A novel low melting point Pb-free solder: a SnBiX solder showed a melting point of about 186[degrees]C and cut the material cost.

Many organizations, such as iNEMI, the National Center for Manufacturing Sciences, Improved Design life and Environmentally Aware manufacture by Lead-free Soldering (IDEALS) and the Japan Electronic Industry Development Association (JEIDA) have recommended certain Pb-free alloys (1-4). The most common recommendations are SnAg (including SnAgCu, or SAC), SnCu, SnZn and SnBi solders. SnAg solders, e.g., SnAgAg, SnAg3.0Cu0.5 (as recommended by JEIDA and NCMS), SnAg3.8AgCu0.7 (recommended by IDEALS) and SnAg3.9AgCu0.6 (recommended by iNEMI) have good mechanical and thermal properties, but their wettability to copper surfaces is poor and their cost is high. Furthermore, their liquidus temperature (around 217[degrees]C) is 34[degrees]C higher that of SnPb37 at 183[degrees]C; thus, they are not suitable for traditional soldering equipment and processes. SnCu solders (e.g., SnCu0.7, as recommended by iNEMI, IDEALS and JEIDA) have relatively lower price points and better wetting characteristics, but their melting points are much higher (221[degrees]C). SnZn solders (e.g., SnZn9, recommended by JEIDA) have good mechanical properties, are low in price, and feature a (low) melting point (198[degrees]C) near that of SnPb37; however, zinc's easy oxidation is a fatal weakness.

SnBi solders come closer to meeting existing parameters. However, existing SnBi solders are not accepted for all applications. The key issue with SnBi solders is their wide liquidus temperature range. At this range, solid and liquid can coexist around 190[degrees]C. When the solders cool at a normal rate, a dendritic segregation of bismuth can form easily, and eutectic bismuth's brittleness could worsen the mechanical and soldering properties. (The fillet-lifting phenomenon occurs easily during wave soldering.)

The purpose of our study was to fabricate a novel solder (called SnBiX) with a low melting point (close to 183[degrees]C), good mechanical and thermal properties, wettability, solderability, reliability, and reasonable cost. Based on SnBi20, silver, copper, germanium, cerium and tin are added via several steps to form solid solution. The solid solution can enhance bismuth's microstructure and the solder's mechanical properties. Meanwhile, the faster cooling rate restrains bismuth's dendritic segregation.

Experimental Method

Table 1 shows the SnBiX compositions. The total amount of added metals is less than 1.5 wt%. According to their melting points from high to low, silver, copper, germanium, cerium, antimony and bismuth, respectively, are melted in different groups and then mixed with tin. The mixture is fabricated as continuous strips via a melt spinning process (Figure 1). This process is also called Rapid Solidification (RS).

After being micro-alloyed with SnBi20, silver enhanced the mechanical strength and electric conductivity; copper reinforced the thermal intensity; germanium prevented tin from oxidation and prevented pad lift; cerium enhanced the microstructure and restrained intermetallic compound (IMC) growth, and antimony stopped the phase change of tin (5).

Melting Points and Microstructures

Differential scanning calorimetry (DSC) was used to determine the melting points of solders. As shown in Figure 2, the liquidus temperature range of SnBi20 is 191.9[degrees]C to 206.3[degrees]C when tested at 2[degrees]C per sec. (Figure 2a). Around 135[degrees]C SnBi57 eutectic produces an obvious endothermic peak. The endothermic peak is the main reason for bismuth dendritic segregation when the solder is remelted. When the metals shown in Table 1 are added to SnBi20, the liquidus temperature range narrows from 189.9[degrees] to 205.2[degrees]C, and the melting point also decreases. The endothermic peak around 135[degrees]C still exists but becomes smaller (Figure 2b). If SnBiX solder cools by RS (at a cooling rate of [10.sup.3]~[10.sup.4][degrees]C per sec.), the liquidus temperature range narrows from 186.1[degrees] to 203.1[degrees]C and the endothermic peak around 135[degrees]C is small (Figure 2c). When the cooling rate is more than [10.sup.4][degrees]C, the endothermic peak disappears (Figure 2d).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Adding microelements can decrease the solder melting point because these microelements can dissolve in the solid solution of SnBi20 and alter the space between tin and bismuth atoms, therefore altering the combination force between atoms. RS is a better method to decrease melting point and eliminate bismuth dendritic segregation because RS changes the solder microstructure.

Figure 3 shows x-ray diffraction (XRD) profiles of SnBi20 and SnBiX by RS. The main phases of the solders are tin and bismuth, without other intermediate phases. Because the cooling rate affects bismuth segregation and the solid solution formation, the lattice parameter of the two solders are different (Table 2). Adding microelements can increase solid solubility and lattice parameter, as does RS.

