- Buyers Guide
A New Low Loss, Laser Ablatable Substrate for Microwave Circuitry
The use of polytetraflouroethylene (PTFE) as a dielectric substrate represents a mature technology in the field of RF/microwave circuit design. The growth in cellular communication infrastructure during the latter part of the twentieth century placed demands on laminated substrate manufacturers to develop both the capacity and products to make low loss PWB materials in sufficient volumes at the right economies. These products are widely used as PWB substrates for power amplifiers, filters, couplers and combiners, antennas and others.
During the same period, PWB manufacturers and allied equipment suppliers developed technologies and processes to cope with the volume requirements for cellular handsets, viz. micro-via, thin-cores and feature-to-feature accuracy. This is perhaps a simplistic representation of events; however, there exists a high technology industry capable of producing complex multilayer circuitry using photo-lithographic and laser machining techniques in volume.
Taconic Advanced Dielectric Division has developed a new non-reinforced thin-core laminate called Taclamplus. Using PWB manufacturing techniques it can be used to form mm-wave circuits allowing the formation of microwave components and is capable of accommodating MMICs and surface-mount devices. With 350°C laminating capability, multilayer structures are possible enabling greater circuit density, stripline componentry and shielding techniques.
Thin-core materials are difficult to process; while reinforcing agents such as woven glass can improve handling and general material stability, their loss characteristics preclude their use in high frequency applications. In addition, laser ablation of such materials is hampered due to the preferential energy absorption over plastics. As a result, plated vias can appear rough, and with unpredictable surface area copper volumes can vary.
To overcome the handling problems, the new thin-core material is supplied laminated to a copper plate. While experimental builds have used 1 mm thick copper, thinner plate or thick electrodeposited copper foil could be used. Matched coefficient of thermal expansion (CTE) materials such as copper-invar-copper or aluminum-silicon alloys is also a possibility. In addition to providing mechanical support, the base metal plate can provide thermal dissipation (heat-sink) and, of course, electrical grounding.
The dielectric is composed of a PTFE/ceramic composite; presently the dielectric constant is 2.2. However, other values are envisaged allowing designers to create multilayer structures with varying dielectric constant dielectric layers. Dielectric thickness is typically 0.1 mm, although others are easily accommodated; presently 0.05 mm is the thinnest available. It should be noted, however, that the lamination requires temperatures in excess of 330°C, and there is limited high temperature press capability in the PCB manufacturing industry, although the capital costs for upgrade are not prohibitive.
The unique composition of the dielectric allows laser ablation with no residue; this makes processing relatively easy in a single step without the need for its removal. Laser ablation can be used for the formation of via-holes where small-diameters can be drilled more economically than with mechanical drilling. The self-limiting nature of laser drilling makes it ideal for sequentially laminated structures (blind/buried vias).
Copper-foil adhesion is comparatively high; this is a pre-requisite for fine-feature resolution. Good foil adhesion is achieved without the need for high RMS copper foils. Low RMS foils make for better conductor losses and this is especially important at high frequencies. Copper-peel strength is typically 10 lb/in (1.8N/mm) for 17 µm ED copper. Indeed, unsupported 9 µm ED copper is available as is 3 µm supported ED foil. The latter two foils make better the opportunity for repeatable micro-gaps (the resolution of printed filter structures, for example).
Laser ablation is relatively new to microwave circuit manufacturing. Broadly speaking the laser-drilling machines used in PWB manufacturing have a combination of UV and CO2 lasers.1 The latter work in the infrared region and ablation is largely thermal (material is heated and evaporated), making them limited in “spot size.” CO2 lasers are not capable of ablating copper foils above 3 µm. UV lasers can resolve relatively small “spot sizes” and use photo-ablation. Each photon is capable of breaking a bond in the bulk material and therefore has the ability to remove atoms or molecules without thermal effects. As a result, the ablated area is of better quality with limited damage to surrounding areas. Typically UV lasers are used to “trapan” the copper foil to create an aperture. This defines the via diameter. Thereafter, the CO2 laser, with the larger beam, sweeps over the copper foil and selectively ablates the exposed dielectric material. Micrographs of plated 6 mil (0.15 mm) ablated vias are shown in Figures 1 and 2.
