Cracking of IMC in ENIG Electroless Nickel-gold Plating

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ENIG (Electroless Nickel Immersion Gold) is a widely used process for PCB surface treatment, also known as immersion gold or electroless nickel gold. The gold layer of ENIG is thinner and less dense than that of electroless gold plated boards. Generally, the thickness of the nickel layer is required to be 3-5 ¦Ìm, and the thickness of the gold layer is 0.05-0.15 ¦Ìm (usually controlled in the range of 0.07-0.1 ¦Ìm). When it is necessary to carry out the soldering process on the surface of the ENIG, it is necessary to increase the thickness of the nickel layer to 4-7 ¦Ìm. ENIG treatment of the pad surface flat, in the close-pitch device printing and mounting process has a good use of results. The gold layer has good conductivity and wear resistance and can be used directly as a contact surface. In addition, the gold layer also has the ability to resist oxidation and corrosion, and can effectively protect the underlying nickel layer from oxidation, so as to maintain good solderability.

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                                                                                           Figure 1. ENIG nickel-gold pads

However, when soldering with ENIG electroless nickel-gold plates, the problem of abnormal growth and cracking of intermetallic compounds (IMC) on the pads may occur. This paper will discuss the causes of this problem and possible solutions.
IMC growth and thermal embrittlement
Tin-copper intermetallic compounds (IMCs) are thermally brittle at high temperatures for long periods of time. This is due to liquid solder and copper formed by the IMC is pebbled (sliced scalloped), there are gaps between the grain boundaries, so that the liquid solder and pad copper foil continues to generate Cu6Sn5, and the smaller grains and into the larger grains. When the grain growth to a certain size, the bottom of the substrate has been unable to withstand, the grains will break their own fall into the solder joints, this phenomenon is known as drop. Therefore, it is very important to control the thickness of the IMC layer, too thick IMC layer will lead to thermal embrittlement and cracking problems.

IMC Layering and Falling
The problem of tin-nickel IMC delamination and dropout arises from the multiple growth of IMC. The Sn-Ni IMC layer formed during the first reflow process will be pushed up by the newly generated IMC layer at the bottom during the second reflow process. If the first IMC layer is too thick, and the second IMC layer is also very thick, then the later generated IMC will crowd out the original IMC layer, resulting in the separation of the intrinsic IMC layer and shedding into the solder joint. Therefore, it is necessary to control the thickness of both Sn-Cu IMC and Sn-Ni IMC during the soldering process. the IMC layer should be as thin as possible, but must be present, otherwise it will not form a reliable solder joint.
Factors affecting IMC growth
£¨a£©Excessive soldering time and temperature: Excessive soldering time and high soldering temperature will accelerate the growth rate of IMC, resulting in IMC layer thickness exceeding the normal range.
£¨b£©Excessive roughness of the electroless nickel layer: A rough electroless nickel layer accelerates the dissolution of IMC and thus promotes the growth of IMC.
£¨c£©Presence of nickel uplift: If the nickel layer is uplifted, the free nickel will dissolve directly into the solder, further increasing the thickness of the IMC.
By strictly controlling the soldering time and temperature, controlling the roughness of the electroless nickel layer, and avoiding the phenomenon of nickel layer uplift, the thickness of IMC can be reduced, and the reliability and mechanical strength of solder joints can be improved. Further research and optimisation of ENIG process parameters are important for solving the problem of abnormal IMC growth and cracking to meet the demands of high reliability electronic manufacturing.