Rapid Sintering Nanosilver Joint by Pulse Current for ... - IEEE Xplore

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IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY, VOL. 13, NO. 1, MARCH 2013

Rapid Sintering Nanosilver Joint by Pulse Current for Power Electronics Packaging Yunhui Mei, Yunjiao Cao, Gang Chen, Xin Li, Guo-Quan Lu, Member, IEEE, and Xu Chen

Abstract—Sintering of nanosilver paste has been extensively studied as a lead-free die-attach solution for bonding semiconductor chips. The bonding process typically consists of a lowtemperature drying step to remove organic solvents in the paste followed by sintering at around 250 ◦ C. Normally, a soak time of several minutes at the sintering temperature is necessary to establish strong bond strength by the conventional heating method. In this paper, we tested the feasibility of applying pulses of alternating electrical current through the nanosilver bonding layer to achieve strong joints in less than a second, not minutes. Experiments were carried out by joining rectangular copper blocks that were either coated with a layer of electroplated silver or without. A layer of nanosilver paste was stencil printed on one block, dried at temperature below 100 ◦ C, before the other copper block was placed on. The bonding members were then inserted under an alternatingcurrent spot-welding machine for rapid joining with current pulses. Die-shear test was used to quantify the joint strength. Investigated processing variables on the joint strength were current level, current-on time, nanosilver bondline thickness, predrying temperature and time, and copper surface finish. Scanning electron microscopy was used to characterize the joint microstructure. It is suggestive that the current sintering of nanosilver paste could be used for rapid joining of metal-to-metal connection, such as bonding copper bus bars onto power electronics modules. Index Terms—Current sintering, lead-free, low-temperature sintering, nanosilver paste, rapid joining.

I. I NTRODUCTION

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OR DECADES solder had been used as the material of interconnection of power electronics. Its widespread use was mainly due to its low cost and satisfactory strength properties. However, traditional soldering no longer meets the requirements of high-temperature applications, and lead-containing solder is harmful to the environment. Silver nanoparticles have seen dramatic applications in recent years due to the combined advantages of polymer matrices Manuscript received October 30, 2012; revised December 15, 2012; accepted December 28, 2012. Date of publication January 3, 2013; date of current version March 7, 2013. This research was supported by the National Natural Science Foundation of China under Grant 51101112, Grant 10802056, Grant 11172202, Grant 11072171, and Grant 51175375. (Corresponding author: G. Chen.) Y. Mei and X. Li are with the Tianjin Key Laboratory of Advanced Joining Technology and the School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China. Y. Cao, G. Chen, and X. Chen are with the School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China (e-mail: [email protected]). G.-Q. Lu is with the Department of Materials Science and Engineering, and Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 USA. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TDMR.2012.2237552

(i.e., low processing temperature and good physical and mechanical properties) and nanosilver fillers (i.e., good electrical, thermal, and dielectric properties and highly reactive surface area) [1], [2]. The silver nanoparticles can be used in metal_metal bonding [3] die-attached interconnection [4] relying on those advantages. Silver is also unique among all cost-effective metals by nature of its conductive oxide (Ag2 O). In addition, silver nano-particles are relatively easy to be formed into different sizes (a few nanometers to 100 nm) and shapes (such as spheres, rods, wires, disks, and flakes) [1], [5] and well dispersed in a variety of polymeric matrix materials. Therefore, silver nano-composites are widely accepted as a promising material in the electronic industry, such as for leadfree conductive adhesives in electronic interconnects and high dielectric constant composites in embedded passives components. Unlike the widely used soldering or adhesive bonding technologies [6], this new technology, often referred to as the low-temperature joining technology [7], [8], is based on the sintering of micrometer-size silver powder at temperatures below 300 ◦ C. Soldering process must be carried out beyond the melting temperature which cannot be very high. This feature limits its application in high-temperature area. While for sintering, it can be realized far below the melting temperature. Usually, a screen- or stencil-printed layer of micron-scaled silver flakes is used as interconnection material. A mechanical pressure of about 40 MPa on a 100 mm2 chip is required to get such low sintering temperatures. Under the applied pressure for a few minutes, the silver die-attach layer undergoes significant densification to a density of 80% at 250 ◦ C. However, under such pressures, even the slightest irregularities, e.g., Griffith cracks can lead to cracking of the brittle silicon [9], [10]. The sintered silver joints were reported to have excellent thermal and electrical conductivities [11], strong shear strength in excess of 30 MPa, and a high reliability. Recently, we have demonstrated [12]–[17] a strategy of replacing the high mechanical pressure with a chemical driving force by using nanosilver powder to lower the sintering temperature. The nanosilver powder has been made into the paste form to offer a one-to-one replacement for solder or silver epoxy. The introduction of the nanosilver paste significantly simplifies the low-temperature joining or sintering technology and has paved its way for widespread adaptation by power electronics manufacturers. In the past, a low sintering temperature of about 250 ◦ C and a mechanical pressure of about 1–5 MPa on a 100 mm2 chip was required to get enough shear strength of nanosilver dieattach layer, i.e. (≥ 35 MPa). Under the above conditions for a few minutes, the silver die-attach layer underwent significant densification to a density of 80% at 250 ◦ C. The densification

