Open access

Introductory Chapter: Overview of Pb-Free Solders and Effect of Multilayered Thin Film of Sn on the Lead-Free Solder Joint Interface

Written By

Monalisa Char and Abhijit Kar

Submitted: March 7th, 2019 Published: October 2nd, 2019

DOI: 10.5772/intechopen.86713

Chapter metrics overview

974 Chapter Downloads

View Full Metrics

1. Introduction

All electronic devices starting from a small remote-controlled car to large aerospace vehicles require numerous interconnects within the circuit to complete the electrical pathways for its smooth functionality. For making these contact points, soldering plays the most important role. Until recently, Sn63-Pb37 (Sn-Pb eutectic) solders were used widely for making these contact points. The whole electronic industry was depending on the Sn-Pb alloy due to their low cost, good solder ability, low melting temperature, and satisfactory mechanical and functional properties. However, it has been observed that there arise serious environmental and health hazards caused due to the extensive use of electronic gadgets containing lead. So, there came strict restrictions imposed by the Restriction of Hazardous Substance (2002/95/EC (RoHS 1)) and Waste Electrical and Electronic (WEEE) directives and other similar bodies in use of lead in the electronic gadgets. Hence, the need for the development of lead-free solder alloy for electronics and microelectronic devices has come into the limelight.

However, the miniaturization of electronic devices aiming at high performance has made the task very complex for all the researchers to develop reliable and cost-effective electronic joining materials/technologies. Scientists have developed different lead-free soldering alloys for making consumer electronic devices; however, after more than couples of decades, it is yet to conclude the best possible solder that can withstand different harsh environmental conditions and provide good reliability and improve the service life of the device/joints (compared to the Sn-Pb eutectic solder). In recent days, we have observed that the service life of any electronic gadgets have been reduced drastically. These early failures of various consumer electronic gadgets have been often linked with the absence of a good lead-free solder material, which can withstand various service-exposed conditions [1, 2, 3, 4, 5, 6, 7, 8].

After many years of research, different lead-free solder alloys like SAC105, SAC205, SAC305, SAC405, SN100C, Sn-Zn-, Sn-Cu-, and Sn-Bi-based solders have been explored by the industry and academia [9, 10, 11]. However, SAC305 (Sn-3.0Ag-0.5Cu), a lead-free solder alloy, has been identified as a nearest substitute of the conventionally used Sn37Pb solder.

Although SAC305 alloy is widely used in electronic industry, there is still a large scope for its modification or introduction of other joining technologies/procedures in order to get the optimized reliable joints, which we lack even now. Joints produced using such alloys have been investigated extensively by different researchers. It has been found that the interfacial microstructure, orientation of IMC at the interface, and its volume fraction are the key factors for optimizations of the mechanical and functional properties of the joints [12, 13, 14, 15].

It has been realized that focusing only on Pb-free solder alloy development would certainly reduce the environmental pollution. But while achieving this, rise in processing temperature (because of the higher melting point of the lead-free solder) would in turn enhance the energy consumption while manufacturing the solder joints, thereby causing thermal pollution again. So, we have focused on finding suitable method/s for lowering the processing temperature and controlling the interfacial intermetallics growth of lead-free soldering, thereby increasing the reliability of the gadgets.

In this regard for lowering the processing temperature, we have used the transient liquid phase (TLP)-like soldering technique to join two copper substrates along with solder alloy Sn-3.0Ag-0.5Cu (SAC305). In case of high-temperature metal joining (welding/brazing), it is observed that the TLP bonding along with some interlayers generally produces IMC layer at the interface with lesser thickness compared to that in the conventional brazing/welding. Also, it produces homogeneous interface [16]. By dint of TLP-like soldering technique, we have prepared lead-free solder joint of Cu using thin multilayer structure of tin along with SAC305 solder alloy, hereafter termed as Cu-Sn/SAC/Sn-Cu solder system. In order to compare the effect of this thin multilayered structure, conventional solder joint of copper pads using SAC305, hereafter termed as Cu-SAC-Cu, has been prepared. We have produced the solder joints at 230°C, which is about 15–30°C lower than that of the conventional reflow soldering process generally used for making the lead-free solder joints [17].

Already reported studies in the literature show that joints produced at high processing temperature consist of higher volume fraction of IMCs with inhomogeneous interfacial microstructure (mostly scallop shaped), which essentially produce internal stress and exhibit premature failure [18].

It is found that the interfacial morphology and distribution of the IMC across the interface plays the key role in determining the quality and reliability of any solder joints [4, 19, 20]. Therefore, researchers have attempted to develop homogeneous interfacial microstructure with restricted growth of intermetallics in order to produce solder joints with better mechanical and functional properties. But, it is yet to conclude the best suitable and optimized temperature or processing technique that would give us reliable lead-free solder joint with improved service life of the electronic gadgets [21, 22, 23, 24].

