Bismuth-Based Transient Liquid Phase (TLP) Bonding as High-Temperature Lead- Free Solder Alternatives

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1 2017 IEEE 67th Electronic Components and Technology Conference Bismuth-Based Transient Liquid Phase (TLP) Bonding as High-Temperature Lead- Free Solder Alternatives Junghyun Cho, Roozbeh Sheikhi, and Sandeep Mallampati Materials Science and Engineering Program Binghamton University (State University of New York) Binghamton, NY 13902, USA Liang Yin and David Shaddock GE Global Research Niskayuna, NY 12309, USA Abstract Predominant high melting point solders for high temperature electronics contain lead (Pb), which will soon be banned by environmental regulations as in most of consumer electronics. In an effort to replace the Pb-based solders with a new high-temperature capable material, we developed a transient liquid phase (TLP) bonding of bismuth (Bi) and nickel (Ni). A molten Bi (m.p. of 271 C) strongly reacts with Ni to form a Bi3Ni or BiNi intermetallic layer, both of which can withstand over 400 C. To study microstructural developments and their influences on reliability performance, the die attached coupons were assembled using Bi-Ni TLP bonds. It was shown that Bi3Ni is the first phase to form, after which the diffusion of Bi controls the growth kinetics of this intermetallic phase with the activation energy of 65.5 kj/mol for the temperature range from 160 to 240 C. The solid-state transformation of Bi3Ni to BiNi follows, which is a slower process and only occurs at a long-term aging (activation energy of kj/mol, for the temperature range from 260 to 300 C). When tested at high temperatures (up to 350 C) or exposed under long-term storage at 200 C over 1000 hours, such bonds showed no degradation in die shear strength. It indicates the potential of the Bi-Ni TLB bonds as a Pb-free alternative to replace high-pb solders for high temperature electronics that operate at 200 C or higher. however, it is now imperative to consider Pb-free alternatives even for such high-temperature, harshenvironment electronics [8]. One potential solution can be found from a transient liquid phase (TLP) bonding [9-12] that can form from a Bi interlayer with Ni, which is a common metallized surface for the die as well as the substrate. After Bi is melted above 271 C, the Bi layer in contact with Ni is converted to a Bi 3Ni or BiNi intermetallic phase via dissolution of Ni, followed by isothermal solidification at a reflow temperature [13,14]. These intermetallic phases, once formed, can be stable up to ~ 470 C (with Bi 3Ni), or ~ 617 C (with BiNi) according to its phase diagram (Fig. 1), thus making this TLP bond useable in extremely high temperatures. The transformation kinetics involved in these two reaction products and their microstructure developments will play a key role in maintaining the reliability of such TLP bonds. Keywords-transient liquid phase (TLP) bonds; hightemperature electronics; Pb-free; die attach; Bi3Ni; BiNi I. INTRODUCTION Materials developments are crucial for next generation power semiconductor devices [1-5], downhole drilling systems [6] and automobile electronic modules [7] that operate at or are exposed to higher temperatures (> 200 C) than conventional electronics. Apart from high temperature electronics, higher melting point materials are also needed for die attach materials in the first level interconnects where the Sn-Ag-Cu (SAC) alloys with reflow peak temperatures of C are used for the subsequent board level assembly. In addition, various metal sealing needs can be found in hermetic packaging of MEMS devices that are also exposed to high temperatures. Traditionally, high lead (Pb)- based solders fit favorably to the above categories with melting temperatures in the range of C, and used extensively in such fields. With government regulations mandating industry toward Pb-free solders in all electronics, Figure 1. Calculated Bi-Ni binary phase diagram Between these two thermodynamically stable intermetallic phases, it was shown that Bi 3Ni is the first phase to form, and that the solid-state transformation of Bi 3Ni to BiNi only takes place above 250 C [15]. We also confirm that such solid-state transformation is rather a slow process and occurs at long-term aging at the interfacial region when there is a plenty of Ni supply. Our testing /17 $ IEEE DOI /ECTC

