- Email: [email protected]
(Cu–5 at.%Ni) interconnections
Materials Chemistry and Physics 142 (2013) 682e685
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Growth kinetics of the intermetallic phase in diffusion-soldered (Cue5 at.%Ni)/Sn/(Cue5 at.%Ni) interconnections A. Wierzbicka-Miernik a, *, K. Miernik b, J. Wojewoda-Budka a, K. Szyszkiewicz c, R. Filipek c, L. Litynska-Dobrzynska a, A. Kodentsov d, P. Zieba a a
Institute of Metallurgy and Materials Science, Polish Academy of Sciences, 25 Reymonta Str., 30-059 Krakow, Poland Institute of Materials Engineering, Cracow University of Technology, 37 Jana Pawla II Av., 31-864 Krakow, Poland Faculty of Materials Science and Ceramics, AGH University of Science and Technology, 30 Mickiewicza Av., 30-059 Krakow, Poland d Laboratory of Materials and Interface Chemistry of Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands b c
h i g h l i g h t s Fast growth of (Cu1xNix)6Sn5 phase in the Cue5at%Ni/Sn/Cue5at%Ni joint. The (Cu1xNix)6Sn5 phase occurs almost simultaneously in the whole reaction area. The n parameter was found to be 0.27e0.15 in the temperature range 240e260 C. The n value indicates grain boundary diffusion during soldering.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 December 2012 Received in revised form 12 July 2013 Accepted 16 August 2013
A stereological analysis was carried out in order to obtain the kinetics parameters of the (Cu1xNix)6Sn5 growth in the diffusion soldered (Cue5 at.%Ni)/Sn/(Cue5 at.%Ni) interconnections where previously anomalous fast growth of this phase was described. The n-parameter in the equation x ¼ ktn was found to be 0.27e0.15 in the temperature range 240e260 C, respectively. This is far away from the volume control process (n ¼ 1 if total surface of forming phase is the reference). The TEM/EDX microanalysis made across the (Cu1xNix)6Sn5/Sn e solder showed sudden change of Sn and Cu content typical for the grain boundary diffusion as the rate controlling mechanism. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Electronic materials Intermetallic compounds Diffusion Interfaces Electron microscopy (TEM and SEM)
1. Introduction One of the main stream of the environmentally friendly technologies which is intensively developed is the soldering of modern materials. In the past, conventional lead containing solders (SnePb eutectic alloys) have been widely used in electrical and electronic devices, but nowadays are banned by two European Union regulations (Waste from Electrical and Electronic Equipment and Restriction of Hazardous Substances) because of the toxic effect on the human health and the environment [1]. Lead-free solders have nearly replaced SnePb alloys [2e6]. The proposed substitutes are mostly based on the multi-component systems, in which * Corresponding author. Tel.: þ48 12 2952817; fax: þ48 12 2952804. E-mail addresses: [email protected], [email protected] (A. Wierzbicka-Miernik). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.08.022
interaction among individual elements should be described in details, both from the side of kind of forming phases, their microstructure and chemical composition and also in the aspect of rate of phenomena occurring at the interphase boundaries. So far, all the effort has been focused on the SAC type materials containing mostly Sn with 3 wt.% Ag and 0.3 wt.% Cu [7e9]. The second important direction of the environmentally friendly soldering technologies is finding the new joining methods which could be applied in the electrical and electronic industry. One of this technique seems to be the diffusion soldering process (low and high temperature) [10e18]. However, very short time of soldering process on the automatic production line of electronic components and electrical engineering is not sufficient to completely fill the joint area by the intermetallic phases (IPs). Therefore, investigations focused on finding such additions which would either bring about the acceleration of diffusion
A. Wierzbicka-Miernik et al. / Materials Chemistry and Physics 142 (2013) 682e685
683
Fig. 1. SEM images of (Cue5 at.%Ni)/Sn/(Cue5 at.%Ni) diffusion soldered joint obtained at 250 C for a) 5 min, b) 10 min, c) 15 min.
