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JANUARY 2021 I DESIGN007 MAGAZINE 71 II. Control electromagnetic fields by using dual stripline mixed signal/plane pour layers to isolate critical signals. In Figure 3, there are three technolo- gies on the same stripline layers. This is accomplished by using a combination of copper pours on one or the other signal layers. Trace width and spacing can also be modified to trim the impedance. There are many advantages to this configuration including: • This configuration isolates critical signals and accommodates all the necessary power supplies in complex designs • DDR3/4 data or address busses can be routed on separate regions using the DDR supply and GND as reference planes • Adjacent signal traces for the noncritical digital signals should be routed orthogonally to avoid crosstalk III. Closely couple the signal layers to the reference planes to reduce crosstalk and radiation. After reading all the above, you will now be extremely knowledgeable about stackup and material planning. But what is crucial for a good stackup design? A. Field Solver Precision As mentioned, closed loop equations are in reality just approximations. You can make do with approximations on low frequency, non- critical designs but they just don't cut it in today's high-speed environment. Equations particularly come unstuck in dual asymmetric stripline configurations. The most essential design tool for optimizing the stackup of a PCB is the 2D field solver (e.g. Figure 4). It is used to predict the character- istic impedance, edge-coupled and broadside coupled differential impedance for all topolo- gies, including microstrip, stripline and dual stripline. In addition to the accuracy, the other advantage is its ability to include second-order effects such as trace thickness and the influ- ence of air, solder mask, and multiple adjacent prepreg dielectrics. 3D field solvers are not more accurate than 2D field solvers in stackup design. When interconnects have a uniform cross-section, a 2D field solver can be more precise, faster and much easier to use. To give you an idea, iCD now has a choice of over 700 series of rigid and flexible dielec- tric materials, from over 60 different manufac- turers, in its dielectric materials library. When each material is used for the right target appli- cation, the resultant PCB will have the lowest possible cost while still satisfying the design and performance goals of the project. Choos- ing the best material for an application is often a daunting task. However, you can quickly sort through the vast array of choices, to make an informed decision, with the right tools. Typically, when the impedance of a sub- strate is first calculated, virtual materials are used as the basis. In other words, we choose a Figure 3: Three technologies accommodated on the same stripline layers.