Investigation of interfacial reactions between Sn–Zn solder with electrolytic Ni and electroless Ni(P) metallization

Journal of Alloys and Compounds 440 (2007) 117–121

Investigation of interfacial reactions between Sn–Zn solder with electrolytic Ni and electroless Ni(P) metallization Ahmed Sharif, Y.C. Chan ∗ Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong Received 27 July 2006; received in revised form 6 September 2006; accepted 6 September 2006 Available online 9 October 2006

Abstract In this study, interfacial reactions of electrolytic Ni and electroless Ni(P) metallization of the ball-grid-array (BGA) substrate with the molten Sn–9Zn (wt.%) eutectic solder alloy were investigated, focusing on the shear strengths and the identification of the intermetallic compound (IMC) phases at various reflow periods. Zn-containing Pb-free solder alloys were kept in molten condition (240 ◦ C) on the bond pads for different durations ranging from 1 to 60 min to render the ultimate interfacial reaction and to observe the consecutive shear strength. After the shear test, fracture surfaces were investigated by scanning electron microscopy equipped with an energy dispersive X-ray spectrometer. Cross-sectional studies of the interfaces were also conducted to correlate with the fracture surfaces. The solder ball shear-load for the Ni(P) system during extended reflow increased with the increase of reflow time. The consumption of the electroless Ni(P) layer in Sn–9Zn was also lower than that of the electrolytic Ni. It was evident that the Sn–Zn solder/electrolytic Ni system was more vulnerable than the Sn–Zn solder/electroless Ni(P) system in high temperature long time liquid state annealing. Sn–Zn solder with electroless Ni(P) metallization appeared as a good combination in soldering technology. © 2006 Elsevier B.V. All rights reserved. Keywords: Sn–Zn solder alloy; Surfaces and interfaces; Mechanical properties; Microstructure; Intermetallics

1. Introduction Due to recent legislative, environmental, public sentiment and market drivers around the world, traditional Pb-containing (SnPb) solders in electronic industry are under strict restriction. Extensive investigations are on-going over the last few years to find an acceptable Pb-free solder for various electronic attachment applications [1–5]. Recently, Sn–Zn solder has become highly recommended as a substitute for Sn–Pb eutectic solder due to its lower melting point [6–9]. Sn–Zn solder can also be used without replacing the existing manufacturing lines or electronic components [10]. Moreover, Sn–Zn is advantageous from an economic point-of-view because Zn is a low cost metal. However, Sn–Zn eutectic solder is difficult to handle practically due to its highly active characteristics [10]. The basic microstructure of Sn–9Zn binary alloys and their interfaces with Cu and Ni were investigated [10–12]. Date et al. developed a ductile-

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to-brittle transition mode of Sn–9Zn joints on Cu metallization by impact test after aging, in which Cu5 Zn8 thickening and void formation accounted for the brittleness of solder/Cu test coupon with increasing aging time [13]. The most common surface finishes on ball-grid-array (BGA) pads are electrolytic Ni/Au and electroless Ni(P)/Au plated over the Cu pad. The electrolytic Ni/electroless Ni(P) acts as a diffusion barrier layer and Au acts as an oxidation barrier layer. The use of new materials will necessitate high standards for reliable, high density, assembly. Especially, a flip chip under bump metallurgy (UBM) comprising few micron thick metal or alloy layers which requires precise designing so that adequate diffusion barriers and good adhesion functions can be fulfilled [14]. At the same time, the UBM layers should not dissolve too strongly into liquid solders and react excessively with them. The metallurgical behaviors of Sn–Zn solder in joints with different surface finishes and the related joints reliability at package level have not been sufficiently studied yet. Therefore, the present study was carried out to investigate the effect of reflow on the reliability of Sn–9Zn solder with electrolytic Ni and electroless Ni(P) metallization.

