Eutectic Sn-Zn Research Articles

Solderability and intermetallic compounds formation of Sn-9Zn-xAg lead-free solders wetted on Cu substrate

The eutectic Sn-9Zn alloy was doped with Ag (0 wt.%-1 wt.%) to form Sn-9Zn-xAg lead-free solder alloys. The effect of the addition of Ag on the microstructure and solderability of this alloy was investigated and intermetallic compounds (IMCs) formed at the solder/Cu interface were also examined in this study. The results show that, due to the addition of Ag, the microstructure of the solder changes. When the quantity of Ag is lower than 0.3 wt.%, the needle-like Zn-rich phase decreases gradually. However, when the quantity of Ag is 0.5 wt.%-1 wt.%, Ag-Zn intermetallic compounds appear in the solder. In particular, adding 0.3 wt.% Ag improves the wetting behavior due to the better oxidation resistance of the Sn-9Zn solder. The addition of an excessive amount of Ag will deteriorate the wetting property because the glutinosity and fluidity of Sn-9Zn-(0.5, 1)Ag solder decrease. The results also indicate that the addition of Ag to the Sn-Zn solder leads to the precipitation of e-AgZn3 from the liquid solder on preformed interfacial intermetallics (Cu5Zn8). The peripheral AgZn3, nodular on the Cu5Zn8 IMCs layer, is likely to be generated by a peritectic reaction L + γ-Ag5Zn8 → ɛ-AgZn3 and the following crystallization of AgZn3.

Thermal and mechanical properties of Sn–Zn–Bi lead-free solder alloys

Sn–Zn eutectic alloy has been considered as one of the lead-free solder materials that can replace the toxic Sn–37Pb solder without increasing soldering temperature. This study investigates the effect of Bi content on the microstructure, thermal, and mechanical properties of the eutectic Sn–Zn alloy. The results indicate that the microstructures of the Pb-free solders were eutectic. Addition of Bi decreased the melting point and heat of fusion of the solders. 1% Bi addition enhanced the solid solution effect and raised the tensile strength of the as-cast alloys. Notably, with increasing Bi content, the SnBi inter-metallic compound (IMC) precipitates and the ZnO phase are extracted from the β-Sn matrix with non-uniform microstructure. The ZnO and SnBi IMC precipitates are considered to be the crucial reasons for rapid deterioration of the tensile properties of the specimens. Furthermore, the SnBi IMC particles are transformed into Bi-rich phase during the annealing process, which resulting in an increase of the susceptibilities of Zn to oxygen for the annealed Sn–9Zn–4Bi alloys. Compared with the other alloys, the newly developed Sn–9Zn–1Bi alloy showed some fine properties and this lead-free solder could substitute some Pb-containing solder alloys in microelectronic components packaging and interconnects.

Effect of rare earth element additions on the impression creep of Sn–9Zn solder alloy

Creep behavior of the Sn–9Zn–RE alloys containing 0.1, 0.25 and 0.5 wt.% rare earth (RE) elements was studied by impression testing and compared to that of the eutectic Sn–9Zn alloy. The tests were carried out under constant punching stress in the range 40–135 MPa and at temperatures in the range 298–420 K. Results showed that for all loads and temperatures, Sn–9Zn–0.25RE had the lowest creep rate, and thus the highest creep resistance among all materials tested. This was attributed to the formation of Sn–RE second phase precipitates which act as the main strengthening agent in the RE-containing Sn–Zn alloys. RE contents higher than 0.25 wt.%, resulted in a lower creep resistance due to a reduction in the volume fraction of Zn-rich phase caused by the formation of Sn–Zn–RE intermetallics. The average stress exponents of 6.8, 6.9, 7.1, 6.8 and activation energies of 42.6, 40.6, 43.0 and 44.9 kJ mol−1 were obtained for Sn–9Zn, Sn–9Zn–0.1RE, Sn–9Zn–0.25RE, and Sn–9Zn–0.5RE, respectively. These activation energies were close to 46 kJ mol−1 for dislocation climb, assisted by vacancy diffusion through dislocation cores in the Sn. This, together with the stress exponents of about seven suggests that the operative creep mechanism is dislocation climb controlled by dislocation pipe diffusion.

