Sn structures can grow rapidly within the liquid

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1 Research Summary Lead-Free Solder Plate Formation in the Solidification of Near-Ternary Eutectic -Ag-Cu Sung K. Kang, Won Kyoung Choi, Da-Yuan Shih, Donald W. Henderson, Timothy Gosselin, Amit Sarkhel, Charles Goldsmith, and Karl J. Puttlitz Near-ternary eutectic -Ag-Cu alloys are leading lead-free candidate solders for various applications. These alloys yield three phases upon solidifi cation: β-,, and Cu 6 5. Large, plate-like, pro-eutectic structures can grow rapidly within the liquid phase, potentially adversely affecting the mechanical ehavior and reducing the fatigue life of solder joints. This article reports on the formation of such plates in -Ag-Cu solder alls and joints and demonstrates how large plate formation can e minimized. INTRODUCTION The electronics industry will make sustantial progress toward a full transition to lead-free soldering technology in the near future. At present, nearternary eutectic -Ag-Cu alloys are the leading candidate solders. 1,2 The ternary-eutectic composition is now thought to e close to -3.5Ag-0.9Cu (y wt.%), 3,4 with a melting point of 217 C. The near-eutectic, commercially availale alloys are exemplified y the -3.8Ag-0.7Cu and -4.0Ag-0.5Cu compositions. The electronics industry has egun to study oth the processing ehaviors and the thermomechanical fatigue properties of these alloys to understand their applicaility in the context of electronic card reliaility requirements. 5 The near-eutectic ternary -Ag-Cu alloys yield three phases upon solidification: β-,, and Cu 6 5. Attempts to characterize the solidification ehavior and define the ternary eutectic composition in the -Ag-Cu system have een reported y several investigators. 3,4,6 Using differential-thermal analysis (DTA) methods, 3 it was reported that plate nucleation and ensuing growth may occur with minimal undercooling. In contrast, the β- phase required significantly greater undercooling in order to induce nucleation and ring aout final solidification. 3,6 As a consequence of this disparity in the required undercooling for nucleation, large, plate-like structures can grow rapidly within the liquid phase during cooling, efore the final solidification of solder joints, as reported previously. 7 9 Large plates can adversely affect the plastic deformation properties of the solder 8 and cause plastic-strain localization at the oundary etween the plates and the ounding β- phase. 7 Recently, the authors have also reported several phenomena related to the presence of large plates in thermomechanical fatigue testing of ceramic all-gridarray (CBGA) solder joints. 9 In this study, strain localization at the oundary etween the plates and the β- phase and preferred crack growth at the interface is descried as well as control of large plate formation y increasing cooling rate or y reducing silver content in the -Ag-Cu alloys. An example of a CBGA solder joint, failed y thermomechanical fatigue, is shown in Figure 1, where the interface etween a large plate and the β- phase provided a preferential crack-propagation path. See sidear for experimental details. a c 50 µm 10 µm Figure 1. Cross-sectional views of -3.8Ag-0.7Cu CBGA solder joints failed under accelerated thermal cycling test. Crack propagation along interfaces etween plates and β- phase is indicated y arrows (A, B, C, and D). 61

2 Figure 2. Optical micrographs of -3.8Ag-0.7Cu BGA solder alls: (a) and () as-received, and (c) and (d) slow-cooled (0.02 C/s). c d ALLOY TESTING RESULTS -3.8Ag-0.7Cu Solder Balls Figure 2 shows optical micrographs of the microstructure of as-received and slow-cooled (0.02 C/s) -3.8Ag-0.7Cu (SAC) solder alls. Typical tin dendrites are oserved in oth samples, and the ternary eutectic structure of the - Ag-Cu system is found in the region etween the dendrite arms. In slowcooled alls, a large, rod-shaped structure is noticed in the cross section. To reveal its true morphology, the matrix of slow-cooled SAC solder alls was dissolved in a solution of 50% HCl in H 2 O for two to three days. The extracted structure is seen as a large plate µm wide and µm thick. The size and population of large plates are dependent on the cooling rate and amount of undercooling required for nucleation of β-. Tale I. Melting Temperature and Undercooling of -Ag-Cu Solder Alloys Alloy Melting Composition Temperature Undercooling (wt.