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2. Experimental procedures The solder mask defined copper bond pad of a substrate of the BGA package was used as a base for electrodeposition of Ni and electroless deposition of Ni(P). The P content in the Ni(P) layer was around 16 at.%. The thickness of Ni(P) and electrolytic Ni plating was about 3.5 and 5 ␮m, respectively. Au plating was immediately layered on top of the electrolytic Ni and electroless Ni–P to avoid oxidation of the nickel surface. In both cases, Au plating thickness on the surface finish was about 0.5 ␮m. Lead-free 91Sn–9Zn (wt.%) solder balls with a diameter of 0.76 mm, were placed on the prefluxed bond pad of the substrates and reflowed at a temperature of 240 ◦ C for 1 min in a convection reflow oven (BTU VIP-70N). The flux used in this work was a commercial rosin activated (RA) flux. The assembled packages then reflowed isothermally at 240 ◦ C for time of 5, 10, 30 and 60 min in a high temperature oven to examine the interfacial reactions between the solder and the thin-film UBM. Shear tests were performed on both the as-reflowed and extended-reflowed samples using a Dage Series 4000 Bond Tester. The shear tool height and the test speed of the shear test in this work were about 40 ␮m and 300 ␮m/s, respectively. About 40 randomly chosen solder balls were sheared to obtain the average and the extent of deviation. The fracture surfaces after the ball shear tests were investigated thoroughly by scanning electron microscopy (SEM) in the secondary electron mode as well as by energy dispersive X-ray spectrometer (EDX). To investigate the microstructures, the as-reflowed and extended-reflowed samples were mounted in 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 crosssectioned samples were obtained using a Philips XL 40 FEG scanning electron microscope equipped with an energy dispersive X-ray spectrometer. The accuracy of the compositional measurement was about ±5%. To find out the formula composition of the intermetallic compounds (IMCs), the chemical analyses of the EDX spectra were corrected by standard ZAF software. The back-scattered electron (BSE) imaging mode of the SEM was used for the interfacial study.

3. Results and discussion The mechanical strength of the solder joints with two different pad materials was measured for each reflow condition. The change of the average shear loads and their standard deviations are shown in Fig. 1. Regardless of the bond pads, both of the as-reflowed joints exhibited high strength. It should be noted that Sn–Zn solder with Au/Ni(P) pads maintained the high strength even after extended reflow. However, Fig. 1 also shows that the solder ball shear load for Ni system during extended reflow increases with the increase of reflow time up to certain stage and then turns to decrease. The Ni(P) metallization gave relatively better ball shear load at about 1.91–2.03 kgf over the whole duration of reflow.

Fig. 1. Variation of average shearing forces of solder joints with respect to the different times of reflow.

Fig. 2 shows the fracture surface of the interface formed between the molten Sn–Zn solder alloy with the electroless Ni(P)/Cu and electrolytic Ni/Cu bond pad at 240 ◦ C for 1 min. Mostly ductile fracture occurred within the solder with a high degree of plastic deformation. This fracture occurred at a location near but lower than the shearing height, leaving a thick layer of solder on the pad. This indicates that the solder/pad bond is much stronger than the shear strength of the bulk solders. EDX analysis from these surfaces confirmed that fracture occurred through the solder alloys. For the Ni(P) systems, fracture mode remained ductile even after long time reflow within the reported period. It is consistent with the high value of their shear strengths. Yoon et al. also observed no deterioration of the joint strength of the Sn–Zn solder/Ni(P) system even after 120 min of reflow [15]. Fig. 3(a) shows a typical fracture surface of the interface of the Sn–Zn alloy with the electroless Ni(P) bond pad for 60 min of reaction time. Since the fracture occurred inside the solder, the difference in shear strength should be independent of the interface structure. The increase in shear load of the solder joint may be related to the strengthening effect of the solder alloy due to the homogenization at the time of reflow [16]. It was also found that after extended reflow, the lower portion of the solder near the bond pad was enlarged and the height of the solder joints was reduced. Due to the solder mask, the contact area between the solder and the bond pad remained the same. However, the area along the shearing height was increased after long time reflow condi-

Fig. 2. Pad side fracture surfaces of as-reflowed Sn–Zn solder joint with: (a) electroless Ni(P) layer and (b) electrolytic Ni layer.

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Fig. 3. Pad side fracture surfaces of Sn–Zn solder joint after 1 h of reflow with: (a) electroless Ni(P) layer and (b) electrolytic Ni layer.

