1. Development trends of lead-free solder

Lead and its compounds will bring great harm to the living environment and safety of human beings; the large use of sn/Pb alloy solder in the electronics industry is one of the major sources of pollution. Japan first developed lead-free solder and applied it to actual production, and proposed to ban its use in 2003. The United States and Europe proposed a ban in 2006. In addition, special emphasis is placed on the recycling of electronic products. The export products of some sole proprietorships and joint ventures in my country have also been applied. Lead-free solder has entered the stage of practicality.

2. Requirements for lead-free solder

1. The melting point is low, and the eutectic temperature of the alloy is similar to the eutectic temperature of Sn63/Pb37 at 183 °C, which is roughly between 180 °C and 220 °C.

2. Non-toxic or very low toxicity, the selected materials will not pollute the environment now and in the future. Laser Stencil

3. Thermal conductivity and electrical conductivity should be comparable to Sn63/Pb37 eutectic solder, with good wettability.

4. The mechanical properties are good, and the solder joints should have sufficient mechanical strength and thermal aging resistance.

5. To be compatible with existing welding equipment and processes, welding can be carried out without changing the equipment or changing the current process.

6. It is easy to repair each solder joint after welding.

7. The cost should be low, and the selected material can ensure sufficient supply.

3. Alloy materials that may replace Sn/Pb solder at present

The non-toxic alloys that are likely to replace Sn/Pb solders are Sn-based alloys, mainly Sn, with the addition of Ag, Zn, Cu, Sb, bi, In and other metal properties to improve solderability.

At present, Changhe’s lead-free solders are mainly based on Sn-Ag, Sn-Zn, Sb, Bj as the matrix, adding appropriate amount of other metal elements to form ternary alloys and multi-component alloys.

1. Sn-Ag solder

Sn-Ag based solder has excellent mechanical properties, tensile strength, creep properties and thermal aging slightly worse than Sn-Pb eutectic solder, but there is no problem of ductility deterioration with time; Sn-Ag based solder The main disadvantage is that the melting point is high, 30-40 °C higher than Sn-Pb eutectic solder, poor wettability and high cost.

2. Sn-Zn solder

Sn-zn solder has good mechanical properties, better tensile strength than sp-pb eutectic solder, and can be drawn into wire; it has good creep properties, slow deformation speed, and long time to fracture; the disadvantage is that ZN is very easy to Oxidizing, poor wettability and stability, and corrosive.

3. Sn-Bi solder

Sn-Bi solder is an alloy solder composed of SN-AG (CU) alloy as the base and an appropriate amount of BI added; the advantage is that the melting point is reduced, making it similar to the SN-PB eutectic solder; the creep properties are good, and the The tensile strength of the alloy is increased; the disadvantage is that the ductility is deteriorated, it becomes hard and brittle, and the workability is poor, and it cannot be processed into wire.

4. Currently widely used lead-free solder

SN3.2Ag-0.5Cu is currently the most widely used lead-free solder. Its melting point is 217-218°C.

5. Lead-free soldering brings problems

1. Components: The component body is required to withstand high temperature and be lead-free. That is, lead-free plating should also be used for the soldering terminals and lead-out lines of the components.

2. PCB: The PCB substrate is required to withstand higher temperatures, not deform after welding, and lead-free plating on the surface of the pad is compatible with lead-free solder for assembly and welding, and requires low cost.

3. Flux: To develop a new type of flux with better wettability, it must match the preheating temperature and soldering temperature, and meet the environmental protection requirements.

4. Welding equipment: To adapt to higher welding temperature requirements, the preheating zone of the reflow soldering furnace should be lengthened or replaced with new heating elements; the materials of the solder tank, solder wave nozzle and guide rail transmission claw of the wave soldering machine should be resistant to high temperature corrosion. When necessary (such as high density and narrow spacing), new solder oxidation inhibition technology and inert gas N2 protection soldering technology are used.

5. Process: The printing, patching, welding, cleaning and testing of lead-free solder are all new topics, which must be adapted to the requirements of lead-free solder.

6. Scrap recycling: Recycling BI, CU, and Ag from lead-free solder is also a new topic.