Reverse polarity effect from effective charge disparity during electromigration in eutectic Sn–Zn solder interconnect

Polarity effect on the interfacial reactions from high-density electric currents was investigated in a solder interconnect with a large disparity in the effective charge between the solder constituents. A reverse polarity effect was found where the intermetallic compound layer at the cathode grew significantly thicker than that at the anode under electric loading. Such an abnormal polarity effect was shown to result from electromigrations of Sn and Zn along opposite directions as dictated by the disparity in their effective charges. As Sn migrated to the anode under electron wind force, the resulting back stress drove Zn atoms to drift to the cathode. A kinetic analysis of the Zn mass transport explained the differential growth of the intermetallic compounds at the two electrodes, in good agreement with the experimental data.

Effects of Ag and Al Additions on the Structure and Creep Properties of Sn-9Zn Solder Alloy

Creep behavior of the eutectic Sn-9Zn, Sn-9Zn-0.5Ag, and Sn-9Zn-0.5Al solder alloys was studied by impression testing under constant punching stress in the range of 60 MPa to 130 MPa and at temperatures in the range of 298 K to 370 K. Analysis of the data showed that, for all loads and temperatures, Sn-9Zn-0.5Al had the lowest creep rates and thus the highest creep resistance among all materials tested. The creep resistance of Sn-9Zn-0.5Ag was slightly lower than that of the Al-containing alloy. The enhanced creep behaviors of the ternary alloys are attributed to the presence of AgZn3 and very fine Zn particles, which act as the main strengthening agents in the Sn-9Zn-0.5Ag and Sn-9Zn-0.5Al alloys, respectively. Assuming a power-law relationship between the impression rate and stress, average stress exponents of 6.9, 7.1, and 7.2 and activation energies of 42.1 kJ mol−1, 42.9 kJ mol−1, and 43.0 kJ mol−1 were obtained for Sn-9Zn, Sn-9Zn-0.5Ag and Sn-9Zn-0.5Al, respectively. These activation energies are close to 46 kJ mol−1 for dislocation climb, assisted by vacancy diffusion through dislocation cores in the Sn. This, together with the stress exponents of about 7, suggests that the operative creep mechanism is dislocation climb controlled by dislocation pipe diffusion.

A comparison of impression, indentation and impression-relaxation creep of lead-free Sn–9Zn and Sn–8Zn–3Bi solders at room temperature

Creep behavior of Sn–9% Zn and Sn–8% Zn–3% Bi solder alloys was studied by impression, indentation, and impression-relaxation tests at room temperature (T > 0.6Tm) in order to evaluate the correspondence of the creep results obtained by different testing techniques, and to evaluate the effect of Bi on the creep response of the eutectic Sn–9Zn alloy. Stress exponent values were determined through these methods and in all cases the calculated exponents were in good agreement. The average stress exponents of 8.6 and 9.9, found respectively for the binary and ternary alloys, are close to those determined by room temperature conventional creep testing of the same materials reported in the literature. These exponents imply that dislocation creep is the possible mechanism during room temperature creep deformation of these alloys. The introduction of 3% Bi into the binary alloy enhanced the creep resistance due to both solid solutioning effect and sparse precipitation of Bi in the Sn matrix.

