**Abstract**

Intermetallics are intermediate compounds formed between two metals. They are usually brittle. The presence of intermetallics leads to deterioration in mechanical properties. This chapter reviews the intermetallic compounds formed during the manufacture of lead-free alloys. Intermetallic compounds formed in ordinary lead-based alloys are also discussed. The role of rare earth, especially indium and lanthanum, additions on intermetallic formation is examined. Microstructures of intermetallics are analysed. Hardness values of lead-free alloys are compared with emphasis on type and nature of intermetallics. SEM photographs of lead-free solders are discussed with regard to type of fracture, and the role of intermetallics in nature of fracture is examined. Lastly, the general mechanisms of formation of intermetallics are touched upon, and these mechanisms are extended to intermetallic formation in lead-free alloys.

**Keywords:** lanthanum based alloy, indium based alloy, hardness, characterization

### **1. Introduction**

Intermetallic is a type of metallic compound that exhibits definite stoichiometric ratio of atoms. It also has a definite crystal structure. Their role is twofold. In small quantities, they can strengthen the soldering joint. However, when the amount of intermetallics increases, they may render the joint brittle. For example, La3Sn intermetallics are known to be present in lanthanum-based lead-free alloys. These are known to increase the hardness of the alloys. Ag3Sn, Cu3Sn and Cu6Sn are some known intermetallics in tin-, copper- and silver-based lead-free alloys. Mostly, the interaction of these intermetallics with the substrates has to be studied in detail in order to determine the exact effect on the solder properties. The growth of these intermetallics with time is determined by solid-state diffusion rules and varies from system to system.