2 results from the TLP bonded assemblies made mostly of the Bi 3Ni phase showed no noticeable degradation of the die shear strength when tested at 350 C. The strength was also not diminished even after thermal aging at 200 C over 1000 hours. These results clearly show the potential of this Bi-Ni TLP bonding as high-temperature Pb-free solder alternatives and warrant further process optimization and more testing data. This work presents current progresses found in a new TLP bonding system based upon the Bi interlayer. Key microstructural developments at various bonding and aging conditions, as well as their mechanical performance and reliability data, are reported. The reliability testing from these Bi-Ni TLP systems includes high temperature storage data. II. EXPERIMENTAL PROCEDURE In this work two types of assemblies were employed using Si die (Ni)/Bi/(Ni) Si die and Si die (Ni)/Bi/Ni foil sandwiched coupons. The latter provides an infinite supply of Ni during the bonding process which is desired for the growth kinetics studies. Pure Bi pieces cut from its ingot rod (99.999%; Goodfellow, Huntington, UK) were used as an interlayer in both cases. Si dies were sputtered for backside metallization of Ti (70 nm) / Ni (1000 nm) / Au (70 nm). Dimensions of these dies were 1/8 1/8. As for the Ni substrate, annealed 0.5-mm thick Ni (99.98%) foils with dimensions of 50 mm 50 mm were purchased from Goodfellow and then cut into same dimensions as of the Si dies using a diamond band saw. Prior to bonding, all of the components were cleaned using ethanol and acetone, followed by a deionized water for rinsing. The reflow and aging processes were carried out on a hot plate inside a glove box with N 2 flow. The amount of O 2 and moisture in the glove box was kept under 0.1 ppm during the bonding process. Components were bonded using ~ 3mg of Bi piece at 300 C and various holding times, and subsequently cooled by removing them from the hot plate. Figure 2 shows the schematic of the sandwiched assembly. The aging process was carried out in a range of temperatures (160 C-300 C) for up to 48 hours. During the reflow and aging treatment, about 1 MPa of bonding pressure was exerted on the sandwiched coupons. Figure 2. Schematics of the sandwiched assembly at below T m of Bi and at bonding temperature (300 C) Reflowed and aged samples were prepared for cross sectional microstructure observations. Samples were embedded in an epoxy resin and polished with SiC abrasive papers. They were then fine polished with diamond slurries and colloidal silica solution, down to 0.02µm level. The microstructures were examined using optical microscope (Zeiss Axio Imager) and field emission scanning electron microscope (FE-SEM; Zeiss, Supra 55VP). Backscatter electron (BSE) detector was used to obtain a composition contrast, and energy dispersive X-ray spectrometer (EDS was used to acquire chemical compositions of different phases in the microstructure. Die shear strength tests were carried out on the same sandwiched coupons at room temperature up to 350 C using the Nordson Dage 4000 plus tester. These tests were performed on both as-reflowed and high temperature storage samples (stored at 200 C up to 1000 hours). III. RESULTS AND DISCUSSION A. Reactivity and Microstructure Developments between Bi and Ni through Isothermal Solidication As mentioned earlier, according to the Bi-Ni phase diagram two intermetallic compounds are possible when Bi and Ni react. Reflow temperature was 300 C, in which as soon as Bi melts on the Ni surface, the reaction between these two elements begins to occur. Before melting, both elements have very limited solubility in each other s phase, so no significant solid state reaction is expected to occur. The cross-sectional microstructure of Si (Ni)/Bi/(Ni) Si sandwiched coupons bonded at 300 C and various times is shown in Figure 3. As seen here, the volume fraction of intermetallic phase within the Bi matrix increases with an increasing bonding time, thus indicating more transformation to the intermetallic phase. (c) 50 µm 50 µm 50 µm Figure 3. Cross-sectional images