For Sn-3.0Ag-0.5Cu/Cu solder joint prepared at 250°C, Tong et al. showed that after long hours of aging (˃288 h) at 150°C, the growth rate of Cu3Sn surpasses the growth rate of Cu6Sn5 phase, where the total IMC thickness increases by almost 97.56% after aging of only 500 h [25].

Compared to conventional Cu-SAC-Cu solder joints (Figure 1), we have been able to produce Cu-Sn/SAC/Sn-Cu solder joints at 230°C (Figure 2), which exhibit a reduced growth rate with respect to the total IMC thickness and show homogeneous interfacial microstructure structure across the solder joint interface. Interestingly, even after aging of 1200 h at 150°C, it has been observed that there is only 68.85% increment in the total IMC thickness for Cu-Sn/SAC/Sn-Cu solder joint (Figure 3a), whereas there is an increment in the total IMC thickness of about 78% for conventional Cu-SAC-Cu solder joint (Figure 4a). It has also been observed that the growth rate of conventional Cu-SAC-Cu solder joint is increasing rapidly from 0.005 to around 0.01 μm/h at 1200 h of aging (Figure 4b). Whereas a reduced growth rate of total IMC thickness has been detected for Cu-Sn/SAC/Sn-Cu solder joint across the solder joint interface, which is gradually decreasing from 0.008 (as soldered condition) to 0.001 μm/h at 1200 h of aging, suggesting that the IMC formation has been controlled with the help of thin multilayer of tin film as shown in Figure 3b.

Figure 1.

Optical images of the IMC microstructure of Cu-SAC-Cu solder joint with varying aging time at 150°C. (a) As cast, (b) 100 h aging, (c) 300 h aging, (d) 500 h aging, (e) 700 h aging, and (f) 1200 h aging.

Figure 2.

Optical images of the IMC microstructure of Cu-Sn/SAC/Sn-Cu solder joint with varying aging time at 150°C. (a) As cast, (b) 100 h aging, (c) 300 h aging, (d) 500 h aging, (e) 700 h aging, and (f) 1200 h aging.

Figure 3.

(a) Total IMC thickness of Cu-Sn/SAC/Sn-Cu solder joint with variation of aging time in hours. (b) First derivative of total IMC thickness of Cu-Sn/SAC/Sn-Cu solder joint with aging time in hours.

Figure 4.

(a) Plot of total IMC thickness of Cu-SAC-Cu solder joint with variation of aging time in hours. (b) Derivative plot of total IMC thickness of Cu-SAC-Cu solder joint with variation of aging time in hours.

In order to control the IMC thickness across the solder joint interface, many researchers have also tried to add the fourth element, which would get mixed up with the solder alloy and would restrict the diffusion of Cu from substrate toward solder and thereby restricting tin to get diffused from solder toward substrate. But, an optimized growth rate of IMC formation has not been achieved yet. Ni-doped SAC solder alloy has been studied by Benabou et al. Ni is considered to be one of the best elements in restricting the diffusion of copper across the interface [26]. Study shows that though Ni-doped solder alloy produces joints with thinner IMC layer, it could not be able to restrict the formation of voids in Cu3Sn layer with aging time and it produces cavities in (Cu, Ni)6Sn5 phase, which would be the point of initiation of failure of solder joint. Also, it increases the electrical resistivity of the solder joint system due to the discontinuity in the electrical pathways because of the presence of the voids.

Kang et al. have reported SAC solder alloy with surface finish of Ni(P)/Au, which produces Ni3Sn4 IMC layer, which grows faster than Cu-Sn IMC (Cu6Sn5 and Cu3Sn phases) on bare Cu and remains detached from the substrate. This detachment of the Ni3Sn4 phase from the substrate decreases the shear strength as well as increases the electrical resistivity of the solder joint due to the discontinuity in the electrical pathways [27]. However, our proposed multilayer structure in the Cu-Sn/SAC/Sn-Cu solder joint shows neither void at the Cu3Sn phase nor any cavities at Cu6Sn5 phase; moreover, a controlled rate of IMC formation has been observed. This may be attributed due to the formation of solid solution of β-Sn phase in the interface, which restricts the Cu diffusion from substrate to solder. On the other hand, a significant improvement in the electrical properties has been observed for the solder joints produced using this TLP-like soldering and multilayered structure.


