7.6.1 Solid Materials.
Solid material such as dust, dirt, filings, lint, sawdust and packing materials impede metallic contact by
providing mechanical stops between the surfaces. They can affect the reliability of the connection by fostering
corrosion. Dust, dirt, and lint will absorb moisture and will tend to retain it on the surface. They may even
promote the growth of molds, fungi, and bacteriological organisms which give off corrosive products. Filings of
foreign metals can establish tiny electrolytic cells (see Section 7.8) which will greatly accelerate the
deterioration of the surfaces.
The bond surface should be cleaned of all such solid materials. Mechanical means such as brushing or wiping are
generally sufficient. Care should be exercised to see that all materials in grooves or crevices are removed. If a
source of compressed air is available, air blasting is an effective technique for removing solid particles if they
are dry enough to be dislodged.
7.6.2 Organic Compounds.
Paints, varnishes, lacquers, and other protective compounds along with oils, greases and other lubricants are
nonconductive and, in general, should be removed. Commercial paint removers can be used effectively.
Lacquer thinner works well with oil-based paints, varnish, and lacquer. If chemical solvents cannot be used
effectively, mechanical removal with scrapers, wire brushes, power sanders, sandpaper, or blasters should be
employed. When using mechanical techniques, care should be exercised to avoid removing excess material from
the surfaces. Final cleaning should be done with a fine, such as 400-grit, sandpaper or steel wool. After all of
the organic material is removed, abrasive grit or steel wool filaments should be brushed or blown away. A final
wipe down with denatured alcohol, dry cleaning fluid, or lacquer thinner should be accomplished to remove any
remaining oil or moisture films.
Many paint solvents such as lacquer thinner and acetone are highly
flammable and toxic in nature. They should never be used around open
flames and adequate ventilation must be present. Inhalation of the
fumes must be prevented.
Oils, greases, and other petroleum compounds should be wiped with a cloth or scraped off. Residual films
should be dissolved away with an appropriate solvent. Hot soapy water can be used effectively for removing
any remaining oil or grease. If water is used, however, the surfaces must be thoroughly dried before completing
the bond. For small or intricate parts, vapor degreasing is an effective cleaning method. Parts to be cleaned
are exposed to vapors of trichlorethylene, perchlorethylene, or methylene chloride until the surfaces reach the
temperature of the vapor. In extreme cases, further cleaning by agitation in a bath of dry chromic acid, 2 lbs
per gallon of water, and sulfuric acid, 4 oz per gallon of water, (7-7) may be necessary. The average dip time
should be restricted to less than 30 seconds because prolonged submersion of parts in this bath may produce
severe etching and cause loss of dimension. This bath must be followed by a thorough rinse with cold water and
then a hot water rinse to facilitate drying.