The Intermetallic Compound (IMC) layer grows in the interface between Cu-based, Ni-based and Sn-solder is the certain result of the chemical reaction that forming the soldering joint. But the IMC layer is also the most weakness location of the solder bonding-force.
As we meniotned in previous page that PCBA bonding-force shall include 5 kinds of defect positions at least. The IMC layers usually is the most weakness locations show on the solder bonding-force base on forming good PCBA solder. Why?
The IMC is the abbreviation of InterMetallic Compound in the electronica manufacturing industry. The IMC layer is the certain result of the chemical reaction to form the soldering joint. Working-Bear describes the IMC layer just like the girl and boy marry then have a child.
As working-Bear described before 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 with the best strength, but too thick of the cement or unevenly painted will make the wall easily collapse. The IMC layer is the same as cement. It is the key to form the good soldering but it is also the weakest layer in the whole soldering structure. Thinking where the brick wall will be broken from if use hammer to hit it. Most of the crack will happen on the cement layer. The crack will come from the IMC layer first just like cement layer once the stress acts on the soldering point.
If you still not yet really understand what is the IMC then recommending you read the article of [What is IMC (Intermetallic Compound) in the electronic manufacturing industry?] first.
So, if the cement is not well painted or unevenly, partially painted, the cement layer is too thick or too thin will affect to the soldering bonding-force? The answer is “Yes”. If the solder crack happens at the IMC layer then we must check the IMC grows continuously and evenly between the metal interface or not. Usually, engineer will do cross-section and SEM (Scanning Electron Microscope) to check the IMC growth status. And use EDX (Energy Dispersive Spectrometer) to do the elemental analysis or chemical characteraztionzation for further judgment.
Generally speaking, you hard to find the IMC layer grow unevenly or discontinuously unless reflow or soldering process didn’t give enough thermal energy or there is oxidization on the PCB pad finished and component solder foot.
No mater the IMC layer growth too thick or too thin will affect the strength of bonding-force of solder, but there is nothing SMT process can do to improve it. What SMT process can do is to grow the IMC layer between the interface of metal and make sure it is evenly. This is because once the IMC layer grows then its thickness will become thicker with time and heat accumulation. When the IMC layer grows too thick, the soldering strength will become worse and brittle. It just like the cement applies between bricks. The right amount of cement can make different bricks joint together strongly, but if the cement is too thick then it is easily to be pushed down from the cement layer. This also explains why the reliability of electronic products will become worse after period of use.
There are many people keep chasing working-bear what is the ideal thickness of IMC layer? I will say the best thickness of IMC layer shall be 1~3um but the 1~5um is the regular acceptable thickness for the intermetallic compound of Copper-Tin (CuSn) and Copper-Nickel (CuNi).
There is one factor from the SMT process need to be taken care of that is the voids in the solder joints deleteriously impact solder joint reliability. The voids are the absence of solder within the solder joints and it will weaken the solder joint strength. Most of the void typically contain nothing but air. Some voids may contain flux residues or cured resins when epoxy flues or resin containing pastes are used. These voids were trapped gases (air, humidity or chemical ingredients of flexes) within the joint which have insufficient time to escape during the reflow process when the solder is molten and solid.
Two obvious features can help to identify whether the voids came from the air-traps:
1. The inner surface of the voids are smooth.
2. The voids show round shape in the solder joint.
The larger the void, the more unfavorable the soldering strength. How can the hollowed-out wood be able to withstand folding & bending? Unfortunately, the voids in the solder joint are difficult to avoid completely during the soldering process, especially the BGA, QFN, LGA kind of package need a large volume of solder. Since the voids in solder joints can’t be eliminated then think differently to define how many voids or its size can be acceptable at a certain ratio. Once there is better technology then tighten this specificatoin will be ok. It is a trade-off.
According to the requirements of IPC-7095B and IPC-A-610D, the diameter of the total voids of the BGA solder ball can not exceed 25% of the total diameter of the solder balls. Most electronics factories also follow this used to determine the yield of the voids. The specification may be updated in the future and please refer to the latest IPC specifications.
Therefore, only when the IMC cannot grow or the IMC layer didn’t distribute evenly will be correlative with the quality of the SMT process. These may be related to insufficient thermal energy in the reflow oven and may be related to the poor surface treatment of the PCB or the storage environment, or it may be related to the quality of the electronic components. This requires cross-section and elemental analysis solution to provide evidence and judgment.
Now you will be be an expert for the soldering strength now. You shall do your peroanl anaysis and judgment first next time while you meet the sodlering crack.