Effects of Bi Content on Mechanical Properties and Bump Interconnection Reliability of Sn-Ag Solder Alloys

Transcription

1 Effects of Bi Content on Mechanical Properties and Bump Interconnection Reliability of Sn-Ag Solder Kazuki Tateyama, Hiroshi Ubukata*, Yoji Yamaoka*, Kuniaki Takahashi*, Hiroshi Yamada** and Masayuki Saito Microelectronics Packaging Research Center Corporate Manufacturing Engineering Center Toshiba Corporation 33, Shin-isogo-cho, Isogo-ku Yokohama, Japan Phone: Fax: *Packaging Technology Section, Ome Operations Digital Media Equipment & Services Company Toshiba Corporation **Display Materials and Devices Laboratory Corporate Research and Development Center Toshiba Corporation Abstract The effects of the Bi content on the mechanical properties (temperature dependence of tensile strength and elongation, stress relaxation) of Sn-Ag solder alloys were investigated in this work to optimize the Bi content. It has been shown that the addition of Bi up to 5% to Sn-Ag solder alloys increased tensile strength at 297 K, but there was no further improvement by Bi addition beyond 5%. As the temperature rises, the tensile strength enhancement by Bi addition is reduced. At 423 K, in particular, a Bi addition of 3% or more hardly affects the tensile strength. Bi addition to Sn-Ag solder alloys makes breaking elongation decrease and stress relaxation occur more easily. Therefore, in order to make the inelastic strain generated in the Sn-Ag-Bi solder bumps smaller than that in the eutectic solder bumps, it is considered necessary to limit the Bi content to 3% or less. Thus, interconnection reliability evaluation under the temperature cycle test conditions was performed on the bumps of the T-BGA mounted on the substrate in which Sn-Ag solder alloys containing 3% or less Bi was applied. The results showed that the reliability of the above solder bumps was superior to that of the eutectic solder bumps. Furthermore, it was confirmed from Finite Element model simulation results that the enhancement of the strength of the solder alloy by Bi addition improved the bump interconnection reliability of T-BGA by reducing the inelastic strain in bumps. Key words: 1. Introduction Pb-free Solder, Sn-Ag Solder, Mechanical Properties, Interconnection Reliability, and FEM Simulation. As Pb-free solder, a series of Sn-based alloys has been investigated 1 4. Among these materials, Sn-Ag eutectic solder (Sn- The International Journal of Microcircuits and Electronic Packaging, Volume 23, Number1, First Quarter 2 (ISSN ) International Microelectronics And Packaging Society 131

2 Intl. Journal of Microcircuits and Electronic Packaging Ag) has many promising characteristics, but its relatively high melting temperature may preclude its use as replacement for Pb- 2. Optimization of Bi Content in Sn-Ag Sn eutectic solder () 1,2. Therefore, Sn-Ag solder alloys in Solder which the melting point was reduced by Bi addition has been studied as a leading candidate 5 7. However, it has also been reported that Bi addition to Sn-Ag solder alloys affects mechanical properties greatly, and degrades the strain-controlled low cycle 2.1. Solder Alloy Compositions fatigue at room temperature 8. On the other hand, the solder bump Evaluated solder alloy compositions are listed in Table 1. The interconnection in electric equipment is broken by the interaction melting points were measured by a differential scanning calo- of thermal fatigue and creep, since the interconnection is rimeter (DSC). Figure 1 shows the effect of Bi addition to the exposed to periodic temperature cycles. Therefore, in order to Sn-Ag solder alloys on their melting points. The solidus- and improve the bump interconnection reliability, it is necessary to liquidus temperatures were lowered at a rate of -.8 K and -3.9 K make the inelastic strain generated in bumps smaller by raising per 1% Bi added to Sn-Ag solder alloys, the difference between the solder alloy strength. In this case, the solder alloy strength is the two temperatures ( T) becoming larger as the Bi content in divided into two parts; time-independent strength and time-dependent Sn-Ag solder alloys increases. strength. The time-independent strength dominates the strain of the temperature rising and falling periods of periodic Table 1. Compositions of solder alloys. temperature cycles, while the time-dependent