Effect of indium addition in Sn-rich solder on the dissolution of Cu metallization

Transcription

1 Journal of Alloys and Compounds 390 (2005) Effect of indium addition in Sn-rich solder on the dissolution of Cu metallization Ahmed Sharif, Y.C. Chan Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong, PRChina Received 23 July 2004; received in revised form 13 August 2004; accepted 17 August 2004 Abstract An investigation has been carried out to study the dissolution of the Cu pad of the ball-grid-array (BGA) substrate into the molten Sn 9%In 3.5%Ag 0.5%Cu, Sn 3.5%Ag 0.5%Cu and Sn 0.7%Cu (wt.%) solder alloys. A fixed volume of BGA solder ball (760 m dia) was used on a 13 m thick Cu pad with a diameter of 650 m. The dissolution measurement was carried out by measuring the change of Cu pad thickness as a function of time and temperature. Scanning electron microscopy was used to examine the microstructure of the solder joint and to measure the consumed thickness of Cu. The dissolution of Cu in Sn 3.5%Ag 0.5%Cu solder is higher than the other two lead-free solders. The presence of indium in the solder plays a major role in inhibiting the consumption of Cu in the soldering reaction. The intermetallic compounds (IMCs) formed at the Sn 9%In 3.5%Ag 0.5%Cu/Cu interface are determined as a scallop-shaped Cu 6 (Sn, In) 5. Bulk of the Sn 9%In 3.5%Ag 0.5%Cu solder also contains Cu 6 (Sn, In) 5 and Ag In Sn precipitates embedded in the Sn-rich matrix. It is also found that more Cu-containing Sn 0.7%Cu solder shows lower Cu consumption than Sn 3.5%Ag 0.5%Cu solder at the same heat treatment condition Elsevier B.V. All rights reserved. Keywords: Lead-free solder; SEM; Dissolution; Intermetallic compound 1. Introduction Based on increasing pressures to achieve environmentally friendly electronic materials and processes, and indeed, growing governmental regulations around the world, the drive is strong to use lead-free solders in electronic assemblies [1 3]. This push has highlighted the fact that the industry has not yet arrived at a decision for lead-free solders. The reliability of the lead-free solders has been studied a lot recently, but its knowledge is still not well understood and many issues related to them are under heavy debate. The present research is motivated by a desire to characterize the interfacial reaction of Cu with Sn Ag Cu In alloy, which may be a possible replacement for the eutectic Pb Sn alloy. Two of its binaries and a ternary have already been successfully used in electronic soldering for some time, the eutectic Sn Cu and the eutectic/near eutectic Sn Ag Cu at high tempera- Corresponding author. Tel.: ; fax: address: eeycchan@cityu.edu.hk (Y.C. Chan). tures (T m = 227 and 217 C, respectively) and the eutectic Sn In at low temperatures (T m = 120 C). The reasonable price of the Sn Cu solder make it an essential alloy for wave soldering applications [4]. Owing to several advantages, the Sn Ag Cu alloy has been recommended by the National Electronic Manufacturing Initiative (NEMI) to replace the eutectic Sn Pb solder in reflow processing [5]. An attractive approach to lower its melting temperature is to use an additive such as indium. In application, indium added solder has high ductility, improved fatigue resistance and good wettability [6,7]. There already exist some well-developed solders (e.g. Sn 2.8%Ag 20%In) from the Sn Ag In system with a melting temperature range close to that of the eutectic Pb Sn [8]. Unfortunately, the cost of the In content within these solders is notably high. A compromise may be reached by consolidating the features of Sn Ag In and Sn Ag Cu alloys with the Sn 9%In 3.5%Ag 0.5%Cu solder as a substitute for the traditional Sn 37%Pb solder. The use of new materials will necessitate high standards for reliable, high-density, assembly. Especially a flip chip /$ see front matter 2004 Elsevier B.V. All rights reserved. doi: /j.jallcom

2 68 A. Sharif, Y.C. Chan / Journal of Alloys and Compounds 390 (2005) A flexible BGA substrate with a 13 m thick 99.9 wt.% Cu pad with a diameter of 650 m surrounded by solder mask were used to attach the solder ball. All substrates were cleaned in a dilute solution of hydrochloric acid and then rinsed in water prior to soldering to remove the surface oxide. Differential scanning calorimetry analysis showed the solidus and liquidus temperatures of the Sn 9%In 3.5%Ag 0.5%Cu solder at 203 and 208 C, respectively. Lead-free 87%Sn 9%In 3.5%Ag 0.5%Cu, 96%Sn 3.5%Ag 0.5%Cu and 99.3%Sn 0.7%Cu (wt.