Effects on the microstructure and mechanical properties of Sn-0.7Cu lead-free solder with the addition of a small amount of magnesium

1 Introduction

Soldering technology provides most of the interconnection methods between electrical and electronic components. For the last 50 years, a great number of electronic packaging industries throughout the world have been effectively soldering with the Sn-37Pb eutectic alloy in the manufacturing of computer chips, circuitry and other electronic equipment, because of its low processing temperatures [1], [2]. With the realization that lead (Pb) is a contaminant both to the environment and to human health, the use of lead in electronic products has been banned, especially since the passage of EU legislation regarding waste in electrical and electronic equipment on 1 July 2006 [3]. In response to the restrictions on the use of lead in electronic products and in manufacturing environments, many ongoing works have attempted to find acceptable lead-free solders for various electronic attach applications. Of the many lead-free solder alloys, the eutectic Sn-0.7Cu (in wt.%) solder is one of the most promising replacements for lead due to its low cost [4], [5], [6]. Several low-temperature solders are commercially available, including indium- and bismuth-containing alloys and, at present, dozens more are being reported in open literature [7], [8], [9], [10], [11], [12]. Lead-free solders containing Bi have a low melting temperature and high tensile strength. However, Bi makes the solder less ductile and can even exhibit the serious defect called “fillet-lifting”. Solder with indium has the necessary ductility to be molded into various shapes. However, indium is too expensive to use in the quantities required for a solder base. Although several commercial and experimental lead-free solder alloys are available as replacement for Sn-Pb solder [13], [14], [15], the melting point of these lead-free solders are between 208°C and 227°C, which is about 30°C higher than the melting point of the eutectic Sn-Pb solder alloy. High reflow temperature has a significant impact on solder joint performance and can affect the reliability of microelectronic package [16]. Lowering the reflow temperature directly reduces the total thermal damage on components joined by solder.

In open literature, various methods have been discussed to improve the properties of lead-free solder alloys, and one of the most effective ways is believed to be micro-alloying [17]. Many researchers have investigated the effect of magnesium on the melting point and wettability of lead-free solders [18], [19], [20], [21]. Although some results on magnesium-containing lead-free solder alloy have been reported, only a few full experiments and evaluation studies have been conducted. Therefore, the first objective of the current work was to present the effect on the bulk microstructure and mechanical properties of a minimal addition of magnesium to Sn-0.7Cu alloy. The second objective was to evaluate magnesium-containing solder joint characteristics, including wettability, mechanical properties, microstructure, and the effect of isothermal aging on interfacial intermetallic compound (IMC) growth and fracture.

2 Materials and methods

To make Sn-0.7Cu-0.01Mg solder alloy, first, solder ingots of Sn-0.7 wt.% copper (used as basic solder components) and magnesium turnings (used as alloying elementa) were carefully weighed and placed in a graphite crucible. The melting process was carried out at 750°C for several hours in an inert argon gas atmosphere using a resistance-heated furnace. The ingots were re-melted at 400°C, cast into dog bone-shaped tensile specimens in a graphite mold and cooled with water. The chemical composition of the produced alloys was determined by X-ray fluorescence spectrometry. Their chemical compositions were shown in Table 1. The tensile tests of the Sn-0.7Cu and Sn-0.7Cu-0.01Mg solders were conducted at a strain rate of 1 mm/min and at room temperature. Next, the melting point of the solders was investigated with differential scanning calorimetry (DSC) by using small pieces of alloys about 20 mg in weight and a heating rate of 0.5°C/min from room temperature to 350°C in a nitrogen atmosphere.

Small solder cube beads (2 mm cubic) were cut from an ingot. The beads were cleaned with acetone in an ultrasonic bath. The substrate consisted of copper plates of 99.9% purity and an area of 10 mm square. A soldering paste flux comprising vaseline 80–90 wt.%, zinc chloride 4–6 wt.%, paraffin 6–9 wt.%, ammonium chloride 1–3 wt.%, and water 2–4 wt.% was used. The wetting tests were conducted on a hot plate at 250°C–290°C for 10 s. The contact angles and spread area were measured on solidified samples at room temperature, as shown with the schematic diagram in Figure 1.

Figure 1:

Schematic drawing of a method for measuring contact angle and spreading area.

