WETTABILITY OF BiAg11 SOLDER DURING FLUX APPLICATION

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1 WETTABILITY OF BiAg11 SOLDER DURING FLUX APPLICATION Michal CHACHULA a, Roman KOLEŇÁK a, Robert AUGUSTÍN a, Monika KOLEŇÁKOVÁ a a Slovak University of Technology, Faculty of Materials Science and Technology, Paulinska 16, , Trnava, Slovakia; michal.chachula@gmail.com, roman.kolenak@stuba.sk, robert.augustin@stuba.sk, monika.kolenakova@stuba.sk Abstract Recently does not exist any adequate substitute for high lead content PbSn5 solder. Therefore, new alternative solder alloys, such as BiAg11 are developed. The aim of this contribution is to identify wettability and interactions between alternative BiAg11 solder and base materials Cu, Ag and Ni using the flux and reflow soldering (temperature 380 C) without shiel ding gas. Holding time at the temperature was 10 seconds. Wettability measurement on the specimens was performed. Measured wettability angles for the BiAg11 and Cu base material solder joints was 25 ( very good wettability), BiAg11 - Ag base material reached the value of 18 (perfect wettability) and BiAg11 - Ni base material was approximately 98 (unsufficient wettability). Optical microscopy and EDX analysis were used for the observation of specimen cross sections. Values of wavy interface were measured in the case of all samples. BiAg11 solder - Cu interface was thick only 1 µm, BiAg11 solder - Ag interface 3.5 µm and the last BiAg11 solder - Ni interface was about 5 µm. As follows from reached results, Ag and Cu base materials are the most suitable materials for the BiAg11 solder. The application of Ni as the base material is inappropriate, because of the formation of brittle intermetallic compounds. In the case of Ag and Cu base materials, only a weak eutectic reaction occured. Key words: wettability, BiAg, transitional area, flux 1. INTRODUCTION Sn - Pb based alloy was the most commonly used solder alloy for electronic industry. The harmful effects of lead are generally well known, and therefore, all soldering materials are being substituted for lead - free ones. However, PbSn5 and PbSn10 solders with high lead content are still utilized in the applications where higher temperature is required because no existing substitutes has been developed. But there are alternative solders whose qualities are similar to lead-containing solders [1, 2, 3]. These include precious metals such as gold (AuSn20, AuGe12) and silver (BiAg11, SnAg25Sb10). BiAg11 solder was chosen for the experiment, because of more affordable price than Au based solders and higher melting point in comparison to SnAg25Sb10 solder [2]. Many contributions focused on BiAg11 solder properties. The interaction among BiAg11 solder with metallic substrates were studied at Tsingua university in Beijing. Institute of Multidisciplinary research for advanced materials at Tohoku university deals with thermodynamic properties and evaluations of lead free solders for higher working temperatures. Tensile strength and ductility of BiAg11 solder was tested [2, 4, 5]. 2. EXPERIMENT Cu, Ag and Ni substrates as well as solder consisting of 89 % of Bi and 11 % of Ag were chosen for the experiment. Not only the melting point must have been in the range between 260 C and 450 C but also solder must have had satisfactory tensile strength and be at reasonable price. Vacuum casting was 1

