Reaction of Sn to Nanocrystalline Surface Layer of Cu by Near Surface Severe Plastic Deformation

Transcription

1 Solid State Phenomena Vol. 127 (2007) pp Online available since 2007/Sep/15 at (2007) Trans Tech Publications, Switzerland doi: / Reaction of Sn to Nanocrystalline Surface Layer of Cu by Near Surface Severe Plastic Deformation Y.Minamino 1a, Y.Koizumi 1b, N.Tsuji 1c, Y.Nakamizo 1, T.Shibayanagi 2d and M.Naka 2e 1 Department of Adaptive Machine Systems, Osaka University, 2-1,Yamadaoka, Suita, Osaka 565, Japan 2 Joining and Welding Research Institute, Osaka University, 11-1, Mihogaoka, Ibaraki, Osaka , Japan a minamino@ams.eng.osaka-u.ac.jp, b koizumi@ams.eng.osaka-u.ac.jp, c tsuji@ams.eng.osaka-u.ac.jp, d toshiya@jwri.osaka-u.ac.jp, e naka@jwri.osaka-u.ac.jp Keywords: reaction, diffusion, intermetallic compounds, layer growth, electroplated tin, OFHC-Cu, near surface ultrafine grains, near surface severe plastic deformation The near surface ultrafine grains (NSUFG) layer with grain sizes of about 35nm to 200nm from surface to about 10μm was prepared in the OFHC-Cu sheet with coarse grains size of about 7.2μ m (CG-Cu) by Near Surface Sever Plastic Deformation method. The solid reactions of Sn to NSUFG layer and CG-Cu were basically investigated at 379K to 493K for 1x10 3 to 6x10 6 s. The Cu 6 Sn 5 (η ) and Cu 3 Sn ( ε ) layers were formed between Cu and Sn. The thickness of the ε layer in NSUFG-Cu/Sn was similar to that in CG-Cu/Sn one, while the thickness of the η layer in NSUFG-Cu/Sn reaction was about two times thicker than that in CG-Cu/Sn one. This enhancement of the η layer growth in NSUFG-Cu/Sn reaction was due to the large supply of Cu atoms to the reaction layer by the grain boundary diffusion in the NSUFG-Cu. The rate-controlling processes of layer growth were boundary diffusion mechanism in reaction layer at lower temperatures in shorter annealing time, and volume diffusion mechanism at higher temperatures in longer annealing times. Introduction Recently, a near surface ultrafine grain (NSUFG) layer has been able to be made in the near surface layer of metallic materials by a near surface severe plastic deformation (NSSPD) method [1,2]. These NSUFG metallic materials are expected to exhibit the superior properties such as suppression of fatigue crack initiation from surface, stress corrosion cracking, increase in proof strength, improvement of coating, promotion of surface reaction and so on, because the surface covered with NSUFG layer is closely related with above mentioned various properties. At interface between copper-base conductor alloys and Sn-base solder alloys, binary intermetallic compounds in the Cu-Sn system are formed during soldering and subsequent heating, although the Sn solders are alloyed with various elements to lower their melting points and to improve their mechanical properties of interconnections [3-5]. The formation of such compounds with high brittleness and high electrical resistivity is inevitable for interconnection. Therefore, it is necessary to lower the soldering temperature and to suppress the reaction on the interface to improve the performance of soldering. The NSUFG layer near surface of copper would be able to change the mode of the reaction between copper-base alloy and Sn-base solder and lower the soldering temperature. The purposes of this study are to make the NSUFG layer in OFHC-Cu sheet (NSUFG-Cu sheet) by using the NSSPD method, and to investigate and discuss the reactivity of Sn to this NSUFG in the wider temperature and annealing period range of 373K to 493K and 1ks to 6000ks, view of comparing reacting with that of Sn to coarse grain of OFHC-Cu sheet (CG-Cu sheet). In the present study, the OFHC Cu and pure Sn metals were used instead of the Cu-base conductor alloy and Sn-based solder alloy, because the binary intermetallic compounds were formed during soldering and the keeping out the effect of the various elements added to solder on the reaction was desirable to analyze and discuss their reactions. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: , Pennsylvania State University, University Park, United States of America-04/06/14,20:18:39)