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MEI et al.: RAPID SINTERING NANOSILVER JOINT BY PULSE CURRENT

Fig. 1.

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Brief description of manufacture of nanosilver.

Fig. 3. As-printed specimen before sintering.

Fig. 4. Schematic illustration of (a) current-assisted sintering technology (CAST), and (b) die-shearing tester.

Fig. 2.

TEM micrograph of silver nanoparticles.

rate is closely related to the particle size, with the particle size decreases the densification rate increases obviously [18]. Current-assisted sintering technology (CAST) is an innovative sintering method with short holding times that allows compaction of ceramics and powdered metals, but its application in sintering nanosilver has not been reported yet. The CAST process is similar to the traditional hot-pressing process for sintering. However, instead of using an external heating source, current is allowed to pass through the sample and generates heat in the sample. Thus, CAST can generate fully dense samples by providing rapid heating and short holding time. The rapid densification is attributed to three factors: (1) the application of mechanical pressure; (2) the use of rapid heating; and (3) the use of current. The mechanical pressure can remove pores from compacts and enhance diffusion. However, it is frequently argued that the improved densification rates stem mostly from the use of current of high energy. Thus, the aim of the present study is to characterize the CAST for sintering nanosilver paste. The shear strength and microstructure of sintered nanosilver joints were discussed. CAST will provide a cost-effective way to bond electronic devices by using nanosilver paste in the future. II. E XPERIMENTAL The nanosilver paste was made by adding selected organic surfactant, binder, and thinner into 30 nm nanosilver particles, as shown in Fig. 1. Fig. 2 shows TEM micrograph of nanosilver paste with particle size ranging from 30 nm to 50 nm. The particles are mostly separated instead of forming aggregates. It seems that there is almost no aggregation or agglomeration in this nanosilver paste.

Preparation of the Specimens: Both the substrate and dummy die are made of bare copper or silver-plated copper. The bare copper was first deoxidized by polishing with sand paper and then polished by a solution containing CH3 COOH (40 ml/L), H2 SO4 (100 ml/L), H2 O2 (160 ml/L), and C2 H5 OH (40 ml/L). The bare copper was then rinsed with de-ionized water before being used as the dummy die or substrate. The silverplated copper was cleaned by only alcohol and an ultrasonic cleaner. The nanosilver paste was stencil printed onto the bare copper or silver-plated copper substrate (22 mm × 15 mm × 1.5 mm) to form a square silver film (7 mm × 7 mm × 0.09 mm). The printed specimens were first predried at 70 ◦ C on a hot plate for 10 minutes in order to remove the solvent in the nanosilver paste to ensure a certain wettability of the paste film [19] and a good contact with the copper dummy die. A dummy die (5 mm × 5 mm × 1.5 mm) was then placed on the predried nanosilver layer. One of the as-printed specimens before sintering is shown in Fig. 3. Sintering Process: A schematic illustration of the currentassisted sintering technology used in this study is shown in Fig. 4(a). The dummy die was bonded with the substrate by sintering the nanosilver at a couple of electrodes that could manually adjust the alternating current (ac) output from 5.5 kA to 8.25 kA, driving time from 50 ms to 1000 ms, and loading pressure from 5 MPa to 10 MPa. Since the higher the heating rate the denser the sintered silver joint [17] a high heating rate produced by ac was used to sinter nanosilver paste in a short time, e.g., 800 ms. For small area chip attachment, there is no need for predrying the silver paste and applying additional external pressure because the organics can be burned up easily owing to the short pathway [20]. However, for a 5 mm × 5 mm or even larger joint, a pressure of 5 to 10 MPa is required during the sintering. The pressure serves two purposes: (1) to improve the contact between the die/substrate and silver paste and prevent the silver

