STUDY OF PROPERTIES OF LEAD-FREE SOLDER TYPE Au-20Sn AT ULTRASOUND ASSISTANCE

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1 8th International DAAAM Baltic Conference "INDUSTRIAL ENGINEERING April 2012, Tallinn, Estonia STUDY OF PROPERTIES OF LEAD-FREE SOLDER TYPE Au-20Sn AT ULTRASOUND ASSISTANCE Chachula M. & Koleňák R. Abstract: The main aim of this work was to determine the soldering, thermal and mechanical properties of joints fabricated by use of Au-20Sn solder. Soldered joints were fabricated by application of a flux-free process with assistance of power ultrasound. By acting of power ultrasound, the Au-20Sn solder wets the surface of Cu, Ag and Ni substrates. The contact wetting angle on all substrates varied within 1 α 10 interval. The melting point of solder was determined using of DSC analysis and soldering temperature was then selected. The shear strength of soldered joints was determined. The highest shear strength (195 MPa) was observed on Cu substrate and the lowest (20 MPa) was observed on Ni substrate. It is supposed that this solder may be a suitable substitution for Pb-5Sn solder for higher application temperatures. Key words: lead- free solder, solderability, EDX analysis, shear strength. 1. INTRODUCTION The work deals with the study of solders for higher application temperatures. The Sn-Pb alloys have been for a long time the most spread soldering material in electronic industry. However, owing to harmful effect of lead, to be mentioned also in the scientific work of a collective of authors [1], all these soldering materials are gradually substituted by the lead-free solders. But at applications where higher soldering temperature is required, there is still used the Pb-5Sn solder with a high lead content. Fortunately, also alternative soldering alloys exist, which compete with the lead solders by their properties. These may be based on precious metals as Au, but also on the basis of metals as Zn and Bi. The scientific work [2], has dealt with the study of Au and Bi based solders. The Au-20Sn and Bi-11Ag solders, with acceptable soldering temperature were considered for perspective ones. Thermodynamical properties of mentioned solders were studied in work [3]. Based on the results of above mentioned scientific works, but also regarding the other publications [4-7], the primary Au-Sn base was suggested for experiments. It was actually the Au-20Sn solder. 2. EXPERIMENT 2.1 Preparation of specimens The studied material combination consisted of Cu, Ag and Ni metal substrates. The metal substrates were in the form of rings 15 mm in diameter and 1.5 mm in thickness. The Au-20Sn solder was manufactured by vacuum casting from the mentioned initial elements with 4N purity. Soldered joints were fabricate by application of power ultrasound from ultrasonic generator type Hanuz UZP2 with an output power of 400 W/ 40 khz. Description of this soldering equipment was published in work [8]. Ultrasonic soldering meets the requirements for soldering speed and quality of joint fabrication. After fabrication, the soldered joints were cross cut for metallographic study 616

2 and the cross sections were prepared by wet grinding. 2.2 Assessment of experiments Since an experimentally prepared solder was used for soldering, it was necessary to determine the solidus and liquidus temperature. Due that reason the DSC analysis was performed. Based on the results of DSC analysis, the soldering temperature for fabrication of test specimens was suggested. The transient intermetallic phases were studied on microscope Neophot 32. Chemical microanalysis of boundary in soldered joints was carried out by EDX analysis. This is a part of electron scanning microscope type JEOL 7600F. The shear strength was determined on LabTest 5.250SP1-VM equipment at room temperature. For the change in direction of force acting on specimen, a special shearing device was developed. This jig assures a uniform shear loading of specimen in the plane of solder and substrate boundary. The shear gap was selected to 0.1 mm, what corresponds to 2 % of the sheared cross section of the roller formed of solder. The uniform testing rate was 0.5 mm/min. 2.3 Properties of Au-20Sn solder Eutectic solder Au-20Sn is suitable from the viewpoint of melting temperature, a good thermal and electric conductivity, a high tensile strength and a good wettability. The solder properties mentioned in publication are in Tab. 1 [9]. Tab. 1 Properties of Au-20Sn solder [9] Melting point 280 C Soldering temperature 320 C Tensile strength 275 MPa Density g/cm 3 Coef. of therm. expansion C Therm. conductivity 57 W/m.K Specific therm. capacity 388 J.kg -1 K -1 Young s modulus 68 GPa Ductility 2 % Electric resistance Ω.m 3. RESULTS 3.1 DSC analysis DSC analysis was performed in order to identify the melting point of Au- 20Sn solder. Tab. 2 shows the results from DSC analysis measurement. Tab. 2 The results of DSC measurement Fig. 1 shows the DSC curve of Au- 20Sn solder. The solder starts to melt at temperature C. The solder is fully molten at C. Fig. 1 DSC curve of Au-20Sn solder on heating 10K/min 3.2 EDX analysis of joints fabricated by Au-20Sn solder The structure of Au-20Sn solder is formed of a eutectic mixture of Au 5 Sn and AuSn phases. The lighter phase shown in Figs. 2 and 3 is Au 5 Sn phase. EDX analysis has revealed approximate composition of this phase. The phase consist of 11.2 wt. % Sn and 88.8 wt. % Au. The dark phase shown in Figs. 2 and 3 is AuSn phase and consist of 38.4 wt. % Sn and 61.6 wt. % Au. The concentration profiles of Ag, Au, Sn elements determined by EDX analysis are documented in Fig