strength dominates the strain during temperature hold periods of these cycles. In Composition (weight %) order to obtain the bump interconnection reliability of Sn-Ag-Bi Designation Sn Pb Ag Bi Cu solder alloys comparable or superior to that of, it is necessary to make the both strengths of Sn-Ag-Bi solder alloys equal SABC Bal Sa-5Bi Bal to or greater than those of. Sa-3Bi Bal Much work has been done on the relationship between the Bi Sa-1.5Bi Bal content in Sn-Ag solder alloys and the tensile strength, which is time-independent, at room temperature 6 SAC Bal However, it has not been investigated how the Bi addition to the alloys affects the Sn-Ag Bal temperature dependence of strength. Since the microstructure of Bal solder alloys undergoes intense variations at room temperature, their tensile strength and ductility depend greatly on temperature. For exact evaluation of the tensile strength of solder alloys, therefore, its temperature dependence needs to be investigated. Solder alloys easily suffer creep deformation, which is timedependent. 5 3 Further, the creep deformation processes in bumps during the temperature hold periods closely resemble those involved in the stress relaxation of solder alloys 2,9. However, al though the stress relaxation of Sn-Ag eutectic solder has been 15 studied 1 44, it has not been investigated how Bi addition to Sn-Ag Sn-Ag-Cu(Solidus) 1 Sn-Ag-Cu(liquidus) solder alloys affects the stress relaxation. Hence, it is necessary 42 (Solidus) 5 to perform the stress relaxation tests of the Bi-containing to Sn- (liquidus) Ag solder alloys to evaluate their time-dependent strength. 4 Sn-Ag-Cu(ƒ T) This paper describes the effects of the Bi content on the mechanical properties and the bump interconnection reliability of Sn-Ag solder alloys. Firstly, the effects of the Bi content on the temperature dependence of the tensile strength and the breaking elongation of Sn-Ag solder alloys were evaluated to optimize the Figure 1. Relation between melting points and Bi content. amount of Bi in Sn-Ag solder alloys. In addition, the effect of the Bi content on creep characteristics was evaluated by the stress relaxation tests. Then, a tape Ball Grid Array (T-BGA) was 2.2. Microstructure of Sn-Ag-Bi Solder built on the substrate using Sn-Ag solder alloys with the optimized Bi content and was evaluated for the bump interconnec- The microstructure of the solder alloys was investigated using a Scanning Electron Microscope (SEM) and an Energy Distion reliability under temperature cycle test (TCT) conditions. Finally, Finite Element model (FEM) simulations of the T-BGA persion X-ray micro analyzer (EDX). mounted on the substrate were performed to confirm whether the Figures 2(a) (d) show the microstructures of the Sn-Ag solder alloys. The microstructure of SAC consists of a matrix of the inelastic strain generated in bumps becomes smaller as the tensile strength of the solder alloys is enhanced. The International Journal of Microcircuits and Electronic Packaging, Volume 23, Number1, First Quarter 2 (ISSN ) 132 Temperature, T /K International Microelectronics And Packaging Society T, K

3 Sn phase, intermetallic Ag 3 Sn and Cu precipitates. As the Bi content is increased, Bi dissolves in the Sn phase (Figure 2 (b), (c)). When the Bi content exceeds the maximum soluble amount of Bi in the Sn phase, Bi begins to precipitate in the Sn phase (Figure 2(d)). Since the EDX results show that the amount of Bi in the Sn phase at the moment when Bi precipitates appear is in the range of %, it is clear that the maximum soluble amount of Bi in the Sn phase is about 3 4% at 297 K. (a) SAC (b) Sa-1.5Bi tensile strength. In order to evaluate the dependence of the solder alloy strength on deformation temperature, tensile strength (σ B ) has been defined as Equation (1) as a function of deformation temperature (T) considering the dislocation migration. σ A BT (1) B In this case, A is a constant and B is the temperature strengthening coefficient that indicates the degree of the tensile strength changes with temperature. The Bi content dependence of A and B is listed in Table 2. It is clear that the addition of Bi up to 5% to Sn-Ag solder alloys increases B, but there is no further change beyond 5%. The increase of B corresponds to the fact that the tensile strength decrease with a greater rate as the deformation