2 116 Designing of Interfacial Structures in Advanced Materials and their Joints Experimental An OFHC-Cu sheet in size of 200mm in length, 110mm in width and 2mm in thickness was annealed at 623K for 3.6ks in vacuum of 2.7x10-2 Pa. Its chemical compositions are listed in Table 1, and its average grain size was 7.2μm, and After being slightly polished by alumina powder of 0.3 μ m, the NSUFG layer was formed on surface of one side of the sheet by the NSSPD method: the sheet was wire-brushed at ambient temperature by the Friction Stir Welding (FSW) machine with a 304 stainless-wire bevel brush Fig.1 NSSPD apparatus (FSW with wire blush, thermo couple, load cells). of 85 mm in diameter with wires of 0.3 mm in diameter as shown in Fig.1. The brush was 6 degree tilted from the normal direction of the sheet to the moving direction of sheet, its rotation speed was 2000 rpm, its average pressing road of brush against sheet was 130 N and the feeding speed of the sheet was 50 mm/s. During NSSPD, the sheet was dipped in water for cooling, and its temperature during the NSSPD was measured to be 370K by the thermocouple. This NSSPD Cu sheet has two surfaces; one with NSUFG layer on one side of the sheet (Hereafter, this surface is referred to as NSUFG-Cu surface) and the other with coarse grains on the other side (CG-Cu surface). For the measurement of the grain sizes near surfaces the microstructures of near surface region were observed perpendicularly to surface by a transmission electron microscopy (TEM), while the cross sections of the sheets were observed by EBSP in a field emission-scanning electron microscopy (FE-SEM). Two surfaces of the sheet slightly polished with alumina powder (0.05μm) and degreased by acetone were electroplated with Sn of 80 μm in thickness. Samples of about 3 mm x 3mm x 2mm in size were prepared from the centeral part of the Sn electroplated sheet, and they have the interface between the CG-Cu and Sn (CG-Cu/Sn interface) and that between NSUFG -Cu and Sn (NSUFG-Cu/Sn interface). Each of samples was clamped by special SUS holders in order to prevent them from the peeling away of Sn plate during treatments, and was sealed into glass tubes with Ar gas and Sn particles for suppression of vaporization of Sn plate. They were then annealed at 373K to 493K for 1~6x10 3 ks in a silicon oil. The annealed samples were ground in the perpendicular direction to the interfaces. The cross sections were polished with diamond paste. The reaction layers were observed by FE-SEM, and their width was evaluated from the FE-SEM microphotographs. The diffusion profiles of the reaction layers were measured by the electron probe microanalyzer (EPMA). Results Fig.2 shows the orientation imaging micrographs (OIMs) of the cross section perpendicular to the surface of CG-Cu, NSUFG-Cu, and NSUFG-Cu annealed at 493K for 500ks. In CG-Cu, the coarse grains of about 7.2μm in diameter exist just from the surface to their inner parts (Fig.2a). On the other hand, the NSUFG layer is observed from the surface to the inner part up to the depth of about 10μm in NSUFG-Cu, and the grain size increases with depth from surface (Fig.2b). Fig.3 shows the grain size distribution of NSFUG-Cu against the depth from the surface, where the grain size at the surface was measured to be 35nm by TEM and the grain sizes in inner part was measured from

3 Solid State Phenomena Vol the photo in Fig.1b. The grain size is quite fine of 35 nm to 100 nm from surface to the depth of 3μ m, and then it increases to about 200 nm in the depth of 10μm. In deeper part, the grain size steeply increases, and the grain size in the depth of 30μm is almost equal to the 7.2μm of the CG-Cu. After annealing at 493K for 500ks, the grain growth occurs, but the NSUFG layer exists and its grain sizes retain sub-micron sizes (Fig.2c). This indicates that the NSUFG layer is quite stable against the long annealing of 500ks even at 493K. Fig.3 Distribution of grain size from surface to inner part of NSUFG-Cu. Fig.2 Orientation imaging micrographs by EBSD: (a) CG-Cu, (b) NSUFG-Cu and (c) NSUFG-Cu annealed at 493K for 500ks. Fig.4 SEM microstructures of (a) CG-Cu/Sn and (b) NFUFG-Cu/Sn annealed at 493K for (1) 1ks and (2) 400ks. The SEM microphotographs of reaction layers at the CG-Cu/Sn and NSUFG-Cu/Sn interfaces annealed at 493K for 1ks and 400ks as an example shown in Fig.4. Two kinds of layers are formed

