For many years the analysis of contaminant residues on PWB surfaces has been of major importance to the industry. While the identification of residues left on metallic surfaces has proven to be relatively straightforward, the analysis of organic contamination of similar composition to that of the underlying board surface has not been as successful. Through the use of modern XPS instrumentation, the non‐ionic component of water soluble flux has been identified and differentiated from the chemically similar FR‐4 and soldermask substrates. This paper presents the XPS results for a series of experiments aimed at determining the location and relative concentration of water soluble flux residues on standard surface insulation resistance (SIR) comb patterns. The data show that the water soluble flux residue is not present as a uniform coating on the board surface but appears in localised sites in high concentrations while being absent in other locations. Through more aggressive cleaning procedures the sites of high residue concentration can be significantly reduced.
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