398 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY PART B, VOL. 21, NO. 4, NOVEMBER 1998

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1 398 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY PART B, VOL. 21, NO. 4, NOVEMBER 1998 Aging Effects on Shear Fatigue Life and Shear Strength of Soldered Thick Film Joints G. Y. Li and Y. C. Chan, Senior Member, IEEE Abstract The effects of tin diffusion, silver and palladium dispersion, and intermetallic compound growth on the shear fatigue of solder joints between thick film mixed bonded conductor Pd Ag and solder 62Sn 36Pb 2Ag are investigated. Microstructural analysis reveals that the intermetallic compounds (IMC s) Pd 3 Sn 2,Pd 3 Sn, Pd 2 Sn, Pd 3 Sn 2, PdSn, PdSn 2, PdSn 4,Ag 5 Sn, Ag 3 Sn, PbPd 3, and Pb 3 Pd 5 are formed after aging. X-ray dot maps demonstrate that the longer the aging time, the more serious the silver and palladium dispersion into the solder and the tin diffusion into the conductor. It is observed that the tin diffuses to the interface of the substrate/conductor after 120 h aging. Shear strength tests with different strain rate show that the adhesion strength decreases with prolongation of aging time. Shear cycling tests indicate that the fatigue lifetime of the solder joints depends on the diffusion depth of the silver and palladium, especially the tin diffusion into the thick film conductor. The above results mean that the more serious is the tin and silver interdiffusion, and the more IMC s is formed in the solder joint (which effects are the results of prolonged storage at high temperature or of long term operation in real SMT assemblies), the more sensitive is the solder joint to stress. Eventually fatigue failure of the joint may result. It is argued that volume change and increased brittleness caused by the intermetallic formation, and volume swelling of the conductor layer due to tin diffusion, are major factors in the decrease of fatigue lifetime and degradation of the shear strength of the solder joints. Index Terms Intermetallics, reliability, shear fatigue, shear strength, SMT, solder joints, thick film conductor. Fig. 1. Schematic of the surface mount solder joint used for shear stress tests. I. INTRODUCTION THICK film hybrid circuit technology is widely used in hybrid microelectronics. This has been an extremely important technology because of the applications in which it is used, robustness to hostile environments and its functionality. Due to its increased use, especially in hostile environments, the failure mechanisms and reliability of thick film hybrid circuit surface mount solder joints require better understanding in the electronics industry. Ag/Pd-based conductors and Sn/Pbbased solders are probably the most commonly used material system in thick film hybrid circuits [1]. Palladium tin and silver tin intermetallics are present after soldering of thickfilm conductors and components in this system [2] [4]. Especially during reflow soldering, the growth of the intermetallics layers is important, because of the relatively long process and operation times. The solid state growth processes of intermetallic compounds are generally complex and continue Manuscript received July 22, 1997; revised April 9, This work was supported by a Hong Kong Research Grants Council CERG grant under Project The authors are with the Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong. Publisher Item Identifier S (98)09149-X. Fig. 2. Shear load versus displacement at different cross head speed. in the long term during the service life of the connection. As intermetallic layers are brittle, such layers can have a determining effect on the mechanical properties of soldered joints, especially on the fatigue strength and lifetime [5]. Numerous investigations of tin diffusion, silver dispersion, intermetallic formation and effects of temperature cycling tests on solder joint adhesion strength have been done previously [2], [4], [6] [9]. However, the effects of tin diffusion, silver /98$ IEEE

2 LI AND CHAN: AGING EFFECTS ON SHEAR FATIGUE LIFE AND SHEAR STRENGTH 399 (a) (b) Fig. 3. (c) (d) Backscattered SEM picture and X-ray elements mapping of a cross-sectional view of the solder joints before aging. (a) (b) Fig. 4. (c) (d) Backscattered SEM picture and X-ray elements mapping of a cross sectional view of the solder joints after 2 days aging.