4 118 Designing of Interfacial Structures in Advanced Materials and their Joints in both interfaces. The concentrations of the wider and thinner layers measured by EPMA are respectively 45.2at%Sn and 25.8at%Sn. According to these concentrations and phase diagram of Cu-Sn system, the wider layer is Cu 6 Sn 5 (η ) phase and the other is Cu 3 Sn ( ε ) phase. These phases are in accordance with the results of previous reports by Takenaka et al, Vianco et al and Ohishi et al [3-5]. The widths of η and ε layers are plotted on the linear scales in Fig.5. Two reaction layers grow with annealing time. Large difference between the NSUFG-Cu/Sn and CG-Cu/Pt reactions is the width of the η phase: the width of η in NSUFG-Cu/Sn reaction is about two times larger than that in CG-Cu/Sn one, although the layer widths of ε phase in both reactions are similar to each other. The phase growth is represented by following equation: W=kt n (1) where W is the layer width, k the constant, t the annealing time, and n the exponent. The widths of η and ε layers are also plotted on the logarithmic scales in Fig.6. The plotted points for the η phase, ε phase and total widths at 493K as well as 473K fall on two lines with the gradients of about 0.3 in shorter annealing times and 0.5 in longer annealing times with some scatter. On the other hand, they lie well on straight line at lower temperatures below 453K. From the gradients of these lines of layer growths, the n values in Eq. 1 can be evaluated and shown in Fig.7. The n values of width of η layer and total width of η and ε layers are about 0.25 near 373K and they monotonically increase to about 0.35 at 453K. At higher temperatures of 473K and 493K, the n values are about 0.35 for shorter annealing times and about 0.5 for longer annealing times. The n values of ε layer width slightly increase from about 0.4 to 0.45, and they reach to 0.5 at 493K for longer annealing times as well as n values of η phase and total phase widths. Fig.5 Phase growth of reaction layers at 493K in linear scale. Fig.6 Phase growth of reaction layers at 493K in the logarithmic scale. Discussion The layer growth occurs due to the rearrangement of the Cu and Sn atoms at the interfaces required for the growth of layer that may involve a reaction barrier, and the diffusion mass flux across the layer where the diffusion flux slows down with increasing layer thickness. The n values of Eq. 1 corresponds to 0.25 for layer growth mechanism due to grain boundary diffusion with grain growth, 0.5 due to volume diffusion and 1 for interface reaction [3]. As shown in Fig.7, the n values of present results are from about 0.25 to 0.5. These values indicate that the layer growth mechanisms are governed by volume diffusion mechanisms and/or grain boundary diffusion with

