Ageing Effects On Shear Fatigue Life Of Solder Joint Between Pd/Ag ...

Ageing Effects on Shear Fatigue Life of Solder Joint between Pd/Ag Conductor and Sn/Pb/Ag Solder G. Y. LI and Y. C. CHAN Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong 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. To reflect the real situation in industry, the samples were prepared by assembling leadless ceramic chip carrier (LCCC) passive components on a 96% A1203 substrate and ageing at 15OOC for varying times. Mcrostructural analysis reveals that the intermetallic compounds (IMC) Pd3Sn2, Pd3Sn, PdzSn, Pd3Sn2, PdSn, PdSn2, PdSQ, AgSSn, Ag$n, PbPd3 and Pb3Pds are formed after ageing. X-ray dot maps demonstrate that the longer the ageing time, the more serious the silver and palladium dispersion into the solder and the tin Qffusion into the conductor. It is observed that the tin diffuses to the interface of the substratekonductor after 120 hours ageing. Shear strength tests with different strain rate show that the adhesion strength decreases with prolongation of ageing time. Shear cycling tests indicate that the fatigue lifetime of the solder joints depends on the diffusion range of the silver and palladium, especially the tin diffusion into the k c k film conductor. The above results mean that the more serious is the tin and silver interdiffusion, and the more IMC is formed in the solderjoint (which can be caused and formed by prolonged storage at high temperature or after long term operation of modern SMT electronic assemblies) the more sensitive is the solder joint to stress, eventually leading to possible fatigue failure of the joint. It is argued that volume change and increased brittleness caused by the intermetallic formation, volume swelling of the conductor layer suffered from tin diffusion into the conductor are major factors in the decrease of fatigue lifetime and degradation of the shear strength of the solder joints. Introduction Thick film hybrid circuit technology is widely used in hybrid microelectronics. This has been an extremely important technology because its attributions of applications, environments, and functions. Due to its increased use, especially in hostile circumstance, the failure mechanisms and reliability of surface mount solder joints require better understanding in the electronics industry. AgPd-based conductors and Sn/Pb-based solders are probably the most commonly used material system in thick film hybrid circuits [11. Palladium-tin and silver-tin intermetallics are present after soldering of thick-film conductors and components in this system [2-41. Especially during reflow soldering, the growth of the intermetallics layers is important, because of the relatively long process and

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operation times. The solid state growth process of intermetallic compounds are generally complex and continue 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 fatigued joint 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 12, 4, 6-81. However, the effects of tin diffusion, silver and palladium dispersion and intermetallic compound growth on degradation of fatigue lifetime and shear strength for real industrial assemblies are not yet well understood. The purpose of this work is to study the effects of tin diffusion, silver and palladium dispersion and intermetallic compounds growth on the shear fatigue of solder joints between thick film mixed bonded conductor Pd-Ag and solder 62Sn-36Pb-2Ag. This investigation also aims at trying to gain a better understanding of the failure mechanism of such solder joints. To reflect the behaviour in real applications, the samples are prepared by assembling LCCC passive components on a 96% AlzO3 substrate. 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 [9]. In this work, a shear cycling method is employed to test the lifetime of a solder joint prepared by the usual SMT process. Metallographic, scanning electron microscopy (SEM) and X-ray difiaction (XRD) methods are used to examine the microstructure of the solder joint at different ageing times. The results of this work may contribute a basis for predicting solder joint lifetime and checking assembly quality through monitoring the state of tin diffusion into the conductor, silver and palladium dispersion into the solder and the degree of intermetallic formation during the electronic assembly process.