3 400 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY PART B, VOL. 21, NO. 4, NOVEMBER 1998 (a) (b) Fig. 5. (c) (d) Backscattered SEM picture and X-ray elements mapping of a cross-sectional view of the solder joints after 5 days aging. (a) (b) Fig. 6. (c) (d) Backscattered SEM picture and X-ray elements mapping of a cross-sectional view of the solder joints after 32 days aging.

4 LI AND CHAN: AGING EFFECTS ON SHEAR FATIGUE LIFE AND SHEAR STRENGTH 401 Fig. 7. Silver X-ray mapping of the X1000 cross section of the solder joints for different aging times: (a) before aging; (b) two days; (c) five days; (d) 11 days; (e) 20 days; and (f) 47 days. and palladium dispersion, and IMC growth on degradation of fatigue lifetime and shear strength for real industrial assemblies are not yet well understood. To prevent such degradation, these mechanisms must be probed and understood. This work aims at exploring the effects of elements diffusion and IMC growth on the shear fatigue life and shear strength of solder joints between thick film mixed bonded conductor Pd Ag and solder 62Sn 36Pb 2Ag. To reflect the behavior in real applications, the samples are prepared by assembling LCCC passive components on a 96% Al O substrate. High temperature isothermal aging is an accelerated model for surface mount assembly aging during operation and is widely used [5], [10]. To investigate the joint degradation, the aging test is employed to study the degradation behavior. Possible mechanisms for operational aging may be determined on the basis of such observations. Mechanical fatigue is important not only as a mode of solder joint failure, such as that due to vibrations, but also insofar as it can be used to describe and predict the more complex problem of thermal fatigue [11]. In this work, shear cycling is employed to test the lifetime of a solder joint prepared by the usual SMT procedures which has been subsequently isothermally aged. This experiment is one of the more effective methods of simulating the effects of fatigue in real surface mounted

5 402 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY PART B, VOL. 21, NO. 4, NOVEMBER 1998 Fig. 8. Palladium X-ray mapping of the X1000 cross section of the solder joints for different aging times: (a) before aging; (b) two days; (c) five days; (d) 11 days; (e) 20 days; and (f) 47 days. assemblies and provides a useful basis for predicting solder joint lifetimes. II. EXPERIMENTAL PROCEDURE Commercial DuPont Pd/Ag (ratio: 1 : 3) conductor 6120 was printed on a 96% Al O substrate (Kyocera corporation, Japan) to form the electric circuit by using thick film printing technology. After drying at 150 C for 10 min, the samples were air fired in a belt furnace. The total firing cycle time was 30 min with 10 min of peak firing at a temperature of 850 C. Surface mount passive components (LCCC 1812 capacitor) were then assembled on the substrates by means of standard infrared reflow using solder paste 62Sn 36Pb 2Ag (Electro- Science Laboratories, INC. USA: NC-3701-J-90). The test samples were aged in an oven at 150 C for periods of 0, 2, 5, 11, 20, 32, and 47 days. The metallographic preparation of the solder joints was done according to the method described in our previous work [12]. The microstructure of the specimens was investigated with a scanning electron microscope (SEM JSM-820) and X-ray diffractometer (XRD Siemens D500). Element diffusion depth measurements were taken from the samples, the mean being recorded of 10 readings taken at different locations on each sample. The shear stress and the cyclic shear fatigue tests