processes and chemical reactions occurring on the SneCu interface or change the sequence of formed intermetallic phases. In the classical Cu/Sn/Cu joints, a Cu6Sn5 phase forms as the first one, and then growth of a Cu3Sn phase occurs. However, the Ni addition to SAC alloy, already in 0.01%, resulted in effective retardation of undesirable Cu3Sn phase growth [14]. Similar effect was observed at 0.05 wt.% Ni to the Sn0.7Cu solder [15]. On the other hand, the Ni addition in the amount of 0.02e0.1 wt.% to the Sn þ 0.7 wt.% Cu in the reaction with the Ni substrate leads to the acceleration of Cu6Sn5 phase growth and to the change of its shape from cylindrical to so called faceted one [16]. Preliminary works carried out on (Cue5 at.%Ni)/Sn/(Cue5 at.% Ni) diffusion soldering interconnections showed that in the presence of 5 at.% Ni the reaction runs more quickly and only growth of the (Cu1xNix)6Sn5 phase is observed [17]. Moreover, the phase does not grow in the form of scallops from the Cu substrate towards the interconnection center but its growth occurs almost simultaneously in the whole interconnection area. Therefore, the main purpose of this study was to obtain more information on the kinetics of the (Cu1xNix)6Sn5 growth in the diffusion soldered (Cue5 at.%Ni)/Sn/(Cue5 at.%Ni) interconnections which possibly should help us to explain why this process occurs so fast. 2. Experimental To investigate the growth kinetics of the (Cu1xNix)6Sn5 intermetallic phase in the reaction zone between Cue5 at.%Ni substrate and a pure Sn foil (0.127 mm in thickness), the (Cue5 at.%Ni)/Sn/ (Cue5 at.%Ni) interconnections were prepared by the diffusion soldering process [9] at 240, 250 and 260 C for different times: 2, 5, 10, 15 and 20 min in an argon protective atmosphere. The chemical composition characterization of the structural components in the obtained joints was done using a FEI XL30 environmental scanning electron microscope (SEM) equipped with an X-ray energy dispersive (EDX) spectrometer (EDAX GEMINI 4000). The chemical analyses were performed using 20 kV accelerating voltage, 75 mA beam current, 35 take off angle and 100 s time per single measurement. At least 15 samples were investigated; for each one, the same phase was observed and the average composition value of three measurements was calculated taking into account the Ni-Ka, Cu-Ka, Sn-La series. The microstructure observations of the crosssections of soldered samples for stereological analysis were performed on a JEOL JSM 5510 LV scanning electron microscope operating at the backscattered electron (BSE) regime and 20 kV accelerating voltage. This enabled to have a relatively strong phase contrast between elements of the microstructure: Cue5 at.%Ni substrate, (Cu1xNix)6Sn5 intermetallic phase (IP) and solder material e pure Sn. To determine the growth kinetics of the (Cu1xNix)6Sn5 phase, the stereological analysis was carried out using the software ImageJ by Wayne Rasband 1.45s [18] from the National Institutes of Health, USA. For a better presentation of the specific
character of the IP growth, the Sn, presented in the reaction zone after soldering process, was selectively etched away with a solution of 1 cm3 HCl and 100 cm3 C2H5OH. Samples prepared in such a way were then observed using a Quanta 3D scanning electron microscope (FEI) in the secondary electrons mode (SE). The details concerning the changes of the chemistry accompanying the growth of the (Cu1xNix)6Sn5 phase were determined using a TECNAI G2 FEG super TWIN (200 kV) transmission electron microscope (TEM) equipped with a high angle annular dark field detector and integrated with an energy dispersive X-ray (EDX) spectrometer manufactured by EDAX. The thin foils were cut out from the accurately selected regions of the (Cu1xNix)6Sn5 phase using a Quanta 3D focused ion beam.
3. Results and discussion The overall view of the whole soldering area is shown in Fig. 1. The new intermetallic phase grows from the copper side towards the center of the interconnection. This phase was identified using the energy dispersive X-ray spectroscopy technique as containing: 50.7 at.% Cu, 45.1 at.% Sn and 4.2 at.% Ni, which corresponds to the h e (Cu1xNix)6Sn5. The occurrence of this phase was confirmed using selected area electron diffraction as described in Ref. [17]. Further SEM observations performed after longer time of soldering indicated that the h e (Cu1xNix)6Sn5 was the only one phase formed and it covered whole the interconnection (Fig. 1c). This observation confirmed the conclusion coming from the previous research [17] that addition of Ni to the Cu substrate leads to the suppression of formation and growth of the Cu3Sn phase, causes the substantial increase of the h e (Cu1xNix)6Sn5 volume fraction and changes its morphology. The main information about the intermetallic growth kinetics is falling under the simple relation between the IP thickness, x, and time of annealing, t, at appropriate temperature:
xðtÞ ¼ k$t n
(1)
where k is the growth rate constant and n e the exponential factor. However, the (Cu1xNix)6Sn5 intermetallic phase was formed not in the shape of scallops growing from the substrate into the joint center. On the contrary, it appeared as the isolated areas, so called “porous needle shape” [19], especially clearly visible after selective etching (Fig. 2). This made the measurements of the IP width problematic and inaccurate. However, if we consider the growth of the (Cu1xNix)6Sn5 phase as the moving boundary problem for spherically symmetric growth of a particle b in the surrounding phase a under diffusion-limited regime, then we can write the following equation for the c (r,t) e the concentration outside of the growing particle:
vc v2 c 2 vc ¼ Da þ vt vr 2 r vr
! for r > RðtÞ
(2)
684
A. Wierzbicka-Miernik et al. / Materials Chemistry and Physics 142 (2013) 682e685
Fig. 2. SEM images of (Cue5 at.%Ni)/Sn/(Cue5 at.%Ni) diffusion soldered joint obtained at 260 C for 10 min after selectively etching away the Sn.
