**4. The interfacial intermetallic compound (IMC) layers**

In connected metals, all the common base materials, coatings, and metallizations, such as Cu, Ni, Ag, and Au, form intermetallic compounds (IMC) with Sn, which is the major element in Sn solders. Cu is the material most frequently used for leads and pads on flip chip substrates and printed wiring boards. It is now known that in the solder/Cu interfacial reaction, Sn reacts rapidly with Cu to form Cu3Sn (ε-phase) and Cu6Sn5 (η-phase) [25]. Other metal substrate/solder interfacial reactions form IMCs, such as Ag3Sn[26] (Sn solder/Ag ), Sn-Ni [27] (Sn solder/Ni), Ag-In[28] (In solder/Ag) and Cu-In IMC[29] (In solder/Cu). These intermetallic compounds are generally more brittle than the base metal, which can have an adverse impact on the solder joint reliability. Excessive thickness may also decrease solder joint ductility and strength [30-34]. Recently, we found that a great number of nano-Ag3Sn particles form on the Cu6Sn5 IMC when the solders contain Ag3Sn precipitate phase after a Pb-free Sn3.5Ag0.5Cu (SAC) nano-composite solder/Cu substrate interface reaction[30, 31]. These nanoparticles apparently decrease the surface energy and hinder the growth of the Cu6Sn5 IMC layer during soldering and aging. All these results indicate that Gibbs absorption theory can be used to explain the formation of these nanoparticles and their effects on the surface energy of the IMC. Many studies have reported that nano-sized, nonreacting, noncoarsening oxide dispersoid particles, such as TiO2 [30-32], Al2O3 [33], Y2O3[34], CNTs [35], and ZrO2[36] can affect the growth rate of interfacial IMC.

Sn4In4.1Ag0.5Cu Nil 56±6 60±8 37±7 [22]

Sn3.5Ag0.7Cu Nil 31±2 35±1 41±8 [23] 0.01wt.% MWCNTs 36±2 47±1 36±2 0.04wt.% MWCNTs 36±4 46±6 37±2 0.07wt.% MWCNTs 33±3 43±5 35±4 Sn3.5Ag0.5Cu Nil 45.96±1.14 54.34±1.42 49.2±1.3 [16]

Sn3.5Ag0.25Cu Nil 53.2 55.7 48.6 [14]

Table 2. The data showing the enhancement of the mechanical properties of nano-composite

In connected metals, all the common base materials, coatings, and metallizations, such as Cu, Ni, Ag, and Au, form intermetallic compounds (IMC) with Sn, which is the major element in Sn solders. Cu is the material most frequently used for leads and pads on flip chip substrates and printed wiring boards. It is now known that in the solder/Cu interfacial reaction, Sn reacts rapidly with Cu to form Cu3Sn (ε-phase) and Cu6Sn5 (η-phase) [25]. Other metal substrate/solder interfacial reactions form IMCs, such as Ag3Sn[26] (Sn solder/Ag ), Sn-Ni [27] (Sn solder/Ni), Ag-In[28] (In solder/Ag) and Cu-In IMC[29] (In solder/Cu). These intermetallic compounds are generally more brittle than the base metal, which can have an adverse impact on the solder joint reliability. Excessive thickness may also decrease solder joint ductility and strength [30-34]. Recently, we found that a great number of nano-Ag3Sn particles form on the Cu6Sn5 IMC when the solders contain Ag3Sn precipitate phase after a Pb-free Sn3.5Ag0.5Cu (SAC) nano-composite solder/Cu substrate interface reaction[30, 31]. These nanoparticles apparently decrease the surface energy and hinder the growth of the Cu6Sn5 IMC layer during soldering and aging. All these results indicate that Gibbs absorption theory can be used to explain the formation of these nanoparticles and their effects on the surface energy of the IMC. Many studies have reported that nano-sized, nonreacting, noncoarsening oxide dispersoid particles, such as TiO2 [30-32], Al2O3 [33],

Y2O3[34], CNTs [35], and ZrO2[36] can affect the growth rate of interfacial IMC.

**0.2%YS (MPa)** 

1.0 vol.% Al2O3 72±6 75±6 21±3 3.0 vol.% Al2O3 73±3 77±6 11±3 5.0 vol.% Al2O3 74±3 76±2 10±0

0.25 wt.% Al2O3 48.81±1.23 60.20±1.84 47.3±0.8 0.5 wt.% Al2O3 52.56±1.56 62.44±1.76 44.0±1.2 1.0 wt.% Al2O3 57.22±1.8 68.05±1.63 43.5±2.1 1.5 wt.% Al2O3 61.45±2.3 70.05±2.06 32.5±3.2

0.25 wt.% TiO2 59.5 61.5 40.5 0.5 wt.% TiO2 67.6 69.1 32.1 1.0 wt.% TiO2 69.3 70.1 25.2

**Mechanical properties** 

**Elongation (%)** 

**References** 

**UTS (MPa)** 

**Solder matrix Reinforcement** 

solders[13, 14, 16, 22, 23].

**nanoparticles** 

**4. The interfacial intermetallic compound (IMC) layers** 

Fig. 2. Top view of the IMC at the interfaces of the nano-composite solder joints on Cu substrate after aging for 7 days at 175oC: (a) SAC and SAC- TiO2 [31].
