Lead-based solders have been used extensively in the electronics industry for a long period until hazards associated with the heavy metal caused it to be gradually phased out from usage. This article compares lead and lead-free solders based on their physical, chemical, environmental, and functional properties.

Introduction

Soldering is the process of joining two metal structures with the help of a metal alloy. The alloy (also known as the solder) is heated to high temperatures with a handy tool called soldering iron, which causes the alloy to melt. The molten solder is applied to the metal joint, and once it solidifies, a strong bond is formed.

Solder joints are used in electronic manufacturing to connect various components such as resistors, capacitors, inductors, integrated circuits (ICs), and more on printed circuit boards (PCBs).

The soldering process differs from welding in the way that it doesn’t require the base metal to melt. The solder joint is much weaker but can be formed at a lower temperature than welding.

Types of Solders

Several classifications exist for solders, but the major ones are based on the form in which they are sold, the type of cores they use, and the elements present in them. 

Based on the physical structure in which they arrive, solders are classified as:

• Solder wire wound on a spool

• Solder pellets

• Solder rods

• Solder foils

• Solder paste

Based on the type of cores, solders are classified into:

• Solid core solder: Soldering wires made of just the soldering alloy are called solid core solder. Flux needs to be added separately for this type of solder. Solid core solders are mainly used for electronics.

• Acid core solder: Soldering wires having a hollow core filled with acid are called acid core solders. Acid is an aggressive flux that removes the metal oxide layer from the metal surface on which it is applied. After soldering, the surface needs to be cleaned to prevent the excess flux from corroding the surface. Acid core solder is used for making strong joints in steel structures but is not recommended for electronics.

• Rosin core solder: In resin core solder, instead of filling the core of the solder wire with acid, it is filled with rosin, a semi-transparent resin obtained from trees that acts as a mild flux.

The most significant classification, however, is based on whether the solder uses lead as one of its components:

• Lead-based solder: Lead-based solder is formed by mixing Tin-Lead (Sn-Pb) in approximately 60:40 ratio. Lead-based solders have a comparatively lower melting point of around 183°C/361F than lead-free solders. It is also the eutectic temperature for the Sn-Pb mixture as, at this point, the solder can melt and freeze back too. The melting point for lead-based solder depends on the ratios in which Sn and Pb are present. Lead-based solders are now depreciated owing to the toxic properties of lead and several health and environmental hazards associated with them.

• Lead-free solder: Many different lead-free solder alloys have been considered as potential alternatives to the classical lead-based alloy. The options include Tin Bismuth (Sn-Bi), Tin-Silver (Sn-Ag), Tin-Gold (Sn-Au), Tin-Indium (Sn-In), Tin-Antimony (Sn-Sb), Tin-Palladium (Sn-Pd), and Tin-Zinc (Sn-Zn). For practical use, lead-free solders may seem less attractive than lead-based solders. Still, the fact that they are safer for the health and the environment makes them the natural choice for use in the industrial space.

Lead Solder vs. Lead-Free Solder

Let’s now check out the key differences between lead and lead-free solders to understand how the choice of the solder impacts real-world applications.

Melting Point

Lead-free solders tend to have a higher melting point than lead-based alloys. It is because of this reason lead-free solders are hard to work with. Lead alloys typically have a melting point of 83°C/361°F, which is far less than most lead-free alloys' melting points.^[1]

Table 1 lists the melting temperatures of lead-based and some commonly used lead-free solders. Note that the number present in front of the element name denotes the percentage of the respective element in the solder (E.g. Pb-37Sn contains 37% lead).

Table 1: Melting points of lead and different lead-free solders ^[2]. Refer to ^[1] in the references section below to check out the effect of different concentrations of elements on the melting point of the solder alloy.

Wettability

Wettability is a critical characteristic of the solder in a liquid state that directly impacts the strength of the metallurgical bond created by the soldering process.^[2] It quantifies how well the melted solder can be applied and spread over the contact surface of the base metal.

