Journal of ELECTRONIC MATERIALS, Vol. 33, No. 4, 2004 Regular Issue Paper
374
Mechanisms for the Intermetallic Formation during the
Sn-20In-2.8Ag/Ni Soldering Reactions
T.H. CHUANG,1,2K.W. HUANG,1and W.H. LIN1
1.—Institute of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan. 2.—E-mail: [email protected]
The morphology and growth kinetics of intermetallic compounds formed during the interfacial reactions between liquid Sn-20In-2.8Ag solder and Ni substrates are investigated. Energy-dispersive x-ray (EDX) analysis identifies the composition of the interfacial intermetallics as Ni3(In0.99In0.01)4. The
sol-dering reactions at lower temperatures (225–275°C) result in the predominant formation of a homogeneous intermetallic layer whose growth is diffusion controlled. At higher soldering temperatures (300–350°C), the interfacial in-termetallics appear to be long needlelike crystals, and the grooves in between the intermetallics provide fast-diffusion paths for Ni atoms to react with Sn atoms at the intermetallic front, which leads to interface-controlled growth kinetics. The intermetallic needles turned out to be flat slablike after selective etching of the unreacted solder. Kinetics analysis showed that they not only lengthened in the longitudinal direction, but also coarsened transversely by the Ostwald ripening mechanism.
Key words: Sn-20In-2.8Ag, Ni substrate, intermetallic slabs, growth
kinetics, transverse ripening
(Received October 30, 2003; accepted January 6, 2004)
INTRODUCTION
The development of Pb-free solders is needed in the electronics industry due to environmental concerns. Among an array of promising candidates, Sn-3.5Ag solder possesses the advantages of high strength, good ductility, long thermal fatigue life, improved creep resistance, and low cost.1–3However, the high
melting point of this alloy (221°C) as compared to the traditional Sn-37Pb solders (183°C) is an issue in electronic packaging. On the other hand, there is Sn-51In, a typical low-melting-point solder (118°C). In application, it can also boast such merits as high ductility, improved fatigue resistance, and good wet-tability.4,5Furthermore, an In-containing solder has
been found to form an Au dissolution barrier AuIn2at
the solder/pad interface, which can prevent the for-mation of brittle intermetallic compounds such as AuSn4in the solder matrix.6Unfortunately, the cost
of the In content within these solders is notably high, thereby limiting their applications to those cases
where production costs are of a lesser concern. A compromise may be reached by consolidating the fea-tures of Sn-3.5Ag and Sn-51In alloys, which gives rise to the development of the Sn-20In-2.8Ag solder.
Soldering is a low-temperature joining process widely employed for electronic packaging. Moreover, this technique is also used to bond recycled sputter-ing targets with their backsputter-ing plates. Dursputter-ing the soldering reaction process, the formation of inter-metallic compounds at the interfaces is a sign of a sound joint. However, an overgrowth of such inter-metallics will cause cracking along the interfaces. Nickel is one of the most popular substrate materi-als for soldering, so the solid/liquid reactions of Ni with certain commonly used solders (such as pure Sn,7–9 pure In,10 Sn-51In,11,12 Sn-58Bi,8 Sn-9Zn,13
and Sn-37Pb14 alloys) have been investigated. In
this present study, the interfacial reactions between liquid Sn-20In-2.8Ag solder and Ni substrates take place at temperatures ranging from 225°C to 350°C for 15–90 min, with the aim that the mechanism of soldering reactions for this newly developed solder alloy be clarified.
EXPERIMENTAL
The Sn-20In-2.8Ag (wt.%) solder was prepared by vacuum melting (105 torr) at 600°C. The ingot was homogenized for 120 h at 100°C and then rolled into a 0.2-mm-thick foil. The melting temperature of this Sn-20In-2.8Ag solder was taken at a heating rate of 10 K/min by means of differential scanning
calorimetry (DSC). The Ni substrates with dimen-sions 8 mm 12 mm were cut from a 1-mm-thick Ni plate (99.5% purity), ground with SiC paper and polished with 1 µm and 0.3 µm Al2O3 powders.
