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July 2014 • The PCB Magazine 37 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 (direction to the pos- itive TCC) toward the end of each curve at high temperature region. We mixed fillers with the polymer in an arbitrary way. Thus, in this phys- ical construction of the polymer compositing, the dielectric is comprised of filler and polymer with the filler occupying the majority of the di- electric. The filler selected ended up slight posi- tive TCC and the plain polymer (without filler) showed negative TCC. The filler contribution to the net composite TCC is dependent on its volume fraction and distribution. Therefore, it is assumed that the filler can control complete TCC by compensating for negative TCC of the polymer and positive TCC at higher tempera- ture could be regarded as being responsible for the uniform filler distribution in the polymer matrix. SEM photos in Figure 4 supports 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 is use- ful to measure Dk for isotropic mixtures (when fillers are randomly oriented) [8,9] . The product is the copper clad laminate (CCL) with a standard panel, 18 x 24 inches. It was composed of two sheets of copper foils on both ends with organic based composite dielectrics having fillers dis- persed into polymer in between. Copper foils are available in various thicknesses, 0.5 ounce and 1 ounce being the dominant thickness, but thinner copper foil would help to minimize the variation in capacitance during etching pro- cess in PCB manufacturing. The typical Dk and DF of the RF capacitor laminate at 1GHz were measured, 7.8 and 0.0022, respectively. We are expanding the product's capacitance density range up to 670 pF/cm 2 thinning dielectrics and process optimization. The standard reliability tests including solder shock, solder float, time to delamination and THB (temperature, humid- ity and bias) testing were performed and all these tests passed. C. Uniformity of Capacitance Capacitance tolerance for the organic-based RF capacitor laminate is critical for the appli- cation of forming discrete type embedded ca- pacitors inside the organic packaging substrate. In this case, capacitance tolerance can be ex- pressed as 3 sigma in the form of (mean of ca- pacitance) ± (3 sigma) for the foot print size of the capacitor. Smaller tolerance in capacitance is desirable to achieve better yield performance of the RF device in manufacturing [2] . However, when forming discrete embedded capacitors inside the organic substrate for RF module, the materials and processes don't currently allow for the tight tolerances due to the material and the process variation. The dispersion techniques for the ceramic fillers in polymer and the right coating method for putting the ceramic-filled polymer composite material on the copper can minimize the material variation. In addition to the material variation, the process variation (mainly etching variation) in formation of the electrodes by the etching process in PCB manu- facturing will add to the tolerance. In order to understand the capacitance tolerance of the RF CAPACITOR MATERIAL FOR USE IN PCBS continues Figure 5: Frequency dependence of dk and dF of the developed rF capacitor laminate.