Assemblies manufactured with Electroless Nickel Immersion Gold (ENIG) plated PCBs and components can succumb to a failure mode known as “Black Pad”. Black Pad is a corrosion of the Ni layer during the immersion gold step. The end result can vary with complete de-wetting of the pad/component surface, to suitable wetting with weakened solder joints (that do not often reveal themselves until later in the field). As a result, routine IPC 610 inspection efforts are not sufficient for Black Pad screening.

There is some difference of opinion as to the Black Pad mechanism.1,2,3,4,5 The end result is clear, with either weakened solder joints or complete solder separation at the interface between either the solder and pad or solder and lead. The characteristic of a Black Pad failure, as the name states, is a blackened appearance of the pad surface. As a result of the variation in the failure mode, the following is a list of characteristics that should collectively be observed to be considered a true Black Pad failure.

    1. The fracture will occur at the interface and not within the bulk solder as indicated by the presence of Ni-Sn intermetallic compound (IMC) at the fracture interface (Figure 4-1).

    2. High levels of phosphorus at the fracture interface as indicated in Figure 4-2 (in which the P content was 28.25%). It is known that at the IMC region, P levels do become naturally enriched. In Black Pad, however, the Ni corrodes as a result of increased activity of the immersion gold step. This hyperactive immersion gold step is induced by pH, solder mask contamination, and the electric fields on the board. The last affect can cause particular pads or regions on a pad to be selectively effected.

    3.  Nickel on the surface of the PCB pad is another indicator of Black Pad.  Nickel corrosion is detected through Auger Spectroscopy.

    4.  Mud cracks at the fracture interface and a lack of Ni-Sn IMC compound as a result of the Ni being tied up
as Ni-P compound. Figure 4-3 displays the “mud crack” appearance with very little Sn present at the fracture
interface.

The difficulty in establishing a black pad mechanism stems from how the degree of Ni corrosion will influence what is observed. Figure 4-4 displays ENIG pads in which Ni corrosion was induced. (Note, the Au was removed by a cyanide etch). As result, a Black Pad failure may not be established, only that the Ni became corroded.