In the previous post, we were talking about how the world of high-speed design split. You can see in this: Electrical engineering problem – part 1
This part we are going to see what happens makes PCB is a challenge.
Circuit element sizes
All components mounted in packages have their performance degraded due to unwanted package parasitics, such as lead inductance, load capacitance, and lead-to-lead crosstalk. The faster a component must operate, the more these unwanted parasitics degrade performance. Minimizing the size of the package in which the component is mounted minimizes these parasitics. RF and microwave components are often mounted directly on a PCB without a package. Logic circuits often have hundreds of leads. In order to make room for all of the contacts, the package cannot be small. Therefore, the design rules used with logic circuits must allow for these parasitics.
Layer count
Minimum layer count is always a desirable goal as it keeps the cost of a PCB down. Most of the time, it is possible to implement RF and microwave designs in two or three layers. High-speed digital designs cannot be done in two layers for several reasons. The most obvious is the need for layers to hold all of the signal wires or transmission lines. A not so obvious reason is the need for a robust power distribution system of low inductance planes and the capacitance that exists between them.
Feature accuracy
Odd shapes are visible on the left-hand side of the PCB. These are bandpass filters, power splitters, directional couplers, tuned circuits, and other components. They are formed by etching precise shapes in the copper foil of the outer layer of the PCB. The frequency at which these functions are executed is directly dependent on how accurately they are etched. In a digital PCB, these functions will be performed by discreet components so this kind of feature accuracy is not required. What is needed in a digital PCB is accurate trace width and dielectric thickness control in order to ensure that transmission line impedances are kept within allowable tolerances.
Source: Right the first time
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