Soldering reactions were carried out in an infrared furnace under a vacuum of 103 torr. The heating processes were done at various temperatures from 225°C to 350°C for 15–90 min. The specimens could Fig. 1. Microstructure of the as-cast Sn-20In-2.8Ag solder
contain-ing coarse Ag2In intermetallic islands in the Sn-22.6In-1.2Ag matrix.
Fig. 2. DSC analysis of the Sn-20In-2.8Ag solder gives solidus/ liquidus at 176.33°C and 187.90°C, respectively.
Fig. 3. Morphology of the intermetallic compounds formed during the interfacial reactions between Sn-20In-2.8Ag solder and Ni substrates at 225°C for (a) 30 min, (b) 45 min, (c) 60 min, and (d) 90 min.
376 Chuang, Huang, and Lin
be cooled to room temperature in 2 min via a water-cooling system installed within the furnace. A rosin mildly activated type flux was used to eliminate the oxide layer on the surfaces of the solder and Ni substrates.
For metallographic analysis, the soldered speci-mens were cross sectioned, ground with SiC paper, and polished with 1 µm and 0.3 µm Al2O3 powders.
The morphology of intermetallic compounds formed in the solder matrix and at the Sn-20In-2.8Ag/Cu in-terface was observed by scanning electron microscopy. The chemical compositions of the intermetallic com-pounds were identified with energy-dispersive x-ray (EDX) spectroscopy. For kinetic analysis, the growth length (∆X) of the intermetallics was determined by measuring the needle length at 30 equally-spaced points and calculating the average value. The stan-dard deviation for various intermetallic compound layers was between 1.2 µm and 3.5 µm. For further in-vestigation of the three-dimensional morphology of the intermetallic compounds, the unreacted solder matrices of these soldered specimens were selectively etched away in an aqueous solution of 35% HF 10% HNO3 55% H2O. The thickness (∆W) of the exposed
intermetallic wickers after selective etching was also measured. The kinetics and mechanism of such a
transverse coarsening phenomenon in intermetallics were analyzed.
RESULTS AND DISCUSSION
The microstructure of the as-cast Sn-20In-2.8Ag solder is shown in Fig. 1, which contains islandlike precipitates embedded in the Sn-rich matrix. These islandlike precipitates possess a composition (at.%) of Ag : In 67.7 : 32.3, which corresponds to the Ag2In phase. A previous study indicated the
appear-ance of fine Ag3Sn particles in large quantities in an
as-cast Sn-3.5Ag solder,15but those particles are
ab-sent in this Sn-20In-2.8Ag solder. The EDX analysis also shows that the formation of Ag2In precipitates
causes the composition (wt.%) of the matrix to deviate from Sn-20In-2.8Ag to Sn-22.6In-1.2Ag. Differential scanning calorimetry analysis in Fig. 2 indicates that the solidus and liquidus tempera-tures of this solder are 176.33°C and 187.90°C, respectively, which are quite near the eutectic point of traditional Sn-37Pb solder (183°C).
There are two types of intermetallic compounds formed at the Sn-20In-2.8Ag/Ni interfaces after soldering reactions at various temperatures. As shown in Fig. 3, the interfacial intermetallic layers are relatively homogeneous at lower temperatures, Fig. 4. Morphology of the intermetallic compounds formed during the interfacial reactions between Sn-20In-2.8Ag solder and Ni substrates at 350°C for various times: (a) 15 min, (b) 30 min, (c) 45 min, and (d) 90 min.
though accompanied with the presence of a certain number of elongated intermetallic grains at their reaction fronts. The presence of such elongated inter-metallic compounds begins to dominate, enabling
these intermetallic compounds to further evolve into long needle shapes at a higher soldering temper-ature, as revealed in Fig. 4. Figure 5 demonstrates more clearly the change in morphology for inter-Fig. 5. Morphology of the intermetallic compounds formed during the interfacial reactions between liquid Sn-20In-2.8Ag solder and Ni sub-strates for 60 min at various temperatures: (a) 250°C, (b) 275°C, (c) 300°C, and (d) 325°C.