6 LI AND CHAN: AGING EFFECTS ON SHEAR FATIGUE LIFE AND SHEAR STRENGTH 403 Fig. 9. Diffusion depth of Ag and Pd versus aging times. were performed using an Instron machine (INSTRON-mini 44) at room temperature. The schematic of the surface mount solder joint used for shear stress tests is shown in Fig. 1. To estimate the fatigue failure of the solder joint arising from mechanical and thermal strains, the ideal cyclic shear testing should be performed with a load imposed in a direction parallel to the line joining the component terminations. In this work we were unable to apply such a load without damaging the joints, due to the presence of solder at the component terminations after assembly on the substrate. For this reason, the shear test was performed as shown in Fig. 1. Although the strain tensor is different in the solder joints for the two force directions, the fatigue failure data can still provide a useful basis for exploring the effects of microstructure changes on the shear fatigue failure of the solder joints [13]. For the shear stress test, failure stress was determined by pulling the components with two different cross head speeds of mm/min and 0.1 mm/min along the up direction shown in Fig. 1. A typical recorded shear load behavior is shown in Fig. 2. It is obvious that the results of the strength tests are dependent on the strain rate (cross head speed), with increase of cross head speed from to 0.1 mm/min leading to an increase in shear strength from 70 to 105 N. For the cyclic shear fatigue life test, a cyclic shear load was imposed on the LCCC components to move the components up and down relative to the substrate at a constant speed of 8 mm/min and maximum displacement from 0.12 to 0.12 mm. The shear load changes were continuously sampled by a computer during the test. III. RESULTS AND DISCUSSIONS A. Microstructure of Soldered Thick Film Joints SEM micrographs and X-ray element mapping of crosssectional views of surface mount solder joints aged at 150 C for different times are illustrated in Figs Silver dot mapping of solder joints before aging, shown in Fig. 3, reveals that a distinct layer of metallization conductor remains next to the ceramic substrate. The thickness of the metallization layer was approximately 10 mm. Figs. 3 6 are of the X-ray maps for lead, tin, and silver, respectively, taken from the same area Fig. 10. X-ray diffraction patterns of the fracture surface of the failure solder joint after 32 days aging at 150 C. of the backscattered SEM electron image. Comparing the tin and silver X-ray maps, tin diffusion into the Pd/Ag conductor is evident. It may be observed that the diffused tin reaches the conductor/substrate interface after aging for 120 h. Ag and Pd X-ray mapping (Figs. 7 and 8) reveal that the longer the aging time the more serious the silver and palladium diffusion into the solder. The relationships between mean diffusion depth and aging time are given in Fig. 9. The graphs show that the rate of silver and palladium diffusion is initially large and becomes smaller after about 120 h, in an approximately parabolic time dependence. The diffusion rate of the silver in solder joints can be determined and was found to be nm/s. X-ray diffraction (XRD) patterns produced by the fracture surface of the solder joint after 32 days isothermal aging at 150 C are shown in Fig. 10. The XRD data reveals the coexistence of intermetallic compounds Ag Sn, Ag Sn, Pd Sn, PdSn, Pd Sn, PdSn, PdSn, PdSn, PbPd, and Pb Pd. However, no evident layer structure is observed in the IMC s from the backscattered SEM image. Because the silver palladium metallization reacts strongly with the solder alloy, the intermetallic compounds may exist throughout the total tin and silver interdiffusion thickness. B. Fatigue and Shear Strength Test Samples aged at 150 C for 0, 2, 5, 11, 20, 32, and 47 days were subjected to shear strength and shear cycling tests in order to evaluate the influence of IMC formation, tin diffusion, and silver and palladium dispersion on the solder joint adhesion strength and failure. Mathematically, reliability is defined as the probability of a product (a solder joint in this work) performing its intended purpose without failure for a specified period of time under a given operating condition. Numerically, reliability is the percentage of survivors, i.e., [10] where is the lifetime distribution which can be modeled by the Weibull cumulative distribution function, and is the value of the random variable. In this work, is defined as the number of cycles to failure of the solder joint. In practice, however, we look for a drop in shear load. Fracture occurs soon after load drop during cycling, as demonstrated by the (1)