temperature rises. Therefore, it is considered that the addition of Bi has little effect on the tensile strength at higher temperatures. (c) Sa-3Bi (d) Sa-5Bi Figure 2. Microstructures of Sn-Ag solder alloys Effect of Bi Content on Tensile Deformation Tensile test specimens had a total length of 12 mm, a gauge length of 25 mm and a thickness of 3.5 mm. Tensile tests were carried out in a deformation temperature range from 297 to 423 K. A strain rate of 1-4 s -1 was chosen for this work considering the TCT conditions. Figure 3 shows the relation between the tensile strength and the Bi content. The tensile strength of is shown on the right vertical axis. The plots on the left vertical axis are SAC s data. The tensile strengths of Sn-Ag solder alloys are higher than that of for every temperature from 297 to 423 K. At 297 K, the addition of Bi up to 5% to Sn-Ag solder alloys increased tensile strength, but there was no further improvement beyond 5%. Since the maximum soluble amount of Bi in the Sn phase is about 3 4%, the cause of tensile strength enhancement by the addition of Bi up to 3% to Sn-Ag solder alloys is presumably solution hardening. Further increase of the tensile strength by the Bi addition beyond 3% appears to be caused by precipitate dispersion hardening. Furthermore, the rate of the tensile strength increase by the addition of Bi up to 3% is larger than that by the Bi addition beyond 3%. As a result, it has been clarified that solution hardening is more dominant than precipitate dispersion hardening as the cause of tensile strength enhancement of Sn- Ag solder by Bi addition. As the deformation temperature rises, the tensile strength enhancement by Bi addition is reduced. At 423 K, in particular, a Bi addition of 3% or more hardly affects the The International Journal of Microcircuits and Electronic Packaging, Volume 23, Number1, First Quarter 2 (ISSN ) International Microelectronics And Packaging Society 133 Tensile strength, B/MPa Figure 3. Relation between tensile strength and Bi content. Table 2. Bi content dependence of A and B. Designation Bi content (%) A B SABC Sa-5Bi Sa-3Bi Sa-1.5Bi SAC Sn-Ag Breaking elongation is the main index of material ductility. Figure 4 shows the Bi content dependence of breaking elongation. The plots on the left vertical axis are SAC s data. Breaking elongation decreases with the increase in the amount of Bi addition for every temperature from 297 to 423 K. The cause is presumably that cracks are generated more easily, since the number of points where stress concentrates increases by the increase of the Bi- solute and precipitates. Moreover, when Bi precipitates arise by superfluous Bi addition to the Sn-Ag solder, the ductility of Sn-Ag-Bi solder alloys is reduced extremely at temperatures beyond 411 K since the Sn-Bi eutectic crystal arises 5.

4 Intl. Journal of Microcircuits and Electronic Packaging Therefore, it is considered necessary to restrict the amount of Bi so that no Bi precipitation takes place in the Sn phase. the inelastic strain generated in Sn-Ag-Bi solder alloy bumps smaller than that in bumps during the holding time, it is considered necessary to limit the Bi content to 3% or less. Breaking elongation, f Stress, /MPa Holding time, t /s Figure 4. Relation between breaking elongation and Bi content. From these results, it is considered that the effective Bi content in Sn-Ag solder alloys for the enhancement of bump interconnection reliability is 3% or less Effect of Bi Content on Stress Relaxation The stress relaxation tests were conducted on the same specimens as those for tensile tests under constant-strain conditions with 5% strain and in the holding temperature range from 297 to 423 K. Figure 6. Relation between relaxation stress and Bi Figure 5 shows a typical example. Most of stress relaxation content. is observed in the first 1 seconds, and then the stresses level off within a holding time of 1, seconds except for a holding In order to carry out quantitative evaluation of the stress relaxation characteristics, the percentage reduction from the initial temperature of 297 K. The time until stress relaxation is completed becomes shorter as holding temperature rises. Therefore, stress (R) has been defined as Equation (2), stress relaxation is considered to be dominant in the thermal activation process. Figure 6 shows the stress at