**1. Introduction**

The adoption of Restriction of Hazardous Substances Directive 2002/95/EC (RoHS 1) in 2003 was one of the driving forces mandating the electronic industries to look for alternatives to replace the tin-lead eutectic alloy (SnPb-6337), which had been used for decades. RoHS 1 has limited the content of Pb to be less than 1000 ppm by weight in each homogenous material, which is including the soldering material. The SnPb-6337 contains 37% of Pb, which is an element with known harmful effect to human health and the environment. Thus, SnPb-6337

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

has to be replaced in order to meet the RoHS compliance. Pb is just one of the six banned substances under RoHS 1 [1]. The banned list of RoHS has expanded to 10 under Directive (EU) 2015/863 (RoHS 2), which was published in 2015. The full list of banned substances in RoHS 2 is shown in **Table 1**. PBB and PBDE are mainly used as frame retardant in plastics and hexavalent chromium is used in chrome plating, chromate coatings and primers, and in chromic acid. DEHP, BBP, DBP, and DIBP are phthalates, which are being used as plasticizers. These substances are rare in solder materials. Out of the 10 substances listed in RoHS 2, only Pb is the most concerned substance for solder industry. This explains why some of us refer RoHS compliance solder as Pb-free solder. Although the RoHS compliance cannot help in reducing the electronics waste, which has increased tremendously in recent years, it helps to minimize the hazardousness of such trash to environment.

Since 2006, Pb-free soldering has been in the main stream of industries because RoHS 1 took effect on 1st July 2006. Many changes in alloy composition are adopted in order to match the requirement of solder joint with Pb-free definition. Tin Copper (SnCu), Tin Silver (SnAg), and Tin Silver Copper (SnAgCu or SAC) are the dominating Pb-free solder alloys adopted by the industries.

SAC alloy had received most publicity in the initial stage of conversion from SnPb-6337 to Pb-free solder. One of the reasons was because SAC alloy with 3.0% silver (Ag) and 0.5% copper (Cu), SAC305 was endorsed by the IPC Solder Value Product Council as the preferred option for SMT assembly. Other than SAC305, the industries had also adopted other SAC alloys with higher silver content such as Sn3.8Ag0.7Cu (SAC387) and Sn4.0Ag0.5Cu (SAC405), which are believed to be the true eutectic ternary solder alloys, which have single melting point at 217°C. SAC305 is a hypo-eutectic ternary alloy with melting range of 217–219°C. However, the high cost of Ag compels the industries to consider other alternatives such as low silver solution, SAC0307, or no silver solutions such as SnCuNi (SN100C®). A more detailed discussion on this topic will be covered in Section 2 of this chapter.


**Table 1.** Ten substances which are banned in Restriction of Hazardous Substances Directive 2015/863 (RoHS 2).

The requirement of solder joint performance is getting stringent in order to support the modern electronification. It is especially for automotive industry. In a modern car system, the usage of advanced electronic components is getting common, for example, in logic control, switches, and sensors. The adoption of electronic components has significantly boosted the speed and accuracy of these systems. The electronification of car system such as advanced driver assistance systems (ADAS) has given another level of driving experience to consumer. On the other hand, the incorporation of multimedia system in car system, which is to increase the comfort level of driver has further increased the usage of electronic components in a car. It explains the growth of auto electronics has surpassed other industries in recent years [2]. Due to the harsh use environment where the under-hood temperature is much higher than other applications, the requirement on the solder joint performance is higher. Moreover, these systems are considered as life-critical system, which has increased the challenges in selecting an appropriate solder alloy. Due to this kind of new requirements, solder manufacturers and researchers are striking to invent new alloy, which can support this platform. It also implies that the conventional Pb-free solder alloys such as SnCu, SAC, and low SAC are showing limitations in fulfilling the demand of such applications. The various strengthening mechanisms in Sn-based solder alloy which have been deployed by the industries will be discussed in Section 3 of this chapter.

has to be replaced in order to meet the RoHS compliance. Pb is just one of the six banned substances under RoHS 1 [1]. The banned list of RoHS has expanded to 10 under Directive (EU) 2015/863 (RoHS 2), which was published in 2015. The full list of banned substances in RoHS 2 is shown in **Table 1**. PBB and PBDE are mainly used as frame retardant in plastics and hexavalent chromium is used in chrome plating, chromate coatings and primers, and in chromic acid. DEHP, BBP, DBP, and DIBP are phthalates, which are being used as plasticizers. These substances are rare in solder materials. Out of the 10 substances listed in RoHS 2, only Pb is the most concerned substance for solder industry. This explains why some of us refer RoHS compliance solder as Pb-free solder. Although the RoHS compliance cannot help in reducing the electronics waste, which has increased tremendously in recent years, it helps

