Interfacial Properties of Zn–Sn Alloys as High Temperature
Lead-Free Solder on Cu Substrate
Jae-Ean Lee
1;*, Keun-Soo Kim
2, Katsuaki Suganuma
2, Junichi Takenaka
3and Koichi Hagio
3 1Department of Adaptive Machine Systems, Osaka University, Suita 565-0871, Japan
2Institute of Scientific and Industrial Research, Osaka University, Ibaraki 567-0047, Japan 3Nihon Genma MFG Co. Ltd., Osaka 532-0032, Japan
The potentials of the newly designed Zn–xSn (x¼40, 30, and 20 mass%) alloys as high temperature lead-free solders and their interface properties on Cu substrate were investigated, focusing on the interface microstructure and mechanical properties. Hypereutecic alloys show two endothermic peaks in differential scanning calorimetry (DSC), one appears at 200C and the other varies from 365 to 383C with decreasing Sn content. These peaks are well associated with the eutectic and liquidus temperatures of binary Zn–Sn alloys, and little undercooling were observed on cooling. Two Cu–Zn compound layers are formed at the Zn–Sn alloys/Cu interface. The reaction phases are identified as-Cu5Zn8
and"-CuZn5phases from the Cu side, and no Cu–Sn compound was identified. The thickness of the reaction layers and the joining strength
increased with decreasing Sn content. Each joint shows a different fracture pattern, which gradually changes from transgranular in Zn–Sn alloys near the interface to the at"-CuZn5/-Cu5Zn8reaction layers with decreasing Sn content.
(Received May 27, 2005; Accepted September 28, 2005; Published November 15, 2005)
Keywords: lead-free solder, zinc-tin alloy, high temperature solder, intermetallic compound, interfacial microstructure, joining strength
1. Introduction
High temperature solders are widely used as interconnect-ing materials for Si dies to packaginterconnect-ing lead-frames, and also for flip-chip technology. Conventional high temperature solders have been high lead-bearing alloys, typically 85– 97 mass%Pb–Sn, and Au- or Bi-based alloys.1–5)Over the last 10 years, scientists have made much effort in respect to developing substantially free solders to replace lead-bearing solders, eutectic Sn–37 mass%Pb. Successful devel-opments have almost led to the point where lead-free electronics assembly is accomplished. In contrast, the works on replacing of high lead-bearing solders have been relatively little.1–5) Au- and Bi-based alloys as representative high
temperature lead-free alloys also have been faced with several serious problems during using electronic industries until now.1–7) The high cost and the formation of massive
intermetallic compound (IMC) or the brittle nature of Bi prevents widespread adoption of Au- and Bi-based alloys. High lead-bearing solders need to be replaced by lead-free alloys as soon as possible, because the use of high lead solder influences the recycling possibilities of electronic circuit boards.
Recently, Sn–Ag–Cu alloy has been recognized as the standard lead-free solder for use in the middle temperature range due to its excellent reliability and compatibility with the current components.8)The melting temperature of Sn–
Ag–Cu ternary eutectic alloy is higher approximately 34C
than that of eutectic Sn–Pb binary alloy. Accordingly, the overall reflow temperature should be increased up to about 240–250C. Thus, a new lead-free high temperature solder
that has no degradation at about 260C is needed.
Recently one of the authors developed new Zn based alloys; ones that have no IMC and allow a certain amount of liquid at 260C.9)The formation of adequate liquid phase is
expected to provide relaxation of thermal stress between Si die and metallic substrate in power device package. Among the various candidates for use as high temperature lead-free solders, Zn–Sn alloys are expected to be one of the most forgiving solders because they can provide good mechanical and electrical properties as well as excellent economy. However, Sn–Zn near eutectic alloy, a high Sn alloy is known to have the serious weakness of active oxidation and dross formation. The recent developments have improved this drawback with newly designed flux and alloying designs. Such improvements are also expected for high Zn alloys; and further, an easily oxidizing nature is not such a big problem because many applications for high temperature solders are currently fabricated in inert atmospheres. Therefore, to establish this basic Zn–Sn alloy as a high temperature solder, an understanding of the fundamental properties of the alloy and its interface microstructure with various substrates is a necessity.