of die attach assembly using Bi as an interlayer; bonded at 300 C for 2min, 3 min and (c) 8 min To more clearly identify the phase formed within the matrix, BSE images were obtained from the same samples (Fig. 4). The matrix phase was confirmed to be untransformed Bi. It also shows that a 1-μm thick Ni layer was completely consumed during the reflow process. A top Au layer used for oxidation resistance on top of Ni did not get involved in the reaction. Second phase formed inside the Bi matrix was confirmed to be Bi 3Ni via EDS. It showed no evidence of BiNi formation. As bonding time increases, more interdifussion occurs which results in more IMC formation provided that there is continuous Ni supply. Another observation was bulk nucleation of Bi 3Ni in the matrix, rather than surface nucleation of Bi 3Ni from Ni, as the Ni layer was completely consumed. No interfacial Bi 3Ni layer thus formed during the bonding on the die/bi interface (see Fig. 4), which might be advantageous since brtittleness 1554

3 of interfacial intermetallics leave the joint more prone to interfacial failure. supply, BiNi can form at a later stage of the bonding time. The reaction of BiNi will be further discussed in section D. Figure 4. SEM BSE Image of interfacial area for the sample bonded at 300 C for 3 minutes The same bonding conditions were applied to Si (Ni)/Bi/Ni foil coupons. This time, intermetallic compounds formed in the bonded Bi/Ni interfacial area, rather than in the matrix. Bi 3Ni was found to grow vertically with columnar morphology on the Ni substrate as shown in Fig. 5. It was also found that a very thin layer of BiNi forms at the interface of Bi 3Ni/Ni (see Fig. 5 ). It should be noted that no BiNi layer was observed for shorter bonding times. Si Die 10 µm Ni Figure 5 cross sectional image of die (Ni)/Bi/Ni foil sandwiched coupon bonded at 300 C for 8 minutes via polarized optical microscope, SEM BSE image of the Ni interfacial area that shows BiNi B. Mechanism for IMC formation Based on the observations from optical and scanning electron micrographs, a proposed mechanism for nucleation and growth of intermetallics in the Bi-Ni system is shown in Fig. 6. It explains the steps in which intermetallic compounds are being formed through isothermal solidification that occurs at the reflow temperature. In the case of Si (Ni)/Bi/(Ni) Si coupons, a limited amount of Ni on Si die is quickly dissolved into molten Bi. When the concentration of Ni reaches a critical value needed for Bi 3Ni formation, this phase nucleates and subsequently grows. Bulk nucleation of IMCs is dominant, thus making equiaxed microstructure. When one of the bonding components is the Ni foil, it acts as an infinite source of Ni since it cannot be completely consumed during the reflow process. Ni foil side can, therefore, provide the surface nucleation sites of the Bi 3Ni phase. Once it nucleates, it will grow into the molten Bi with columnar morphology, as shown in Fig. 5. On the other hand, the top interface formed between the Si die (Ni) and molten Bi, where Ni was completely consumed. It resulted in equiaxed microstructure of Bi 3Ni due to bulk nucleation on the die side. With further exposure at bonding times, a thin layer of BiNi begins to form at the Bi 3Ni/Ni interface, but not at the Bi 3Ni and Si. It indicates that with sufficient Ni Figure 6. Schematics for IMC formation mechanism Bi/(Ni) Si die and Bi/Ni foil C. Die Shear Strength of Bi-Ni TLP bonds In order to evaluate mechanical reliability of TLP bonds, Si die (Ni)/Bi/(Ni) Si die sandwiched coupons bonded at 300 C for 3 minutes and then aged for 4 hours at 200 C, were sheared using a Dage tester. For these samples, microstructures were not fully converted into Bi 3Ni yet, having residual Bi in the bonded region. Some of these samples were exposed to high temperature storage for 250 and 1000 hours. The shear strength results of these samples are also presented in Fig. 7 (c) Figure 7. Die shear strength of Si die/bi/si die bonds made at 300 C for 3 minutes, same conditions then HTS at 200 C for 250 hours and (c) same conditions and then HTS at 200 C for 1000 hours The shear data show that the average strength of TLP bond is close to 17 MPa. This value is a bit lower than high- 1555

