PCB Classification and How Each Type Is Made
The word printed circuit board covers a very wide range of products. A single sided board in a power supply and a twenty layer backplane share the same manufacturing logic but almost none of the same process steps, and a flexible circuit or a metal core board diverges from both. Understanding the classification is useful because the category determines which capabilities a supplier needs and which design rules apply.
The classification can be approached from three directions: the number of conductive layers, the mechanical nature of the base material and the way the layers are joined. Most boards can be placed in a category from each of those axes, and the combination determines the manufacturing route.
Classification by Layer Count
A single sided board has conductors on one face and often a printed legend on the other. The manufacturing route is simple: drill or punch the holes, print and etch the copper, apply the mask and the legend, then finish and profile the panel. It is the cheapest board to make and is used where the circuit is simple and the volume is large.
A double sided board adds conductors on the second face and plated through holes to connect them. Plating is the step that changes the process, because it requires a conductive path through the hole and therefore a seed layer, either by electroless deposition or by a conductive graphite or polymer process. Multilayer boards extend the same idea, laminating several double sided cores together with prepreg and drilling through the whole stack to connect the layers that need it.
Classification by Base Material
Rigid boards use a woven glass fabric with a thermosetting resin, of which FR4 is the most common, or a specialised laminate where the dielectric properties matter. The material is supplied as a copper clad sheet and processed in flat panels, which is what allows the fine registration that multilayer boards need.
Flexible boards use a polyimide film with rolled or electrodeposited copper, and they are thin enough to bend. A flexible circuit may be a single layer, a multilayer stack with a defined bend area or an adhesiveless construction for the highest flex life. Rigid flex combines the two by embedding flexible cores inside a rigid stack, with the flexible material emerging from the rigid section at the bend.
Classification by Joining Method and Structure
The way the layers are connected produces another set of categories. A conventional multilayer uses plated through holes that pass through the entire stack. A build-up board adds layers sequentially with microvias that connect only the outermost pair, and repeating the sequence produces the higher order HDI constructions in which a via may connect any two adjacent layers.
Metal core and metal backed boards place the circuit on a dielectric bonded to an aluminium or copper base, which changes the process because the base cannot be drilled and plated in the same way as a laminate. Ceramic substrates replace the organic laminate entirely with a material that is metallised by bonding, brazing or thick film printing, and they are used where the thermal and insulation requirements are beyond what a polymer can provide.
How the Classification Drives Design Rules
Each category has its own minimum feature size, its own tolerances and its own set of process limitations. A single sided board can tolerate a wide conductor and a large clearance because the process is cheap and the tooling is simple. A fine line multilayer requires a controlled etch and a controlled lamination to hold the registration, and its minimum line width is correspondingly smaller and its tolerances tighter.
HDI boards push the same limits further, with smaller vias, thinner dielectrics and a stack-up that is built rather than laminated once. The design rules for a high order build-up include restrictions on where a microvia can land and how it must be captured, which do not exist on a conventional multilayer. Applying the wrong rule set is a common source of a design that cannot be built at the quoted price, which is why the classification should be decided before the layout rather than inferred from it.
Choosing Between Categories
The choice usually starts with the density requirement. If the components and the routing fit on two layers, that is the cheapest answer. When they do not, the next question is whether more layers solve it or whether the escape from a fine pitch package requires a build-up structure. A multilayer board with more layers is often cheaper than an HDI board with fewer, until the fanout of the largest device makes the multilayer impossible.
Thermal and mechanical requirements can override the density argument. A board that has to remove several hundred watts will need a metal core or a ceramic substrate whatever its routing density, and a product that has to fold will need a flexible or rigid flex construction. Where several requirements point in different directions, the usual resolution is to use a rigid flex assembly with a metal backed section, which is more expensive than any single category but avoids a separate interconnect. The trade should be documented, because the reasoning is easy to lose and expensive to reconstruct, just as in any other pcb design and fabrication decision.
How the Panels Are Sized and Profiled
The classification also determines how the board is delivered. Rigid boards are produced in standard panels, typically eighteen by twenty four inches or a metric equivalent, and the customer artwork is stepped up to fill the panel as efficiently as possible. Panel utilisation drives a large part of the cost, which is why a board outline that leaves awkward margins is more expensive than the same area in a shape that nests well.
Flexible circuits are produced in rolls or in long panels that follow the shape of the part, and rigid flex parts are usually profiled by laser or by routing after lamination. The profiling method matters because the flexible area must not be damaged, and because the cut edge of a polyimide has to be clean enough not to initiate a tear. Each category therefore has its own set of rules about what the outline may contain, and these belong in the fabrication drawing along with the stack-up.
Where the Categories Overlap
In practice the categories are not exclusive. A rigid board can contain a flexible core; a metal backed board can be a multilayer; a ceramic substrate can carry a build-up layer. The overlap is where most of the interesting engineering happens, and it is also where the process becomes hardest to control, because the requirements of two categories have to be satisfied at once.
Where categories are combined, the fabrication drawing has to state which rules apply in which region. A bend area inside a rigid flex board has a different minimum feature size from the rigid section, and the tolerances around the transition are tighter than either. Making that explicit, rather than assuming the supplier will infer it, is what keeps a combined design manufacturable and keeps the manufacturing tolerances achievable in every region of the part.
Additional Considerations for This Build
Practical attention to PCB classification pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating PCB classification explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.
Deliberate attention to HDI build-up pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating HDI build-up explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.
Related reading: our fabrication notes, board quality and design release notes cover the same ground.
FAQ
Is a four layer board always better than a two layer board? No. A two layer board is cheaper, has fewer process steps and often has better yield. It becomes inadequate when the routing cannot be completed or when a reference plane is required for signal integrity.
What is the difference between HDI and a high layer count board? HDI describes how the layers are added and connected rather than how many there are. An HDI board may have fewer layers than a conventional multilayer but achieve higher density through microvias and thin dielectrics.
Can a flexible and a rigid board be made by the same supplier? Not always. Flexible circuits need different handling, different lamination and often different equipment. Rigid flex requires both, which is why the number of suppliers able to make it is smaller.