Fig. 4. Schematic drawing of BGA solder joints of: (a) as-reflowed and (b) after 60 min of reflow.

tion. For long time molten condition, sagging of the bulk solder occurred, i.e. the bulged molten solder on the pad slowly bowed down due to its weight. As a result, the forces became higher with extended reflow because of the larger sheared area. The extent of deformation of the bulk solder is shown schematically in Fig. 4. For electrolytic Ni metallization, a mixed mode of both ductile and brittle fracture was observed after 10 min of reflow. The brittleness of the solder joints increased with the increase of reflow time. For long time molten reaction, mostly brittle fracture occurred within the IMCs for Ni metallization. Fig. 3(b) shows a typical fracture surface of the interface of the Sn–Zn alloy with the electrolytic Ni bond pad for 60 min of reaction time. Fig. 3(b) shows that the fracture surface is almost flat without any visible plastic deformation. Failure mode was very brittle. EDX analysis revealed that the brighter layer on the pad side consisted of Ni–Zn IMCs—which might be Ni5 Zn21 . EDX

results on the black region of the pad showed only Ni—which might be the unreacted Ni on the substrate. Fig. 5 shows the statistical representation of the different failure modes in a chart form. It is evident that all of the Sn–Zn solder joints on the electroless Ni(P) exhibited bulk fracture independent of the reflow time. Beyond 10 min of reflow, a mixed mode of ductile and brittle fracture was observed for electrolytic Ni. The shear test had revealed a transition of fracture mode from a ductile to a brittle manner as a function of the reflow time. Detailed cross-sectional studies were carried out to investigate the relationship between the shear strength and the interfacial morphologies of the solders with the metallization pads. Fig. 6 shows SEM images of the Sn–9Zn/pad joints after 1 min reflow. Most interestingly, layer-type spalling at the interface was clearly observed in the Ni system from the initial reflow (Fig. 6(b)). EDX analysis of this spalled IMC layer revealed that the IMC was composed of Au and Zn and the Au percentage of this layer was about 25 at.%. This observation implies that the spalled IMC layer is the AuZn3 compound. A very thin layer of IMC was noticed at the interface of the Sn–Zn solder/(Au) Ni system. The active nature of the Zn confirms an instant reaction zone at the interface to maintain the bonding between the solder and the substrate. Chiu et al. observed Ni–Zn compound at the interface of Sn–Zn-based solder/Ni system without the Au layer on the Ni layer [17]. In this study, the presence of Au above the Ni layer change the reaction chemistry with the Sn–Zn solder. After initial reflow, no spalling was noticed at the interface of the Sn–Zn solder/Ni(P) system (Fig. 6(a)). Most interestingly,

Fig. 5. Failure modes of the reflowed solder joints of: (a) electroless Ni(P) layer and (b) electrolytic Ni layer.

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Fig. 6. SEM micrographs showing the interfaces after soldering for 1 min at 240 ◦ C on: (a) electroless Ni(P) layer and (b) electrolytic Ni layer.

there was no P-rich Ni compound observed at the resolution limit of the SEM used. The absence of P-rich Ni layer at the interface of the Sn–Zn solder/Ni(P) system was also confirmed by other researchers [15,18]. The average thickness of intermetallic layers was 0.8–1.2 ␮m. In the initial reflow condition, the IMCs on the Ni(P) metallization were mainly composed of Au–Ni–Sn–Zn. The atomic percentage of the Au element in these compounds was higher than that in the interfacial IMCs on the electrolytic Ni layer after initial reflow. A little increase in the IMCs thickness was observed after extended period of reflow (Fig. 7(a)) with Ni(P) metallization. Interestingly, the morphology of the IMCs changed to more scallop type from the initial planner type. Again no P-rich Ni layer was observed at the interface after a long time in the molten reaction state. As the reaction rate of Ni(P) with Sn–Zn-based solder was very low, the formation of the crystallized P-rich Ni layer was also retarded. The same Au–Ni–Sn–Zn quaternary compounds were observed on the Ni(P) metallization. Through extended reflow Sn became more involved in the IMCs and the percentage of Sn was increased in the IMCs for Ni(P) system. In the initial IMCs, around 4–6 at.% Ni was present, whereas in the formed later IMCs, the atomic percentage of Ni was around 7–10. The composition of the interfacial IMCs layer near the substrate side was determined to be 51–58 Zn, 8–15 Au, 7–10 Ni and 15–22 Sn (at.%). After 10 min reaction in molten condition, it was seen that IMCs thicknesses grew gradually with the increasing of time with the electrolytic Ni metallization. According to EDX, the IMC at the interface of the electrolytic Ni system after 10 min reflow also consisted of Au, Ni, Sn and Zn. Over the original Ni layer, a thin new type of IMC was also observed after 10 min of

