PCB Layer Stack: What Every Layer Does

The Anatomy of a Board

A printed circuit board is a sandwich of conductive layers separated by insulating ones. The conductor is copper foil, patterned into traces, planes and pads. The insulator is a resin reinforced with glass fibre, supplied either as a core, which is a fully cured sheet with copper on both sides, or as prepreg, which is a partially cured sheet that flows and bonds during lamination.

On the outside, a solder mask protects the copper and defines the openings where joints are made, a legend identifies the components, and a surface finish protects the exposed copper and makes it solderable. Those layers are visible, and the internal structure is not, which is why the stack has to be specified rather than assumed.

The Two Kinds of Copper Layer

Signal layers carry the routing. They may also carry a copper pour, which is a region of copper connected to a net, usually ground, that fills the unused area. A pour improves the thermal spreading and provides a reference surface, but it is not a substitute for a proper plane when the return path has to be controlled.

Plane layers are largely unbroken copper connected to a supply or to ground. A ground plane gives every signal above or below it a defined return path and a controlled impedance, and it shields the layers on either side from each other. A power plane distributes a rail with a very low impedance, and a power and ground plane pair forms a distributed capacitance that helps at high frequency.

Most stacks also contain mixed layers, where a plane is used for power distribution in one region and routing occupies another. A plane split is acceptable when it does not cross the path of a signal that references it; otherwise, the return current has to detour and both the impedance and the radiation suffer.

Core, Prepreg and Thickness

The dielectric thickness between two copper layers determines the impedance of a trace on either side of it, and it is built up from the core and prepreg thicknesses available. The fabricator has a defined set of materials, and the stack is assembled from them, which is why a stack design usually starts with the laminate supplier and the plant rather than with an arbitrary dimension.

Two properties of the dielectric matter for the design: the dielectric constant, which sets the geometry for a given impedance, and the glass transition temperature, which sets the thermal margin through assembly and operation. A higher glass transition temperature usually comes with a higher cost, and it is worth paying for when the board runs warm or the assembly is demanding.

How a Stack Is Built

The layers are laminated under heat and pressure. The prepreg flows and cures, bonding the cores together and filling the space around the copper features. The sequence, the pressure, the temperature and the time determine the final thickness and the integrity of the bond.

Two rules govern the design of the stack itself. The first is symmetry: the construction above the mechanical centre should match the construction below it, so that the stresses from the copper and the resin cancel and the board stays flat through lamination and reflow. The second is copper balance: a layer with a large copper area opposite a layer with very little produces a local stress that shows as warpage, and the imbalance is corrected by adding a balancing copper feature rather than by changing the stack. Our notes on PCB manufacturing describe how the lamination is controlled.

Typical Stacks by Layer Count

• Single sided: one copper layer on a rigid substrate. No planes, no vias, the cheapest construction, used for very simple circuits.
• Double sided: two copper layers with plated through holes. No dedicated plane, but a ground pour on one side improves the noise behaviour considerably.
• Four layers: the classic arrangement is signal, ground, power, signal. Every signal layer has an adjacent plane, the power distribution is low impedance and the stack is symmetrical.
• Six layers: typically signal, ground, signal, signal, power, signal, or signal, ground, signal, power, ground, signal. It provides four routing layers and two planes in a symmetrical stack.
• Eight layers and above: additional plane and signal layers arranged so that every signal layer has an adjacent reference, used for dense boards with several high speed interfaces and multiple supply rails.

The specific arrangement matters more than the count. A six layer board with a poor arrangement can perform worse than a well designed four layer board, because the value of the planes depends on what is adjacent to what.

The Reference Plane Principles

Keep a plane adjacent to every signal layer. The return current of a trace flows in the nearest plane, and the impedance depends on the distance to it. A signal layer with no adjacent plane has an undefined impedance and a return path that can take a long and radiating route.

Do not cross a plane split. Where a plane is divided, the return current cannot follow the trace across the boundary, so it detours around the split, which increases the loop area and the radiation. Where a signal must cross a split, provide a stitching capacitor close to the crossing.

Provide a return via at every layer change. A signal that changes layer changes its reference plane, and the return current has to change with it. A via connecting the two planes close to the signal via gives the return path somewhere to go.

Keep the plane pair close. A thin dielectric between the power and ground planes increases the distributed capacitance and lowers the impedance, which helps the high frequency decoupling.

Special Layers

Stacks can include layers that are not simply signal or plane. An embedded capacitance layer uses a thin high dielectric constant material to form a large distributed capacitor. A shielding layer is a copper plane placed specifically to isolate two regions of the board. A thermal layer is a thick copper area used to spread heat from a device. A stiffener is a mechanical layer bonded to a flex circuit, not a copper layer at all.

Specifying the Stack

A stack specification should give the fabricator the layer order, the function of each layer, the dielectric thicknesses and materials, the copper weights, the finished thickness and its tolerance, the impedance targets and the layer in which the impedance is referenced, the via types and the surface finish. With that information the fabricator can confirm that the stack is buildable, propose an alternative from the materials available and produce the coupon that proves the impedance.

The design should not be released with a stack that has not been confirmed by the plant that will build it, because the dielectric thicknesses that the fabricator actually uses determine the impedance the design will get. Our notes on PCB design and layout cover the design side of that conversation, and our notes on PCBA testing cover the verification of the finished board, while our notes on quality management describe how the process is controlled in production.

FAQ

What is the difference between a core and prepreg? A core is a fully cured sheet of laminate with copper on both sides; prepreg is a partially cured sheet that flows and bonds during lamination. The stack is built from a combination of both.

How many layers does a board need? A simple circuit needs two. A design with high speed signals and several supply rails usually needs four or six, so that every signal layer has an adjacent reference plane.

Why does the stack have to be symmetrical? Because the stresses from the copper and the resin have to cancel. An asymmetrical stack warps during lamination and again during reflow.

Can a plane be split? Yes, if no signal that references it crosses the split. Otherwise the return current has to detour, which increases the loop area and the emissions.

Who should design the stack? The designer defines the function of each layer and the impedance targets, and the fabricator translates them into the core and prepreg combination that produces the required thicknesses.

Conclusion

A PCB layer stack is a mechanical and an electrical structure at the same time. The signal layers carry the routing, the plane layers provide the reference and the power distribution, the core and prepreg set the dielectric thickness and therefore the impedance, and the symmetry and copper balance keep the board flat. Decide the function of each layer first, let the fabricator build the physical stack from it, and confirm the impedance on a coupon before the design is released.