The purpose of the present work is to evaluate the potential of Zn–Sn alloys for use as a high-temperature lead-free solder in the electronic industries. In particular, microstructures and the solderability of Zn–Sn alloys and their joining interface microstructures with Cu substrate are discussed; this involves the identification of the relationship of the Zn–Sn alloy composition and the fracture pattern by examining the joining tensile strength.
2. Experimental Procedures
Zn–xSn (x¼40, 30, and 20 mass%) binary alloys were provided by Nihon Genma MFG Co. Ltd. The chemical compositions of the Zn–Sn alloys are listed in Table 1. Hereafter, the composition unit ‘‘mass%’’ is omitted. The alloy ingots were cold rolled into 300mmthick sheets. Cu substrates were 99.99% pure and were prepared into cubes of 15mm15mm15mm. The surface of the Cu substrates to be soldered and the rolled Zn–Sn sheets were mechanically
*Corresponding author, E-mail: jelee@eco.sanken.osaka-u.ac.jp
polished and finished by using aluminum oxide powders with a mean diameter of 0.3mm.
Thermal analysis of the Zn–Sn alloys was carried out in temperatures up to 400C in an argon gas flow of 30 mL/min
at a constant heating and cooling rate of 3C/min by
differential scanning calorimetry (DSC). A polished solder sheet of 200mm thickness was placed between two Cu substrates. The sandwich specimen was heat-treated at 420C
in 750 mmHg vacuum, and then air-cooled at a cooling rate of 1.8C/s. The temperature profile of the soldering is shown
in Fig. 1. After soldering, rectangular specimens with
dimensions of 1mm3mm30mm were cut from each
sandwich specimen for tensile testing. The interfacial micro-structure of the joints was observed by scanning electron microscope (SEM). The mean reaction thickness was measured using ten Zn–Sn/Cu interface photographs. X-ray
diffraction (XRD) and electron probe micro-analysis
(EPMA) were carried out to identify intermetallic com-pounds formed at the interface between the Zn–Sn alloys and the Cu substrates. The tensile strength of the joint specimens was measured at a crosshead speed of 0.5 mm/min at room temperature by a universal testing machine. Data was obtained from at least ten samples.
3. Results and Discussion
3.1 Thermal properties of Zn–Sn alloys
To ascertain the fundamental thermal reaction properties of Zn–Sn alloys, we carried out DSC analysis. Figures 2(a) and (b) show the typical DSC curves of Zn–Sn alloys on heating and cooling, respectively. These hypereutectic alloys show two endothermic peaks, one appears approximately at 200C and the other varies from 365 to 383C with
decreasing Sn content. Each endothermic peak corresponds well to the melting point of the Zn–Sn binary alloys. The lower temperature peak associates with the Zn–Sn eutectic temperature and higher temperature peaks are the liquidus temperatures. On cooling from a liquid state, the under-cooling of all alloys exhibited about 4–7C at higher
temperatures and 3C at lower temperatures. None of the
Zn–Sn alloys showed any other significant reaction peak. The results of DSC coincided with the binary phase diagram of the Zn–Sn alloy. It is possible to control the alloy phase for this alloy system, especially the liquid fraction by using thermodynamic information.
3.2 Joining interface of Zn–Sn alloys and Cu substrate
Figure 3 shows the microstructure of the interface between Zn–Sn alloys and a Cu substrate. In solder part, the dark and bright color phases are primary-Zn and-Sn/-Zn eutectic phase, respectively, as shown in Figs. 3(a), (b), and (c). All of the samples, primary -Zn grains were surrounded with -Sn/-Zn eutectic phase, in which fine Zn platelets disperse in
a-Sn matrix. A fraction of the primary-Zn area increased
[image:2.595.314.540.72.401.2]with decreasing Sn content over the whole composition range, while that of -Sn/-Zn eutectic decreased with decreasing Sn content, as expected from the phase diagram. Figures 3(d), (e), and (f) shows the typical interfacial microstructures of Zn–40Sn/Cu, Zn–30Sn/Cu, and Zn– 20Sn/Cu joints, respectively. Two reaction layers were formed between Zn–Sn alloys and the Cu substrate.
