Thermomigration and electromigration in Sn58Bi ball grid array solder joints

J Mater Sci: Mater Electron (2010) 21:1090 1098 DOI 10.1007/s10854-009-9992-2 Thermomigration and electromigration in Sn58Bi ball grid array solder joints X. Gu K. C. Yung Y. C. Chan Received: 18 August 2009 / Accepted: 30 September 2009 / Published online: 13 October 2009 Ó Springer Science+Business Media, LLC 2009 Abstract In the present study, individual effect of thermomigration (TM) and combined effects of TM and electromigration (EM) in Sn58Bi ball grid array (BGA) solder joints were investigated using a particular designed daisy chain supplied with 2.5 A direct current (DC) at 110 C. Driven by the electric current, Bi atoms migrated towards the anode side and formed a Bi-rich layer therein. With a thermal gradient, Bi atoms tended to accumulated at the low temperature side. When the effects of TM and EM were in same direction, TM assisted EM in the migration of Bi, otherwise it counteracted the effect of EM. The effect of electron charge swirling were detected when the electric current passed by the Cu trace on the top of the solder bump instead of entering into it. For the joint without current passing by or passing through, only TM induced the migration of the Bi atoms. 1 Introduction Electromigration (EM) of solder joints has become a most persistent reliability issue in interconnects of microelectronic devices due to continuous miniaturization and the X. Gu Y. C. Chan (&) Department of Electronic Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong, People s Republic of China e-mail: eeycchan@cityu.edu.hk K. C. Yung Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, People s Republic of China requirements for enhanced performance of devices [1]. Recently, thermomigration (TM) has been regarded as another reliability concern as it has been found to accompany EM in flip chip solder joints [2]. TM may assist or counteract EM depending on the direction of the thermal gradient. Most of the studies are focused on the TM of Sn Pb solder joints [2 6]. The results showed that with a substantial thermal gradient, Pb atoms in the solder migrated towards the lower temperature side. Due to the increasing awareness of the toxicity of Pb, Pb-free solders are being widely used in the electronic industry to replace Pb-containing solders in recent years. Among all the Pb-free solders, the Sn58Bi solder alloy is a promising candidate for low-temperature applications due to its low melting point (139 C) [7], so it is important to study the reliability of the Sn58Bi solder joints. Especially for the similar bi-phase structure of Sn58Bi alloy to that of Sn37Pb alloy, the investigation of TM and EM in Sn58Bi solder joint is very necessary. Many studies have reported the EM behavior of Sn58Bi solder joints [8 11]. Generally, these investigations have found that a continuous Bi-rich layer formed near the anode side, while a Sn-rich band formed at the cathode side after the current stressing. However, very few studies have yet been published to explore the TM behavior of Sn58Bi solder. In our previous study [12], the combined effects of TM and EM in Sn58Bi solder joints were investigated by using a line-type structure supplied with direct current (DC) in opposite directions. The results showed that Bi migrated to the lower temperature side driven by the thermal gradient. This study is intended to study the individual effect of TM and the combined effects of EM and TM in Sn58Bi ball grid array (BGA) solder joints using a particular designed daisy chain structure supplied with DC current. In addition, to help understand the mechanism of TM, a three-dimensional

