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Photoetching’s device applications include haptics for intraocular lenses used in cataract surgery and other corrective implants.

The photochemical etching process (photoetching) has characteristics that appeal to many industries. It allows for rapid design changes, low tooling costs, and quick turnaround in the manufacture of small, light-gauge metal parts. It can also deliver exact repeatability, making parts with intricate patterns, precise tolerances, and burr-free edges. Photo- etching can even produce patterns on tough corrosion-resistant alloys and on components that are impossible to duplicate by other production methods. Common metals that can be photoetched include silver, copper alloys, beryllium copper, stainless steels, aluminum, nickel alloys, spring steels, and other steel alloys.

Some firms that specialize in photoetching have focused on process development so that the process can be used on specialty materials with applications in the medical device industry. Such materials include Elgiloy, titanium, tungsten, molybdenum, nitinol, niobium, and polyimide. The photoetching process has applications for a variety of devices and device components, including:

• Maxillofacial screens and cranial closure implants.
• High-transmission cellular grids.
• X-ray collimators.
• X-ray ID tags.
• Anode and cathode grids for batteries.
• Stents.
• Vascular stiffeners and closures.
• Haptics for intraocular lenses.
• Screens for filtration.
• Needles.
• Surgical blade blanks.
• Contact and retention springs.
• Electrical interconnects.

Parts can be manufactured from metals as thin as 0.0005 in. and as thick as 0.1 in. (or thicker when etching is combined with laser machining). This article provides an overview of the photoetching process as well as some of the desirable properties and unique processing characteristics of many of today’s most appealing materials. It also covers some of the most common value-added operations performed on photoetched parts.

Photoetching: The Basics

Electrical interconnects are one of the various device components that can be photoetched.

Photoetching involves coating an acid-resistant photosensitive film on metal, and exposing and developing the image of parts on the film. The exposed metal is chemically etched away with an acid specifically formulated for the metal. Next, the residual photoresist film is removed, and the parts undergo cleaning. At this point, the parts are ready for inspection or subsequent value-added operations, such as plating or forming.

Light-gauge metals are typically photoetched either as sheets of material or in a reel-to-reel format. In either process, the parts are repeated across the entire surface, except for a narrow border on the periphery, which is required for stability. This allows for

an economy of scale—i.e., many parts are affected each time a sheet is processed. This also allows suppliers to produce intricate and difficult parts.

The tooling used for the photoetching process is typically an image of the part plotted in emulsion on a Mylar film. This tooling is suitable for most applications. For designs that require extremely tight feature controls, the emulsion is plotted onto a glass plate instead of Mylar. The glass offers a stable platform for the tooling and removes any variation that could be attributed to the Mylar film.

This etched and formed grounding spring is used in medical imaging machines.

In either case, the tooling is plotted from computer-assisted drawing files, which eliminates the need for expensive hard tooling that is usually associated with other metal-manufacturing methods. Mylar tools are typically available within a day, whereas glass tooling can be ready for manufacturing within a week.

Another advantage of Mylar tooling is that it greatly shortens lead times, and design revisions can be quickly and economically retooled during the prototyping phase. Design changes can be tooled and manufactured in a few days instead of weeks or months. By adding nomenclature to the photo tool, items such as part numbers or logos can be added to the part surface with no effect on tooling or part cost.

Photoetching can be used to create a variety of screens for filtration.

Practical limits for the sizes of slots, spaces, or holes are determined by the thickness of the material being etched. The accuracy of features and minimum feature-size capability increases as material thickness decreases. Generally, allowing for a minimum feature size that is 25% greater than the material thickness results in a design that can be produced at high yields. The tooling, in conjunction with the photoetching process itself, gives device designers the freedom to specify extremely intricate geometries on a wide range of thin materials without worrying about the burring and stress problems that may arise with stamping or machining processes.

The process is best suited for small metal parts found in medical devices, and it can help improve overall design-to-production time. Photoetching can handle large production runs as well as prototypes with equally high levels of quality, consistency, and performance.

Specialty Materials

Several unique materials have characteristics that make them desirable for medical devices. These materials include tungsten, molybdenum, niobium, titanium, Elgiloy, nitinol, and polyimide. However, photoetching some of these materials can be challenging, which has led photoetching companies to adjust their process development. For instance, alternative photoresist films may be required, and in some cases, new etch chemistry formulations must be developed.

Tungsten and Molybdenum. For example, tungsten and molybdenum are refractory metals that are typically considered difficult to etch. Both of these materials offer excellent strength and stiffness at high temperatures as well as good thermal conductivity, low thermal expansion, and low emissivity.

Tungsten is most commonly known for its use as a filament in lamps and in applications for the aerospace industry. But its high-temperature strength and ability to withstand x-rays make it appealing for electronics applications, including collimators and shielding components in medical therapy and detection equipment. The metal also has applications for x-ray opaque markers in implantable devices.

Molybdenum has good electrical resistivity and corrosion resistance at high temperatures, making it suitable for electrical-contact and electrode applications. Photoetching has been shown to be a viable approach for producing light-gauge parts from these metals.

Niobium. Niobium is another refractory metal that is lightweight and has excellent high-temperature durability and corrosion resistance. Niobium is ductile and easily malleable, but it can also be difficult to stamp—it has a propensity for causing premature tool wear when compared with more standard materials. This material also has excellent welding characteristics that make it appropriate for applications such as electrical leads. Keeping these characteristics in mind, photoetching can be an effective alternative for producing thin niobium parts.