Table 1 Chemical compositions of Zn–Sn alloys (mass%).
Composition
Zn–40Sn Zn–30Sn Zn–20Sn
Components fraction
Zn 59.097 69.184 79.048
Sn 40.880 30.810 20.950
Pb 0.018 — —
Ag 0.004 0.003 0.001
Cu — 0.002 —
Cd 0.001 0.001 0.001
Time, t/min
T
emper
ature
,
T
/
°
C
Cooling rate = 1.82 °C/s
0 5 10 15 20 25 30 35 40 45
0 50 100 150 200 250 300 350 400 450
Fig. 1 Temperature profile for a Zn–Sn alloy/Cu joint.
Endother
mic
(a)
100 150 200 250 300 350 400
365 °C
374 °C
383 °C
Exother
mic
Temperature,
T
/
°
C
359 °C
367 °C
(b)
Zn-40Sn
Zn-30Sn
Zn-20Sn
Zn-40Sn
Zn-30Sn Zn-20Sn
379 °C 200 °C
197 °C
100 150 200 250 300 350 400
[image:2.595.49.288.79.352.2]To clarify the reaction layer phase, we carried out EPMA element mapping analysis for all samples, as shown in Fig. 4. From the result of EPMA element mapping analysis, it was found that the reaction layer contains only Cu and Zn without
Sn; being clearly shown that the reaction layer consists of two Cu–Zn intermetallic compounds. The thickness of the reaction layer facing to the Cu substrate is thicker than that of the reaction layer that forms adjacent to the solder.
(b)
(e)
(c)
(f)
(d)
Reaction la
y
e
r
Cu
20 µm
(a)
α-Zn+β-Sn α-Zn
Eutectic Zn
10 µm
20 µm
Fig. 3 SEM micrographs of Zn–Sn alloys and Zn–Sn alloy/Cu interfaces: (a) Zn–40Sn, (b) Zn–30Sn, (c) Zn–20Sn, (d) Zn–40Sn/Cu, (e) Zn–30Sn/Cu, and Zn–20Sn/Cu.
Cu
Zn
Sn
a
b
10
µ
m
c
[image:3.595.84.514.73.287.2] [image:3.595.114.482.335.696.2]Moreover, it is interesting to note that the-Sn/-Zn eutectic phase exists as layers along the interface between the reaction layer and the solder for all alloys, as indicated the Fig. 4(c). The results of quantitative analysis are summarized in Table 2. The compositions of the two phases coincide well with the "-CuZn5 phase for the thinner layer and the
-Cu5Zn8 phase for the thicker one. To clarify the phases,
polished surfaces of the reaction layers parallel to the interface were examined by X-ray diffraction analysis of all the joints. A typical result is shown in Fig. 5; the formation of the reaction phases mentioned above, i.e., "-CuZn5 and
-Cu5Zn8, were confirmed.
One of the present authors10,11) reported that two Cu–Zn intermetallic compound layers, such as -Cu5Zn8 and 0
-CuZn, are formed at reflow temperature without any Sn for the Sn–Zn eutectic solder/Cu interface. The reaction thick-ness of 0-CuZn is very thin, less than 1mm as seen in
Ref. 10). In the present study,0-CuZn cannot be identified.
Unfortunately, there is no report of a ternary Cu–Sn–Zn phase diagram in the temperature range between 350 to 400C. Based on the fact that Sn is difficult to solve in the
Cu–Zn system, it seems to be useful to discuss this reaction based on the Cu–Zn binary system. From the binary Cu–Zn phase diagram, three intermetallic compounds,i.e.,0-CuZn,
-Cu5Zn8, and "-CuZn5, can be expected for this reaction
system. In the present reaction system, the amount of Zn is much larger than in the Sn–Zn eutectic alloy/Cu reaction system, which can promote the formation of"-CuZn5. Higher
resolution imaging techniques, such as TEM are required for further study.
