Tin/Lead-based solder is the most commonly used solder in electronics assembly, but in the last year there has been a push across the industry to switch to lead-free solder bar. The rationale is that there is a growing understanding of lead use and its adverse effects on human health.

Health hazards associated with lead include neurological and reproductive disorders, and neurological and physical developmental delays. Lead poisoning is particularly harmful to the neurodevelopment of young children.

There are laws to control the use of lead. For example, the use of lead in plumb bobs, gasoline, and oil paintings has strict regulations. In the United States, the use of lead in consumer oil paintings has been banned since 1978. Other related regulations are America, Europe and Japan are in the process of giving birth. Table 1 shows the amount of lead used in various products. Batteries account for 80% of lead, and electronic solder accounts for about 0.5% of all lead. Even if the use of lead in electronic solder is banned, it cannot solve all the problems. Lead poisoning problem. However, the 0.5% lead in electronic solder is still a considerable amount.

Table 1. Consumption of Lead in Products Product Consumption Compression-Formed Products Solder (Non-Electronic Solder Electronic Solder Other Elements to Replace Lead Alloys should provide similar physical, mechanical, temperature and electrical properties to tin/lead eutectic solders. Table 2 shows metals that can replace lead and their relative costs.

Table 2. Materials that substitute for lead and their relative prices. The relative price of lead-substituting elements. The relative price of lead (reference value, antimony, bismuth, copper, indium, silver, tin, zinc, etc.) The starting point of the available resources is hopeless, and the bismuth supply now available may be completely used up if this alloy is widely used in the booming electronics industry.

Table 3. US Bureau of Mines data on world usage and production of different elements World usage of elements (tons of world production (tons of remaining production (tons Note: current world solder consumption = 60,000 tons, or 6,600,000 liters from the potential shown in Table 2) Looking at the relative prices of the alternative metals, it is clear that many lead-free solders will be significantly more expensive than the tin/lead solders they replace. For example, indium (In) is one of the main elements used to replace lead, but it is a secondary Precious metals, almost as expensive as silver. It should be noted, however, that the high cost of the proposed solder alloys is not as important in determining the final product price as initially shown. Because of the small quantities required, in assembly, and Solder cost is almost insignificant compared to other cost factors such as components, board and assembly. The properties of the selected alloy are very important.

Lead-free solder and its properties are the same as temperature, mechanical, creep, and fatigue properties. The melting temperature point is one of the most important solder properties. Table 4 provides a list of lead-free solders currently available.

Table 4. Lead-free solder and its characteristics Lead-free solder chemical composition melting point description ° C eutectic high strength, good temperature fatigue properties ° high strength, good temperature fatigue properties ° high strength, high melting point ° good shear strength and temperature fatigue properties °  high strength ° high melting point ° C co- Melting high strength, high melting point It should be noted that the chemical composition of lead-free solder is still being optimized to achieve the desired characteristics. The solder chemistries in Table 4 may differ slightly from commercially purchased solders. For example, Table 5 shows some solder brands purchased from different suppliers.

Table 5. Lead-free solder solder names from different suppliers, chemical composition, melting point description Above wave soldering temperature °C Potential In/Pb incompatibility, requiring lead-free plating of PCB pads and component pins °C Liquid temperature too high, requiring wave soldering temperature above 260°C °C Liquid temperature too high , which requires a wave soldering temperature above 260°C. Lead-free solders containing high amounts of indium (In) (such as the first alloy in Table 5) have potential incompatibility of indium and lead. If there is lead. To get a true lead-free process, it may be necessary to use a lead-free finish on the PCB if indium-containing alloys are used. The industry is focusing on developing alternative electroplating layers. Examples include Alpha Metal's AlphaLevel flash silver plating, and s tin/bismuth plating for board layers and component leads.

From Table 4, we can see that the melting point of lead-free solder is much lower than that of tin/lead eutectic alloy, or it is much higher. Table 5 shows mostly higher temperature lead-free solders. When using low temperature solder, special fluxes are required because standard fluxes may be inactive at low temperatures. Another problem related to low temperature soldering is the reduction in wetting characteristics due to lower flow at sub-eutectic temperatures.

For low temperature applications, indium-containing solder is gaining acceptance. Some companies are using a 52In/48Sn indium-containing solder because of its better rework/rework characteristics. Because the alloy's melting point is 118°C (244°F), rework is performed at low temperatures and generally does not cause temperature damage. If the printed circuit board is gold plated to prevent oxidation, then indium-containing solder can be used to prevent gold loss.

Another low melting point lead-free solder is 58Bi/42Sn. If we look at the metallographic diagram of the Sn/Bi alloy, we find that its melting point is 138°C. Bismuth is used in welding alloys to achieve low welding temperatures, but the alloys generally exhibit poor liquefaction characteristics.

Many of the other alloys listed in Table 4 have a much higher melting point than the tin/lead eutectic of 183°C. For example, zinc/tin high temperature lead-free solder has a melting point of 198°C.

The high melting point solder will not fuse with the widely used substrate materials, such as FR-4. In addition, rework has to use high temperature, which will greatly increase the possibility of damage to the board.

There are currently no mix-in lead-free solder alternatives, although some suppliers describe their solders as "almost mix-in". Even these soldering irons that require rework are at temperatures as high as 400°C (750°F), which is too high for some applications and can cause potential thermal damage.

Also, one of the key issues with using high melting point solder in wave soldering is the increased likelihood of capacitor breakage. The wave soldering temperature needs to be maintained at about 230~245°C, which is about 45~65°C above the melting point of tin/lead solder. A lead-free solder with a melting point of 220°C requires a wave soldering temperature of 265~280°C, which increases the temperature difference between preheat and wave, increasing the possibility of capacitor breakage.

In general, almost all lead-free solders have poorer wetting properties (diffusivity) than the tin/lead eutectic, causing poor solder fillets. To improve wetting properties, special flux formulations are required. Pb-free solder also has poor fatigue properties, although in one study no degradation of solder joint integrity was observed after temperature cycling with high temperature 95.6Sn/3.5Ag (the last alloy in Table 4).

The ideal solder melting point should be around 180°C, so that the reflow temperature is 210~230°C, the wave oven temperature is 235~245°C, and the hand soldering temperature is 345~400°C (650~700°F). Only skilled operators can handle higher hand soldering temperatures without temperature damage.

The Electronics Industries Council (IPC) standard, J-STD-006, provides a detailed listing of tin/lead and lead-free solders. However, no lead-free solder has been identified as a drop-in tin/lead eutectic replacement. The industry is still looking for the correct lead-free solder that can actually replace the tin/lead eutectic. This is a challenge that the industry has to contend with.