(Received March 3, 1999; accepted May 14, 1999). Effect of Cooling Rate on Microstructure and. Mechanical Properties of Eutectic Sn-Ag Solder Joints with and ...
1184 of ELECTRONIC MATERIALS, Vol. 28, No. 11, 1999 Journal
Sigelko, Choi, Subramanian, Lucas, Special andIssue Bieler Paper
Effect of Cooling Rate on Microstructure and Mechanical Properties of Eutectic Sn-Ag Solder Joints with and without Intentionally Incorporated Cu6Sn5 Reinforcements JEFF SIGELKO, S. CHOI, K.N. SUBRAMANIAN, JAMES P. LUCAS, and T.R. BIELER Department of Materials Science and Mechanics, Michigan State University, East Lansing, MI 48824-1226 USA
Solidification of eutectic Sn-Ag solder, with and without Cu6Sn5 composite reinforcements, on copper substrates, was investigated at two different cooling rates. The size, orientation, randomness, and overall morphology of the dendritic microstructure were examined as a function of cooling rate. Cu6Sn5 particle reinforcements were found to act as nucleation sites for dendrites, in addition to sites on the substrate/solder interface. The mechanical properties of these solders were also examined as a function of cooling rate. Solder joints with a lower load-carrying area were found to exhibit higher shear strength, but reduced ductility when compared to solder joints with more load carrying area. Key words: Solder joints, eutectic Sn-Ag, Cu6Sn5 reinforcements, microstructure
INTRODUCTION Over the past decade, there has been significant interest in eliminating lead in solders because of its hazardous nature to life and the environment. Certain lead-free solders such as eutectic Sn-Ag and Sn3.5Ag-.5Cu1 solders have emerged after extensive considerations as likely candidates to replace lead containing solder joints. One of their advantages is that they melt at temperatures near 221°C instead of at 183°C like the eutectic 63Sn-37Pb alloy does. This property has led to higher service temperature applications. Other important properties are strength and ductility. The strength and ductility of a solder joint are very important for in-service applications. Solder joints need to demonstrate a good balance between strength and ductility in order to withstand thermal, mechanical, and thermomechanical loading. In addition to compositional variations, other avenues that are pursued for improving mechanical properties include a composite approach.2–5 One such variation is incorporation of intermetallic reinforcements by insitu methods.6,7 Such reinforcements can alter the properties of the solder joint, and can also alter the solidification structure by providing additional heterogeneous nucleation sites for dendrites. The effect of the solidification structure on the (Received March 3, 1999; accepted May 14, 1999) 1184
mechanical behavior has been documented in normal Pb-Sn solder joints.8,9 The main goal of this study is to correlate the solidification structure in non-composite and composite solder joints solidified at different cooling rates. Another goal is determine the effect of cooling rate on the mechanical properties of these solder joints. The mechanical properties studied in detail were shear strength and simple shear. Simple shear is defined as the displacement of the solder joint divided by the thickness of the solder joint. Simple shear is used instead of shear strain because the shear displacement is too high to describe with a parameter such as shear strain. The solder joints studied were representative of realistic solder joints used in surface mount technology, with respect to geometry, size and solidification parameters. Eutectic Sn-Ag solder was the solder of choice in this investigation. This solder is referred to as noncomposite solder, and the same solder containing 20 vol.% Cu6Sn5 particle reinforcements in the solder is referred to as a composite solder. The patent is pending for the methods used to produce the composite solder. EXPERIMENTAL PROCEDURE The solder joints used in these experiments were prepared by using two half dogbone shaped copper strips made by Electro Discharge Machining (EDM). Copper dogbones were masked to maintain a solder
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the number of solder preforms used to make the solder joints. A thick solder joint was obtained with three preforms while a thin solder joint had one preform. Solidified solder joints were prepared for microstructural evaluation by standard metallographic techniques. Scanning electron microscopy (SEM) was used to reveal the microstructural features of the solder joints. An Instron tensile testing machine was used to load single shear-lap solder joint samples at a cross-head speed of 1.27 mm/min (.05 in/ min) to obtain mechanical property data. RESULTS/DISCUSSION Fig. 1. Representative cooling curves for all samples.