for 0 < r < RðtÞ
(3)
where R(t) denotes the time dependent radius of the growing particle, cb is the constant concentration in the particle, Da is a diffusion coefficient, with boundary and initial conditions
cðRðtÞ; tÞ;
lim cðr; tÞ ¼ cN
(4)
for r > R0 ;
(5)
r/N
cðr; 0Þ ¼ cN
where cN is the concentration in the bulk of the material, R0 is the initial radius of the particle and Stefan’s mass balance condition at a e b interface
ca cb
dR dt
þ Da
vc ðRðtÞ; tÞ ¼ 0: vr
(6)
The system (2)e(6) has a unique solution for the time evolution of the particle size, R(t), of the following form
RðtÞ ¼ R0 þ K
pffiffiffiffiffiffiffiffiffiffiffi 4Da t :
(7)
The parameter K > 0, which appears in this equation can be obtained as the solution to the special transcendental equation
pffiffiffiffi 2 ca cN 2K 2 1 pKeK erfcðKÞ ¼ ca cb
(8)
where erfc is the complementary error function: erfcðxÞ ¼ ZN pffiffiffiffi 2 es ds: 1 erfðxÞ ¼ ð2= pÞ x
Thus, the general conclusion from the solution pffiffi of this model is that increase of the radius is proportional to t ; and in consequence the growth of the volume is proportional to t3/2:
4 4 pffiffiffiffiffiffiffiffiffiffiffi3 VðtÞ ¼ pRðtÞ3 ¼ p K 4Da t ¼ k3 $t 3=2 : 3 3
pffiffiffiffiffiffiffiffiffiffiffi2 SðtÞ ¼ 4pRðtÞ ¼ 4p K 4Da t ¼ k2 $t Thus, for the diffusion controlled growth we have
1
2
ðgrowth of thickness of IPÞ
(11)
S ¼ k2 $t 1
(12)
V ¼ k3 $t 3=2
(13)
The main phenomenological law describing the diffusion controlled growth is written as [20]:
z ¼ 1 expðk$t n Þ;
(14)
where z stands in our case for the fraction of transformed matter from Sn solder to h e (Cu1xNix)6Sn5 phase. The relation of the type S ¼ ktn can be seen as the limit case of this more general law for the short times, because for small ktn we have approximation exp(ktn) z 1 ktn, so
z ¼ 1 expðk$t n Þz1 ð1 k$t n Þ ¼ k$t n :
(15)
Therefore, in order to determine the growth kinetics of the h e (Cu1xNix)6Sn5 phase, series of 20 SEM images were taken after various time (t) of soldering. Then, the regions of IP and Sn were isolated in the joint zone and total relative area was determined in all the images. Next, only the IP field was separated and again the relative area of IP in the reaction zone was measured. All the stereological analyses were performed in an automatic way on the basis of all pixels of each photo. This provided full information on the percentage of structural components in the interconnections. Values of the exponent factor, n together with the growth rate constant, k can be easily found when plotting log S versus log t. The equation describing the straight line fitted to the data points:
log S ¼ n log t þ log k
(16)
provides the direct information about n and k (see Table 1).
(9)
For the time evolution of the area S(t) of the particle external surface can be written in the following form: 2
x ¼ k1 $t
=
cðr; tÞ ¼ cb
(10)
Table 1 Values of k and n parameters for (Cu1xNix)6Sn5 phase in (Cue5 at.%Ni)/Sn/(Cue 5 at.%Ni) joint soldered at 240, 250 and 260 C. Temperature ( C)
n
k (h1)
240 250 260
0.27 0.08 0.24 0.08 0.15 0.09
20.71 1.19 25.07 1.19 33.66 1.18
A. Wierzbicka-Miernik et al. / Materials Chemistry and Physics 142 (2013) 682e685
685
Fig. 3. a) TEM image of (Cue5 at.%Ni)/Sn/(Cue5 at.%Ni) diffusion soldered joint obtained at 260 C for 10 min and b) corresponding EDX line scan.