reflow. After 30 min of reaction in molten condition, it was seen that IMCs thickness increased whereas original Ni layer thickness decreased. It was noticed that the new dark IMC formed a layer over the original Ni layer. EDX analysis revealed that the new layer was composed of 78–80 Zn and 20–22 Ni (at.%). This observation also confirmed the presence of Ni5 Zn21 compound in the Sn–Zn solder after 30 min of reflow. After 60 min of reaction in molten condition, it was seen that Ni–Zn IMCs thickness increased (Fig. 7(b)). The thickness of the bright interfacial Au–Ni–Sn–Zn IMC and the dark Ni–Zn IMC was about 1.0–1.3 and 1.4–1.9 ␮m, respectively, on the Ni layer after 60 min of molten reaction. After 60 min of reflow, the composition of the interfacial IMC became quite similar to that on the Ni(P) system. By measuring the remaining metallization thickness from the SEM micrograph and by subtracting it from the initial thickness, the consumed metallization thickness is deduced. Fig. 8 shows a comparison between the two-metallization layers as regards thickness reduction due to dissolution in the Sn–9Zn solder. Initially, the consumption was similar for both the metallization layers. It was seen that the consumption of the electroless Ni was lower in the Sn–9Zn solder than that of the electrolytic Ni during extended reflow. Only about 0.3–0.5 ␮m of the electroless Ni(P) layer took part in the reaction with the Sn–9Zn molten solder in 1 h. Whereas, about 1.3–1.5 ␮m of the Ni layer was consumed after 60 min of molten reaction. The more participation of the Ni later on to form Ni–Zn IMC was the source of the consumption of the electrolytic Ni layer. It was confirmed that the Au–Zn IMC layer started to spall-off from the interface of the Sn–Zn eutectic solder and Au/electrolytic Ni/Cu system even after the initial reflow. It can be stated that the high surface tension of the Au–Zn compounds

Fig. 7. SEM micrographs showing the interfaces after soldering for 1 h at 240 ◦ C on: (a) electroless Ni(P) and (b) electrolytic Ni layer.

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4. Conclusion

Fig. 8. The consumed thickness of metallization layer vs. reflow time at 240 ◦ C.

with the electrolytic Ni layer in the Sn–Zn solder may cause the Au–Zn layer to spall off from the interface in the molten state. Electroless Ni was deposited from an acidic hypophosphite bath which introduced P in the Ni metallization and made the layer amorphous [19]. It can also be concluded that the presence of P and also the amorphous structure of the electroless Ni(P) layer reduces the surface energy of the interfacial IMC, as even after 1 h of reflow there is no spalling observed at the interface. In flip-chip packaging, several re-flows, often up to seven or eight times are needed. Each of them brings the solder alloys above the melting temperature for a period of about 30 s–1 min [20]. The collective effect of such multiple reflows on the reaction between molten solder and UBM has been a reliability issue. Although the IMC spalling started very early in the reflow, it did not influence greatly the shear strength of the Ni system. The highly reactive nature of the Zn confirmed an instant IMC formation at the interface with the spalling of the Au–Zn compound layer. The interfacial IMC together with the unreacted Ni provided the adhesion between the solder and the substrate. The spalling of the initial Au–Zn IMC from the very early reflow, actually increased the reaction between Ni and Zn during extended reflow. With the increase of reaction time, the thick Ni–Zn compound layer created the weakest link with original Ni layer. It was found that spalling of the IMCs was retarded with the introduction of Ni(P) metallization layer. Thus it was expected that the interfacial IMC over the original Ni(P) layer hindered the supply of Zn atoms to form the Ni–Zn IMC. As the composition of the interfacial IMC for both the metallization layers was found to be quite similar after extended reflow, the amorphous nature of the Ni(P) layer might play a major role in inhibiting the reaction between Ni and Zn. By changing the Ni metallization deposition technique, the formation of Ni–Zn compound is retarded which in turn increases the reliability of the solder joint to the higher extent.