%) ( C) ( C)* -3.8Ag-0.7Cu Ag-0.9Cu Ag-0.9Cu Ag-0.9Cu Ag-0.9Cu * Undercooling is defined as the difference in temperature etween the melting point and the point on a cooling trace where the sample temperature rises rapidly from the ase line due to the heat lierated from the solidification. The amount of undercooling and melting point measured for five - Ag-Cu alloys with a different silver content are measured y DSC and listed in Tale I. The undercooling required for the β- phase is determined as the temperature difference etween the melting point on the heating curve and the onset temperature of the β- solidification on the cooling curve. This measurement reports the undercooling required for nucleation of β- phase eing 18 C to 34 C for the -XAg- 0.9Cu alloys, where X = 2.0 to 3.8. The microhardness test was conducted on specific phases within the cross sections of SAC solder alls heat treated at various conditions, as listed in Tale II. The microhardness of the tin-rich phase in the as-received SAC solder alls is aout 21.9 (Vickers hardness numer), while it reduces to 17.3 after annealing at 150 C for 500 h. The microhardness of a large plate itself is measured to e 126.5, which is much higher than the β- matrix found in slow-cooled SAC solder alls, which measured In the SAC solder alls, where large plates were completely dissolved y reflowing at 250 C for 3 min., the microhardness of the fine dendrite structure is found to have a higher value (26.50) than the coarse dendrite (21.5). The higher hardness of the fine dendrite may e attriuted to a higher silver content in the region resulting from the dissolving plate compared to the average silver content of the coarse dendrite. -Ag-Cu Alloys with Reduced Silver Content To investigate the effect of silver content on the formation of large plates, several -Ag-Cu solder alloys with a different silver content were otained commercially as BGA alls: -3.4Ag-0.9Cu, -3.0Ag-0.9Cu, - 2.5Ag-0.9Cu, and -2.0Ag-0.9Cu. The copper content is kept constant at 0.9 wt.%, while the silver content varies from 2.0 wt.% to 3.4 wt.%. When the commercial SAC alloy is included, the silver content ranges from 2.0 wt.% to 3.8 wt.%. The melting point and undercooling of each alloy were measured and listed in Tale I. Each group of solder alls was heated to 250 C, held there for 10 min., and then cooled slowly at a rate of 0.02 C/s to room temperature. The microstructure of aout 100 solder alls from each group was examined to count the numer of Tale II. Microhardness Measurement on -3.8Ag-0.7Cu Solder Balls SAC Solder Balls Microhardness Numer (VHN)* As-Received 21.9 Annealed (150 C, 500 h) 17.3 β- Dendrites (0.02 C/s) 15.2 Large Plates (0.02 C/s) Fine β- Dendrites ( 26.5 dissolved + quenched) Coarse β- Dendrites ( 21.5 dissolved + quenched) * Vickers hardness numer (VHN) is reported as an average value of ten indentations made on the cross sections of solder alls using a 10 gf load. 62 JOM June 2003

3 solder alls containing at least one large plate of or longer. The frequency of oservation of large plates is reported in Tale III. This tale displays clearly the effect of silver content on the population of large plates. For the SAC alloy with 3.8% Ag, 76 out of 100 alls are counted to contain large plates, while almost no alls contain large plates in the slow-cooled alloys with a silver content less than 3.0 wt.%. When the microhardness variation as a function of silver content in asreceived and slow-cooled -Ag-Cu alloys is measured, the microhardness value of as-received (fast cooled) solder alls decreases steadily as silver content is lowered. In the slow-cooled alloys, the microhardness does not vary much regardless of silver content. This may e ecause the solder matrix has approximately the same silver content in the slow-cooled samples due to the precipitation of large plates. -Ag-Cu Alloys with Reduced Copper Content Figure 3 shows optical micrographs from the -Ag-Cu alloys, slow-cooled (0.02 C/s), with a reduced copper content: -3.8Ag-0.7Cu, -3.8Ag- 0.35Cu, and -3.5Ag. Since these samples were in the form of a small ingot, the oservation for large plates was performed qualitatively, as shown in Tale III. There is no difference in the amount of large plates in 0.7Cu and 0.35Cu alloys. However, when there is no copper such as in -3.5Ag (and -Ag-Bi), a few plates are detected, ut much less frequently and they are smaller in size. This suggests that the formation of large plates is also influenced y the presence of copper, ut not as