Electric current effect on microstructure of ball grid array solder joint

Transcription

1 Journal of Alloys and Compounds 392 (2005) Electric current effect on microstructure of ball grid array solder joint B.Y. Wu, Y.C. Chan Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, PRChina Received 18 August 2004; received in revised form 20 September 2004; accepted 21 September 2004 Available online 10 November 2004 Abstract The electric current effect on the microstructure of ball grid array (BGA) solder joint was investigated at room temperature with current density of the order of 10 2 A/cm 2. Sandwich-like samples of Au/Ni/Cu pad-sn37pb solder ball-au/cu pad structure were used for current stressing with different time/current density combinations. Extensive dissolution of pad metal, massive formation of (Cu, Ni) 6 Sn 5 and Cu 3 Sn intermetallic compounds (IMC), slight accumulation of (Cu, Ni) 6 Sn 5 IMC, nucleation of large dendrite of (Cu, Ni) 6 Sn 5 IMC, localized dissolution of the pad metal, as well as melting of the solder joint were observed for different cases, which can be ascribed to joule heating produced by the electric current flow in the circuit. A compositional gradient-controlled solid-state diffusion process with assistance of electromigration was suggested for a current density of A/cm 2. And, a reaction-dominated liquid-state dissolution process at very high temperature caused by joule over-heating occurred for A/cm 2. Super-heating induced instant break down of the solder joint and/or circuit break was found for A/cm Elsevier B.V. All rights reserved. Keywords: BGA solder joint; Joule heating; Diffusion; Electromigration; Dissolution 1. Introduction In the past decade, area array technologies, for example, BGA and flip chip interconnects, have been attracting ever-increasing applications in electronic packaging to achieve smaller size, higher reliability and perfect performance. However, with trends towards higher integration and further miniaturization of area array structure, electromigration becomes a new reliability concern for the solder joint due to high current density. Electromigration has been understood as an enhanced diffusion process of metal atoms in the current direction, i.e. atomic flux due to electron momentum transfer arising from an applied electric current [1]. Extensive efforts on the failure behavior of electromigration have previously been based on stripes or thin films of Al and Cu in integrated circuits [2], while little work has focused on solder joints. Corresponding author. Tel.: ; fax: address: eeycchan@cityu.edu.hk (Y.C. Chan). Recently, Tu et al. found that in flip chip solder joint, when the current density reached the range of 10 4 A/cm 2, drift of metal atoms occurred seriously, causing dissolution of under bump metallization at the cathode and the accumulation of IMCs at the anode, meanwhile the current crowding accelerated the migration process. As a result, microvoids formed at the cathode end and hillocks piled up at the anode end [3 7]. Kao and co-workers found a rapid, asymmetrical, and localized dissolution of Cu at the cathode side of flip chip solder joints under a current density of A/cm 2 [8]. Basaran and co-workers [9] characterized the Pb phase coarsening and concluded that electric currents have greater influences on phase growth than temperature or thermomigration caused by temperature gradient due to joule heating during current stressing. However, most of the work was conducted on flip chip solder joints with high current density of A/cm 2 and evaluated at temperature higher than 100 C. Chen and Chen reported directional polarity effect of electric current on the IMC thickness at the interfaces of Sn/Ni, Sn/Ag, Sn/Cu, and /$ see front matter 2004 Elsevier B.V. All rights reserved. doi: /j.jallcom

