Under normal handling conditions, warp yarns are under tension and remain straight, while fill yarns should remain at right angles to them. However, for some reason, if some of the fill yarns in the fabric move away from their 90° position, the laminate or multilayer may develop a ripple or twist. For the fabricator, it is very important to have raw fabric with undistorted yarns. However, even for fabric with undistorted yarns, warpage can still occur unless the laminator takes care to align the yarns in one sheet of prepreg relative to another.
It is not easy to specify or measure the dielectric constant of the laminate, as it depends not only on its intrinsic properties, but also on the method of testing, conditioning of the sample before and during the test, and the test frequency. Moreover, dielectric constant tends to vary with temperature.
At ABP, we determine the characteristic impedance of a PCB based on the laminate thickness, its dielectric constant, and the height and width of the etched line height. Impedance matching and control are critical to linked functional modules when dealing with high-speed devices and designs.