strongly as silver content. CBGA Modules Joined y Reduced-Silver SAC Alloys To demonstrate the effects of cooling rate and silver content, several silverreduced -Ag-Cu alloys were used to join CBGA modules to a FR4 test card having copper onding pads. Two different cooling rates (0.6 C/s vs. 1.2 C/s) were employed after reflow at the peak temperature of 240 C. Figure 4 compares two cross-sectional EXPERIMENTAL PROCEDURES The -Ag-Cu solder alloys investigated in this study are mostly in the form of solder alls, aout 890 µm in diameter, commercially produced for all-grid-array (BGA) applications. Solder compositions investigated include -3.8Ag-0.7Cu, -3.4Ag-0.9Cu, -3.0Ag-0.9Cu, -2.5Ag-0.9Cu, -2.0Ag-0.9Cu, -3.5Ag-3.0Bi, and -3.5Ag (with a nominal composition variation of ± 0.2 wt.%). In a few cases, chill-cast ingots of various compositions (-3.5Ag, -3.8Ag-0.35Cu, and -3.8Ag-0.7Cu) were also used. The thermal analysis of each solder alloy was carried out using differential scanning calorimetry (DSC) to measure the melting temperature and the amount of undercooling associated with solidification. To examine the effects of variations in the solidification kinetics on the formation of large plates, several cooling rates ranging from 0.02 C/s to 3 C/s were applied for BGA solder alls (not attached to any sustrate) after reflow at 250 C for 10 min. The microstructures of BGA solder alls and solder joints solidified at various cooling rates were examined to find the population of plates as well as any changes in the general microstructure due to the different cooling rates. Microhardness tests were performed using a 10 gf load on cross sections of multiple solder alls to detect the cooling-rate effects. Some BGA solder alls with varying silver content were used to join a few ceramic BGA modules to mid-tg FR4 test cards in a forced convection oven under a N 2 atmosphere with peak temperatures ranging from 230 C to 250 C. During reflow processing, different cooling rates (0.6 C/s vs. 1.2 C/s) were applied to control the growth of large plates in the solder joints. views of the joints made of -3.8Ag- 0.7Cu cooled at 0.6 C/s and 1.2 C/s. Several large plates are noted in oth joints. In addition, many small Cu 6 5 rods (seen as particles) are oserved near copper pads, indicating a significant dissolution of copper from the onding pads to occur during the reflow. Figure 5 illustrates the crosssectional views of the joints of - 2.5Ag-0.9Cu cooled at 0.6 C/s and 1.2 C/s. No large plates are detected in the joints with the reduced silver content. This oservation confirms the enefits of a reduced silver content in -Ag-Cu alloys on controlling large plate formation. EFFECT OF COOLING RATE Under manufacturing process conditions, solder joints composed of the -3.8Ag-0.7Cu alloy require 15 C to 30 C undercooling for the nucleation of the β- phase. The DSC measurements undertaken in the present study reflect similar undercooling for the nucleation of the β- phase as shown in Tale I. The final solder-joint solidification event is of such a short duration that significant growth of the plates during this period is extremely unlikely ecause growth must involve long-range diffusion. It is more likely that during the much longer undercooling process, the plates can grow to a large size in the liquid phase efore the final joint solidification event. Accordingly, the cooling rate is critical in determining the formation of large plates in the SAC solder joints. At a high cooling rate, there would e not enough time for plates to grow into a large size if even they were nucleated. This is consistent with the oservation that large plates are not commonly oserved in as-received BGA solder alls and in solder joints cooled rapidly, at a rate of 1 C/s or higher. Hence, the formation of large plates can e kinetically controlled y employing a cooling rate of 1.5 C/s or higher during a reflow process. However, providing a high cooling rate to prevent large plate formation is not always practical, especially with a sustrate of large thermal mass. The high-coolingrate operation can also ring some unwanted side effects, such as thermal stress or strain in a sustrate. Another cooling-rate effect is oserved with the microstructure of β- dendrites in -Ag-Cu alloys. The slow-cooled alloys reveal a coarse β- structure, while the rapidly cooled ones exhiit a fine β- structure, as shown in Figure 2. As reported in Tale II, the microhardness of the slow-cooled alloys is more or less the same regardless of silver content. This suggests that formation of large plates in the 63