Figure 4 shows the metallograph of solders (magnification: 200x). The white structure is bismuth rich, a Bi-based solid solution combined of bismuth and a little tin. The black structure is tin rich, an Sn-based solution combined from tin and a little bismuth. Because of its large amount, bismuth cannot totally dissolve in a solid solution of tin; the undersaturated bismuth can exist solely outside the base of solid solution easily and form a brittleness phase. As shown in Figure 4a, the segregation of bismuth is very serious. On the contrary, the microstructure of SnBiX by RS is fine and symmetrical without any other segregation phase (Figure 4b). This means RS can obviously restrain the segregation of bismuth. Actually, the cooling rate's effect on segregation is complicated. At a low cooling rate (less than [10.sup.2][degrees]C per sec.), the higher the cooling rate, the more insufficient the diffusion between solid phase and liquid phase and the more serious the segregation of bismuth. In this case, the primary phase of tin is rich in the center of the crystal grain, and bismuth accumulates near the surface of the crystal grain, finally forming SnBi eutectic between crystal grains. However, when the cooling rate is extremely high (more than [10.sup.4][degrees]C), undercooling occurs, which promotes crystal core formation and refines crystal grains, thus leading to the ultra fine and symmetrical microstructure (Figure 4b). Once this microstructure is formed, its microstructure characteristic can be "hereditable." Figure 4c shows that after remelting twice, the microstructure is basically unchangeable, showing only the growth of the bismuth crystal grain. The heredity properties of SnBiX by RS are important, as they mean the solder can be used in wave soldering and reflow as conventional solder without special parameters.

[FIGURE 4 OMITTED]

Mechanical and Soldering Properties

The stress load of welding in practical applications comes mainly from shearing force, so shear strength is one of the important mechanical factors of solders. Table 3 shows the alloys' shear strength. The reinforcement of solid solution after adding microelements leads to an aberration of crystal lattices and forms stronger inner stress, which can increase solder shear strength. Meanwhile, bismuth dispersedly distributes on the solder's base frame and reinforces its mechanical properties; therefore, the shear strength increases. SnBiX by RS has the greatest shear strength compared to traditional SnPb37 solder and SnBi20. The remelted SnBiX solder also has good shear strength, close to that of SnPb37.

Solderability was tested by means of through-hole soldering. The PCB laminate was Nelco N4000-13Si; the test board was a 20-layer board with a 1.5 mm hole diameter and 0.9 mm component lead diameter. Traditional soldering iron and flux were used. PCB pads and holes had OSP finish. As shown in Figure 5, solder filled the total hole without the liftoff phenomenon. Thus, the alloy met the solderability requirements.

[FIGURE 5 OMITTED]

Conclusions

Our study showed that a SnBiX solder fabricated by adding microelement based on SnBi20 reduced the cost and lowered the melting point to around 186[degrees]C, close to traditional SnPb37 eutectic. This means that users could use the solder as a drop-in, without altering soldering equipment and processes.

The added microelement can increase the solid solubility of SnBi20. The reinforcement restrained bismuth segregation, enhanced its shear strength and improved mechanical and thermal properties to close those of SnPb37.

The fine and symmetrical microstructure can be gained by using rapid solidification technology, and the microstructure characteristic is "hereditable." Therefore, the solder can be used in both wave and reflow processes without an apparent change in either the microstructure or the mechanical or soldering properties. Thus, SnBiX solder by RS would be a practical Pb-free solder.

References

1. Cynthia Williams, "NEMI's Lead-Free Assembly Project Reports Latest Results," IPC Apex Proceedings, January 2002.

2. D.W. Bergman, "NCMS Lead-Free Solder Project," Journal of SMT, February 2000.

3. Soldertec, Second European Lead-free Soldering Technology Roadmap, February 2003.

4. Katsuaki Suganuma, "Current Lead-free Soldering in Japan," http://www.nepss.org/presentations/SEMI_Europe/leadfreesolderingjapan.pdf, 1998.

5. Katsuaki Suganuma, Technology of Lead Free Soldering [M], (in Chinese), Beijing: Science Publishing Co., 2004. pp. 26-56.

Yuanshan Li has a master's in electrochemistry and is a senior SMT engineer at the National University of Defense Technology; [e-mail]. Xiaojuan Lei and Zhenhua Chen are with the School of Material Science and Engineering at Hunan University.

Table 1. Compositions of SnBiX solder (%wt)

Sn       Bi  Ag       Cu       Ge       Ce       Sb

Balance  20  0.1~0.5  0.1~0.5  0.1~0.5  0.1~0.5  0.1~0.5

Table 2. Lattice Parameters of Alloys

Solders  SnBi20  SnBiX by RS  Standard PDF cards

Sn  a     5.832   5.845        5.831
    c     3.186   3.185        3.182
Bi  a     4.539   4.539        4.547
    c    11.865  11.875       11.861

Table 3. Alloy Shear Strength

Solder               SnPb37  SnBi20  SnBiX by RS  Remelted SnBiX

Shear strength /MPa  50      45      55           50