One can notice that the n coefficient varies from 0.15 to 0.27 in the whole annealing temperature range. This is a very large deviation from the volume diffusion controlled process (n ¼ 1, if total surface of growing phase is the reference). There is no possibility to compare the obtained results with other data as the latter were determined based on the thickness of growing phases (where n ¼ 0.5 stands for volume diffusion controlled process). However, the fact is that a similar tendency was reported by Bader et al. [21]. They examined growth of the h e Cu6Sn5 phase in diffusion soldered Cu/Sn/Cu joints and found the n values to be 0.21 and 0.25 for 240 C and 300 C, respectively. Takenaka et al. [22] in Sn/Cu/Sn diffusion couples determined n ¼ 0.319 at 200 C and n ¼ 0.363 at 160 C. The growth of the h phase in diffusion soldered Cu/Ine 48 at.%Sn/Cu interconnections resulted in n parameter between 0.26 and 0.31 in the temperature range 180e250 C [23]. Such small values of n were explained in Refs. [17,21] by the morphology of the h especially in early stage of soldering. The deep grooves separating particular scallops were considered to be fast diffusion paths. Bader et al. [21] also pointed out that the grooves become larger after longer time of the process and the transport capacity decreases due to the increase of diffusion paths and reduction in the concentration gradient. However, the n parameter in Sn/Cu/Sn diffusion couples was determined after relatively long period of annealing, even 1200 h. Therefore, Takenaka et al. [22] suggested that the grain boundary diffusion may contribute to the rate-controlling process at lower annealing temperatures. They claimed that below n equal to 0.25 the process is controlled exclusively by the grain boundary diffusion and grain size of the intermetallic phase is proportional to the square root of the annealing time. In our research on (Cue5 at.%Ni)/Sn/(Cue5 at.%Ni) interconnections it is difficult to identify clearly the mass transport mechanism because of the specific way of the (Cu1xNix)6Sn5 phase growth. However, some useful information can be derived based on the results of EDX microanalysis made across the grain of (Cu1xNix)6Sn5. Fig. 3a is TEM micrograph showing an area adjacent to the Cue5 at.% Ni substrate. The grain of (Cu1xNix)6Sn5 is located between two grains of Sn e solder. The EDX microanalysis performed for such location showed (Fig. 3b) sharp (limited to few nanometers) decrease of Sn-content accompanied by simultaneous sharp increase of Cu e content. This is unlike for volume diffusion solute concentration profile and indicates that the distance for the diffusion is really limited to (Cu,Ni)6Sn5/Sn grain boundary. 4. Conclusions The studies of the growth kinetics of the (Cu1xNix)6Sn5 in the (Cue5 at.%Ni)/Sn/(Cue5 at.%Ni) diffusion soldered interconnections
revealed the large deviation of the n-parameter from 1, appropriate for the volume diffusion control if the fraction of surface transformed into the intermetallic phase is the reference. The n parameter obtained from the stereological measurements of the total area occupied by the (Cu1xNix)6Sn5 was found to be 0.27e0.15 in the temperature range 240e260 C. Such behavior is attributed the substantial contribution of grain boundary diffusion operating during whole process of soldering and confirmed by the TEM/EDX high spatial resolution microanalysis. Acknowledgments The research was financially supported by the Polish Ministry of Science and Higher Education under the grant no. 378/N-Eindhoven/2009/0. The SEM and TEM studies were performed in the Accredited Testing Laboratories at the Institute of Metallurgy and Materials Science of the Polish Academy of Sciences as well as at the Institute of Materials Engineering, Cracow University of Technology. References [1] ‘Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment’, Directive 2002/95/EC, Off. J. Eur. Union (13 February 2003). [2] M. Abtew, G. Selvaduray, Mat. Sci. Eng. R 27 (5e6) (2000) 95e141. [3] K. Zeng, K.N. Tu, Mat. Sci. Eng. R 38 (2) (2002) 55e105. [4] T. Laurila, V. Vuorinen, J.K. Kivilahti, Mat. Sci. Eng. R 49 (1e2) (2005) 1e60. [5] K. Suganuma, Curr. Opin. Solid State Mater. Sci. 5 (1) (2001) 55e64. [6] K.N. Tu, A.M. Gusak, M. Li, J. Appl. Phys. 93 (3) (2003) 1335e1353. [7] T.Y. Lee, W.J. Choi, K.N. Tu, et al., J. Mater. Res. 17 (2) (2002) 291e301. [8] S. Chada, R.A. Fournelle, W. Laub, D. Shangguan, J. Electron. Mater. 29 (10) (2000) 1214e1221. [9] A.R. Fix, G.A. Lopez, I. Brauer, W. Nuchter, E.J. Mittemeijer, J. Electron. Mater. 34 (2) (2005) 137e142. [10] P. Zie˛ ba, J. Wojewoda, Recent Research Developments in Materials Science, vol. 4, Research Signpost, Kerala, 2003, pp. 261e282. [11] W.F. Gale, D.A. Butts, Sci. Technol. Weld. Joining 9 (4) (2004) 283e300. [12] J. Wojewoda, Solid State Phenom. 138 (2008) 165e174. , J. Wojewoda-Budka, R. Filipek, P. Zie˛ ba, Arch. Metall. [13] P. Skrzyniarz, A. Sypien Mater. 