Experimental procedure Commercial DuPont PdAg (ratio: 1:3) conductor 6120 was printed on a 96% A 1 2 0 3 substrate (Kyocera corporation, Japan) to form the electric circuit by using thick film printing technology. After drying at 150°C for 10 minutes, the samples were air fired in a belt furnace. The total firing cycle times is 30 minutes with 10 minutes 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 then aged in an

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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 accordng to the method described in our previous work[lO]. The microstructure of the specimens was investigated with a scanning electron microscope (SEM JSM-820) and Xray deactometer (XRD siemens D500). The shear stress and the shear cycle fatigue tests were performed using an Instron machne (INSTRON- mini 44). For the shear stress test, failure stress was determined by pulling the components and with two Merent cross head speeds of 0.005 " i n . 0.1 "/min. along the drection shown in Fig. 1. For the

Fig. 1 Schematic of the surface mount solder joint used for shear stress tests

Fig. 2 Backscattered SEM picture and x-ray elements mapping of a cross sectional view of the solder joints before ageing.

shear cyclic 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 "/min. and displacement from -0.12 to 0.12". The shear load changes were continuously sampled by a computer during the test. Results and discussions 1. Mlcrostructure 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 Merent times are illustrated in Fig. 2-5. The results show that the tin diffusion into the PdAg conductor is evident. After ageing 120 hours, it is observed that the diffused tin reaches the conductor/substrateinterface. Ag and Pd X-ray mapping reveal that the longer the ageing time the more serious the silver and palladium diffusion into the solder. The relationships between dispersion depth and ageing time are given in Fig. 6. The graphs show that the rate of silver and palladium diffusion is initially large and becomes much smaller after about 120 hours. X-ray daaction patterns of the cross-section of the solder joint after 32 days isothermal ageing at 150" are shown in Fig. 7. The XRD data reveals the coexistence of intermetallic compounds Ag5Sn, Ag3Sn, Pd3Sn2, Pd3Sn, Pd2Sn, PdSn,, P d S q PdSn, PbPd3 and Pb3Pds. 2. Fatigue and shear strength test The samples aged at 150" for 0, 2, 5, 11, 20, 32 and 47

Fig. 3 Backscattered SEM picture and x-ray elements mapping of a cross sectional view of the solder joints after 2 days ageing.

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t O t

. . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

100 200 300 400 500 600

700

800

900 1000 1100 1200

A gang times (hours)

Fig. 6 Diffusion depth of Ag and Pd Vs ageing times. days were subjected to shear strength and shear cycling tests in order to evaluate the influence of IMC, 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 percent age of survivors, i.e. [ 111:

R ( x ) = 1- F ( x )

(1)

F(x) is the lifetime distribution which can be modelled by the Weibull cumulative distribution function, x is the value of the random variable. In this work, x 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 representative peak cyclic load verus cycle number graph in Fig. 8. The load drop and fracture process lasts only about a hundred cycles. Thereby, it is reasonable to regard the number of cycles at 50% load drop as the fatigue lifetime of the solder joint [9,13,14]. Fig. 9 . shows the fatigue lifetime (average number of cycles to failure) dependence on ageing time using the 50% load drop criterion, and 5% load drop criterion for comparision. The 50% and 5% load drop lifetime curves are very close. The rate of decrease of lifetime is much large before 5 days of ageing time than after. Comparing Fig. 9 and Fig. 2-5, 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. After the diffused tin reaches the interface of the substrate/conductor and as the silver and palladium dispersion gradually slow, the rate of decrease of fatigue lifetime also become slow. The failure shear stress of solder joints as a function of the ageing time for different cross head speed is shown in Fig. 10. The trend of the curves have a turning point at 5

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g-

2- a.d. AI& Sn; 3

a

25

45

35

55

b. BaTio,; e.Ag&n; g. Pb3Pds; h. PbPd3; j. PdSn; k. Pd2Sn; m.PdSn4

75

65

c. Pb;

f.Ag3Sn; i. Pd3Sn2; I. PdSnz;

85

Diffraction angle (2-theta scale)

Fig. 7 X-ray Mraction patterns of a cross-section of the solder joint after 32 days ageing at 170OC.