The best way to determine the quality of wettability is to check the contact angle of the liquid solder with the surface of the metal. Solder droplets with high wettability form a smaller angle with the surface compared to the ones with lower wettability.

Fig. 1: A droplet with poor wettability forms a large angle with the surface (left), whereas a droplet with good wettability forms a small angle with the surface and spreads nicely over it.

Surface tension is another parameter that determines the extent of wettability for a solder. A solder in the liquid state must have a low surface tension so that the force of attraction between the molecules of the solder and the metal surface is higher than the intermolecular forces of the solder, making the liquid spread more and wet the surface properly.

The joints formed between the electronic components and PCB pads must be strong enough to provide both electrical conductivity and mechanical support to the circuit board. Temperature, atmosphere, and surface compositions can be optimized along with the solder composition to achieve good wetting behavior and, thereby, a strong bond.

Tin Whiskers

Tin whiskers are microscopic thread-like structures that randomly emanate from pure tin surfaces and form electrically conductive paths. It is not yet known why or how they are formed, but they momentarily cause short circuits wherever they grow.

Fig. 2: Microscopic view of whiskers on the tin surface

A large number of whiskers arise from pure tin; hence, it is mixed with some amount of other elements, such as lead to mitigate this phenomenon. The effect of tin whiskers is more severe in lead-free solders, which is why they have to be treated to improve reliability.

Costs

Lead is a heavy metal that costs roughly $2 per kg, and Tin costs around $30 per kg (at the time of writing this article). On the other hand, Silver is a precious metal. It costs over $750. Lead can be acquired at a fractional cost of other solder elements, and hence the price of the alloy comes down as the percentage of lead goes up.

Although the price of acquiring materials depends on many factors like market conditions, region, quantity required, and more, there’s no way for lead-free solders to be available at a price lesser than lead-based solders.

The cost of raw materials is central to the formulation of business policies. An increase in the cost of solder directly impacts the cost of the PCB and the product's profitability. 

Without any doubt, lead-based solders are cost-effective monetarily but pose a severe threat when it comes to health and environmental impact.

Environmental Impact and Health

Just like other metals, lead bioaccumulates in nature. It is a toxic element that can be released into the air and water throughout its life cycle. When soldering, lead-based solders release dust and fumes, which are highly toxic to humans. Lead can worsen asthma or cause irritation to the eyes. The element is also known to be carcinogenic. Fig. 3 lists out some symptoms of lead poisoning in humans.

Fig. 3: Symptoms of lead poisoning in humans

It is always advised to wear a mask, avoid eating food, avoid drinking anything, and follow all safety instructions when soldering with lead alloys. 

The European Union (EU) introduced the Waste Electrical and Electronic Equipment Directive (WEEE) and the Restriction of Hazardous Substances Directive (RoHS) in the early-mid 2000s to ban the use of lead and various other toxic elements in electronics manufacturing. Commercial products sold in the EU must comply with the RoHS directive to be sold in various countries across the world. Lead solder is banned even for hobbyist projects in many countries as well.

Longevity

Longevity is another aspect that determines the reliability of a solder. At the interface of the solder and the base metal, a substance called an intermetallic compound is formed. The compound is brittle and continues to expand with time. Its expansion increases the stress in the area and forms cracks in the solder joint. Eventually, this leads to a reduction in the mechanical strength, which could cause it to get less reliable or even fail completely.

When switching happens in electronic circuits, the amount of current flowing through the solder joint changes, which causes the solder to be exposed to higher temperatures during heavy load and low temperatures at other times. Apart from the regular switching operations, the sudden change in current during switch-on and switch-off operations causes thermal shocks to the solders. During its lifecycle, a solder joint has to endure repeated cycles of heating, cooling, and thermal shocks resulting in microcracks formation. Another common cause of cracks in solders is vibration. The solder joints weaken over time when an electronic product is subjected to mechanical vibrations. Vibrations and sudden shocks also result in the formation of cracks in solders.