55 (2010) 123e130. [14] Y.W. Wang, C.C. Chang, C.R. Kao, J. Alloys Compd. 478 (2009) L1eL4. [15] N. Nishikawa, J.Y. Piao, T. Takemoto, J. Electron. Mater. 35 (2006) 1127e1132. [16] C.H. Wang, H.T. Shen, Intermetallics 18 (2010) 616e622. [17] A. Wierzbicka-Miernik, J. Wojewoda-Budka, L. Litynska-Dobrzynska, A. Kodentsov, P. Zieba, Sci. Technol. Weld. Joining 17 (1) (2012) 32e35. [18] Web site: http://rsbweb.nih.gov/ij/. [19] S.W. Chen, S.-H. Wu, S.-W. Lee, J. Electron. Mater. 32 (2003) 1188e1194. [20] J.W. Christian, The Theory of Transformations in Metals and Alloys, Part 1, third ed., Pergamon, UK, 2002, pp. 538e546. [21] S. Bader, W. Gust, H. Hieber, Acta Metall. Mater. 43 (1) (1995) 329e337. [22] T. Takenaka, S. Kano, M. Kajihara, N. Kurokawa, K. Sakamoto, Mat. Sci. Eng. A 396 (2005) 115e123. [23] J. Wojewoda, P. Zie˛ ba, B. Onderka, R. Filipek, P. Romanów, Arch. Metall. Mater. 51 (2006) 345e353.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Growth kinetics of the intermetallic phase in diffusion-soldered (Cue5 at.%Ni)/Sn/(Cue5 at.%Ni) interconnections A. Wierzbicka-Miernik a, *, K. Miernik b, J. Wojewoda-Budka a, K. Szyszkiewicz c, R. Filipek c, L. Litynska-Dobrzynska a, A. Kodentsov d, P. Zieba a a
Institute of Metallurgy and Materials Science, Polish Academy of Sciences, 25 Reymonta Str., 30-059 Krakow, Poland Institute of Materials Engineering, Cracow University of Technology, 37 Jana Pawla II Av., 31-864 Krakow, Poland Faculty of Materials Science and Ceramics, AGH University of Science and Technology, 30 Mickiewicza Av., 30-059 Krakow, Poland d Laboratory of Materials and Interface Chemistry of Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands b c
h i g h l i g h t s Fast growth of (Cu1xNix)6Sn5 phase in the Cue5at%Ni/Sn/Cue5at%Ni joint. The (Cu1xNix)6Sn5 phase occurs almost simultaneously in the whole reaction area. The n parameter was found to be 0.27e0.15 in the temperature range 240e260 C. The n value indicates grain boundary diffusion during soldering.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 December 2012 Received in revised form 12 July 2013 Accepted 16 August 2013
A stereological analysis was carried out in order to obtain the kinetics parameters of the (Cu1xNix)6Sn5 growth in the diffusion soldered (Cue5 at.%Ni)/Sn/(Cue5 at.%Ni) interconnections where previously anomalous fast growth of this phase was described. The n-parameter in the equation x ¼ ktn was found to be 0.27e0.15 in the temperature range 240e260 C, respectively. This is far away from the volume control process (n ¼ 1 if total surface of forming phase is the reference). The TEM/EDX microanalysis made across the (Cu1xNix)6Sn5/Sn e solder showed sudden change of Sn and Cu content typical for the grain boundary diffusion as the rate controlling mechanism. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Electronic materials Intermetallic compounds Diffusion Interfaces Electron microscopy (TEM and SEM)
1. Introduction One of the main stream of the environmentally friendly technologies which is intensively developed is the soldering of modern materials. In the past, conventional lead containing solders (SnePb eutectic alloys) have been widely used in electrical and electronic devices, but nowadays are banned by two European Union regulations (Waste from Electrical and Electronic Equipment and Restriction of Hazardous Substances) because of the toxic effect on the human health and the environment [1]. Lead-free solders have nearly replaced SnePb alloys [2e6]. The proposed substitutes are mostly based on the multi-component systems, in which * Corresponding author. Tel.: þ48 12 2952817; fax: þ48 12 2952804. E-mail addresses: [email protected], [email protected] (A. Wierzbicka-Miernik). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.08.022
interaction among individual elements should be described in details, both from the side of kind of forming phases, their microstructure and chemical composition and also in the aspect of rate of phenomena occurring at the interphase boundaries. So far, all the effort has been focused on the SAC type materials containing mostly Sn with 3 wt.% Ag and 0.3 wt.% Cu [7e9]. The second important direction of the environmentally friendly soldering technologies is finding the new joining methods which could be applied in the electrical and electronic industry. One of this technique seems to be the diffusion soldering process (low and high temperature) [10e18]. However, very short time of soldering process on the automatic production line of electronic components and electrical engineering is not sufficient to completely fill the joint area by the intermetallic phases (IPs). Therefore, investigations focused on finding such additions which would either bring about the acceleration of diffusion
A. Wierzbicka-Miernik et al. / Materials Chemistry and Physics 142 (2013) 682e685
683
Fig. 1. SEM images of (Cue5 at.%Ni)/Sn/(Cue5 at.%Ni) diffusion soldered joint obtained at 250 C for a) 5 min, b) 10 min, c) 15 min.
