**4. Hardness results**

*Lead Free Solders*

**3. Growth kinetics**

range of 1.6–2.3 microns [3].

kinetics [4]:

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

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,

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

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

The growth of these intermetallic layers can be modelled using parabolic growth

where w = thickness of the intermetallic layer, w0 = initial thickness of the layer,

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

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/

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/

w = w0 + D√t (1)

appeared to be applicable to this metallic system [1].

microwave energy has been used for soldering [2].

dendritic solidification morphology, just like in castings.

D = diffusion coefficient and t is the time of annealing. Arrhenius type of growth kinetics is seen.

and then approaches the parabolic law [5].

thermomigration-controlled at the hot end [6].

**10**

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 presence of intermetallics.

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].

## **4.1 Indium-based Sn▬Zn alloys**

All hardness values described in this section are taken along two perpendicular diagonals of a square-shaped specimen. Results are shown in (**Table 1**).

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 lead-based soldering alloys.

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

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 size and improve hardness [13, 14].


**Table 1.**

*Results of hardness testing.*


#### **Table 2.**

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

### **4.2 Lanthanum-based lead-free alloys**

Two solders are analysed for their hardness. Both are lanthanum-based alloys with similar composition. Results are given in **Tables 4** and **5.**

The average hardness for sample 1 is **17.8 HV**.

The average hardness for sample 2 is **18.4 HV**.

Muhammed Aamir et al. [15] investigated that the inclusion of 0.4 wt.% of La in to Sn▬Ag▬Cu (SAC) solder system results in intermetallic growth. Intermetallics are hard in nature. Hence, our findings are in line with reported work, which indicates increase in hardness on addition of lanthanum.

#### **4.3 Sn▬Cu lead-free alloys**

Referring **Table 6** above silver and manganese content are more in sample 1. Manganese is known to increase hardness. This could be the reason for higher hardness in sample 1. Since hardness of sample 1 is higher than sample 2, this could indicate the presence of more intermetallics in sample 1. With the addition of La, the microhardness of -Sn and eutectic area was enhanced from 13.8 to 16.4 Hv and from 16.8 to 18.8 Hv, respectively [16]. Cu6Sn5, Ag3Sn and MnSn2 are present in dendritic Sn-rich solid solution (ẞ Sn). These intermetallics are found in both samples as seen in the microstructures of the samples. However, % age of intermetallics in each of the samples may be different. Average hardness of the second sample is considerably lower than the first one. This indicates that the presence of intermetallics is lesser in the second sample. More detailed analysis using XRD is required to


#### **Table 3.**

*Hardness value of specimen 88Sn***▬***7.5Zn***▬***2.5Al***▬***2In.*


#### **Table 4.**

*Microhardness value for sample 1 of composition 1 Sn-87.5% Zn-7.5% Bi-4% La-1%.*


**13**

analysed.

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

*Hardness values for the two alloys.*

*Microhardness value for sample 1.*

*Microhardness value for sample 2.*

**Table 6.**

**Table 7.**

**Table 8.**

come to a final conclusion. Zhang et al. showed that addition of 0.05% rare earth Yb suppressed the growth of intermetallics and the morphology of Cu6Sn5 layer can

**S No 1 2 3 4 5 6 Avg** Sample 1 26.7 26.4 28.4 32.5 28.5 30.4 28.8 Sample 2 18.1 19.0 17.8 18.3 17.8 15.4 17.7

**Diagonal 1 (μm) Diagonal 1 (μm) Microhardness value (HV 0.5)**

**Diagonal 1 (μm) Diagonal 1 (μm) Microhardness value (HV 0.5)**

17.5 18.6 16.8 20.9 21.6 17.3 22.42 21.6 20.5 22.52 22.41 19.1 23.44 24.03 17.2

22.42 22.42 19.3 22.46 19.19 16.7 24.32 25.21 18.2 19.92 16.89 17.4 22.32 19.05 20.4

Two samples with slight compositional differences have been analysed, and the

For Sn▬8Zn▬3Bi, with increasing temperature, the amount of hard Bi segrega-

Microstructral analysis done in a recent research work by the authors has been

Sample 1 has a composition of **Sn-87.5% Zn-7.5% Bi-4% La-1%**.

