How to understand IMC and why intermetallic compound is important for PCB assembly

How to understand IMC

In the process of assembly and welding of circuit boards, the term IMC is often mentioned in the analysis reports of engineers or laboratories. What is IMC? What role does IMC play in the PCB Assembly factory welding process? Does it affect the strength of the weld? I often hear people say that this IMC is too thin or too thick. What is the reasonable thickness of IMC?

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    What is IMC

    IMC is an intermetallic compound, which is not an alloy (but some regard IMC as a kind of alloy) and is not a pure metal.Since IMC is a molecular component of chemical reactions, the formation of IMC must give energy, which is why amount of solder paste needs to be heated during the welding process.

    In addition, only pure tin (Sn) in the composition of solder paste will have diffusion reaction with copper base (such as OSP, I-Ag, I-Sn surface treatment board) or nickel base (ENIG surface treatment board) in strong heat, and then generate solid interface IMC

    What are differences between alloy and IMC

    IMC is a compound formed by two or more metal elements in a fixed proportion, is a result of a chemical reaction, belongs to the pure substance. For example, Cu6Sn5, Ni3Sn4, AuSn4, etc.
    Alloy, on the other hand, is a mixture of two or more metals. The proportions are not fixed and can be adjusted at any time, but the different elements are evenly mixed together.

    Why there are other metal in solder paste

    This is because the melting point of pure tin is up to 232°C, which is not easy to be used for general assembly and welding of PCB board. In other words, current electronic parts cannot reach such high temperature. 

    Therefore, tin must be used as the basis, and other alloy solder must be added to reduce its melting point, so as to achieve the main purpose of mass production and energy saving. The second purpose is that certain metals can improve the toughness and strength of the solder joint.Add a small amount of silver and copper to make.

    How does Cu 6 Sn 5 form in IMC

    organic welding protection film

    Surface treatment PCB based on copper, such as OSP(organic welding protection film), I-Ag(silver dipping), I-Sn(tin dipping), HASL (tin spraying) and solder paste welding, will form benign IMC Cu 6 Sn 5 compound in the high heat back welding furnace, but with the aging of time, or PCB through the back welding furnace for too long.

    It goes from Cu6Sn5 to Cu3Sn of the inferior IMC. form in IMC Cu6Sn5 is basically generated by the liquid reaction of Cu and Sn, and will first grow between the copper base and the solder, while Cu3Sn is basically generated by the solid reaction of the generated Cu6Sn5 and Cu, so it generally grows at the interface of Cu and Cu6Sn5.

    Nickel-based surface treatment PCBS, such as ENIG, ENXG, and ENEPIG, will generate Ni3Sn4 of benign IMC after combining with solder paste in high heat return furnace.Gold (Au), silver (Ag), palladium (Pd) and tin (Sn) can also form AuSn 4, Ag3Sn and PdSn 4 and other compounds, but it is a wandering IMC, which is harmful to the strength of the solder joint but not beneficial.

    The biggest role of the gold and silver on the welding pad is to protect the bottom nickel and copper from air contact and rust, the thicker the gold and silver, The weaker the solder joint, but not so thin that it does not cover the nickel or copper, otherwise it will not be able to protect the nickel or copper.

    What is the strength of different kinds of IMC

    welding pad of copper base

    Welding is a kind of chemical reaction.Taking the welding pad of copper base as an example, good welding will immediately generate η-phase(read Eta) benign Cu 6 Sn 5, and will also thicitate with the accumulation of welding heat and aging time.

    During the aging process, the solder joint will grow malignant ε-phase (read Epsilon) on the original Cu 6 Sn 5 malignant Cu 3 Sn. In general, the welding strength of copper base is better than that of nickel base, and the reliability is relatively high, but the strength will deteriorate with time aging.

    Nickel base nickel leaching and plating nickel gold will form Ni3Sn4 copides with solder paste, but the gold plating nickel gold is usually thicker, the thicker the gold layer, the solder joint IMC generation will be relatively thin, and when gold does not completely escape, but easy to form gold brittle, because in the welding process, only after AuSn4 migration, The nickel base will form Ni 3 Sn 4 with the solder paste, but the strength of Ni3Sn4 is not as strong as Cu6Sn5.

    Should the thickness of IMC be thicker

    Should the thickness of IMC be thicker​

    As long as the interface IMC grows out and grows evenly, it is OK, because the IMC will grow longer and thicker with the accumulation of time and heat. When the IMC grows too thick, the strength will become worse and become easy to crack. IMC is a bit like the cement between the bricks in building a wall. The right amount of cement can hold the different bricks together, but the cement is too thick and easily knocked down.

    The generation speed of IMC is basically proportional to the square of time and temperature. In addition, the thickness of IMC is suggested to fall within 1 ~ 3um. According to the formation principle of IMC, it will be known that there is no IPC industrial standard for the thickness of IMC.

    FAQ

    IMC is an intermetallic compound, which is not an alloy (but some regard IMC as a kind of alloy) and is not a pure metal.

    Welding is a kind of chemical reaction.Taking the welding pad of copper base as an example, good welding will immediately generate η-phase(read Eta) benign Cu 6 Sn 5, and will also thicitate with the accumulation of welding heat and aging time.
    As long as the interface IMC grows out and grows evenly, it is normal.Because the IMC will grow longer and thicker with the accumulation of time and heat. When the IMC grows too thick, the strength will become worse and become easy to crack. IMC is a bit like the cement between the bricks in building a wall. The right amount of cement can hold the different bricks together, but the cement is too thick and easily knocked down.

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