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TABLE I S UMMARY OF S INTERING C ONDITIONS IN FABRICATION OF S PECIMENS

joint from voids or cracks caused by out-gassing during the sintering, and (2) to lead to a uniform and dense microstructure of the sintered silver joint. A summary of sintering conditions used in the fabrication of specimens is given in Table I. At least three specimens were prepared for each sintering condition. Die-Shear Testing: The shear strength of the sintered specimen was obtained by using a bond tester (XTZTEC Condor 150) at a rate of 4 × 10−4 m/s. Fig. 4(b) shows a schematic of the die-shearing test. The specimen was fixed on the surface of the platform of the die-shearing setup by a vacuum pump that was connected to the die-shearing setup. Microstructures Analysis: The microstructures of the sintered specimen were analyzed by scanning electron microscopy (SEM). From the fracture surface of the specimen we could see that some of the facture interface happened in the silver paste and some happened in the interface between Cu and Ag paste. SEM micrograph was taken on the fracture surface in the silver paste of the sintered specimen after being sheared. Comparison of the microstructures of the sintered specimens

with different conditions was discussed to explain variation of die-shear strength of the sintered specimens.

III. R ESULTS AND D ISCUSSION A. Effect of Silver Metallization Fig. 5 shows that the copper dummy die is bonded with the copper substrate by sintered nanosilver. The bondline is uniform and with the thickness of approximately 25–30 μm. Fig. 6 shows the die-shear strength of the specimens with different metallized materials. Specimens were fabricated under four different sintering conditions (conditions No. 1 to No. 4 in Table I). All four types of specimens were sintered at 7 kA for 500 ms. We can see that the shear strength of a specimen bonded by two silver-plated copper surfaces is greater than that of specimens of other types, probably because bare copper is easy to oxidize, especially at high temperatures. The oxidation of copper can prevent the diffusion of silver particles into the copper. As a result, the die-shear strength of the


Fig. 5.

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Metallographic image of cross section of sintered silver joint.

Fig. 8. Comparison of shear strength of specimens with different ac’s.

Fig. 9. Comparison of size of sintered silver particles in sintered specimens with (a) 6.50 kA, and (b) 8.25 kA. Fig. 6. Comparison of shear strength of specimens with different metallization.

Fig. 7. SEM micrographs of fracture surfaces of sintered nanosilver joints by (a) current-assisted sintering technology, and (b) traditional hot-pressing method.

bare-copper_bare-copper specimens is almost half of that of the silver-plated-copper_silver-plated-copper specimens, as shown in Fig. 6. On the other hand, the silver plating on the silverplated-copper_silver-plated-copper specimens can prevent the copper from being oxidized. In addition, the silver oxides, e.g., Ag2 O, start to be decomposed at 175 ◦ C [21]. Little oxidation can occur on the surface of the substrate/dummy die. Therefore, robust bonding can be generated at the interfaces between the sintered silver and copper. The shear strength of the CAST specimen is 3 times that of the traditional hot-pressing sintered specimen [22]. Fig. 7 shows less elongated shape on the fracture surface of the traditional hot-pressing sintered silver joint compared with that of the CAST joint. Moreover, the whole process of the traditional method takes approximately 60 min or even more while the new method only need 10 min for predrying and less than 1 second for sintering. The current sintering method was therefore cost effective and could achieve high shear strength. Compare with the traditional hot-pressing process, the CAST provides rapid