Titanium. Titanium is extremely strong and lightweight. It provides for some of the highest strength-to-weight ratios available on the market. In addition to its biocompatibility, titanium also has excellent corrosion resistance to a broad range of acids, alkalis, and industrial chemicals. One of its unique characteristics is its inherent ability to osseointegrate, meaning it supports a direct structural and functional connection to living bone. This makes titanium particularly suitable for dental implants, reconstructive meshes, and cranial closure implants. Photoetching can be used to help create these devices, given that these products are typically manufactured from very thin materials and often have complex geometries. Other common applications for photoetched titanium parts include anode and cathode battery grids used in implantable devices as well as in electrical interconnect components. Because of the material properties already discussed, titanium is also used in the manufacture of metal parts for applications in chemical etching equipment. Some firms have developed chemical etching processes for titanium that support an array of designs using specially built etching machines and proprietary chemistry.

Elgiloy can be etched to create vascular stiffeners and closures.

Elgiloy. Elgiloy is highly resistant to corrosion and has high fatigue strength. Its corrosion resistance makes it one of the most difficult materials to chemically etch. These characteristics make the metal a highly desirable material for closures and vascular stiffeners. However, effective etching of Elgiloy can only be achieved through proprietary processes and chemical etchants—meaning a device manufacturer would have to develop a process on its own or have it done by another company.

Nitinol. Nitinol is known as the shape-memory alloy because its unique characteristics allow it to return to a predetermined shape after undergoing deformation. The material has excellent biocompatibility, good spring characteristics, and high corrosion resistance. This alloy is commonly used for implantable stents, although many other creative applications continue to be developed.

Polyimide. Polyimide is a polymer film that exhibits good physical, chemical, and electrical insulation properties over a wide temperature range. Its electrical and chemical-resistance properties are excellent even at unusually high temperatures. The material is used to make haptics for intraocular lenses and is well suited for foldable contact and acrylic lenses. Polyimide offers flexibility comparable to polypropylene and polymethylmethacrylate haptics, with greater tensile strength and superior shape memory.

Value-Added Operations

For some applications, photoetching is just a part of the total process. Photoetched parts that are manufactured from non-corrosion-resistant materials typically require some type of additional finish plating to protect the surface. In other instances, designers seeking a precision-stamped, light-gauge, formed metal part should consider using a photoetched blank and value-added forming for their design. In addition to plating and forming, other common value-added operations performed on photoetched parts include electropolishing, welding, lamination, heat treatment, and laser marking.

Plating. Plating added to photo-etched parts improves corrosion or wear resistance, enhances electrical contact properties, or improves aesthetics. Gold and silver are often used for plating in electronic applications, while palladium and nickel are used for parts that require high wear resistance. Other common finishes are tin, copper, and electroless nickel. Elaborate designs may need two surface finishes on the same part to support different requirements of the application. For example, an electrical contact may require a tin plate on one end, where it will be attached to a printed circuit board, and gold or nickel on the other end (if good wear resistance is required).

Two surfaces can be achieved with a selective plating process in which a plating mask is applied to the manufacturing panel, isolating the area for the first plating deposit. The mask is removed after the first plating process is complete, and the process is repeated for the second plating requirement. The result is a selective finish on each part. When this process occurs while the parts are still in the manufacturing panel, it can minimize cost.

Electropolishing. Electropolishing is another useful finishing process for applications that require ultrasmooth edges or surface finishes. Metal removal is minimal, usually about 0.0002–0.0003 in. Electropolishing targets high-current-density areas such as sharp edges or peaks in surface grain structure at a higher rate, which rounds off the sharp edges and levels the surface. This process is highly beneficial for surgical implants and devices intended for operating rooms. (For more on this technique, see the last article in this machining section, “Modifying Metallic Implants with Magnetoelectropolishing.”)

Forming. Second to finishing, forming is the most common value-added process performed on a photoetched part. Some companies offer extensive in-house forming, ranging from simple bending performed with universal tooling to very precise bending that is specific to the customer’s design and is performed using customized tooling. When combined with value-added forming, photoetching enables the production of precision component metal parts with minimal tooling. Tooling can be tailored for quantities ranging from quick-turn prototyping through volume production.

Heat Treatment. Heat-treatable copper and steel alloys can be used when formed material must have exceptional hardness or spring properties. In such cases, parts are etched and subsequently formed in the soft state, then heat-treated in inert-atmosphere furnaces to achieve the required hardness and spring characteristics.

For bends that do not require structural strength or for cases in which a sharp internal radius is needed, such as board-level shielding applications, depth-etched bend lines or grooves are used to support hand forming. This eliminates forming tools altogether and can further lower the cost of bringing a design to production.

Lamination. Some designs require an electrically insulating layer to be applied to the part. Manufacturers can selectively laminate a layer of polyimide film to the photoetched part. The bond achieved from this process is robust and nearly permanent, and the polyimide film itself is also strong and has good wear characteristics. Polyimide film can also be laminated to a sheet prior to photoetching so the polyimide can be used as a carrier, binding many small etched features together. This technique enables etchers to maintain appropriate spacing on the carrier and facilitate handling during installation.

Not all manufacturers can offer the convergence of a photoetched part with the value-added operations discussed. For medical applications in which part quality and consistency are of the utmost concern, OEMs should look for companies that provide the processes needed for their application.

Conclusion

Photoetching allows for quick turnaround and minimal tooling costs. It is compatible with the most desirable materials currently used in medical applications and enables efficient production of both prototype and production quantities of these materials. The process allows the required parts to be fabricated with precision and repeatability. Demands for finer features in photoetched parts continue to drive the suppliers of resist films and imaging equipment to improve their product offerings to meet these challenges. And as new metals with desirable properties are developed or identified, companies with photo-etching experience should contribute the process development expertise required to master the art of etching these special materials.

John Daniel is the marketing manager for Tech-Etch Inc. He can be contacted via e-mail at jdaniel@tech-etch.com.