%) solder balls with a diameter of 0.76 mm, were placed on the prefluxed Cu bond pad of the substrates as shown in Fig. 1 and then soldered at four different temperatures; 230, 240, 250 and 260 C for times (t) of 1 (as-reflow), 5, 10 and 20 min in a convection reflow oven. As the melting temperature of the Sn 0.7%Cu solder is around 227 C, the reflow operation at 230 C for Sn 0.7%Cu solder was not taken into consideration for subsequent characterization. The flux used in this work was a commercial no-clean flux. To investigate the microstructure, the as-reflowed and extended reflowed samples were mounted with resin, cured at room temperature, mechanically ground and then polished in order to obtain the cross sections of the solder/ubm interfaces. The chemical and microstructural analyses of the gold-coated cross-sectioned samples were obtained by using a Philips XL 40 FEG scanning electron microscope (SEM) equipped with an energy dispersive x-ray spectrometer (EDX). The accuracy of the compositional measurement was about ±5%. 3. Results and discussion Fig. 1. Schematic diagram of solder ball attachment on Cu pad (before reflow). under bump metallurgy (UBM) comprising few micron thick metal or alloy layers which requires precise designing so that good adhesion functions can be fulfilled [9]. At the same time, the UBM layers should not dissolve too strongly into liquid solder bumps and react excessively with them. Interaction and interdiffusion between molten solder and Cu has been one of the most important metallurgical joining processes. Since the amount of Cu is limited and some of the Cu must remain intact through all the reflows and subsequent reworks, the understanding of the consumption rate of Cu is very important. This was a strong motivation for us, to have a closer look at the consumption rate of Cu in the soldering reaction with lead-free solders. 2. Experimental procedures The substrate area to solder volume ratio was higher in previous works on the dissolution [10 12]. Thermodynamically the Cu Sn compound that forms on the existing IMCs possesses less free energy of formation than that forming from new nuclei. So the chance of finding the existing IMCs site is higher for the solder with higher substrate area to solder volume ratio. In this experiment, the real manufacturing situation is adopted where the Cu substrate area to solder volume ratio is much lower. So, it is important to realize the dissolution behavior of the small-predefined area of substrate metal pad in the large volume of molten solder ball. Fig. 2 represents the backscattered electron micrographs for the interfaces of Sn 9%In 3.5%Ag 0.5%Cu, Sn 3.5%Ag 0.5%Cu and Sn 0.7%Cu solder balls on Cu substrate during the soldering operation at 240 C for 5 min. Only the Cu 6 Sn 5 intermetallic layer was observed in the interfaces of the Sn 3.5%Ag 0.5%Cu and Sn 0.7%Cu solders using SEM. For Sn 9%In 3.5%Ag 0.5%Cu solder, the IMCs are Cu 6 (Sn x In 1 x ) 5. The EDX analysis shows that the composition (at.%) of the interfacial intermetallics is Cu:Sn:In = 54.5:42.7:2.8, which corresponds to Cu 6 (Sn 0.94 In 0.06 ) 5. Cheng and coworkers found that Cu 6 (Sn 0.64 In 0.36 ) 5 and Ag 2 In were the stable phases in the Sn 20%In 2%Ag 0.5%Cu soldering reaction [13]. In the present study, as the indium content in the solder is less, the indium content in the interfacial IMCs is also less. The bulk of the Sn 9%In 3.5%Ag 0.5%Cu solder contains Cu 6 (Sn 0.94 In 0.06 ) 5 and Ag In Sn precipitates embedded in the Sn-rich matrix. No Ag 2 In or Ag 3 Sn IMCs are observed in the In-containing solder. On the other hand, the composition (at.%) of the Ag In Sn IMCs is Ag:In:Sn = 75.1:11:13.9, which corresponds to the Ag 3 (In 0.56 Sn 0.44 ) phase. An interesting point to be noted is that according to the EDX analysis the Sn-rich solution phase contains around 7 at.% of In. For this high solubility of In in Sn, small amounts of In have the chance to be involved in the Cu Sn IMCs formation. A little bit of lead (around 0.02 wt.%) is found in the Sn 0.7%Cu solder as impurity and appeared as a white region (Pb-rich phase) in the structure (Fig. 2c). These commercially available Sn Cu solder balls might not have been produced with necessary measures to make them 100% lead-free. To produce lead-free Sn base solders, Sn should be treated by electrolytic refining before alloying with other elements. Fig. 3 shows a comparison between different solder alloys as regards thickness reduction due to consumption of the Cu pad from the substrate at different temperatures. By

3 A. Sharif, Y.C. Chan / Journal of Alloys and Compounds 390 (2005) Fig. 2. SEM micrographs showing the interface after reflowed for 5 min at 240 C of (a) Sn 9%In 3.5%Ag 0.5%Cu, (b) Sn 3.5%Ag 0.5%Cu and (c) Sn 0.7%Cu solders. Fig. 3. The consumed thickness of Cu vs. reflow time at different temperatures for (a) Sn 9%In 3.5%Ag 0.5%Cu, (b) Sn 3.5%Ag 0.5%Cu and (c) Sn 0.7%Cu solder systems.

