Preface

This book deals with some of the most advanced research observations and their applications in electronic device manufacturing in a very concise and structured way. It will help readers to gain in-depth knowledge about lead-free solder and its significance. Generally, the electronics world has been shifting towards lead-free electronic devices because of strict rules, government legislation, and serious health and environmental issues arising out of the use of lead in electronic devices.

It is now a great challenge for academia and industry to study and analyze lead-free solder joints and to recommend the manufacturing of reliable lead-free electronic devices. Moreover, the task becomes more complicated when miniaturization hit the electronics world recently.

All five chapters were written by acclaimed researchers from all over the world describing detailed and necessary facts and figures on all aspects and scopes of lead-free soldering, its application, and the immense possibilities of advanced materials development.

*Chapter 1* gives a general overview and discusses the need for lead-free solder research. It also proposes some of the very recent observations towards achieving reliable lead-free solder joints. Kar et al. observe very interesting effects of using multilayers of Sn for controlling intermetallic compounds growth at the lead-free solder join interface. *Chapter 2* describes the importance of intermetallics in leadbased and Pb-free solder alloys starting from their formation, characteristics, and effects on material joints. Since the growth of intermetallics over time is determined by solid-state diffusion rules and varies from system to system, readers will get a broad overview of the interaction of intermetallics with substrates and will be able to determine the exact effect on solder properties. The chapter explains how the mechanical properties and growth kinetics of intermetallics at the solder joint interface can be altered by the addition of rare earth elements. *Chapter 3* discusses the soldering of metallic and ceramic materials by lead-free active Sn and Bi-Inbased solders. Readers will gain detailed knowledge of the possibilities of soldering ceramic materials, which is limited due to the poor wettability of ceramic substrates with commercial solders at classical soldering technologies because of the different thermal expansions of soldered materials. This chapter describes how the technology of soldering with active solders is selected for joining ceramic materials along with applications of ultrasound for mechanical activation of solders, which disrupts the surface oxides, changes the surface energy of ceramic materials, and supports the diffusion processes in the interface. *Chapter 4* explains the room temperature formation of intermixing layers between a Cu/glass stack. For fabricating highly reliable Cu/glass structures, atomically scaled interface bonding is currently necessary. This chapter describes how these nanoscale ZnO adhesion layers are prepared and used for reducing the processing temperature so that it does not affect the packaging technologies. *Chapter 5* discusses ways of improving lead-free micronsized solder joint cracks. These can be used in flex-on-board interconnections, which have become very popular in mobile electronics applications. However, crack formation within these micron-sized solder joints remains a major problem, which

occurs in low-temperature curable acrylic polymer resins after bonding processes. The authors describe how the elastic modulus of an anisotropic conductive film resin assembly can be altered for low-melting solder materials and electronic device packaging so that these cracks can be controlled and reliability maintained. In this study, the mechanism of an Sn-58Bi solder joint crack with low-temperature curable acrylic adhesive is investigated.

The main focus of this book is to make scientists/researchers aware of the most advanced developments and applications in lead-free solders. We hope this book will be helpful not only for those who work in the field of solder jointing and its applications but also for researchers and scientists working on photonics and semiconductor devices. Finally, we would like to thank all the contributing authors for their hard work in the preparation of this book.

> **Abhijit Kar** Jagadis Bose National Science Talent Search, Kolkata, India

> > **1**

**Chapter 1**

Interface

**1. Introduction**

has come into the limelight.

conventionally used Sn37Pb solder.

*Monalisa Char and Abhijit Kar*

Introductory Chapter: Overview

of Multilayered Thin Film of Sn

All electronic devices starting from a small remote-controlled car to large aerospace vehicles require numerous interconnects within the circuit to complete the electrical pathways for its smooth functionality. For making these contact points, soldering plays the most important role. Until recently, Sn63-Pb37 (Sn-Pb eutectic) solders were used widely for making these contact points. The whole electronic industry was depending on the Sn-Pb alloy due to their low cost, good solder ability, low melting temperature, and satisfactory mechanical and functional properties. However, it has been observed that there arise serious environmental and health hazards caused due to the extensive use of electronic gadgets containing lead. So, there came strict restrictions imposed by the Restriction of Hazardous Substance (2002/95/EC (RoHS 1)) and Waste Electrical and Electronic (WEEE) directives and other similar bodies in use of lead in the electronic gadgets. Hence, the need for the development of lead-free solder alloy for electronics and microelectronic devices

However, the miniaturization of electronic devices aiming at high performance has made the task very complex for all the researchers to develop reliable and costeffective electronic joining materials/technologies. Scientists have developed different lead-free soldering alloys for making consumer electronic devices; however, after more than couples of decades, it is yet to conclude the best possible solder that can withstand different harsh environmental conditions and provide good reliability and improve the service life of the device/joints (compared to the Sn-Pb eutectic solder). In recent days, we have observed that the service life of any electronic gadgets have been reduced drastically. These early failures of various consumer electronic gadgets have been often linked with the absence of a good lead-free solder material, which can withstand various service-exposed conditions [1–8]. After many years of research, different lead-free solder alloys like SAC105, SAC205, SAC305, SAC405, SN100C, Sn-Zn-, Sn-Cu-, and Sn-Bi-based solders have been explored by the industry and academia [9–11]. However, SAC305 (Sn-3.0Ag-0.5Cu), a lead-free solder alloy, has been identified as a nearest substitute of the

of Pb-Free Solders and Effect

on the Lead-Free Solder Joint
