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PCB-Apr2017

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38 The PCB Magazine • April 2017 signals in millimeter wavelengths have a direct influence on the choice and thickness of sub- strate, PCB feature (and feature-to-feature) po- sitional accuracy and feature dimensional accu- racy. The requirements for matching manufac- turing and metrology capacities are discussed in this article. Match Needs Raw Material Selection Microstrip, stripline and co-planar wave- guide transmission line technologies are all de- ployed in mmW PCB design. Increasingly, de- signers are using substrate integrated waveguide (SIW) and this was true of circuitry used in Mi- WaveS. SIW sees a rectangular waveguide cre- ated within the dielectric by adding a top metal over the ground plane and fencing the structure with rows of plated vias on either side. A sketch depicting the SIW PCB feature configuration is depicted in Figure 1. Benefits of SIW are po- tentially lower losses than with microstrip and coplanar waveguides since dielectric losses are typically lower than conductor losses at milli- meter waves. The via fence needs to be dense enough to prevent field leakage and signal loss from the waveguide to the substrate. Many tra- ditional waveguide components such as power dividers, signal couplers, filters and antennas can be realized by SIW technology. Component performance approaches conventional air-filled waveguide performance and has the advantage of low radiation leakage and interference com- pared to microstrip and coplanar circuits. A defining requirement for raw material se- lection is minimizing loss (dielectric loss, con- ductor loss). Much has been published regard- ing raw materials and loss [4]. Several low-loss materials were used in MiWaveS and in some instances combined with FR-4 to form so-called mixed-dielectric multilayers (FR-4-based layers being used to satisfy the digital function of the designs with consideration for the long-term economic aspects). However, liquid crystal polymer (LCP), a low-loss thermoplastic base material, was identified at the outset for the mmW functions. LCP is a good candidate for mmW multilayer PCB structures; it has a sta - ble dielectric constant through the frequency range, exhibits low moisture absorption, and has comparatively low loss. Rogers Corporation offer a non-woven-based copper-clad laminate and matching bond-ply [3] . The combination of laminate and bond-ply simplifies the design process inasmuch it provides opportunity for homogenous dielectric properties. A mechani - cal benefit of such a combination is the oppor- tunity to maximize MLB planarity (flatness). The challenge from a fabrication perspective (with LCP) however is coping with material movement: • Typically, mmW MLB dielectric separa- tions are comparatively thin. For transmission, the dielectric spacing (signal to ground plane) is governed by wavelength. In MiWaveS 100 µm thick laminates and bond-plies were deployed. The combination of thin substrate without wo- ven-glass reinforcement has a negative effect on dimensional stability. Innerlayers change dimensionally during processing (etching and so forth). Typically, they shrink in both X and Y planes; the degree of retained metal of course has an influence on the degree of shrinkage. • Being a thermoplastic, the materials soften during thermal excursion, MLB lamination in- duces further mechanical distortion, often lo- calized. • Further dimensional change occurs during outer-layer processing. Since positional alignment of drilled/plated features to printed feature is a critical attribute (some mmW designs required positional ac- PCB TECHNOLOGY REQUIREMENTS FOR MILLIMETER-WAVE INTERCONNECT AND ANTENNA Figure 1: Sketch depicting SIW configuration; ground plane either side of dielectric with PTH (via) fence.

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