Plating fog is often observed when metallizing a latent pattern by electroless copper plating. Metallic elements in the nuclei of plating fog particles are analysed using electron probe microanalysis (EPMA). Silver and iron are invariably found in the particles. The iron is driven into the board during the liquid honing process and reduces silver ions in the photosensitive material to silver metal which causes plating fog. Plating fog also arises from reduced copper particles or generated hydrogen gas bubbles when they are trapped on the unpatterned region. Mechanical stirring accompanied by substrate oscillation is an effective method of avoiding plating fog.