Relationship between the Dielectric Properties and Thickness of FR-4 Laminates and Implications for Controlled Impedance Applications.

The relationship between the dielectric properties and thickness of FR-4 laminates was investigated. Thickness variations were predominantly due to differences in the glass- to-resin ratio. An empirical equation was derived that gives the dielectric constant of an FR-4 laminate, epsilon' sub (lam), as a function of the volume fraction of resin, V sub (rsn), and the frequency at which the measurement was performed (between 1 kHz and 1 GHz). An alternative expression was also determined that permits the calculation of epsilon' sub lam in the same frequency range from just the dielectric thickness of the laminate (providing the effective thickness of the reinforcement is known). These relationships should be a valuable aid to electrical designers and others who need to know dielectric properties of laminates at different frequencies. The impact of changes in dielectric thickness, and consequent changes in epsilon' sub (lam), upon characteristic impedance, Z sub 0, was assessed. Since, epsilon' sub (lam) increases with decreasing laminate thickness, variations in Z sub 0 produced by changes in thickness are compounded by consequent changes in epsilon' sub (lam). The current thickness specification (8+-1.5 mils) used by AT&T-TS/RG for purchasing nominal 8 mil core C-stage FR-4 laminates allows a large tolerance in Z sub 0 of typically 20-30%. This tolerance in Z sub 0 does not include possible line width variations. Close attention to substrate thickness is necessary for impedance control.