PCB Types and How Each One Is Built

Printed circuit boards are usually discussed as if there were one kind. In practice there are several constructions, each with its own process, its own cost structure and its own appropriate use. Choosing between them is one of the first decisions in a design, and it determines a great deal of what follows.

The Simplest Construction: Single Sided

A single sided board carries copper on one face only. Components are placed on that face, and the circuit is completed without any connection to the other side.

Its advantages are cost and simplicity. There is one imaging step, one etching step and no plating through holes unless holes are needed to mount components. Its limitation is that conductors cannot cross without a jumper, so it suits simple circuits: power supplies, control panels, and products where the routing is naturally planar. As soon as the circuit needs a crossover, the design has to move up to the next construction.

Double Sided Boards

A double sided board carries copper on both faces, with plated holes connecting them. The plated through hole is what makes this possible: a hole is drilled, the barrel is metallised, and the two layers become one electrical network.

That single feature removes the crossover limitation entirely and allows dense routing on both faces. Double sided construction is the workhorse of consumer electronics and industrial control, and it is usually the cheapest way to build a circuit of moderate complexity. Our fabrication notes describe the plating step that makes it work.

Multilayer Boards

A multilayer board stacks several double sided cores with insulating prepreg between them, pressed into a single rigid panel. The inner layers are imaged and etched before lamination, and the outer layers are processed after.

Two things drive the move to multilayer construction. The first is routing density: inner layers provide additional routing space that cannot be obtained any other way. The second is electrical performance: an inner layer can be dedicated to a power or ground plane, which gives every signal a nearby reference. Those two reasons often appear together, and the layer count follows from whichever demands more. Our layer assignment article describes how the arrangement is decided.

Rigid, Flexible and Rigid Flex

Most boards are rigid, using a glass reinforced epoxy laminate as the base. A rigid board holds its shape, supports components without additional structure and is the default for anything that will be mounted in a case.

A flexible PCB uses a thin polymer film as its base, with copper bonded to it and covered by an insulating layer except where solder joints are needed. Because the base is thin and compliant, the board can be folded and bent to fit a shape that a rigid board cannot. Where a design needs rigid sections for components and flexible sections for the connection between them, the two are combined in a rigid flex construction. Our article on flexible PCB construction covers the process detail.

Metal Core Boards

A metal core board replaces the usual insulating base with aluminium or copper, separated from the circuit by a thin thermally conductive dielectric. The purpose is heat removal rather than routing, and the construction is usually single sided because the metal base has no room for a second circuit layer.

These boards are used where heat flux is high and local, such as under power LEDs and large semiconductors. The metal base spreads the heat laterally and conducts it into whatever the board is mounted against, which is a job that copper planes on a conventional board can only do less effectively. Our metal core PCB article describes the trade-offs.

HDI and High Density Construction

High density interconnect construction uses finer features, smaller vias and often laser drilled microvias to achieve routing density that conventional multilayer boards cannot reach. The vias may be blind, buried or stacked, and the layers may be built up sequentially rather than laminated in one operation.

The cost rises sharply with each additional feature, so the construction is used where the product genuinely requires it: dense processor boards, mobile devices and modules where the total volume is small. The structural considerations overlap with those described in our blind and buried via article.

Choosing the Right Construction

The choice follows from the circuit rather than from a preference. Count the nets that must cross each other, decide whether a reference plane is needed, check whether the product must bend or fit a curved space, and establish whether heat has to be removed through the board.

Those four questions narrow the options quickly. A design that needs a reference plane and has moderate density is a four layer rigid board. One that must fold into a housing is a flexible board, possibly with stiffeners where components are placed. One that must survive a high local heat flux may need a metal core. Starting from those answers rather than from a familiar construction is what keeps the cost proportionate.

Cost and Where It Comes From

Cost tracks process steps rather than board area. A single sided board has the fewest steps, a double sided board adds drilling and plating, a multilayer board adds imaging, lamination and registration control for every layer, and HDI adds laser drilling and sequential build.

Flexible materials cost more than rigid laminates and are harder to handle, and metal core boards require machining of the metal base. Understanding this makes the cost comparison between constructions predictable, and it explains why reducing layer count or feature size is usually the largest available saving. Our lead time article describes the schedule side of the same trade.

Substrate Choices and What They Change

The substrate determines the electrical and mechanical behaviour of the finished board before any circuit is drawn on it. Standard FR-4 is adequate for the majority of designs, and it is stocked in a range of thicknesses and copper weights that cover most requirements. Where the design needs lower dielectric loss, higher thermal performance or tighter dielectric constant tolerance, a different substrate is specified and the cost rises accordingly.

The choice should follow from the requirement rather than from habit. A substrate selected for a dielectric constant the design does not rely on, or for a loss figure the interface does not approach, adds cost to every board without improving the product. Recording which requirement drove the choice is what prevents it from being removed later by someone optimising cost.

How Construction Affects Assembly

The construction decided at design stage follows the board through assembly. A thicker board needs a reflow profile that accounts for its thermal mass, a flexible circuit needs a carrier to hold it flat, and a metal core board needs support because it will not flex into a fixture.

Panelisation and thickness also determine whether the assembly line can handle the board at all. These are not details that can be settled after the design is frozen, because they affect the tools and the fixtures that have to be ordered. Our PCBA production flow article describes where those decisions sit in the sequence.

FAQ

Is a multilayer board always better than a double sided board? No. It offers more routing space and better reference planes, and it costs more. Where a double sided board can carry the circuit, it is the cheaper and faster answer.

What makes a PCB flexible? The base material. A thin polymer film with thin copper bends without cracking, while a glass reinforced epoxy laminate does not.

Can a flexible PCB carry the same circuits as a rigid one? Yes, within limits on current and on the number of layers. Where components must be mounted, the flexible section usually needs a stiffener.

Why is a metal core board usually single sided? Because the metal base is solid. Adding a second circuit layer requires another insulating layer, which adds thermal resistance and cost.

Which construction is best for a prototype? The one the product will use, unless the design is still in doubt. Prototyping in a different construction hides the problems that the chosen construction would reveal.