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Intermetallic compounds
Intermetallic compounds (IMC) are compounds formed between metals and metal elements.
In modern electronic assembly technology, electronic assembly welding is mainly composed of tin based soft solder, which generates intermetallic compounds (IMC) between the melted solder alloy and the surface of the welded metal to achieve electrical and mechanical connections between the welded metal.
This process involves surface wetting, interatomic diffusion, dissolution, metallurgical bonding, and multiple disciplines such as physics, chemistry, metallurgy, and materials science. It is a complex chemical reaction, and the formation of suitable intermetallic compounds (IMC) is a sign of good solder joint formation.
The determination of solder joint quality usually adopts the appearance inspection standards of QJ or IPC, especially the quantitative indicator "contact angle". However, relying solely on the appearance of solder joints is not perfect, and the microstructure of solder joints also needs to be observed; The microstructure of the solder joint determines its performance. Figure 5 shows that the appearance of the solder joint is good, with a contact angle of less than 90 °. According to IPC standards, it is qualified. However, when observing the metallographic morphology of the solder joint after slicing, it was found that there were obvious cracks at the interface. The solder joint did not form intermetallic compounds (IMC) at all, and did not achieve the purpose of welding. It firmly connected the two welded objects. This solder joint is an unqualified solder joint.
The internal structure of a qualified solder joint, IMC, is the key to connecting two materials. The compounds formed by different base materials and solder materials are different, and the IMC generated by Cu and Sn solder alloys is Cu6Sn5 and Cu3Sn.
The IMC of the solder joint interface of high reliability electronic products after welding should be flat, uniform, and continuous. The optimal thickness of Cu Sn intermetallic compound is 1.5-3.5 μ mm。
When the thickness of IMC is less than 0.5 μ m, it has almost no strength due to its thinness. When the thickness of the IC is greater than 4 μ m, the IMC is too thickCausing the connection to lose elasticity, due to the loose and brittle structure of IMC, it can also result in low strength and easily cause cracking of the solder joints under external forces, leading to component fracture.