Materials Chemistry and Physics 85 (2004) 63 67 Effects of different printed circuit board surface finishes on the formation and growth of intermetallics at thermomechanically fatigued small outline J leads/sn Pb interfaces Meng-Kuang Huang, Pei-Lin Wu, Chiapyng Lee Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10672, Taiwan, ROC Received 29 August 2003; received in revised form 17 November 2003; accepted 4 December 2003 Abstract The effects of printed circuit board (PCB) surface finish and thermomechanical fatigue (TMF) on the formation and growth of intermetallic compounds (IMCs) between Small Outline J (SOJ) leads and Sn 37Pb solder were investigated. The thickness of the IMC layer formed initially at as-soldered SOJ/Sn Pb interface over Ni/Au PCB surface finish was about 1.3 times of that over OSP PCB surface finish. The parabolic TMF cycle dependence clearly suggests that the growth processes are controlled primarily by solid-state diffusion. The diffusion coefficient for the growth of total IMC layer at SOJ/Sn Pb interface over Ni/Au PCB surface finish is the same as that over OSP PCB surface finish and, thus, the total IMC layer at the SOJ/Sn Pb interface over Ni/Au PCB surface finish is thicker than that over OSP PCB surface finish. Using the Cu Ni Sn ternary isotherm, the anomalous phenomenon that the presence of Ni retards the growth of the Cu 3 Sn layer while increasing the initial growth of the Cu 6 Sn 5 layer can be addressed. 2004 Elsevier B.V. All rights reserved. Keywords: Sn Pb; Intermetallic compound; Surface finish; Ni/Au; Thermomechanical fatigue; Cu Ni Sn ternary isotherm 1. Introduction Solder joint interconnects are mechanical means of structural support for bridging the various electronic components and providing electrical contacts and a thermal path for heat dissipation. The tin lead (Sn Pb) solder alloy has been widely used as interconnection materials in electronic packaging due to low cost, low melting temperatures and good wetting behavior on several substrates such as Cu, Ag, Pd and Au [1 8]. Though lead-free solders are under development in view of health consideration, understanding solder substrate reactions is still an important aspect of solder joint processing. Over the last several years, the microelectronics industry has shown significant interest in new PCB surface finishes. The new PCB finishes such as immersion gold over electroless nickel [9,10] (Ni/Au) and organic solderability preservative [11,12] (OSP) are evaluated for their soldering performance. Recent studies [9,10] show that Ni/Au coating provides enhanced solder wettability. The function of the Ni layer is to prevent rapid reaction between the solder and the Corresponding author. Fax: +886-2-27376644. E-mail address: cl@ch.ntust.edu.tw (C. Lee). Cu layer, which is under the Ni layer and is a part of the internal conducting trace. The purpose of gold is to prevent the oxidation of the underlying nickel layer, thereby enhancing solderability. Therefore, an evaluation of the compatibility of Sn Pb solder with the PCB and component lead terminations is necessary. Solder joints in service conditions experience internal thermal stresses due to coefficients of thermal expansion mismatches between the component leads, solder, and the substrate. Thermomechanical fatigue (TMF) occurs due to such thermal stresses that develop during temperature excursions encountered during service. As a result, mechanical deformation occurs in the solder joints. There have been studies of TMF on lead tin solder during the last decade [13 16]. The objective of the present study is to examine the impact of different PCB surface finishes and TMF on the formation and growth of intermetallic compounds (IMCs) between the Small Outline J (SOJ) leads and Sn 37Pb solder. In this study, SOJ leads/sn Pb joint was subjected to TMF between 233 and 398 K, and metallographic examination was performed to investigate the evolution of intermetallics. We also seek to better understand the fundamental aspects regarding the formation and growth of different intermetallics. 0254-0584/$ see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2003.12.003