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6000 -

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5000

-

4000 -

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2000

3000

4000

5000

6000

7000 8000

Shear cycles

Aging time (hrs)

Fig. 8 Representative shear peak load versus cycle number. days ageing time from fast rate of decrease to slow for both cross head speeds. It is worth noting that the diffising tin reaches the interface of the conductorhbstrate at 5 days of ageing. This may mean of that tin difision plays an important role in the degradation of the shear strength. 3. 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. Microstructural changes cause changes in the mechanical properties of the solder alloy, thereby affecting the reliability of the joints.

105

Fig. 9 The shear fatigue lifetime Vs ageing times.

SEM micrographs and X-ray element mapping of the cross-sectional views of the solder joints show that tin from the solder diffuses to the conductorhbstrate interface after ageing 120 hours and that silver and palladium dispersion into the solder are fast. X-ray diffraction patterns of the fracture surface of failured solder joints further reveal the coexistence of intermetallic compounds AgSSn, Ag3Sn, Pd3Sn2,Pd3Sn,Pd2Sn,PdSn2, PdSn4,PdSn,PbPd3 and PbsPds. It is evident that the microstructure of the solder joints undergo a big change in the ageing process. The crystal structure, lattice parameter, unit cell volume and the density of the elements and alloys Pb, Sn, Ag, Pd, Pd-

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Sn, Pb-Pd and Ag-Sn are summarized in Table 1 [16]. After tin diffuse into the conductor and the PdAg conductor disperse into the solder, intermetallics are formed, the volume of the conductor layer and the solder layer must change. The more serious the interdiffusion and the more the intermetallic compounds formed, the greater the volume changes. The fatigue lifetime and the shear strength decreased quickly prior to the diffusing tin reaching the interface of the substratekonductor and while dispersion of the silver and palladium is fast. This 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 --

eo I

t

= s

6o 40

0

100 200

300 400 500 600 700 800 900 1000 1100 1200

Aging time (hrs)

Fig. 10 Failure shear load as a function of ageing time for different cross head speeds.

Table 1 Crystal structure, lattice parameter and density of some Pd-Sn, Pb-Pd and Ag-Sn alloys Alloy

I

Clystal structure (space group)

I

Lattice Parameter (nm)

I

Density (Mg/m3)

0

Unit cell Volume (A3) 108.15

I

1

58.86

3 6 . 4 3 1 65-70 454.397

197.73

262.86

intermetallics are less ductile, and less thermally and electrically conductive [ 1I]. When these intermetallic confounds become too thick, due to the increased brittleness of the joints, the reliability of the joint can be jeopardized by cracking. 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 [ 111. The results of the fatigue lifetime tests show that the difference in the average number of the cycles to failure for the 50% load drop and the 5% load drop criterion decrease from hundreds to tens of cycles when the intermetallics layer becomes thick. T h s may mean that the fatigue lifetime of the

solder joint is related 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. In addition, when these compounds form as continuous layers at the solder-substrate interface, the intermetallics can interrupt electrical currents with their high resistivity, effectively isolating the metals that were to be electrically joined [ 1I]. Another possible degradation mechanism is related to the mode of shear deformation. It is well known that the super-plastic eutectic Sn-Pb alloy deforms by grain rotation and grain boundary sliding [17]. The IMC layer is harder

