How to Classify Lead-Free Solder Pastes and Achieve Lead-Free Soldering?

2022-03-25

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1. Why Develop Lead-Free Solder Pastes？

In the past, electronic product packaging adopted leaded solders. Widely-used Tin-lead eutectic solders have a series of advantages such as low melting point, good wettability, and bright solder joints. However, lead is a toxic substance with multi-affinity. Lead can damage the human nervous, hematopoietic, and digestive systems. Additionally, lead poisoning may induce serious diseases such as leukemia, kidney disease, heart disease, and mental disorders, causing great harm to people's health.

Metal Lead severely pollutes water, soil, and air. Once the environment is polluted, it will be difficult to recover. Lead pollution management is complex, and the recovery cycle is long, which requires huge expenditure.

2. How to Classify Lead-Free Solder Pastes?

2.1 According to melting point and soldering temperature

Lead-free solder pastes include high-temperature lead-free solder paste, medium-temperature lead-free solder paste, and low-temperature lead-free solder paste. Fitech defines a lead-free solder paste with a melting point higher than 240°C as high-temperature lead-free solder paste because it can form a temperature gradient with the SAC series solder paste and complete multiple-reflow soldering. Fitech’s high-temperature lead-free solder pastes include the FH-280 series, FH-260 series, and FT-901 series. Lead-free solder paste with a melting point lower than 180°C is called low-temperature lead-free solder paste. Similarly, lead-free solder paste with this melting point can create a temperature gradient with SAC solder paste and complete multiple-reflow soldering. Fitech's high-temperature lead-free solder pastes include FL-200/180/170 series, FT-574 series, FACA-138 anisotropic conductive paste, etc. Finally, the solder with a melting point between high-temperature and low-temperature solder pastes is called medium-high-temperature lead-free solder paste, such as the SAC305 series.

2.2 By alloy types

It can be classified into tin-silver-copper solder paste (SAC), gold-tin solder paste (Au80Sn20), tin-bismuth-silver alloy lead-free solder paste, tin-bismuth-copper solder paste, tin-antimony solder paste, tin-indium solder paste, and so on.

2.3 By process types

It can be divided into printing lead-free solder paste, dispensing (syringe) lead-free solder paste, jetting (syringe) lead-free solder paste, pin-transferred lead-free solder paste, laser lead-free solder paste, etc. Different processes have different requirements for the solder paste characteristics.

2.4 According to Washability

The solders pastes include no-clean types, solvent-soluble types, water-soluble types, and water-based cleaning types. Different processes have different requirements for packaging cost, reliability, etc. The available types of lead-free solder paste are also different.

2.5 Special lead-free solder pastes

It refers to a lead-free solder paste product developed for a specific application scenario to satisfy the unique process. It is characterized by strong pertinence to meet the technological needs in specific fields. For example, Fitech launched the Fitech mLED™ series lead-free solder paste for new mini-LED and micro-LED packaging.

3. Why Is It Complex to Completely Replace Leaded Solders by Lead-Free Solder Pastes？

Leaded solder pastes were popular because of their excellent low melting point, wettability, reliability, and temperature gradient formation in the application. However, lead-free solder pastes are prone to various problems.

For example, the most widely-used lead-free solder pastes are SAC series solder pastes. It has a melting point of 217-219°C, which is 36°C higher than that of the tin-lead eutectic solder. Higher reflow curing temperatures introduce many issues and variables to the electronics manufacturing process and supply chain. Meanwhile, the wetting property of SAC solder paste is not as good as that of tin-lead eutectic solder.

In order to achieve a low-soldering temperature similar to that of tin-lead eutectic soldering, tin-bismuth and tin-bismuth-silver alloys were considered as the alternatives. However, tin-bismuth and tin-bismuth-silver alloys have problems such as solder joint brittleness and poor mechanical properties.

Therefore, the substitution of lead-free solder for leaded solder is a complex system engineering. It requires a full understanding of the impacts of lead-free solder substitution on the manufacturing process. Some of these issues require further investigation and understanding, such as design, PCB\components and consumables, process, equipment, quality, and cost.

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