Degradation of solderability of electroless nickel by phosphide particles

Available online at www.sciencedirect.com

Surface & Coatings Technology 202 (2007) 268 – 274 www.elsevier.com/locate/surfcoat

Degradation of solderability of electroless nickel by phosphide particles Jian-Jun Guo a , Ai-Ping Xian a , Jian Ku Shang a,b,⁎ a

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China b Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA Received 26 February 2007; accepted in revised form 13 May 2007 Available online 24 May 2007

Abstract Wetting of electroless nickel by the eutectic SnAgCu solder alloy was investigated in the as-deposited and annealed conditions. In the as-deposited state, P content in the nickel had a minimal effect on the solderability of the electroless nickel. Upon annealing, numerous nickel phosphide particles precipitated out of the electroless nickel with a high-P content. The presence of these phosphide precipitates reduced the solderability of the electroless nickel both in terms of the wetting force and wetting kinetics. While weakening in the wetting force is related to the inferior contact condition, the slower wetting kinetics is explained in terms of the reduction in the dissolution rate of the electroless nickel from non-soluble phosphide particles. © 2007 Elsevier B.V. All rights reserved. Keywords: Solderability; Electroless nickel; Wetting; Pb-free solder; SnAgCu alloy

1. Introduction Flip-chip packages are being widely used in the microelectronic industry because they offer one of the highest I/O number densities among the current packaging technologies [1]. A basic element of the flip-chip packaging consists of a solder bump sitting atop an under bump metallization (UBM). While Cu-based UBMs have found widespread applications in the past decades, their rapid reaction with solders has raised serious reliability concerns [2,3], especially with high melting-temperature Pb-free solders. Ni-based alloys provide attractive alternative UBMs to Cu in the high density packaging technology, because of their slower reaction with most solders [4–6]. When deposited by catalytic reduction without electric current, electroless nickel (EN) offer additional advantages such as selective deposition, uniform thickness, low residual intrinsic stress, and high conformity. As a diffusion barrier and/or UBM, electroless nickel has become an essential element of the device packaging structures based on Cu/Ni/Au. Unlike the Ni films prepared by vacuum deposition or electrolytic plating, EN is often made as an alloy, most commonly from the Ni–P system. During reflow soldering, phosphorus (P) in the EN plating not only influences the interfacial reactions with ⁎ Corresponding author. Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA. E-mail address: [email protected] (J.K. Shang). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.05.055

solders [5,7], but can also change the wetting process so that solderability of EN may be quite different from that of pure Ni. Young et al. [6] studied the static wetting of EN by Sn–Bi and Sn-

Fig. 1. Schematic illustration of wetting process in solderability test.

J.-J. Guo et al. / Surface & Coatings Technology 202 (2007) 268–274

269

Fig. 2. EDX spectra of nickel deposits: (a) Ni, (b) Ni–5.4P, and (c) Ni–16.6P.

rich alloys and found that the wetting angle decreased with P content and soldering temperature, but increased with high temperature annealing time. However, the solderability of a metal is often measured by both the wetting force and by kinetics of wetting. It is not clear how the static wetting angle may be directly related to the wetting force. Furthermore, wetting kinetics of Pbfree solders on EN must also be considered to assess how P may affect the solderability of EN. In this study, a wetting balance was used to determine the solderability of ENs with various P contents in the as-deposited and annealed state. In the as-deposited condition, we found that P content had very little effect on solderability of EN both in terms of the maximum wetting force and the wetting time required for the wetting force to balance the buoyancy force. A

significant effect of P concentration on solderability was only found when high-P EN was annealed to induce precipitation of numerous Ni3P particles. The degradation in solderability from Ni3P particles is explained by analyzing wetting forces and dissolution kinetics. 2. Experimental procedures Nickel plating was deposited on a high-purity copper substrate. Prior to deposition, Cu sheets in dimension of 20 mm × 10 mm × 0.12 mm were polished to 1000 mesh grit and then electropolished for 2 min. Pure Ni plating was prepared by electrodeposition and Ni–P alloys were deposited by electroless plating.