7 404 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY PART B, VOL. 21, NO. 4, NOVEMBER 1998 Fig. 11. Representative shear peak load versus cycle number. Fig. 13. Shear fatigue lifetime as a function of total interdiffusion thickness. Fig. 12. The shear fatigue lifetime versus aging times. representative peak cyclic load versus cycle number graph in Fig. 11. The load drop and fracture process lasts only about a hundred cycles, therefore it is reasonable to regard the number of cycles at 50% load drop as the fatigue lifetime of the solder joint [9], [14], [15]. Fig. 12 shows the mean fatigue lifetime (mean number of cycles to failure) dependence on ageing time using the 50% load drop criterion, and a 5% load drop criterion for comparison. The 50 and 5% load drop lifetime curves are very close. The shorter the aging time, the greater the probable number of cycles to failure. The rate of decrease of lifetime slows with increasing aging time. Comparing Fig. 12 and Figs. 3 6, it is evident that the high rate of decrease corresponds to both tin diffusion prior to reaching the interface of the substrate/conductor and the stage of faster dispersion of silver and palladium. Fig. 13 displays the quantitative relationship between shear fatigue life (N %) and total interdiffusion thickness. It is evident that the thinner the total interdiffusion, the greater the probable numbers of cycle to failure. This quantitative relationship is useful for fatigue life prediction. The failure shear stress of solder joints as a function of the aging time for different cross head speed is shown in Fig. 14. The trends of the curves show an initial fast rate of decrease for both cross head speeds. It is worth noting that the Fig. 14. Failure shear load as a function of aging time for different cross head speeds. diffusing tin reaches the interface of the conductor/substrate at five days of aging. This correspondence may mean that tin diffusion plays an important role in the degradation of the shear strength. For the shear strength test, most failures occur at the substrate/conductor and solder/conductor interfaces. C. Mechanism of Fatigue Failure and Shear Strength Loss The above results indicate that the fatigue lifetime and the shear strength is directly dependent on microstructural changes in the solder alloy. The microstructure of the solder joints evidently undergoes a big change in the aging process. Microstructural changes cause changes in the mechanical properties of the solder alloy, thereby affecting the reliability of the joints. The crystal structures, lattice parameters, unit cell volumes, and the densities of the elements and alloys Pb, Sn, Ag, Pd, Pd Sn, Pb Pd, and Ag Sn are summarized [16] in Table I. After tin diffuses into the conductor and the Pd/Ag conductor disperses into the solder, intermetallics are formed, which change the volume of the conductor and solder layers. The more serious the interdiffusion and the more intermetallics is formed, the greater the change in volume. The experimental observation that fatigue lifetime and the shear strength de-

8 LI AND CHAN: AGING EFFECTS ON SHEAR FATIGUE LIFE AND SHEAR STRENGTH 405 TABLE I CRYSTAL STRUCTURE, LATTICE PARAMETER, DENSITY, AND UNIT CELL VOLUME OF SOME ELEMENTS AND Pd Sn, Pb Pd, Ag Sn ALLOYS Alloy Crystal Structure Lattice Parameter (nm) Density (Mg/m 3 ) Unit Cell Volume (Å 3 ) (space group) Sn Tetragonal (14 1 /amd) a = c = Pb Cubic (Fm 3 m) a= Ag Cubic (Fm 3 m) a= Pd Cubic (Fm 3 m) a= Ag 3 Sn Orthorhombic (Pmmn) a = b = c = Ag 5 Sn Hexagonal (p6 3 /mmc) a = c = = PbPd 3 Cubic (Fm 3 m) a= Pb 3 Pd 5 Monoclinic (C 2 ) a= b = c = = Pd 3 Sn 2 Hexagonal (p6 3 /mmc) a = c = = 120 PdSn Orthorhombic (Pnma) a = b = c = Pd 2 Sn Hexagonal (Pnma) a = b = c = PdSn 