processes and chemical reactions occurring on the SneCu interface or change the sequence of formed intermetallic phases. In the classical Cu/Sn/Cu joints, a Cu6Sn5 phase forms as the first one, and then growth of a Cu3Sn phase occurs. However, the Ni addition to SAC alloy, already in 0.01%, resulted in effective retardation of undesirable Cu3Sn phase growth [14]. Similar effect was observed at 0.05 wt.% Ni to the Sn0.7Cu solder [15]. On the other hand, the Ni addition in the amount of 0.02e0.1 wt.% to the Sn þ 0.7 wt.% Cu in the reaction with the Ni substrate leads to the acceleration of Cu6Sn5 phase growth and to the change of its shape from cylindrical to so called faceted one [16]. Preliminary works carried out on (Cue5 at.%Ni)/Sn/(Cue5 at.% Ni) diffusion soldering interconnections showed that in the presence of 5 at.% Ni the reaction runs more quickly and only growth of the (Cu1xNix)6Sn5 phase is observed [17]. Moreover, the phase does not grow in the form of scallops from the Cu substrate towards the interconnection center but its growth occurs almost simultaneously in the whole interconnection area. Therefore, the main purpose of this study was to obtain more information on the kinetics of the (Cu1xNix)6Sn5 growth in the diffusion soldered (Cue5 at.%Ni)/Sn/(Cue5 at.%Ni) interconnections which possibly should help us to explain why this process occurs so fast. 2. Experimental To investigate the growth kinetics of the (Cu1xNix)6Sn5 intermetallic phase in the reaction zone between Cue5 at.%Ni substrate and a pure Sn foil (0.127 mm in thickness), the (Cue5 at.%Ni)/Sn/ (Cue5 at.%Ni) interconnections were prepared by the diffusion soldering process [9] at 240, 250 and 260 C for different times: 2, 5, 10, 15 and 20 min in an argon protective atmosphere. The chemical composition characterization of the structural components in the obtained joints was done using a FEI XL30 environmental scanning electron microscope (SEM) equipped with an X-ray energy dispersive (EDX) spectrometer (EDAX GEMINI 4000). The chemical analyses were performed using 20 kV accelerating voltage, 75 mA beam current, 35 take off angle and 100 s time per single measurement. At least 15 samples were investigated; for each one, the same phase was observed and the average composition value of three measurements was calculated taking into account the Ni-Ka, Cu-Ka, Sn-La series. The microstructure observations of the crosssections of soldered samples for stereological analysis were performed on a JEOL JSM 5510 LV scanning electron microscope operating at the backscattered electron (BSE) regime and 20 kV accelerating voltage. This enabled to have a relatively strong phase contrast between elements of the microstructure: Cue5 at.%Ni substrate, (Cu1xNix)6Sn5 intermetallic phase (IP) and solder material e pure Sn. To determine the growth kinetics of the (Cu1xNix)6Sn5 phase, the stereological analysis was carried out using the software ImageJ by Wayne Rasband 1.45s [18] from the National Institutes of Health, USA. For a better presentation of the specific
character of the IP growth, the Sn, presented in the reaction zone after soldering process, was selectively etched away with a solution of 1 cm3 HCl and 100 cm3 C2H5OH. Samples prepared in such a way were then observed using a Quanta 3D scanning electron microscope (FEI) in the secondary electrons mode (SE). The details concerning the changes of the chemistry accompanying the growth of the (Cu1xNix)6Sn5 phase were determined using a TECNAI G2 FEG super TWIN (200 kV) transmission electron microscope (TEM) equipped with a high angle annular dark field detector and integrated with an energy dispersive X-ray (EDX) spectrometer manufactured by EDAX. The thin foils were cut out from the accurately selected regions of the (Cu1xNix)6Sn5 phase using a Quanta 3D focused ion beam.
3. Results and discussion The overall view of the whole soldering area is shown in Fig. 1. The new intermetallic phase grows from the copper side towards the center of the interconnection. This phase was identified using the energy dispersive X-ray spectroscopy technique as containing: 50.7 at.% Cu, 45.1 at.% Sn and 4.2 at.% Ni, which corresponds to the h e (Cu1xNix)6Sn5. The occurrence of this phase was confirmed using selected area electron diffraction as described in Ref. [17]. Further SEM observations performed after longer time of soldering indicated that the h e (Cu1xNix)6Sn5 was the only one phase formed and it covered whole the interconnection (Fig. 1c). This observation confirmed the conclusion coming from the previous research [17] that addition of Ni to the Cu substrate leads to the suppression of formation and growth of the Cu3Sn phase, causes the substantial increase of the h e (Cu1xNix)6Sn5 volume fraction and changes its morphology. The main information about the intermetallic growth kinetics is falling under the simple relation between the IP thickness, x, and time of annealing, t, at appropriate temperature:
xðtÞ ¼ k$t n
(1)
where k is the growth rate constant and n e the exponential factor. However, the (Cu1xNix)6Sn5 intermetallic phase was formed not in the shape of scallops growing from the substrate into the joint center. On the contrary, it appeared as the isolated areas, so called “porous needle shape” [19], especially clearly visible after selective etching (Fig. 2). This made the measurements of the IP width problematic and inaccurate. However, if we consider the growth of the (Cu1xNix)6Sn5 phase as the moving boundary problem for spherically symmetric growth of a particle b in the surrounding phase a under diffusion-limited regime, then we can write the following equation for the c (r,t) e the concentration outside of the growing particle:
vc v2 c 2 vc ¼ Da þ vt vr 2 r vr
! for r > RðtÞ
(2)
684
A. Wierzbicka-Miernik et al. / Materials Chemistry and Physics 142 (2013) 682e685
Fig. 2. SEM images of (Cue5 at.%Ni)/Sn/(Cue5 at.%Ni) diffusion soldered joint obtained at 260 C for 10 min after selectively etching away the Sn.
