Technology Focus
Jack Browne, Technical Director
MISSILE SEEKERS and other military applications have long relied on millimeter-wave frequency bands for operation. The fine circuit traces and spacing that are required for millimeter-wave (30 GHz and higher) circuitry have long been in the manufacturing domain of just a handful of companies, with Filtran Microcircuits, Inc. (FMI, www.fi l-tranmicro.com) one of the best of the bunch.
What kind of skills and processes are needed for fabricating millimeter-wave circuitry? Every component and transmission line at those higher frequencies is a significant percentage of the wavelengths of the frequencies of interest, and the electrical lengths of these circuits can impact phase performance in radar systems. In addition to being well equipped in terms of the production and measurement equipment needed to produce fine-geometry printed-circuit boards (PCBs), FMI boasts a veteran engineering and production team obsessed with quality and performance.
The “build-to-print” PCB fabricator has produced small-signal and large-signal circuitry at frequencies to 100 GHz and beyond. FMI can process single-sided circuits on panels up to 24 24 in. and multilayer circuits on panels as large as 18 24 in. Panels as large as 12 12 in. can be
processed with gold plating. The firm offers extensive machining capabilities with machining tolerances as fine as ±0.001 in. The firm also offers CO2 laser-machining capability for complex shapes and circuit features. Filtran accepts a wide range of standard circuit layout files, including DXF, Gerber, and IGES formats, and can even help a customer optimize their layouts for the lowest cost and highest yields possible.
Using proprietary processing techniques, including magnetron sputtering and semiconduc-tor-grade microlithography, and precision machining, FMI can realize printed-circuit line widths and spacing as fine as 1 mil (0.001 in. or 25 µm) with ±0.2-mil accuracy on both hard (ceramic) and soft polytetrafluoroethylene (PTFE) circuit-board materials. The company works with a wide range of circuit-board materials, including microstrip, bonded stripline, thick-metal-backed PTFE, and even multilayer circuits with homogeneous and mixed-dielectric layers. The firm’s fabrication capabilities are backed by other facilities within the parent company, Merrimac Industries ( www.merri-macind.com), including design engineering at West Caldwell, NJ and high-volume manufacturing at the firm’s facility in San Jose, Costa Rica.
FMI adds embedded resistors for soft substrates and sputtered nickel-chromium resistors for hard
substrates at standard resistance values of 50 Ω/square, although other values are also available. The company features a proprietary liquid photoresist process for creating single-sided, double-sided, and multilayer circuits with plated through holes and, for less demanding designs, FMI can apply less costly dry photoresist techniques. For thermal and ground interconnections, FMI can realize filled viaholes as small as 0.010 in. in diameter.
FMI can metalize viaholes through a variety of techniques, including its proprietary sputtered-blind-hole (SBH) process. The unique process involves using a thin-film copper “seed” metalization on a viahole, reinforced by electroplated copper. A surface finish meeting a customer's requirements, such as tin, tin-lead, nickel, or gold, is added on top of the copper layers. The SBH process has been well received by commercial, military, and satellite customers for viaholes as small as 0.025 in. in diameter on metal-backed PCBs, without additional machining of counterbores on the backside of the circuit’s metal carrier. For more on FMI, visit the “Design Guide” section of their website at www.filtranmicro.com.
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