**6. Summary**

nowadays. Gold and silver based contacts provide optimum wear resistance in sliding contacts but they are not economical. Therefore, the sliding wear and tear are important for Sn based solders may limit their applications. [101] Tin based contacts are generally susceptible to fretting wear which is a prime concern in automobile applications.[102, 103] Sn based connec‐ tors are very ductile and more sensitive to fretting wear. Hammam *et al*. investigated the wear and frictional properties of various Sn coating prepared hot dipping, electroplating, and reflow processes. They suggested that the different processing routes for Sn deposition produce different thickness of the Sn coatings and hence the intermetallic compounds on a metallic substrates. [104] In case of nanocomposite solders, Jun *et al*. reported that tin bronze reinforced with carbon fibers improves the resistance against the fretting wear appreciably.[105] Sharma *et al*. have recently found a significant enhancement on the wear resistance of Sn and Sn–Ag

Electrochemical behavior of Sn based coatings is important in microelectronic packaging devices. The corrosion of Sn base coatings may deteriorate the microstructural properties of the solder joints and ultimately lead to entire failure of the device. For example, in marine applications, the chlorides ions from the sea can dissolve the solder joints by forming soluble compounds.[106] There are various forms of corrosion such as pitting, crevice, and/or galvanic corrosion of solder and substrate material. Therefore the corrosion of electronic devices may impose a serious threat among microelectronics community to produce corrosion resistant solder materials. However, there is a scarcity of information of electrochemical corrosion of the solder joints.[107, 108] It has been shown that presence of Bi in Sn–Bi solder can increase the dissolution of Sn slightly in sulfuric acid solution and vigorously in nitric acid solution compared to that of pure tin.[108, 109]. It has been noticed that lead-free solders are highly corrosion resistant towards chloride ion attack compared to conventional Sn-Pb solder. [110] Recently, Sharma *et al.* proposed that use of nanoparticles is better idea to avoid the localized dissolution of Sn coatings. If the nanoparticles are added in an optimum concentration in the solder matrix, the corrosion resistance of the solder matrix can improve significantly by setting up of homogeneous corrosion instead of preferential and sudden damage. The high surface energy nanoparticles in the solder matrix reduce the corrosion potential, corrosion current,

and diffusion capacitance, thus increasing passivity of the solder coating.[111]

Tin whisker growth in the Sn based coatings is a serious issue in microelectronic packaging devices. The driving force of these whisker growth is the generation of compressive stresses in the coatings on storage for a long time. When plated on a metallic substrate like copper, the Cu-Sn interdiffusion across the interface may give rise to the formation of Cu6Sn5, Cu3Sn etc., IMCs and grow with time causing a volume expansion of the interface. The expansion of the interface causes the setup of compressive stress in the coatings. Recently, it has been identified by various researchers that addition of nanoparticles may suppress the growth of IMCs and restrict the formation of whiskers in the matrix. Generally, the ceramic nanoparticles are very

matrices reinforced with CeO2 nanoparticles.[41, 95]

**5.7. Electrochemical corrosion**

264 Electrodeposition of Composite Materials

**5.8. Whisker growth mitigation**

Addition of nanoparticles in the solder matrix improves the microstructural properties due to the refinement in the grain size as well as the thickness of the IMC (Cu6Sn5, Ag3Sn) in the solder matrix. Wettability is improved due to the decrease in interfacial energy in presence of high surface energy nanoparticles. The higher hardness of nanosolder composites as compared to monolithic alloys can be attributed not only to the grain size and dispersion strengthening effect, but also to the refinement of the IMCs such as Cu6Sn5, Ag3Sn by ceramic nanoparticles. The melting point of the solders is minimum form an optimum reinforcement amount in the matrix, which indicates its possibility to use without any change in the existing soldering procedures. There is a rise in the resistivity of the composite matrix compared to the monolithic materials. However, the resistivity of the composites falls within the usable limits as reported for other Sn and Sn–Ag-based composites, used for electrical contact applications. The nanoscale reinforcements are added in minute concentrations and do not degrade the resis‐ tivity much if their distribution is uniform and electromigration phenomena in the composite solders. The addition of reinforcement in the Sn matrix also improves the wear resistance, which ultimately increases the coating life for application. The wear resistance of the composite coatings is better than that of the monolithic materials, and it is associated with an enhancement in the microhardness of the composite. It is also observed that composite solders possess the better corrosion resistance as compared to monolithic ones. The presence of fine Ag3Sn compounds in composite increases the passivation of the matrix which acts as a noble barrier in addition to ceramic reinforcements against corrosion propagation. It is also observed that an incorporation of CeO2 nanoparticles in the composite matrix reduces the compressive stresses developed in the coatings. The residual stresses of monolithic materials are negative, i.e., compressive in nature. A decline in residual stress indicates that the driving force for whisker growth can be minimized by choosing an optimum concentration of ceria, and thus, the coating life can be improved.
