**3. Joints**

The interconnectivity of device components is very important in terms of electrical and mechanical properties. Most of these components are connected by solder joints. Solder joint is a very interesting subject to study in detail, and many new problems arise as electronic devices change almost daily.

Within this joint, IMCs are an important issue and must receive proper attention. Formation of an IMC layer at the solder/substrate interface after reflowing used to indicate a good joint between the solder and the substrate.

However, IMCs continue to achieve higher working temperatures and longer times. Thick IMC negatively affect the long‐term reliability of solder joints due to their brittle nature [4, 5]. Even worse, to accommodate microelectronic components, current smaller and thinner sizes of solder joints are needed. The formation of smaller joints indirectly means that the volume fraction of the formed IMC layer tends to increase.

To inhibit interfacial IMC growth, several types of substrate finishes have been developed. The most popular surface finish for Cu substrates for high‐end electronic applications is electroless nickel electroless palladium immersion gold (ENEPIG). ENEPIG is a tri‐layered structure consisting of a layer each of electroless Ni, electroless Pd and immersion Au. The electroless Ni layer serves as an efficient diffusion barrier between the solder and the Cu pad, which can effectively inhibit the growth of interfacial IMCs [6].

Another approach to reducing the growth of IMCs uses composite solders. Inert reinforce‐ ments i.e. TiO2 , Al2 O3 , CeO2 , Fe<sup>2</sup> NiO4 , TiC, TiO2 , ZnO, ZrO2 and a carbon base i.e. graphene and carbon‐nanotube are among the popular materials to be used in composite solders. These inert particles (mostly of nanosize) are nonreacting with the molten solder during the reflow process, which helps to refine the IMC's structure and consequently improve the mechanical and other properties [7–9]. Composite solders have therefore attracted consid‐ erable attention. For a full review of nanocomposite solders, the reader is referred to Shen and Chan [7].
