What is the IMC? We always heard the manufacturing engineer talking about this keyword when discussing to the soldering quality. What is the role of IMC layer in the soldering? Does IMC layer will affect the soldering strength and solderability? What is the best IMC layer thickness?
The IMC is the abbreviation of InterMetallic Compound in the electronic manufacturing industry. It grows between the solder alloy and Cu substrate for the Printed Circuit Board Assembly (PCBA). The IMC is neither a pure metal element nor alloy (Some expert think IMC is a kind of alloy). The IMC is a chemical compound that chemically combined by two or more elements. So, you may see the chemical formula of IMC like Cu6Sn5、Ni3Sn4、AuSn4…etc.
Why I say IMC is not an alloy. This is because the behavior is different between IMC and alloy. The alloy is a mixture of two or more metal elements evenly together just like mix the boys and girls in the same classroom. it maybe substitutional or Interstitial alloys. We can say the IMC is the child after girl and boy together have a chemical reaction.
Since the IMC is a chemical reaction result, so give thermal enige to form the IMC is necessary. This is why the solder paste need to go reflow oven with high temperature to melt the form the IMC with bonding. Only the pure tin (Sn) in the solder paste composition and copper base (for example OSP, I-Ga, I-Sn finished) or nickel bass (ENIG finished) in the PCB can cause the diffusion in high temperture and form the firmed interfacical IMC layer.
Where the IMC layer grow?
In the beginning, the interface will grow the good IMC layer of Cu6Sn5. As time and temperature effect that Cu6Sn5 will slowly evolve into poor brittle IMC layer of Cu3Sn. The IMC layer of Ni3Sn4 will grow on the Nickel base PCB. There is no strength degrade risk of IMC layer for the Nickel base PCB except there is black pad quality issue.
The thicker of IMC layer grows the better soldering?
Well, there is a misunderstanding that people think the IMC layer grows the thicker the more strong soldering strength. Actually, the IMC layer can be grown and distributed evenly at the interface location will be good enough. Once the IMC layer grows too thick and over 5u” then it will become brittle and soldering strength will be recuded.
What is the role of IMC layer in the soldering?
How to describe the role of IMC layer in the soldering? The IMC layer growth is the index of good soldering. If there is no IMC layer grows between the solder alloy and Cu substrate then the electronic component and PCB will not join together, but the IMC layer is also the weakest point in whole soldering strength structure.
I can say that the IMC layer just like the cement applies between different bricks and have them construct as a wall. The right amount of cement and paint it evenly can make the bricks join together with the best strength, but too thick of the cement or unevenly painted will make the wall easily collapse.