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APRIL 2024 I PCB007 MAGAZINE 87 ity for 1993. e solution to the loss of nearly 5.8 square inches out of 17.5 square inches was to employ microvias and microvia-in-pads. e new microvia board (called Lynx) was designed with a reduced surface area and as a six-layer design (1+4+1), two fewer layers than the original. Reading about the IMPS topolog y from HiDEC in 1994, the Lynx board was again rede- signed to a four-layer construction. To mini- mize the microvias, the outer two layers (1 and 4) were flooded with ground, and only power and signals were placed on the inner layers. Figure 1 shows the new power-signal routing architecture, which was called power mesh to differentiate it from IMPS. Electrical Model e original Lynx board was not a controlled impedance, but additional PCB designs were that used power mesh. e consensus is that power mesh is an offset coplanar stripline. Figure 2 shows this cross-section of the offset coplanar stripline. e table shows the values for 50-ohm single-ended and 100-ohm differential imped- ances for different trace widths, spacings, core thicknesses, and overall thicknesses. e crosstalk model indicates that the PMA creates a naturally low crosstalk condition. Each signal trace of X width is approx- imately 3X or 4X distance from the next signal, depending on the power trace width. is cre- ates horizontal crosstalk of less than 2%. The vertical crosstalk is extremely low. From 15 mV/V for thin cores (0.012") to 2.6 mV/V for a thick core (0.051"). PMA Application e first PMA was completed in 1994. e Lynx multilayer is shown in Figure 3. e inner layer FR-4 core (Figure 3a) was 12 mils thick (0.012"). e initial design used epoxy-resin coated copper foil of about 2-mils thick as the microvia layers 1 and 4. e micro- vias were 7 mils in diameter with 14-mil pads. Traces and spaces were 5 mils. Figure 3b shows the finished power mesh multilayer. Without any traces on the sur- face—as all components had via-in-pads—the unbroken ground plane serves as the effec- tive ground return and impedance reference. It was also highly effective as an RFI/EMI shield. Switching noise was reduced because the ground connections were micro-resistance and had no inductive or capacitive elements in series to the ground connection. Similarly, noise budgets were improved because the con- nection to power had the minimum induc- tance and capacitance, nearly 1/10 that of a through-hole and trace connecting the com- ponent land. Designing With PMA e one discouraging characteristic of the PMA is that EDA tools do not recognize nor automatically design with the PMA. at does Figure 3: Example of a power mesh design: a) Inner layer (L2 & L3) with buried vias; b) Finished four-layer power mesh multilayer with microvia-in-pad and unbroken ground plane. a b