4 Bi based solders ( 25 MPa) and commercially available high-temperature solders ( 26 MPa for Pb-Ag-Sn alloy; 35 MPa for Bi-Ag-X alloy) [16]. However, the strength was not diminished as the test was done at higher temperatures up to 350 C. In fact, testing at 350 C shows even a higher strength than the samples tested at room temperature, which can be attributed to a further conversion of the remaining Bi into intermetallics at 350 C exposure. Furthermore, the strength was maintained even after 1000 hours of HTS at 200 C. These high temperature data demonstrate the high temperature capability of the TLP bond made of Bi-Ni system when it is exposed at 200 C or higher, up to 350 C. an Arrhenius relation, its temperature dependence can be given by Q k 2 = k02 exp RT, (2) where Q is the activation energy, k0 is the material constant, and R is the gas constant. By plotting ln (k2) vs. 1/RT, the activation energy can be found from its slope. In the case of Bi3Ni formation, Q was 65.6 kj/mol (Fig. 10). This value is very close to the activation energy reported in Bi-Ni system by Duchenko et al. [17]. They have obtained 67.1 kj/mol for activation energy of Bi3Ni formation. D. Growth Kinetics of Bi3Ni and BiNi Si die (Ni)/Bi/Ni foil sandwiched coupons were reflowed at 300 C for 6 minutes and then exposed to high temperature aging for various times to grow the two IMCs. Growth kinetics for both Bi3Ni and BiNi were investigated by measuring the thickness of the IMCs grown across the interface formed between Bi interlayer and Ni foil. 1) Growth Kinetics of Bi3Ni Aging of TLP bonds were done at 160 C, 200 C and 240 C. Sandwiched coupons were exposed to these temperatures for different holding times. Figure 8 shows the crosssectional microstructure for 6, 24 and 48 hours at 200 C. It can be seen that a continuous layer of Bi3Ni is formed at the interface, which grows towards Bi side with its thickness increasing with aging time. The thickness of this layer was measured at various points and a mean thickness was acquired for each aging time. Figure 9. Average intermetallic (Bi3Ni) layer thickness vs. aging time: ( ) 240 C, ( ) 200 C and ( ) 160 C (c) Bi Bi3Ni Ni Figure 8. Cross sectional optical microscope image for samples aged at 200 C for 6 hours, 24 hours and (c) 48 hours Figure 10. ln (k2) vs 1/RT, where slope represents the activation energy for Bi3Ni formation. Figure 9 shows the plot of the Bi3Ni thickness vs. aging time at three temperatures. A parabolic growth behavior with time is seen in all three temperatures, which is an indication that growth of Bi3Ni is diffusion controlled. In this case, the growth kinetics can be expressed with h = kt n (1) where h is the thickness of the IMC, k is the growth constant and n is the growth exponent. By fitting the data points to Eq. (1), k and n values are obtained (in Table I). Growth exponent is close to 0.5 in all temperatures, which indicates that lattice diffusion of Bi is the responsible mechanism. With increasing temperature, n value is slightly decreasing that may suggest the potential for the mechanism change at higher temperatures. As the growth rate constant (k) follows TABLE I. KINECTIC PARAMETERS EXTRACTED FROM FITTING DATA AT THREE TEMPERATURES (ASSUMING LATTICE DIFFUSION). μm Temperature ( C) n k( h ) 2) Growth Kinetics of BiNi Same process as in Bi3Ni was carried out for the growth kinetics of BiNi. In this case, aging was done in two temperatures, 260 C and 300 C. Since the thickness of BiNi 1556

5 at these aging conditions was much less than that of Bi 3Ni, SEM was used to measure average thicknesses of the BiNi layer at various times and temperatures. Figure 11 shows the thickness of BiNi at 300 C for 6, 24 and 48 hours. Thickness of this layer is increasing but the range for thickness is an order of magnitude less than that of Bi 3Ni even at these high temperatures. Bi3Ni BiNi Ni In the case of BiNi, there is also a decrease in growth exponent value with increasing temperature as is the case for Bi 3Ni. It indicates the potential mechanism change away from lattice diffusion of Bi when temperature further increases. The activation energy was estimated using these two temperatures, from which it was close to kj/mol (Fig. 