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The Why and How of Switching Layers – from 1 or 2-layer to 4-layer and higher

By pcbpower      November 07,2016

The ever persistent miniaturization of electronic products triggers the manufacture of more densely packed printed circuit boards with increased electronic capabilities. Unfortunately, the lack of enough available space makes it challenging for the single sided and double sided PCBs to match the increasing assembly density.

The Significance of available space for routing the traces in switching from 1 or 2-layer to multilayer

It is a well-known fact that the most basic Single Sided boards have their components mounted on one of their sides and the conductor pattern on the opposite. Such boards have severe limitations when it comes to routing the traces in the conductor pattern. Since it is only on one side that no traces can cross, and have to be routed around each other, it is used merely in very primitive circuits.

On the other hand, the double sided boards have the advantage of sporting a conductor pattern on both sides of the board so that the surface available is two times larger than that in a single sided board. Also the traces can cross by routing them to opposite side of the board with electrical connection between them through vias.

For an even larger available area for the traces the Multilayer boards are equipped with one or more conductor pattern, bonding together several double sided boards by means of heat and pressure. Pre-preg acts as the bonding medium besides serving as the dielectric between the layers. The number of separate conductor patterns indicates the number of layers.

In the case of layer count the upper limit has no restrictions regarding the number of layers. However, 4- and 6-layers dominate the multilayer area occupying up to 90% of the market. Multilayer (28 or more layers) PCBs are rare, highly complicated, and expensive.

In multi-layer PCBs most often the entire layers are exclusive to Ground and Power being given the names – Signal, Power or Ground planes.

In double sided PCBs the vias are known to penetrate the whole board. So if there are multiple layers of conductive patterns of which only a few need to be connected then such vias would waste the space that could otherwise be used for routing the traces on unconnected layers. However, a correct choice of blind and buried vias which penetrate only the necessary layers this problem can successfully be avoided.

Other vital aspects of the PCB layout:

Apart from the above mentioned aspect of routing the traces, there are a couple of other aspects too such as noise, stray capacitance and cross talk which are equally important in the designing of the PCB layout.

It is mandatory that the design of PCB meets the guidelines, minimizing the length of the signal line as also the effect of crosstalk besides avoiding the parallel routes. Moreover, the critical signals need to be routed between power and ground planes to prevent unwanted coupling.

The limited achievable intersections prevent the single-sided PCB or double-sided PCB from meeting the requirement and since the demand for interconnections and crosses is much more, the PCB can achieve a satisfactory performance only if it is expanded to more than two layers.

The number of layers you use for the best resulting PCB depends on how you route the sensitive signals. It is normally just two layers required for the use of micro-strip traces, one of them being a GND plane. But again a four-layer PCB, with a GND and a VCC plane and two signal layers would always be more preferable. If the circuit is complex and signals must be routed as strip-line due to the propagation delay and/or characteristic impedance, a six-layer stack-up would be ideal.

Multilayer PCBs are widely used in various electronics devices for their multiple advantages such as high assembly density, small size, reliability, good quality and so on. They also offer superior reductions in distortion and signal propagation in applications where signal integrity and “cross talk” levels are critical. Although the initial costs of Multilayer PCBs are higher than that of Single Sided or Double Sided PCBs, their benefits are extensive. They seem to have revolutionized the electronics industry by defining the future of printed circuit board as a whole.