2 238 B.Y. Wu, Y.C. Chan / Journal of Alloys and Compounds 392 (2005) other couples with DC current density of A/cm 2 [10]. For BGA interconnects, especially in high-density power electronic applications, although its feature size is much larger than that of the flip chip interconnects, it may also be susceptible to electromigration-related issues due to its simple solder-pad metallization configuration, low melting point of Sn37Pb solder, poor thermal dissipation, and possible movement of reactive Sn and Cu atoms in the structure. In the present study, the electric current effect on the microstructure of BGA solder joint was investigated. Since Au/Ni and Au are most commonly used as coatings on Cu pad, Sn37Pb solder balls were soldered on substrates with Au/Ni/Cu pad and Au/Cu pad for current stressing in this work. 2. Experiments Two flexible PCB substrates with designed circuit patterns and BGA solder balls were assembled to prepare the sandwich-like test module. The two flexible substrates have different thin film coatings on the Cu pad, one of which is 2 m electroplated Ni and then 0.5 m immersion Au (Au/Ni/Cu pad); and the other is only 0.5 m immersion Au (Au/Cu pad). The diameter of the Cu pad on both substrates is 680 m. At first, eutectic Sn37Pb solder balls, with diameter of 680 m, were coated with flux and attached to the desirable Au/Ni/Cu pads on one substrate to conduct reflow soldering. Then, the other substrate with corresponding Au/Cu pads was flip down to cover on the balls. Following that, the whole assembly was reflowed again to form BGA solder joints, providing both mechanical support and electrical interconnect. In the temperature profile of reflow, the peak temperature was 220 C and dwell time above liquid was about 60 s. At last, the sample was ultrasonically washed for 10 min in alcohol to remove the flux residue and then dried for current stressing. In total 24 samples were prepared as described above using the same process parameters to assure the results were reproducible. In this experimental design, only one solder joint per sample is subjected to current stressing, which is electrically connected to electrodes by daisy chains. Other solder joints function only as supporting pillars. The daisy chain Cu trace is 300 m wide and 15 m thick. Length of the Cu trace from solder joint to electrodes is about 4000 m. At room temperature, a constant electric current was applied to the solder joint through conductive wire soldered to the electrodes, in which the Au/Ni/Cu pad is the anode side, and Au/Cu is the cathode side. A precision current source, IXL-LDX-3500, was used to assure constant and stable electric current, in any case of resistance fluctuation of the circuit. Different currents strengths, 1, 1.5 and 2 A, were applied and maintained for different time periods for different samples. After the test, each sample was molded and polished to observe the microstructure of the solder joint by using scanning electron microscopy (SEM). 3. Results and discussion The typical cross-sectional SEM image of a BGA solder joint just reflowed is shown in Fig. 1. The arrow labeled e indicates the direction of electron flow. The average size of the opening window at the interface of pad/solder for current flow is about 650 m in diameter, and the average standard height is about 600 m. The microstructure views of the anode side A and cathode side B in Fig. 1 before current stressing are enlarged in Fig. 2(a) and (b), respectively. It is known that the thin Au coating is dissolved and consumed away quickly into the Sn-based solder bulk during soldering Fig. 1. SEM of cross-section of BGA solder joint: (a) At anode and (b) At cathode.

3 B.Y. Wu, Y.C. Chan / Journal of Alloys and Compounds 392 (2005) Fig. 2. Solder joint interfaces as reflowed without current stressing: (a) At anode and (b) At cathode. [11,12]. At the anode side, between Sn37Pb solder and Ni coating, Ni 3 Sn 4 was confirmed by energy-dispersive X-ray (EDX) to form and mix with the irregular Ni layer. It is necessary to mention here that the surface of the electro-plated Ni coating is not smooth and the solder to the Ni interface experienced two times the above-described reflows, producing ani 3 Sn 4 /Ni mixture layer with an average thickness value of about 4.5 m. At the cathode side, the solder to the Cu interface only underwent one time a reflow, which generated a layer about 3 m thick of Cu 6 Sn 5 IMC. According to earlier study [13], a thin layer of Cu 3 Sn IMC may exist at the Cu to Cu 6 Sn 5 interface although it was not visible due to resolution limitation of the SEM. The current stressing results are summarized in Table 1 to show the IMC evolution in the solder joint. The average current density through the opening window of solder joint to pads was calculated to be about , and A/cm 2 when 1, 1.5 and 2 A current were applied, respectively. Fig. 3 shows the image of the sample after 168 h of A/cm 2 stressing. A few small (Ni, Cu) 6 Sn 5 particles were found to distribute close to the Ni 3 Sn 4 layer at the

