Lead-free brazing alloy properties summary

What is lead-free solder?

Lead-free solder (Pb-free solder) means that the chemical composition of the solder does not contain lead (Pb), and the Rohs Directive requires that the Pb content does not exceed 1000 PPM, or 0.1%. The main representative of Pb-free solder is tin-based solder, and the most used Pb-free solder is tin-silver-copper Pb-free solder, which is regarded by the industry as the best choice for replacing tin-lead solder.

The following is a summary of the practical and physical properties of lead-free brazing alloys:

I. Requirements for industrial practicality The application of lead-free brazing alloys in the electronics industry needs to meet the following requirements in order to ensure their widespread use and reliability in actual production:

1. Melting point: the melting point should be as close as possible to SnPb eutectic brazing materials (about 183°C), which can avoid substantial modifications to existing equipment, process parameters and substrates, and help lead-free brazing materials to be used. Modification, help the rapid promotion and application of lead-free brazing material.

2. conductivity: due to the electronic assembly components on the brazing soldering joints are small, brazing joints of the conductivity is particularly important. Good electrical conductivity can avoid the generation of resistance heat, the reliability of brazed joints adversely affect.

3. wetting erformance: good wetting performance is a prerequisite for the formation of good brazed joints. Good wettability brazing material can better cover and fill the welding surface, forming a more reliable connection.

4. mechanical properties: brazed joints in electronic assembly play a role in mechanical connection, so lead-free brazing alloy should have good mechanical properties, including strength, toughness, etc., in order to cope with thermal shock, mechanical vibration, high temperature and other harsh environments.

Second, the physical properties of the main elements of lead-free brazing alloys Lead-free brazing materials usually use Ag, Cu, Sb, Bi, In and other metal elements to replace Pb in SnPb brazing materials. these metal elements have different physical properties, and these properties will have an impact on the performance of the alloys composed of them.

The following are the physical properties of some of the main elements (including Pb as a comparison):

Silver (Ag): high electrical and thermal conductivity, often used to improve the electrical conductivity of brazing materials.

Copper (Cu): also highly conductive and thermally conductive, and a commonly used additive element in lead-free brazing materials. Antimony (Sb): used to improve the mechanical properties and wettability of brazing materials.

Bismuth (Bi): lower melting point, can reduce the melting point of the brazing material, but poor electrical conductivity.

Indium (In): also has a lower melting point but is more costly. The combination and proportion of these elements will directly affect the overall performance of the lead-free brazing alloy.

Therefore, when developing lead-free brazing materials, it is necessary to comprehensively consider the impact of various elements and optimise the performance of the alloy by adjusting the elemental composition and proportion. The following table shows the basic physical properties of the metal elements related to lead-free brazing materials (for comparison, the physical properties of Pb are also included).