Interfacial Reaction and Microstructure Evolution of Sn-9Zn / Ni(Cu) Solder Joints

: Sn-9Zn solder is a promising Pb-free solder, but it tends to form bulky intermetallic compounds (IMC) grains at the interface when soldered with common simple metal Cu or Ni substrates. Interfacial reaction between Sn-9Zn solder and Ni(Cu) solid solution substrates at 250 ◦ C and 350 ◦ C were systematically probed in this study. Results showed that when soldered at 250 ◦ C, a Ni 5 Zn 21 layer is formed at Sn-Zn / Ni-20Cu and Sn-Zn / Ni-40Cu joints; and Ni 2 Sn 2 Zn + Cu 5 Zn 8 and Cu 5 Zn 8 phases are formed in Sn-Zn / Ni-60Cu and Sn-Zn / Ni-80Cu joints, respectively. Fine-grained IMCs formed at the interface are formed even when the soldered time is prolonged to 16 h. This result indicates that Ni(Cu) solid solution substrates inhibit the rapid growth of IMC at the Sn-Zn / Ni-Cu interface. Ni(Cu) solid solution substrate can also provide various combinations of reaction products at the Sn-Zn / Ni-Cu joints. The Ni 5 Zn 21 transfers to Ni 2 Sn 2 Zn + Cu 5 Zn 8 phases when the Cu content increased to 60%, and a bi-layered structure Ni 2 Sn 2 Zn + Cu 5 Zn 8 IMCs was formed in Sn-Zn / Ni(Cu) joints at 350 ◦ C regardless of the Cu content in Ni(Cu) substrate (20–80%). These results would provide an e ﬀ ective support in designing Sn-Zn soldering system with optimized IMC layer to improve mechanical performance.


Introduction
Soldering is one of the most important packaging methods for electronic industry. During soldering, intermetallic compounds (IMCs) form at the solder/substrate interface. A thin, continuous, and uniform IMC layer provides good bonding for soldered joints. The Sn-9Zn (weight percent) alloy, with the melting point of 183 • C, is a promising Pb-free solder because of its environment-friendly, cost-effective, and good mechanical properties [1][2][3][4]. Nevertheless, Sn-9Zn alloy tends to form bulky IMC grains at the interface when Sn-9Zn is soldered with common substrates, such as Ni and Cu; however, the soldered Sn-9Zn/Cu and Sn-9Zn/Ni joints presented poor mechanical performance [5][6][7]. Generally, with the brittle nature of IMC and the tendency to generate structural defects near interface, the bulky IMC grains deteriorate joint reliability [8][9][10][11].
Minor alloying elements are added in Sn-9Zn solder to enhance joint reliability. Most studies focused on the alloying effect on Sn-9Zn solder performance; additional elements can enhance the corrosion resistance, tensile strength, and wettability of Sn-9Zn solder [12][13][14][15][16][17]. Only limited attention was given to interfacial reaction in soldered joints. However, bulky and irregular IMC grains are still formed at the Sn-Zn/Cu interface when Al and Ag are added in the solder [18].
Multielement substrate provides an effective way to change IMC species, morphology, and growth kinetics [19][20][21]. The interfacial reaction between Ni(Cu) or Ni(Co) multielement substrates, and and Au-20Sn or Sn-Cu solders has been extensively studied [21][22][23][24]. Cu alloying in Ni substrate alters the reaction path and induces the formation of thin uniform IMC layers, thereby improving the mechanical reliability of soldered joint [20]. In other words, the mechanical reliability of the joint by using Ni(Cu) substrate to modify the interfacial reaction products is expected to improve. However, only a few publications reported the interfacial reaction between Sn-Zn and Ni(Cu) substrates. To explore the feasibility and implementation of enhancing the reliability of the Sn-Zn soldered joints by using multielement substrates, we carefully investigated the interfacial reactions between Sn-9Zn solder and Ni(Cu) substrate at 250 °C and 350 °C in this study.

