Thermal Effects on PCB Laminate Material Dielectric Constant and Dissipation Factor

Values for printed circuit board (PCB) laminate dielectric constant (Dk) and dissipation factor (Df) used in circuit design and signal integrity (SI) modeling are typically those presented on laminate maker datasheets. In most cases, these values are derived from measurements on samples which have not been exposed to thermal stresses representative of the printed circuit board (PCB) assembly process. This paper discusses the changes in Dk and Df values for a variety of laminate materials following simulated assembly thermal exposure of test vehicles to six SMT cycles at 260°C (Pb-free) or 225°C (SnPb eutectic). An additional concern arises around an effect of operating temperatures upon the effective Dk and Df of PCB materials. Due to thermal radiation from active IC devices, power supplies, etc., the operating temperature of PCBs within a network equipment chassis is typically higher than the 23-25°C value at which Dk and Df are measured and reported. This paper also describes the changes in Dk and Df observed when the test samples were measured at temperatures of 50°C and 75°C.