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46 The PCB Magazine • October 2017 The low dielectric constant results from the use of many new HDI materials. Many of these materials are not glass-reinforced and thus have lower dielectric constants than glass-reinforced laminates. Many of the dielectrics are liquid such as the high-Tg epoxy or polyimide, or the photodielectric resins (PDR). Some materials are thin, vacuum-laminated dielectrics with high thermo-plastics contents. However, all materi- als are uniformly thin—this contributes to reductions in wiring delays and re- duction in noise. Several of these new materials and their electrical character- istics are shown in Table 2. The relative cost is based on high-volume usage in Asia, including dealer markups. Figure 3 shows the characteristic im- pedance for a fine-line HDI microstrip (3-mil trace) with two dielectrics of Dk equal to 3.5 and 4.5. The lower Dk al- lows the dielectric thickness to be near- ly one to one-half mil thinner. Crosstalk between Two or More Nets HDI miniaturization provides shorter interconnect lengths, and if the lower dielectric constant materi- al is used, then cross talk in HDI sub- strates is reduced. Bogatin provides the following example: "A typical line width in HDI technology is 3 mils (75 microns). Figure 3 shows the char - acteristic impedances of 3-mil traces for vari- ous dielectric thicknesses. The dielectric thick- ness will be less for a lower dielectric constant. This means a lower dielectric constant material system will either result in less crosstalk for the same spacing, or the traces can be moved closer together and have the same amount of crosstalk" [3] . Bogatin continues, "The variation in the near-end cross talk coefficient with separation for two, 50-ohm microstrip traces is shown (Fig- ure 4). In the two cases studied, the line-width was 3 mils, and the dielectric thickness was ad- justed so that for the two different dielectric constants, the line impedance was the same. From these curves, it can be seen that if the rout- ing pitch is cross talk constrained, just the low- er dielectric constant of the HDI material sys- tem may allow a board to shrink up to 28%. For coupled lengths less than the saturation length, the magnitude of the near-end voltage noise will scale with length. The saturation length will depend on the rise time. For a rise time of one nanosecond, the saturation length with an effective dielectric constant of 2.5 is about 7.6 inches, which would include many of the trac- es in a small card application. The relative cou- HDI'S BENEFICIAL INFLUENCE ON HIGH-FREQUENCY SIGNAL INTEGRITY Table 2: HDI materials and their electrical characteristics. Figure 3: Characteristic impedance of lower dielectric constant materials.