Planning the multilayer PCB stackup configuration is one of the most important aspects in achieving the best possible performance of a product. A poorly designed substrate, with inappropriately selected materials, can degrade the electrical performance of signal transmission increasing emissions and crosstalk and can also make the product more susceptible to external noise. These issues can cause intermittent operation due to timing glitches and interference dramatically reducing the products performance and long term reliability .The good thing to built right PCB substrate can effectively reduce electromagnetic emissions, crosstalk and improve the signal integrity providing a low inductance power distribution network. And, looking from a fabrication point of view, can also improved manufacturability of the product.
Suppressing the noise at the source rather than trying to elevate the problems once the product has been built makes sense. Having the project completed ‘Right First Time’ on time and to budget means that you cut costs by reducing the design cycle, have a shorter time to market and an extended product life cycle.Boards containing copper planes allow signals to be routed in either microstrip or stripline controlled impedance transmission line configurations creating much less radiation than the indiscriminate traces on a two layer board. The signals are tightly coupled to the planes (either ground or power) reducing crosstalk and improving signal integrity.
Planes, in multilayer PCB’s, provide significant reduction in radiated emission over two layer PCBs. As a rule of thumb, a four layer board will produce 15 dB less radiation than a two layer board .
What should we consider when selecting a multilayer stackup ? • A signal layer should always be adjacent to a plane. This limits the number of signal layers embedded between planes to two and top and bottom (outer) layers to one signal. • Signal layers should be tightly coupled (<10 MIL) to their adjacent planes • A power plane (as well as a ground) can be used for the return path of the signal. • Determine the return path of the signals (which plane will be used). Fast rise time signals take the path of least inductance which is normally the closest plane. • Cost (the boss’s most important design parameter).
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