Impression creep of hypoeutectic Sn–Zn lead-free solder alloys

Creep behavior of the hypoeutectic Sn–Zn alloys containing 2.5–6.5 wt.%Zn was studied by impression testing and compared to that of the eutectic Sn–9Zn alloy. The tests were carried out under constant punching stress in the range 50–110 MPa and at temperatures in the range 298–370 K. Assuming a power law relationship between the impression rate and stress, average stress exponents of 6.2, 6.4, 7.3, and 7.4 and activation energies of 41.9, 43.9, 44.5 and 46.0 kJ mol −1 were obtained for Sn–2.5Zn, Sn–4.5Zn, Sn–6.5Zn and Sn–9Zn, respectively. These activation energies are close to 46 kJ mol −1 for dislocation climb, assisted by vacancy diffusion through dislocation core in the Sn. This, together with the stress exponents of about 7 suggests that the operative creep mechanism is dislocation climb controlled by dislocation pipe diffusion. Analysis of the data showed that for all loads and temperatures, the eutectic Sn–9Zn had the lowest creep rates and thus the highest creep resistance among all materials tested. The creep resistance of Sn–6.5Zn was only slightly lower than that of the eutectic alloy while the other two alloys showed much lower creep resistances. This is attributed to the higher volume fraction of Zn-rich second phase precipitates in the more concentrated alloys which acts as the main strengthening agent in the Sn–Zn alloy system.

Interfacial Reactions between Eutectic SnZn Solder and Bulk or Thin-Film Cu Substrates

Interfacial reactions between eutectic SnZn solder and bulk or thin-film Cu substrates are investigated and compared. The thicknesses of bulk and thin-film Cu substrates are 0.5 mm and 4,000 A, respectively. Different dominant reaction products and interfacial microstructures are observed in these two types of interfacial reactions. In the bulk Cu type, the Cu5Zn8 phase is the dominant reaction product under reflow and solid-state annealing. However, the CuZn5 phase becomes the dominant reaction product in the thin-film Cu type. The Cu5Zn8 phase in the bulk Cu type remains as a uniform microstructure after reflow. After solid-state annealing, however, the Cu5Zn8 phase fractures and the Cu6Sn5 and Cu3Sn phases are formed at the Cu5Zn8/Cu interface. The CuZn5 phase in the thin-film Cu type ripens after reflow and the phase morphology is transformed from a uniform layer into separated scallops. In situ observation of the interfacial microstructure after solid-state annealing reveals that prominent deformation occurs in the solder region close to the interface in the bulk Cu type. While in the thin-film Cu type, the CuZn5 grain is extruded out of the interface.

Abnormal polarity effect of electromigration on intermetallic compound formation in Sn–9Zn solder interconnect

Interfacial reactions in a eutectic Sn–Zn solder/Cu interconnect were investigated under electric current stressing. After applying a constant direct current at 4.26 × 104 A cm−2 for 166 h, an abnormal “polarity effect” was found where the intermetallic compound layer at the cathode was about 2.3 μm thicker than that at the anode. Upon increasing the temperature to about 185 °C, Cu–Zn compounds were replaced by Cu–Sn compounds at the anode and by a mixture of Cu–Zn and Cu–Sn compounds at the cathode.

Thermal conductivities of solid and liquid phases in Pb–Cd and Sn–Zn binary eutectic alloys

The thermal conductivities of the Pb solution (Pb–3.3 wt.% Cd), the Cd solution (Cd–0.25 wt.% Pb), the Sn solution (Sn–1 wt.% Zn), the eutectic Pb–Cd (Pb–17.4 wt.% Cd) and the eutectic Sn–Zn (Sn–8.9 wt.% Zn) were measured with a radial heat flow apparatus up to eutectic temperature, T E. The thermal conductivity ratios of the eutectic liquid phase to the eutectic solid phase, R for the Pb–Cd and Sn–Zn binary alloys were found with a Bridgman type directional solidification apparatus at T E. Thus, the thermal conductivities of the eutectic liquid phases, κ L, for the Pb–Cd and Sn–Zn eutectic binary alloys were obtained by using the values of κ S and R at their eutectic temperature. Finally, the results have been compared with previous experimental values for binary alloys and pure materials.

