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December 2017 • The PCB Design Magazine 61 ically by 13-17%. The speed of propaga- tion of digital signals is independent of trace geometry and impedance. If you are aware of this issue, then the trace delays (as shown in Figure 3) can be matched to compensate for the varying flight time, so that at the nomi- nal temperature, all signals running on either microstrip or stripline will arrive at the receiver simultaneously. How- ever, this does not solve the problem of temperature skew between microstrip and stripline layers. Figure 4 shows the impact of temperature on Rogers' high- speed RO4003 laminate. The black curve is measured while the red line is the cal- culated -40 to 90 o C temperature range. Rogers' materials are specifically devel- oped for high-speed designs. However, over this temperature range, the dielec- tric constant (Dk) of other materials can vary as much a 10% depending on the stability of the material. And since the propagation velocity is proportional to the square root of the dielectric con- stant, it will vary about ~5% over the same temperature range. Dielectric loss (Df) also tends to increase as temperature in- creases. In contrast, microstrips react differently as the surrounding medium is a mixture of dielec- tric (prepreg and solder mask) and air. When the temperature changes, the portion of the field travelling in the FR-4 is affected, but the portion of the field in air is not. This is because the dielectric constant of air is steady and does not vary with temperature. As a result, the tem- perature coefficient for microstrip traces is less severe than for stripline. This implies that even if the delay of microstrip and stripline traces are perfectly matched, they will still vary indepen- dently with temperature. This skew could be as much as +/- 20ps, which eats away a large por- tion of the board level timing budget of high- speed signals. The obvious way around this issue is to ei- ther route entirely on microstrip or entirely on stripline. Since microstrip traces tend to ra- diate emissions, and there is limited space on the outer layers due to component footprints, one would be well advised to only route critical signals on inner stripline layers. Microstrip and stripline traces should not be mixed, except for the unavoidable short fanouts from the IC lands to interconnecting vias. Points to Remember: • A transmission line does not carry the digital signal itself, but rather guides electromagnetic energy from one point to another. • Signals travel at the same speed, given the same medium. • Digital signals travel as a transverse wave which is characterized by particle motion that is perpendicular to the wave energy. • The speed of a computer does not depend intrinsically on the speed of electrons, but rather on the speed of energy transfer between electronic components. • The speed of this wave varies depending on the layer, in the multilayer substrate, and the surrounding dielectric materials. SIGNAL FLIGHT TIME VARIANCE IN MULTILAYER PCBS Figure 4: Dielectric constant vs. temperature (top), and dielectric loss vs. temperature (bottom). Source: Rogers Corporation