4 70 A. Sharif, Y.C. Chan / Journal of Alloys and Compounds 390 (2005) Fig. 4. SEM micrographs showing the interface after reflowed for 20 min at 260 C of (a) Sn 9%In 3.5%Ag 0.5%Cu, (b) Sn 3.5%Ag 0.5%Cu and (c) Sn 0.7%Cu solders. measuring the remaining Cu thickness from the SEM micrograph and by subtracting it from the initial thickness, the consumed Cu thickness is deduced. The initial consumption rate is high for all the solder alloys. From this graph, it is also evident that the initial consumption rate is the highest for the highest temperature. At high temperatures, the diffusion is faster so that the Cu atoms can move quickly into the liquid solder. A large flux flowed into the unsaturated liquid phase in the beginning of the reaction, and then the dissolution effect is reduced when a less unsaturated liquid phase is used. The consumption occurs by dissolution through the channels between the scallop grains. With time, these channel areas is reduced and the dissolution rate of Cu is also reduced. It is seen that the consumption of Cu is higher in the Sn 3.5%Ag 0.5%Cu solder than in the other two lead-free solder alloys during extended reflow. Between Sn 9%In 3.5%Ag 0.5%Cu and Sn 0.7%Cu solder, Sn 9%In 3.5%Ag 0.5%Cu solder shows slightly less dissolution of the Cu pad than Sn 0.7%Cu solder. The melting temperatures of Sn 0.7%Cu, Sn 3.5%Ag 0.5%Cu and Sn 9%In 2.8%Ag 0.5%Cu solder are around 227, 217 and 208 C respectively. Thus, the degree of super heating for Sn 9%In 3.5%Ag 0.5%Cu solders at the working temperatures is higher than for the other two lead-free solders. However, the dissolution rate in the Sn 9%In 3.5%Ag 0.5%Cu solder alloy is lower under each condition than in the other two high Sn-containing solders. It is also evident that more Cu-containing Sn 0.7%Cu solder shows lower Cu consumption than Sn Ag 0.5%Cu solder at the same reflow condition. It can be concluded that more Cu addition reduces the concentration gradient which reduces the driving force of dissolution. Fig. 4 represents the backscattered electron micrographs for the interfaces of Sn 9%In 3.5%Ag 0.5%Cu, Sn 3.5%Ag 0.5%Cu and Sn 0.7%Cu solder balls on Cu substrate during the soldering operation at 260 C for 20 min. At this stage a very thin layer of Cu 3 Sn compounds appears between the Cu 6 Sn 5 and the Cu pad. The average thickness of the intermetallic compounds at the solder substrate metallization interface after reflows at various temperatures for different durations are shown in Fig. 5. The initial growth rate of the IMCs is high for all the solder systems. The average IMCs thickness for all the alloys is more or less very close. The morphology of the interfacial Cu Sn In IMCs in Sn 9%In 3.5%Ag 0.5%Cu solder system is very similar to the Cu Sn IMCs in the other two lead-free solders system. Some large Ag 3 Sn intermetallic plates also appear in the Sn Ag Cu solder matrix. It is reported that large plate-like Ag 3 Sn structures can grow rapidly within the liquid phase, during cooling, before the final solidification of solder joints [14]. The appearance of such large compounds in the solder can be detrimental to the mechanical properties of the solder joints [15,16]. With the addition of 9% indium to the solder, the composition and the appearance of the Ag 3 Sn compounds is changed (Fig. 6). About 56% of Sn is replaced by indium to form Ag 3 (In 0.56 Sn 0.44 ). And the large Ag In Sn intermetallics appear in spherical form. So it may be stated that the interfacial energy per unit area between Ag In Sn intermetallics and molten In-containing solder is higher than that between Ag 3 Sn and molten Sn Ag Cu solder. For this reason, Ag In Sn compounds that form in the solder matrix, become spherical to possess less free energy thermodynamically.

