Fast Spreading of Liquid SnPb Solder on Gold-coated Copper Wheel Pattern

Transcription

1 J. Mater. Sci. Technol., 2010, 26(12), Fast Spreading of Liquid SnPb Solder on Gold-coated Copper Wheel Pattern Wei Liu 1), Lei Zhang 1), K.J. Hsia 2) and J.K. Shang 1,3) 1) Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang , China 2) Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA 3) Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA [Manuscript received March 23, 2009, in revised form June 24, 2009] The reactive Sn63Pb37 spreading on patterned film structures was examined in a reducing atmosphere (H 2 5%+Ar 95%). Liquid solder spreading was observed to follow the wheel pattern made of Au/Cu thin film. At the center, a liquid cap was formed around the hub by viscous spreading of the liquid front. Ahead of the main viscous flow front, a liquid film was found to be extended on thin Au-Cu lines at a fast rate to a great distance by rapid Sn-Au chemical reaction. KEY WORDS: Wetting; Reactive wetting; Spreading; Solder; Thin film pattern; Liquid film 1. Introduction Wetting plays a critical role in many industrial and biological processes and has thus drawn much attention in the past years. Recently, with the development of microfabrication technology, the structured surfaces have been actively explored to control wetting and dewetting behaviors of solid surfaces [1 3]. So far, much of the work has been limited to non-reactive systems and very little research has been devoted to the reactive system, where the structured substrate can no longer be considered as an ideal solid but one with local diffusion or dissolution. One of the reactive systems, liquid solders on the thin metallization films, has attracted more attention due to its widespread application in chip-level package; yet the mechanism of the reactive wetting, especially the formation of precursor films is still unclear in the SnPb/Cu and SnPb/(Au/Cu) reactive systems [4,5]. Saiz and Tomsia [6] reported a precursor film in Au-Ni and Ge-Si systems, but in other reac- Corresponding author. Prof., Ph.D.; Tel.: ; Fax: ; address: jkshang@illinois.edu (J.K. Shang). tive wetting systems, there is a sharp triple junction without a precursor film. Until now, little knowledge has been available on the behavior of the liquid precursor film in solder reactive wetting systems. In our study, the Au/Cu layered film was patterned by photolithography into alternating domains of Au/Cu film and Si substrate with different surface morphologies, such as butterfly, sector, and radial design, and so on. These domains provided great wettability contrast for liquid SnPb solder. Flow of the liquid SnPb on the patterned structures was examined in situ on a hot-stage microscope with H 2 protection. For all designs, liquid film could be observed to be extended ahead of the main liquid front. Especially, on a symmetrical radial strip design, we could get the homogeneous and nice spreading of liquid film in every direction, showing a newer peculiar reactive spreading under this special design. 2. Experimental The Au/Cu bilayered thin film was prepared by e-beam evaporation. In the bilayered structure, the lower layer of Cu was firstly deposited on Si (100)

2 1144 W. Liu et al.: J. Mater. Sci. Technol., 2010, 26(12), Fig. 1 A sequence of top view optical images of SnPb spreading on the striped pattern (the viscous flow front marked by the arrows) with the native oxide layer, and subsequently capped by a thinner layer of Au. The thicknesses of Au and Cu were 18 and 100 nm, respectively. The Au/Cu bilayered film was patterned by a photolithography process into a wheel shape. The alternating zones between Au/Cu film and Si, provided wettable and nonwettable domains, respectively to liquid SnPb solder. At the center, the hub of the wheel was a wettable circular pad with a radius of r c =0.25 mm. Around the hub, the spokes were made of thin Cu lines of a uniform rectangular cross-section with a width of w=200 µm and a length of h 2 mm. Wetting experiments were performed by reflowing the eutectic SnPb solder balls with a diameter of 0.5 mm on the patterned surfaces. The eutectic SnPb alloy was chosen for its low melting point and excellent wetting characteristics on Cu and Au. Prior to the experiments, the solder ball and the substrate were cleaned in acetone three times and dried with an air gun. The SnPb solder ball covered with no-clean flux was then placed on the central circular pad. After the furnace chamber was evacuated to a pressure of about 1.33 Pa (10 2 Torr) and refilled with high purity gas mixture of H 2 5%+Ar 95%, the chamber was heated at a ramp of 10 C/min to 250 C, and held for 10 min at this temperature before the furnace was cooled to room temperature at the rate of 20 C/min. The entire spreading process was in situ recorded by a CCD camera and the kinetic data were obtained from the recorded video images. After the wetting experiment, the sample was analyzed by scanning electron microscopy (SEM, JEOL, Tokyo, Japan), and the phases were identified with energy-dispersive spectroscopy (EDS). The cross-sectional structures of the reactive interfaces were examined after cross-sectioning using a focused ion beam (FIB) technique. 3. Results and Discussion The surface structure has a great effect on the spreading of liquid SnPb. As shown in Fig. 1, the liquid flow follows the wheel pattern closely. In Fig. 1(a), solder melting in the hub is evident from the collapse of the solder ball. Subsequently, the liquid solder flows from the hub onto the connected Au/Cu spokes (Fig. 1(b)). Ahead of the liquid flow front, a liquid film is formed (Fig. 1(c)). The liquid film spreads along the Au-coated copper line to its end (Fig. 1(d) (h)). At the same time, the main liquid front (marked by arrows) continues to advance but at a much slower rate. The final wetting shape on the wheel pattern is quite different from the classical wetting behavior of liquid metal on a flat and homogeneous substrate. On bulk gold, spreading of SnPb solder creates a spherical cap that stops expanding when all the molten solder in the cap has reacted with gold to form Au-Sn intermetallic compounds (within several minutes) [7]. On a continuous Cu film, spreading of SnPb solder produces no liquid film, except for a microscopic halo surrounding the spherical cap ahead of the main liquid front. In constrast, as Fig. 1 demonstrates, spreading of SnPb solder on the Au/Cu line follows the pattern of the wetting zone closely. The higher magnification image in Fig. 2(a) shows that the cooled solder pattern consisted of three separate regions: a liquid cap marked 1 around the hub, a continuous film marked 2 along the Cu lines, and a partial dewetting zone marked 3 between the central cap and the solder film. As shown in Fig. 2(b), the dewetting zone is comprised of isolated pieces of solder (light contrast), and the exposed silicon surface (dark contrast). The dewetting apparently happens when the liquid SnPb solder has consumed the Au/Cu film