64 M.-K. Huang et al. / Materials Chemistry and Physics 85 (2004) 63 67 2. Experimental The devices used to evaluate the integrity of the circuit board solder joints were 44 I/O Small Outline J lead integrated circuit packages. The package material was plastic. The leads were copper, with a J lead configuration. The lead pitch and width were 1.27 and 0.55 mm, respectively. Lead-frame was coated with Sn 0.7 mass% Cu solder. The printed circuit board (PCB) is 1.6 mm thick and is made of epoxy-glass multifunctional laminate. The glass transition temperature of the laminate was between 145 and 165 C. Two lead-free surface finishes of PCB were used for the study. One case used the gold over nickel (Ni/Au) finishing and the other used the organic solderability preservative surface finishing applied on the six-layer FR-4 test boards. The surface layer of Au, with a thickness of 0.08 m, is for oxidation protection, and is deposited by immersion. The inner Ni layer is deposited by electroplating, and is about 2.5 m thick. Sn 37 mass% Pb solder paste containing RMA type flux was screen printed onto the circuit boards. The components were then placed on the PCB pads by a pick and place machine. Solder reflow of the test vehicles was performed in a reflow furnace under flowing nitrogen (200 ppm O 2 ). In this study, reflow process was conducted with the use of a forced air convection oven with a reflow time of 75 s. The peak temperature of SOJ during reflow soldering process was maintained at approximately 523 K. Reflow soldering was performed under N 2 atmosphere. The use of nitrogen would improve soldering performance for lead-free materials. Fig. 1 shows the optical micrograph of SOJ/Sn 37Pb solder joints over different PCB surface finishes prior to the TMF test. Fig. 1. Optical micrograph of SOJ/Sn 37Pb solder joints over different PCB surface finishes prior to the TMF test. The TMF test was carried out between 40 and 125 C. A dwell time of 15 min was applied for both the high and the low extremes. The transfer time for temperature ramp up or down was 5 min. Each solder joint sample was sectioned by a tungsten single wire saw. Those sections were grounded with three grades of SiC papers (#500, 1200 and 2400) and then mechanically polished with two grades of diamond paste (3 and 1 m) and colloidal silica suspension. Morphologies of the interfaces were first examined using optical microscopy (OM). The mean thickness of the intermetallic compound layer was calculated from 50 thickness measurements made along the layer. Compositions of the reacted layers were determined using an electron probe microanalyzer (EPMA), and were used as the major identification of phases. 3. Results and discussion Metallographic cross-sections of the SOJ/Sn Pb interface over OSP surface finish were subjected to the following conditions: (a) as-soldered, after (b) 250, (c) 500, (d) 750, and (e) 1000 TMF cycles as shown in Fig. 2. Compared with the as-soldered sample, the IMC layer is obviously thicker after TMF cycles and another sub-layer appears after 500 TMF cycles. The intermetallic layer thickness increases as the number of TMF cycle increases. The lighter sub-layer located closest to the solder field was always present; the darker sub-layer adjoining the SOJ component appeared only after heat treatment in excess of 500 TMF cycles. Fig. 3 shows optical micrographs of the SOJ/Sn Pb interface over Ni/Au surface finish subjected to the same conditions as those of Fig. 2. The micrograph in Fig. 3 shows that the IMC layer consists of only one layer. No evidence of darker sub-layer was found within the interface. The intermetallic layers grow as the number of TMF cycles increases. Comparing