for 0 < r < RðtÞ
(3)
where R(t) denotes the time dependent radius of the growing particle, cb is the constant concentration in the particle, Da is a diffusion coefficient, with boundary and initial conditions
cðRðtÞ; tÞ;
lim cðr; tÞ ¼ cN
(4)
for r > R0 ;
(5)
r/N
cðr; 0Þ ¼ cN
where cN is the concentration in the bulk of the material, R0 is the initial radius of the particle and Stefan’s mass balance condition at a e b interface
ca cb
dR dt
þ Da
vc ðRðtÞ; tÞ ¼ 0: vr
(6)
The system (2)e(6) has a unique solution for the time evolution of the particle size, R(t), of the following form
RðtÞ ¼ R0 þ K
pffiffiffiffiffiffiffiffiffiffiffi 4Da t :
(7)
The parameter K > 0, which appears in this equation can be obtained as the solution to the special transcendental equation
pffiffiffiffi 2 ca cN 2K 2 1 pKeK erfcðKÞ ¼ ca cb
(8)
where erfc is the complementary error function: erfcðxÞ ¼ ZN pffiffiffiffi 2 es ds: 1 erfðxÞ ¼ ð2= pÞ x
Thus, the general conclusion from the solution pffiffi of this model is that increase of the radius is proportional to t ; and in consequence the growth of the volume is proportional to t3/2:
4 4 pffiffiffiffiffiffiffiffiffiffiffi3 VðtÞ ¼ pRðtÞ3 ¼ p K 4Da t ¼ k3 $t 3=2 : 3 3
pffiffiffiffiffiffiffiffiffiffiffi2 SðtÞ ¼ 4pRðtÞ ¼ 4p K 4Da t ¼ k2 $t Thus, for the diffusion controlled growth we have
1
2
ðgrowth of thickness of IPÞ
(11)
S ¼ k2 $t 1
(12)
V ¼ k3 $t 3=2
(13)
The main phenomenological law describing the diffusion controlled growth is written as [20]:
z ¼ 1 expðk$t n Þ;
(14)
where z stands in our case for the fraction of transformed matter from Sn solder to h e (Cu1xNix)6Sn5 phase. The relation of the type S ¼ ktn can be seen as the limit case of this more general law for the short times, because for small ktn we have approximation exp(ktn) z 1 ktn, so
z ¼ 1 expðk$t n Þz1 ð1 k$t n Þ ¼ k$t n :
(15)
Therefore, in order to determine the growth kinetics of the h e (Cu1xNix)6Sn5 phase, series of 20 SEM images were taken after various time (t) of soldering. Then, the regions of IP and Sn were isolated in the joint zone and total relative area was determined in all the images. Next, only the IP field was separated and again the relative area of IP in the reaction zone was measured. All the stereological analyses were performed in an automatic way on the basis of all pixels of each photo. This provided full information on the percentage of structural components in the interconnections. Values of the exponent factor, n together with the growth rate constant, k can be easily found when plotting log S versus log t. The equation describing the straight line fitted to the data points:
log S ¼ n log t þ log k
(16)
provides the direct information about n and k (see Table 1).
(9)
For the time evolution of the area S(t) of the particle external surface can be written in the following form: 2
x ¼ k1 $t
=
cðr; tÞ ¼ cb
(10)
Table 1 Values of k and n parameters for (Cu1xNix)6Sn5 phase in (Cue5 at.%Ni)/Sn/(Cue 5 at.%Ni) joint soldered at 240, 250 and 260 C. Temperature ( C)
n
k (h1)
240 250 260
0.27 0.08 0.24 0.08 0.15 0.09
20.71 1.19 25.07 1.19 33.66 1.18
A. Wierzbicka-Miernik et al. / Materials Chemistry and Physics 142 (2013) 682e685
685
Fig. 3. a) TEM image of (Cue5 at.%Ni)/Sn/(Cue5 at.%Ni) diffusion soldered joint obtained at 260 C for 10 min and b) corresponding EDX line scan.
One can notice that the n coefficient varies from 0.15 to 0.27 in the whole annealing temperature range. This is a very large deviation from the volume diffusion controlled process (n ¼ 1, if total surface of growing phase is the reference). There is no possibility to compare the obtained results with other data as the latter were determined based on the thickness of growing phases (where n ¼ 0.5 stands for volume diffusion controlled process). However, the fact is that a similar tendency was reported by Bader et al. [21]. They examined growth of the h e Cu6Sn5 phase in diffusion soldered Cu/Sn/Cu joints and found the n values to be 0.21 and 0.25 for 240 C and 300 C, respectively. Takenaka et al. [22] in Sn/Cu/Sn diffusion couples determined n ¼ 0.319 at 200 C and n ¼ 0.363 at 160 C. The growth of the h phase in diffusion soldered Cu/Ine 48 at.