270

J.-J. Guo et al. / Surface & Coatings Technology 202 (2007) 268–274

NaH2PO2·H2O, and 0.002 M of C12H25NaSO4 [8]. The P content in the plating was adjusted by controlling the pH value of the solution (pH = 4 for high P and pH = 6.5 for low P). The deposition time was about 1 h, resulting in a thickness of about 6–10 μm for all platings. After deposition, select platings were annealed at 400 °C for 4 h under a vacuum of 10− 3 Torr. The structures of the nickel deposits were determined from a Rigaku D/MAX-2400 X-ray diffractometer with Cu Kα radiation (λ = 0.15418 nm). The surface morphology of the asdeposited plating was examined by a scanning electron microscope (SEM) and by the JB-4C surface roughness tester. Compositional analysis was conducted using energy dispersive X-ray spectroscopy (EDS, Oxford link ISIS 300). The fine microstructure of the plating was examined by the Philip EM420 transmission electron microscope (TEM).

Fig. 3. XRD patterns of Ni platings: a) Ni, b) Ni–5.4P, and c) Ni–16.6P.

In the electrodeposition, a nickel plate was used as the anode and the polished Cu substrate as the cathode. The electrodeposition was carried out in an electrolyte consisting of 0.575 M of NiSO4·6H2O, 0.52 M of H3BO3, 0.24 M of NaCl, 0.006 M of C7H5NO3S, and 0.002 M of C12H25NaSO4 at 45 °C for 1 h under the current of 1.5 A/m2. The solution for electroless deposition was composed of 0.076 M of NiSO4·6H2O, 0.034 M of C6H5O7Na3·2H2O, 0.183 M of CH3COONa, 0.094 M of

Fig. 4. SEM images of Ni platings: a) Ni, b) Ni–5.4P, and c) Ni–16.6P.

J.-J. Guo et al. / Surface & Coatings Technology 202 (2007) 268–274

271

Fig. 5. Wetting curves of SnAgCu solder on Ni platings at 257 °C with RMA flux.

Solderability of nickel platings was determined by a wetting balance. In the wetting balance, the force imposed by the molten solder upon a test specimen is measured as a function of the wetting time, as the specimen is immersed into, held in, and pulled out of, a solder bath. A typical wetting balance curve is shown in Fig. 1. The maximum wetting force, Fmax, is the static force when an equilibrium state is reached during wetting tests. The wetting kinetics is measured by the time required to reach a certain wetting force. For example, as marked on the wetting curve, t0 indicates the time it takes when the wetting angle reaches 90° and is measured from the instant when the solder first comes to contact with the substrate to the time when the wetting force is equal to the buoyancy force. The wetting balance measurements were made on a SKC-8H commercial instrument. To remove the oxide film and contaminants on the sample surface, test coupons were first dipped into a 4% HCl solution, washed in distilled water, cleaned by alcohol, and dried by air flow. A pot of liquid Sn3.8Ag0.7Cu eutectic solder (also known as SAC) was then prepared by melting the solder stock in a furnace attached to the wetting balance. During tests, the liquid solder was kept at a constant temperature of 257 °C (40 °C above the melting point of the solder). For each plating condition, five specimens were tested and the results from them averaged. After the wetting tests, SnAgCu solder layers on select samples were removed by etching in a solution of FeCl3, HCl and ethanol to expose the intermetallic compounds (IMCs) for direct analysis by SEM. 3. Results and discussion The EDS spectra of the nickel platings are shown in Fig. 2. The electrodeposition resulted in a pure Ni coating. The Table 1 Solderability parameters of SnAgCu on as-deposited Ni platings θmin (degree) γLF (mN/m)

Ni

Ni–5.4P

Ni–16.6P

42.6 ± 0.5 437 ± 7

43.2 ± 0.8 420 ± 7

41.9 ± 0.6 423 ± 9

Fig. 6. Wetting curves on annealed Ni platings: a) Ni and b) Ni–16.6P.