heating on the specimen. And rapid heating can increase the rate of densification. Changing the rate of densification is a way to alter grain size [23]. Rapid heating can decrease the grain size. Decreasing grain size decreases the amount of possible cluster of dislocations at the grain boundary, increasing the amount of applied stress necessary to move a dislocation across a grain boundary. The higher the applied stress to move the dislocation, the higher the strength is. So it could be a preferable way instead of press-assisted sintering process to sinter nanosilver in power electronic applications, e.g., bonding bus bar. B. Effect of AC Fig. 8 summarizes the die-shearing results of the specimens fabricated with different ac’s, i.e., 5.5 kA, 6.5 kA, 7.5 kA, and 8.25 kA (conditions No. 5 to No. 8 in Table I). All four types of specimens were sintered for 800 ms. It can be seen that the shear strength of the specimens increases as the ac output increases. The shear strength of the specimen sintered at 8.25 kA can reach 86.5 MPa, which is almost five times as large as that sintered at 5.5 kA, probably because the higher the ac the higher the current density going through the nanosilver joint and the larger the amount of heat created in the nanosilver joint. Most of the organics in the nanosilver paste can be burned up in a short time at high temperatures. These organics act as a barrier to atom diffusion and agglomeration and are very critical to the generation of strong bonding [24]–[26]. Moreover, the high ac provides sufficiently high energy or high temperature, which accelerates the diffusion of silver atoms and grain/particle growth in the nanosilver joint. Fig. 9(a) and (b) show the size of sintered silver particles in the sintered specimens with

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Fig. 10. Comparison of microstructures of fracture surface of specimens with ac of (a) 5.50 kA, (b) 6.50 kA, (c) 7.00 kA, and (d) 8.25 kA. Fig. 12. Comparison of shear strength of specimens with different current-on time.

C. Effect of Current-On Time

Fig. 11. SEM micrograph of cross-section of sintered silver joint.

6.50 kA and 8.25 kA, respectively. It can be observed that the size of the sintered nanosilver particles is larger at 8.25 kA than at 6.50 kA. The microstructure of the silver joint sintered at the higher current is much denser than that sintered at the lower current. Because no crack or void exists in the silver joint sintered at the higher current, as shown in Fig. 9(b), the die-shear strength of the specimen sintered at 8.25 kA is much higher than that sintered at 5.5 kA. In order to investigate microstructures of the sintered nanosilver joint, cross section and fracture surface of the specimen happened in the silver paste were analyzed by SEM. Significant elongated shape can be seen on the fracture surfaces of the sintered silver joints in Fig. 10, probably because strong bonding occurs in the specimens with different ac. Fig. 11 shows that the silver layer is dense. It can be seen that the higher the current, the more significant the elongated shape on the fracture surface of the specimen. The shear strength of the current-assisted sintered silver joint is much greater than that of soldered joints and pressureassisted sintered joint. 86.5 MPa can be obtained when the current is 8.25 kA. The shear strengths of solder joints, e.g., PbSn, AuGe12, ZnAl5, are about 30 MPa [10], [27]. And the shear strengths of the pressure–assisted sintered joint are about 40 MPa [22]. Furthermore, bare copper used to be bonded with bare copper via controlling the oxygen partial pressure during sintering to avoid oxidizing copper. The shear strength of the sintered joint obtained via controlling the oxygen partial pressure was usually lower than 20 MPa [28].

Fig. 12 shows the effect of current-on time on the die-shear strength of the sintered bare-copper_bare-copper specimens. The current-on time ranges from 200 ms to 1000 ms (condition No. 9 to No. 16 in Table I). All the specimens shown in Fig. 12 are sintered at 8.25 kA. The die-shear strength increases with increasing current-on time. The die-shear strength of the sintered specimen with current-on time of 1000 ms is 3 times that of the sintered specimen with current-on time of 200 ms. Since most of the organics in the nanosilver paste cannot be burnt up at an ac of 8.25 kA in the short time of 200 ms, the dieshear strength is low at 24.6 MPa. However, all of the organics in the nanosilver paste can be burnt up if the current-on time is long enough. The longer the current-on time, the longer the time keeps for mass transfer, atom diffusion, and grain growth. These factors are critical to the formation of good bonds at the interfaces between the sintered silver joint and copper dummy die/substrate [4], [10]. In order to investigate the effect of current-on time on microstructures of the sintered nanosilver joint, fracture surface of the specimen happened in the silver paste was analyzed by SEM as shown in Fig. 13. No elongated shape can be observed in Fig. 13(a) and (b). We can even see some voids in Fig. 13(a). However, significant elongated shape can be seen on the fracture surfaces of the sintered silver joints in Fig. 13(c) and (d), probably because strong bonding occurs in the specimens with the current-on time of 800 ms and 1000 ms. It is found that the longer the current-on time, the more significant the elongated shape on the fracture surface of the specimen. D. Effect of Predrying Time Fig. 14 shows the effect of predrying time on the die-shear strength of the bare-copper_bare-copper specimen. The predrying time ranges from 5 min to 20 min (condition No. 17 to No. 20 in Table I). All the specimens shown in Fig. 14 are sintered at 8.25 kA. The die-shear strength increases with increasing predrying time from 5 min to 15 min. Longer


