Flexible low-cost, low-loss analog and digital multilayer circuits can be fabricated well into the millimeter-wave region thanks to a line of LCP laminates and bonding films.
Flexible circuits have long been associated with polyimide films, although such materials have been limited in high-frequency performance. The R/flex® 3000 line of liquid-crystal-polymer (LCP) circuit materials from Rogers Corp. (Chandler, AZ), on the other hand, brings all the benefits of flexible substrates to microwave design, and without the high
dielectric constant and poor moisture absorption that have restricted polyimide-based substrates to lower-frequency circuits.
The Rogers LCP solution is actually a multiproduct family of materials that can be used separately or together to form single-layer or multilayer high-frequency circuits. The R/flex 3000 line includes the R/flex 3600 material with LCP dielectric and single-clad copper laminate, the R/flex 3850 material with LCP dielectric and double-clad copper laminate, and the R/flex 3908 bonding film which can be used as an adhesive layer between copper and the LCP dielectric material to form high-speed, high-frequency multilayer circuits. The single-clad and double-clad laminate materials have melting temperatures of +290 and +315ºC, respectively, while the bonding film has a melting temperature of +280ºC.
An LCP film is essentially a thermally stable thermoplastic material. Since the R/flex 3000 materials share a low dielectric constant of 2.9 at 10 GHz with negligible moisture effects, the flexible-circuit-board materials can be used from audio frequencies to well past 40 GHz. These are low-loss materials, with a miniscule dissipation factor of 0.002 at 10 GHz for all three materials. Since the materials are structurally sound with temperature, circuit traces can be fabricated with extremely tight spacing, supporting extremely dense analog and digital circuitry without fear of crosstalk or signal coupling from signal traces shifting due to thermal effects. In summarizing some of the other pertinent electrical parameters, the two laminate materials feature surface resistivity of 1.0 × 1010 MΩ and volume resistivity of 1.0 × 1012 MΩ-cm while the R/flex 3908 bonding film exhibits surface resistivity of 1.0 × 1010 MΩ and volume resistivity of 1.0 × 1012 MΩ-cm. All three materials offer dielectric breakdown strength of 3500 V/mil (1378 kV/cm).
Because they share the LCP dielectric layers, the laminates and bonding film show low moisture absorption and excellent chemical resistance. The water absorption amount (tested according to the IPC 2.6.2 standard) is a mere 0.04 percent after being maintained at +23ºC for 24 hours. The chemical resistance (tested according to the IPC 126.96.36.199 standard) is 98.7 percent. The coefficient of hydroscopic expansion (at +60ºC) is 4 PPM/percent relative humidity (RH).
The R/flex 3000 laminates and bonding materials show very little dimensional movement with temperature, with a coefficient of thermal expansion (CTE) that is 17 PPM/ºC in both the X and Y directions. The amount of force to initiate a tear in the materials (minimum initiation tear strength) is 3.1 lbs (1.4 kg). The laminates are also characterized by the peel strength of their copper cladding, with values of 5.2 lbs/in. (0.95 N/mm) for the single-clad R/flex 3600 material and 5.2 lbs/in. (0.95 N/mm) for the double-clad R/flex 3850 material. The single-clad material exhibits tensile strength of 17.5 kpsi (120 MPa) and tensile modulus of 350 kpsi (2400 MPa). The double-clad material has tensile strength of 29 kpsi (200 MPa) and tensile modulus of 327 kpsi (2255 MPa). The bonding film exhibits tensile strength of 31 kpsi (216 MPa) and a tensile modulus of 355 kpsi (2450 MPa).
The R/flex 3000 materials meet flame-resistance requirements for consumer products according to Underwriter's Laboratories UL-94 specifications. The LCP materials can be machined much like polyimide or FR-4 circuit-board materials. Since they are thermoplastic materials, however, all drilling operations should avoid overheating the sidewalls (which could leave rough surfaces for plating). The materials are halogen free for minimal environmental impact, and can be processed with both CO2 and YIG lasers. A hot oil vacuum press with good temperature control can be used for fabricating the multilayer circuits.
The R/flex 3000 materials have no special storage requirements and no limits on shelf life. The laminates are available in standard thicknesses of 25 µm (0.001 in.), 50 µm (0.002 in.), and 100 µm (0.004 in.) while the bonding film is available in standard thicknesses of 25 and 50 µm (0.001 and 0.002 in.). All three R/flex 3000 materials can be supplied in sheet sizes of 18 × 12 in. (457 × 305 mm) and 18 × 24 in. (457 × 610 mm). The R/flex 3600 and the R/flex 3908 materials are available on a roll with widths to 20 in. Custom sizes are also available. Rogers Corp., Advanced Circuit Materials Div., 100 North Dobson Rd., Chandler, AZ 85224; (480) 961-1382, FAX: (480) 961-4533, Internet: www.rogerscorporation.com.