The effects of reflow on the interfacial reactions of the substrate metallization with Sn–9Zn BGA solder ball were presented in this paper. Electroless Ni(P) metallization gave better results in terms of shear strength on liquid state annealing than electrolytic Ni metallization with Sn–Zn solder. Less than 0.5 ␮m of the electroless Ni layer was consumed by the Sn–Zn solder with 60 min molten reaction at 240 ◦ C. However, more than 1.4 ␮m thick electrolytic Ni layer was consumed by the same solder within the same reported period. With the increase of reaction time, the thick Ni–Zn compound layer created the weakest link with original electrolytic Ni layer. Neither P-rich Ni layer, nor Ni–Zn compound was observed at the interface of the Sn–Zn solder/Ni(P) system even after a long time molten reaction. Sn–9Zn solder and electroless Ni(P) metallization was identified as a good combination in soldering technology. Acknowledgement The authors would like to acknowledge the financial support provided by City University of Hong Kong for the project “Reliability study of SnZn-based lead-free solders for Green Electronic Manufacturing,” SRG project no. 2004SRG061 (CityU no. 7001822). References [1] C.S. Oh, J.H. Shim, B.J. Lee, D.N. Lee, J. Alloys Compd. 238 (1996) 155–166. [2] M. Abtew, G. Selvaduray, Mater. Sci. Eng. R 27 (2000) 95–145. [3] K. Zeng, K.N. Tu, Mater. Sci. Eng. R 38 (2002) 55–105. [4] Y. Cui, X.J. Liu, I. Ohnuma, R. Kainuma, H. Ohtani, K. Ishida, J. Alloys Compd. 320 (2001) 234–241. [5] K.S. Kim, S.H. Huh, K. Suganuma, J. Alloys Compd. 352 (2003) 226–236. [6] T.C. Chang, M.H. Hon, M.C. Wang, J. Alloys Compd. 352 (2003) 168–174. [7] J.M. Song, T.S. Lui, G.F. Lan, L.H. Chen, J. Alloys Compd. 379 (2004) 233–239. [8] K.I. Chen, S.C. Cheng, S. Wu, K.L. Lin, J. Alloys Compd. 416 (2006) 98–105. [9] J.W. Yoon, S.B. Jung, J. Alloys Compd. 407 (2006) 141–149. [10] K.S. Kim, J.M. Yang, C.H. Yu, I.O. Jung, H.H. Kim, J. Alloys Compd. 379 (2004) 314–318. [11] L.L. Duan, D.Q. Yu, S.Q. Han, H.T. Ma, L. Wang, J. Alloys Compd. 381 (2004) 202–207. [12] J.W. Yoon, S.B. Jung, J. Mater. Res. 21 (2006) 1590–1599. [13] M. Date, T. Shoji, M. Fujiyoshi, K. Sato, K.N. Tu, Scripta Metall. 51 (2004) 641–645. [14] K. Kuloj¨arvi, V. Vuorinen, J. Kivilahti, Microelectron. Int. 15 (1998) 20–24. [15] J.W. Yoon, H.S. Chun, S.B. Jung, Mater. Trans. 46 (2005) 2386–2393. [16] M.N. Islam, Y.C. Chan, A. Sharif, M.O. Alam, Microelectron. Reliab. 43 (2003) 2031–2037. [17] M.Y. Chiu, S.S. Wang, T.H. Chuang, J. Electron. Mater. 31 (2002) 494–499. [18] M. Date, K.N. Tu, T. Shoji, M. Fujiyoshi, K. Sato, J. Mater. Res. 19 (2004) 2887–2896. [19] R.M. Allen, J.B. Vandersande, Scripta Metall. 16 (1982) 1161–1164. [20] C.Y. Liu, C. Chen, A.K. Mal, K.N. Tu, J. Appl. Phys. 85 (1999) 3882–3886.