13). This activation energy is higher, compared to that in Bi 3Ni. Table II shows the kinetics parameters obtained from the above experiments. Albeit with n close to 0.5 (lattice diffusion controlled), k constant is much lower for BiNi formation. Lower k value along with higher activation energy indicates the slower kinetics even at higher temperatures (above 250 C). Q=176.7 kj/mol ln (k 2 ) (c) 1/RT (10-4 mol/j) Figure 13. ln (k 2 ) vs 1/RT, where slope represents the activation energy for BiNi formation. Figure 11. Si (Ni)/Bi/Ni foil coupons aged at 300C for 6 hours, 24 hours and (c) 48 hours Figure 12 shows the plot of BiNi thickness vs. aging time at two different temperatures (260 C, 300 C). For both temperatures, the parabolic thickness behavior with time is observed, which still indicates the lattice diffusioncontrolled mechanism (n close to 0.5). Figure 12. Average intermetallic (BiNi) layer thickness vs. aging time: ( ) 300 C and ( ) 260 C TABLE II. KINETIC PARAMETERS EXTRACTED FROM FITTING DATA AT TWO TEMPERATURES (ASSUMING LATTICE DIFFUSION). μm Temperature ( C) n k ( h ) It is also noted that the voids are observed within the BiNi layer (see Fig. 11). The reaction that is responsible for formation of BiNi layer at the Bi 3Ni/Ni interface is as follows: Bi 3Ni + 2 Ni 3 BiNi (3) By calculating molar volumes of the reacting components and product BiNi, a volume change of -9.4% is obtained Therefore, this reaction leads to contraction when forming a BiNi phase from Bi 3Ni and Ni which can eventually result in the creation of voids along grain boundaries and triple junctions. E. High Temperature Storage Sandwiched die-die coupons were also exposed to high temperature storage at 200 C up to 1000 hours. Figure 14 shows SEM images of the samples taken out at 250 and 500 hours. It can be seen that up to 250 hours most of Bi region 1557

6 was converted to form Bi 3Ni and that some cracks/voids were shown up due to the contraction accompanied with the solidification involved with Bi 3Ni transformation. As for the 500-hour sample, EDS still showed the pockets of the remaining Bi which indicate that due to a limited amount of Ni sputtered on the Si dies, further conversion into Bi 3Ni was ceased. Also internal cracks became more dominant in this sample with the HTS time, but these cracks seemed to be isolated, not continuous throughout the bond line. 2 µm Figure 14. Si die (Ni)/Bi/(Ni) Si die bonded samples after 250 hours, 500 hours for high temperature storage at 200 C As shown in shear strength data for these samples in Figure 7, very small degradation of shear strength was observed even after 1000 hours of high temperature storage. It verifies those internal cracks are mostly isolated and that they are not much degrading the strength. It will, however, be the potential reliability concern, so we explore the possibility of reflow processing optimization to prevent the formation of such cracks. Furthermore, additional reliability data of the TLP bonds are under investigation with thermal shock tests (-55 to +250 C). IV. CONCLUSION Bi-Ni TLP bonds were made at 300 C and microstructure developments and mechanical performances were evaluated at various reflow times and aging conditions. The bonds can be made of Bi 3Ni and Bi 3Ni/BiNi. Bi 3Ni is more dominant in the microstructure since this IMC has the faster kinetics of formation. Formation of both phases is controlled by lattice diffusion of Bi (growth exponent close to 0.5). Full conversion of the bond region with Bi 3Ni phase from the Bi interlayer was also demonstrated. It was shown that this joint made of the Bi 3Ni phase can maintain the bond integrity up to 350 C and under long high temperature aging at 200 C without losing much mechanical strength. Ni metallization can be quickly consumed, so a thinner bond line should be made if the metallized Ni thickness is small. BiNi grows very slowly at lower temperatures, but it begins to appear at higher temperature aging (> 260 C) and becomes more active at 300 C. In such a temperature range, activation energy of BiNi formation was quite high (Q=176.7 kj/mol), compared to that of Bi 3Ni formation at the lower temperature range (Q=65.5 kj/mol). It indicates that BiNi formation can be more