J Mater Sci: Mater Electron (2010) 21:1090 1098 1091 thermal electrical finite element method (FEM) will be conducted to simulate the current density distribution and temperature distribution in the structure. 2 Experimental Both of the small substrates and large substrates used to fabricate the BGA solder joints were made of flame retardant 4 (FR4) materials. The thickness of each substrate was 1.5 mm. The BGA pads were a solder-mask-defined type, and the opening diameters of the pads on the small substrate and large substrate were 280 lm and 250 lm, respectively. The thickness of the Cu pads was 35 lm with organic solderability preservative (OSP) surface finishing. The pads were linked with designed Cu traces with a thickness of 35 lm for electrical continuity. The small substrate was attached to the large substrate using flip chip bonding technology by a Flip-Chip bonder (SUSS, FCM). The assembly process was similar to that of a conventional surface mount technology (SMT) procedure, including flux printing, ball placement, reflow soldering and cleaning. The aligned samples were reflowed using a hot air convection reflow oven (BTU PYRAMAX 100 N). The peak temperature in the temperature profile was 176 C and the duration above the melting point of Sn58Bi alloy was about 82 s. Figure 1a gives images of a small substrate and large substrate before the assembly. Figure 1b gives the image of a prepared structure, and four designed daisy chains formed at the edges of the small substrate. Solder joints 1 6 were prepared for each chain. Only one chain was supplied with a DC current during the testing. In order to create the thermal gradient across the solder joints, two Cu pads with dimensions of (35 lm 9 12000 lm 9 24000 lm) were designed on the other side of the large substrates. Each Cu pad was connected with the BGA pads of joints 2 and 5 through the plating through holes (PTH) (as shown in Fig. 1a). The prepared structure with one chain supplied with DC current and was put into the furnace set at 110 C. Figure 1c shows the electron flow through the solder joint chain. For the solder joints 1, 3, 4 and 6, the electric current passes through the solder bumps. While the electric current passes by the top of the solder bump of joint 2 instead of entering it. No current passes by or passes through the solder bump of joint 5. After predetermined periods of time, samples were taken out of the furnaces and quenched in air. Each one sample was mounted and cross sectioned carefully towards the center of the solder joint. Microstructural observations were performed using a Philips XL 40 FEG Scanning Electron Microscope (SEM) equipped with an energy dispersive x-ray (EDX). Three-dimensional thermal electric FEM simulation was used to calculate the current density distribution and temperature distribution in the test structure. In the simulation model, the height of the solder bump between the two substrates is 150 lm. The physical parameters are listed in Table 1 [3, 13 15]. TCR is the abbreviation of temperature coefficient of resistance. 3 Results and discussion 3.1 Results Figure 2 shows the typical microstructure of one Sn58Bi BGA solder joint aged at 110 C without current stressing for 480 h. It is clear that the two interfaces exhibited nearly the same microstructure. Figure 3 shows the microstructures of six solder joints in one daisy chain supplied with a 2.5 A DC current at 110 C for 480 h. The measured temperatures of the small substrate surface and the large substrate surface were about 112 and 132 C, which showed that a thermal gradient was created between the small substrate and the large substrate during the test. According to the microstructures of the solder joints shown in Fig. 3, substantial mass migration had occurred in the solder joints during the current stressing. In particular for joints 1 and 6, serious separation of Bi atoms had occurred. It is clear that phase separation in Sn58Bi solder is especially prominent for solder joints 1 and 6. Figure 4 shows the detailed microstructures of joint 1 and joint 6 with local magnified images. The Bi atoms accumulated at the small substrate side in solder joint 1, while a large Sn-rich area formed at the large substrate side. By contrast, Bi atoms accumulated at the large substrate side of joint 6. It is well known that Bi atoms migrate in the direction of the electron flow. As the direction of the electron flow was from the large substrate to the small substrate for joint 1, while it was from the small substrate to the large substrate for joint 6, it is reasonable to find the phenomena as the above illustrated. For solder joint 1, as shown in Fig. 4b, the IMC layer at the small substrate side is composed of a Cu 3 Sn IMC layer (near the Cu pad side) about 2.4 lm thick and a Cu 6 Sn 5 IMC layer (near the solder side) about 4.2 lm thick. By contrast, the IMC layer at the large substrate side (as shown in Fig. 4c) is mainly the Cu 6 Sn 5 phase with a thickness of more than 20 lm. About 12 lm of the Cu pad at the large substrate side had been consumed during the test. The IMC layer at the small substrate side is thinner than that at the large substrate side. The enhanced effect of the electric current and the barrier effect of the Bi-rich layer played important roles in the growth of the Cu-Sn IMC layer in

1092 J Mater Sci: Mater Electron (2010) 21:1090 1098 Fig. 1 a Optical images of a small substrate (chip) and large substrate (substrate) before the assembly, b optical image of a sample prepared for subsequent tests, and c schematic diagram of electrified daisy chain Table 1 Thermal conductivities and electrical resistivities for the materials used in the simulation Material Thermal conductivity (W/m C) Resistivity (lx cm) TCR (10-3 / C) Cu 398 1.7 4.3 FR4 0.7 Sn58Bi 19 30 4.4 Sn58Bi/Cu solder joints, which has been reported in our previous study [11]. It is worth noting that big voids formed at the interface between the Bi-rich phase and the Sn-rich phase. However, the solder joints chain did not open after the test. According to Fig. 4a, the solder joint is still partly connected. The formation of the crack can be ascribed to substantial Cu-Sn IMC growth in the solder joint which consumed a large amount of Sn atoms. Fig. 2 SEM image of one solder joint aged at 110 C without current stressing for 480 h