4 240 B.Y. Wu, Y.C. Chan / Journal of Alloys and Compounds 392 (2005) Table 1 Characters of IMCs for different current density/time combination Cases IMC at anode side (Au/Ni/Cu pad) IMC at cathode side (Au/Cu pad) Composition Thickness ( m) Composition Thickness ( m) 0 A and 0 h Ni 3 Sn 4 /Ni mixture 4.5 Cu 6 Sn A/cm 2 Ni 3 Sn 4 /Ni mixture 4.5 Cu 6 Sn h (Ni, Cu) 6 Sn 5 : at.% Cu Dispersed A/cm 2 Ni 3 Sn 4 /Ni mixture 4 Cu 6 Sn h (Ni, Cu) 6 Sn 5 : at.% Cu Dispersed A/cm 2 Cu 3 Sn 2 Cu 3 Sn 2 11 h (Ni, Cu) 6 Sn 5 : at.%ni 18 (Ni, Cu) 6 Sn 5 : at.% Ni 10 Fig. 3. Solder joint interfaces after 1 A and 168 h current stressing: (a) At anode and (b) At cathode.

5 B.Y. Wu, Y.C. Chan / Journal of Alloys and Compounds 392 (2005) anode side. The Cu atomic ratio in the IMC was measured to be in the range at.%, which varied from location to location. The Ni 3 Sn 4 /Ni mixture layer thickness at the anode side is about 4.5 m, the same as that of the samples without current stressing. However, at the cathode side, the Cu pad consumption is apparent and the Cu 6 Sn 5 layer grows to be about 6 m, which is two times as thick as that without current passage. Fig. 4 illustrates the result after 353 h of A/cm 2 current stressing. The interfaces do not show much difference from that of the sample subjected to 168 h of A/cm 2 stressing. The IMC thickness and composition at both sides were found to be similar to those depicted in Fig. 3. In this case, the Cu atomic percentage in dispersed (Ni, Cu) 6 Sn 5 particles at the anode side increased slightly to become at.%. Comparing the grain size in the bulk solder, a grain-coarsening phenomenon of the Pb-rich phase (shown in the picture as a white zone in the bulk solder) was observed. After 353 h of current stressing, the Pb-rich size was obviously larger than in the 168 h samples described in Fig. 3. This result is rather consistent with the current density, time- and temperaturedependent grain-coarsening model suggested by Ye et al. [9]. Fig. 4. Solder joint interfaces after 1 A and 353 h current stressing: (a) At anode and (b) At cathode.

6 242 B.Y. Wu, Y.C. Chan / Journal of Alloys and Compounds 392 (2005) Fig. 5. Solder joint interfaces after 1.5 A and 11 h current stressing. When A/cm 2 of current was applied and lasted for 11 h, significant interfacial reaction between solder and pads occurred, as can be seen in Fig. 5. The pad metals on both sides were dissolved extensively by the solder to form a huge IMC zone. At the anode side, a large amount of IMCs exist as a result of almost half (about 10 m thick) of the pad base metal being consumed away, as proved in Fig. 5(a). A relatively dark layer of about 2 m thick Cu 3 Sn is very clear, whereas the Ni coating disappeared. Meanwhile, a massive (Ni, Cu) 6 Sn 5 block follows the Cu 3 Sn layer to produce a layer with an average thickness of 18 m. The detected Ni atomic ratio in the IMC increased from 3.6 to 20.5 at.% along the direction from pad to solder. Unlike dispersed particles in the above cases, long needle-like (Ni, Cu) 6 Sn 5 protrudes irregularly towards the bulk solder. At the cathode side, Cu 3 Sn and (Ni, Cu) 6 Sn 5 layers were also observed, where they are much more flat than at the anode side. In total about 5 m thick part of the Cu pad was gone due to interfacial reaction, resulting in about 10 m thick IMC, in which (Ni, Cu) 6 Sn 5 with a Ni atomic rate of at.% dominates the main body. The IMC thickness and related pictures of A/cm 2 cases are not shown since the circuit failed with a burning of the polymeric materials or breaking apart of the