As studied by various researchers, this problem is observed in lead-free solders in a much more severe form compared to conventional lead-based solders. ^[3]

Physical Features

It is sometimes possible to differentiate lead solders from others by just a visual inspection. Solders with lead content usually appear shinier than lead-free ones. The shininess of solders was typically associated with their quality. 

Fig. 4: A poor-quality solder (left) doesn’t appear to be as shiny as the one with good quality (right). Source: Androkavo-YouTube

Apart from the physical appearance, lead-based solders also perform a bit better compared to their lead-free alternatives in terms of electrical and mechanical characteristics. Engineers and hobbyists who have worked with lead-based solder sometimes don’t readily accept lead-free solder as the performance, reliability, and ease of use of the latter are inferior compared to the former.

However, it's equally true that things have changed now; the lead-free solders have improved continuously and are almost at par with their lead-based counterparts today. Here is a table comparing the various physical properties of commonly used lead and lead-free solders:

Table 2: A comparison between the physical properties of Traditional lead eutectic Sn37Pb, Lead-free SAC387 and Lead-free Sn0.7Cu solders^[4]

Which one should you use?

For manufacturers carrying out their development, manufacturing, sales, and operations in countries that have restricted the use of leaded electronics, there’s no choice but to use lead-free solders.

If options are available to choose between lead and lead-free solders, hobbyists may choose lead-based solders for small-scale applications, as lead solders are easy to work with and are cost-effective. But as far as commercial usage is concerned, lead-free solders are a sensible choice. 

Despite all the advantages of lead-based solders, the toxic nature of the heavy metal makes it difficult to justify its usage. 

As always, the decision to use a particular type of solder must be taken after considering all the factors such as health, environmental safety, costs, design requirements, and more. 

Key Takeaways

Different types of solders are available for use in the electronics industry. Lead-based solders were the most popular ones of available options until the mid-2000s. They were affordable and exhibited excellent physical properties. 

The EU released the RoHS directive in 2006, which restricted the use of toxic elements like lead in the electronics industry, after which lead-free alternatives started to gradually replace the toxic solders. 

The newer lead-free solders are a little costly, have a higher melting point, a lower wettability, suffer from tin whiskers, and are not as long-lasting as lead-based solders; however, a continued research in the domain ensures the properties of the newer soldering alternative keep improving. 

The fact that users are not going to dump toxic waste at the end of the product’s life cycle makes lead-free solders the right choice for medium to large-scale use.

References

[1] Solder Alloy Melting Temperature, Technic Inc., [Online], Available from: https://www.technic.com/sites/default/files/resources/Solder%20Alloy%20Melting%20Temperature.pdf

[2] Mhd Noor, E.E., Mhd Nasir, N.F. and Idris, S.R.A. (2016), "A review: lead free solder and its wettability properties", Soldering & Surface Mount Technology, Vol. 28 No. 3, pp. 125-132. https://doi.org/10.1108/SSMT-08-2015-0022

[3] Jiang, Nan; Zhang, Liang; Liu, Zhi-Quan; Sun, Lei; Long, Wei-Min; He, Peng; Xiong, Ming-Yue; Zhao, Meng  (2019). Reliability issues of lead-free solder joints in electronic devices. Science and Technology of Advanced Materials, 20(1), 876–90. doi: 10.1080/14686996.2019.1640072

[4] Comparison between Lead Soldering and Lead-Free Soldering Manufacturing Procedure in PCBA, Pcbcart,  [Online], Available from: https://www.pcbcart.com/article/content/lead-soldering-and-lead-free-soldering.html

[5] Maya Posch, Lead-Free Solder Alloys: Their Properties And Best Types For Daily Use, 28 Jan 2020, Hackaday,   [Online], Available from: https://hackaday.com/2020/01/28/lead-free-solder-alloys-their-properties-and-best-types-for-daily-use/