J Mater Sci: Mater Electron (2010) 21:1090 1098 1093 Fig. 3 SEM images of Sn58Bi solder joints in one daisy chain under 2.5A DC current stressing at 110 C for 480 h, the red arrows indicate the electron current Fig. 4 SEM images of solder joints 1 and 6 under 2.5 A current stressing at 110 C for 480 h: a cross section of joint 1, b local magnified image of joint 1 at the small substrate side, c local magnified image of joint 1 at the large substrate side, d cross section of joint 6, e local magnified image of joint 6 at the small substrate side, and f local magnified image of joint 6 at the small substrate side For solder joint 6, as the direction of the electron flow was from the small substrate (cathode) to the large substrate (anode), most of the Bi atoms accumulated at the large substrate side. The IMC layer at the cathode is mainly Cu 6 Sn 5 with a thickness of about 8.3 lm. By contrast, the IMC layer at the anode is mainly Cu 3 Sn with a thickness of 3.1 lm, and only a very thin (about 0.5 lm) Cu 6 Sn 5 formed at the solder side. It is worth noting that more than 90% of the Bi atoms had accumulated at the anode side for joint 1 and joint 6, which was due to the rapid transfer of the Bi atoms driven by the electron flow. Also, the IMC formed at the anode

1094 J Mater Sci: Mater Electron (2010) 21:1090 1098 side is composed of the Cu 3 Sn phase and Cu 6 Sn 5 phase, it is different from the results reported in previous study [11], which showed the main phase of the IMC at the interface of the Cu/Sn58Bi solder is Cu 5 Sn 6. Two causes are responsible for this. One is the barrier effect of the Bi-rich layer. After long periods of current stressing, the Bi-rich layer became thick enough to block the transfer of Sn atoms and further IMC development was limited due to the lack of Sn atoms at the anode side, while more Cu atoms could be supplied by the Cu pad and more Cu 6 Sn 5 IMC turned into Cu 3 Sn. As the Bi-rich layer developed in the joints illustrated in the previous report was thinner than that in solder joint 1 and joint 6 considered here, substantial amounts of Sn atoms in the bulk solder could diffuse to the interface and react with the Cu atoms to form Cu 6 Sn 5. The other reason is that the actual annealing temperature in the solder joints in the previous report was lower than that in the case considered here. According to Laurila et al. [16], the high temperature will be helpful for the growth of the Cu 3 Sn. It was also reported that Kirkendall voids always tend to form inside Cu 3 Sn during the anneling of the Cu/solder [16, 17] interconnections. But no Kirkendall voids could be observed in our results, which maybe due to the shorter annealing time in this test than that cited in the above literature [16, 17]. For the cathode side, there were enough Sn atoms supplied for the Cu-Sn IMC formation and Cu 6 Sn 5 IMC was the main product. Figure 5 gives the magnified images of solder joint 3 and joint 4. The upper side of the image is the small substrate while the lower side is the large substrate side. There is no big difference in the microstructure between the two solder joint, except the Bi separation occurred at opposite substrate sides. The average thickness of the Bi-rich layer in solder joint 3 is about 24 lm, and that in solder joint 4 is about 22 lm. A layer of Cu 3 Sn IMC with a thickness of about 1 lm was detected at the interface of the anode side for each joint, but the main phase of the IMC layer is Cu 6 Sn 5. The IMC at the cathode interface is mainly Cu 6 Sn 5 for each solder joint. Figure 6 shows the detailed microstructure of solder joint 2. The electric current passed by on the top of the solder bump instead of entering into it. Despite the current did not pass through the solder joint, a Bi-rich phase also formed at the larger substrate side during the test. A major difference of the microstructure from the joints those shown in Figs. 4 and 5 is that two Bi-rich phase regions can be observed in Fig. 6. One Bi-rich phase region formed at the interface of the large substrate (as shown in Fig. 6d). The other one with small area formed at one side of the contact between the solder and the Cu trace on the small substrate (as shown in Fig. 6c), while the Sn-rich phase formed at the other side of the contact between the solder and the Cu trace (as shown in Fig. 6b). The mechanism of formation of the Bi-rich areas in solder joint 2 may be ascribed to the TM and the electron charge swirling, and this will be discussed in a following section. The average thickness of the Bi-rich layer at the large substrate side is about 15.5 lm. The IMC layer at the small substrate is about 4.9 lm, while that at the large substrate is about 4.3 lm which is only a slightly thinner than that at the small substrate side. Figure 7 shows the microstructure of solder joint 5. During the test, there was no current passing through this solder joint or pass by the Cu pads those contacted with the solder bump. It is clear that a Bi-rich layer with an average thickness about 5.9 lm formed at the large substrate side, which is thinner than that in solder joint 2, which also indicates that the migration of Bi in this joint is not prominent. No obvious Sn-rich phase can be observed at the small substrate side. As the Sn-rich phase is induced by the migration of Bi, the small amount of Bi atoms migration towards the large substrate could not leave a visible Sn-rich phase at the small substrate side. IMC layers at the two interfaces are nearly the same thickness, with the thickness of about 3.4 lm. For the Sn58Bi solder joint with current stressing, the polarity effect of the electric current and the barrier effect of the Bi-rich layer played critical roles in the growth of the IMC layers, so the thickness of Fig. 5 SEM images of the cross section of solder joints under 2.5 A DC current stressing at 110 C for 480 h: a solder joint 3 and b solder 4

