**2. Intermetallics in lead-based and lead-free solders**

Way back in 1972, Lois Zackreysak has reported intermetallics in lead-free alloys. Tin-based lead-free alloys showed the presence of Sn▬Cu intermetallic when soldered on a copper-based substrate. It was found that throughout the early stages of the growth process, the Cu3Sn and the Cu6Sn5 thicknesses are approximately equal. As tin is depleted from the solder matrix, Cu6Sn5 growth slows down, while the Cu3Sn continues to grow at the expense of the tin-rich intermetallic. The presence of insoluble alloying elements affects the intermetallic growth rate to a minor degree. The general theory for the diffusion-controlled growth of plates appeared to be applicable to this metallic system [1].

Just after soldering, Cu6Sn5 formed between the solder and copper pad. It should be noted that in most of the soldering joints, there are generally three layers formed. The three layers are substrate, intermetallic and solder. The intermetallic layers are sandwiched between the two substrate layers. In the study mentioned above, microwave energy has been used for soldering [2].

## **3. Growth kinetics**

A study of growth kinetics of any solidification process will be useful to predict the type of microstructure formed and the morphology of the microstructures. Generally, as the alloys solidify under normal cooling conditions, we can expect a dendritic solidification morphology, just like in castings.

During soldering of lead-free alloys, the Cu6Sn5 intermetallic sublayer is clearly visible, and the Cu3Sn sublayer is noticeable only for the sample annealed at 150°C [1].

Comparison of the 63Sn▬37Pb joints and the lead-free joints shows that the initial thickness of the intermetallic layer is not significantly impacted by the higher temperature used during the lead-free assembly process. All the values are in a range of 1.6–2.3 microns [3].

The growth of these intermetallic layers can be modelled using parabolic growth kinetics [4]:

$$\mathbf{w} = \mathbf{w}\_0 + \mathbf{D}\sqrt{\mathbf{t}} \tag{1}$$

**11**

**Table 2.**

**Table 1.**

*Results of hardness testing.*

*Role of Intermetallics in Lead-Free Alloys DOI: http://dx.doi.org/10.5772/intechopen.85515*

were clearly observed.

**4. Hardness results**

presence of intermetallics.

**4.1 Indium-based Sn▬Zn alloys**

lead-based soldering alloys.

size and improve hardness [13, 14].

*Hardness value of specimen 88Sn*▬*7Zn*▬*2Al*▬*2.5In.*

Cu solder joints during reflow at 260°C on a hot plate, due to the rapid migration of Cu atoms under a simulated temperature gradient of 51°C/cm. Qu et al. [1, 8] in situ studied the soldering interfacial reactions under a temperature gradient of 82.2°C/ cm at 350°C using synchrotron radiation real-time imaging technology, and asymmetrical growth and morphology of interfacial IMCs between the cold and hot ends

Hardness, especially microhardness is a very simple engineering measurement that gives information about the material being tested. In the case of volume of measurement being small as is the case of solders, then microhardness measurements are done from the periphery to the centre along any diagonal. In the case of solders, hardness values are an indication of whether intermetallic phases are present or not. For example, if the hardness of a material of a particular composition is higher than the normal solder alloy of the same composition, there is a possibility of intermetallic formation. Microstructural analysis should also be used to confirm the

These hardness results are taken from the author's research work on lead-free alloys. All the values quoted here are experimental values obtained by the author and his team of coresearchers, and they have been published recently [9, 10].

All hardness values described in this section are taken along two perpendicular

Sn▬37Pb solders normally used for soldering have a hardness of 12 HV [11]. So, on an average, both the new alloys have hardness higher than the traditionally used

Small changes in Zn and Al do not change the hardness much, and literature has shown that even small additions of lanthanum affect the hardness. So, it can be assumed with reasonable certainty that the hardness changes are due to changes in lanthanum content. Small additions of indium have been shown to improve the microhardness and also produce considerable changes in the microstructure [12]. Small amounts of indium addition have been found to refine grain

**Specimen Hardness, HV 0.2 Average** Sn▬7Zn▬3Al▬3In 18.4 19.9 19.1 18.8 19.3 19.1 Sn▬6Zn▬2Al-2.5In 18.6 17.7 18.1 18.3 17.9 18.1

Diagonal 1 22.42 22.46 24.32 19.92 20.53 Diagonal 2 22.42 19.19 25.21 16.89 19.5

diagonals of a square-shaped specimen. Results are shown in (**Table 1**).

**Specimen 88Zn**▬**7Zn**▬**2Al**▬**2.5In Hardness, HV 0.2**

Hardness values are shown below in **Tables 2** and **3**.

where w = thickness of the intermetallic layer, w0 = initial thickness of the layer, D = diffusion coefficient and t is the time of annealing.

Arrhenius type of growth kinetics is seen.

Li et al. have studied the growth kinetics of Sn-based lead-free solders on copper substrate. The results show that IMC layer is formed at solder-Cu substrate interface within a short time. It was found that Fick's law was not followed. Fick's law states that the mean total thickness increases linearly with the square root of the time. In most of the systems during solidification processing, welding and other manufacturing processes tend to follow Fick's law under equilibrium or near equilibrium conditions. In the case of soldering, cooling is very fast, especially when copper is used as substrate. It deviates from Fick's law at the early stage of the growth process and then approaches the parabolic law [5].

The growth behaviour of intermetallic compounds (IMCs) at the liquid-solid interfaces in Cu/Sn/Cu interconnects during reflow at temperatures in the range of 200–300°C on a hot plate, which was investigated by Zhao et al. The interfacial IMCs showed clearly asymmetrical growth during reflow. The growth of Cu6Sn5 IMC at the cold end was significantly enhanced while that of Cu3Sn IMC was hindered especially at the hot end. It was found that the temperature gradient had caused the mass migration of Cu atoms from the hot end towards the cold end, resulting in sufficient Cu atomic flux for interfacial reaction at the cold end while inadequate Cu atomic flux at the hot end. The growth mechanism was considered as reaction/thermomigration-controlled at the cold end and grain boundary diffusion/ thermomigration-controlled at the hot end [6].

Guo et al. [7] found that the interfacial Cu6Sn5 was much thicker at the cold end, whereas the consumption of Cu was much faster at the hot end in Cu/Sn▬2.5Ag/

Cu solder joints during reflow at 260°C on a hot plate, due to the rapid migration of Cu atoms under a simulated temperature gradient of 51°C/cm. Qu et al. [1, 8] in situ studied the soldering interfacial reactions under a temperature gradient of 82.2°C/ cm at 350°C using synchrotron radiation real-time imaging technology, and asymmetrical growth and morphology of interfacial IMCs between the cold and hot ends were clearly observed.