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than the bulk solder. During shear cycling, when the substrate is Qsplaced relative to the component, the shear stress may easily be concentrated at the interface of intermatallic compounds, formed hgher stress regions, increased susceptibility of the joint to cracking and leading to decrease of the fatigue lifetime. Since the interdmsion of the conductor and solder elements and subsequent intermetallics formation can greatly affect the fatigue lifetime and the adhesion strength of the solder joints, assembly quality and lifetime can be checked by monitoring the state of tin diffusion into the conductor, silver dispersion into the solder, and degree of intermetallics formation during the electronic assembly process. Conclusions The effects of tin difision, 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 62Sn36Pb-2Ag are investigated. Microstructural analysis clearly reveals the layer structure of interdiffusion of the Pd/Ag conductor and Pb/Sn/Ag solder after ageing. It is observed that tin diffused to the interface of conductorhbstrate after 120 hours ageing. X-ray diffraction results reveal the formation of the intermetallic compounds AgSSn, Ag3Sn, Pd3Sn2,Pd3Sn,Pd2Sn,PdSn2, PdSn4,PdSn,PbPd, and Pb3Pds. Conductor layer swelling caused by tin diffusion and volume changes caused by the interdiffusion and subsequent intermetallic formation are the major reasons for the degradation of the fatigue lifetime and shear strength of soldered thick film joints. The brittlenss of the intermetallic compounds layer is another important factor affecting the fatigue lifetime. The high resistivity of the intermetallic compounds layer may accelerate the degradation of the solder joints electrical contact quality. Shear stress may easily be concentrated at the interface of intermatallic compounds, forming higher stress regions, increasing susceptibility of the solder joints to cracking and leading to decrease of the fatigue lifetime. Acknowledgements The authors would like to acknowledge the financial support of a CITYU Strategic Research Grant (project no.9040106) for this work. References 1. Sea Fue Wang, Joseph P. Dougherty, Wayne Huebner and John G. Pepin, “Silver-palladium thick-film conductors”, J. Am. Ceram. Soc., 77[12], 3051-72 (1994).

2. C. J. Thwaites and M. Woodall, “Silver-palladium metallisation interactions with reflowed solder pastes”, Brazing & soldering (12), 57-60 (1987). 3. C. J. Thwaites, “Some metallurgical studies related to the surface mounting of electronic components”, circuit World 11, (l), 8-12 (1984). 4. Bi-Shiou Chiou, K. C. Liu, Jenq-Gong Duh and P. Samy Palanisamy, “Intermetallic formation on the fracture of S o b solder and PdAg conductor interfaces”, IEEE Trans. on CHMT, vo1.13, n0.2, pp.267-274, 1990 5. R. J. Klein Wassink, “Soldering in electronics”, 2nd Edition, Electrochemical Publications Limited, England 1989. 6. S. J. Muckett, M. E. Wanvick and P. E. Davis, “Thermal ageing effects between thick film metallizations and reflowed solder creams”, Plating & Surface Finish. 73, (l), 44-50 (1986). 7. R. J. Klein Wassink, “Notes on the effects of metallisation of leadless components on soldering”, Hybrid circuits (13), 9-12 (1987). 8. Bi-Shiou Chiou, K. C. Liu, Jenq-Gong Duh and P. Samy Palanisamy, “Temperature cycling effects between S o b solder and thick film PdAgl conductor metallization”, IEEE Trans. on CHMT, vol. 14, no. 1, pp.233-237, 1991 9. John H. Lau, “Solder Joint Reliability-theory and application”, Van Nostrand Reinhold New York 1991. 10. Alex C. K. So and Y , C. Chan, “Reliability Studles of surface mount solder joints--effect of Cu-Sn intermetallic compounds”, IEEE Trans. on CHMT, vo1.19, no.3, , pp.661-668, 1996. 11. Frear, S. N. Burchett, H. S. Morgan and J. H. Lau, “The Mechanics of Solder Alloy Interconnects”, Van Nostrand Reinhold New York 1994. 12. R. J. K. Wassink, “soldering in electronics”, London, U. K.: Electro-chemical Publ., 1989, ch. 4, pp 149-159. 13. H. D. Dolomon, “Isothermal fatigue of LCCPWB interconnections”, Journal of Electronic Packaging, Vol. 144, pp. 161-168, June 1992. 14. 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. on CPMT Part B, Vol. 18 Iss: 4, pp. 715-719, 1995. 15. R. G. Loasby, N. Davey and H. Barlow, “Enhanced property thick-film conductor pastes”, Solid-state Technol., pp.46-50, 1972. 16. Villars, A. Prince and H. Okamoto, “Handbook of Ternary Alloy Phase Diagrams”, ASM International, The Materials Information Society, 1995 17. D. Frear, D. Grivas, J. W. Morris, “A Microstructure study of the thermal fatigue failures of 60Sn-40Pb solder joints”, J. of Electronic Materials, vol. 17, No. 2, pp. 171180,1988.

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