Issue link: https://iconnect007.uberflip.com/i/1285883
32 DESIGN007 MAGAZINE I SEPTEMBER 2020 requirements, one should ensure that the fol- lowing rules are followed to avoid a possible debacle: • All signal layers should be adjacent to and closely coupled to an uninterrupted refer- ence plane, creating a clear return path and eliminating broadside crosstalk. • There is good planar capacitance to reduce AC impedance at high frequencies. • High-speed signals should be routed between the planes to reduce radiation. • The substrate should be symmetrical with an even number of layers, which prevents the PCB from warping during fabrication and reflow. • The stackup should accommodate a number of different technologies. • Cost—the most important design parameter—should also be addressed. Take charge of the impedance selection pro- cess and learn to control the variables rather than just handing the decisions over to the fab shop. This should be done very early in the design process to eliminate the need for change at a later stage, which could require the rerout- ing of critical nets and result in delays in the development schedule. Key Points • Impedance is the key factor that controls the stability of a design. • The substrate is the most important com- ponent of the assembly and needs to be planned correctly to maintain consistent impedance across layers, avoid uninten- tional signal coupling, and reduce electro- magnetic emissions. • PCB fabricators know their production process extremely well, and their manufacturing expertise is invaluable to produce a high-performance substrate. • Trace width increases as impedance decreases, so start with the highest impedance technology first. • 1.4 mil is a good first choice for copper thickness unless you have a high layer count stackup. • Differential signals that are closely coupled will operate mainly in the differential mode with some common-mode radiation from imbalances in the signals. • If the two traces are separated enough to prevent coupling, then both act as single- ended signals. A 100-ohm differential pair becomes two individual 50-ohm single- ended signals. • The combination of trace clearance and dielectric thickness provides the point of coupling. Use the point of coupling to define the crosstalk trace clearance constraints. • All signal layers should be adjacent to and closely coupled to an uninterrupted reference plane. This provides the shortest loop area and the lowest inductance for the return current path. • Glass weave skew and pile unevenness can be prevented by using two plies of 1067 glass prepreg combined. Table 2: The actual materials for the three different technologies.