Fig. 6. Log plot for the growth length (∆X) of the intermetallic compounds Ni3(In0.99In0.01)4formed during the interfacial reactions between Sn-20In-2.8Ag solder and Ni substrates as a function of reaction time (t).
Fig. 7. Growth length (∆X) of the intermetallic compounds Ni3(In0.99In0.01)4formed at Sn-20In-2.8Ag/Ni interfaces after solder-ing reactions at lower temperatures rangsolder-ing from 225°C to 275°C versus the square root of reaction time (t1/2).
378 Chuang, Huang, and Lin
metallic compounds formed in the high (300°C to 350°C) and low (225°C to 275°C) temperature ranges. The EDX analyses indicate that both types of intermetallics are of the same composition (at.%):
Ni : Sn : In 45.0 : 54.4 : 0.6, which corresponds to the Ni3(Sn0.99In0.01)4 phase. Figure 6 shows the log
plots of the growth length (∆X) of the intermetallic compounds as a function of reaction time (t). The n values in the kinetic relation (∆X tn) are calculated from the slopes of the plots for various soldering tem-peratures. The results indicate that the n values range from 0.60 to 0.65 with an average of 0.63 at lower temperatures (225–275°C). In this tempera-ture range, the intermetallic growth during the soldering reactions tends to be diffusion controlled. On the other hand, higher soldering temperatures (300–350°C) yield quite larger n values ranging from 0.83 to 1.03 (average 0.91), which leads to the infer-ence of an interface-controlled reaction mechanism. The discrepancy in kinetics for intermetallic growth at lower and higher temperatures can be clarified by the different morphology of intermetallic compounds formed in both temperature ranges. At the lower soldering temperatures, there is an intermetallic layer with a homogenous matrix existing in between liquid Sn-20In-2.8Ag solder and the Ni substrate. The rate-controlling step in the growth of inter-metallic compounds is believed to be the diffusion of Ni atoms through the “dense” intermetallic layer. When the dense intermetallic layer is transformed Fig. 8. Arrhenius plot of the reaction constant (K) for the
intermetal-lic growth during the interfacial reactions between liquid Sn-20In-2.8Ag solder and Ni substrates at lower temperatures ranging from 225°C to 275°C.
Fig. 9. Appearance of intermetallic slabs formed at the Sn-20In-2.8Ag/Ni interfaces after selective etching away of the unreacted solder. Solder-ing reactions took place at 225°C for various times: (a) 30 min, (b) 45 min, (c) 60 min, and (d) 90 min.
into a layer of “loose” intermetallic needles with the increase of soldering temperature, as evidenced in Fig. 5, the grooves in between the parallel-aligned intermetallic needles provide fast-diffusion paths for Ni atoms to react with the Sn atoms. A linear kinetic relation (interface controlled) to the growth of interfacial intermetallic can be inferred.