5 A. Sharif, Y.C. Chan / Journal of Alloys and Compounds 390 (2005) Fig. 5. Growth of intermetallic compound layers in (a) Sn 9%In 3.5%Ag 0.5%Cu, (b) Sn 3.5%Ag 0.5%Cu and (c) Sn 0.7%Cu solder systems at different temperatures. Fig. 6. SEM micrographs showing the interface after reflowed for 5 min at 250 C of (a) Sn 9%In 3.5%Ag 0.5%Cu and (b) Sn 3.5%Ag 0.5%Cu solders. Fig. 7. Examples of Cu 6 Sn 5 IMCs in the bulk of solder after 10 min of reflow at 250 C of (a) Sn 9%In 3.5%Ag 0.5%Cu and (b) Sn 3.5%Ag 0.5%Cu.

6 72 A. Sharif, Y.C. Chan / Journal of Alloys and Compounds 390 (2005) A solder ball with a volume of m 3 can consume much Cu from the substrate. Intermetallic compound formation by Cu/Sn reaction is not fast enough to consume all the Cu atoms that have entered the liquid solder. The unreacted portion will diffuse out to enter the bulk of the molten solder. When the percentage of Cu reaches the threshold limit of Cu 6 Sn 5 compound formation, the Cu Sn IMCs also nucleate heterogeneously in the bulk solder. During solidification as the solubility of Cu in the bulk solder decreases, Cu and Sn react to form Cu 6 Sn 5 that deposits on the existing IMCs in the bulk solder. Fig. 7 shows this type of IMC in the bulk solder. The structure of the Cu Sn IMCs in the bulk solder is more or less the same for all the alloy compositions. The only difference is that for Sn In Ag Cu solder alloy, Cu Sn IMC contains some In in it. Assuming the total dissolution of Cu from the substrate is equal to the Cu in the bulk liquid solder and the Cu in the Cu Sn compounds, from the mass balance of Cu [17], haρ Cu = nvρ L + f Cu ρ C V C (1) where h is the consumed thickness of Cu, A the total interfacial area between solder and Cu, n the weight fraction of Cu in liquid solder, V the total volume of liquid solder, f Cu the weight fraction of Cu in the Cu Sn compound, V C the total volume of the Cu Sn intermetallics and ρ Cu, ρ L and ρ C the density of the Cu, liquid solder and Cu Sn intermetallic compounds, respectively. After saturation of Cu in the molten solder, the rate of consumption of Cu through the remaining channel area depends on the volume change of Cu Sn IMCs [17]. This equation is given by haρ Cu = f Cu ρ C V C (2) The Cu Sn IMCs are formed both in the solder/substrate interface and in the bulk of the solder. Then, haρ Cu = f Cu ρ C (V I + V B ) (3) where V I and V B are the volume of IMCs in the interface and in the bulk, respectively. V B = Aρ Cu h V I (4) f Cu ρ C After reflow, using Eq. (4), the IMC that formed in the bulk can be calculated. For simplicity, IMC grains are assumed to be hemispherical [11,18] and distributed all over of the surface evenly with the same radius. From the thickness of the IMCs in the interface, the total volume of the IMC in the interface can be calculated as V = (2/3)πr 3 m, where m is the number of scallops that form at the interface. From the experimental measurement we found the total interfacial area to be equal to A = cm 2. For the model calculations we also used the material constants ρ Cu = 8.96 g/cm 3 and ρ C = 8.27 g/cm 3 [11]. As the atomic weight and density of Sn and In is very close, it is assumed that the density of Cu Sn and Cu Sn with small amount of indium is the same. In this Fig. 8. Comparison of volume of IMCs vs. reflow time at 250 C with solder Sn 9%In 3.5%Ag 0.5%Cu, Sn 3.5%Ag 0.5%Cu and Sn 0.7%Cu both in the bulk and at the interface. calculation, only the dissoluted Cu atoms are taken into consideration. Fig. 8 shows the calculated volume of