Sample 2 has a composition of **Sn-87% Zn-7% Bi-4.5% La-1.5%**.

**5. Microstructural and EDAX analysis of Sn**▬**Zn**▬**Al-In alloys**

Results of experimental work on 88Sn▬7.5Zn▬2.5Al▬2In and 88Sn▬7Zn▬2Al▬2.5 in lead-free soldering alloys are presented below.

be changed to a relatively flat morphology (**Table 6**) [17].

The average hardness for sample 1 is **18.4 HV**.

The average hardness for sample 2 is **18.2 HV**.

tion increases which is the main cause of the rise in hardness.

**4.4 Sn▬Zn-lanthanum lead-free alloys**

results were presented in **Tables 7** and **8**.

**5.1 Analysis of low Al-low indium alloys**

#### **Table 5.**

*Microhardness value for sample 2 of composition 2 Sn-87% Zn-7% Bi-4.5% La-1.5%.*

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


#### **Table 6.**

*Lead Free Solders*

**4.2 Lanthanum-based lead-free alloys**

**4.3 Sn▬Cu lead-free alloys**

*Hardness value of specimen 88Sn***▬***7.5Zn***▬***2.5Al***▬***2In.*

Two solders are analysed for their hardness. Both are lanthanum-based alloys

Muhammed Aamir et al. [15] investigated that the inclusion of 0.4 wt.% of La in to Sn▬Ag▬Cu (SAC) solder system results in intermetallic growth. Intermetallics are hard in nature. Hence, our findings are in line with reported work, which

Referring **Table 6** above silver and manganese content are more in sample 1. Manganese is known to increase hardness. This could be the reason for higher hardness in sample 1. Since hardness of sample 1 is higher than sample 2, this could indicate the presence of more intermetallics in sample 1. With the addition of La, the microhardness of -Sn and eutectic area was enhanced from 13.8 to 16.4 Hv and from 16.8 to 18.8 Hv, respectively [16]. Cu6Sn5, Ag3Sn and MnSn2 are present in dendritic Sn-rich solid solution (ẞ Sn). These intermetallics are found in both samples as seen in the microstructures of the samples. However, % age of intermetallics in each of the samples may be different. Average hardness of the second sample is considerably lower than the first one. This indicates that the presence of intermetallics is lesser in the second sample. More detailed analysis using XRD is required to

**Specimen 88Sn▬7.5Zn▬2.5Al▬2In Hardness, HV 0.2**

 17.8 17.0 17.4 17.45 18.45 17.9 20.1 20.5 20.3 17.5 15.5 16.2 17.8 16.8 17.4

*Microhardness value for sample 1 of composition 1 Sn-87.5% Zn-7.5% Bi-4% La-1%.*

*Microhardness value for sample 2 of composition 2 Sn-87% Zn-7% Bi-4.5% La-1.5%.*

16.5 16.5 16.5 17.3 18.1 17.7 18.6 22.6 20.2 19.5 19.0 19.3 17.6 19.6 18.2

Diagonal 1 17.5 20.9 22.42 22.52 21.9 Diagonal 2 18.6 21.6 21.6 22.41 22.2

**S. No Diagonal 1 (μm) Diagonal 2 (μm) Microhardness value (HV 0.5) average**

**Diagonal 1 (μm) Diagonal 2 (μm) Microhardness value (HV 0.5)**

with similar composition. Results are given in **Tables 4** and **5.**

The average hardness for sample 1 is **17.8 HV**. The average hardness for sample 2 is **18.4 HV**.

indicates increase in hardness on addition of lanthanum.

**12**

**Table 5.**

**Table 3.**

**Table 4.**

*Hardness values for the two alloys.*


#### **Table 7.**

*Microhardness value for sample 1.*


#### **Table 8.**

*Microhardness value for sample 2.*

come to a final conclusion. Zhang et al. showed that addition of 0.05% rare earth Yb suppressed the growth of intermetallics and the morphology of Cu6Sn5 layer can be changed to a relatively flat morphology (**Table 6**) [17].
