Pure Cu-(Sn37Pb) and Cu-(Sn3.5Ag0.5Cu) soldered interface has been characterized by Scanning Electron Microscopy 

(SEM)  and  Transmission  Electron  Microscopy  (TEM)  fitted  with  EDX.  TEM  analysis  of  the  interface  confirms  the presence of different intermetallic compounds (IMCs) in the interface, such as  (Cu 6 Sn 5 ),   (Cu 3 Sn) and Ag 3 Sn phases. Microstructural observations are correlated with the electrical and mechanical properties of the joints. (Lead free solder)-Cu  joint  exhibits  better  electrical  conductivity  (0.28x10 6 ohm-1cm-1)  and mechanical  strength  ~68MPa  compare  to  the conventional (lead-tin solder)-Cu joint which exhibits electrical conductivity and mechanical strength as 0.22x10 6 ohm-1cm-1

 and ~55MPa respectively

Ghosh, M

Ghosh, R N

Kar, Abhijit

Ray, Ajoy K

English

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 1 Comparative study of tin lead and lead free solder joint interface  Abhijit Kar, Mainak Ghosh, Rabindra Nath Ghosh and Ajoy Kumar Ray Materials Science and Technology Division National Metallurgical Laboratory (CSIR); Jamshedpur-831 007, India   Abstract:  Pure Cu-(Sn37Pb) and Cu-(Sn3.5Ag0.5Cu) soldered interface has been characterized by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) fitted with EDX. TEM analysis of the interface confirms the presence of different intermetallic compounds (IMCs) in the interface, such as  (Cu6Sn5),   (Cu3Sn) and Ag3Sn phases. Microstructural observations are correlated with the electrical and mechanical properties of the joints. (Lead free solder)-Cu joint exhibits better electrical conductivity (0.28x106 ohm-1cm-1) and mechanical strength ~68MPa compare to the conventional (lead-tin solder)-Cu joint which exhibits electrical conductivity and mechanical strength as 0.22x106 ohm-1cm-1 and ~55MPa respectively.   Key Words: Lead free solder, interface, TEM, electrical conductivity, IMC.  Corresponding Author: e-mail: karabhijit@hotmail.com Tel: +91-657-2271709, Fax: +91-657-2270527  Introduction:  Eutectic tin-lead solder has been extensively used as the most convenient joining materials for the electrical interconnects for last few decades [1-3]. It has now been realized the hazardous effects of using such lead based alloy in the electronic and packaging industry. During processing of electronic materials lead vapours and while the electronic wastes are disposed, lead causes serious environmental pollution and health hazards. Therefore, it is going to be mandate to replace conventional eutectic tin-lead solder by some appropriate lead free solder in near future [1-3]. During the preparation of lead free solders, the most vital factors considered are melting point of the alloy, wetability, corrosion resistance, electrical conductivity and mechanical strength of the joint interface [1]. Several lead free alloy including Sn-9Zn, Sn-Zn-In, Sn-9Zn-xAg, Sn-Zn-Al, Sn-Ag-In have been studied. Sn-9Zn eutectic solder shows melting point of 198oC, which is closer to Sn-37Pb eutectic i.e. 183oC, but due to poor mechanical strength, wetability and corrosion resistance limits its use. Study shows that the addition of silver improves the wetability of the solder [4]. Chang et al. [1] reported that the Sn-9Zn-xAg /Cu interface  (with x≤ 3.5 %) exhibits better solder joint reliability than Sn-9Zn/Cu joint interface. Present study aims to investigate the microstructural nature, mechanical property and the electrical conductivity of the Sn37Pb/Cu and Sn-3.5Ag-0.5Cu/Cu joint interface and revels the superiority of the later over former.  Experimental: 2nd International Conference on Recent Advances in Composite Materials (ICRACM-2007), February, 20-23, 2007, New Delhi,  Paper No: P-130   2 Sn (99.98 wt%), Ag (99.99 wt%), Pb (99.95 wt%), and Cu (99.95 wt%) pure ingots are used for the preparation of two different alloys i.e. Sn-37Pb (wt %) and Sn-3.5Ag- 0.5 Cu (wt %). For preparing of 50g solder alloy, respective quantity of metals were taken in a quartz tube and the tubes were sealed keeping a high purity argon atmosphere (IOLAR 1) inside. Before sealing, tubes were evacuated and purged with argon gas for three times. The sealed quartz tubes containing Sn-Pb and Sn-Ag-Cu were heated in a furnace at 700oC. For complete homogenization, the samples were kept at 700oC for 3 hours. After cooling, the tubes were broken and the alloys were taken out. Subsequently the alloys were rolled to make strips of 0.2 mm thick. These were cleaned with acetone and used for soldering purposes. The compositions were confirmed by bulk SEM-EDX analysis. The melting points of Sn-37Pb and Sn-3.5Ag-0.5Cu were found to be 183 and 217oC respectively.  