Effect of Cu addition on interfacial reaction between Sn–9Zn solder and Ag

The effect of Cu added eutectic Sn–9Zn solder reacting with the Ag substrate has been investigated in this study. Three Ag–Zn intermetallic compounds (IMCs), ∈–AgZn3, γ–Ag5Zn8, and ζ–AgZn, were formed on the Sn–9Zn/Ag interface at 260 °C. While Cu was gradually added to the Sn–9Zn alloy, microstructures of intermetallic compounds changed dramatically. The intermetallic compound microstructures became loose and Sn and Cu atoms in the Ag-Zn intermetallic compounds increased. If more than 3 wt% of Cu was added to the Sn-9Zn alloy, Ag-Sn intermetallic compounds were formed on the Ag surface and massive spalling of Ag–Zn IMC layers from the Ag surface occurred in a short reaction time.

Effect of small additions of alloying elements on the properties of Sn-Zn eutectic alloy

Sn-Zn alloys have been considered for use as lead-free solders. Their poor wetting and oxidation resistance properties are the main obstacles that prevent them from becoming commercially viable solders. The effects of alloying elements, such as lanthanum, titanium, aluminum, and chromium, on oxidation resistance, wetting properties, and tensile properties of eutectic Sn-Zn solder are described herein. Results show the addition of alloying Ti, Al, and Cr can improve the oxidation resistance of Sn-9Zn solder. La, Ti, and Cr do not have much effect on the wetting properties, whereas Al worsens the wetting. Differential scanning calorimetry investigations reveal the solidus temperature of these solders to be ∼200°C. Addition of Cr improves ductility while maintaining tensile strength.

On the advantages of using a hypoeutectic Sn–Zn as lead-free solder material

Hypoeutectic Sn–Zn may be a better choice than the eutectic Sn–9Zn as a lead-free solder. We checked the non-equilibrium melting behaviors of a series of Sn–Zn alloys (2.5–9 wt.% Zn) by differential thermal analysis, and found that at a heating rate of 5 °C/min, Sn–6.5Zn behaves the same way as the eutectic Sn–9Zn in melting. Dipping and spreading tests were carried out to characterize the wettability of Sn–Zn alloys on Cu. Both tests indicated that Sn–6.5Zn has significantly better wettability to Cu than Sn–9Zn does. The reaction layers formed during the spreading tests were examined. For all samples with 2.5–9 wt.% Zn, two reaction layers are formed at the interface, a thick and flat Cu 5Zn 8 layer adjacent to Cu and a thin and irregular Cu–Zn–Sn layer adjacent to the alloy. The total thickness of the reaction layers between the alloy and Cu was found to remarkably decrease with decrease of the Zn concentration.

Composition control of tin–zinc deposits using experimental strategies

The robust electroplating settings of a direct-current (dc) plating mode for the co-deposition of Sn–Zn deposits with their composition close to the eutectic point (i.e., Sn–9Zn) from the chloride solutions were achieved and investigated by using experimental strategies, including the fractional factorial design (FFD) and central composite design (CCD) coupled with the response surface methodology (RSM). The temperature of the plating bath, pH, and the metallic ion ratio (i.e., Sn 4+/Zn 2+ ratio) were found to be the key factors affecting the composition of Sn–Zn deposits in the FFD study. The effects of pH and temperature of the plating solution on the composition of Sn–Zn deposits were examined using a regression model in the CCD study. This model, represented as contour plots, showed that pH 5.0 and temperature = 78 °C were the robust electroplating settings for the co-deposition of the eutectic Sn–Zn alloys, which was independent of the substrates. In addition, based on the robust plating settings, the composition of Sn–Zn alloys could be precisely controlled and predicted by adjusting the composition of the plating baths. From the morphologies and crystalline information, the binary Sn–Zn deposits prepared in this work should belong to heterogeneous alloys.