area of about 1 mm2 during soldering. The masking compound was painted around the potential solder area, and baked at 150°C for about one h. The regions on the copper dogbones to be soldered were cleaned with a solution of 1 part nitric acid and 1 part water, followed by a wash in methanol. The solder area was fluxed with alpha 200 L flux. Solder preforms, roughly 1 mm2 in area and approximately 40 microns thick, were placed between the two fluxed copper strips. An aluminum fixture was used to prepare several identical samples simultaneously. The entire fixture was heated to 280°C to melt the solder preforms. This same procedure was used to make both composite and non-composite solder joints to facilitate comparisons. In order to impose two different cooling rates on the molten solder, the aluminum fixture containing several identical samples was cooled on either a steel block or a firebrick after the solder had melted. Temperature readings of the aluminum fixture were recorded every 20 seconds once the aluminum fixture was placed on the cooling block. Since aluminum is a very good thermal conductor, it was assumed that the temperature of the solder joints in the aluminum fixture was the same as the aluminum fixture itself. Cooling curves were obtained by plotting the temperature readings obtained as a function of time. The thickness of the solder joints was varied by varying
As shown in Fig. 1, the cooling rates of samples cooled on the steel block and the firebrick are different. The cooling rate for samples cooled on a steel block was about twice as fast as the cooling rate for samples solidified by cooling on a firebrick. The initial cooling rate for samples cooled on the steel block was about 330°C/min. The initial cooling rate for samples cooled on the firebrick block was about 140°C/min. The Sn-Ag solder joints solidified at 221°C. The dendritic microstructure of each sample was evaluated to determine possible dendrite nucleation sites. It was found that the solder/substrate interface is a very common site for dendrites to nucleate. Cu6Sn5 particles in the bulk solder also served as dendrite nucleation sites as shown in Fig. 2a. Other sites that dendrites could nucleate were in the regions of the interface where there was no intermetallic layer formation as shown in Fig. 2b. Large pores also provided heterogeneous nucleation sites for dendrites as shown in Fig. 2c. Overall, the dendritic microstructure in non-composite samples tended to be more oriented along a given direction, while the dendrites in the composite samples were very random with respect to directional orientation. Typical microstructures of non-composite and composite solder joints cooled at chosen rates are provided in Figs. 3 and 4. The width of the dendrites was calculated for both the composite Sn-Ag and noncomposite Sn-Ag samples. Overall, the samples cooled
a b c Fig. 2. Other dendrite nucleation sites besides substrate/solder interface. (a) Dendrite nucleation from particle; (b) dendrite nucleation from in between interfacial Cu6 Sn5 particles; (c) dendrite nucleation from large pore present in microstructure.
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a
a
b Fig. 3. Typical non-composite solder joint microstructures. (a) Thin joint—cooled on firebrick; (b) thick joint—cooled on steel.
on steel had the largest dendrite width in both composite and non-composite samples. Conversely, the samples cooled on firebrick had the smallest dendrite width in both types of solder joints. However, this trend was reversed in thick, non-composite solder joint samples. Another observation is that all the composite samples had a smaller dendrite width than their non-composite counterparts. This is likely due to the presence of the Cu6Sn5 particle reinforcements. These particles tended to limit the size of the dendrites while they form. As a result, the dendrites formed in the composite solder have a smaller width. The dendrites also do not grow very long because the mean free path for dendrite growth is much shorter in the composite solder. Table I shows the results of the measured dendritic width. The density of the dendrites in the non-composite samples was calculated by overlaying a grid pattern on SEM micrographs of the microstructure. The density of the dendrites is defined as the percent area of microstructure that is inhabited by actual dendrite arms. Density of the dendrites in the microstructure ranged from about 58% to 90%. Overall, samples cooled on steel had the lowest density in the micro-
b Fig. 4. Typical composite solder joint microstructures. (a) Thin joint— cooled on firebrick; (b) thick joint—cooled on steel.
Table I. Width Measurements Cooling Block/ Joint Geometry
Thin
Thick
Non-Composite Dendrite Steel 5.16 µm Firebrick 3.63 µm
5.18 µm 5.76 µm
Composite Dendrite Steel Firebrick
5.10 µm 4.05 µm
4.47 µm 3.69 µm
structure with an average density of about 65.6%, and samples cooled on firebrick had the highest dendrite density with a density of 75.3%. Similar studies were not carried out on composite solder joints. The data obtained from the mechanical testing of
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Table II. Average Mechanical Properties Data for Identical Cooled Samples Sample Geometry
Max. Shear Stress (MPa)
Simple Shear at Max. Sh. St.
Total Simple Shear
Effective Area (mm2)
% Porosity
Thin Thick Thin Thick
41.2 45.4 48.1 58.0
1.59 1.13 .818 1.15
6.30 3.80 2.54 2.10
.853 .719 .488 .543
.5 1 15.2 25.7
Cooled on Firebrick Noncomp. Thin Noncomp. Thick Comp. Thin Comp. Thick
44.8 36.7 51.0 51.3
2.03 .762 1.06 .554
4.03 1.96 2.09 1.49
.922 .798 .597 .572
13.5 .5 20.7 20
Solder Cooled on Steel Noncomp. Noncomp. Comp. Comp.
a
b
c Fig. 5. (a) Influence of thickness on mechanical properties in similar solder joints with similar cooling rate, porosity, and effective area; (b) influence of solder type on mechanical properties of solder joints with similar cooling rate, porosity, and effective area; (c) influence of effective area on mechanical properties in similar solder joints with similar thickness, porosity, and cooling rate.