electroless platings contained 5.4 at.% P and 16.6 at.% P, hereafter referred to as low-P and high-P electroless nickels. The diffraction patterns of the EN platings are shown in Fig. 3. Both pure Ni and low-P platings produced sharp diffraction peaks, which can be indexed to the face-centered crystal (FCC) structure. For pure nickel plating, the preferred plane was (200) crystal plane, while (111) plane became the preferred crystal plane in the Ni–5.4P plating. For high-P EN, a broad hump was observed in the XRD analysis, typical of an amorphous structure. These observations agree with the previous reports that the supersaturation of P in Ni has resulted in the variation of the structure from crystalline to amorphous as the P content increases [8,9]. In the as-deposited state, all platings had a shiny and smooth surface. From the analysis of JB-4C surface roughness tester, the average roughness of Ni, Ni–5.4P and Ni–16.6P was very small, about 0.114 μm, 0.067 μm and 0.088 μm, for the three platings respectively. The low roughness values of the platings are consistent with the SEM observations in Fig. 4. Since the surfaces of platings were rather smooth, the effect of the roughness on wetting should be small [10] (as confirmed by the solderability measurements below). The wetting curves for the three nickel platings in the asdeposited state are presented in Fig. 5 using the SnAgCu eutectic solder as the wetting liquid phase. Despite the presence

272

J.-J. Guo et al. / Surface & Coatings Technology 202 (2007) 268–274

Fig. 8. XRD pattern of Ni–16.6P plating after annealing at 400 °C for 4 h.

force and the buoyancy force. The wetting angle between the liquid solder and the substrate can be calculated from Eq. (1) [11–13]: " h ¼ sin  1

4w2  ð qgPH 2 Þ

2

4w2 þ ð qgPH 2 Þ2

# ð1Þ

where H is the height of the solder rise on the side of the test sample, ρ is the solder density, g is the acceleration of gravity, and P is the perimeter of the sample. The interfacial tension between the solder and flux, γLF, may be determined from Eq. (2) [11–13]: ! qg 4w2 2 gLF ¼ þH : ð2Þ 4 ð qgPH Þ2 Table 1 lists the values of the minimum wetting angle corresponding to the maximum wetting force and the solder surface tension as determined from Eqs. (1) and (2). The contact angle of the SnAgCu solder on all substrates was about 43° irrespective of the P concentration. The surface tension, γLF, Fig. 7. Intermetallic compounds at the solder interfaces with: a) Ni, b) Ni–5.4P, and c) Ni–16.6P.

of P and the difference in the atomic structure, all three platings exhibited very similar solderability. The maximum wetting forces were nearly identical and the times to return to the zero wetting force were also very close for the three platings. The rise of the wetting force to the maximum was slightly faster for the electroless nickel. This may be related to a slightly smoother surface for the electroless nickels. When the steady-state wetting force is reached, the weight of the solder meniscus, w, is balanced by the maximum wetting Table 2 Composition of intermetallic compounds at the solder interfaces with Ni platings

Ni Ni–5.4P Ni–16.6P

Ni

Cu

Ag

Sn

24.4 28.8 21.5

32.2 31.0 34.8

2.6 3.4 1.0

40.5 36.8 42.7

Fig. 9. TEM dark field image of high-P electroless Ni after annealing (courtesy of Dr. P. L. Liu).

J.-J. Guo et al. / Surface & Coatings Technology 202 (2007) 268–274

Fig. 10. Schematic of transport processes at the wetting tip.

of the SnAgCu in the presence of the flux was about 430 mN/m. This is higher than the surface tension of SnPb eutectic alloy but slightly smaller than that of SnAg eutectic alloy [14,15]. While the nickel substrates were annealed, clear differences in wetting behavior were observed. As shown in Fig. 6, pure Ni showed a minimal effect of annealing on the wetting properties. However, a significant degradation of wetting properties occurred for the high-P electroless Ni after annealing. The maximum wetting force was reduced by roughly 30% (from 296 mN/m to 223 mN/m) and the wetting time to the zero force nearly doubled from 0.75 s to 1.14 s. On the low-P electroless Ni, the degradation was about the same as on the Ni plating. In a reactive wetting system, the contact angle is determined not only by the balance of interfacial tensions, but also by the physicochemical properties of the final state at the interface so that the contact angle is modified as follows [16]: cos h ¼ cos h0 

Dgr þ Ddr dlv

ð3Þ

where θ0 is the contact angle without reaction, ▵gr is the Gibbs energy released by the interfacial reaction, ▵δr is the interaction between the initial phases and the reaction product phases, and δlv is the surface tension of solder. The reaction products at the interfaces are shown in Fig. 7, where the intermetallic compounds were exposed by chemical removal of the solder alloy. The IMC

Fig. 11. Wetting kinetics as a function of temperatures.