Materials and Methods
Pure Sn (99.99 wt.%), Zn (99.9 wt.%), Ni (99.99 wt.%), and Cu (99.99 wt.%) shots were carefully weighed to prepare the Sn-9Zn solder and Cu-Ni (20,40,60, or 80 at.% Ni) substrates. The Sn-9Zn solder and Ni(Cu) substrates were fabricated by inducing melting in quartz tubes filled with Ar and molten solder casting in the graphite mode. The as-obtained Sn-9Zn and Ni(Cu) plates, with the thickness of 5 mm, were rolled to 0.1 and 1 mm thickness, respectively. The Ni(Cu) was cut into sheets of 5 mm × 5 mm × 1 mm, while Sn-Zn were cut into foils of with the dimension pf 5 mm× 5 mm× 0.1 mm.
Before soldering, Ni(Cu) sheets were mechanically polished. The substrate sheets and solder foils were cleaned with acetone in an ultrasonic washer for 5 min. The Sn-9Zn foil was placed between two Ni(Cu) sheets ( Figure 1) in a furnace in Ar atmosphere at 250 °C for 1, 4, 9, and 16 h or at 350 °C for 2, 10, and 60 min and then cooled in air. The soldered samples for electro-probe microanalysis (EPMA) were mechanically polished in the direction normal to the soldering interface. Shimadzu XRD-6000 X-ray diffraction (XRD) instrument, with Cu Kα radiation (wavelength 0.154 nm) at 40 kV and a tube current of 30 mǺ, was used to measure the phase constitution of the reaction layers with the scanning rate of 1 deg/min. To analyze the ternary reaction products accurately, we tensile fractured the soldered joints. Then, the Sn-Zn solder matrix was removed using 5% HCl water solution and cleaned with acetone in an ultrasonic washer for 5 min. The etched surface was perpendicular to cross-section. The etched interfacial microstructure of the joints was further analyzed by scanning electron microscopy (SEM) equipped with an energy dispersive spectrometer.

Reaction Products in Sn-Zn/Ni-20Cu Joint
Two reaction layers formed in Sn-Zn/Ni-20Cu joints after soldering at 250 °C Ni-2016 h according to the back-scattered electron (BSE) micrograph (Figure 2a). EPMA result revealed that dark phase-composited Ni-74.7Zn-11.4Cu-1.3Sn (at.%) can be designated as Ni5Zn21 phase according to Ni-Zn-Cu isothermal section at 250 °C. The reaction product formed between Ni5Zn21 phase and Ni-20Cu substrate, with the composition of Ni-20.2Zn-7.7Cu-40.8Sn. According to the equilibrium phase diagram of Sn-Zn-Ni and Sn-Zn-Cu system at 250 °C, several ternary phases contained The soldered samples for electro-probe microanalysis (EPMA) were mechanically polished in the direction normal to the soldering interface. Shimadzu XRD-6000 X-ray diffraction (XRD) instrument, with Cu Kα radiation (wavelength 0.154 nm) at 40 kV and a tube current of 30 mǺ, was used to measure the phase constitution of the reaction layers with the scanning rate of 1 deg/min. To analyze the ternary reaction products accurately, we tensile fractured the soldered joints. Then, the Sn-Zn solder matrix was removed using 5% HCl water solution and cleaned with acetone in an ultrasonic washer for 5 min. The etched surface was perpendicular to cross-section. The etched interfacial microstructure of the joints was further analyzed by scanning electron microscopy (SEM) equipped with an energy dispersive spectrometer.