the IMCs in the interface and in the bulk of the solder for all the systems for a temperature of 250 C. It indicates that the soldering reaction with Cu is not conservative. The volume of IMCs in the bulk is found to be much higher than that in the interface. The substrate area to solder volume ratio was higher in previous works [9,10,15], where IMCs formation in the interface was dominating and the interfacial reaction was more conservative. It is also evident that as the Cu substrate dissolution is the highest, the IMCs formation within the solder is also the highest in the Sn 3.5%Ag 0.5%Cu solder. 4. Conclusion The effects of reflow (230, 240, 250 and 260 C) on the dissolution of a Cu substrate in Sn 9%In 3.5%Ag 0.5%Cu, Sn 0.7%Cu, and Sn 3.5%Ag 0.5%Cu BGA solder ball are presented in this paper. As the temperature and time increases, the dissolution of Cu increases. A very fast dissolution of the substrate is observed in the early stage of the molten solder/solid Cu reaction for the solder alloys. In-containing leadfree solders exhibit less dissolution of Cu during the long time reflow condition. Between the other two high Sn-containing solders, the more Cu-containing Sn 0.7%Cu solder shows lower Cu consumption than the Sn 3.5%Ag 0.5%Cu solder. The Cu 6 Sn 5 IMC is also observed in the bulk of the solder. With the addition of 9% indium in the Ag-containing solder, the composition and the appearance of the Ag 3 Sn compounds is changed. From the model calculations, it is found that the amount of Cu 6 Sn 5 in the bulk solder is much greater than the amount of IMCs present at the interface for

7 A. Sharif, Y.C. Chan / Journal of Alloys and Compounds 390 (2005) all the solder alloy systems. The IMC formation within the Sn 3.5%Ag 0.5%Cu solder is also the highest. Acknowledgements The authors would like to acknowledge the financial support provided by Research Grant Council of Hong Kong for the project Wetting Kinetics and Interfacial Interaction Behavior between Lead-free Solders and Electroless Nickel Metallizations, CERG project no. CityU 1187/01E (CityU ref ) and the research studentship of City University of Hong Kong. The technical support from Mr. Boyi Wu is gratefully acknowledged. References [1] K. Suganuma, Mater. Trans. 45 (2004) 605. [2] K. Zeng, K.N. Tu, Mater. Sci. Eng. Rep. 38 (2002) 55. [3] M. Abtew, G. Selvaduray, Mater. Sci. Eng. Rep. 27 (2000) 95. [4] J.W. Yoon, Y.H. Lee, D.G. Kim, H.B. Kang, S.J. Suh, C.W. Yang, C.B. Lee, J.M. Jung, C.S. Yoo, S.B. Jung, J. Alloys Comp. 381 (2004) 151. [5] J.E. Sohn, Circuits Assembly 13 (2002) 32. [6] Z. Mei, J.W. Morris, J. Electron. Mater. 21 (1992) 401. [7] Z. Mei, J.W. Morris, J. Electron. Mater. 21 (1992) 599. [8] T.H. Chuang, K.W. Huang, W.H. Lin, J. Electron. Mater. 33 (2004) 374. [9] K. Kulojärvi, V. Vuorinen, J. Kivilahti, Microelectron. Int. 15 (1998) 20. [10] H.K. Kim, H.K. Liou, K.N. Tu, Appl. Phys. Lett. 66 (1995) [11] H.K. Kim, K.N. Tu, Phys. Rev. B 53 (1996) [12] A.G. Ward, J.W. Taylor, J. Inst. Met. 86 (1957) 36. [13] M.D. Cheng, S.Y. Chang, S.F. Yen, T.H. Chuang, J. Electron. Mater. 33 (2004) 171. [14] D.W. Henderson, T. Gosselin, A. Sarkhel, S.K. Kang, W.K. Choi, D.Y. Shih, C. Goldsmith, K.J. Puttlitz, J. Mater. Res. 17 (2002) [15] T.Y. Lee, W.J. Choi, K.N. Tu, J.W. Jang, S.M. Kuo, J.K. Lin, D.R. Frear, K. Zeng, J.K. Kivilathi, J. Mater. Res. 17 (2002) 291. [16] J. Haimovoich, AMP J. Technol. 3 (1993) 46. [17] H.K. Kim, K.N. Tu, Appl. Phys. Lett. 67 (1995) [18] J.H. Kim, S.W. Jeong, H.M. Lee, Mater. Trans. 45 (2004) 710.