Issue link: https://iconnect007.uberflip.com/i/1285883
SEPTEMBER 2020 I DESIGN007 MAGAZINE 31 50/100, while DDR3/4 is 40/80 single-ended/ differential impedance. Controlling impedance simultaneously on each signal layer with a number of different technologies can become a challenge. Unfortunately, your preferred PCB fabricator will not stock these values, so the next step is to convert the virtual materials to actual mate- rials that are available (in the partial stackup of Figure 8). This will also give you the actual materials for the three different technologies (Table 2). This is the step that most, if not all, designers avoid, leaving it to the fab shop CAM engineer. To do this, we first need to determine an operating frequency that the product needs to perform. Generally, we need to consider the bandwidth up to the fifth harmonic of the fun- damental. If we have a clock running at 800 MHz, then 4-GHz material would be appropri- ate. I have chosen the ITEQ IT-180A, 5-GHz material, which is readily available from Asian fabricators and is suitable for low-cost, high- speed applications. The iCD Materials Planner can be used to narrow down the selection from the fabricator's stock. The configuration of the PCB stackup depends on many factors. But whatever the Table 1: The three different technologies using virtual materials. Figure 8: The actual materials using ITEQ, IT-180A (iCD Stackup Planner).