Since 2006, Pb-free soldering has been in the main stream of industries because RoHS 1 took effect on 1st July 2006. Many changes in alloy composition are adopted in order to match the requirement of solder joint with Pb-free definition. Tin Copper (SnCu), Tin Silver (SnAg), and Tin Silver Copper (SnAgCu or SAC) are the dominating Pb-free solder alloys adopted by

SAC alloy had received most publicity in the initial stage of conversion from SnPb-6337 to Pb-free solder. One of the reasons was because SAC alloy with 3.0% silver (Ag) and 0.5% copper (Cu), SAC305 was endorsed by the IPC Solder Value Product Council as the preferred option for SMT assembly. Other than SAC305, the industries had also adopted other SAC alloys with higher silver content such as Sn3.8Ag0.7Cu (SAC387) and Sn4.0Ag0.5Cu (SAC405), which are believed to be the true eutectic ternary solder alloys, which have single melting point at 217°C. SAC305 is a hypo-eutectic ternary alloy with melting range of 217–219°C. However, the high cost of Ag compels the industries to consider other alternatives such as low silver solution, SAC0307, or no silver solutions such as SnCuNi (SN100C®). A more detailed

**Table 1.** Ten substances which are banned in Restriction of Hazardous Substances Directive 2015/863 (RoHS 2).

to minimize the hazardousness of such trash to environment.

discussion on this topic will be covered in Section 2 of this chapter.

**Substance RoHS limit** Lead (Pb) 1000 ppm/0.1% Mercury (Hg) 1000 ppm/0.1% Cadmium (Cd) 100 ppm/0.01% Hexavalent chromium (Cr6+) 1000 ppm/0.1% Polybrominated biphenyls (PBB) 1000 ppm/0.1% Polybrominated diphenyl ether (PBDE) 1000 ppm/0.1% Bis(2-ethylhexyl) phthalate (DEHP) 1000 ppm/0.1% Butyl benzyl phthalate (BBP) 1000 ppm/0.1% Dibutyl phthalate (DBP) 1000 ppm/0.1% Diisobutyl phthalate (DIBP) 1000 ppm/0.1%

the industries.

92 Recent Progress in Soldering Materials

Besides in-car system, the effort of popularization of electric vehicles has further promoted the usage of electronic components in automotive industry. In view, the fact that this kind of system involves high current and high temperature, conventional SAC Pb-free alloys cannot fulfill the basic requirement. This is the reason why the exemption for RoHS compliance has been further extended to 2021 for Pb use in high-melting temperature type solders (i.e., Pb-based solder alloys containing 85% by weight or more Pb, Category 7a). Nowadays, most of the electronic devices for such application are still adopting high Pb solder as the die attach material. There are various options available in replacing high Pb solder at die attach process. But, there are still gaps to be filled up before such new materials can be fully commercialized and mass adoption of such new materials in current production.

Internet of things (IoT) has caught high attention from industries. Many industry players believe that the IoT will boost the growth of electronic component to another level, the era of IoT. The IoT has reshuffled the business process of many big organizations in preparing themselves to ride on this wave. To interpret the IoT in a simpler way, it is an application which allows the wide use of sensor in our daily life. In IoT, there is also a platform, which can connect all the relevant sensors for data analyzing and actuation. The objective of IoT is to realize a smarter world. According to *Business Insider*, there are 1.9 billion interconnected devices in 2014, with an expected 9 billion by 2018. By then, the number of IoT devices will surpass the summation of mobile phone, tablet, and PCs, which are the current major consumer market. With such high quantity of IoT device, the usage of solder as interconnect is also prospective. The selection of solder alloy to fulfill the requirement of this sector will focus in meeting the specific criteria for manufacturing, reliability, toxicity, cost, and availability.

In view of prospective growth of electronic component and new solder joint requirement, solder manufacturers and researchers are working together in developing new alloy or modifying existing alloys to close the gap between the user and supplier of solder material. In the chapters that follow the evolution of Pb-free alloy and the techniques used in enhancing solder alloy strength are discussed.