dominant if the reflow temperature is further raised or the bond is exposed at high temperature (over 300 C) for an extended period. Both Bi 3Ni and BiNi formation showed the voids and cracks developed in the microstructure. It is therefore necessary to further optimize the reflow process and better understand microstructure evolution and volume change associated with phase transformations during high temperature aging and thermal stress conditions. ACKNOWLEDGMENT This project was financially supported by Integrated Electronics Engineering Center (IEEC) of Binghamton University (State University of New York). In addition, we used the equipment facilities at Analytical and Diagnostics Laboratory (ADL) of Small Scale Systems Integration and Packaging (S 3 IP) Center for some of materials characterization data presented here. We also thank Russell Tobias from Binghamton University for his initial die shear testing data, and Kaustubh Nagarkar and Arun Gowda from GE for their support on this project. REFERENCES [1] J. B. Casady and R. W. Johnson, "Status of silicon carbide (SiC) as a wide-bandgap semiconductor for high-temperature applications: A review," Solid-State Electronics, vol. 39, pp , [2] A. B. Lostetter, F. Barlow, and A. Elshabini, "An overview to integrated power module design for high power electronics packaging," Microelectronics Reliability, vol. 40, pp , [3] P. O. Quintero and F. P. McCluskey, "Temperature Cycling Reliability of High-Temperature Lead-Free Die-Attach Technologies," Device and Materials Reliability, IEEE Transactions on, vol. 11, pp , [4] J. Valle-Mayorga, C. P. Gutshall, K. M. Phan, I. Escorcia- Carranza, H. A. Mantooth, B. Reese, et al., "Hightemperature silicon-on-insulator gate driver for SiC-FET power modules," IEEE Transactions on Power Electronics, vol. 27, pp , [5] C. Buttay, D. Planson, B. Allard, D. Bergogne, P. Bevilacqua, C. Joubert, et al., "State of the art of high temperature power electronics," Materials Science and Engineering: B, vol. 176, pp , [6] K. D. Lyons, S. Honeygan, and T. Mroz, "NETL Extreme Drilling Laboratory Studies High Pressure High Temperature Drilling Phenomena," Journal of Energy Resources Technology, vol. 130, p , [7] R. W. Johnson, J. L. Evans, P. Jacobsen, J. R. Thompson, and M. Christopher, "The changing automotive environment: high-temperature electronics," Electronics Packaging Manufacturing, IEEE Transactions on, vol. 27, pp , [8] A. Z. Miric and A. Grusd, "Lead-free Alloys," Soldering and Surface Mount Technology, vol. 10, pp , [9] M. Hou and T. W. Eagar, "Low temperature transient liquid phase (LTTLP) bonding for Au/Cu and Cu/Cu interconnections," Transcations-American Society of Mechanical Engieers Journal of Electronic Packaging, vol. 114, pp , [10] H. A. Mustain, W. D. Brown, and S. S. Ang, "Transient Liquid Phase Die Attach for High-Temperature Silicon Carbide Power Devices," Components and Packaging Technologies, IEEE Transactions on, vol. 33, pp ,

7 [11] P. O. Quintero, T. Oberc, and F. P. McCluskey, "High Temperature Die Attach by Transient Liquid Phase Sintering," in HiTEC 2008, Albuquerque, NM, 2008, pp [12] N. S. Bosco and F. W. Zok, "Critical Interlayer Thickness for Transient Liquid Phase Bonding in the Cu-Sn System," Acta Mater., vol. 52, pp , [13] S. Mallampati, H. Schoeller, L. Yin, D. Shaddock, and J. Cho, "Developments of High-Bi Alloys as a High Temperature Pb- Free Solder," in IEEE Electronic Components and Technol. Conf. (ECTC), Lake Buena Vista, FL, May 27-30, 2014, pp [14] J. Cho, S. Mallampati, R. Tobias, H. Schoeller, L. Yin, and D. Shaddock, "Exploring Bismuth as a New Pb-Free Alternative for High Temperature Electronics," in IEEE Electronic Components and Technology Conference (ECTC), Las Vegas, NV, May 31-June 3, 2016, pp [15] M. S. Lee, C. Chen, and C. R. Kao, "Formation and Absence of Intermetallic Compounds during Solid-State Reactions in the Ni Bi System," Chemistry of Materials, vol. 11, pp , 1999/02/ [16] J. Cho, S. Mallampati, and H. Schoeller, "Design of Bismuth Alloys for High-Temperature Solder Materials," p. Unpulished Work, [17] V. I. Dybkov and O. V. Duchenko, "Growth kinetics of compound layers at the nickel-bismuth interface," Journal of Alloys and Compounds, vol. 234, pp , 1996/02/