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Fig. 13. Comparison of microstructures of fracture surface of specimens with current-on time of (a) 200 ms, (b) 400 ms, (c) 800 ms, and (d) 1000 ms.

Fig. 15. Comparison of microstructures of fracture surface of specimens with predrying time of (a) 5 min, (b) 10 min, (c) 15 min, and (d) 20 min.

Fig. 14. time.

Fig. 16. Shear strength of specimens with different thickness of nanosilver paste.

Comparison of shear strength of specimens with different predrying

predrying time allows more time for solvent to burn out, but the paste will be too hard to deform and the origin contact between copper surfaces and nanosilver layer will not be sufficiently good if the time is too long. Therefore, there is a little drop in die-shear strength as prolonging the predrying time to 20 min. The die-shear strength of the sintered specimen with a predrying time of 15 min is only 17.6% larger than that of the sintered specimen with a predrying time of 5 min. Since most of solvent in the nanosilver paste can be evaporated at 70 ◦ C in the short time of 5 min, the die-shear strength is significantly high at 87.1 MPa. In order to investigate the effect of predrying time on microstructures of the sintered nanosilver joint, fracture surface of the sheared specimen was analyzed by SEM, as shown in Fig. 15. Significant elongated shape can be observed in Fig. 15(c). However, less significant elongated shape can be seen on the fracture surfaces of the sintered silver joints in Fig. 15(d). The longer the predrying time the more the elongated shape on the fracture surface of the sheared specimen. E. Effect of Thickness of As-Printed Nanosilver Film Fig. 16 shows the effect of thickness of as-printed nanosilver paste on the die-shear strength of the bare-copper–bare-copper

specimen. Two different thicknesses, i.e., 50 μm and 90 μm, of as-printed nanosilver film were studied. (condition No. 21 to No. 22 in Table I). All the specimens in Fig. 16 are sintered for 500 ms at 8.25 kA. There is a drop in the die-shear strength as decreasing thickness of as-printed nanosilver paste from 50 μm to 90 μm, probably because the binders in the thinner silver film could be burnt out more sufficiently within a short time of only 500 ms. The residual binders might be barriers for the silver grain to spread.

F. Effect of Predrying Temperature Fig. 17 shows the effect of predrying temperature on the dieshear strength of the bare-copper_bare-copper specimen. The predrying temperature ranges from 70 ◦ C to 160 ◦ C (condition No. 23 to No. 26 in Table I). All the specimens shown in Fig. 17 are sintered at 8.25 kA. The die-shear strength decreases with increasing predrying temperature. Theoretically, higher predrying temperature allows solvent to burn out easily, but the paste becomes too hard to deform and the contact between copper surfaces and nanosilver layer is not sufficiently good as increasing predrying temperature. Therefore, nanosilver paste prefers to evaporate the solvant at 70 ◦ C for 10 min.

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It seemed that CAST could be a good way to bond bare copper with bare copper in power electronic applications, e.g., bus bar connection. 4) Silver metallization could benefit current-assisted sintered nanosilver joint on shear strength. The shear strength of current-assisted sintered nanosilver joint increased with increasing ac, current-on time, and predrying time, but decreased with increasing as-printed thickness of nanosilver film and predrying temperature. 5) The sintered specimen fabricated by CAST showed dense texture. It was observed that the well-bonded sintered silver joint showed significant elongated shape. R EFERENCES Fig. 17. Shear strength of specimens with different predrying temperatures.

Fig. 18. Comparison of microstructures of fracture surface of specimens with predrying temperature of (a) 70 ◦ C, (b) 100 ◦ C, (c) 130 ◦ C, and (d) 160 ◦ C.

