Soldering is a metallurgical process used since the ancient times. Diffusion soldering is an attractive joining method for the formation of thermally and mechanically stable Pb-free bond in electrical and electronic at relatively low process temperatures, which involves inter-diffusion, reaction diffusion, and isothermal solidification. The fundamental requirement for the diffusion soldering process is the existence of at least one inter-metallic compound between the components to be joints. In short, the diffusion soldering process can be described as an interlayer of alloy melting component is providing as a foil or thin film coating between the high melting joint’s components. In an electronic packaging, lead has been banned due to environmental issues and healthy concern. It is widely known that lead is related to certain health risks. If lead particles are inhaled or ingested, its can accumulate in the human body causing damage to the blood and central nervous systems. Solder alloys have been manufactured with liquidus temperatures as low as 10.70C, as exemplified by the ternary eutectic Ga62.5In21.5Sn16 alloys and as high as 4240C as in case of the Ge55Al45 binary eutectic composition (Vianco, 1999). Solder can be categorized in three group’s base on melting point of the solder as shows in table 2.1.
Table 2.1: Classification of solder (Indium Cooperation)
Solder
Temperature (0C)
Low temperature solder
≤138
Mid-range temperature solder
138 < t ≤ 235
High Temperature solder
235 < t ≤ 450
Most of the assemblies currently use solder in order to create connection between components and the printed circuit board. Although conductive adhesive can be pressed into service for this application, solder remain far and away the most widespread joining medium. Familiarity, low cost, high reliability and ease of use mean that solder will continue to be an important joining material. Solder interconnects perform three functions as mechanical, electrical and thermal. Solder provides an electrical connection path from the silicon chip to the circuitry on the substrate within the package (first level interconnection), between the different packages and between the package and the Cu traces on the printed wiring board or PWB (second level interconnection) (Shangguan, 2005). It also serves as a path for dissipation of heat generated by the semiconductor (Abtew and Selvaduray, 2000). Solder provides a mechanical and electrical joint that is essential to keep components in place once a circuit has been assembled. While the mechanical strength is important, it is also necessary to ensure that the soldered joint provide a good electrical connection is made between the two connections which require joining. This can only be achieved satisfactorily if the medium, i.e. the solder joining the two conducts electricity well.
Soldering materials
2.1.1 Tin lead solder
From the very beginning of the electronics industry, solder joints have been made primarily by tin and lead alloy. In particular, the eutectic tin-lead alloy (Sn63Pb37) has been used almost exclusively in electronic due to its unique characteristic (cost, availability, ease of use, low melting point, excellent wetting on Cu and it alloys and electrical/thermal/mechanical/chemical characteristic) (Shangguan, 2005; Zhang et. al 2007; Hui et al. 2000 and Chen et al. 2007). The eutectic tin -lead solder provides an excellent electrical conductivity and suitable mechanical strength for joining (Shangguan, 2005). It is also a critical material in virtually all electronics because it is uniquely capable of meeting high technology performance in cost efficient manner.
2.1.2 Lead-free solder requirement
When trying to identify an alternative to lead solder, it is important to ensure the properties of lead-free solder are comparable or superior. According to recent report (Shangguan, 2005; Ramayani, 2007; and Nurfazlin, 2009), lead in solders contributes outstanding properties to overall reliability of tin-lead solder such as;
Reduces the surface tension of pure tin to improve the wettability.
Reduces the rate at which substrates are dissolved by tin.
Enable tin and copper to rapidly form inter-metallic compounds by diffusion
Provide ductility to tin-lead solders.
Addition of lead prevents the transformation of β-tin to α-tin. If the transformation occurs, it will cause increasing in solder volume and loss of structural integrity.
Tin-lead solder have low melting point of 1830C for eutectic solder, which allows the use of low re-flow temperature in the electronic packaging process and ensures reliability of the package.
Low cost.
However, the new lead-free solders needs to have closer melting temperature to existing tin-lead solder, particularly eutectic and near eutectic solder in order to have similar re-flow profile during the manufacturing process. The other requirements need to be fulfilling by lead-free solder such as:
Non-toxic or less toxicity compare with lead solder. Any elements that choose to replace lead in solder must not be harmful to people and environment.
Acceptable processing temperature. Any design of alloy must be able to perform a good wettability under the current processing temperature or close.
Narrow plastic range. This will minimize any reliability or formation of weak phase that may have resulted by the large plastic range.
Good wettability on substrates. Compatible with a standard surface finish.