7 B.Y. Wu, Y.C. Chan / Journal of Alloys and Compounds 392 (2005) solder joint instantly within a few seconds after applying a 2 A current. The critical condition for triggering electromigration, derived mostly from study of flip chip solder joint, is at above 100 C and of the order of 10 4 A/cm 2 [14,15]. However, the results in this work indicated that even at room temperature and low current density of 10 2 A/cm 2, some electromigration-like behaviors in flip chip solder joints can still occur in Sn37Pb BGA solder joints. A dissolution of the Cu pad could be found to be comparable to UBM and/or Cu trace dissolution at the cathode side, and slight dispersion of (Ni, Cu) 6 Sn 5 particles was similar to the IMC accumulation at the anode side. However, it was not as apparent as expected in high-density current stressing test of flip chip solder joints. For A/cm 2 cases, at the cathode side, Cu dissolution and Cu 6 Sn 5 thickening were found at the cathode side, whereas these reactions did not proceed significantly as time increased. As reported above, 168 and 353 h stressing gave almost the same Cu 6 Sn 5 thickness, which indicated that in the two constituent fluxes of migration, an atomic flux driven by a concentration gradient dominated the growth rate of the IMC over that driven by electron wind [16]. At an early stage of current stressing, many Cu atoms in the pad were driven to leave the original positions and move towards the structurally and compositionally heterogeneous IMC layers. Therefore, some Cu atoms reacted with the IMC to form Cu 3 Sn and the latter was gradually transformed to Cu 6 Sn 5 by addition of Sn atoms. Some Cu atoms passed through the inter-granular spacing in the IMC thin layers and then combined with Sn atoms to form new Cu 6 Sn 5. At the same time, a small amount of Cu 6 Sn 5 was dissolved to release some Cu atoms, of which some reacted with Sn atoms and some others could possibly reach the cathode side. The dissolution of the Cu pad and the nucleation of new Cu 6 Sn 5 competed with the dissolution of Cu 6 Sn 5, contributing to the thickening of the IMCs. After the IMCs kept growing to become dense and thick enough as a barrier, it retarded the movement of atoms and only a few Cu atoms could travel so far to make the IMC grow continuously. The IMC layers reached a saturated and stable status, thus the addition of Cu could not induce continuous and remarkable IMC growth any more. To some extent, it could be said that the thick and compact Cu 6 Sn 5 IMC is much more resistant to the atomic migration and reaction. At the anode side, the IMC thickness did not change much due to an insignificant movement of Cu atoms from the cathode and a low diffusion rate of Ni from the coating. Under a low current density, a small amount of Cu atoms could arrive at the anode side even after a long time of current stressing. In addition, Ni 3 Sn 4 and Ni are well known as diffusion-resistant layers, only a few of them dissolved to release Ni atoms to migrate for combining with Sn atoms and arrived Cu atoms. That is why no appreciable IMC appeared to accumulate at the anode side, but only a few (Ni, Cu) 6 Sn 5 particles dispersed near the original Ni 3 Sn 4 /Ni mixture layer. So, we suggested that for the A/cm 2 case, the microstructure characteristics were the result of a solid-state reaction which was dominated by a concentration gradient and aided by electromigration and temperature increase due to current flow. For the A/cm 2 case, it is interesting to mention that for only 11 h stressing, massive IMC grew at both sides, which was quite different from the A/cm 2 cases. It was reported elsewhere that during current stressing experiments, the dissolution of either the base metallization or IMC occurs only at the cathode side, from where electrons are leaving [17]. But for our case, at the anode side, not only the Ni coating was dissolved completely, but remarkably also the Cu base beneath it. At the center of the solder joint, very large dendrite like (Ni, Cu) 6 Sn 5 was distinct as shown in Fig. 6. In addition, (Ni, Cu) 6 Sn 5 was also detected at the cathode side, which requires diffusion of Ni from the anode side to the cathode side against the direction of electron flow. We tend to believe that the tremendous dissolution of pad metal, huge (Ni, Cu) 6 Sn 5 formation in this case was governed by the liquid reaction between molten solder and pad metal. It was not a solid-state migration induced mechanism any more, but a liquid-state dissolution controlled interaction. The melting of solder is a result of the high temperature caused by joule over-heating during the current stressing. If the solder remained in solid status, we can apply the solid-state diffusion coefficient D s of Ni in the solder, of the order of cm 2 /s [18], to determine the time required for Ni atoms at 175 C to move from the anode side to the cathode side. Simply using the relationship χ 2 = Dt, we conclude that it would be impossible by solid-state diffusion to let Ni travel across 600 m in a time of less than 10,000 h. In our case, Ni was detected at the cathode within 11 h. Only if the solder was in molten condition could this happen. If the solder changed into liquid status, the Ni atoms can very easily move due to the chemical concentration gradient. Since the diffusivity of Ni is nearly the same as that of Cu, using a liquid-state diffusion coefficient D l, of the order of cm 2 /s [19], the time was calculated ranging from 6 min to 1 h. Thus, it is not surprising that some Ni atoms competed with the electron driving force to move towards and reach the cathode in short time. Herein, the electric current effect on liquid solder is not clear and further investigation is needed. Moreover, the Cu 3 Sn phase was abnormally apparent and was about 2 m thick at both sides, which could also prove the liquefaction of the solder. Generally, in a typical reaction of a Sn37Pb/Cu couple at a reflow soldering temperature of C for about 60 s, Cu 6 Sn 5 phase growth with a favorable energy is much more apparent than Cu 3 Sn growth, which is too thin to be detected. It was also reported by Prakash and Sritharan that high temperature and low Sn content in solder could enhance the nucleation of Cu 3 Sn between SnPb solder and Cu [20]. In our case, it was supposed that the temperature was raised to melt the solder and reaching more than 230 C, which makes it thermodynamically beneficial to form the Cu 3 Sn phase. Thus, after complete dissolution of the Ni coatings, the Cu atoms reacted rapidly with Sn. The Sn37Pb solder nearby the pad was saturated by Cu atoms, inducing a