Reaction Products in Sn-Zn/Ni-20Cu Joint
Two reaction layers formed in Sn-Zn/Ni-20Cu joints after soldering at 250 • C Ni-2016 h according to the back-scattered electron (BSE) micrograph ( Figure 2a). EPMA result revealed that dark phase-composited Ni-74.7Zn-11.4Cu-1.3Sn (at.%) can be designated as Ni 5 Zn 21 phase according to Ni-Zn-Cu isothermal section at 250 • C. The reaction product formed between Ni 5 Zn 21 phase and Ni-20Cu substrate, with the composition of Ni-20.2Zn-7.7Cu-40.8Sn. According to the equilibrium phase diagram of Sn-Zn-Ni and Sn-Zn-Cu system at 250 • C, several ternary phases contained approximately 20 at.% Zn and 40 at.% Sn, as presented in Table 1. It may be a ternary compound, but directly defining the phase constitution only by EPMA measurement is difficult.  Table 1. It may be a ternary compound, but directly defining the phase constitution only by EPMA measurement is difficult. To analyze the ternary reaction products accurately, we removed the Sn-Zn solder matrix, and the etched surface was further characterized by SEM (FEI Corporation, Hillsboro, OR, USA) and XRD (Rigaku Corporation, Tokyo, Japan). All the three layers, that is, solder matrix, Ni5Zn21, and undefined ternary compounds layer, can be directly observed in the etched surface ( Figure 2b). The XRD pattern of the etched surface is presented in Figure 3. The Ni-20Cu (PDF, 04-0850), Sn (PDF, 04-0673), and Ni5Zn21 (PDF, 04-0850) phases can be identified directly. Other diffraction peaks are identical with the τ3 phase, as reported by Schmetterer [25], which can be designated as the Ni2Sn2Zn phase [26].  To analyze the ternary reaction products accurately, we removed the Sn-Zn solder matrix, and the etched surface was further characterized by SEM (FEI Corporation, Hillsboro, OR, USA) and XRD (Rigaku Corporation, Tokyo, Japan). All the three layers, that is, solder matrix, Ni 5 Zn 21 , and undefined ternary compounds layer, can be directly observed in the etched surface ( Figure 2b). The XRD pattern of the etched surface is presented in Figure 3. The Ni-20Cu (PDF, 04-0850), Sn (PDF, 04-0673), and Ni 5 Zn 21 (PDF, 04-0850) phases can be identified directly. Other diffraction peaks are identical with the τ3 phase, as reported by Schmetterer [25], which can be designated as the Ni 2 Sn 2 Zn phase [26].     Figure 5 shows the BSE micrographs and XRD patterns of Sn-Zn/Ni-Cu couples after reaction at 250 °C for 9 h, and the compositions of each reaction products are listed in Table 2. As shown in the table, the Cu content in the substrate exerted considerable effect on the interfacial reaction products. When Sn-9Zn solder reacted with Ni-40Cu substrate, a uniform Ni5Zn21 layer was formed at the     Figure 5 shows the BSE micrographs and XRD patterns of Sn-Zn/Ni-Cu couples after reaction at 250 °C for 9 h, and the compositions of each reaction products are listed in Table 2. As shown in the table, the Cu content in the substrate exerted considerable effect on the interfacial reaction products. When Sn-9Zn solder reacted with Ni-40Cu substrate, a uniform Ni5Zn21 layer was formed at the  Figure 5 shows the BSE micrographs and XRD patterns of Sn-Zn/Ni-Cu couples after reaction at 250 • C for 9 h, and the compositions of each reaction products are listed in Table 2. As shown in the table, the Cu content in the substrate exerted considerable effect on the interfacial reaction products. When Sn-9Zn solder reacted with Ni-40Cu substrate, a uniform Ni 5 Zn 21 layer was formed at the interface. However, two reaction layers, namely, Ni 2 Sn 2 Zn and Cu 5 Zn 8 , were formed in Sn-Zn/Ni-60Cu, and Cu 5 Zn 8 and CuZn layers appeared in Sn-Zn/Ni-80Cu. interface. However, two reaction layers, namely, Ni2Sn2Zn and Cu5Zn8, were formed in Sn-Zn/Ni-60Cu, and Cu5Zn8 and CuZn layers appeared in Sn-Zn/Ni-80Cu.  Cu content in substrate also altered the morphology of the products. Figure 6 illustrates the etched surface of Sn-Zn/Ni-Cu joint after reaction at 250 °C for 9 h. In the Ni5Zn21 phase in Sn-Zn/Ni-20Cu and Sn-Zn/Ni-40Cu joints, extremely fine Ni5Zn21 grains constituted the layer in Sn-Zn/Ni-20Cu joint, and the spherical Ni5Zn21 particles formed at interface in Sn-Zn/Ni-40Cu joints. A dense Cu5Zn8 layer was formed in the Sn-Zn/Ni-60Cu joint, while a dense Cu5Zn8 layer with some CuZn grains was formed in the Sn-Zn/Ni-80Cu joint.