2 performed in order to ensure the % purity of the solder alloy [3]. The sizes of samples were 40 x 40 x 2 mm. A production procedure of the samples began with putting 1 gram of BiAg11 solder to the centre of clean and degreased surface of each sample. Than we added 1 gram flux of the Soldaflux type 7000 (3.1.1.A). Samples were soldered in an air furnace at the temperature of 380 C and during 10 seconds at this temperature. The aim of the experiment was to learn about the interactions between BiAg11 solder and Cu, Ag and Ni substrates and also to determine the influence of flux and its effects. The level of interaction was determined on the basis of wetting angles [10]. Tests of wettability of BiAg11 solder were conducted with the help of goniometric method. The lower is the angle, the better interaction can be expected. Flux can help the interaction but it depends also on the type of substrate [1, 2]. Samples were prepared metalographically with the aim to determine the contact wetting angles and perform observations at the solder - substrate interface. Because of the fact that samples were in corroded state the results proved the presence of phases and visible wavy interfaces. The EDX analysis was utilized to monitor the change in concentration of elements on determined line through defined area of phase interface. These areas included the areas of dark phases, matrix and the areas of phase interface [6, 7, 8]. 3. RESULTS 3.1 Optical microanalysis Areas of phase boundaries and the microstructure, which are obtained by optical microscopy are clearly visible in following figures. Various precipitated particles of the second phase rich in Ag, can be observed in the Bi rich primary phase (fig. 1 a, b, c) [11, 12]. Fig. 1 Cu (a), Ag (b) and Ni (c) substrate - solder phase interface 3.2 Measurement of wettability angles First, the sample with Cu substrate were measured and wetting angle was 25 which means good wettability (fig. 2 a). 2

3 Fig. 2 Angles of wettability BiAg11 solder on Cu (a), Ag (b) and Ni (c) substrates Satisfactory results were recorded in case when sample with Ag substrate were utilized. The sample had the wetting angle of 18 which means good wettability (fig. 2 b). The result of sample regarding wetting angle with Ni substrate were as follows. The angle was 146 which means unsatisfactory wettability (fig. 2 c). Visible gap between BiAg11 solder and Ni substrate proves that there are unwetted areas. Negative effects also included the formation of visible hollows [7, 9, 10]. 3.3 EDX analysis Based on the images from the EDX analysis it is visible that Cu does not dissolve neither in Bi nor Ag. Independently on the fact whether flux has been applied or not, there was no intermetallic compound between Bi and Cu elements, and only a eutectic reaction occurred with expected weak interaction between solder and the Cu surface at the temperature of C. The size of wavy interface reached 1 µm and was insignificant (fig. 3). Eutectic reaction was not significant as well. [13, 15, 16]. 3

4 Fig. 3 EDX analysis of BiAg11 solder on Cu substrate and line profiles of elements present During soldering Ag substrates with BiAg11 solder, a eutectic reaction with more significant interaction between solder and surface occurs. There is a higher probability that primary material will be wetted. The more significant eutectic reaction occurs, the bigger is the waving effect. The size of wavy interface of 3.5 µm is depicted on the (fig. 4). The complexity of this process is determined by low solubility of Ag in Bi in the liquid state [12, 14, 17]. Fig. 4 EDX analysis of BiAg11 solder on Ag substrate and line profiles of elements present During the interaction of BiAg11 solder with Ni substrate, the formation of inter - layer of chemical compound is expected. According to Ni - Bi binary diagram, the formation of NiBi 3 intermetallic compound is the most 4

5 probable at the temperature of 271 C [18, 19]. Then Ni dissolves to NiBi 3. The width of formed NiBi 3 intermetallic compound was about 20 µm (fig. 5). [19]. Fig. 5 EDX analysis of BiAg11 solder on Ni substrate and line profiles of elements present 4. CONCLUSION Better results were recorded with Cu and Ag substrates. The results of wetting angles were 25 for Cu substrate and 18 for Ag substrate which means that both materials are suitable for BiAg11 solder and for its potential application in the practice. In case of Ni substrate is better not to use it for application to practice [4]. The results of wetting angles are only then satisfactory. In the near future, BiAg11 solder has a good perspective to be used in electrical engineering in soldering printed circuit boards directly on conductive surface [3, 12]. It is the first economically advantageous lead - free solder which can be used to solder printed circuit boards and which is able to withstand the temperature of 260 C during reflow soldering [16]. ACKNOWLEDGEMENTS The author thanks to prof. Ing. J. Lokaj, PhD. for EDX microanalysis. 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. REFERENCES [1.] Shi, Y. at al. (2009). Investigation of rare earth - doped BiAg high - temperature solders. Journal of Materials Science: Materials in Electronics, Vol. 8., No. 12, ( ). [2.] Lalena, N. J.; Nancy, F. J. & Weiser, W. M. (2002). Experimental investigation of Ge-doped Bi-11Ag as a new Pbfree solder alloy for power die attachment. Journal of Electronic Materials, Vol. 31., No. 11, ( ). [3.] Song, M. J.; Chuang, Y. H. & Wu, M. Z. (2006). Interfacial reactions between Bi - Ag high - temperature solders and metallic substrates. Journal of Electronic Materials, Vol. 35., No. 5, ( ). 5

