Do you first think of stamping for manufacturing flat metal parts? It may not be the best choice. In fact, a process called photochemical machining (PCM) provides a better alternative for parts that are about 0.0002 to 0.090-in. thick and intricate, brittle, or have low production volumes. PCM, also known as chemical milling, is an etching method in which a mask defines locations where the metal will be removed.

PCM typically produces parts from sheets of a wide range of metals including copper, titanium, Nitinol, stainless steel, and other Ferrous alloys. Part sizes range from a single component occupying an entire sheet to several thousand pieces per sheet. Common applications include springs, antennas, connectors, EMI/RFI shields, and fine-resolution screens.

Probably the biggest advantage to PCM is the use of photo tooling. It's quickly made so that design modifications just involve changing artwork. Stamping, on the other hand, requires fabricating precision dies from tough materials such as tungsten carbide. Also, PCM easily makes intricate patterns that cannot be duplicated by other processes. PCM does not alter the temper, stress, or other physical properties of the metal. The process, however, does require a relatively high level of operator skill.

Because PCM uses corrosive chemicals, etching equipment is isolated from other plant machinery. The equipment is made from polyvinyl chlorides and various materials that withstand strongly corrosive and other process chemicals. Etchant is located in a sump at the machine base. A heating element and cooling coil help maintain the etchant at a constant temperature. The machine pumps the chemical through a manifold system to oscillating spray nozzles, which direct the chemical to each side of the sheet. Machines with conveyors allow continuous processing and are most widely used. Machining in batches or one sheet at a time can be effective for short runs and high-precision parts.

Making the photo tool

The first step to PCM prepares the material. Sheets are cleaned and degreased. They may be scrubbed by machines with horizontal, cylindrical brushes to remove contaminants. The sheets are then tested to assure all contaminants have been removed.

After receiving sketches, drawings, or electronic files, engineers use CAD to make a pattern of repeated part designs. Patterns are typically drawn with compensations to allow for the isotropic nature of the alloy to be etched and to eventually separate parts from the sheet. Some designs include small tabs that hold parts onto the sheet for ease in handling in later operations.

The pattern is then laser-plotted onto photographic film, making the photo tool, which can contain from one to several thousand images. Exposing the film to light transfers the images to a photosensitive polymer or photoresist coating the metal sheet.

Each side of the work piece can be exposed individually, or the two sides can be exposed at the same time between a pair of mirror-image masters in precise register. A vacuum printing frame is used. A vacuum of about 500 mm of mercury ensures good contact. The wavelength of the light source should correspond to the maximum wavelength of the photoresist absorption spectrum. Common light sources include high-pressure mercury, metal halide, and mercury xenon lamps.

Conveyorized equipment then sprays the metal sheets with liquid developer, often a proprietary solution formulated for a specific photoresist. The sheets are then rinsed. In some cases, sheets are baked in convection or infrared ovens to help evaporate remaining solvents and secure a tougher molecular bond to the substrate. The final result is a panel in which areas to be etched are bare metal and the areas to be protected are covered with a tightly adhering coating.

The panel is then immersed in a chemical bath agitated mechanically or by air. An alternative sprays the panel with heated acid. A chemical reaction takes place as the acid oxidizes the exposed metal to form a soluble substance. The force of the spray washes this away so a fresh metal surface is always in contact with fresh etchant. An opening is created when the etchant penetrates each surface halfway. Etchant continues to run through the opening, smoothing the part edges and producing a vertical sidewall.

While the etchant continues to work, the lateral width of the etch line increases and undercuts the photoresist. The thicker the metal, the greater the amount of undercut. A properly designed photo-tool compensates for this so resulting parts are in tolerance. Most production operations regulate the composition and concentration of the etchant for a removal rate of about 0.01 to 0.05 mm/min. Etchant compositions can be adjusted to meet the requirements of specific applications. In addition, proprietary additives control foaming or wetting characteristics, increase or decrease etching rate, or make etching more uniform. Many formulations are similar to those used in bright dipping, chemical polishing, and electropolishing.

Cleaning finished parts

After etching, panels must be stripped of the photoresist. The art of good stripping involves removing the photoresist completely without staining or corroding metal surfaces. Some stripping formulas merely soften and lift coatings, which then have to be gently brushed off surfaces.

Handling parts during stripping is easier when they remain attached to the metal sheet. Pattern designs usually include one or more small connecting tabs located so they do not interfere with the part's function. Sheets of tabbed parts make electroplating easier. When part requirements preclude using tabs, parts are designed to drop out during etching. However, this may require the use of screens to carry the parts, and can cause difficulty in maintaining dimensions.

Although PCM does not produce burrs, part edges may be irregular and rough to varying degrees. Edge uniformity depends on factors such as local stresses, edge orientation, and grain structure of the workpiece. Slower etching rates usually produce more-uniform edges. Subsequent bright dipping or chemical or electrolytic polishing can also improve edge quality.

EFFECTS OF POOR-QUALITY METAL ON PCM

Metal defect	Effect on PCM process
Coil set (metal curved along its length) and dents	Difficult to coat with photoresist and to contact print against the phototool, resulting in loss of registration and detail
Gage variation (for example, crown, where the thickness increases from the center of a strip of metal)	Difficult to determine optimum etching time as edges to the this is dependent on metal thickness
Too large a grain size	Loss of resolution in etched features
Surface scratches	Difficult to coat with photoresist. Etchant thus flows into the scratches and produces cosmetic defects on the surfaces of the product.
Embedded particles and inclusions such as oxides,sulfides, and silicates	Can produce defects on etching (for example, pits, pimples, and loss of resolution in product) because they etch at different rates compared with the base material.

Etchability ratings

Good	Good to fair	Fair to poor	Poor
Copper (rolled)	AISI 215 stainless steel	Molybdenum	Tungsten
Copper (electrolytic)	AISI 301 stainless steel	Nichrome	Hastelloy C
Beryllium copper	AISI 302 stainless steel	Udimet alloys	Titanium
Brass	AISI 304 stainless steel	Vanadium	René 41
Oxygen-free (Colombium) high-conductivity copper	AISI 305 stainless steel	Chromium	Niobium
Phosphor bronze	AISI 316 stainless steel	Gold
90-10 copper-nickel zinc	AISI 321 stainless steel Lead	Tantalum
Zinc	AISI 347 stainless steel	Manganese
Carbon steel	PH 15-7 stainless steel	Rhenium
Kovar	PH 17-7 stainless steel	Zirconium
Nickel	AISI 410 stainless steel
Monel	AISI 420 stainless steel
Nickel silver	AISI 430 stainless steel
Magnesium	Inconel alloys
Aluminum	Hastelloy B