3 W. Liu et al.: J. Mater. Sci. Technol., 2010, 26(12), Fig. 3 SEM image and associated EDS analysis of the spreading front of liquid SnPb on Au-coated Cu line after cooling Fig. 2 (a) SEM images of the reactive wetting and dewetting shape of SnPb solder on the wheel pattern after cooling, (b) magnified SEM micrograph of dewetting zone, (c) SEM cross-sectional image (with the tilt of 52 deg.) of the reflowed copper line with a width of 30 µm using the FIB technique completely on the Cu line. Figure 2(c) shows the FIB cross-sectional image (with the tilt of 52 deg.) of the copper line with a width of 30 µm covered by the solder film. The cross section of the solder film takes the shape of the classical spherical cap with a nominal contact angle of 17 deg., however, the real contact angle is obtained to be 21 deg. by arctan (tan17 deg./sin52 deg.), indicating the reactive wetting on the line is still controlled by the surface energy minimization. However, the contact angle of 21 deg. is larger than the equilibrium wetting angle of 12 deg. on the copper film. The larger contact angle reflects the confinement of the liquid sol- der by the structured surface, where the contact line is pinned to the domain boundaries between copper film and Si substrate. Such a confinement effect was also observed by Lipowsky et al. [3,8] in non-reactive wetting system, who suggested that the contact angle did not satisfy the Young s equation but fulfilled the inequalities θ Cu <θ<θ Si. The very tip of the spreading front is shown in Fig. 3 along with the EDS compositional analysis. The white particles, marked 1 in the SEM image, are determined to be a Pb-rich phase with the Pb content of 83 wt pct. The formation of these particles is due to the surface segregation of Pb when liquid Sn reacts selectively with Au or Cu. The gray regions marked 2 are determined to be a mixture including Cu, Au and Sn components. The kinetics of solder spreading is given in Fig. 4 in terms of the spreading distance R vs time t for the spreading process shown in Fig. 1. R was measured from the center of the hub to the spreading front, and time zero was set at the point when the solder ball melt and collapsed. The two sets of curves represent