6 [4.] Anonymous authors (2010). Materials Science; Research from Beijing University of Technology has provided new information about materials science. Electronics Business Journal, Vol. 3., No. 2, (89-92). [5.] Matsumoto, T. & Nogi, K. (2008). Wetting in Soldering and Microelectronics. Annual Reviews (Nonprofit Scientific Publisher), Vol. 38., No. 1, ( ). [6.] Rottenmayr, M. at al. (2005). High Melting Pb - Free Solder Alloys for Die - Attach Applications. Advanced Engineering Materials, Vol. 7., No. 10, ( ). [7.] Song, M. J.; Chuang Y. H. & Wu, M. Z. (2007). Substrate Dissolution and Shear Properties of the Joints between Bi - Ag Alloys and Cu Substrates for High - Temperature Soldering Applications. Journal of Electronic Materials, Vol. 36., No. 11, ( ). [8.] Joseph B. at al. (1998). Rapid penetration of liquid Bi along Cu grain boundaries. Scripta Materialia, Vol. 39., No. 6, ( ). [9.] Kim, H. J.; Jeong, W. S. & Lee, M. H. (2002). Thermodynamics - Aided Alloy Design and Evaluation of Pb - free Solders for High - Temperature Applications. Materials Transactions, Vol. 43., No. 8, ( ). [10.] Lee, S. M.; Liu M. C. & Kao, R. C. (1999). Interfacial reactions between Ni substrate and the component Bi in solders. Journal of Electronic Materials, Vol. 28., No. 1, (57-62). [11.] Yamada, Y. at al. (2006). Pb - free high temperature solders for power device packaging. Microelectronics and Reliability, Vol. 46., No. 9, ( ). [12.] Digges Jr., G. T. & Tauber, N. R. (2002). Structure of Bi - Ag eutectic alloy. Journal of Crystal Growth, Vol. 8., No. 1, ( ). [13.] Brunetti, B. at al. (2006). Bismuth activity in lead - free solder Bi In Sn alloys. Calphad, Vol. 30., No. 4, ( ). [14.] Kenichiro, F. at al. (2006). Liquidus temperature design of lead - free solder. Materials Transactions, Vol. 47., No. 4, ( ). [15.] Hecht, M. V. & Wang, L. (2002). Interface stability and microsegregation in bismuth - tin alloys. Journal of Crystal Growth, Vol. 7., No. 2, ( ). [16.] Weiping, F. at al. (2009). Research development of high temperature lead - free solders for electronic assembly. Journal of Alloys and Compounds, Vol. 28., No. 3, (71-74). [17.] Xi, Z. at al. (2010). Transition to Lead - Free Products in the US Electronics Industry: A Model of Environmental, Technical, and Economic Preferences. Environmental Modeling and Assessment, Vol. 16., No. 1, ( ). [18.] Muriel, T. (2007). Die - attach Materials and Processes. Advanced Packaging, Vol. 16., No. 3, (32-36). [19.] Dybkov, V. I. & Duchenko, V. O. (1995). Determination of NiBi 3 reaction - diffusion constants in Ni - Bi couples. Journal of Materials Science Letters, Vol. 14., No. 24, ( ). 6