the solder joints is given in Table II. The entries provided in this table are the averages based on about three samples for each condition. The soldered area was determined from fractured samples for converting the shear load into shear stress. The porosity in the actual sheared cross-sectional area was subtracted out to find the load-carrying area subjected to the shear stress. This load-carrying area is defined as the effective area. As can be seen from shear test data, the shear strength of the thicker solder joints is consistently higher than corresponding thinner joints in both composite and non-composite solder joints that were cooled with the fast cooling rate employed by the steel block. The differences in strength of non-composite and composite samples cooled at the slower cooling rate are not as significant as compared to samples cooled on steel. Also, composite solder joints showed consistently higher strength than the noncomposite samples for similar joint thicknesses— especially in samples cooled on steel. The differences in strength between composite and non-composite samples were not significant in samples cooled on a firebrick. An attempt was made to correlate the shear strength of the samples to the corresponding microstructure with respect to the following dendrite parameters: dendrite width, dendrite orientation, and dendrite directional randomness. No clear trend was evident. As stated earlier, non-composite samples tended to have large dendrites with some degree of preferred orientation while the dendrites in the composite samples were smaller in size, and showed complete randomness with respect to orientation. Correlating these significant differences in the microstructure to the shear strength of the solder joints was difficult. Since the thicker solder joints had a higher shear strength than the thinner solder joints in both composite and non-composite samples, the size, orientation, and randomness of the dendritic microstructure were found not to be dominant factors that affect the shear strength of the as-fabricated solder joints. Influence of solder thickness, solder type, and solder effective area (while maintaining other parameters similar) on the shear properties are presented in Fig. 5a, b, and c, respectively. A correlation was found
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between the effective area and the shear strength of each sample. It was found that samples with a lower effective area generally had a higher strength that occurred at a lower simple shear. Likewise, samples that had a higher effective area had a lower shear strength, but fractured at a higher simple shear. This may be attributed to the concentration of stress over a smaller area that can work harden the solder material during the shear testing. Work hardening of the solder material can strengthen the solder material, which can lead to higher shear strength. The consequence of work hardening the material is reduced ductility. Another explanation for the observed trends could be given on the basis of higher strain rate. Since the shear strain is being accommodated by less solid area in specimens with higher porosity, they may be experiencing a higher shear strain rate. This may also enhance the shear strength at a cost of lower ductility. CONCLUSIONS Based on the results, the dendrite size, orientation, and randomness do not significantly affect the mechanical properties of as-fabricated solder joints. This conclusion was arrived at after it became apparent that noticeable differences in the microstructure of non-composite and composite solder joints did not correlate with the observed trends in mechanical properties data obtained with solder joints of two different thicknesses and cooled at two different rates from the molten state. Solder joints with lower effective areas generally had a higher shear strength, probably due to work hardening of the solder material. Work hardened solder joints that had a higher shear strength also failed at a lower simple shear resulting in reduced ductility. In the range of cooling rates utilized in this study,
the mechanical property variations due to geometrical constraints, and available ductile region to undergo plastic deformation, as dictated by the thickness of the joint, seem to be more significant than the corresponding contributions due to microstructural variations resulting from different cooling rates. ACKNOWLEDGEMENTS The authors would like to thank the Composite Materials and Structures Center of Michigan State University for the financial support of this project. REFERENCES 1. C.M. Miller, I.E. Anderson, and S. F. Smith, J. Electron. Mater. 23, 595 (1994). 2. M. McCormack, S. Jin, and G.W. Kammlott, IEEE Trans. Components, Packaging and Manufacturing Technology, Part A 17, 452 (1994). 3. C.G. Kue, S.M.L. Sastry, and K.L. Jerina, Proc. First Int. Conf. Microstructures and Mechanical Properties of Aging Materials, ed. P.K. Liaw et al. (Warrendale, PA: TMS, 1993), p. 409. 4. R.B. Clough, R. Patel, J.S. Hwang, and G. Lucey, Proc. Technical Program III, National Electronic Packaging and Production Conference (Des Plaines, IL: Cahner-Exposition Group, 1992), p. 1256. 5. M.A. Wasynczuk and G.K. Lucey, Proc. Technical Program III, National Electronic Packaging and Production Conference (Des Plaines, IL: Cahner-Exposition Group, 1992), p. 1245. 6. A.W. Gibson, S.L. Choi, K.N. Subramanian, and T.R. Bieler, Design and Reliability of Solders and Solder Joints, ed. R.K. Mahidhara et al. (Warrendale, PA: TMS, 1997), p. 97. 7. S.L. Choi, A.W. Gibson, J.L. McDougall, T.R. Bieler, and K.N. Subramanian, Design and Reliability of Solders and Solder Joints, ed. R.K. Mahidhara et al. (Warrendale, PA: TMS, 1997), p. 241. 8. Z. Mei, J.W. Morris, Jr., M.C. Shiniamo, and T.S.E. Summers, J. Electron. Mater. 20, 599 (1991). 9. Z. Mei and J.W. Morris Jr., Trans. ASME, 114 (New York: ASME, 1992), pp. 104–108.
