How do chemical milling and precision electroforming compare in medical device design and manufacturing? Although the technologies can and do compete, the benefits and limitations of each define exclusive areas of involvement with minimum overlap. For design situations that could use either process, cost is usually the deciding factor. In general, per device unit or per unit area, chemical milling is often less costly.

That said, electroforming provides extreme precision, small part sizes, and multilevel parts beyond the capability of chemical milling. Electroforming is used in a variety of applications such as implanted fluid capillary systems and micro radio frequency induction data-transfer devices.

Chemical milling

This process starts with preformed metal sheet stock. Sheet sizes are measured in square feet, with thicknesses from 0.0005 to 0.125 in. and tolerances typically +/- 0.001 in. or greater. Structures built are almost always flat, single-layer devices. Overall unit sizes of X,Y dimensions can range from 0.025 in² to 2 ft².

A subtractive technology, chemical milling removes metal by chemically etching it to create a specific shape or structure. First, a UV-sensitive photoresist material, usually in dry film sheet form, is laminated to both sides of the sheet or foil to be etched. Photo masks of repetitive part patterns, usually on a flexible film, are aligned and fixtured to both sides of the coated sheet. These get exposed to UV light, which cures and hardens the photoresist. A developing solution washes away the uncured areas, leaving bare metal in the areas to be etched. The remaining photoresist protects the rest of metal and defines the part shape. Spraying the appropriate chemistry on the sheet etches out the parts.

The process is isotropic, meaning the deeper the etch, the greater the undercut material. Because of this effect, the wall profile of chemically milled structures has a characteristic hour-glass shape and mask artwork must incorporate a compensation factor for final dimensions.

Chemical milling is well suited for creating devices with minimum tolerances between +/- 0.0005 and +/- 0.0030 in., depending on thickness. Minimum X,Y feature size is typically equal to or greater than the thickness of the foil being etched. Minimum through-hole sizes are typically equal to or greater than the foil thickness.

Minimum unit size is also dependent on the thickness of the sheet being etched. Simple shapes such as washers or springs can have overall dimension as small as 0.030 in. Compatible materials for the process include nickel, pure copper, copper alloys, and for implantable medical applications, titanium, and stainless steel.

Precision electroforming

This process starts with sheet sizes that rarely exceed 12 in². Thicknesses range from 0.0002 to 0.010 in. Devices built using precision electroforming, sometimes referred to as 3D micro structures or 3D Micros, are typically dimensioned in microns. Overall unit sizes range from 0.0004 to 1.0 in., and can be single-level (flat) structures or multi-level complex structures.

Precision electroforming is an additive process in which 3D micro structures are formed by electrochemically depositing metal into a precisely formed photoresist mold. The electroforming process is a highly refined and precisely controlled traditional dc anode-cathode-electrolyte system. Plating parameters such as bath composition, concentration, current density, duty cycle, temperature, filtration, and agitation are optimized to result in the desired metal characteristics. The most commonly used deposited materials are nickel cobalt, nickel, pure gold, copper, and hard or bright gold. Material selection is based on the application requirements.

Building a single-level structure entails first preparing a carrier plate by sputter depositing a thin (< 5000 angstroms) adhesive and conductive seed metal layer onto a glass blank. The seed layer provides controlled adhesion between the glass base and the electrochemically deposited metal and forms the electrical conduit for the subsequent electroforming deposition. The particular seed metal used varies with the metal being plated.

Next comes creating a photoresist “mold” of the intended structure. The mold is formed by depositing and imaging the X,Y plane features of the intended structure into UV-sensitive photoresist on the seed-metal-coated glass carrier The Z dimensions are controlled by the thickness of the photoresist mold. The selected metal is electrochemically deposited into the photoresist mold. Once the electoforming is completed, the photoresist mold is removed. Finally, the completed electroformed micro structures are removed from the glass carrier.

Multilevel structures are formed by sputter depositing a secondary seed metal layer over the first layer and then repeating the steps. Successive layers usually have different X,Y feature dimensions requiring unique photomasks for each layer. To date, this process has been used to create complex four-level structures. Structures with more layers are possible, but the interlayer planarity becomes more critical. Planarization processes are available.

Minimum feature size for electroformed micro structures is 0.000080 in. Maximum aspect ratio is generally considered to be 2.5:1. In this case, aspect ratio is defined as thickness (Z dimension) divided by minimum feature size (X,Y dimensions). Greater aspect ratios are possible depending on the particular structure being built.

Critical feature and dimensional tolerances are ± 0.000040 in. Critical dimension values, tolerances, and aspect ratios are interrelated. The higher the aspect ratio, the greater the tolerance needed. Depending on the particular structure, feature sizes as small as 0.000040 in., and aspect ratios as high 10/1 are possible.

Wall profiles are typically perpendicular to the X,Y plane of the structure. For fluid-jetting applications such as high-pressure water jet surgical scalpels, the electroforming process can generate funnel-shaped aperture holes.