The melting points of the alloys were determined using a thermal analyzer (SDT Q600, TA Instruments-USA). The transition joints between solder alloy and Cu substrate were prepared by solder reflow process. 0.2 mm thick solder alloy was placed between two pre-heated (200 for Sn-37Pb and 230oC for Sn-3.5Ag-0.5Cu) organic solderability preservative (OSP) finished Cu substrates and allowed to re-flow for 120sec at 230oC. The Cu substrates (8x8x5 mm3) were preheated to avoid quenching of the solder and to maintain desired flowability.  The transition joints in as reflowed condition were sectioned transversely in a precision cut off machine (MECATOME P100, Presi, France) using 0.25mm thick diamond wafer blade. Metallographically polished and cleaned samples were examined in a scanning electron microscope (Jeol JSM 840A) using back scattered mode (BS) at 15kV. The compositions of chemical species (wt%) in the reaction layer and in the different structural features were determined by energy dispersive spectroscopy (KAVEX). Shear testing was carried out at room temperature using a specially designed jig mounted on a tensile testing machine (Honsfield, H10K-S, 10KN capacity) at a crosshead speed of 0.1mm min-1.  Reproducibility of the results has been checked by performing tests on five samples from each of the solder joints. A set of joints was mechanically ground to obtain 0.1mm thickness and subsequently ultrasonic cutting, dimpling and ion milling were done. To identify the phases present at the interface, transmission electron microscope (Philips, CM200) at 200kV using EDS (EDAX) and to examine their morphology as well as distribution. Electrical conductivity (% IACS) of the solder joints was measured using single probe technique (Forster Sigma Tester, Model 2.067,W.Germany) after necessary calibration. % IACS is defined as the percentage ratio of international standard copper electrical conductivity to the electrical conductivity of the material used.  Results and Discussions:  The melting points (MP) of the Sn-Pb eutectic solder and Sn-Ag-Cu solder alloy are 183 and 217oC respectively (Fig.1a & b). The reflow temperature (RFT) in the present investigation is 230oC. This is close to the MP of pure tin. If the difference between MP and RFT is higher, the mass transport becomes more profuse across the interface of any transition joint. Hence, for the Sn37Pb solder alloy, the diffusion of chemical species is more pronounced than Sn3.5Ag0.5Cu alloy. This factor on the other hand enhances the thickness of the IRL in case of Sn37Pb/Cu solder joint (Fig.2a) Incase of Sn3.5Ag0.5Cu/Cu solder alloy the presence of Ag in the IRL is minimal and does not take part in the formation of intermetallic compound. It can be assumed, that the major quantity of Ag is entangled  3 with Sn to form Ag3Sn within the solder alloy and thus reducing the extent of diffusion of Sn towards IRL further makes it thinner (Fig. 2b) [5]. The formation of Cu3Sn observed in the IRL of Sn3.5Ag0.5Cu/Cu solder joint, is                governed by the phase stability according to the reaction Cu6Sn5 + 9Cu  5Cu3Sn [5-6]. It has been found, that Cu6Sn5 acts as a diffusion barrier for the formation of other Cu-Sn compounds [7]. In Sn-Pb/Cu joint, the IRL contains mainly Cu6Sn5 with substantial thickness. This is perhaps responsible for the absence of Cu3Sn in that couple. The shear strength of Sn3.5Ag0.5Cu/Cu solder joint exhibits ~1.75 times higher than that of Sn-Pb/Cu solder joint.                      -0.6-0.4-0.20.00.20.40.60.8Heat Flow (W/g)0 50 100 150 200 250Temperature (°C)Exo Up Universal V3.9A TA InstrumentsFig.1. (a) DSC plot for Sn37Pb solder alloy (b) DSC plot for Sn3.5Ag0.5Cu solder alloy -1.0-0.50.00.51.0Heat Flow (W/g)0 50 100 150 200 250 300Temperature (°C)Exo Up Universal V3.9A TA Instruments(b) (a) Fig.2. (a) SEM BS of Sn37Pb/Cu interface (b) SEM BS of Sn3.5Ag0.5Cu/Cu interface Interface Cu Substrate Cu Substrate Interface  Cu Substrate  (a) (b)  4 Fig.3a exhibits the selected area diffraction pattern (SADP) of Cu3Sn as well as polycrystalline Ag3Sn intermetallic phases identified at the Sn3.5Ag0.5Cu/Cu solder joint interface. The continuous IRL at the Sn37Pb/Cu joint interface have been observed from SEM-BSE image, Fig.2b.  