the end point of R ( 1 ) 1 (2) σ ( t) stress relaxation (relaxation stress) at each holding temperature. σ () The relaxation stress of is shown on the right vertical axis. where σ(t) is the stress at holding time t, and σ() is the initial Data for % Bi signify those of Sn-Ag. At a holding temperature stress. Figure 7 shows the relation between the percentage reduction in stress and the Bi content. It is clear that the percent- of 323 K, the relaxation stresses of all Sn-Ag solder alloys are twice that of and do not depend much on the amount of age reduction in stress increases with the increasing Bi content. Bi. On the other hand, for holding temperatures over 373 K, it Therefore, it appears that Bi addition to Sn-Ag solder alloys makes turns out that relaxation stress becomes smaller as the amount of stress relaxation occur more easily. On the other hand, stress Bi addition to Sn-Ag solder alloys increases. This indicates that relaxation is caused by the migration of defects produced at the the thermal energy beyond 373 K is sufficient to move the defects introduced at the time of deformation. Furthermore, the time of deformation. Therefore, it is considered that the increase of the percentage reduction in stress corresponds to the increase relaxation stress of Sa-3Bi is equivalent to that of at a in the quantity of defects introduced at the time of deformation. holding temperature of 423 K, while the relaxation stress of Sa- In other words, the increase of the percentage reduction in stress 5Bi is equivalent to, but lower than, that of at holding by Bi addition to Sn-Ag solder alloys appears to be evidence that temperatures of 373 and 423 K, respectively. It can be considered that the inelastic strain generated in bumps becomes larger Bi addition to Sn-Ag solder alloys makes the strain-controlled low cycle fatigue characteristics degrade. as the relaxation stress decreases. Therefore, in order to make The International Journal of Microcircuits and Electronic Packaging, Volume 23, Number1, First Quarter 2 (ISSN ) 134 Figure 5. Stress relaxation of Sa-1.5 Bi alloy. Relaxation stress, R/MPa International Microelectronics And Packaging Society

5 Reduction in stress, R Figure 7. Relation between reduction in stress and Bi content. Consequently, it is considered that the Sn-Ag solder alloys containing 3% or less Bi is most practical for solder bump interconnection. 3. Bump Interconnection Reliability of T-BGA Applying Sn-Ag-Bi Solder In consideration of the above optimization of the Bi content in Sn-Ag solder alloys, evaluation was performed on the interconnection reliability of bumps made of the Sn-Ag-Bi solder alloys on the T-BGA mounted on the substrate under TCT conditions. The specification of the T-BGA and substrate used for evaluation is listed in Table 3. In the TCT condition, temperature is changed cyclically between 218 and 398 K with a 3 minute dwell at each temperature extreme. The samples were regarded to have failed when the resistance change of each package exceeds 1%. The change of bump resistance during TCT was monitored by the four probe method. Table 3. Specification of T-BGA and substrate. Figure 8 shows the dependence of cumulative failure on the number of TCT cycles. This result shows that the interconnection reliability of the bumps which have been made of Sn-Ag Assuming that the bump interconnection reliability is improved solder alloys containing 3% or less Bi is superior to that of the by the enhancement of the strength, the Bi content in Sn-Ag bumps. In addition, it is clear that the bump interconnec- The International Journal of Microcircuits and Electronic Packaging, Volume 23, Number1, First Quarter 2 (ISSN ) tion reliability is enhanced by the increase of the Bi content. The result agreed with expectations, indicating the bump interconnection reliability is dominated by the strength of solder alloys. Cumulative failure, % Sn-Ag-.7%Bi alloy Sn-Ag-3%Bi alloy Number of TCT cycles, N /cycle Figure 8. Dependence of cumulative failure on the number of temperature cycles. Figures 9(a) (c) show the cross-sectional photographs of bump interconnections, (a) and (b) being taken after 25 cycles and (c) after 2 cycles. It is clear that the crack becomes larger as the tensile strength of solder alloys decreases. In addition, the bump fracture occurs at the part closer to the electrode as the tensile strength of solder alloys increases. Therefore, it is considered that the mode of bump fracture changes from the fatigue destruction of solder to the destruction of the interface between the solder bump and the electrode for greater solder alloy strength. Crack (a) Sn-Ag-3%Bi alloy Crack Crack (b) Sn-Ag-.7%Bi alloy BGA type BGA ball pitch Tape-BGA 1.27 mm BGA ball number 48 BGA size 35 mm? Substrate material Substrate size Substrate thickness FR mm 1.2 mm (c) Figure 9. Cross-sectional view of T-BGA solder bump interconnection. 4. Estimation of Inelastic Strain by FEM Simulation International Microelectronics And Packaging Society 135

