**7. Black pad effect**

The black pad effect is a characteristic for Ni/Au surface finishes, where Ni contains higher amount of phosphor. During the Ni/Au surface layer creation, the nickel layer is covered with a thin layer of gold; this plating process may lead to corrosion of the nickel surface. The final Au coating can provide good wetting for the solder alloy, even though it has an oxidised Ni-P layer under it. Another cause of the black pad effect is the solder pads reacting with a lead-free alloy with a higher tin content at a higher temperature (longer reaction time). This reaction produces a thicker layer (rich in phosphorus) on the interface, which has a defective structure (microfractures, microvoids). Due to the black pad effect, the soldered joint is considerably weakened mechanically and ultimately will break the conductive connection between the component's terminal and the soldering pad. This fault is very difficult to detect; thus it may occur on devices that have passed output control tests and have already been sent to market (**Figure 11**).

**Figure 11.** *Photo of black pad effect after stripping off the immersion gold layer.*