To consider the strength of lead-free solder alloy and its soldering using copper lead frames, the bulk material and the Cu/solder/Cu joints were used as test specimens. Figure 2 shows (A) tensile specimens of the alloy cast into a graphite mold, and (B) two copper rods fixed in a holder and positioned with the two end surfaces separated by a distance of 0.2 mm. The end surface of specimens were pre-coated with a flux, dipped in a solder bath for 10 s, and then cooled in air. Following the soldering process, some of the solder joints were retained in the as-soldered condition, while the others were heat treated at 150°C for 25, 100 and 200 h, respectively. The as-soldered and aged samples were then tensile tested using a micro-tensile tester at room temperature under a strain rate of 0.05 mm/min up to a complete fracture. The fracture surfaces of the tensile specimens were observed with a scanning electron microscope (SEM) and energy dispersive X-ray (EDX) spectroscopy. For cross-sectional SEM, samples were cold mounted in epoxy and polished down to a 0.05 μm finish. After polishing, solder etching was carried out to reveal the microstructure in cross section. Etching was done with 5% HCl–95% C₂H₅OH solution for a few seconds. EDX spectroscopy was performed in the SEM to analyze the chemical composition of the interfacial layers.

Figure 2:

Configuration of the tensile testing specimens (A) bulk specimen (B) joining specimen.

3 Results and discussion

3.1 Thermal analysis

DSC analysis was performed in order to identify the melting points of the lead-free Sn-0.7Cu and Sn-0.7Cu-0.01Mg solders. The DSC curves obtained for the two alloys are shown in Figure 3. The onset temperature of the DSC heating curve is the temperature at which the solder starts to melt, or it can also be marked as the solidus temperature of the solders or the eutectic temperature of a eutectic solder alloy. For the Sn-0.7Cu alloy, the onset temperature is 229.96°C, the addition of 0.01 wt.% magnesium into the Sn-0.7Cu solder lowers the onset temperature from 229.96°C to 227.88°C. Comparing the two samples, we find that the Sn-0.7Cu-0.01Mg sample has the lowest melting temperature peak. Furthermore, two samples are nearly compositionally eutectic and no shoulder peaks are observed under these measurement conditions.

Figure 3:

Differential scanning calorimetric profiles of the two solder alloys.

3.2 Microstructure

The microstructure of the as-solidified eutectic Sn-Cu solder, shown in Figure 4, consists of two types of regions. The white dendritic regions are pure β-Sn grains; they are surrounded by a gray eutectic network band, with an extremely fine Cu₆Sn₅ IMC layer located in the interdendritic region. In the magnesium-containing solder, the tin dendrites become slightly coarser. However, the microstructural differences are not significant. In addition, the EDS microanalysis detects magnesium in some instances within the intermetallic phase and the β-Sn phases. The number of these magnesium-containing particles in the alloy is very small at a magnesium content of 0.01 wt.%. Typical EDS elemental mapping analysis is shown in Figure 4D, where it can be observed that the magnesium is not uniformly distributed in the eutectic and the β-Sn phases.

Figure 4:

Microstructure of as-solidified solder alloys.

(A) Sn-0.7Cu, (B) Sn-0.7Cu-0.01Mg, (C) SEM micrographs for Sn-0.7Cu-0.01Mg; elemental mapping results by EDS for (D) Sn, (E) Cu and (F) Mg.

3.3 Tensile strength

The tensile curves of the Sn-Cu solders are presented in Figure 5; the ultimate tensile strength (UTS) and elongation are also summarized. By comparing the Sn-0.7Cu-0.01Mg solder synthesized in this study with the commercial eutectic Sn-0.7Cu solder, the Sn-0.7Cu solder performed with the greatest strength. The addition of 0.01 wt.% magnesium slightly decreases UTS from 38.49 MPa to 29.93 Mpa, while the elongation increases from 52.08% to 62.12%. Figure 6 represents the fractograph of the tensile specimen of Sn-0.7Cu and Sn-0.7Cu-0.01Mg solders, respectively. Microscopic analysis of the tensile fractured surface of the Sn-0.7Cu solder shows the presence of spherical dimples. Only a few dimples have been observed in the Sn-0.7Cu-0.01Mg solder and the sample has tearing-type fractures. Nevertheless, there is an appreciable improvement in ductility.

Figure 5:

Load vs. displacement curves of two solders in the as-solidified condition.

Figure 6:

SEM fractographs of bulk alloys.

(A) Sn-0.7Cu and (B) Sn-0.7Cu-0.01Mg bulk alloys.

