PCB Technical - How to design PCB laminated signal plane stacking

Each layer in a PCB plays a specific role in determining electrical behavior. The signal plane layer carries power and electrical signals between components, but unless you place the copper plane correctly in the inner layer, they may not work properly. In addition to the signal layer, your PCB needs a power supply and ground layer, and you need to place them in the PCB stack to make sure the new board works.

Where are the power, grounding and signal layers placed? This is one of the long-debated issues in PCB design, forcing designers to carefully consider the intended use of their boards, the function of their components, and the signal tolerance on the boards. If you understand the limits of impedance variation, jitter, voltage ripple and PDN impedance, and crosstalk suppression, you can determine the correct arrangement of signal layers and plane layers to be placed on the board.

In general, if your proof-of-concept works on a breadboard, you can use any preferred layout technique on a two-layer board, and the board will most likely work. You may need to use a grid grounding approach to handle high-speed signals to provide a degree of EMI suppression. For more complex equipment running at high speed or high frequency (or both), you need at least four PCB layers, including the power layer, the ground layer, and two signal layers.

When determining the required number of signal plane layers, the first consideration is the number of signal networks and the approximate width and spacing between signals. When you try to estimate the number of signal layers required in a stack, you can take two basic steps:

Determine the net count: The number of signal layers required on the board can be estimated using the simple net count in th

e schematic and the proposed board size. The number of layers is usually proportional to the fraction (net * line width)/(board width). In other words, more networks with wider lines require larger boards or more signal layers. You must use the experience here by default to determine the exact number of signal layers needed to accommodate all the networks at a given board size.

Add your flat layer: If you need to route the signal layer with controlled impedance, you now need to place a reference layer for each controlled impedance signal layer. If the components are densely packed, a power plane is required below the component layer, because there is not enough space on the surface layer to accommodate the power guide. This may result in a double digit number of surface layers required for a high net worth HDI board, but the reference layer will provide shielding and consistent characteristic impedance.

Once the correct number of layers for the multilayer is determined, you can proceed to arrange the number of layers in the PCB stack.

Design PCB lamination

The next step in PCB lamination design is to arrange each layer to provide routing. Your laminations are usually arranged symmetrically around the central core to prevent warping during high-temperature assembly and operation. The layout of the plane layer and the signal layer is critical for impedance-controlled wiring because you need to use specific equations for different wiring arrangements to ensure impedance control.

For rigid-flexible laminate design, you need to define different regions in the laminate for rigid-flexible regions. The layer-stack design tool in Allegro makes this process easy. After capturing the schematic as a blank PCB layout, you can define the layer stack and transition through the different layers. You can then proceed to determine the wiring dimensions required for controlled impedance wiring.

Ribbon lines and microstrip lines and controlled impedance

In order to control the impedance, the wiring of the inner layer between two planar layers should be designed using the strip line impedance equation. The equation defines the geometry required for a ribbon line to have a specific characteristic impedance value. Since there are three different geometric parameters in the equation to determine the impedance, the simple approach is to first determine the number of layers required, as this will determine the layer thickness for a given plate thickness. The copper weight of the internal signal plane layer is usually 0.5 or 1oz./sq. The ft. This uses the line width as a parameter to determine the impedance of a particular characteristic.

The same process applies to microstrip lines on the surface layer. After determining the layer thickness and copper weight, you only need to determine the line width used to define the characteristic impedance. PCB design tools include an impedance calculator that can help you determine the size of the wiring so that they define the cha

racteristic impedance. If a difference pair is required, simply define the lines in each layer as a difference pair and the impedance calculator will determine the correct spacing between the lines.

They may be electrically or inductively coupled to other traces and conductors during wiring on the actual board. Parasitic capacitance and inductance from nearby conductors can change the wiring impedance in the actual layout. To ensure that you have reached your impedance targets for all layers in the stack, you need an impedance analysis tool to track the impedance across the entire selected signal network. If you see unacceptable large changes in the PCB layout, you can quickly select wiring and adjust the wiring to eliminate these impedance changes in the interconnect.

The large impedance changes along the trace are marked in red. The spacing between traces in this area should be adjusted to eliminate this impedance change or to bring it within acceptable tolerances. You can define the desired impedance tolerances in the design rules, and after layout the impedance calculator tool will check the wiring against the desired impedance values.

In the above discussion, we have studied only digital signals because they are more demanding than analog systems. What about a full analog board or a mixed signal board? For analog boards, power integrity is much easier, but signal integrity is much more difficult. For mixed-signal boards, you need to combine the digital approach shown above with the analog approach described here.

Signal isolation

Another option is more, requiring the use of grounded copper powder or fencing to ensure isolation between different parts of the board. If ground casting is performed next to analog wiring, a coplanar waveguide with high isolation has just been created and is a common choice for routing high-frequency analog signals. If fences or other high-frequency conductive isolation structures are to be used, an electromagnetic field solver should be used to check isolation and determine whether isolation in a different signal layer should be chosen.

The return trip plan

The mixing of analog and digital signals on the plate imposes strict requirements on tracking the displacement current of the grounding loop and the isolation between the digital and analog plate parts. The circuit board shall be arranged to ensure that analog return paths do not cross near the digital components and vice versa. This simply divides the digital and analog signals into different layers separated by their respective ground layers. Although this adds to the cost, it ensures isolation between the different parts.

If the analog component is extracted from an AC power supply, the analog component may also require a dedicated analog power board. Outside of power electronics, this is a rare situation, but conceptually easy to handle as long as you can analyze return path planning. A single power plane can be dedicated to both signals if the analog power portion is placed upstream of and separated from the digital signal portion. If the return path is properly planned, interference between the different power and grounding parts can be prevented. For a DC power supply with a switching regulator, switching noise from the DC part needs to be separated from the AC part, just as digital signals need to be separated from analog signals.