It has been found that when the PCB uses ENIG plating, sometimes there is a phenomenon of non-wetting or de-wetting, and the non-wetting place appears black or dark gray which is a black pad phenomenon.
Black pad phenomenon sometimes manifests as poor wetting, sometimes good appearance but weak solder joint strength. The latter poses a serious hidden danger to the reliability of PCB solder joints, because currently there is no way to identify them by inspection, but in use, the solder joints will be disconnected when they are exposed to a larger stress, causing circuit board product failure. Therefore, in some products with high reliability requirements, such as aviation and life support systems, gold-plated surface treatment cannot be used, and gold removal is required.
The black pad is a bad surface defect caused by the electroplating process. Due to the fierce reaction of the Au plating solution and the Ni layer, the Ni layer is deeply corroded and grain boundary corrosion occurs (commonly known as mud cracks). Black pad phenomenon has the following typical characteristics:
(1) After the Au layer is peeled off, the surface of the NI layer shows a "mud crack" phenomenon, as shown in the figure.
(2) If sliced, you can see that the Ni layer is deeply corroded and corrodes the groove like acupuncture.
(3) An abnormally high P-rich layer.
The influence of black pad on the reliability of solder joints depends on the severity of the black pad, such as the distribution area of mud cracks and the depth of needling. Generally, it is difficult to weld on the black disk, and it can be welded well on the non-black pad. Therefore, as long as not more than 50% of the area is a black pad, it is possible to obtain a solder joint with good appearance, but the strength of the solder joint is very weak, which is the danger of a black pad.