3 By interaction of Au-20Sn solder with the surface of Cu substrate (Fig. 3) a new combined (Au,Cu)Sn phase is formed, which assures the surface wetting by the solder. 3.3 Measurement of shear strength of joints Based on knowledge obtained from the DSC analysis, the test specimens for shear test were fabricated by assistance of power ultrasound. The results of shear strength achieved with Au-20Sn solder on Cu, Ag and Ni substrates are documented in Fig. 4. Fig. 2 EDX analysis of Au-20Sn/Ag joint Fig. 3 EDX analysis of Au20-Sn/Cu joint Fig. 4 Shear strength of joints High shear strength was achieved with the joint fabricated by Au-20Sn solder on Cu substrate, namely up to 195 MPa. Lower value of shear strength, namely 151 MPa, was achieved on Ag substrate. The lowest value of shear strength was observed on Ni substrate, namely 20 MPa in average. 3.4 Fractographic analysis of soldered joints The morphology was defined by fractographic analyses by use of SEM. Fracture morphology was investigated on the specimen fabricated on Cu substrate with ultrasound assistance. Ductile failure of specimen has occurred. Fig. 5 shows the fractured surface of Au-20Sn solder on intermetallic boundary. In Au-20Sn solder a tearing-off of metallic substrate was observed. The fracture was formed partially in the solder and partially in the Cu substrate. 618

4 5. ACKNOWLEDGEMENTS This contribution was prepared with the support of the project VEGA 1/0211/11 Development of lead- free solder for higher application temperatures and research of material solderability of metallic and ceramic material. The author thanks to Ing. Martin Sahul for EDX analysis. Fig. 5 Fractured surface of Au-20Sn solder with a partial tearing-off of Cu substrate. 4. CONCLUSIONS The attained results have shown that interaction with joints occurred and surface wetting by Au-20Sn solder took place. The contact wetting angle at soldering with ultrasound attained the value below 10. The boundaries of soldered joints were smooth and free from defects. The melting point is very important at selection of a correct alternative substitution. The DSC analysis has proved that the experimental Au-20Sn solder had the melting point of C. This solder temperature approaches to the melting point of PbSn5 solder with a high lead content, which melting point is 298 C. The shear test of soldered joints has revealed that the highest shear strength up to 195 MPa was achieved with the joint of Au-20Sn solder on Cu substrate. The Au-20Sn solder is a suitable substitution for the lead solder type Pb-5Sn for higher application temperatures. However, rather high price of this solder seems to be a certain disadvantage in practical applications. 6. REFERENCES [1] Bath, J. Lead-free soldering, Springer, [2] Song, J. M., Chuang, H. Y. & Wui, Z. M. Interfacial Reactions between Bi-Ag Solders. Jour. of Elect. Mat., 2006, 9, [3] Kim, H. J. Jeong, W. S. Lee, M. H.: Thermodynamics-Aided Alloy Design and Evaluation of Pb-free Solders for High- Temperature Applications, Mat. Trans., 2002, 8, [4] Yamada, Y.- Takaku, Y, et. al.: Pb-free High Temperature Solders for Power Device Packaging, Micro. Relia. 2006, 46, [5] Andersson, C.- Lai, Z., Liu, J., Jiang, H., Yu, Y.: Comparison of isothermal mechanical fatigue properties of lead-free solder joints and bulk solders. Mat. Sc. And Tech. 2005, 2, [6] Wu, C. M., Yu, D. Q., Wang, L. Properties of lead-free solder alloys with rare earth element additions. Jour. of Mat. Sc. and Eng. 2004, 44, [7] Zeng, K. T., Six cases of reliability study of Pb-free solder joints in electronic packaging technology, Mat. Sc. and Eng., 2002, 38, [8] Koleňák, R., Šebo, P., Provazník, M., Koleňáková, M., Ulrich, K. Shear strength and wettability of active Sn3.5Ag4Ti(Ce,Ga) solder on Al 2 O 3 ceramics, Mat. & Des., 2011, 32, [9] Yoon, J. W., Chun, H. S., Noh, I. B. (2008) Mechanical reliability of Sn-rich Au Sn/Ni flip chip solder joints fabricated by sequential electroplating method, Mic.. Relia., 2008, 48,

5 7. ADDITIONAL DATA ABOUT AUTHORS 1) Michal Chachula, MSc., PhD. student, Roman Koleňák, Assoc. prof. 2) Study of properties of lead- free solder type Au-20Sn at ultrasound assistance 3) Slovak University of Technology, Faculty of Materials Science and Technology in Bratislava, Paulínska 16, Trnava, , Slovac Republic, (address is same for all authors) 4) Michal Chachula, Slovak University of Technology, Faculty of Materials Science and Technology in Bratislava, Paulínska 16, Trnava, , Slovac Republic 620