Form reliable joint. Reliability of solder alloy depending on the coefficient of thermal expansion, elastic modulus, yield strength, shear strength, tensile strength fatigue and creep behavior of the alloy.
Available and affordable. All elements chosen as the candidate for the lead free alloy must be available at a reasonable price.
2.1.3 Lead free solder
A relatively large number of lead-free solder alloys has been developed from binary, ternary, quaternary and even more systems. It can be notice that, a very large number of these solder alloys is based on Sn as the primary material or major constituent. Table shows the lead-free solder that has been developed:
Table 2.2: Proposed lead-free solder alloys with their melting temperature (T m = melting temperature, T s= solidus temperature, T l = liquidus temperature, T e = eutectic temperature
Alloy composition
(Wt %)
Ts (0C)
T1 (0C)
Tm (0C)
Te (0C)
Reference
Sn98Ag2
221
225
Abtew and Selvaduray., 2000
Sn50In50
117
125
Cheng and Lin, 2000
Sn97Cu3
227
275
Abtew and Selvaduray., 2000
Sn96Ag4
221
225
Abtew and Selvaduray., 2000
Sn58Bi42
170
139
Abtew and Selvaduray., 2000
Sn58In42
117
140
Abtew and Selvaduray., 2000
Sn64In36
117
165
Abtew and Selvaduray., 2000
Sn99.3Cu0.7
227
Wu et al., 2004
Sn96.5Ag3.5
221
Wu et al., 2004
Sn43Bi57
139
Cheng and Lin, 2000
Sn95.5Ag3.5In
217
Abtew and Selvaduray., 2000
Sn94.9Ag3.6Cu1.5
225
Abtew and Selvaduray., 2000
Sn89Ag4Sb7
230
Abtew and Selvaduray., 2000
Sn95.5Ag4.0Cu0.5
218
Anonymous., 2000
Sn91.7Ag3.5Bi4.8
208
215
Anonymous., 2000
Sn93.5Ag3.5Bi3
216
220
Anonymous., 2000
Sn77.2Ag2.8In20
179
189
Anonymous., 2000
Sn88Sb4Zn8
198
204
Abtew and Selvaduray., 2000
Sn81Zn9In10
178
Abtew and Selvaduray., 2000
Sn88Zn6Bi6
127
Abtew and Selvaduray., 2000
Sn93.6Ag4.7Cu1.7
217
Abtew and Selvaduray., 2000
Sn95.5.Ag4Cu0.5
216
222
Abtew and Selvaduray., 2000
Sn91.2Ag2Zn6Cu0.8
217
217
Abtew and Selvaduray., 2000
Sn81Zn9In10Cu
Abtew and Selvaduray., 2000
Sn80Zn8In10Bi2
175
Abtew and Selvaduray., 2000
Sn96.2Ag2.5Cu0.8Sb0.5
213
219
Anonymous., 2000
Sn89.2Ag2Cu0.8Zn8
215
215
Abtew and Selvaduray., 2000
Sn96.7.Ag2.8Cu0.5
218
Rizvi et al.,2006
Sn95.7.Ag2.8Cu0.5Bi
214
Rizvi et al.,2006
Sn89Zn9Ag1.5Bi0.5
215
215
Liu et al., 2006
Sn90.5Zn9Ag0.5
199
Chen et al., 2006
Sn90.5Zn9Ga0.5
196
Chen et al., 2006
According to the table 2.2, majority of lead free solders alloys have a melting point around 2000C-2200C. Sn-Cu, Sn-Ag and Sn-Ag-Cu system has liquidus temperature that significantly higher than eutectic tin lead solder. Increasing in melting point will cause the higher processing temperature.
2.1.3.1 Sn42Bi58 solder alloys (1380C)
The low melting point of this alloy makes it suitable for soldering temperature-sensitive components and substrates. Sn42Bi58 solder has reasonable shear strength and fatigue properties, low-temperature eutectic solder with high strength, particularly strong but very brittle (Anton and Angela, 1997). It used extensively in through-hole technology assemblies in IBM mainframe computers, where a lower soldering temperature was required. This solder was good for electronics application and used in thermoelectric applications due to excellent thermal fatigue performance.
2.1.3.2 Sn91.8Ag3.4Bi4.8 solder alloys (200-2160C)
Generally, bismuth is added to Sn-Bi-X solder alloys in order to depress the melting point. Another benefit of adding Bi is greater joint strength. This particular alloy was developed by Sandia National Labs (Anton and Angela, 1997). The result shown that there were no electrical failures on the surface mount devices after 10000 thermal cycles at temperature 750C which is good as tin lead solder.