The morphology of"-CuZn5is the scallop-like wavy layer
while that of -Cu5Zn8 is flat. Such reaction morphologies
are quite similar to those of Cu–Sn compounds such as -Cu6Sn5/"-Cu3Sn formed at eutectic Sn–Pb, and many of
the lead-free alloys with Cu substrates at 200–250C, which
is explained by the mechanism that the-Cu6Sn5compound
grows as a scallop-like morphology from the dissolution of Cu into the liquid solder.12–15) For the present reaction
system, it can be said that Cu dissolution into the liquid solder forms the scallop-shaped "-CuZn5 layer. Moreover, the
growth of scallop-shaped"-CuZn5is expected that the Cu is
predominantly transported to molten solder along the "-CuZn5 grain boundaries during soldering at high
temper-ature by ripening process, not by solid state reaction. Figure 6 shows the thickness change of the reaction layers as a function of the Sn content. The-Cu5Zn8reaction layer
was widely formed as compared with "-CuZn5 one. The
thickness of the-Cu5Zn8 reaction layer is over twice that
[image:4.595.45.290.84.182.2]of the"-CuZn5one,i.e., about 7 to 11mmfor"-CuZn5and 18 Table 2 EPMA quantitative analysis of reaction layers.
Solders Point# Zn (at%) Identified phases
Zn–40Sn a 65:490:40 -Cu5Zn8
b 82:661:19 "-CuZn5
Zn–30Sn a 65:480:24 -Cu5Zn8
b 82:431:03 "-CuZn5
Zn–20Sn a 65:970:31 -Cu5Zn8
b 82:861:80 "-CuZn5
#Pointing out reaction layers in the Fig. 4.
Fig. 5 X-ray diffraction of a Zn–40Sn/Cu interface.
Zn-40Sn
Thic
kness of reaction la
y
ers
,
d
/
µ
m
ε -CuZn5 Total
γ-Cu5Zn8
0 10 20 30 40
Zn-30Sn Zn-20Sn
Fig. 6 Thickness of the"-CuZn5and-Cu5Zn8reaction layers formed at
the interface (Error bars show the standard deviation).
0 10 20 30 40
T
ensile strength,
σ
/MP
a
Zn-40Sn Zn-30Sn Zn-20Sn
[image:4.595.52.284.205.389.2] [image:4.595.314.543.298.510.2]to 30mm for -Cu5Zn8 layer. The thickness of overall
reaction layer increases with decreasing Sn content, from 26 to 41mm.
3.3 Joining strength and fracture properties
Figure 7 shows the tensile strength of the joints as a function of Sn content measured at room temperature. The strength gradually increased to about 22, 28, and 32 MPa with decreasing the amount of Sn content; the values equal to those for Sn–37Pb/Cu joint strength.10) However, the
strength is slightly lower than that for Sn–9Zn alloy or Sn– 8Zn–3Bi paste/Cu joints soldered at 230–250C, i.e., 40–
50 MPa.10,11)The thickness of the intermetallic compounds at the interface for the present system is much thicker than for the high Sn case, i.e., less than 10mmin thickness.10)This seems to provide one of the reasons for the lower strength.