4 c 200 µm 200 µm 200 µm Figure 3. Optical micrographs of the slow-cooled -Ag-Cu alloys with a reduced copper content: (a) -3.8Ag-0.7Cu, () -3.8Ag-0.35Cu, and (c) -3.5Ag. slow-cooled alloys would reduce the silver content effectively to the level of approximately 2 wt.%, regardless of the initial silver content. EFFECT OF SILVER CONTENT To suppress thermodynamically the formation of large plates in - Ag-Cu alloys, silver content has een reduced to 2 wt.% with a fixed copper content of 0.9 wt.%. Large plate formation has een sustantially reduced in alloys with a silver content less than 3 wt.%, even in an extremely slow-cooled condition (i.e., 0.02 C/s) as shown in Tale III. This dependence on silver composition can e explained y a simple thermodynamic calculation in an isopleth phase diagram of -XAg- 0.7Cu. The liquidus is determined in the metastale region y direct extrapolation of the liquidus from the ternary eutectic temperature. As discussed earlier, the solidification process in the -3.8Ag-0.7Cu alloy requires an undercooling of 15 C to 30 C efore the tin phase solidifies. The metastaility of the liquid tin phase depends on the availaility of sites for heterogeneous nucleation to take place. At a nominal copper concentration of 0.7% in the liquid, the composition of the liquidus intersecting at 20 C undercooling is approximately 2.7 wt.% Ag. Thus, for compositions equal to or less than 2.7 wt.% Ag, the phase cannot nucleate or grow within the liquid in a solder joint without undercooling in excess of 20 C. Since the typical undercooling required for nucleation found in the present study is approximately 20 C, plate formation can e therefore suppressed significantly, independent of cooling rate. EFFECT OF COPPER CONTENT The population of large plates was similar in -3.8Ag-0.35Cu and -3.8Ag-0.7Cu, ut much less in -3.5Ag, as shown in Figure 3. The presence of copper in the solder appears to promote the formation of large plates, ut it is not explicitly clear how this effect occurs. One possile explanation is that more copper atoms are detected in a large plate (aout 0.36 wt.% Cu) than in β- dendrites (aout 0.16 wt.% Cu), as reported y Moon, et al. 3 This suggests the role of copper atoms in nucleating a large plate. Since only a small amount of copper atoms may e needed to nucleate a plate in an undercooled -Ag-Cu alloy, there would e no difference in the population of large plates for a different copper composition, such as 0.35 wt.% vs. 0.7 wt.%, as long as some copper atoms are availale. In case of actual solder joints, there would e additional copper atoms dissolved from copper metallization, as shown in Figures 4 and 5. Therefore, it would e difficult to determine the effect of copper content in the solder on the formation of large plates. The additional copper dissolved from the copper metallization can cause excessive precipitation of Cu 6 5 particles near the interface and possily harden the solder joint. Another important effect of copper content can e discussed in terms of pasty range of actual solder joints made of -Ag-Cu alloys. The pasty range T is the difference etween the liquidus Tale III. Population of Large Plates in -Ag-Cu Solder Alloys Solidified at a Rate of 0.02 C/s Solder # of Solder Balls Composition Solder with Large (wt.%) Form* Plates -3.8Ag-0.7Cu 1 76/ Ag-0.9Cu 1 5/100, 10/ Ag-0.9Cu 1 6/100, 3/ Ag-0.9Cu 1 1/100, 0/ Ag-0.9Cu 1 0/100, 0/ Ag 2 Few -3.8Ag-0.35Cu 2 Many -3.8Ag-0.7Cu 2 Many a Figure 4. Cross-sectional views of CBGA solder joints of -3.8Ag-0.7Cu alloys (a) slow cooled at 0.6 C/s and () normal cooled at 1.2 C/s. * 1 = BGA alls; 2 = ingot 64 JOM June 2003

5 Figure 5. Cross-sectional views of CBGA solder joints of -2.5Ag-0.9Cu alloys (a) slow cooled at 0.6 C/s, and () normal cooled at 1.2 C/s. (T L ) and the solidus temperature (T S ) for the SAC alloy. Tale IV shows T at two copper concentrations (0.7 and 0.9 wt.%) and at five silver concentrations (2.1, 2.3, 2.5, 2.7, and 2.9 wt.%). It is noted T appears to e highly sensitive to copper content, ut not very sensitive to silver content in this hypoeutectic silver composition range. The large paste range may not e acceptale ecause the solder defect rate in solder joints (such as fillet lifting or pad lifting) could go up dramatically. Thus, depending on how significant the pasty range is in an application, it may e desirale for the copper content in the SAC solder alloy to e no higher than 0.7 wt.