Lasers Carve MM-Wave Circuits

Oct. 14, 2010
A laser-based circuit-milling system capable of working with a wide range of circuit materials provides the feature resolution needed to reliably fabricate circuits for millimeter-wave applications.

Millimeter-wave frequencies represent largely uncharted bandwidth. Hoping to explore the region further, Germany's Fraunhofer Institute for Applied Solid State Physics, named after Joseph von Fraunhofer who investigated the properties of visible light as early as 1820, has enlisted the aid of a ProtoLaser S system from laser-machining specialist LPKF Laser & Electronics for rapid fabrication of prototype circuits on a wide range of printed-circuit-board (PCB) materials.

The Fraunhofer Institute is involved in not only engineering new solutions for millimeter-wave devices and components, but in enabling their use in practical applications. For example, highfrequency radiometers can be used to continuously measure the temperature and humidity of the atmosphere using a ground-based system. Millimeter-wave circuits are also used in radar, in airport body scanners, in industrial metrology, safety engineering, astronomy, and in environmental surveys.

In order to explore the commercial deployment of its millimeter-wave developments, the Fraunhofer Institute needed the means of fabricating fine-featured circuits from a variety of different PCB substrate materials. The ProtoLaser S system from LPKF uses high-energy pulsed laser beams to selectively ablate the conductive layer (usually copper) of a PCB laminate to form the thin transmission lines and circuit structures needed in support of millimeter-wave frequencies.

The LPKF ProtoLaser S is a compact, laboratory-qualified laser system designed specifically for prototyping as well as on-demand small batch production of PCBs. It is equipped with casters for mobility and requires only compressed air, a vacuum, and a standard 110-VAC power outlet. Using a patented process, the laser can structure circuitry with great precision on most circuit substrates. The laser beam circumscribes the contours of circuit artwork on fully clad PCB material, thus producing the wiring structure.

The laser system can process circuit layouts as large as 229 x 305 mm with a structuring speed of about 1 in.2 per minute (6 cm2 per minute). It works with flexible and rigid substrate materials, including copper-coated FR-4, polytetrafluoroethylene (PTFE), and ceramic substrates. The contact-free process results in handling sensitive materials without damage, resulting in high yields. The best precision is when using ceramic materials. On ceramic materials, the conductive layer is vaporized, allowing for conductor tracks with a width of only 50 m (0.002 in.) and spacing as small as 25 m (0.001 in.). Process parameters can be stored in memory for future research or small production runs. The ProtoLaser incorporates an internal forced-air cooling system for long-term reliability. The system measures 34.5 x 56.3 x 29.5 in. (875 x 1430 x 750 mm) and weighs 573 lbs. (260 kg).

The Fraunhofer Institute has used the ProtoLaser S system to fabricate circuits for millimeter-wave applications, including a four-channel intermediatefrequency (IF) amplifier for a 94-GHz radar system (Fig. 2). The system has also been used to produce a 15-GHz voltagecontrolled-oscillator (VCO) circuit for a multiplexer that generates signals in the 90-GHz range (Fig. 3).

Engineer Michael Kuri, one of the specialists at the Fraunhofer Institute on combining RF semiconductors with circuit carrier systems, confirms the productivity gain when using the ProtoLaser S system: "Our measurements demonstrate that boards produced with the LPKF ProtoLaser S are in no way inferior to prototypes produced externally. However, in-house prototyping facilitates several iteration cycles per day and production on demand." Having the ProtoLaser S system on site at the Fraunhofer Institute allows its engineers to react quickly when modifications to a design are needed. For example, Kuri mentions the development of a slot antenna for an innovative radar system. The antenna required virtually radii-free cut-outs in order to achieve the radiation properties needed for the radar system. The production time to produce the antenna with previous machining services and methods was three days or more; with the ProtoLaser S system, this was reduced to two minutes. Fraunhofer Institute for Applied Solid State Physics (Fraunhofer-Institut fr Angewandte Festkrperphysik, IAF), Tulla Strasse 72, 79108 Freiburg, Germany, +49 (761) 5159-458, Internet: LPKF Laser & Electronics AG, 12555 SW Leveton Dr., Tualatin, OR 97062; (503) 454-4212, FAX: (503) 682-7151, Internet:

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