The typical fracture surfaces and the fracture paths of the joints are shown in Figs. 8 and 9. The Zn–40Sn/Cu joint shows a ductile fracture in a solder layer near the interface
while the Zn–20Sn/Cu joint exhibits an interfacial fracture. The ductile nature of the Zn–40Sn alloy, in which-Sn/-Zn eutectic phase seem to play a key role for deformation, should contribute the lower strength as seen in Fig. 8(a). As a high temperature solder, the softness of the soldered layer is one of the benefits to provide stress relaxation of that caused by the thermal expansion mismatch between Si dies and metallic substrates. In contrast, the considerable amount of Zn grains maintains the strength until the stress reaches the strength of the interface between the two intermetallic compound layers. The fracture path of the Zn–20Sn/Cu joint with brittle nature lies at the interface between the "-CuZn5 and -Cu5Zn8 reaction layers. On the other hand,
the fracture pattern of Zn–30Sn/Cu joint exhibited the complex pattern which is mixed each fracture behavior displaying at Zn–40Sn/Cu and Zn–20Sn/Cu joints.
As a result, we schematically suggest the fracture model obtained for the fracture behavior of Zn–Sn/Cu joints, as seen the Fig. 10. The fracture pattern of the Zn–Sn/Cu joints
(e)
(f)
200 µµm
(a)
(b)
(c)
(d)
10 µm
Fig. 8 SEM fractographs of Zn–Sn alloys/Cu joints after tensile test: (a) Zn–40Sn/Cu, (b) Zn–30Sn/Cu, and (c) Zn–20Sn/Cu by low magnification and (d) Zn–40Sn/Cu, (e) Zn–30Sn/Cu, and (f) Zn–20Sn/Cu by high magnification.
(b)
Cu
γ-Cu5Zn8
(a)
γ-Cu5Zn8 γ-Cu5Zn8
ε-CuZn5 ε-CuZn5
ε -CuZn5
(c)
Zn-Sn solder
[image:5.595.87.514.317.533.2] [image:5.595.84.511.586.757.2]is primarily governed by amount of soft-Sn/-Zn eutectic phase. In case of Sn–9Zn/Cu or Sn based solder/Cu joints generally take place ductile fracture in solder near reaction layer or solder/reaction layer interface owing to ductility in Sn itself and crystalline mismatch.6,10)The Zn–40Sn/Cu joint still maintained ductile nature and occurred transgranular fracture in the Zn–Sn alloy layer near the interface. However, the Zn–Sn/Cu joint almost undergo brittle fracture at "-CuZn5and-Cu5Zn8interface when amount of Sn content
decreases less than approximately 20. Consequently, the Zn– Sn/Cu joint could be obtained enough interfacial strength as well as fracture pattern changed from ductile to brittle with decreasing Sn content. Also, we confirmed that Zn–Sn alloy have the great potential as high temperature lead-free solder on Cu substrate.
4. Conclusions
In the present work, we have examined Zn–xSn (x¼40, 30, and 20) alloys on Cu substrate as candidates for high temperature lead-free solders, especially focusing on the interface properties. The results can be summarized as follows:
(1) All alloys show two endothermic and exothermic peaks. The endothermic peak at lower temperature indicates the eutectic reaction, while that at high temperature would be associated with the liquidus temperature of binary Zn–Sn alloy. These alloys did not show large undercooling.
(2) Two reaction layers are formed at the Zn–Sn alloys/Cu interface. The reaction phases are identified as"-CuZn5
phase adjacent to the solder, and -Cu5Zn8 formed
facing to the Cu substrate, respectively. The reaction thickness is decreased as a function of Sn content. (3) The joining tensile strength is in the range of about 20 to
30 MPa and the interfacial strength increases with decreasing Sn content.
(4) With decreasing Sn content, the facture patterns are gradually changed from transgranular fracture in the Zn–Sn alloy to interfacial fracture at the "-CuZn5/
-Cu5Zn8interface.
Thus, it is concluded that the use of Zn–Sn alloy as a high temperature solder could be possible on Cu substrate from the present work. In order to establish more reliable solder alloys, further works are required to assess the compatibility between Zn–Sn alloys and various substrates.
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(a)
α
-Zn+
β
-Sn
α
-Zn
Cu substrate
ε
-CuZn
5(b)
γ
-Cu
5Zn
8Cu substrate
ε
-CuZn
5 [image:6.595.113.483.73.293.2]γ
-Cu
5Zn
8