%, 0.8 wt.%, or 0.9 wt.%. OPTIMIZATION OF SN-AG-CU ALLOY COMPOSITION Silver and Copper Composition The enefits of a reduced silver content in -Ag-Cu alloys are well demonstrated in the present work, especially for silver content less than 3 wt.%, in minimizing the formation of Tale IV. Calculation of Pasty Range Variations as a Function of Ag and Cu Content in -Ag-Cu Alloy Composition Pasty Range ( T = T l T s ) (wt.%) 0.7%Cu 0.9%Cu -2.1Ag-Cu Ag-Cu Ag-Cu Ag-Cu Ag-Cu large plates. The copper content in -Ag-Cu alloys appears to e less sensitive than silver in forming large plates. However, it is shown that a high copper content can lead to the formation of large Cu 6 5 rods, especially when copper metallization is used in a solder joint. In addition, a high copper content in SAC alloys may enlarge the pasty range as discussed earlier. Thus, reducing oth silver and copper content can e eneficial in suppressing oth large plates and Cu 6 5 particles and limiting its pasty range, and therey in reducing any reliaility risk factors associated with the -Ag-Cu solder joints. Tin Pest Another reliaility concern raised with tin-rich solder alloys is the allotropic transformation of white tin (β-, tetragonal) to gray tin (α-, cuic) at temperatures elow 13 C to form tin pest, as reported y Kariya et al. 10 To confirm this transformation, a ulk sample of -0.7 wt.% Cu alloy was held at 40 C for five months or longer until gray tin was oserved. 11 A scanning-electron micrograph investigation was also conducted in oth the transformed and untransformed area of the sample stored for 11 months at 40 C. The transformed area (tin pest) displayed the individual grains of gray tin with a roken surface due to a large volume change caused y the transformation. The corresponding x-ray diffraction analysis from the gray tin also confirmed the characteristic lines due to the cuic tin (tin pest). 11 Thus, it would e desirale to modify the composition of -Ag-Cu alloys to prevent the tin transformation upon exposure at a low temperature. Based on the present investigation, the authors have proposed a new series of modified -Ag-Cu alloys, 12 of which the -2.3Ag-0.5Cu-0.2Bi (wt.%) BGA alloy for card assemly using standard -3.8Ag-0.7Cu solder pastes is an example. A small amount of ismuth content in the alloy can reduce the possiility of tin transformation as claimed in the literature. 13 Thermal fatigue testing with the modified -Ag-Cu alloys is in progress to demonstrate the eneficial effects of the new alloys. References 1. C.M. Miller, I.E. Anderson, and J.F. Smith, A Viale Tin-Lead Solder Sustitute: -Ag-Cu, J. Electronic Materials, 23 (7) (1994), pp I.E. Anderson et al., Alloying Effects in Near-Eutectic -Ag-Cu Solder Alloys for Improved Microstructural Staility, J. Electronic Materials, 30 (9) (2001), pp K.W. Moon et al., Experimental and Thermodynamic Assessment of -Ag-Cu Solder Alloys, J. Electronic Materials, 29 (10) (2000), p M.E. Loomans and M.E. Fine, Metall. Mater. Trans. A, Phys. Metall. Mater. Sci., 31A (4) (2000), p J. Bartelo et al., Proc. APEX 2001 (Northook, IL: ILF-2, 2001). 6. I. Ohnuma et al., Phase Equiliria and the Related Properties of -Ag-Cu Based P-Free Solder Alloys, J. Electronic Materials, 29 (10) (2000), p D.R. Frear et al., P-Free Solders for Flip-Chip Interconnects, JOM, 53 (6) (2001), p K.S. Kim, S.H. Huh, and K. Suganuma, Materials Science and Engineering, A333 (2002), pp D.W. Henderson et al., Plate Formation in the Solidifi cation of Near-Ternary Eutectic -Ag-Cu Alloys, J. Materials Research, 17 (11) (2002), pp Y. Kariya et al., Tin Pest in -0.5 wt.% Cu Lead-Free Solder, JOM, 53 (6) (2001), p S.K. Kang et al., Proc. 53rd ECTC (Piscataway, NJ: IEEE) to e pulished. 12. D.W. Henderson et al., Lead-Free Tin-Silver- Copper Alloy Solder Composition, U.S. patent fi led 15 Feruary W.G. Burgers and L.J. Groen, Faraday Soc. Discussions, 23 (1957), p Sung K. Kang and Da-Yuan Shih are with IBM T.J. Watson Research Center in Yorktown Heights, NY. Donald W. Henderson, Timothy Gosselin, and Amit Sarkhel are with IBM Corporation in Endicott, NY. Charles Goldsmith and Karl J. Puttlitz are with IBM Corporation in Hopewell Junction, NY. Won Kyoung Choi is with Samsung Advanced Institute of Technology, Suwon, Korea. For more information, contact S.K. Kang, IBM T.J. Watson Research Center, P.O. Box 218, Yorktown Heights, New York 10598; (914) ; fax (914) ; kang@us.im.com. 65