In order to further verify the diffusion-controlled mechanism for the growth of intermetallic com-pounds Ni3(Sn0.99In0.01)4at the Sn-20In-2.8Ag/Ni
in-terfaces in the temperature range between 225°C and 275°C, the growth distance of intermetallics is plotted against the square root of reaction time (Fig. 7). The growth rate constant (K ∆X/t1/2) of
intermetallic compounds is plotted in an Arrhenius diagram, as shown in Fig. 8. The activation energy as calculated in Fig. 8 is 25.4 KJ/mol. This value is close to the activation energy for the lattice diffu-sion of Ni in Sn (18.1 KJ/mol), as reported by Yeh and Huntington.16
Selective etching of the unreacted Sn-20In-2.8Ag solder after soldering reactions at 225°C leads to the exposure of elongated grains at the front of the homogeneous intermetallic layer. Figure 9 demon-strates the three-dimensional morphology of these elongated intermetallic grains shaped in the form
of flat slabs. The slab shape of such intermetallic compounds becomes more pronounced for the selec-tively etched specimens when soldering reactions take place at higher temperature, as shown in Fig. 10. It is also shown in Figs. 10 and 11 that these intermetallic flat slabs will coarsen with reaction time and temperature. In order to clarify the mecha-nism for such transverse coarsening of the inter-metallic slabs during the Sn-20In-2.8Ag(l)/Ni(s)
interfacial reactions, the thickness (∆W) of the inter-metallic slabs formed at various soldering tempera-tures is measured and shown in the log plots in Fig. 12. The growth rate of intermetallics in the transverse direction is much lower than that in the longitudinal direction, as shown in Fig. 6. The n val-ues in the kinetic relation ∆W tn for transverse
coarsening, as calculated from the slopes of the plots, range from 0.30 to 0.44 with an average of 0.38, i.e.,∆W t1/3. The results imply that the
trans-verse coarsening of intermetallic slabs is governed by the Ostwald ripening mechanism.17,18
CONCLUSIONS
Intermetallics that have two types of morphology with the same composition Ni3(Sn0.99In0.01)4 appear
at the Sn-20In-2.8Ag/Ni interface after soldering Fig. 10. Appearance of intermetallic slabs formed at the Sn-20In-2.8Ag/Ni interfaces after selective etching away of the unreacted solder. Sol-dering reactions took place at 300°C for various times: (a) 30 min, (b) 45 min, (c) 60 min, and (d) 90 min.
380 Chuang, Huang, and Lin
reactions at various temperatures. At lower tempera-tures (225–275°C), a homogeneous layer of inter-metallic compounds is formed, accompanied by the presence of a certain number of elongated grains at
its front. In this temperature range, the rate-control-ling step in intermetallic growth is the lattice diffu-sion of Ni atoms through the dense intermetallic layer, thereby establishing a parabolic kinetic rela-tion between intermetallic growth length (∆X) and reaction time (t). The activation energy for this type of intermetallic growth is about 25.4 KJ/mol, which is quite consistent with the activation energy reported by Yeh and Huntington16 for the lattice diffusion of
Ni in Sn (18.1 KJ/mol). The interfacial intermetallic compounds formed during the soldering reactions at higher temperatures (300–350°C) appear in long needle shapes. The grooves between the intermetallic needles provide fast-diffusion paths for Ni atoms to react with Sn atoms at the intermetallic front. In this case, the intermetallic growth kinetics is governed by a linear relation, signifying an interface-controlled mechanism. After selective etching away of the unreacted solder, the exposed interfacial inter-metallic compounds appear in the shape of elongated flat slabs. Through such a three-dimensional mor-phology, a transverse growth of intermetallic slabs accompanied with the lengthening of the slabs in the longitudinal direction can be identified. Kinetic analysis reveals that such a transverse coarsening phenomenon of Ni3(In0.99In0.01)4 intermetallics at
Fig. 11. Appearance of intermetallic slabs formed at the Sn-20In-2.8Ag/Ni interfaces after selective etching away of the unreacted solder. Sol-dering reactions took place for 60 min at various temperatures: (a) 250°C, (b) 275°C, (c) 325°C, and (d) 350°C.
Fig. 12. Log plot for the thickness (∆W) of the intermetallic slabs formed during the interfacial reactions between Sn-20In-2.8Ag sol-der and Ni substrates at various temperatures.
the Sn-20In-2.8Ag/Ni interfaces is governed by the Ostwald ripening mechanism.
ACKNOWLEDGEMENT
The authors sincerely thank the National Science Council, Taiwan, for the sponsorship of this research under Grant No. NSC-91-2216-E002-036.
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