Fig. 2 with Fig. 3, the thickness of the IMC layer formed initially at as-soldered SOJ/Sn Pb interface over Ni/Au PCB surface finish was about 1.3 times of that over OSP PCB surface finish. An electron probe microanalyzer was performed on the intermetallic compound layers of two samples aged after 1000 TMF cycles (Fig. 4). Fig. 4a is the EPMA backscattered electron image (BEI) taken at the SOJ/Sn Pb interface over OSP surface finish of the specimen aged after 1000 TMF cycles. The sub-layer next to the SOJ lead had the Cu 3 Sn stoichiometry. The sub-layer that bordered with the solder field had the Cu 6 Sn 5 stoichiometry. Fig. 4b shows the EPMA BEI micrograph taken at the SOJ/Sn Pb interface over Ni/Au surface finish. The layer between SOJ and Sn Pb had the Cu 6 Sn 5 stoichiometry. It was observed that Cu 3 Sn was not present in the intermetallic compound layer. The solid-state growth kinetics of the intermetallic compound layer formed between SOJ leads and Sn Pb solder over different PCB surface finishes were determined from the thickness measurements. To investigate the kinetics of
M.-K. Huang et al. / Materials Chemistry and Physics 85 (2004) 63 67 65 Fig. 2. Optical micrographs of the SOJ/Sn Pb interface over OSP surface finish subjected to the following conditions: (a) as-deposited, after (b) 250, (c) 500, (d) 750, and (e) 1000 TMF cycles. the intermetallic growth, the total thickness of the intermetallic layer, including Cu 6 Sn 5 and Cu 3 Sn at SOJ/Sn Pb interfaces TMF cycled for various times were measured and the results are shown in Fig. 5 as a function of the square root of TMF cycle. The same slope of the IMC growth curves at both SOJ/Sn Pb interface over Ni/Au surface finish and SOJ/Sn Pb interface over OSP finish indicates that the diffusion coefficients in these two systems are the same. The regression analysis indicates that the data is well-fitted with parabolic TMF cycle dependence with a linear correlation coefficient, R 2, greater than 0.95. The parabolic TMF cycle dependence clearly suggests that the growth processes are controlled primarily by the solid-state diffusion process. The intermetallic thickness follows the empirical relation x = x 0 + (Dt) 1/2 Fig. 3. Optical micrographs of the SOJ/Sn Pb interface over Ni/Au surface finish subjected to the following conditions: (a) as-deposited, after (b) 250, (c) 500, (d) 750, and (e) 1000 TMF cycles. where x is the layer thickness at time t, x 0 the initial thickness of the layer and D the nominal diffusion coefficient. The nominal diffusion coefficient is derived to be 6.83 10 15 cm 2 s 1 for both SOJ/Sn Pb interface over Ni/Au surface finish and SOJ/Sn Pb interface over OSP surface finish. This value is in good agreement with previously published data [17]. The similarity in the kinetics parameters indicated that different PCB surface finishes did not alter the solid-state growth processes both at SOJ/Sn Pb interface over Ni/Au surface finish and at the SOJ/Sn Pb interface over OSP surface finish. There were five elements present in the systems studied: Au, Pb, Ni, Cu, and Sn. The Au Pb Ni Cu Sn phase diagram would be very helpful to understand the driving force for the retardation of the growth of Cu 3 Sn at the interface.
66 M.-K. Huang et al. / Materials Chemistry and Physics 85 (2004) 63 67 Fig. 6. The Cu Ni Sn ternary isotherm at 240 C. This isotherm was adapted from Lin et al. [18]. Fig. 4. Backscattered electron images taken at the SOJ/Sn Pb interfaces of the specimens aged after 1000 TMF cycles over (a) OSP surface finish and (b) Ni/Au surface finish. Unfortunately, pentamerous phase diagrams are not available. There is, however, a very recent Sn Cu Ni ternary isotherm at room temperature determined by Lin et al. [18] reproduced in Fig. 6. The solder joint is relatively large so that the Au concentration in solder is relatively small. The present study showed that Au and Pb did not participate in the chemical reaction, and were inert as far as the chemical reaction was concerned. In the following, we assume that Au and Pb indeed did not play any significant role, and use the Sn Cu Ni ternary to rationalize the observed behaviors. We further assume that the general geometry of Sn Cu Ni isotherms at 240 and 125 C is the same. The isotherm in Fig. 6 clearly reveals that Cu 6 Sn 5 can dissolve up to 25 at.