8 244 B.Y. Wu, Y.C. Chan / Journal of Alloys and Compounds 392 (2005) Fig. 6. Dendrite IMC in center of the solder joint (1.5 A and 11 h). lower percentage of Sn in the solder. Therefore, after massive Cu 6 Sn 5 nucleation, the supply of Sn to Cu was inadequate while the Cu atoms were active at high temperature, compelling Cu 3 Sn formation and survival that contributed to the thickness promotion. Large amount of joule heating produced in the circuit and poor thermal dissipation properties of the material/structures could account for the temperature increase and melting of the solder joint. The heat generation in the Cu trace would be much greater than that in the solder joint with the same current flow since the joule heat produced is proportional to the resistance. Furthermore, the Cu trace is enclosed by insulating materials, which implies a poor heat conductive condition. The resistance R cu and quality M cu of a daisy chain Cu trace with crosssectional area of 4500 m 2 and a total length of 8000 m are calculated to be and g, respectively. The specific heat C cu of Cu is about J/(g C). The heat Q r needed for the Cu trace to have a temperature rise T of 200 C could be simply determined as J, according to Q r = M cu C cu T. It is equal to only about 11 s of total joule heating Q j produced in the Cu trace by a 1.5 A current, when adopting the equation of Q j = I 2 R cu t. Thus, it is expected that when the joule heating was larger than heat dissipation, the temperature of the Cu trace exceeded the melting point of the Sn37Pb (183 C), and the solder joint connected to the Cu trace become melted. It is even more interesting that under a stereomicroscope, we observed cyclical solidification liquefaction transition in terms of rapid changing of the color, curvature and reflection of the solder joint, as shown in Fig. 7. This can partially be explained as follows. For liquefaction, the solder joint absorbed the latent heat rooted in the Cu traces that reached higher temperature due to the joule heating. After melting of the solder joint, the temperature of the system decreased a little bit. Simultaneously, heat dissipation from the solder ball and the Cu trace was also responsible for reduction of the temperature of the system. Thus, due to decrease of the temperature, the solder ball was solidified again, especially the outside layer of the solder joint. During solidification, the solder ball released the latent heat. In the meantime, the temperature of the Cu trace also increased because of the joule heating. Thus, the cumulative heat could increase the temperature of the system again to melt the solder ball. With increasing current flow, Fig. 7. Melting of solder joint by over-joule-heating.

