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PCB-July2014

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34 The PCB Magazine • July 2014 RF CAPACITOR MATERIAL FOR USE IN PCBS continues maximum DC electric field strength applied across the dielectrics in RF capacitor) for a high voltage rating which is essential especially for servers, pico cell and femto cell in base station market space. Generally, fillers are widely ac- cepted in various applications because of their advantage in addressing several limitations of polymers, making its way onto benefits such as better dimensional stability for polymer com- posite membrane, lower coefficient of thermal expansion (CTE) for build-up layer, increasing thermal conductivity for thermal interface ma- terials (TIM), and improved stiffness for under- fill materials. As for the fillers in RF capacitor application, it is presently based almost entirely on the simple perovskite BaTiO 3 (barium tita- nate), but there is strong demand on the class of materials known as paraelectrics, mainly due to the fact that their dielectric properties are much more stable with regard to most operating con- ditions such as frequency, temperature and DC bias [5] . Well-dispersed fillers in polymer composite matrix play a crucial role in achieving RF ca- pacitor requirements as described above and thus factors that determine fillers distribution should be controlled and optimized. Any exis- tence in the particle agglomerates is accompa- nied by the formation of possible defects such as trapped porosity that make the dielectrics vulnerable under practical operating condi- tions. Figure 1 shows typical example of severe filler agglomerates clearly visible, concentrated on the coating surface as discrete protuberances and appearing to be in greater numbers. Filler agglomeration is easy to observe by improper usage of dispersion agents and their mismatch- ing to a solvent composition in formulation. It could degrade electrical properties, in particu- lar dielectric strength and temperature stability of capacitance (both will be described in next section). Various types of coupling agents can be added to the polymer compositing to take advantage of the absorption of a functional polymer to the particle surface to modify the filler/polymer interface chemistry, giving rise to complete de-agglomeration of the fillers and subsequent elimination of the air void [6] . It is also necessary to adjust the solvent composi- tion combined with the coupling agents, which is associated with coupling, adhesion and dis- persion. In this study, a titanium-based cou- pling agent and a typical solvent mixture were selected and formulated with a paraelectic filler and a phenylene-based polymer. The optimum amount of the coupling agent was determined by the viscosity response for various levels of the application of the coupling agent. The de- agglomeration effect was pronounced when sig- nificant reduction in suspension viscosity was observed. Optimal combination was formulat- ed to make RF capacitor laminate that used thin dielectrics having thickness range of 12–25 µm. II. Results and Discussion A. Effect of Ceramic Functional Particles (Fillers) For practical device application, the contri- bution of the filler to the dielectric strength and the temperature stability of the capacitance are of greatest interest. The dielectric strength was examined by measuring dielectrics breakdown voltage (BDV) of the samples having fillers with different size (Figure 2). A circular pattern with 0.5 inch diameter was placed on a bottom elec- trode grounded to the DC tester. A probe was placed on the center of a circle and subjected to the voltage. The electrode material was made on one side of the copper of a RF capacitor lami- Figure 1: SeM image (at tilt angle) of the typical filler agglomerates of ceramic filled polymer composites (on the coating surface).

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