The presence of Cu6Sn5 intermetallic within the IRL has been confirmed by TEM analysis, Fig.3b. It has been observed, that the shear strength of the Sn37Pb/Cu solder is almost at par with the earlier reported value [8], where as that for the Pb-free solder alloy with Cu substrate is substantially higher with respect to the earlier reported value (~61MPa) in as reflowed condition. Such a high strength has not been reported earlier even with other type of substrate surface finish [8]. The electrical conductivity through the interface has been found to be 0.22x106 Ohm-1cm-1 for Sn37Pb/Cu joint and 0.28x106 Ohm-1cm-1 for Sn3.5Ag0.5Cu/Cu solder joint interface. This substantial increase may be attributed due to of two reasons. Firstly in case of Sn3.5Ag0.5Cu/Cu solder joint interface the IRL thickness have been found to be in the range of ~2-3 μm, which is lesser than that of the IRL thickness found in case of Sn37Pb/Cu solder joint i.e. ~4.5-5 μm. Secondly intermetallics are considered to be less conducting material; thus, thinner IRL indicates the volume fraction of intermetallics present at the interface is less in case of Sn3.5Ag0.5Cu/Cu solder joint which in response exhibits the higher electrical conductivity in compare to Sn37Pb/Cu solder joint. The experimental findings are tabulated in the Table 1.                              Fig.3 (a) SADP of Cu3Sn (spot pattern) with the zone axis as B=z=[001] and SADP of           Ag3Sn intermetallic (ring pattern) with the zone axis B=z=[011]. (b) Selected area diffraction pattern of Cu6Sn5 having zone axis B=z=[211].  (b) }311{ }420{ }113{    }433{ (a) {111} {112} {311} 004 103  101 000  5  Table 1. Experimental results of solder alloys and joint interfaces.                     Conclusions:  Cu substrate-solder alloy interface has been characterized. The lead free alloy-Cu interface consists Cu6Sn5, Cu3Sn and Ag3Sn intermetallic phases, whereas lead bearing alloy-Cu interface consists of only Cu6Sn5 intermetallic phase. Thicker layer of Cu6Sn5 acts as a diffusion barrier and prevents formation of Cu3Sn. All these intermetallic phases are confirmed by TEM analyses. Sn3.5Ag0.5Cu/Cu joint exhibits higher shear strength (~68MPa) than that of Cu/Sn-37Pb joint (~55MPa). It is found that the Sn3.5Ag0.5Cu/Cu solder shows superior electrical conductivity than that of the eutectic Sn-Pb/Cu solder. Hence in terms of mechanical property and electrical conductivity Sn3.5Ag0.5Cu solder is more futuristic material compare to conventional Sn37Pb solder.  Acknowledgement: Authors would like to thank Prof S.P.Mehrotra, Director National Metallurgical laboratory for his kind permission to publish this work.  References:  [1] Tao-Chih Chang, Jian-Wen Wang, Moo-Chin Wang, Min-Hsiung Hon; Journal of Alloys and Compounds ….DOI: 10.1016/j.jallcom.2005.09.094 (2006) in press  [2] Xiuqin Wei, Huizhen Huang, Lang Zhou, Meng Zhang, Xiaodong Liu; Materials Letters DOI: 10.1016/j.matlet.2006.05.029 (2006) in press  Properties Sn37Pb  Sn3.5Ag0.5Cu Melting point (oC) of the solder alloys  183 217 Joint Strength (MPa)  54.6±3 67.9±3 Phases present at the interface  Cu6Sn5 Cu6Sn5, Cu3Sn, Ag3Sn Electrical conductivity through the interface (x106 Ohm-1cm-1)  0.22 0.28 Interface thickness (μm)  ~4.5-5 ~2-3  6 [3] Hwa-Teng Lee, Heng-Sheng Lin, Cheng-Shyan Lee, Po-Wei Chen; Materials Science and Engineering A 407 (2005) 36-44 [4] F.Gao, T.Takemoto, J.Alloys & Compounds, doi: 101016/j.jallcom 2005.11.030  [5] F. Gao, T. Takemoto and H. Nishikawa, Mater. Sci. Eng. A. 420 (2006), pp.39-46.  [6] Luhua Xu, John H.L. Pang; Thin Solid Films 504 (2006) 362-366  [7] W.C. Luo, C.E. Ho, J. Y. Tsai, Y. L. Lin and C. R. Kao, Mater. Sci. Eng. A. 396 (2005), pp.385-391. [8] J. J. Sundelin, S. T. Nurmi, T. K Lepisto and E. O. Ristolainen, Mater. Sci. Eng. A. 420 (2006), pp.55-62. 

Comparative study of tin lead and lead free solder joint interface

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Comparative study of tin lead and lead free solder joint interface

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