2.1.3.3 Sn90Bi7.5Ag2.0Cu0.5 solder alloys 1380C (198-2120C)
Although the addition of bismuth to the Sn-Ag-X system imparts greater strength and improves wetting, too much bismuth (more than 5%) will cause segregation of bismuth rich leads to the presence of a small DSC peak around 1380C corresponding to the binary Sn-Bi eutectic or ternary Sn-Ag-Bi eutectic at 136.50C (Anton and Angela, 1997). This eutectic peak has an uncertain effect on joint reliability of the solder as temperature approach 1380C.
2.1.3.4 Sn96Ag3.5 solder alloys (2310C)
This alloy exhibits adequate wetting behavior and strength and is used in electronic industries as well as soldering water mains. According to Anton and Angela, 1997, several sources have also reported good thermal fatigue properties compared to lead solder. In a tin-lead system, the relatively high solid solubility’s of lead in tin and vice versa, especially at elevated temperatures, lead to microstructural instability due to coarsening mechanism. These regions of inhomogeneous microstructural coarsening are known as crack initiation sites. It is well documented that these types of microstructure in tin-lead alloys fail by the formation of a coarsened band in which fatigue crack grows. By comparison, the tin-silver system has limited solid solubility of Ag in Sn, making it more resistant to coarsening. As a result, Sn96Ag3.5 solder forms a more stable and uniform microstructure that is more reliable. Although the Sn96Ag3.5 solder alloys itself exhibit good microstructural stability, when solder on substrate. The combination of higher tin content compared tin-lead solder and higher re-flow temperature environments accelerates the diffusion rate of copper substrate in tin. As its corresponding composition is reached, the brittle Cu6Sn5 IMCs is nucleated and begin to grow lower its mechanical properties of the joint.
2.1.3.5 SAC solders alloys
SAC solder alloys become most promising solder and the best alternative for tin-lead solder replacement (Handwerker, 2005; Nurmi et al., 2005). The alloy has been recommended by several industry consortiums, including Inter-National Electronics Manufacturing Initiative (iNEMI), EU consortium known as IDEALS (Improved Design Life and Environmentally Aware Manufacturing of Electronic Assemblies by Lead-Free Soldering), and the Japan Electronics and Information Technology Industries Association (JEITA). The melting temperatures of SAC solder range from 2160C-2200C according to the composition of the alloys. Because the mechanical stability of the joint degraded when the melting point is approached, elevated temperature cycling produces more damage for tin-lead solder (melting temperature 1830C) as compared to higher melting point of the solders. The higher melting points of SAC solders make SAC solders an ideal solder in high operating temperature application up to 1750C. However, there is a fear of bad effect on reliability of solders caused by over-heating part at the time of melting the solder because of higher re-flow temperature. As for wettability, SAC solder does not wet the substrates as well as tin-lead solder.
2.1.3.6 In52Sn48 (1180C)
The melting temperature of In52Sn48 solder alloys makes it suitable to low temperature application. With regard to properties of Indium in In52Sn48 solder, it displays good oxidation resistance, but is susceptible to corrosion in a humid environment. In52Sn48 solder is very soft metal that has a tendency to cold weld. This solder displayed a poor high temperature fatigue behavior due to its low melting temperature. The high Indium solder usage is limited due to cost and availability constraints.
2.1.4 Material selection regarding health risk
When choosing alternative metals, consideration must also be given regarding environmental issues and health risk. Previous studies in USA and Europe came to following conclusions concerning toxicology of some alternative metal.
Cadmium is extremely toxic and should not be used (high risk)
Antimony is very toxic and should not be considered as a major alloying element (medium risk in Europe-this material considered as a potentially carcinogenic)
Ag and Cu are used in the lead free alloys in small concentration- in Europe these materials are seen as low risk.
Sn and Zn are essential elements in a human diet, yet may be toxic if exposures are sufficiently high (low risk).
Bi is a relatively benign metal with a history of medicinal uses (low risk)
Basically, the main alloying elements like Zn, Bi, Ag and Cu are considered to be a green material except for In that has lower toxicity compare to lead. Table toxicity effect on human of the alloying elements that choose in making the solder alloys.
Table 2.3: A toxicity effect of alloying element used in making the solder
Element
Toxicity
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