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JULY 2021 I SMT007 MAGAZINE 87 surface oils. Alkaline cleaning is followed by a water rinse, an acid rinse, another water rinse, and a drying step. A "water break test" can be employed to judge the completeness of the cleaning step: a sheet of water across the sur- face will break up in water beads within less than thirty seconds if hydrophobic organics are present on the surface. Electrochemical cleaning is commonly used to remove residual traces of contaminants. Depending on the type of metal, the surface is either cleaned anodically or cathodically. Some metal surfaces, such as steel, can absorb hydrogen, especially during cathodic cleaning, which can lead to metallurgical defects. To drive off hydrogen, it is common practice to prebake the material for about 10 minutes at 120°C before resist lamination. is baking step has little to do with resist adhesion, except that it could generate sur- face oxides which might interfere with dry film adhesion. Occasionally, "conversion coatings" are applied to the metal surface prior to resist lam- ination to enhance resist adhesion and etch quality. e following list gives examples. Compatibility with the resist should be eval- uated in the selection of a specific conversion coating: • Aluminum: Proprietary solutions from a variety of chemical suppliers • Magnesium: 2-3% phosphoric acid at room temperature for about 30 seconds, followed by a rinse • Nickel: 2-3% phosphoric acid at 70-80°C for 2-4 minutes, followed by a rinse • Stainless Steel: 20% nitric acid at 65-70°C, followed by rinsing. Alternatively, heat to 260°C for 5 minutes and cool to room temperature • Steel and Ni/Fe Alloys: 25% phosphoric acid at 70-80°C long enough to lightly phosphatize the alloy providing a slightly matte or dull finish • Zinc: 2% phosphoric acid for 2-3 minutes at room temperature Surface cleaning methods vary from metal to metal. Aluminum is typically degreased with an alkaline cleaner, followed by a sodium hydroxide microetch and an "etching smut" removal with nitric acid. is method is not applicable to anodized aluminum. Chromium finishes are cleaned with warm alkaline clean- ers. Strong acid cleaning or reverse-current cleaning will attack this surface finish. Cop- per alloys and brass cleaning is similar to that practiced in PWB fabrication, but microetch- ing or mechanical roughening may not be acceptable on all work pieces. Gold is nor- mally alkali cleaned, followed by an acid dip to neutralize the surface. Stainless steel is cleaned with a hot alkaline cleaner, followed by acid (see above). Kovar/Alloy 42 can be cleaned in a similar fashion. Magnesium is degreased with a mild alkaline cleaner, rinsed, and dried. Pumice can be used if a matte finish is acceptable. Nickel is cleaned similarly, oen followed by an acid "surface activation" step. Silver is pumiced for good resist adhesion if a matte surface is acceptable; otherwise use an alkaline soak cleaner followed by acid neu- tralization. Lamination of Dry Film Resists Manual hot roll laminators are much more common than automatic sheet laminators. Specific lamination conditions are provided by the resist product data sheet. Wet lamination has been tried successfully to improve resist adhesion and conformation to stainless steel surfaces. Surface cleaning methods vary from metal to metal.