Microstructure and Mechanical Properties of Sn-8.55Zn-1Ag-XAl Solder Alloys

The microstructure and mechanical properties of as cast lead-free solders including eutectic Sn-9Zn, Sn-8.55Zn-1Ag and Sn-8.55Zn-1Ag-XAl (X=0.01∼0.45 mass%) alloys were investigated. Microstructures of the Sn-Zn-Ag-XAl alloys consist of AgZn3 compound, Zn-rich phase, Al-rich segregation and hypoeutectic phase. The addition of Al dramatically improves the 0.2% offset yield stress, Vickers hardness, ultimate tensile stress (UTS) and total tensile strain of the Sn-8.55Zn-1Ag-XAl alloys. The increase in the Al content of the Sn-8.55Zn-1Ag-XAl alloy from 0.01 to 0.45 mass% increases the average yield stress from 49.9 to 54.0 MPa (∼47.6 MPa for eutectic Sn-9Zn) and the Vickers hardness from 17.0 to 18.3 Hv (∼16.8 Hv for eutectic Sn-9Zn). The average values of UTS of the Sn-8.55Zn-1Ag-XAl alloys with X=0.01, 0.1, 0.25 and 0.45 mass% Al are 55, 58, 55 and 60 MPa, respectively, while those of the tensile strain are 47%, 52%, 58% and 45%, respectively (52 MPa and 50% for the eutectic Sn-9Zn alloy). Fracture mechanisms of the Sn-8.55Zn-1Ag-XAl alloys are correlated with the Zn-rich phase and Al segregation.

Pasty Ranges and Latent Heat Release Modes for Sn-9Zn-<i>x</i>Ag Lead-free Solder Alloys

The pasty ranges and latent heat release modes of Sn-9Zn-xAg alloys, where x varies between 0.5 and 3.5, are examined in this study. The effects of alloy composition and cooling rate on the pasty range and latent heat release modes are also investigated. A Computer Aided-Cooling Curve Analysis (CA-CCA) technique is used to determine the pasty ranges and latent heat release modes for the alloys. To comply with the requirements of CA-CCA, the heat transfer conditions of the experimental setup must closely resemble of a lumped system, in which the alloy is cooled in a uniform fashion. To confirm this, two thermocouples are inserted into the crucible where the alloy is melted and subsequently resolidified. The crucible is used with and without a covering of insulating material to obtain different cooling rates. The temperature readings show that the heat transfer conditions in the experimental setup indeed meet the requirements of a lumped system. The solid fraction versus temperature relationship (f s -T) for the various alloy compositions and cooling rates are obtained from the temperature data through CA-CCA. The experimental results show that as the silver content of the Sn-9Zn-xAg alloy increases, the liquidus temperature rises and the pasty range broadens. The pasty range is approximately 2°C for 0.5Ag, 9°C for 1.5Ag, 14°C for 2.5Ag, and 18°C for 3.5Ag. As long as the silver content is below 0.5 mass%, silver has little effect on the microstructure, which is basically the eutectic Sn-9Zn and the f s -T relationship is nearly a vertical line. However, as the silver content exceeds 1.5 mass%, the formation of Ag-Zn intermetallic compounds becomes obvious. This causes the alloy composition to deviate from eutectic and to lean towards the tin side of the Sn-Zn phase diagram. This in turn causes the proportion of the primary tin phase to increase and that of the zinc-tin eutectic phase to decrease. This is reflected on the plot of the f s -T relationship by two distinct vertical regions. One corresponds to the primary tin phase and the other to the eutectic phase. As the silver content further increases, the effects of intermetallic compound formation become even more obvious. As an alternative to CA-CCA, Huang's model can be used to obtain a quantitative f s -T function. As the silver content increases, the primary solid fraction for Huang's model increases. As the cooling rate increases, the primary solid fraction and the nonlinearity factors n e and n p decrease.