4 1146 W. Liu et al.: J. Mater. Sci. Technol., 2010, 26(12), R / mm Stage 1 Liquid film (H) Liquid film (V) Liquid flow (H) Liquid flow (V) t / s Stage 2 Stage 3 Fig. 4 Spreading of SnPb solder on the striped pattern as a function of elapsed time Fig. 5 Schematic diagram depicting a proposed mechanism for extrusion of liquid film two branches of Cu lines, one in the horizontal direction (H) and the other in the vertical direction (V). The general trends of the two curves are basically the same, indicating that the spreading is homogeneous in every direction. Each set of curve can be divided into three stages, marked 1, 2 and 3 in Fig. 4. At stage 1, the R vs t curve is characterized by an initial rapid increase in R with t. The average triple line velocity is up to 20 µm/s from 0 to 20 s. In this regime, corresponding to Fig. 1(a) and (b), the liquid solder is forced to flow from the hub onto the connected Cu lines under the dominant control of capillary force, due to the geometric constraints of the substrate. The capillary pressure differential for flow onto the strip line has been given by Hosking et al. [9] to be a function of the temporal contact angle on the circular pad, the equilibrium contact angle on the strip line, and the size parameters of the pattern. At stage 2, corresponding to Fig. 1(c) (h), a liquid film is extended ahead of the viscous flow front and spreads to the end of the strip line. The average spreading velocity of the liquid film is about 3 µm/s from t=50 s to t=480 s with a spreading distance of 1.3 mm. This is far slower than that of the viscous flow, indicating that an entirely different spreading mechanism operated in this regime. As reviewed by Moon et al. [10], the existence of thin films attached to microscopic contact lines has been discovered since 1919, and the material transport mechanism in the liquid film was strongly suggestive of a diffusive growth. In the diffusive growth, R (Dt) 1/2, here D is the diffusivity. For R=1.3 mm and t=430 s, D would be approximately cm 2 /s, which has the same magnitude as the diffusivity, D L = cm 2 /s, estimated for Au in liquid Sn at 250 C by Bruson and Gerl [11]. In addition, the average spreading velocity 3 µm/s was a little larger than the spreading velocity 2 µm/s of liquid Sn on bulk gold at 250 C by Yin et al [12]. At stage 3, the viscous flow front does not stop moving, but spreads on the liquid film as a lubrication layer. However, the R vs t curve is basically flat in contrast with the steep curve at stage 1, indicating that the fluid flow velocity is much slower. Based on the observations and analysis above, the following mechanism is suggested for SnPb liquid film spreading on the gold-coated Cu thin film. Figure 5 indicates the schematic diagram for depicting the proposed mechanism for extrusion of liquid film. Due to the strong Au-Sn affinity, diffusive reaction between Sn and Au occurred rapidly, resulting in fast spreading of liquid SnPb solder. Because of a much higher solubility of Au in liquid SnPb solder, almost no Au- Sn compounds formed during the liquid film spreading. On the other hand, even though the thinner gold layer was entirely dissolved into the molten solder, the liquid solder should still have the enough Sn concentration gradient to sustain its further spreading. Underneath the Au layer, Cu was dissolved into the molten solder very slowly compared with Au. Since the forward spreading rate was much faster than the formation rate of Cu-Sn compounds, neither Au- Sn compounds nor Cu-Sn compounds could develop ahead of the liquid front and become a diffusion barrier for liquid spreading. Moreover, at the liquid-solid interface, slower dissolution rate of Cu into liquid solder also resulted in lower consumption rate of Sn, and thus enough Sn concentration gradient was remained in liquid solder for further spreading. However, with spreading time going on, more Cu was dissolved into the liquid solder on wetting channel. When all the Cu was consumed completely, dewetting would occur accompanied with the evolution of interfacial compounds. In conclusion, reactive spreading of liquid SnPb mainly resulted from the special Au/Sn bilayered structure. Using this proposed mechanism, we could also successfully explain why no liquid film spread on pure bulk Au. In the limiting condition, much more Au fast dissolved into the liquid solder and even consumed all the reacted species Sn, so no enough Sn concentration gradient could sustain further spreading. 4. Conclusion In summary, reactive wetting and spreading of liquid SnPb have been examined on the structured Au/Cu film with H 2 protection. The liquid solder was found to follow the wetting zones in the patterned

5 W. Liu et al.: J. Mater. Sci. Technol., 2010, 26(12), structure. On the wheel pattern, liquid solder flow produced three distinct regions: a central cap, a partially dewetting zone, and thin films along the thin Cu lines. While the liquid front of the central cap spread by viscous flow, and the liquid film advanced on the thin Au-Cu lines by diffusive growth. The partial dewetting zone was formed as the Au-Cu line was partially consumed by the liquid solder near the hub. Acknowledgements This study was supported by the National Natural Science Foundation of China (No ) and the National Basic Research Program of China (No. 2004CB619306). The authors appreciate the helpful discussion with Prof. Z.G. Wang and experimental assistance of John P. Daghfal. REFERENCES [1 ] R. Lipowsky, P. Lenz and P.S. Swain: Colloid. Surface. A, 2000, 161, 3. [2 ] Y.N. Xia, D. Qin and Y.D. Yin: Colloid Interface Sci., 2001, 6, 54. [3 ] R. Lipowsky: Curr. Opin. Colloid Interface Sci., 2001, 6, 40. [4 ] D.W. Zheng, W.J. Wen and K.N. Tu: J. Mater. Res., 1999, 14, 745. [5 ] D.W. Zheng, W.J. Wen and K.N. Tu: Phys. Rev. E, 1998, 57, [6 ] E. Saiz and A.P. Tomsia: Nat. Mater., 2004, 3, 903. [7 ] P.G. Kim and K.N. Tu: J. Appl. Phy., 1996, 80, [8 ] R. Lipowsky: Interf. Sci., 2001, 9, 105. [9 ] F.M. Hosking, F.G. Yost, E.A. Holm and J.R. Michael: J. Electron. Mater., 1996, 25, [10] J. Moon, S. Garoff, P. Wynblatt and R. Suter: Langmuir, 2004, 20, 402. [11] A. Bruson and M. Gerl: Phys. Rev. B, 1980, 21, [12] L. Yin, S.J. Stephan and T.J. Singler: Acta Mater., 2004, 52, 2873.