6 Intl. Journal of Microcircuits and Electronic Packaging solder alloys was optimized. In order to confirm this assumption, the effect of the Bi content on the inelastic strain in bumps produced under the TCT conditions was evaluated by FEM simulations, disregarding the deformation during the holding time. Linear solder alloy properties used for analysis are listed in Table 4. Figure 1 indicates a typical example of the plastic data, namely the true stress vs. true strain curve of Sa-1.5Bi used for analysis. Table 4. Linear properties. Elastic modulus( GPa ) Poisson s ratio Coefficient of thermal expansion( ppm/k) ) Sa-5Bi Sa-3Bi Sa-1.5Bi Sn-Ag Cumulative equivalent inelastic strain, Figure 11. Relation between cumulative equivalent strain and Bi content. It is concluded from the results described above that the enhancement of tensile strength by Bi addition to Sn-Ag solder alloys improves the bump interconnection reliability of the T- BGA by reducing the inelastic strain in the bumps. 1 (Bi=x%) / (Bi=%) True stress, t /MPa Tu re s trai n, ε t Figure 1. True stress-true strain curves of Sa-1.5 Bi alloy. From the simulation results, the element with the maximum cumulative equivalent inelastic strain was found to be identical for all solder alloys, and agrees with the fractured part of solder bumps located by the actual experiment. For each solder alloy, the cumulative equivalent inelastic strain after 3 cycles and the ratio of the strain in the Sn-Ag-Bi solder bump to that in the Sn- Ag bump are indicated in Figure 11. The cumulative equivalent inelastic strain of is shown on the left vertical axis. Disregarding the deformation during the holding time, it is clear that cumulative equivalent inelastic strain in the bumps made of Sn- Ag-Bi solder alloys is smaller than that in the bumps. In addition, this strain in the bumps is reduced as the Bi addition to Sn-Ag solder alloys increases. Moreover, the reduction in the above strain ratio becomes remarkable by the addition of Bi up to 3% to Sn-Ag solder alloys. 5. Conclusion This study has investigated the effects of the Bi content on the mechanical properties and bump interconnection reliability of Sn-Ag solder alloys. The effective Bi amount for the enhancement of the mechanical properties of Sn-Ag solder alloys is 3% or less. The interconnection reliability of the bumps made of the Sn-Ag solder containing 3% or less Bi was superior to that of the bumps. The enhancement of tensile strength by Bi addition to Sn-Ag solder alloys improves the bump interconnection reliability of the T-BGA mounted on the substrate by reducing the inelastic strain in the bumps. In the future, it is necessary to investigate the bump interconnection reliability when the bumps made of the Sn-Ag solder containing 3% or less Bi is used for highly rigid packages, namely the ceramic BGA or the plastic BGA. Acknowledgment The authors are grateful to Mr. K. Atsumi and Mr. S. Makita for their encouragement thoughtout this work and would also like to thank other members of their group for assistance with the experiment. The International Journal of Microcircuits and Electronic Packaging, Volume 23, Number1, First Quarter 2 (ISSN ) 136 International Microelectronics And Packaging Society