In order to investigate the effect of predrying temperature on microstructures of the sintered nanosilver joint, fracture surface of the sheared specimen is analyzed by SEM, as shown in Fig. 18. Elongated shape can be observed in all the images of Fig. 18. IV. C ONCLUSION An innovative technology, i.e., CAST, was studied for sintering nanosilver. In this paper, we obtained the conclusions as the following: 1) Compared with the traditional hot-pressing way to sinter nanosilver, CAST was able to sinter nanosilver in a very short time, i.e., less than one second. Therefore, it should be efficient to use CAST for sintering nanosilver in power electronic application instead of hot-pressing method. 2) The die-shearing strength of the sintered silver specimens fabricated by CAST increased as the current and the time. The die-shear strength of nearly 100 MPa was measured for the specimen sintered at the current of 8.25 kA for 1000 ms. 3) CAST could be used to bond bare copper with bare copper without any protective gas atmosphere, e.g., nitrogen. No copper oxide was observed on the surface of bare copper as soon as the specimen was sintered by CAST.

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Yunhui Mei received the B.S. and Ph.D. degrees in process equipment and controlling engineering from Tianjin University, Tianjin, China, in 2006 and 2010, respectively. He was an Exchange Student with the Center for Power Electronics Systems, Virginia Polytechnic Institute and State University, Blacksburg. He is currently a Faculty Member with the Tianjin Key Laboratory of Advanced Joining Technology and the School of Material Science and Engineering, Tianjin University. His current research interests include high-temperature packaging for high-power-density applications.

Yunjiao Cao received the B.S. degree in mechanical engineering from Tianjin University of Technology, Tianjin, China, in 2010. She is currently working toward the M.S. degree in chemical engineering and technology at Tianjin University, Tianjin. Her current research interests include processing and characterization of nanoscale silver paste for high-temperature device attachment.

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Gang Chen received the Ph.D. degree in process equipment and machinery from Tianjin University, Tianjin, China, in 2006. He was a Teacher with the School of Chemical Engineering and Technology, Tianjin University, where he has been an Associate Professor since 2008. In 2009, he was a Visiting Professor with the Department of Material Science Engineering, Virginia Polytechnic Institute and State University, Blacksburg. He has authored or coauthored over 30 papers in journals and international conferences. His current research interests include reliability of microelectronics, finiteelement analysis, creep fatigue of solders, and constitutive modeling for electronic and conventional structural materials.

Xin Li received the M.S. and Ph.D. degrees in materials processing engineering from Tianjin University, Tianjin, China, in 2012. Since April 2012, she has been a Lecturer with the School of Materials Science and Engineering, Tianjin University. She is currently mainly engaged in the research work on the high-power electronic packaging technology and reliability.

Guo-Quan Lu (M’97) received the B.S. degrees in physics and in materials science and engineering from Carnegie Mellon University, Pittsburgh, PA, in 1984 and the Ph.D. degree in applied physics and materials science from Harvard University, Cambridge, MA, in 1990. He is currently a Professor with the Department of Materials Science and Engineering and the Bradley Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University, Blacksburg. He was with the Alcoa Technical Center, Alcoa Center, PA. Since 2007, he has been holding a Cheung Kong Guest Professorship with the Tianjin Key Laboratory of Advanced Joining Technology and the School of Material Science and Engineering, Tianjin University, Tianjin, China. His current research interests include materials and processing development for electronic packaging of microelectronics, power electronics, and optoelectronics. Prof. Lu was a recipient of the National Science Foundation Career Award and the Research and Development 100 Award in 2007.

Xu Chen received the B.S., M.S., and Ph.D. degrees in applied mechanics from Southwest Jiaotong University, Chengdu, China, in 1982, 1986, and 1992, respectively. He is currently a Professor with the School of Chemical Engineering and Technology, Tianjin University, Tianjin, China, where he is the Head of the Department of Process Equipment and Control Engineering. He has authored or coauthored over 100 papers in international journals and conferences. His current research interests include mechanical properties of newly developed materials, creep-fatigue and constitutive modeling for electronic and conventional structural materials, thermal management, and reliability of die attachment in electronic packaging. Prof. Chen was a recipient of the Teaching and Research Program Award for Outstanding Young Teachers in Higher Education Institutions of the Ministry of Education, China, in 2002, and the Thomson Scientific Research Fronts Award in China in 2008.