**4. Nanoindentation**

used to tackle problems regarding to pure Sn, to control the formation of intermetallic com‐ pounds (IMC), to keep the melting point at eutectic or near eutectic, to improve corrosion

Several publications have already reported on these elements used to form various alloy compositions. Examples of basic compositions are Au‐Sn, Bi‐Sn, Sn‐Ag, Sn‐Cu, Sn‐In and Sn‐Zn. However, the properties of these binary alloy systems are far away from any applica‐ tions [3]. In the next development, ternary solders are proposed, such as Sn‐Ag‐Cu, Sn‐Ag‐ Bi and Sn‐Zn‐Bi solders. However, the formation of brittle failure behavior of IMC during soldering of both phases remains a problem. Now, trending in solder research is composite

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

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

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

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,

Another approach to reducing the growth of IMCs uses composite solders. Inert reinforce‐

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

, ZnO, ZrO2

and a carbon base i.e. graphene

, TiC, TiO2

resistance and improve mechanical properties and so on [1, 2].

solders.

**3. Joints**

ments i.e. TiO2

and Chan [7].

, Al2 O3 , CeO2

devices change almost daily.

2 Recent Progress in Soldering Materials

between the solder and the substrate.

fraction of the formed IMC layer tends to increase.

which can effectively inhibit the growth of interfacial IMCs [6].

, Fe<sup>2</sup> NiO4 Many physical/mechanical characterizations of solder joint have been proposed. However, since the miniaturization of devices reduces solder joint size, the effectiveness of characterization also needs to follow this trend. Nanoindentation hardness testing, using nanometer‐sized indents, is another popular tool that is accorded with smaller component properties. It is used to estimate the physical and mechanical properties of materials [6]. An example of nanoindentation testing involves curve marks and plotting of the force that is applied to solder materials (**Figure 1**) [9].

**Figure 1.** Areas of the nanoindentation marks on the cross‐sectioned surface of (a) SAC305 and (b) the hysteresis plot of the loading and unloading indenters for SAC305, adapted from Ref. [9].