3.4 Contact angle

Wetting is the ability of a liquid to make contact with a solid surface. A low contact angle indicates that the liquid can easily “wet” the surface and spread over a large area on the solid surface. The wetting properties are investigated by measuring the final contact angles and spread areas, as shown in Figure 7. Contact angles between the solder and copper substrate decreased with the increase in temperature for both Sn-0.7Cu and Sn-0.7Cu-0.01Mg solders. Furthermore, spreading area generally increases with respect to rising temperature. Comparing the wettability of both solder alloys, the Sn-0.7Cu-0.01Mg solder presents excellent wetting properties with a smaller contact angle and a larger spreading area.

Figure 7:

Contact angles and spreading areas of solders melted on a copper substrate.

3.5 Aging

The relationship between the mean thickness of the interfacial IMC layer and aging time is given in Figure 8. The graphs show that the IMC’s thickness gradually increases with the increase in aging time. Both solder alloys have the same growth rate, i.e., initially fast and becomes slower after about 100 h. Figure 9 presents a collection of the cross-section images of two solders with a copper substrate as-soldered, and thermally aged for 0 h and 200 h. In the as-soldered joints, the Cu₆Sn₅ IMC located at the solder/Cu interface is identified using EDS analysis. In addition, the solder is in the molten state, and the formation of Cu₆Sn₅ IMC has a round scallop-type morphology. In solid-state aging, all these scallops change to a layered-type morphology. However, in terms of IMC growth, there is no difference between the Sn-0.7Cu and Sn-0.7Cu-0.01Mg solders after aging. The IMC thicknesses of the Sn-0.7Cu and Sn-0.7Cu-0.01Mg solders are almost the same.

Figure 8:

The average thickness of the IMC layer at the interface vs. aging time at 150°C of the two solder alloys.

Figure 9:

SEM micrographs showing the interface after aging for 200 h at 150°C.

(A) Sn-0.7Cu and (B) Sn-0.7Cu-0.01Mg solders.

3.6 Joint strength

A joint tensile test was performed to evaluate the effect of the interfacial reactions on the mechanical reliability of the Cu/solder/Cu joints as a function of aging time. Figure 10 illustrates the relationship between the tensile properties and aging time for the solders experiencing isothermal aging at 150°C. The Sn-0.7Cu solder joint has a higher tensile strength compared with the Sn-0.7Cu-0.01Mg solder joint. In all test conditions, the tensile strengths of both solder joints decrease significantly with increased aging times. To verify the variations in tensile strength, fracture surfaces of the tensile-tested specimens were examined using SEM. As shown in Figure 11, the as-soldered tensile testing specimen shows a ductile failure mode consisting of mere solder at the fracture surface. Generally, in the solder joint tensile test, a fracture occurs at the interface or in the solder region with the lowest strength. In the last few years, many studies have been published concerning the mechanical properties of solder joints [14]. The authors propose that excessively thick IMC layers formed between solder and substrate could significantly degrade the mechanical properties of the solder joints. From the results mentioned above, the IMC’s thickness gradually increased with the increase of aging time, significantly decreasing the tensile strength of the solder joints. Again, it can be briefly concluded that the tensile strength of the solder joints is affected by both the microstructural change of bulk solder and the increase in the interfacial IMC thickness.

Figure 10:

Variations of the tensile strength as a result of aging temperature.

Figure 11:

Fractography of the joints broken in tensile testing after aging for 200 h at 150°C.

(A) Sn-0.7Cu and (B) Sn-0.7Cu-0.01Mg solders.

4 Conclusions

1. For comparison, the DSC results of bulk samples show that the measured melting temperatures of Sn-0.7Cu-0.01Mg decreased by about 2°C.

2. The initial microstructure of eutectic Sn-0.7Cu consists of dendritic β-Sn and the Cu-Sn intermetallic compound Cu₆Sn₅ has been finely dispersed in the matrix of β-Sn. In magnesium-containing alloys, the intermetallic phase has become coarser than the Sn-0.7Cu solder.

3. The Sn-0.7Cu-0.01Mg alloy shows very good elongation, which is better than that of the Sn-0.7Cu lead-free solder alloy; however, there is no significant improvement in tensile strength.

4. Magnesium addition to the Sn-0.7Cu alloy results in a decrease in the mean contact angle and a higher spreading area, which indicates excellent wettability.

5. A significant increase in the intermetallic layer thickness during isothermal aging is observed for both the Sn-0.7Cu and Sn-0.7Cu-0.01Mg solders.

6. After a long thermal aging time, the tensile strength of the Sn-0.7Cu and Sn-0.7Cu-0.01Mg solder joints decreases slightly due to the growth of IMC layers. Furthermore, Sn-0.7Cu-0.01Mg exhibits the lowest tensile strength.