2. Electrical conductivity

The electrical resistivity across the solder joints has been investigated by four probe methods. For the Cu-Sn/SAC/Sn-Cu solder joint it was found 1.4*10−5 Ω-mm for as soldered samples and 2.66*10−5 Ω-mm after 1200 h of thermal aging, whereas the electrical resistivity of the conventional Cu-SAC-Cu solder joint has been found to be 4.9*10−5 Ω-mm at 1200 h of thermal aging, which is almost twice that of Cu-Sn/SAC/Sn-Cu solder joint aged up to 1200 h. Thinner interfacial IMC layers cause lower associated localized Joule heating, resulting in enhanced electronic transport across the joints.

The rate of increment of resistivity is also slow in case of Cu-Sn/SAC/Sn-Cu solder joints than that of the Cu-SAC-Cu solder joint, as revealed from the slope of the curve (Figure 5). This may be attributed due to controlled IMC formation at the interface achieved by using thin multilayers of Sn film for Cu-Sn/SAC/Sn-Cu solder joints. Sn has lower resistivity than SAC solder paste (resistivity value: Sn ≈ 1.15 and SAC ≈ 3.57 μΩ cm). Also, ionization enthalpy of Sn is lower than that of Cu (CuIonization Energy = 745.5 and SnIonization Energy = 708.6 kJ/mol); therefore, Sn loses its outermost electron much earlier than Cu after getting little amount of energy, so it provides more number of charge carriers to conduct electricity, thereby reducing its resistance and increasing the conductivity in comparison with the Cu-SAC-Cu solder joints where no multilayers are been used.

Figure 5.

Resistivity plot of Cu-Sn/SAC/Sn-Cu solder system and Cu-SAC-Cu solder joint system with varying aging time.


3. Conclusion

This introductory chapter discusses the overview of the lead-free solders, its importance, and necessity in the cutting-edge research carried out across the globe for making environment friendly electronic devices. For any solder joints, the interfacial characteristics like mechanical and functional properties along with the microstructural morphology play the mostly vital role. Therefore, a systematic and detailed knowledge of the interfacial products and their structures are of fundamental interest for understanding the behavior of the solder joints. The concept of transient liquid phase-like soldering technique has been discussed, and the effect of thin multilayered film of Sn has been reported toward obtaining a more reliable lead-free solder joint.

We have demonstrated that incorporation of thin multilayer of Sn film along with SAC305 paste could produce good solder joints at 230°C. Cu pads have been joined by TLP-like soldering. For Cu-Sn/SAC/Sn-Cu solder joints, homogeneous interfacial microstructures have been formed across the solder joint interface as compared to scallop like IMC in the conventional Cu-SAC-Cu solder joint. Incorporation of thin multilayers of Sn film is capable of reducing the IMC growth rate up to 10% compared to the conventional Cu-SAC-Cu solder joint. Thinner interfacial IMC layers cause lower associated localized Joule heating, resulting in enhanced electronic transport across the joint, thereby reducing the electrical resistivity and increasing the conductivity across the solder joint interface.


