A customer called the EMPF Help Line and reported that they had a black residue that was coming up through their plated through holes. They suspected that it was Black Pad. They were using an RMA flux and had completely assembled the boards. ACI has commonly seen this electronics manufacturing scenario, and after receiving the circuit board, performed a brief analysis. The board was visually inspected, a section was then removed from the board, and Scanning Electron Microscopy with Energy Dispersive Spectroscopy was utilized.

The results showed that under back scatter electron imaging, the solder had dewetted from the surface of the annular ring, Figure 1. Back scatter electron imaging is useful as it shows in elemental compositions. Higher atomic elements such as lead (Pb) and tin (Sn) appear brighter, and lower atomic elements such as carbon (C), nickel (Ni) appear darker.

The board finish was an electroless nickel immersion gold (ENIG) over the external copper foil. Energy dispersive spectroscopy of the annular ring detected nickel (Ni), phosphorus (P), and oxygen (O). Oxygen implicates that there is oxidation of the nickel. Minor tin was also detected in this area from the formation of the copper tin intermetallic. The level of P detected was lower than what is associated with a black pad failure. When high levels of P are concentrated near the intermetallic, the solder joint becomes embrittled. This is the danger of black pad. In the field, board components having relatively brittle solder joints may fail under stress. This can occur even though the solder joints appear to have good solderability and wetting. When black pad occurs, typically the entire manufactured lot may be recalled. However, in this case, oxidation is apparent which did allow a proper and acceptable solder connection to be formed.

ACI discussed the results with the customer who was discussing options with the printed circuit board manufacturer. In this case, the materials cost of the board made up a small percentage of the overall cost to manufacturer. There were several expensive components on the board, however, and when the final product cost hundred of dollars and many are made, losses add up quickly. In addition, the components cannot be reused, as they cannot endure additional thermal excursions for removal and replacement, and the risk of latent failures occurring was too great. The customer said that this analysis would be used to support their point of view of what was occurring on the boards, and how they could work with the board manufacturer to come to an agreement.