Printed Circuit Design & Fab - August 2008 - (Page 32)

MICROWAvE laMinatE There are a number of factors that can influence conductivity (ρc) and surface roughness (Rq). Plating and surface treatment of the copper conductors are major factors. Conductors, therefore, can have different conductivity depending on these treatments. To obtain the effective conductivity, the skin effect needs to be included because a homogeneous conductivity is assumed in the loss calculation. To derive the effective conductivity, one can calculate the distribution of current density in the cross section of CBCPW and the ratio of current density in each conductor using a 3D EM simulator. In modeling the effective conductivity, nickel plating followed by gold-flash plating was completed on the transmission line before measurement. It was determined, however, that the calculation associated with the gold flash need not be considered in the calculation because of the negligible effect of the thin gold coating. The surface roughness derived was the effective roughness from a ratio of current density on the top and the bottom of the conductor because the top and bottom surface roughness was different. It was necessary to consider the roughness of both sides and the affect to the transmission loss according to the ratio of the current density. fiGurE 2 shows measured and calculated results of the fluoropolymer copper clad laminate with the roughness of copper foil where Rz was 1.8µm (fiGurE 2a), and Rz was 3.6µm (fiGurE 2b). The copper clad laminate shown in Figure 2b uses a copper foil categorized as low profile, but the roughness loss in the mmwave band is relatively large. On the other hand, the copper clad laminate shown in Figure 2a uses a profile-free copper foil, and the roughness loss is the same level as the dielectric loss. Since the results indicated that the surface roughness of the copper foil greatly influenced the transmission loss, profile-free copper foil was used for the development of the new copper clad laminate material. conventional PTFE, the adhesive fluoropolymer allows for a completely dry process for copper-clad laminate fabrication, reducing the environmental impact of the material. It can also further reduce the process cost with the elimination of the wet-process surface preparation steps. Transmission Loss Measurement Transmission lines were measured on a network analyzer that could sweep continuously up to 110 GHz and was outfitted with a probe head. This measurement system provided a highly accurate measurement technique for low-loss transmission lines. The contact pads of transmission lines are CBCPW. Maximum sequence length (MSL) was also tried as an evaluation technique for the transmission line. However, because of the transition of the influence of the line resonance, it is relatively large compared to the transmission loss in the high-frequency range. CBCPW proved to be a better method because the evaluation lines did not need transitions. The measured transmission lines were nickel and gold plated. Through-reflect-line (TRL) calibration of the network analyzer was executed, and the measurement frequency range was from 2 to 110 GHz. The length of measured lines was 8 mm. Several transmission lines with eight different kinds of impedance were measured and analyzed up to 110 GHz in order to separate the transmission loss into dielectric loss, the conductor loss and roughness loss. Due to the mismatch with 50 ohms, the transmission loss exhibited a characteristic ripple. To compare the lines, the ripple characteristic needed to be corrected by matching with input and output impedance. After that, the measured data still showed slight ripple due to the discontinuities, and time and/or temperature drifting characteristics of the measurement system. But this minute ripple did not affect the characteristics, so it was smoothened mathematically. The above-mentioned data processing was completed, and the transmission losses of both the substrate and impedance were obtained. Developing Copper Clad Laminate Materials Based on the factors above, a fluoropolymer material was developed using the profile-free copper foil (Rz is about 1µm). In addition, the fluoropolymer used had good adhesive characteristics so that it could be laminated to the profile-free copper foil without the use of a special surface treatment or additional surface roughening. The resulting peel strength is more than 1 kN/m. In contrast to Repeatability of the Measurement System We can also confirm the repeatability of the measurement system. A transmission line was measured four times, and the difference between the raw data is shown in fiGurE 3. The repeatability, fiGurE 2. Measured and calculated results. 2a (left): cu foil roughness of rz=1.8um. 2b (right): cu foil roughness of rz=3.6um. (Calculated = total loss = lc + lr + ld ) 32 printEd circuit dEsign & faB AUGUST 2008

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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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