Figure 1 shows SEM photos of a drop of contaminant material on the die pad of a raw lead frame at low magnification (left) and high magnification (right).

EDX analysis needs to be performed on samples such as this to identify the elements present in the contamination.  The presence of foreign elements such as Cl, S, and P can lead to corrosion, and must therefore be considered as a possible ground for rejecting the affected batch of samples.  In fact, in the right SEM photo above, evidence of corrosion in the contaminated area may already be observed.