Photo-Machining

March 1, 2007
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A new process can be used to machine ceramic parts simply, quickly and at a lower cost than many conventional methods.

A 4-in. silicon wafer with .012-in. diameter thru-holes machined through PMP.


Computer-driven machine tools, like computerized numerical control (CNC) machining, ultrasonic machining and laser cutting technologies, have revolutionized the way precision components are machined. However, these advanced equipment systems often come with high price tags, time-consuming setup requirements and increased labor costs.

A new alternative has recently been developed. Called the Photo-Machining Process (PMP),* it provides a lower cost alternative to conventional machining for many applications. PMP can selectively remove material or coatings from a variety of dense, hard or brittle materials, and it can also scribe or mark metal surfaces for labeling identification numbers and logos with pinpoint accuracy.

In addition, PMP can be used to roughen surfaces to promote better adhesion between adjoining parts. This process can reduce setup and machine time, as well as operator skill level. It also benefits ceramic engineers striving for improvements in design capabilities.

*Developed by IKONICS Corp., Duluth, Minn.

Figure 1. UV light passes through the clear or transparent areas of the phototool, changing the chemistry of the photoresist to create a precise and accurately placed pattern that masks the substrate for subsequent abrasive etching.

Process Specifics

PMP combines the precision and accuracy of a photoresist with abrasive etching to selectively remove materials. Key steps include the photoresist imaging operation and the abrasive etching process. First, the design is printed on a transparency-like film with a standard inkjet or laser printer. This film becomes the working tool, or phototool, for the photoresist masking operation.

The phototool allows the transfer of the desired pattern onto the photoresist mask by limiting the ultraviolet (UV) exposure to the selected pattern (see Figure 1). An abrasive-resistant mask that changes physical properties when exposed to UV light, the photoresist mask does just what its name suggests-it resists or repels the abrasive to protect the surface being machined.

Figure 2. Very fine particulates are propelled with compressed air, impacting the substrate not protected by the photoresist. With precise tuning of the particulate size and air pressure, material is removed from the substrate in the areas defined by the photoresist pattern.

During the abrasive etching operation, compressed air propels a fine abrasive, such as silicon carbide, in a closed chamber to accomplish the actual machining (see Figure 2). The abrasive selectively removes material in the open or developed areas of the photoresist.

Figure 3. Etch rates for different material surfaces and relative rate removal differences between materials (not actual time to etch).

The etching system is a dry, low-temperature, low-stress operation that quickly removes large amounts of material from most hard, brittle surfaces. The material removal rate depends on the density of the surface being etched, but examples are shown in Figure 3.

Proprietary photoresist technology is the key to PMP. Photoresist technology has been around for many years, particularly in the electronics and decorative etching industries. The films used in PMP have vinyl or rubber-like properties that repel the impact of abrasives, allowing them to survive aggressive etching by fine particulates in a precision sandblasting environment.

Figure 4. PMP can be used in a variety of applications, including (clockwise from top left): a) selective coating removal, where a conductive material is removed in a precise pattern to reveal a non-conductive layer below; b) surface texturing to improve mechanical adhesion to mating layers or parts and/or creating shallow cavities for lubricants between subsequent layers; c) relief patterns for minimizing contact in mechanical seal applications, with or without channels to introduce lubricants; and d) channel patterns used for transferring fluids, air and/or vacuum in an accurately defined opening.

Capabilities

Brittle materials like ceramics are notoriously difficult to machine because of their high level of hardness and propensity to chip or crack. The benefits realized with PMP vary depending on the size and type of patterns or features that are required. However, detailed patterns and shapes, such as channels, surface pads, blind holes and/or markings, can be created with high accuracy and repeatability, with relatively deep etching on lower density materials (see Figure 4). With film resolution of up to 50 µm (.002 in.), fine detail machining is easy to achieve.

PMP is effective for drilling holes in thin wafer materials, and the process enables hole array patterns to be drilled quickly. All holes are made simultaneously, which is much faster than the one-at-a-time laser or ultrasonic drilling methods. Other uses include individually and indelibly engraving hard-to-mark parts, texturing surfaces for subsequent coatings or to promote adhesive bonds, and selectively removing coatings from surfaces. Where etch depth control is required, an automated etching system can be used to maintain 50 µm (.002 in.) depth variation over a 50 cm (20 in.) length.

Although metals and plastics can be surface engraved or marked, they cannot be machined deeply with PMP due to their inherently ductile nature. However, PMP does work well to selectively remove thin layers (<.005 in.) of these materials, such as copper-clad substrates or metal encapsulated ceramics.

Many of these operations can be accomplished through traditional machining techniques, but PMP often reduces setup and production times. In fact, PMP provides several advantages over conventional machining methods when modifying hard/brittle surfaces or when selectively removing coatings from these surfaces. PMP enables the removal of large amounts of material quickly, with little need, if any, for special tooling fixtures or jigs. The system requires no expensive cutting tools to wear out or replace, and the reduction in localized heat eliminates the need for coolants. PMP also has a reduced tendency for "chipped" edges, causes no pitting or stray markings, and does not require a subsequent deburring step.

Due to the buildup of abrasive in the etched cavity, PMP produces a tapered side wall as the etch deepens. This buildup causes the sides of the feature to etch more slowly than the center, leaving the bottom of the etch area with a slight radius.

Photo Finish

PMP combines proprietary photoresists with fine particulate abrasive etching to create a variety of precisely machined parts. The process is simple, quick and less expensive than many conventional machining methods. While PMP does not entirely displace the need for conventional machining, it opens up opportunities as an excellent alternative in a wide range of applications.

For additional information regarding PMP, contact IKONICS Corp. at 4832 Grand Ave., Duluth, MN 55807; (800) 643-1037; fax (218) 628-2064; or visit www.ikonicsindustrial.com.

SIDEBAR: PMP Possibilities

Below is a partial list of materials and coatings that have been tested with PMP:
  • Alumina
  • Alumina Silicate
  • Aluminum Nitride
  • Anodized Aluminum
  • Boron Carbide
  • Carbon Fiber
  • Ceramic Composites
  • Glass
  • Graphite
  • Mullite
  • Plated/Painted Materials
  • Quartz
  • Silicon Carbide
  • Silicon Nitride
  • Steatite
  • Titanium Nitride
  • Zirconia


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