PCB stackup basic

This Application Note details tried and proven techniques for planning high speed Multilayer_ PCB Stackup configurations. There will probably be as many teories as there are designers out there, but this is a “keep it simple” article to give you a start.

There are two major applications areas, RF/analog and Digital.

The major differences between these two areas are the ability of the PCB involved to tolerate signal losses and the complexity of the PCB.

RF/Analog applications are characterized by the need for low dielectric losses, low leakage, a need for a low and uniform dielectric constant (Dk) followed by a low layer count. Since this type of PCB “tends” to be small, cost of the dielectric material has less effect on overall product cost than on other components. As a result, using more exotic and expensive materials to meet performance goals is acceptable.

For this class of PCB, choosing a material based on it’s Dk characteristics and losses usually dominates over other considerations.

Digital applications are characterized by high layer counts, a large numbers of drilled, plated holes, and they can have buried, blind and very small via holes.

The processing costs associated with registering, laminating, bonding many layers, drilling and plating ease, usually dominate the choice of materials. Absolute Dk value of the insulating material is important, but less important than processing costs and dimensional stability.

As a result, woven glass reinforced materials are nearly always required. The choice of resin system used with the glass reinforcement is made based on keeping Z-axis expansion within acceptable limits. Also CAF materials (Conductive Anodic Filament) shall be considered for very complex PCBs. The thicker the PCB, the higher the Tg must be to produce a reliable PCB. Digital applications are nearly always subjected to pricing pressures, so material choices must be made to just achieve performance requirements without adding extra cost.

Planning the multilayer_ PCB stackup configuration is one of the most important aspects in achieving the best possible performance of a circuit. 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 PCB 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.

In contrast, a properly built 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 improve manufacturability of the board.

Suppressing the noise at the source rather than trying to elevate the problems once the board 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.