Process Basics

The photo-etching process, using a technique similar to what’s involved for manufacturing printed circuit boards, transfers part images onto photo-resist-coated metal by using ultra-violet exposure. Then, the unexposed resist is washed away leaving bare metal where etching is to occur.

In tank etching, the prepared plates are submerged in an etchant; in conveyor etching, the top and bottom of the plates are sprayed with pressurized etchant. The alloy and the thickness of the metal determine the length of exposure to the etchant.

Among the many etching solutions in use, the most common is ferric chloride, which is suitable for a range of ferrous and nonferrous alloys, is comparatively easy to work with and can be readily neutralized. Some precious metals and a number of high-performance alloys require more aggressive etchants, such as hydrofluoric acid, that must be handled with extreme care.

Commercial etching facilities are subject to stringent environmental regulation and monitoring. Most are equipped with waste-treatment capabilities that remove dissolved metals from solution, neutralize all chemistry and restore the process water to technically potable condition before discharge.

Producing a photo tool is the start of the process. Customer files, typically in a 2D CAD format such as DXF or DWG, are compensated for the thickness of the material to be etched, then stepped-and-repeated in software to maximize use of a specified sheet size. Some parts can be partially overlapped or “nested” to improve material utilization. The composite file is sent to a high-resolution digital imaging device, often a laser-photoplotter, onto two sheets of optically clear polyester film. The top and bottom halves of this photo tool are optically and mechanically registered and punched. Photo tools are inexpensive, usually under $300, and can be produced in 24 to 48 hours.

Cleanliness is critical to successful photoetching, and the material used must be cleaned thoroughly. Several cleaning processes may be used, including mild acid wash, abrasive scrub, or alcohol wipe with a final rinse in de-ionized water.

The cleaned sheets are laminated on both sides with thin layers of a UV-activated polymer known as a dry-film resist. The resist reacts to light, especially in the UV spectrum, therefore laminating, imaging, and developing are done under yellow-hued safe lights.

The coated metal sheet is placed between the top and bottom halves of the photo tool that are registered on pins in the printer. Air is then removed with a squeegee, and a vacuum is drawn to ensure intimate contact between the emulsion and the resist. The plate is exposed under a high-intensity UV lamp causing the resist in the exposed areas of the image to begin to harden.

The imaged plate is placed in a developer that washes away the unexposed resist, leaving bare metal in the outlines of the parts. The metal that will become the parts is protected by the developed resist.

The etching machines are about 40-ft long and will take sheets to about 29-in. wide. A series of oscillating spray nozzles are on both the top and bottom of driven-wheel conveyors that run the length of the machine. In the first two chambers of the etching machine, the nozzles are dispensing heated, pressurized etchant. In the last two chambers, the etchant is being neutralized by a series of successively cleaner rinses.

Regenerative chemistry management provides consistency and predictability of the etching process by constantly monitoring etchant strength and fraction of metal salts. The specific gravity of the etchant is maintained automatically within a narrow range allowing more efficient use of both chemistry and process water.

After etching, the remaining resist is removed in another automated spray line in an alkaline solution similar to harsh dishwasher detergent, or, if the parts are very delicate, they may be bath-stripped. In either case, stripping is followed by a de-ionized water rinse and warm-air dry.

Editor’s note: Information and graphics for this article supplied by Conard Corp. (www.conardcorp.com)

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