273

assumed an elongated morphology. The individual IMC grains were slightly smaller for the low-P EN. The compositions of IMC identified by EDS analysis are listed in Table 2. On electroless Ni, no P was detected in the IMC layer. The atom ratios of (Cu,Ni) to Sn were similar to that of the Cu6Sn5 phase. The weak Ag signals in the EDS analysis came from residual solder attached to the IMC after chemical etching. Therefore, the IMC is identified as the (Ni, Cu)6Sn5 phase. The same phase was also found at the interface of solder and Ni-based UBMs [17–22]. Since P did not participate in the interfacial reaction [23–25], it is not surprising that the wetting angle did not change much when P was dissolved in Ni as in the electroless platings. When Ni platings were annealed, some grain growth is expected in the pure Ni sample. The grain growth should increase the surface roughness slightly, resulting in a very small change in solderability as shown in Fig. 6a. However, annealing of electroless Ni allowed P to diffuse out of the Ni lattice. In the high-P plating, Ni3P phase precipitates out as shown by the XRD patterns in Fig. 8 and by the TEM dark field image in Fig. 9. The volume fraction of Ni3P particles was about 0.15. The Ni3P particles were randomly dispersed in the Ni matrix. Some of the Ni3P precipitates grew to about 40 nm. When in contact with a liquid solder, Ni3P particles may influence the wetting behavior on electroless Ni in several ways. First, the surface tension of Ni3P should be smaller than pure Ni, leading to a greater wetting angle according to the Young's equation and correspondingly smaller equilibrium wetting force, as shown in Fig. 6b. Secondly, Ni3P is chemically more stable than Ni so that the reduction in the wetting angle by interfacial reaction would be smaller as suggested by Eq. (3). Thirdly, a distribution of non-wetting Ni3P particles along the wetting front is expected to result in local pinning of the wetting front. Fourthly, Ni3P phase is not as easily dissolved by the liquid solder as Ni is. Since the dissolution of the substrate metal is essential to the continuous fast reaction of Sn and Ni at the wetting tip as schematically shown in Fig. 10, wetting kinetics is expected to slow down when the dissolution rate is decreased by the presence of Ni3P particles.

Fig. 12. Comparison of the predicted wetting times and experimental value.

274

J.-J. Guo et al. / Surface & Coatings Technology 202 (2007) 268–274

The effect of the second phase particles on the wetting kinetics may be analyzed if the dissolution is assumed to be the controlling step in the wetting kinetics. The dissolution increases the Ni concentration in the liquid, which may be estimated from the Nernst–Brunner theory of dissolution [26] as follows:

the approximate nature of the analysis, the agreement is remarkable.

n ¼ 1  eat ns

Solderability of electroless Ni was determined by wetting balance measurements and compared to that of electrodeposited Ni. In the as-deposited state, P in the solid solution had a minimal effect on wetting force and kinetics. Upon high temperature annealing, Ni3P precipitated out of the Ni matrix, resulting in degradation of solderability. While the reduction in the wetting force may be related to the increased contact angle, the slow down in the wetting kinetics is attributed to the inhibition of the substrate dissolution by the liquid solder.

ð4Þ

where n is the concentration at the time of t, ns the saturation concentration, and α, the rate constant. For pure Ni, the time, t0, to reach a critical Ni concentration is: 1

n ns

j ¼e

at0

ð5Þ

c

For dilute solutions, high order terms may be dropped and the rate constant follows an exponential function of temperature so that Eq. (5) may be written as t0 ¼ AeQ=kT

ð6Þ

where Q is the activation energy for dissolution, k, the Boltzmann constant, T, the absolute temperature, and A, a pre-exponential constant. Under the assumption that the dissolution is the ratecontrolling step, the wetting time could be approximated by the dissolution time in Eq. (6). A plot of the wetting time vs. the wetting temperature is presented in Fig. 11, where the activation energy was found to be 89 kJ/mol, nearly the same as the activation energy for Sn diffusion (92 kJ/mol). When the dissolution theory is applied to a composite structure made of two species, Ni3P and Ni, Eq. (5) can be written as: n a1 t ð1−e−a2 t Þ ¼ ns f1 ð1−e−a2 t Þ þ f2 a1 t