Creating milled recessed cavities or pockets for MMICs within microwave printed circuit boards is challenging. Traditional machining using end-mills introduce process tolerances that often exceed the precision required to meet electrical (RF/MW) requirements. Not only is this true in the X and Y planes but also in the Z plane. The use of localized fiducials can help maintain accuracy to printed features, although their use increases machine time. Good Z-axis (depth) control will often require localized “zero-setting,” again adding to machine time. Cavity-depth and its tolerance (Z-axis control) are often critical. It has a bearing on the height of the placed component. With mechanical machining, there is also the tendency to generate burrs that again hinder electrical performance and provide sites for chemistry entrapment that can affect the quality of subsequent metal finishing (electrodeposited gold, for example). This is particularly true with dielectrics that contain woven reinforcement materials.
Here again the use of laser ablation helps achieve the tolerances and feature accuracies that are required for MMIC and power-transistor placement. Taclamplus comprises a media that is easily and cleanly ablated to form such cavities. Positional accuracy of state-of-the-art laser machines offered by the likes of Excellon is typically ±0.001" (±25 mm) across a working area 30" x 24" and within a feature (cavity) the tolerances are < 10 µm. With fine-tuning, the tolerance can be as good as ±2 µm.
The practice of recessing MMICs provides the opportunity to present a near flush surface to attach wire or ribbon bonds. Without a step, the lengths of these connections are kept to a minimum and retain a level of flatness. Flat bonds lower the inductance and thus improve the insertion loss.
The use of laser ablation on Taclamplus allows RF-in/RF-out tracking to be terminated flush to the cavity wall, again presenting the opportunity for flatter and shorter bonds. Figure 3 shows an SEM micrograph of a track terminating flush to a laser-ablated cavity.
As previously detailed, Taclamplus is available in 0.1 mm thickness, that is, the typical profile of MMIC devices. Figure 4 shows a schematic of a one-dielectric-layer structure with MMIC cavity and a two-dielectric-layer structure. Along with buried vias and a grounded platform it is possible to mount MMICs on layer 2 of a given multilayered structure.
Taclamplus was used in the recent PROKOSMOS, a BMBF–funded project.2 Here the substrate was used for a demonstrator module (42 GHz LMDS radio). The module is shown in Figure 5. During this project, project leader EADS Germany GmbH measured the Taclamplus dielectric properties. The details are shown in Table 1.
As previously mentioned, Taclamplus was designed as a laminated material for high volume, mm-wave modules, such as radio-links, automotive sensors, etc. Wave-guide launch techniques are easily accommodated and the materials are compatible with a wide range of assembly techniques, including lead-free assembly. In addition, similar to all Taconic materials, Taclamplus satisfies the UL-94 V0 requirement without the need of any flame-retardants.
Taclamplus represents a cost-effective microwave substrate that can be used to create very low loss structures both with single dielectric layers and multiple layers. Exceptional copper-foil adhesion allows small-feature resolution and the unique composition of the dielectric facilitates clean laser ablation for micro-via and component-cavity formation. The use of metal-plate, such as 1 mm copper, helps maintain dimensional stability and provides a sound ground plane and ideal heat-sinking properties.
The author would like to thank Andreas Schmidheini at Varioprint AG (www.varioprint.ch) for lamination and laser ablation evaluations, Bob Lang at Excellon Automation (www.excellon.com) for laser machining detail, Martin Oppermann at EADS Microwave Factory (www.eads.com) for testing, Roderick MacPherson at ALPS Ltd. (www.alps.com) for MMIC placement/wire-bonding considerations, Euan Lockie at Intrasys Design Ltd. (www.intrasysdesign.com) for module design and Stewart McCracken at MCS Ltd. (www.materials-consult.co.uk) for SEM analysis.
- M. Kauf, L. Ekblad and H. Martinez, “Mechanical and Laser Via Formation,” The Board Authority, July 2000.
- PROKOSMOS, EUREKA Project E! 2448.
Taconic Advanced Dielectric Division,
Petersburgh, NY (518) 658-3202
Mullingar, Co. Westmeath,
Ireland +353 44 38300,