Journal of SMT Article


Authors: Jodi Roepsch, Robert Champaign, Marlin Downey
Company: Raytheon Electronic Systems
Date Published: 7/1/2002   Volume: 15-3

Abstract: The corrosion of immersion gold plated electroless nickel (Ni/P), described as "black pad defect" has been documented in terms of the affect on the surface solderability of printed circuit boards and ball grid arrays (BGAs) [1,2,3]. This article describes anomalies associated with the presence of severe "black pad" on BGA components. Black pad defect can be identified in cross section by an abnormally thick phosphorus-rich layer. This phosphorus-rich layer can deter the formation of nickel/tin (Ni/Sn) intermetallics that form the solder bond. Also, in severe cases, identifiable spikes or corrosion cracks can extend deep into the nickel layer.

Unique to this analysis is the severity of the black pad, as revealed by cracks that extend through the nickel. In severe black pad, corrosion spikes that extend completely through the nickel layer provide a path for the solder to make contact to the copper layer below the electroless nickel. This allows the formation of Cu/Sn intermetallics at the solder-ball-to-BGA-pad interface. In the BGA assemblies studied, the severity of the black pad is extensive, interfering with the formation of Ni/Sn intermetallics observed in a normal solder bond between Sn63Pb37 solder and a nickel pad. The Cu/Sn intermetallics anchored the BGA to the pad. After mechanical shock, separation occurred between the solder ball and BGA component interface. In a finished product, black pad is usually only detected when mechanical separation results in a functional failure and is followed by destructive analysis. Prior to mechanical failure, it can not be identified during visual inspection.

Key words: BGA, black pad, immersion gold, electroless nickel, mechanical shock, field emission scanning electron microscopy / energy dispersive spectroscopy (FE-SEM/EDS).

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