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24 The PCB Design Magazine • August 2014 on one side of the copper of a RF capacitor laminate. The dielectric thickness was set to 25µm for all samples. As shown in Figure 2, the effect of the filler size on the BDV was clearly detectable, and the BDV of the di- electrics composed of the larger size filler re- vealed a considerable improvement in BDV compared to the case of formulating with the smaller size filler. Lower BDV may result from non-uniform dispersion of the filler and it may induce the charging of the weak in- terface chemistry, probably representing the filler agglomerates. We continue to work on improving the material to achieve relatively high BDV even with small size filler by con- trolling the suspension chemistry. Indirect evidence for the improvement of the filler distribution was sought by measur- ing capacitance change with temperature for samples fabricated with different suspension preparation routes. Temperature coefficient of capacitance (TCC) is an important material pa- rameter to meet the tighter RF/microwave de- sign tolerances. (Details on TCC were described previously [7] ) Figure 3 shows that the TCC was rotating counterclockwise (in the direction of the positive TCC) toward the end of each curve at high temperature region. We mixed fillers with the polymer in an arbitrary way. Thus, in this physical construction of the polymer com- positing, the dielectric is comprised of filler and polymer with the filler occupying the ma- jority of the dielectric. The filler selected ended up slight positive TCC and the plain polymer (without filler) showed negative TCC. The filler contribution to the net composite TCC is de- pendent on its volume fraction and distribu- tion. Therefore, it is assumed that the filler can control complete TCC by compensating for negative TCC of the polymer and positive TCC at higher temperature could be regarded as being responsible for the uniform filler dis- tribution in the polymer matrix. SEM photos in Figure 4 support this with different level of dispersion of the filler in polymer between two samples. B. Characteristics of RF Capacitor Laminate Figure 5 shows frequency stability of Dk (di- electric constant) and DF (dielectric loss) of our developed RF capacitor laminate product using the ceramic-particle-filled polymer composite. As for the method for checking Dk and DF, the first point in Figure 5 used the lower-frequency method (LCR meter) and the remaining three used the split post resonator cells, which are use- ful to measure Dk for isotropic mixtures (when feature RF CAPACIToR MATERIAL FoR USE IN PCBS continues Figure 2: BDv variation of polymer compositing with the filler size. Figure 3: Temperature stability of capacitance of two samples prepared by different suspension preparation routes.