J Mater Sci: Mater Electron (2010) 21:1090 1098 1095 Fig. 6 SEM images of solder joint 2: a cross section of solder joint 2 with the electric current passing by on top of it, b, c and d local magnified micrographs of each indicated parts IMC layer developed at the anode side is not the same as that at the cathode side. For solder joint 5, no electric current affected the IMC growth, and the barrier effect is not obvious for a Bi-rich layer with a thickness of 5.9 lm, so the IMC layers at the two sides had similar thicknesses. As a thermal gradient was created across the small substrate and the large substrate, the accumulation of the Bi atoms at the large substrate side was due to the TM effect in the solder joint. This will be discussed later. Figure 8 gives the FEM simulation results of the current density distribution in solder joints 1 6 with a 2.5 A DC current applied. The calculated average current density in the powered solder joints is 2.2 9 10 3 A/cm 2. According to the simulation results, the current density in most parts of solder joints 1, 3, 4, and 6 reached above 2.3 9 10 3 A/cm 2. Current crowding was existed at the contact areas between the Cu traces and the solder bumps. The current density at the entry location was about 10 times of the average current density. For the joint 2, the current passing by the Cu trace on the top of the bump, it is clear that current crowding also existed at the entry location and exit location. The current density in most part of solder joint 2 was above 500 A/cm 2, which was due to the effect of electron charge swirling. The effect of electron charge swirling has been reported by Lai et al. [18], their results showed that for a solder joint with the electric current passing by on top of it, the induced vertical current density field leads to the initiation of a void around the UBM. As no current passed by solder joint 5, no current density existed in solder joint 5. In order to understand the effect of electron charge swirling, a vector display of the current density distribution in solder joint 2 is shown in Fig. 9. It is clear that a swirling current density field was created in the solder bump. The magnitude of the current density in the part near the Cu trace on the top of the solder bump was comparable to that on other solder joints having the electric current going through them. Figure 10 shows the FEM simulation results of temperature distribution in the test module supplied with a 2.5 A DC current at an ambient temperature of 110 C. It is obvious that a temperature differences about 40 C existed between the small substrate and the large substrate. The calculated local temperatures are larger than those measured by thermocouple. This may be because the measured locations are the outside surfaces of the test module. Figure 11 shows the detailed temperature distributions in solder joints 1 6. Thermal gradient and the maximum temperature in each solder joint is shown in Table 2. Itis clear that the largest thermal gradient existed in solder joint 2 during the test. 3.2 Discussion For solder joint 1 and solder joint 6, the maximum temperatures in the solder are 131.2 and 136.8 C, respectively.

1096 J Mater Sci: Mater Electron (2010) 21:1090 1098 Fig. 8 Current density distribution in solder joints (a) solder joints 1 3 and (b) solder joints 4 6 (units: A/m 2 ) Fig. 7 SEM images of solder joint 5: a cross section of solder joint 5 (without current stressing), b local magnified micrograph of the interface at the small substrate side, and c local magnified micrograph of the interface at the large substrate side The thermal gradient across joint 1 is 119.2 C/cm, while that across solder joint 6 is about twice of this. As Bi atoms migrate towards the lower temperature side when the solder joint undergoes the thermal gradient [12], the TM and EM of Bi in solder joint 1 are in the opposite directions, Fig. 9 Vector show of the current density distribution in solder joints 2 (units: A/m 2 ) while those are in the same direction for the solder joint 6. However, EM played the overwhelming effect on the migration of Bi atoms for solder joints 1 and 6, it is rational