Wetting Properties of and Interfacial Reactions in Lead-free Sn-Zn Based Solders on Cu and Cu Plated with an Electroless Ni-P/Au Layer

The wetting time, wetting force, contact angle and interfacial reaction of Sn-8.55Zn-0A5Al-XAg (mass%) and Sn-9Zn (mass%) lead-free solders on Cu and Cu/Ni-P/Au substrates have been investigated. The Ag content, X, of thesolders investigated was 0-3 mass%. The results indicate that the wettability of Sn-8.55Zn-0A5Al-XAg solders decreases with increasing Ag content. The Sn-8.55Zn-0.45Ai-XAg. solders containing low Ag content, less than 1mass%, exhibit better wettability than the eutectic Sn-9Zn solder. Furthermore, it was also found that the wettability of Sn-Zn based solders on Cu substrate is better than that on Cu/Ni-P/Au substrate, The when used on Cu and Cu/Ni-P/Au, Sn-9Zn solder forms Cu 5 Zn 8 and AuZn 3 respectively, while the Al-containing solders form Al 4 . 2 Cu 3 . 2 Zn 0 . 7 and Al 2 (Au, Zn). Further addition of Ag gave rise to AgZn 3 particles attached to the underlying intermetallic compound.

Electrodeposition of eutectic Sn–Zn alloy by pulse plating

A uniform deposition of a Sn–Zn alloy deposit was achieved by pulse plating. Apparently, the relative composition of Sn and Zn in the deposit was affected by the bath compositions and pulse condition. A pulse-plating condition of 99.9 ms on-time and 1.0 ms off-time gave rise to a eutectic Sn–Zn deposit, with a eutectic temperature of 198.8 °C (as analyzed by differential scanning calorimetry) and a uniform composition distribution across the deposit. A mechanism for explaining the pulse-deposition behavior of the Sn–Zn eutectic deposit was proposed. A longer off-time period, 99.9 ms versus 0.1 ms, resulted in a nodular, yet thinner deposit.

Tin–zinc electrodeposition from sulphate–gluconate baths

The electrodeposition of tin–zinc alloys with 20–30% Zn by weight from sulphate–gluconate baths at pH 4 and different [Sn(II)], [Zn(II)] and [gluconate]/([Sn(II)]+[Zn(II)]) ratios was studied. The anodic stripping analysis of the deposits was correlated with their morphology, composition and phase structure. When [Zn(II)] is equal to or not much greater than [Sn(II)], a eutectic type Sn–Zn alloy is obtained, with a Zn content of around 20% by weight, which is greater than that of a thermal eutectic alloy. The morphology of the electrodeposited eutectic Sn–Zn alloy tends to disappear at high deposition charges and a second crystalline coating, richer in Sn, is obtained.

The thermal property of lead-free Sn-8.55Zn-1Ag-XAl solder alloys and their wetting interaction with Cu

The wetting behaviors between the quaternary Sn-8.55Zn-1Ag-XAl solder alloys and Cu have been investigated with the wetting balance method. The Al contents, x, of the quaternary solder alloys investigated were 0.01-0.45 wt.%. The results of differential scanning calorimeter (DSC) analysis indicate that the solders exhibit a solid-liquid coexisting range of about 7-10°C. The solidus temperature of the quaternary Sn-8.55Zn-1Ag-XAl solder alloys is about 198.2°C, while the liquidus temperatures are 205-207°C. The experimental results showed that the wettability of the Sn-8.55Zn-1Ag-XAl solder alloys is improved by the addition of Al. The mean maximum wetting force of the solders with Cu is within 0.75-1.18 mN and the mean wetting time is around 1.0-1.1 sec, better than the ∼1.3 sec of eutectic Sn-9Zn and Sn-8.55Zn-1Ag solder alloys. The addition of Al also depresses the formation of e-Ag-Zn compounds at the interface between Sn-8.55Zn-1Ag-XAl solders and copper.