% Ni at room temperature. In the region where supply of Sn was near, a local three-phase equilibrium Sn Cu 6 Sn 5 Ni 3 Sn 4 can be reached, which corresponded to the Sn Cu 6 Sn 5 Ni 3 Sn 4 three-phase region in Fig. 6 where the phase equilibrium Sn Cu 3 Sn Ni 3 Sn 4 cannot be established. The enhancement in the initial growth of the IMC layer at the SOJ/Sn Pb interface over Ni/Au surface finish is also evident. 4. Conclusions Fig. 5. The growth of total IMC layer (including both Cu 6 Sn 5 and Cu 3 Sn) at SOJ/Sn Pb interfaces as a function of the square root of TMF cycle. The growth of the intermetallic compounds between Small Outline J leads and Sn 37Pb solder depended strongly on the printed circuit board surface finish and thermomechanical fatigue. It is found that the thickness of the IMC layer formed initially at as-soldered SOJ/Sn Pb interface over Ni/Au PCB surface finish was about 1.3 times of that over OSP PCB surface finish. The parabolic TMF cycle dependence clearly suggests that the growth processes were controlled primarily by the solid-state diffusion process. The similarity in the kinetics parameters indicated that different PCB surface finishes did not alter the solid-state growth processes both at SOJ/Sn Pb interface over Ni/Au surface finish and at SOJ/Sn Pb interface over OSP surface finish. Using the Cu Ni Sn ternary isotherm, the anomalous phenomenon that the presence of Ni retards the growth of the Cu 3 Sn layer while increasing the growth of the Cu 6 Sn 5
M.-K. Huang et al. / Materials Chemistry and Physics 85 (2004) 63 67 67 layer can be addressed as follows. In the region where the supply of Sn was closer, a local three-phase equilibrium Sn Cu 6 Sn 5 Ni 3 Sn 4 had been reached, which corresponded to the Sn Cu 6 Sn 5 Ni 3 Sn 4 three-phase region in Fig. 6 where the phase equilibrium Sn Cu 3 Sn Ni 3 Sn 4 cannot be established. The enhancement in the initial growth of the IMC layer at the SOJ/Sn Pb interface over Ni/Au surface finish is also evident. References [1] X.W. Liu, W.J. Plumbridge, J. Electron. Mater. 32 (2003) 278. [2] J. Zhao, Y. Mutoh, Y. Miyashita, T. Ogawa, A.J. McEvily, J. Electron. Mater. 30 (2001) 415. [3] K.L. Erickson, P.L. Hopkins, P.T. Vianco, J. Electron. Mater. 27 (1998) 1177. [4] G. Ghosh, J. Electron. Mater. 28 (1999) 1238. [5] M. Schaefer, R.A. Fournelle, J. Liang, J. Electron. Mater. 27 (1998) 1167. [6] C.-J. Chen, K.-L. Lin, J. Electron. Mater. 29 (2000) 1007. [7] P.L. Hacke, A.F. Sprecher, H. Conrad, J. Electron. Mater. 26 (1997) 774. [8] P.L. Hacke, Y. Fahmy, H. Conrad, J. Electron. Mater. 27 (1998) 941. [9] U. Ray, in: Proceedings of the Surface Mount International, 1995, p. 891. [10] M.S. Lee, C.M. Liu, C.R. Kao, J. Electron. Mater. 28 (1999) 57. [11] R. Ludwig, N.-C. Lee, C. Fan, Y. Zhang, Soldering Surf. Mount Technol. 14 (3) (2002) 19. [12] P. Vianco, J. Rejent, in: IEEE Proceedings of the Conference on Electronic Components and Technology, 1996, pp. 1172 1183. [13] W.W. Lee, L.T. Nguyen, G.S. Selvaduray, Microelectr. Reliab. 40 (2000) 231. [14] A. Dasgupta, P. Sharma, K. Upadhyayui, Int. J. Damage Mech. 10 (2001) 101. [15] Y.H. Pao, S. Badgley, R. Govila, E. Jih, in: S.A. Schroeder, M.R. Mitchell (Eds.), Fatigue of Electronic Materials, ASTM, Philadelphia, PA, 1994, pp. 60 81. [16] H.L.J. Pang, K.H. Tan, X.Q. Shi, Z.P. Wang, Mater. Sci. Eng. A 307 (2001) 42. [17] P.L. Tu, Y.C. Chan, J.K.L. Lai, IEEE Trans. Comp. Packag. Manufact. Technol., Part B 20 (1997) 87. [18] C.H. Lin, S.W. Chen, C.H. Wang, J. Electron. Mater. 31 (2002) 907.