Effect of Cu Content on Reaction Products
The solid solution element in reaction IMCs played a significant role in IMC morphology. As reported in most references, bulky Cu5Zn8 grains are formed in the Sn-Zn/Cu joint during long reaction time, which extremely deteriorates the joint reliability [5,11]. However, the fine-grained Cu5Zn8 layer was formed in Sn-Zn/Ni-60Cu and Sn-Zn/Ni-80Cu joints when 5-9 at.% Ni was dissolved in the Cu5Zn8 phase.  Cu content in substrate also altered the morphology of the products. Figure 6 illustrates the etched surface of Sn-Zn/Ni-Cu joint after reaction at 250 • C for 9 h. In the Ni 5 Zn 21 phase in Sn-Zn/Ni-20Cu and Sn-Zn/Ni-40Cu joints, extremely fine Ni 5 Zn 21 grains constituted the layer in Sn-Zn/Ni-20Cu joint, and the spherical Ni 5 Zn 21 particles formed at interface in Sn-Zn/Ni-40Cu joints. A dense Cu 5 Zn 8 layer was formed in the Sn-Zn/Ni-60Cu joint, while a dense Cu 5 Zn 8 layer with some CuZn grains was formed in the Sn-Zn/Ni-80Cu joint.
The solid solution element in reaction IMCs played a significant role in IMC morphology. As reported in most references, bulky Cu 5 Zn 8 grains are formed in the Sn-Zn/Cu joint during long reaction time, which extremely deteriorates the joint reliability [5,11]. However, the fine-grained Cu 5 Zn 8 layer was formed in Sn-Zn/Ni-60Cu and Sn-Zn/Ni-80Cu joints when 5-9 at.% Ni was dissolved in the Cu 5 Zn 8 phase.  Table 3 summarizes the interfacial reaction products of Sn-Zn/Ni-Cu after soldering at 250 °C for various durations. Hence, Ni5Zn21 is the first formed product in Ni-20Cu and Ni-40Cu joints, whereas the Cu5Zn8 phase is the first formed product in Ni-60Cu and Ni-80Cu joints. After prolonging the reaction time, Ni2Sn2Zn phase was formed between the first formed product and substrate. Ni2Sn2Zn was formed in Sn-Zn/Ni-60Cu joint only after 1 h of soldering but appeared in all joints after 16 h.

Effect of Soldering Temperature on Reaction Products
When the soldering temperature increased to 350 °C, Ni2Sn2Zn + Cu5Zn8 phases appeared at the interface regardless of the Cu content in Ni-Cu substrates after short soldering time, that is, 2 min ( Figure 7). Nevertheless, the volume fraction of Cu5Zn8 increased with the increase in Cu content in the substrate. Only few Cu5Zn8 grains appeared in the Sn-Zn/Ni-20Cu and Sn-Zn/Ni-40Cu joints, but a Cu5Zn8 layer occurred between Ni2Sn2Zn and solder matrix when the Cu content in the substrate was >60%.  Table 3 summarizes the interfacial reaction products of Sn-Zn/Ni-Cu after soldering at 250 • C for various durations. Hence, Ni 5 Zn 21 is the first formed product in Ni-20Cu and Ni-40Cu joints, whereas the Cu 5 Zn 8 phase is the first formed product in Ni-60Cu and Ni-80Cu joints. After prolonging the reaction time, Ni 2 Sn 2 Zn phase was formed between the first formed product and substrate. Ni 2 Sn 2 Zn was formed in Sn-Zn/Ni-60Cu joint only after 1 h of soldering but appeared in all joints after 16 h.