7 References 1. Sung K. Kang et al., Lead (Pb)- Free Solders for Electronic Packaging, Journal of Electronic Materials, Vol. 23, No. 8, pp.71 77, J. Glazer, Microstructure and Mechanical Properties of Pb- Free Solder for Low-Cost Electronic Assembly: A Review, Journal of Electronic Materials, Vol. 23, No. 8, pp , M. McCormack et al., The Design and Properties of New, Pb-Free Solder, 1994 IEEE/CPMT International Electronics Manufacturing Technology Symposium, pp. 7 14, T. Murata et al., The Review of Mounting Technology with Pb-free Solder, Journal of Society for Hybrid Microelectronics, Vol. 12, No. 6, pp. 9 14, K. Tateyama et al., Evaluation of Mechanical Properties of Pb Free Solder, The Journal of Japan Institute of Interconnecting and Packaging Electronic Circuits, Vol. 12, No. 7, pp , I. Mori et al., Effect of Bismuth Addition on Joint Reliability of Sn-Ag-Bi Solders, 5th symposium on Microjoining and Assembly Technology in Electronics, pp , Y. Nakahara et al., Mechanical Properties and Joint Strength of Sn-3.5Ag-3In-xBi Solders, Microjoining and Assembly Technology in Electronics, pp , Y. Hirata et al., Thermal Fatigue of Quad Flat Pack Lead/Sn- 3.5%Ag-X (X=Bi, Cu) Solder Joint with Special Reference to it s Relation to Isothermal Fatigue of Bulk Solder and Solder/ Copper Joint, 5th Symposium on Microjoining and Assembly Technology in Electronics, pp , E. W. Hare et al., Stress Relaxation Behavior of Eutectic Tin-Lead Solders, Journal of Electronic Materials, Vol. 24, No. 1, pp , H. Mavoori., Creep, Stress Relaxation, and Plastic Deformation in Sn-Ag and Sn-Zn Eutectic Solders, Journal of Electronic Materials, Vol. 26, No. 7, pp , About the authors Kazuki Tateyama received B.E. and M.E. Degrees in Applied Physics from Hokkaido University, Sapporo, Japan, in 1992, and 1994, respectively. He joined the Research and Development Center, Toshiba Corporation, Kawasaki, Japan, in He has been engaged in the research and development of Flip Chip interconnection technology. Currently, he is with the Corporate Manufacturing Engineering Center, Yokohama, and is engaged in development of high-density interconnection technology and Lead-free solder for electronic packaging. He is a member of the IEICE and IMAPS. Hiroshi Ubukata received B.E. Degree in Applied Mechanical Engineering from Gunma University, Japan, in He has been engaged in development of high density SMD soldering technology and cooling technology of the PC system. He is now with the Packaging key component section, Ome Operations, Digital Media Equipment & Services Company, Toshiba Corporation. Yoji Yamaoka received B.E. Degree in Applied Chemistry from Sophia University, Tokyo, Japan, in He has been engaged in the development of high density SMD soldering technology, electronic components engineering and instrumental analysis. Currently, he is a senior engineer of the Packaging echnology section, Ome Operations, Digital Media Equipment & Services Company, Toshiba Corporation. Kuniaki Takahashi received the B.E. degree in mechanical engineering from Tokai University, Japan, in 198. He has been engaged in the development of high density SMD soldering technology and its application. Currently, he is a senior specialist engineer of the Packaging key component section, Ome Operations, Digital Media Equipment & Services Company, Toshiba Corporation. He is a member of the JIEP, IEICE and JSME societies. Hiroshi Yamada received BE degree from Department of Synthetic Chemistry Nagoya University, Japan. He joined in Toshiba Corporation in 1986 and working at Display Materials and Devices Laboratory in the Corporate Research and Development Center, where he has been working on research on the assembly and packaging technology. He is currently the research scientist of Display Materials and Devices Laboratory, and has been developing the high density and high-speed interconnection technology. He is a member of the Society of Polymer Science, Japan, the IEICE, the JIEP, the IMAPS, and the IEEE. Masayuki Saito received a B.E. and M.E. Degrees in Electronics Engineering from Muroran Institute of Technology, Japan, in 1978 and 198, respectively. He joined Toshiba Corporation in 198 and had worked in Material and Device Laboratory in Research and Development Center, where he had developed assembly technology. In April of 1999, he transferred to Microelectronics Packaging Research Center in Corporate Manufacturing Engineering Center, and since October of 1999, he has been Senior Manager of that center. He is a member of the Institute of Electronics and Information, and Communication Engineers of Japan and the Japan Institute of Electronics Packaging. The International Journal of Microcircuits and Electronic Packaging, Volume 23, Number1, First Quarter 2 (ISSN ) International Microelectronics And Packaging Society 137