Environmental Friendly Low Mass 20g-Sn58Bi/Cu Solder Alloy as an Alternative to Lead SnPb and Its Properties Study

The SnBi solder system are argued to be the possible replacement for the lead and hazardous SnPb solder alloy. The features that differs in this study from the other types of Sn58Bi/Cu is the low mass usage of 20g. This low mass serves as to preserve materials and reduce waste. Yet, the main objectives to provide substantial data in term key properties of melting temperature, hardness and contact area properties of this Sn58Bi solder. The melting point of the Sn58Bi solder alloy is 142.25°C, close to the eutectic temperature. The average Vickers hardness value produced by the Sn58Bi was 28.8Hv, considerably high and close to the SnPb. The spreading area of the Sn58Bi solder alloy on the Cu substrate was calculated to 14.85mm2, at the soldering temperature 230°C. These properties are based on 20g of SB that can be predicted to be quite standout, and at the same time provides less waste of materials. Insight discussions are elaborated in this paper.


Introduction
Solder alloys are used as medium to provide bonding between the electronic components and the substrate, usually a Printed Circuit Board (PCB) in the electronic packaging industry. This is achieved by a metallurgical joining process of soldering [1]. The temperature dealt in this process is low (< 250°C) and thus, in need of efficient solder alloys to be used in the soldering process [2,3]. In this process, the solder alloy brings the component in contact with the substrate, whereby the preferred melting point of the solder alloy to be less than 425°C [3]. Ever since old days, the SnPb solder's enhanced properties in terms of low melting temperature (183°C ) [4,5], high load resistance (hardness) [5] and good wettability to the PCB made this solder the ideal solder alloy. However, the furthering with the enhancement of the solder alloy on its on properties, the attention has to be given in its aspect to the environment. The SnPb provides greatness in its properties but fails in satisfying the crucial part of being a safe material [6,7]. The Pb contain in the alloy causes debate as the Pb is recognised as harmful element that can cause health problem to human [5,8]. Without a doubt, in the soldering industry the contribution of human is a necessity and thus such exposure to Pb could harm the health. The Environmental Protection Agency (EPA) listed the Pb as among the top 17 chemicals [9]. This prompts to finding alternative lead free solder that could replace the SnPb solder alloy. Numerous studies carried out to find the potential candidate for replacement and among the vast researched solder is the SnAgCu solder alloy [10,11]. One of the standout properties of this solder alloy is its proficiency in the mechanical strength (hardness and shear strength) [12]. Notwithstanding that, this solder alloy produces IMC in the solder alloy itself to contribute to a well-defined microstructure. IOP Conf. Series: Earth and Environmental Science 505 (2020) 012004 IOP Publishing doi:10.1088/1755-1315/505/1/012004 2 Nevertheless, with high melting temperature (217°C ), this solder alloy's credential is still questionable. This is added with thick IMC production in this SnAgCu solder if exposed to high temperature [13,14]. Table 1 shows some of the discussed and researched solder alloy. Then again, some solder alloys such as SnBi that known to have low melting point would be taken in to consideration of replacing the SnPb solder alloy. Although, there are studies conducted on the SnBi solder alloy, not many have reported a low mass composition as taken in to attention in this paper. Studies shows that binary element solder are naturally weak in mechanical aspect and in need of high amount of mass consumption to rectify this situation. Taking this as a contest, this research investigates the melting temperature, hardness and contact area of the low mass consumption of the Sn58Bi solder alloy.

Experimental procedures
The Sn58Bi solder alloy was prepared from 8.4g of tin (99.9%, Alfa Aesar), 11.6g of bismuth (99.9%, Alfa Aesar) that was melted in a furnace at 600°C for 1 hour soaking time to make sure a homogenous mixture. To enable a proper mixing of these elements the solder alloy was re-melted using a hot plate at 300°C. The Sn58Bi solder alloy was flattened into billets for the melting, hardness and wetting test. The wettability test was performed by soldering the Sn58Bi solder alloy on to the Copper (Cu) substrate and the contact spearing area was measured using the VIS Pro software. The spreading rate, K was calculated using the equation (1) and Figure 1 shows the parameter needed. The melting properties analysis meanwhile was studied by using the Differential Scanning Calorimetry (DSC)

Melting properties analysis
The first properties that was concerned in this research was the melting properties. Commonly the solidus, liquidus and peak temperature are important aspects that are reflected from the thermal analysis. These are among the important parameters that could influence the microstructural of the solder alloy, and further related to the mechanical properties. Lower melting temperatures are desirable in the electronic industries mainly to avoid board warpage or deterioration to the PCB board [15,16,17]. According to [17], higher reflow temperature would cause evaporation of entrapped moisture that causes crack. The Sn58Bi solder in this research produces a solidus temperature, T S of 141.18°C , liquidus temperature, T L of 147.45°C and peak temperature, T M of 142.25°C ( Figure 2). The temperature of the Sn58Bi was not as the eutectic temperature (139°C) because some elements of Bi and/or Sn may be left out during the remelting process and such occurrence is usual in an experiment study. One of the important analysis that could be extracted from these thermal properties is the pasty range (T M -T S ), producing 1.07°C and with the range of less than 5°C; the Sn58Bi solder alloy could stimulate good microstructural properties. Although the microstructure aspect of the Sn58Bi solder alloy is not discussed in this study, the pasty range could provide a sight of advantage in the microstructure formation [15]. This observation was comparable to the report made by [16]. The low melting temperature for this solder alloy allows lower reflow temperature which underwrites to a lower thermal environment during soldering and at the same time avoid any thermal damage of other components.

