Issue link: https://iconnect007.uberflip.com/i/1199133
22 DESIGN007 MAGAZINE I JANUARY 2020 microstrip (solder mask coated microstrip), the electromagnetic field propagates partially in the dielectric material, solder mask, and air. Whereas in both stripline structures, the elec- tromagnetic field propagates in the dielectric material sandwiched between the planes. Interconnect impedance is a function of the geometry of the conductors and the dielectric constant of the material adjacent to or sepa- rating them. For PCB traces, the most criti- cal dimension is the ratio of trace width to height above/below the reference plane(s). Impedance is also inversely proportional to the square root of the dielectric constant. Clearly, the accurate control over impedance requires precise management of both the physical ge- ometries and the material characteristics along the entire length of the interconnect. Figure 2 illustrates the variation of imped- ance with the three most influential variables: trace width, dielectric thickness, and dielectric constant. These impedance plots were simulat- ed by multiple passes of the field solver in the iCD Stackup Planner. Note that the microstrip impedance (top row) varies almost twice as much as the stripline impedance (bottom row) to the same changes in the variables. Conse- quently, microstrip transmission lines are more vulnerable to change in impedance, which is another good reason not to route critical sig- nals on the outer layers. Any slight variation in any of the total of five variables (including copper thickness and differential clearance) will dramatically change the localized imped- ance of a microstrip interconnect. These are physical properties of the multilayer PCB that the fabricator must control to maintain a con- stant impedance. Having a PCB fabricated to controlled imped- ance specifications does not necessarily control the impedance of your routed traces; it only con- trols the impedance of the test coupons. Only you can control the impedance of the signal in- terconnect. As technology progresses, develop- ers are specifying controlled impedance boards more frequently. The PCB fabricator does their best to control the impedance, of the bare board, given all the manufacturing variables. The fab- ricator will initially predict the stackup trace im- pedance using a field solver. They should then place impedance test coupons on the outer edge of the PCB to check that the manufactured prod- uct matches the predicted impedance using a time-domain reflectometer (TDR). Figure 2: Comparison of microstrip and stripline impedance variations. (Source: iCD Stackup Planner)