  1. 1. Zhang L, Wang ZG, Shang JK. Current-induced weakening of Sn3.5Ag0.7Cu Pb-free solder joints. Scripta Materialia. 2007;56(5):381-384
  2. 2. Mahmood T, Mian A, Amin MR, Auner G, Witte R, Herfurth H, et al. Finite element modeling of transmission laser microjoining process. Journal of Materials Processing Technology. 2007;186(1):37-44
  3. 3. Tu KN, Thompson RD. Kinetics of interfacial reaction in bimetallic Cu-Sn thin films. Acta Metallurgica. 1982;30(5):947-952
  4. 4. Kar A, Ghosh M, Majumdar BS, Ghosh RN, Ray AK. Interfacial microstructure, shear strength and electrical conductivity of Sn-3.5Ag-0.5In/Cu lead free soldered joints. Materials Technology. 2007;22(3):161-165
  5. 5. Tu K-N. Copper–Tin Reactions in Bulk Samples. In: Solder Joint Technology, Springer Series in Materials Science Vol. 117. New York: Springer; 2007. pp. 37-71
  6. 6. Gao F, Takemoto T, Nishikawa H. Effects of Co and Ni addition on reactive diffusion between Sn-3.5Ag solder and Cu during soldering and annealing. Materials Science and Engineering A. 2006;420(1-2):39-46
  7. 7. Xu L, Pang JHL. Nano-indentation characterization of Ni–Cu–Sn IMC layer subject to isothermal aging. Thin Solid Films. 2006;504(1-2):362-336
  8. 8. Luo WC, Ho CE, Tsai JY, Lin YL, Kao CR. Solid-state reactions between Ni and Sn-Ag-Cu solders with different Cu concentrations. Materials Science and Engineering A. 2005;396(1):385-391
  9. 9. Cheng S, Huang C-M, Pecht M. A review of lead free solders for electronics applications. Microelectronics Reliability. 2017;75: 77-95
  10. 10. Berni R, Catelani M, Fiesoli C, Scarano VL. A comparison of alloy-surface finish combinations considering different component package types and their impact on soldering reliability. IEEE Transactions on Reliability. 2016;65(1):272-281
  11. 11. Coyle R, Sweatman K, Arfaei B. Thermal fatigue evaluation of Pb-free solderjoints: Results, lessons learned, and future trends. Journal of the Minerals, Metals and Materials Society. 2015;67(10):2394-2415
  12. 12. Kim KS, Huh SH, Suganuma K. Effects of intermetallic compounds on properties of Sn-Ag-Cu lead-free soldered joints. Journal of Alloys and Compounds. 2003;352(1-2):226-236
  13. 13. Chan YC, So ACK, Lai JKL. Growth kinetic studies of Cu-Sn intermetallic compound and its effect on shear strength of LCCC SMT solder joints. Materials Science and Engineering B. 1998;55(1-2):5-13
  14. 14. Li JF, Agyakwa PA, Johnson CM. Interfacial reaction in Cu/Sn/Cu system during the transient liquid phase soldering process. Acta Materialia. 2011;59(3):1198-1211
  15. 15. Deng X, Piotrowski G, Williams JJ and Chawla N. “Influence of initial morphology and thickness of Cu6Sn5 and Cu3Sn intermetallics on growth and evolution during thermal aging of Sn-Ag solder/Cu joints,” Journal of Electronic Materials. 2003;12(32):1403-1413
  16. 16. Egar T, Sadeghian M, Ekrami A, Jamshidi R. Transient liquid phase bonding of 304 stainless steel using a Co-based interlayer. Science and Technology of Welding and Joining. 2017;8(12):666-672
  17. 17. Leong YM, Haseeb ASMA. Soldering characteristics and mechanical properties of Sn-1.0Ag-0.5Cu solder with minor aluminum addition. Materials. 2016;9(7):1-17. DOI: 10.3390/ma9070522
  18. 18. Tan AT, Tan AW, Yusof F. Influence of nanoparticle addition on the formation and growth of intermetallic compounds (IMCs) in Cu/Sn-Ag-Cu/Cu solder joint during different thermal conditions. Science and Technology of Advanced Materials. 2015;16:033505
  19. 19. Mohammad Faizan. “Effect of inter metallic phase morphology on substrate dissolution during reflow soldering,” International Journal of Mechanical Engineering and Computer Applications. 2014;2(2):28-32. ISSN 2320-6349
  20. 20. Ghosh M, Kar A, Das SK, Ray AK. Aging characteristics of Sn-Ag eutectic solder alloy with the addition of Cu, In, and Mn. Metallurgical and Materials Transactions A. 2009;40(10):2369-2376
  21. 21. Laurila T, Hurtig J, Vuorinen V, Kivilahti JK. Effect of Ag, Fe, Au and Ni on the growth kinetics of Sn-Cu intermetallic compound layers. Microelectronics Reliability. 2008;49(3):242-247 DOI: 10.1016/j.microrel.2008.08.007
  22. 22. Chia PY, Haseeb ASMA, Mannan SH. Reactions in electrodeposited Cu/Sn and Cu/Ni/Sn nanoscale multilayers for interconnects. Materials. 2016;9:430. DOI: 10.3390/ma9060430
  23. 23. Bui QV, Nam ND, Yoon JW, Choi DH, Kar A, Kim JG, et al. Effect of gold on the corrosion behavior of an electroless nickel/immersion gold surface finish. Journal of Electronic Materials. 2011;40(9):1937-1942
  24. 24. Yoon J-W, Noh B-I, Kim B-K, Shur C-C, Jung S-B. Wettability and interfacial reactions of Sn–Ag–Cu/Cu and Sn–Ag–Ni/Cu solder joints. Journal of Alloys and Compounds. 2009;486:142-147
  25. 25. An T, Qin F. Effects of the intermetallic compound microstructure on the tensile behavior of Sn-3.0Ag-0.5Cu/Cu solder joint under various strain rates. Microelectronics Reliability. 2014;54:932-938. DOI: 10.1016/j.microrel.2014.01.008
  26. 26. Benabou L, Vivet L, Tao QB, Tran NH. Microstructural effects of isothermal aging on a doped SAC solder alloy. International Journal of Materials Research. 2018;109(1):76-82. DOI: 10.3139/146.111578
  27. 27. Kang SK, Choi WK, Yim MJ, Shih DY. Studies of the mechanical and electrical properties of lead free solder joints. Journal of Electronic Materials. 2002;11(31):1292-1303

Written By

Monalisa Char and Abhijit Kar

Submitted: March 7th, 2019 Published: October 2nd, 2019