Effect of Soldering Temperature on Reaction Products
When the soldering temperature increased to 350 • C, Ni 2 Sn 2 Zn + Cu 5 Zn 8 phases appeared at the interface regardless of the Cu content in Ni-Cu substrates after short soldering time, that is, 2 min ( Figure 7). Nevertheless, the volume fraction of Cu 5 Zn 8 increased with the increase in Cu content in the substrate. Only few Cu 5 Zn 8 grains appeared in the Sn-Zn/Ni-20Cu and Sn-Zn/Ni-40Cu joints, but a Cu 5 Zn 8 layer occurred between Ni 2 Sn 2 Zn and solder matrix when the Cu content in the substrate was >60%.  Figure 8 presents the interfacial reaction products of Sn-Zn/Ni-Cu joints after soldering at 350 °C for 60 min. Ni3Sn4 phase formed at the Sn-Zn/Ni-20Cu interface, while mainly Ni2Sn2Zn phase was formed when Cu content in the substrate was >20%. CuZn grains also appeared in Sn-Zn/Ni-60Cu and Sn-Zn/Ni-80Cu joints. The interfacial reaction products in Sn-Zn/Ni-Cu joints after various durations at 350 °C are listed in Table 4.   Figure 8 presents the interfacial reaction products of Sn-Zn/Ni-Cu joints after soldering at 350 • C for 60 min. Ni 3 Sn 4 phase formed at the Sn-Zn/Ni-20Cu interface, while mainly Ni 2 Sn 2 Zn phase was formed when Cu content in the substrate was >20%. CuZn grains also appeared in Sn-Zn/Ni-60Cu and Sn-Zn/Ni-80Cu joints. The interfacial reaction products in Sn-Zn/Ni-Cu joints after various durations at 350 • C are listed in Table 4.  Figure 8 presents the interfacial reaction products of Sn-Zn/Ni-Cu joints after soldering at 350 °C for 60 min. Ni3Sn4 phase formed at the Sn-Zn/Ni-20Cu interface, while mainly Ni2Sn2Zn phase was formed when Cu content in the substrate was >20%. CuZn grains also appeared in Sn-Zn/Ni-60Cu and Sn-Zn/Ni-80Cu joints. The interfacial reaction products in Sn-Zn/Ni-Cu joints after various durations at 350 °C are listed in Table 4.

Discussions
It is generally accepted that joint reaction occurs in two steps: (1) solidification preceded by the substrate dissolution; and (2) some transformations in the solid [32,33]. Considering the evolution of interfacial microstructure, two reaction stages can be roughly determined for the interfacial reaction between Sn-Zn solder and Ni-Cu substrate, as demonstrated in Figures 9 and 10. Given that no quaternary phase in Zn-Sn-Ni-Cu system has been reported or detected thus far, the possible reaction paths of Sn-9Zn/Ni-Cu couples were proposed according to the Ni-Sn-Zn [25][26][27][28][29][30][31] and Cu-Sn-Zn [34] isothermal sections at 250 and 350 • C, and no other phases were suspended. In addition, the reactions are peritectic reaction according to the Ni-Sn-Zn [25][26][27][28][29][30][31] and Cu-Sn-Zn [34] phase diagram at 250 and 350 • C. The solid substrates or reaction product react with liquid and form new reaction products.  Ni2Sn2Zn + Cu5Zn8 Ni2Sn2Zn + Cu5Zn8 Ni2Sn2Zn + CuZn

Discussions
It is generally accepted that joint reaction occurs in two steps: (1) solidification preceded by the substrate dissolution; and (2) some transformations in the solid [32,33]. Considering the evolution of interfacial microstructure, two reaction stages can be roughly determined for the interfacial reaction between Sn-Zn solder and Ni-Cu substrate, as demonstrated in Figure 9; Figure 10. Given that no quaternary phase in Zn-Sn-Ni-Cu system has been reported or detected thus far, the possible reaction paths of Sn-9Zn/Ni-Cu couples were proposed according to the Ni-Sn-Zn [25][26][27][28][29][30][31] and Cu-Sn-Zn [34] isothermal sections at 250 and 350 °C, and no other phases were suspended. In addition, the reactions are peritectic reaction according to the Ni-Sn-Zn [25][26][27][28][29][30][31] and Cu-Sn-Zn [34] phase diagram at 250 and 350 °C. The solid substrates or reaction product react with liquid and form new reaction products.    Ni2Sn2Zn + Cu5Zn8 Ni2Sn2Zn + Cu5Zn8 Ni2Sn2Zn + CuZn