9 B.Y. Wu, Y.C. Chan / Journal of Alloys and Compounds 392 (2005) Fig. 8. Localized dissolution of pad with 2 A in a few seconds. the heat generation would be greater and thus, the duration of the liquid state would also be increased. It is the first time that we have found in this work that the solder joint solidified and liquefied repeatedly when a certain amount of current was added during the current stressing experiments. However, with a further increase of the current density to A/cm 2, when the supply of heat was high enough to abruptly increase the temperature of the liquid solder beyond its melting point, the circuit failed due to superheating. Although it is hard to measure the average thickness of the IMC since it varied from location to location and sample to sample, we obtained a localized dissolution of the Cu pad in a region where current flowed to the solder joint. As seen in Fig. 8, the joint deformed seriously due to gravity and surface tension, and the corners on both sides were locally dissolved. Much dendritic IMC was also found to become distributed over the whole solder joint. Again, it is conceivable that superhot spots induced by a large amount joule heating due to the close contact with the Cu trace and possible current stressing [5,6] at the meeting point of the Cu trace and solder ball can directly account for the surprising localized dissolution. Especially, at locations with defects, such as pinholes in the metallization, the dissolution might be significant. 4. Conclusions The electric current effect on the microstructure evolution of Sn37Pb BGA solder joint has been investigated with current densities of , and A/cm 2.It was found that electromigration-like phenomena could still occur, although not so apparently, even at low current density of the order of 10 2 A/cm 2 and room temperature conditions. In the process, dissolution of the Cu pad at the cathode side, accumulation of (Ni, Cu) 6 Sn 5 IMC at the anode side, and Pb phase coarsening in the solder were observed. For the A/cm 2 case, the reaction in the solder joint was a solid-state diffusion controlled by a Cu concentration gradient with the aid of electric current flow, which was responsible for the thickening of the IMC at the cathode side as well as a slight change at the anode side. For the A/cm 2 case, it was proposed that a liquid-state interaction at high temperature caused by joule heating from the Cu trace accounted for the considerable dissolution of the Cu pad metal, huge (Ni, Cu) 6 Sn 5 formation and abnormal nucleation of Cu 3 Sn at both sides, plus the presence of dendritic IMC in the solder joint. For the A/cm 2 case, overwhelming joule heating led to instant failure/break of the circuit/solder joints in a few seconds, in which local dissolution of the Cu pad and dendritic IMC formation were noticeable in the solder joint for some samples. Acknowledgements This project has been supported by CityU CERG project ( , N CityU 103/03). The discussion with L. Chen at NUS is gratefully acknowledged. References [1] K.N. Tu, J. Appl. Phys. 94 (2003) [2] A. Christous, Electromigration and Electronic Device Degradation, Wiley, United States of America, [3] T.Y. Lee, K.N. Tu, J. Appl. Phys. 90 (2001) [4] H. Gan, K.N. Tu, Proceedings of the 52nd Elec. Comp. Tech. Conference, 2002, pp

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