2 Orthorhombic (Aba2) a = b = c = PdSn 4 Orthorhombic (Aba2) a = b = c = crease quickly prior to the diffusing tin reaching the interface of the substrate/conductor and while dispersion of the silver and palladium is fast, may mean that conductor layer swelling caused by tin diffusion and the volume changes caused by interdiffusion and subsequent intermetallic formation are the major reasons for the degradation of the fatigue lifetime and shear strength. The physical properties of the intermetallics differ significantly from those of the solder or the base metal intermetallics are less ductile, and less thermally and electrically conductive [10]. When these intermetallic layers become too thick, the reliability of the joint can be jeopardized by cracking due to the increased brittleness of the joints. This can be a problem especially if the joint is exposed to any mechanical forces, such as vibration, expansion, or contraction caused by variations in temperature. A generalized fatigue law for metals has been proposed on the basis of cumulative stored visco-plastic strain energy density [17]. During shear cycling, when the substrate is displaced relative to the component, the shear visco-plastic stress may easily be concentrated at the edge of the intermetallic compounds region, forming higher energy density regions, increasing susceptibility of the joint to cracking and leading to decrease of the fatigue lifetime. This idea is supported by our observation that the majority of fractures occur at the interface of conductor and solder while the XRD data reveal that there is IMC at the fracture surface. The results of the fatigue lifetime tests show that the difference in the average number of the cycles to failure between the 50% load drop and the 5% load drop criterion decreases from hundreds to tens of cycles when the intermetallics layer becomes thick. This may mean that the fatigue lifetime of the solder joint is sensitive to the brittleness of the joints, that is, the brittleness of the intermetallic compounds is another important factor affecting the fatigue life of the solder joints. IV. CONCLUSION The effects of tin diffusion, silver and palladium dispersion, and intermetallic compound growth on the shear fatigue lifetime and shear strength of solder joints between thick film mixed bonded conductor Pd Ag and solder 62Sn 36Pb 2Ag are investigated. Microstructural analysis clearly reveals the layer structure of interdiffusion of the Pd/Ag conductor and Pb/Sn/Ag solder after aging. It is observed that diffusing tin reaches the interface of the conductor/substrate after 120 h aging. X-ray diffraction results reveal the formation of the intermetallic compounds Ag Sn, Ag Sn, Pd Sn,PdSn, Pd Sn, PdSn, PdSn, PdSn, PbPd, and Pb Pd. The rate of decrease of lifetime and shear strength of the solder joints correlates with the higher dispersion rate of Ag and Pd at shorter aging time and the time at which Sn reaches the conductor and substrate interface. Conductor layer swelling caused by tin diffusion and volume changes caused by the interdiffusion and subsequent intermetallic formation may be the major reasons for the degradation of the fatigue lifetime and shear strength of soldered thick film joints. The brittleness of the intermetallic compounds layer is another important factor affecting the fatigue lifetime.

9 406 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY PART B, VOL. 21, NO. 4, NOVEMBER 1998 ACKNOWLEDGMENT The authors would like to thank Dr. D. P. Webb for editing the manuscript. [16] A. Prince and H. Okamoto, Handbook of ternary alloy phase diagrams, ASM International, The Materials Information Society, [17] J. D. Morrow, Cyclic plastic strain energy and fatigue of metals, in Proc. ASTM STP 378, Philadelphia, PA, 1964, pp REFERENCES [1] S. F. Wang, J. P. Dougherty, W. Huebner, and J. G. Pepin, Silver palladium thick-film conductors, J. Amer. Ceram. Soc., vol. 77, no. 12, pp , [2] C. J. Thwaites and M. Woodall, Silver-palladium metallization interactions with reflowed solder pastes, Brazing Soldering, vol. 12, pp , [3] C. J. Thwaites, Some metallurgical studies related to the surface mounting of electronic components, Circuit World 11, vol. 1, pp. 8 12, [4] B.