Discussions
It is generally accepted that joint reaction occurs in two steps: (1) solidification preceded by the substrate dissolution; and (2) some transformations in the solid [32,33]. Considering the evolution of interfacial microstructure, two reaction stages can be roughly determined for the interfacial reaction between Sn-Zn solder and Ni-Cu substrate, as demonstrated in Figure 9; Figure 10. Given that no quaternary phase in Zn-Sn-Ni-Cu system has been reported or detected thus far, the possible reaction paths of Sn-9Zn/Ni-Cu couples were proposed according to the Ni-Sn-Zn [25][26][27][28][29][30][31] and Cu-Sn-Zn [34] isothermal sections at 250 and 350 °C, and no other phases were suspended. In addition, the reactions are peritectic reaction according to the Ni-Sn-Zn [25][26][27][28][29][30][31] and Cu-Sn-Zn [34] phase diagram at 250 and 350 °C. The solid substrates or reaction product react with liquid and form new reaction products.   The monolayered IMC structure appeared first after soldering at 250 • C. Meanwhile, bi-layered IMC structure was observed at 350 • C. As listed in Tables 3 and 4, Ni 5 Zn 21 was the first formed product in Ni-20Cu and Ni-40Cu joints, whereas Ni 2 Sn 2 Zn + Cu 5 Zn 8 phases in Ni-60Cu and Cu 5 Zn 8 phase in Ni-80Cu. However, Ni 2 Sn 2 Zn + Cu 5 Zn 8 phases were always formed at 350 • C as it always had Cu content in Ni(Cu) substrate (20-80%).
Thermodynamic and kinetic factors determine the interfacial reaction products during soldering process [35]. The reaction products generally tend to be controlled by the diffusivity of the reactant in the reaction zone [10]. In most cases, only an IMC layer forms at the interface at first, even when solder reacts with binary substrate, because one element in substrate would dominate the interfacial reaction. For example, Ni is the dominant reaction element in Ni-20Cu and Ni-40Cu substrates when soldering at 250 • C. Similar to the reaction product formed in the Sn-Zn/Ni joint [21,36], Ni 5 Zn 21 is still the first formed product in the Sn-Zn/Ni-20Cu and Sn-Zn/Ni-40Cu joints.
By contrast, if both elements in the binary substrate can provide successive supplement toward the reaction zone, then a bi-layered IMC structure would occur at first. The diffusivity of reactants in the reaction zone was dependent upon the content and temperature. The Ni 2 Sn 2 Zn + Cu 5 Zn 8 bi-layered IMC structure only appeared in the Sn-Zn/Ni-60Cu joint after soldering at 250 • C. This result indicated that binary substrate with suitable content can provide successive supplement of both elements toward the reaction zone. This suitable substrate composition range would increase with the increment of soldering temperature. Given that the diffusion coefficient of atomic will dramatically increase with increased temperature, the Ni 2 Sn 2 Zn + Cu 5 Zn 8 bi-layer was formed first at 350 • C when the Cu content in the Ni-Cu substrate ranged from 20% to 80%.
The new reaction products (mainly Ni 2 Sn 2 Zn) occurred when soldering time was prolonged; thus, the original reaction product is no longer in local equilibrium after reacting at a sufficiently long time [24,37]. The formation of Ni 2 Sn 2 Zn phase between Ni and Ni 5 Zn 21 was also reported in Sn-Zn/Ni joint after prolonged soldering time [15]. In the present work, the Zn in solder is depleted in a later reaction stage, while the products formed in the early stage react with liquid solder (mainly Sn) and new phases appear. The Ni 5 Zn 21 phase transferred to Ni 2 Sn 2 Zn, and Cu 5 Zn 8 phase transferred to Ni 2 Sn 2 Zn and CuZn when the reaction time at 250 • C was prolonged.
In brief, when soldering with Sn-Zn solder, Ni(Cu) solid solution substrate can optimize the morphologies of Ni 5 Zn 21 and Cu 5 Zn 8 at the interface, and a thin and uniform reaction layer consisting of fine IMC grains can be obtained. Various reaction product combinations were formed at the interface, that is, Ni 2 Sn 2 Zn + Cu 5 Zn 8 . The formation of thin Ni 2 Sn 2 Zn layer near the substrate enhances the joint reliability [15]. Hence, the adoption of Ni(Cu) solid solution substrate will provide an effective way to improve the joint reliability of the Sn-Zn soldering system.