Microhardness
In the midst of mechanical properties that been investigated by most researches is the mirohardness of the solder alloy. The hardness of a solder alloy provides the information on the ability of the solder to resist penetration upon load. The Sn58Bi solder alloy's Vickers microhardnes is tabulated in Table 2.
The average Vickers hardness of the Sn58Bi solder alloy was calculated to be 28.78Hv. The result is comparably high compared to the Sn1.0AgCu solder alloy that produces an average of 9.78Hv [17] and even compared to the SnPb solder that produces 12.64Hv in a study conducted by [18]. The high hardness value of the Sn58Bi solder in this study could be first attributed to the low pastry range that can improve microstructural properties by producing finer structures [11,19]. Besides that, the SnBi solder system has a unique characteristic that the Sn and Bi element will not diffuse to produce any IMC compound but rather exist as single elements [20]. Also, the low melting temperature of the Sn58Bi solder here permits a better nucleation of grains in the microstructure at the soldering temperature of 230°C [21]. This will be opposing to the high melting temperature solders as these solders would need to be reflowed at even higher temperature to achieve a better grain production to enhance the microstructure. Thus, the Sn58Bi solder could be able to resist further penetration and increase the hardness.

Spreading area and spreading rate
The wettability aspect of the Sn58Bi was investigated using the spreading area ( Figure 3) and the spreading rate. The spreading area of the Sn58Bi solder alloy on the copper substrate was calculated to show the ability of this low melting solder alloy's spreading capability. In fact, the spreading area is influence by the surface tension of the molten solder alloy during soldering. This interaction between the solder and the substrate also determines the production of IMC layers effecting the spreading area. Low surface tensions together with low viscosity of the molten solder are the desired outcome in providing a better wettability [22]. As mentioned by [23], larger spreading area is desirable to produce a good joint, as these features are known to produce thin IMC layer and is void-free. Saying that, no baseline of limitation of the spreading area was mentioned by any studies, hence, this paper will be comparing the literatures availability of the SnPb and other researched solder alloy to relate with the Sn58Bi's spreading area. The average spreading area of the Sn58Bi solder alloy was 14.85mm 2 . The spreading rate of the Sn58Bi was also calculated as per equation (1), and the average was 67.2%, provide in Table 3. As previously mentioned, the spreading area and spreading rates does not have a standard to differentiate the good and bad wettability, nonetheless, the other solder alloy's readings will be compared as to assess this current study. The spreading rate of the Sn3.8Ag0.7Cu was 82.48% and is high since the high melting point of the solder permits lower viscosity due to high thermal energy [23,24]. In a different study, the spreading rate of the Sn3.0Ag0.5Cu was 90.15%, again contributed due the high melting point [24]. Although the spreading rate of the Sn58Bi is only 67.2%, yet this range is acceptable since the difference of comparing the high melting point solder is only about 20% difference. The low melting point is the reason for this spreading rate. Looking at the spreading area, this is also common for the high melting temperature solders to have better spread, for example, the Sn6Zn4Bi solder alloy had a spreading area of 23mm 2 at 230°C [7] compared to 14.9mm 2 for the Sn58Bi solder in this study. The low melting point of the Sn58Bi solder prohibits the larger spreading such as the Sn6Zn4Bi and Sn3.0Ag0.5Cu solder alloys, as the surface tension is high in the current solder. Even so, the spreading rate is not that much of a variance in difference and the Sn58Bi solder can be suggested to also provide satisfactory spreading rate and area [25]. In a study done by [26], the spreading area produced of the SnPb solder was 3.2mm 2 , lower compared to the Sn58Bi solder in this study. Once again, the higher surface tension of the SnPb solder alloy is justified as the reason of this low spreading area. Concerning this, the Sn58Bi solder still manages to produce efficient spreading area and rate.

Conclusion
The scope of the paper to produce the analysis on the properties of the low mas composition Sn58Bi solder alloy was successfully completed. The melting properties shows advantageous influence as the melting temperature was lower than the SnPb solder alloy and accompanied with shorter pasty range. The mechanical aspect in terms of the microhardness was experimented and the outcome of the result showed that the Sn58Bi solder alloy could compete with the SnPb and other lead free solder alloys and the contribution was predicted because the microstructural aspect of Sn and Bi that acts as discrete elements upon solidification. Taking in to concern on the wettability property, the spreading area and rate yields satisfactory result as the Sn58Bi solder alloy harvests better spreading area compared to the SnPb solder alloy and had minimal difference compared to the efficient spreading rate of a high melting temperature lead free solder alloys. Overall, the Sn58Bi low mass composition solder provided positive result based on the thermal, hardness and wettability that could be taken as reference for further investigation and implementation.