-S. Chiou, K. C. Liu, J.-G. Duh, and P. S. Palanisamy, Intermetallic formation on the fracture of Sn/Pb solder and Pd/Ag conductor interfaces, IEEE Trans. Comp., Hybrids, Manufact. Technol., vol. 13, pp , June [5] R. J. K. Wassink, Soldering in Electronics, 2nd ed. London, U.K.: Electrochemical Publ., [6] S. J. Muckett, M. E. Warwick, and P. E. Davis, Thermal aging effects between thick film metallizations and reflowed solder creams, Plat. Surf. Fin., vol. 73, no. 1, pp , [7] R. J. K. Wassink, Notes on the effects of metallization of leadless components on soldering, Hybrid Circuits, vol. 13, pp. 9 12, [8] R. G. Loasby, N. Davey, and H. Barlow, Enhanced property thick-film conductor pastes, Solid-State Technol., pp , [9] B.-S. Chiou, K. C. Liu, J.-G. Duh, and P. S. Palanisamy, Temperature cycling effects between Sn/Pb solder and thick film Pd/Ag conductor metallization, IEEE Trans. Comp., Hybrids, Manufact. Technol., vol. 14, pp , Mar [10] S. N. Burchett, H. S. Morgan, and J. H. Lau, The Mechanics of Solder Alloy Interconnects. New York: Van Nostrand, [11] J. H. Lau, Solder Joint Reliability Theory and Application. New York: Van Nostrand Reinhold, [12] A. C. K. So and Y. C. Chan, Reliability studies of surface mount solder joints Effect of Cu Sn intermetallic compounds, IEEE Trans. Comp., Hybrids, Manufact. Technol., vol. 19, pp , Sept [13] Y. C. Chan, P. L. Tu, A. C. K. So, and J. K. L. Lai, Effect of intermetallic compounds on the shear fatigue of Cu/63Sn 37Pb solder joints, IEEE Trans. Comp., Packag., Manufact. Technol. B, vol. 20, pp , Nov [14] H. D. Dolomon, Isothermal fatigue of LCC/PWB interconnections, J. Electron. Packag., vol. 144, pp , June [15] Y. C. Chan, D. J. Xie, J. K. L. Lai, and I. K. Hui, Application of direct strain measurement to fatigue studies in surface solder joints, IEEE Trans. Comp., Packag., Manufact. Technol. B, vol. 18, pp , Nov G. Y. Li received the B.Sc. degree in applied physicsand the M.Sc. degree in solid state physics from Huazhong University of Science and Technology, Huazhong, China, in 1982 and 1988, respectively, and is currently pursuing the Ph.D. degree at the City University of Hong Kong, Kowloon, Hong Kong. From 1988 to 1994, he was employed as an Assistant Professor with the Department of Physics, Huazhong University of Science and Technology. His current research interests include electronic materials, device and failure mechanism, and reliability of electronic packaging and assembly. Y. C. Chan (M 85 SM 95) received the B.S. degree in electrical engineering, the M.S. degree in materials science, and the Ph.D. degree in electrical engineering, all from the Imperial College of Science and Technology, University of London, London, U.K., in 1977, 1978, and 1983, respectively. He joined the Advanced Technology Department, Fairchild Semiconductor, as a Senior Engineer. In 1985, he was appointed to a Lectureship in Electronics at the Chinese University of Hong Kong. From 1987 to 1991, he worked in various senior operations and engineering management functions in electronics manufacturing [including SAE Magnetic (Hong Kong) Ltd. and Seagate Technology]. He set up the Failure Analysis and Reliability Engineering Laboratory, SMT PCBA, Seagate Technology, Singapore, prior to joining the City University of Hong Kong as a Senior Lecturer in electronic engineering, in 1991, and was promoted to University Senior Lecturer, in He is currently a Professor in the Department of Electronic Engineering and Director of the EPA Center, City University of Hong Kong. He has successfully obtained R&D funding exceeding HK $30 million to support work in electronic packaging and assemblies, failure analysis, and reliability engineering. His current technical interests include surface mount technology, electronic materials, and component reliability.