Printed Circuit Design & Fab - August 2008 - (Page 31)

MICROWAvE laMinatE low-loss Fluoropolymer CoPPer ClAD lAMInATe Improved evaluation methods clarify dielectric loss, including conductor loss and roughness loss on copper-clad laminate materials. by KAzuhIKo nIwAno, MAnuEl REyES, MITSuFuMI ono and KoJI IKAwA In recent years, more applications in the millimeter-wave band range, such as automotive radars, are being implemented. This trend is expected to continue. For a PCB to be used in these applications, it is very important that they exhibit low loss at high frequencies. When constructing a PCB for the mm-wave range, fluoropolymer copper-clad materials are good candidates due to the extremely low-loss characteristics of the fluoropolymer material. PCBs with very small loss tangents exhibit a very low dielectric loss. On the other hand, the conductive loss will be relatively large compared to the dielectric loss, so its influence on the transmission loss will increase. Conductive loss is composed of the loss due to conductor conductivity, and the loss due to the conductor’s surface roughness. Particularly, the loss due to surface roughness increases in the highfrequency range. In this situation, it is very useful to separate the transmission loss associated with the dielectric, and the conductor loss and losses due to surface roughness to better understand the loss factors of a given substrate. In a majority of conventional material data sheets, only the loss tangent of the material is reported. Therefore, we needed to manufacture transmission lines and measure the transmission loss in order to learn the transmission loss at the intended frequency of use and also the frequency response. Moreover, it is impossible to ascertain which substrate will present lower transmission loss using only conventional data sheets. We measured a CBCPW (conductor backed co-planar waveguide) that is often used as a transmission line in a mm-wave band. In this work, we propose a method to separate the transmission loss into dielectric loss (Ld), conductor loss (Lc) and roughness loss (Lr). One purpose is to use the separated loss factors for developing new substrates, and another purpose is to propose the separate procedure to be better able to judge and compare the loss factors of any substrate. This allows the designer and fabricator to better compare different candidate materials. Analytical Calculation of Transmission Loss The calculation method for transmission loss of CBCPW is based on the impedance dielectric loss (Ld) of CBCPW (Z0cp) and is calculated by using quasi-static analysis conductor loss (Lc) based on the conformal mapping method.1, 2 roughness loss (Lr) These calculations follow where εre is the effective dielectric (Z0cp) constant and K(*), K’(*) are the elliptic integrals of the first kind re and its complement, respectively. The frequency dispersion3 and conductivity ( c) dielectric loss (R ) surface roughness(Ldq) the effect of metallization thickness4 are taken into consideration conductor loss (Lc) opt in EQuation 1. roughness loss (Lr) EQ. 1 (Z0cp) 60 1 Z 0 cp re K k1 / K ' k1 K k 6 / K ' k 6 re conductivity ( c) surface roughness (Rq) ( Rq 2 Lopt Since the effect of conductor surface roughness is not considLc tan 1 1.4 r ered in the above calculation, we apply EQuation 2 that is based 60 1 on the ratio of the rms surface roughness (Rq) and the skin depth Z 0 cp K k1 / K ' k1 This equation is as follows5: (δ) to the loss calculation. K k 6 / K ' k 6 re 2 EQ. 2 Rq 2 Lr Lc tan 1 1.4 2 ( ( fiGurE 1. Cross section of CBCPW and input parameters for transmission loss calculation. AUGUST 2008 The input parameters that are necessary to calculate transmission loss are indicated in fiGurE 1. printEd circuit dEsign & faB 31

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Printed Circuit Design & Fab - July 2008 Contents Our Line Market Watch Around the World Happenings ROI Tip Jar Software Performance Interconnect Strategies Final Finish Forum Product Development Challenges in a Global Market Innovative Modeling Supports Co-Design of the Power Supply Chain, Part 2 Low-Loss Fluoropolymer Copper Clad Laminate Qualifying PCBs Outsourced in Asia Copper Plating and Microvia Fill for Advanced PCBs Off the Shelf Marketplace Ad Index BGA Bulletin

Printed Circuit Design & Fab - August 2008

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