5 Solid State Phenomena Vol grain growth. Therefore, although the Cu and Sn atoms contribute to the reaction, the Cu diffusion flux should be discussed in order to understand the rate-controlling process of the layer growth in detail, because the diffusivity of Cu atoms is slower than that of Sn atoms in Cu-Sn system. Generally speaking, below the temperatures of 0.7T m the contribution of the grain boundary diffusion becomes dominant in polycrystalline metal [6]. In Cu metal at the annealing temperatures of Cu this work which is (0.27 ~ 0.36) T m Cu ( T m :melting point of Cu), the volume diffusion is nearly frozen when compared with the grain boundary diffusion. On the other hand, the recrystallization temperature T R is about ( ) T m in general, although it depends on the annealing times and the degree of working. As the T R of Cu is about 473K, it is possible that the recrystallization and grain growth slightly occurs at 473K and 498K of higher annealing temperatures of this work. This is in accordance with the slight grains growth at 493K as shown in Fig.1c. Assuming that the width of grain boundary is 1nm, the volume fractions of the grain boundaries in the CG-Cu with the diameter of 7.2μm and NSUFG-Cu with those of 35nm to 200nm from surface to the depth of 10μm are respectively evaluated to be 0.004% and 8.5%~1.5%. From this, the ratio of these volume fractions of CG-Cu to NSUFG-Cu is 375 to Therefore, the amount of Cu flux at the grain boundary with thin layer is considerably enlarged by the existence of NSUFG layer because of the immensely wide area of grain boundaries, and this larger supply of Cu atoms to the reaction by the larger volume fraction of NFUFG boundaries gives rise to the enhancement of layer growth. The Cu atoms supplied from Cu side diffuse through the ε and η layers crossing the interfaces with rearrangement of Cu and Sn atom, and enter the Sn side. The annealing temperatures of this ε research are (0.39~0.52) T max and (0.54~0.72) T max η for the ε and η phases where T max ε and T are the maximum temperature of existence of ε and η phases. Therefore, it is quite η max possible that both the volume and grain boundary diffusion practically operate in ε and η layers and the grains in these layers grow during annealing. In other words, the Cu atoms in these layers diffuse mainly through the grain boundaries of fine grains at the lower temperatures and in the shorter annealing times, and they diffuse mainly in the large grains at higher temperatures and longer annealing times. In Sn side, the Cu atoms entering to Sn metal diffuse rapidly by the volume diffusion in large Sn grains, because the annealing temperatures of this research are (0.73~ Sn 0.97) T m ; the volume diffusion is dominant even at lowest temperature of 373K and the grains of Sn metal becomes quite large in just beginning of annealing because of the T R of 260K. Thus, as shown in Fig.6, the monotonous increase of n values from about 0.25 near the temperature of 373K to 0.5 at 493K for longer annealing times strongly suggests that the rate controlling processes of layer growth are the mass flow of Cu atoms in the ε and η layers by grain boundary diffusion and grain growth at lower temperatures and volume diffusion in the condition of higher temperatures and long annealing times. Conclusions Fig.7 The exponent n of phase growth in CG-Cu/Sn and NSUFG-Cu/Sn. 1. The near surface ultrafine grains (NSUFG) with grain size of 35nm to 200nm was prepared

6 120 Designing of Interfacial Structures in Advanced Materials and their Joints from surface to about 10μm in depth in OFHC-Cu with coarse grain size of 7.2μm (CG-Cu) by Near Surface Sever Plastic Deformation (NSSPD) method 2. The Cu 6 Sn 5 and Cu 3 Sn layers appeared at the interfaces of NSUFG-Cu/Sn and CG-Cu/Sn after annealing of 1x10 3 to 6x10 6 s at 373K to 493K. The Cu 6 Sn 5 layer at the interfaces of NSUFG-Cu/Sn grows twice thicker than that at CG-Cu/Sn by the enhancement of reaction due to the large amount of mass flux of Cu atoms through the NSUFG boundaries. 3. The exponent n values of layer growth increase from 0.25 at 273K to 0.35 at 493K in shorter annealing and to 0.5 at 493K in longer annealing, where W=kt n (W:layer thickness, k:constant and t:annealing time). These n values strongly suggest that the rate controlling processes are the grain boundary diffusion and grain growth at lower temperatures and volume diffusion at higher temperatures and long annealing times. References [1]M.Sato, N.Tsuji, Y.Minamino and Y.Koizumi: Mater. Sci. Forum, , (2003) p [2]M.Sato, N.Tsuji, Y.Minamino and Y.Koizumi: Sci. and Tech. of Advan. Mater., 5 (2004) p.145. [3]T.Takenaka, S.Kano, M.Kajihara, N.Kurokawa and K.Sakamoto: Materials Science and Engineering A, 396 (2005) p.115. [4]P.T.Vianco, P.F.Hlava and A.L.Kilgo: J.Electron. Mater., 23 (1994) p.583. [5]M.Onishi and H.Fujibuchi: Trans. JIM, 16 (1975) p.539. [6]P.Shewmon, Diffusion in solids, TMS, Pennsylvania, (1989) p

7 Designing of Interfacial Structures in Advanced Materials and their Joints / Reaction of Sn to Nanocrystalline Surface Layer of Cu by Near Surface Severe Plastic Deformation /