ð7Þ

where f is the volume fraction and subscripts 1 and 2 refer to Ni3P and Ni respectively. To reach the same critical Ni concentration at time, tc, Eq. (8) must hold: 1  ea2 t0 ¼

a1 tc ð1  ea2 tc Þ f1 ð1  ea2 tc Þ þ f2 a1 tc

ð8Þ

which relates the wetting time, tc, for the composite structure to the wetting time, t0, of the pure Ni. For the pure Ni, t0 = 0.8 s. The wetting time for the annealed high-P electroless Ni could be predicted from Eq. (8) if the two rate constants, α1 and α2, are known. Since these two constants are not available, the predictions of Eq. (8) are presented in Fig. 12 for a range of α2/α1 ratios. For the annealed high-P electroless Ni, where f1 = 0.15, f2 = 0.85, the predicted wetting times range from 1 s to 2 s over a wide range of dissolution rate constants, α2. While a more accurate analysis would require careful measurements of the two rate constants for pure Ni and Ni3P, the predictions are not too far from the measured wetting time of 1.1 s. In view of

4. Conclusions

Acknowledgment The support for this study was provided by the National Basic Research Program of China under grant no. 2004CB619306. The helpful discussions with Dr. L. Zhang are greatly appreciated. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]

L. Matthew, J. Vardaman, Proc. SPIE Int. Soc. Opt. Eng. 4587 (2001) 571. M. Abtew, G. Selvaduray, Mater. Sci. Eng. R. 27 (2000) 95. K.N. Tu, J. Appl. Phys. 93 (2003) 1335. P.L. Liu, Z. Xu, J.K. Shang, Metall. Mater. Trans., A Phys. Metall. Mater. Sci. 31 (2000) 2857–2866. M. He, A. Kumar, P.T. Yeo, G.J. Qi, Z. Chen, Thin Solid Films 462–463 (2004) 387. B.L. Young, J.G. Duh, B.S. Chiou, J. Electron. Mater. 30 (2001) 543. F. Li, C. Liu, A. Xian, J.K. Shang, Acta Metall. Sin. 40 (2004) 815. H.S. Yu, S.F. Luo, Y.R. Wang, Surf. Coat. Technol. 148 (2001) 143. E.V. Makhsoos, J. Appl. Phys. 51 (1980) 6366. F.G. Yost, J.R. Michael, E.T. Elsenmann, Acta Metall. Mater. 43 (1995) 299. E.P. Lopez, P.T. Vianco, J.A. Rejent, J. Electron. Mater. 34 (2005) 299. E.P. Lopez, P.T. Vianco, J.A. Rejent, J. Electron. Mater. 32 (2003) 254. P.T. Vianco, J.A. Rejent, J. Electron. Mater. 28 (1999) 1138. I. Artaki, A.M. Jackson, P.T. Vianco, J. Electron. Mater. 23 (1994) 757. J. Glazer, J. Electron. Mater. 23 (1994) 693. J. Chen, M. Gu, F. Pan, J. Mater. Res. 17 (2002) 911. C.E. Ho, R.Y. Tsai, Y.L. Lin, C.R. Kao, J. Electron. Mater. 31 (2002) 584. W.C. Luo, C.E. Ho, J.Y. Tsai, Y.L. Lin, C.R. Kao, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 396 (2005) 385. W.T. Chen, C.E. Ho, C.R. Kao, J. Mater. Res. 17 (2002) 263. D.Z. Li, C.Q. Liu, P.P. Conway, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 391 (2005) 95. Y.D. Jeon, S. Nieland, A. Ostmann, H. Reichl, K.W. Paik, J. Electron. Mater. 32 (2003) 548. A. Zribi, A. Clark, L. Zavalij, P. Borgesen, E.J. Cotts, J. Electron. Mater. 30 (2001) 1157. M. He, Z. Chen, G.J. Qi, Acta Mater. 52 (2004) 2047. M.O. Alam, Y.C. Chan, K.C. Hung, Microelectron. Reliab. 42 (2002) 1065. M.L. Huang, T. Loeher, D. Manessis, L. Boettcher, A. Ostmann, H. Reichl, J. Electron. Mater. 35 (2006) 181. S. Chada, R.A. Fournelle, W. Laub, D. Shangguan, J. Electron. Mater. 29 (2000) 1214.