) depositions

Abstract:

Electroless copper deposition using formaldehyde as a reducing agent at 60 °C is widely used in commercial printed circuit board industries. However, formaldehyde, as a carcinogen, has high potential risk to the environment and the plating operators. Therefore, alternatives to formaldehyde used in electroless copper deposition have been proposed. Electroless nickel–phosphorus (Ni–P) deposits are widely used in various industries, in particular as protective and functional coatings because of their unique combination of material properties. Many of these material properties are adjustable through thermal processing when the amorphous structure of the deposits undergoes a crystalline transition process.

Key words

electroless copper

electroless nickel–phosphorus (Ni–P)

formaldehyde

printed circuit board

thermal processing

1.1 Electroless copper deposition

Electroless copper deposition is widely used in the printed circuit board industry for plating onto bare laminate to make non-conductive substrates conductive for further electroplating, and for plating through holes to connect circuits in different layers. The principle of electroless copper deposition is to use the chemical reaction between a reducing agent and a copper salt in a liquid solution so that the copper ion can be reduced to a copper atom. The reaction should be continuous so that sufficient copper can form a film and cover the substrate.

Currently, commercial electroless copper plating solution uses formaldehyde as the reducing agent. However, formaldehyde is a well-known carcinogen with a strong potential hazard to the environment and the staff working around an electroless copper deposition facility. To address this issue, several alternatives have been proposed, amongst which glyoxylic acid has very high potential. The other way to reduce the hazard of formaldehyde is to reduce the plating temperature so that there will be less formaldehyde fumes released from the plating solution to the air. However, before an alternative or a plating condition can be industrially applied, the properties of the deposits acquired in these conditions need to be tested and analysed.

1.2 Electroless nickel–phosphorus (Ni–P) deposition

Electroless nickel–phosphorus (Ni–P) deposits are produced from the electrochemical reduction of a nickel ion through the autocatalytic plating processes in the plating solution. The electroless plating process that led to the practical application of these deposits was first developed by Brenner and Riddell, and the first patent for it was awarded in 1950. Following this in the next decade, the electroless Ni–P deposits were mainly used for the interiors of railroad tankers carrying concentrated corrosive solutions. Because of various investigations over the years, understanding and enhancement of the unique material properties of electroless Ni–P deposits have become significant, and have broadened the deposits’ application and role in varied industries, in particular for the use as protective and functional coatings. There is improvement in material properties, such as solderability, wear and corrosion resistances, and magnetic response through plating and heat-treatment processing. The major industries that have benefited from using electroless Ni–P deposits include the chemical, oil and gas, automotive, electronics and the aerospace industries. Today, most commercially used electroless Ni–P deposits are from a binary system that has a range of 3–14 wt% phosphorus content depending on their application roles. Some properties of the basic elements are given in Table 1.1.

Table 1.1

Elementary properties of phosphorus and nickel

Element	Phosphorus	Nickel
Chemical symbol	P	Ni
Atomic number	15	28
Atomic mass	30.973762	58.6934
Atomic radius (pm)	93	124.6
Electronic configuration*	[Ne]3s^23p^3	[Ar]3d^84s^2
Most common substance	P4	Ni
Substance class	Non-metal	Metal
Crystal lattice	Cubic	Face centred cubic
Density at 25 °C (kg m^−3)	1820	8902
Melting and boiling points (°C)	44 and 280	1453 and 2732
Electrical conductivity at 25 °C (Ohm^−1 m^−1)	1.00 × 10^−9	1.46 × 10^7
Thermal conductivity at 25 °C (Wm^−1 K^−1)	0.235	90.7

^*[Ne] = 1s²2s²2p⁶, [Ar] = [Ne]3s²3p⁶.

The difference in microstructural characteristics of electroless Ni–P deposits has significant effect on the material properties (Li et al., 2001; Taheri et al., 2001; Ten and Chang, 2001), and is subject to the influence of crystallisation and phase transformation behaviour of deposit during thermal processing (Li et al., 2001; Taheri et al., 2001). Besides, it is evident that the crystallisation and phase transformation behaviour is determined by the alloy compositions and thermal processing conditions, such as phosphorus content, heating rate and processing end temperature (Taheri et al., 2001). As a result, the correlation between crystallisation and phase transformation behaviour of electroless Ni–P deposits, and those influencing factors, is of importance to the optimised design of specific material properties for targeted applications. However, despite being the subject matter of many previous investigations, there appear to be conflicting results regarding the microstructural and material properties, and crystallisation and phase transformation behaviour of electroless Ni–P deposits, under those mentioned influencing factors. These variations are likely caused by many other related factors, such as plating condition, material aspect of the substrate, and testing and measuring techniques.

1.3 How to plate the depositions in the laboratory

1.3.1 Electroless copper

Electroless copper deposits can be prepared in the laboratory by using a homemade electroless copper plating line or a mini electroless copper plating line. In the following sections, the ingredients of the plating solutions, procedures of the electroless copper plating, and other relevant essential solutions are described in detail.

Electroless copper plating solutions

In this book, in electroless copper plating solutions, either formaldehyde ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Author contact details
• Preface
• Acknowledgements
• Chapter 1: Introduction to electroless copper and nickel–phosphorus (Ni–P) depositions
• Part I: Electroless copper depositions
• Part II: Electroless nickel–phosphorus (Ni–P) depositions
• Index