%Sn/Cu interconnections resulted in n parameter between 0.26 and 0.31 in the temperature range 180e250 C [23]. Such small values of n were explained in Refs. [17,21] by the morphology of the h especially in early stage of soldering. The deep grooves separating particular scallops were considered to be fast diffusion paths. Bader et al. [21] also pointed out that the grooves become larger after longer time of the process and the transport capacity decreases due to the increase of diffusion paths and reduction in the concentration gradient. However, the n parameter in Sn/Cu/Sn diffusion couples was determined after relatively long period of annealing, even 1200 h. Therefore, Takenaka et al. [22] suggested that the grain boundary diffusion may contribute to the rate-controlling process at lower annealing temperatures. They claimed that below n equal to 0.25 the process is controlled exclusively by the grain boundary diffusion and grain size of the intermetallic phase is proportional to the square root of the annealing time. In our research on (Cue5 at.%Ni)/Sn/(Cue5 at.%Ni) interconnections it is difficult to identify clearly the mass transport mechanism because of the specific way of the (Cu1xNix)6Sn5 phase growth. However, some useful information can be derived based on the results of EDX microanalysis made across the grain of (Cu1xNix)6Sn5. Fig. 3a is TEM micrograph showing an area adjacent to the Cue5 at.% Ni substrate. The grain of (Cu1xNix)6Sn5 is located between two grains of Sn e solder. The EDX microanalysis performed for such location showed (Fig. 3b) sharp (limited to few nanometers) decrease of Sn-content accompanied by simultaneous sharp increase of Cu e content. This is unlike for volume diffusion solute concentration profile and indicates that the distance for the diffusion is really limited to (Cu,Ni)6Sn5/Sn grain boundary. 4. Conclusions The studies of the growth kinetics of the (Cu1xNix)6Sn5 in the (Cue5 at.%Ni)/Sn/(Cue5 at.%Ni) diffusion soldered interconnections
revealed the large deviation of the n-parameter from 1, appropriate for the volume diffusion control if the fraction of surface transformed into the intermetallic phase is the reference. The n parameter obtained from the stereological measurements of the total area occupied by the (Cu1xNix)6Sn5 was found to be 0.27e0.15 in the temperature range 240e260 C. Such behavior is attributed the substantial contribution of grain boundary diffusion operating during whole process of soldering and confirmed by the TEM/EDX high spatial resolution microanalysis. Acknowledgments The research was financially supported by the Polish Ministry of Science and Higher Education under the grant no. 378/N-Eindhoven/2009/0. The SEM and TEM studies were performed in the Accredited Testing Laboratories at the Institute of Metallurgy and Materials Science of the Polish Academy of Sciences as well as at the Institute of Materials Engineering, Cracow University of Technology. References [1] ‘Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment’, Directive 2002/95/EC, Off. J. Eur. Union (13 February 2003). [2] M. Abtew, G. Selvaduray, Mat. Sci. Eng. R 27 (5e6) (2000) 95e141. [3] K. Zeng, K.N. Tu, Mat. Sci. Eng. R 38 (2) (2002) 55e105. [4] T. Laurila, V. Vuorinen, J.K. Kivilahti, Mat. Sci. Eng. R 49 (1e2) (2005) 1e60. [5] K. Suganuma, Curr. Opin. Solid State Mater. Sci. 5 (1) (2001) 55e64. [6] K.N. Tu, A.M. Gusak, M. Li, J. Appl. Phys. 93 (3) (2003) 1335e1353. [7] T.Y. Lee, W.J. Choi, K.N. Tu, et al., J. Mater. Res. 17 (2) (2002) 291e301. [8] S. Chada, R.A. Fournelle, W. Laub, D. Shangguan, J. Electron. Mater. 29 (10) (2000) 1214e1221. [9] A.R. Fix, G.A. Lopez, I. Brauer, W. Nuchter, E.J. Mittemeijer, J. Electron. Mater. 34 (2) (2005) 137e142. [10] P. Zie˛ ba, J. Wojewoda, Recent Research Developments in Materials Science, vol. 4, Research Signpost, Kerala, 2003, pp. 261e282. [11] W.F. Gale, D.A. Butts, Sci. Technol. Weld. Joining 9 (4) (2004) 283e300. [12] J. Wojewoda, Solid State Phenom. 138 (2008) 165e174. , J. Wojewoda-Budka, R. Filipek, P. Zie˛ ba, Arch. Metall. [13] P. Skrzyniarz, A. Sypien Mater. 55 (2010) 123e130. [14] Y.W. Wang, C.C. Chang, C.R. Kao, J. Alloys Compd. 478 (2009) L1eL4. [15] N. Nishikawa, J.Y. Piao, T. Takemoto, J. Electron. Mater. 35 (2006) 1127e1132. [16] C.H. Wang, H.T. Shen, Intermetallics 18 (2010) 616e622. [17] A. Wierzbicka-Miernik, J. Wojewoda-Budka, L. Litynska-Dobrzynska, A. Kodentsov, P. Zieba, Sci. Technol. Weld. Joining 17 (1) (2012) 32e35. [18] Web site: http://rsbweb.nih.gov/ij/. [19] S.W. Chen, S.-H. Wu, S.-W. Lee, J. Electron. Mater. 32 (2003) 1188e1194. [20] J.W. Christian, The Theory of Transformations in Metals and Alloys, Part 1, third ed., Pergamon, UK, 2002, pp. 538e546. [21] S. Bader, W. Gust, H. Hieber, Acta Metall. Mater. 43 (1) (1995) 329e337. [22] T. Takenaka, S. Kano, M. Kajihara, N. Kurokawa, K. Sakamoto, Mat. Sci. Eng. A 396 (2005) 115e123. [23] J. Wojewoda, P. Zie˛ ba, B. Onderka, R. Filipek, P. Romanów, Arch. Metall. Mater. 51 (2006) 345e353.