J Mater Sci: Mater Electron (2010) 21:1090 1098 1097 Table 2 Thermal gradient and the maximum temperature in solder joints Solder joint Thermal gradient ( C/cm) Maximum temperature ( C) 1 119.2 131.2 2 403.5 130.0 3 0 136.8 4 317.3 142.1 5 226.9 124.7 6 239.9 136.8 Fig. 10 Temperature distribution in the test module (units: C) to observe that most of the Bi atoms in the solder accumulated at the small substrate side for joint 1. For solder joint 6, as the thermal gradient is in the same direction of Fig. 11 Temperature distribution in solder joints 1 6 (units: C)

1098 J Mater Sci: Mater Electron (2010) 21:1090 1098 the electron flow, TM assisted the effects of EM on the migration of Bi. For solder joint 2, three factors played important effects on the migration of Bi. One is the effect of the electron charge swirling, which induced a current density above 500 A/cm 2 in the solder bump. The other is that a thermal gradient with magnitude of 403.5 C/cm was created across the solder bump during the test, TM was in the direction from the small substrate to the large substrate side, which drove Bi atoms moving towards the large substrate. The last factor is that the maximum temperature in this joint is about 136.7 C, and this temperature is very near the melting point of Sn58Bi solder alloy. As the migration of Bi is diffusion-controlled process, the temperature played an important effect on the diffusion of Bi atoms. For the above reasons, a Bi-rich layer about 15.5 lm thick formed at the large substrate in solder joint 2. The Bi-rich phase which formed at the contact of Cu trace with the solder bump can be ascribed to the electron charge swirling. For the joint 5, a thermal gradient with magnitude of 226.9 C/cm existed in it, TM is the only driving force for Bi migration. According to the temperature simulation results, the thermal gradient and the maximum temperature are smaller than those for solder joint 2, so a thinner Bi-rich layer with thickness of 5.9 lm formed during the test, which is rational. For solder joint 3, there was no temperature difference in the solder bump, only EM induced the migration of Bi. For the solder joint 4, a thermal gradient about 317.3 C/cm existed in the bump, but it is in the opposite direction of EM. Despite a maximum temperature of 142 C, since the TM counteracted with EM, the migration of Bi was not dramatic and an even thinner layer formed than that in solder joint 3. It is worth noting that the maximum simulation temperature in the solder joint is above the melting point of Sn58Bi alloy, but no obvious melting of the solder joint can be observed. This may be due to the counteract affect of the TM in the solder joint. 4 Conclusions TM and EM in Sn58Bi BGA solder joints were investigated using a particular designed daisy chain supplied with a 2.5 A DC current at 110 C. The individual effects of TM and EM, and combined effects of TM and EM were detected. Driven by the electric current, the Bi migrated towards the anode side and formed a Bi-rich layer therein. With a thermal gradient, the Bi atoms tended to migrate towards the lower temperature side. When the effects of TM and EM were in same direction, TM assisted EM in the migration of Bi, otherwise TM counteracted the effect of EM. The effects of electron charge swirling were detected when the electric current passed by the Cu trace on the top of the solder bump instead of entering into it. After stressed for 480 h, two Bi-rich areas formed in the solder bump. One Bi-rich area formed at one side of the contact between the Cu trace and the solder bump. The other formed at the large substrate side (low temperature side). The electron charge swirling and TM played combined effects on the accumulation of Bi. For the joint without current passing by or passing through, only TM induced the migration of the Bi atoms. A Bi-rich layer with a thickness about 5.9 lm formed at the low temperature side, which was thinner than that with the current passing by the Cu trace on the top of the solder bump. Acknowledgments This project has been supported by RGC General Research Fund (GRF) of Hong Kong (Project No. 9041486). References 1. K.N. Tu, J. Appl. Phys. 94, 5451 (2003) 2. H. Ye, C. Basaran, D.C. Hopkins, Appl. Phys. Lett. 82, 1045 (2003) 3. A.T. Huang, A.M. Gusak, Y.S. Lai, K.N. Tu, Appl. Phys. Lett. 88, 141911 (2006) 4. F.Y. Ouyang, K.N. Tu, Y.-S. Lai, A.M. Gusak, Appl. Phys. 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