Most PCBs are composed of between one and sixteen (or even more) conductive layers separated and supported by layers of insulating material (substrates) laminated (glued) together. Layers may be connected together through drilled holes called vias. Either the holes are electroplated or small rivets are inserted. High-density PCBs may have blind vias, which are visible only on one surface, or buried vias, which are visible on neither.
Low-end consumer grade PCB substrates frequently are made of paper impregnated with phenolic resin, sometimes branded “Pertinax”. They carry designations such as XXXP, XXXPC, and FR-2. The material is inexpensive, easy to machine by drilling, shearing and cold punching, and causes less tool wear than glass fiber reinforced substrates. The letters “FR” in the designation indicate Flame Resistance.
High-end consumer and industrial circuit board substrates are typically made of a material designated FR-4. This consists of a woven fiberglass mat impregnated with a flame resistant epoxy resin. It can be drilled, punched and sheared, but due to its abrasive glass content requires tools made of tungsten carbide for high volume production. Due to the fiberglass reinforcement, it exhibits about five times higher flexural strength and resistance to cracking than paper-phenolic types, albeit at higher cost.
PCBs for high power radio frequency (RF) work use plastics with low dielectric constant (permittivity) and dissipation factor, such as Rogers RO4000, Rogers Duroid, DuPont Teflon(types GT and GX), polyimide, polystyrene and cross-linked polystyrene. They typically have poorer mechanical properties, but this is considered an acceptable engineering tradeoff in view of their superior electrical performance.
PCBs designed for use in vacuum or in zero gravity, as in spacecraft, being unable to rely on convection cooling, often have thick copper or aluminum cores to dissipate heat from electrical components.
Not all circuit boards use rigid core materials. Some are designed to be very or slightly flexible, using DuPont’s Kapton polyimide film, and others. This class of boards, sometimes called flex circuits, or rigid-flex circuits, respectively, are difficult to create but have many applications. Sometimes they are flexible to save space (PCBs inside cameras and hearing aids are almost always made of flex circuits so they can be folded up to fit into the limited available space). Sometimes, the flexible part of the circuit board is actually being used as a cable or moving connection to another board or device. One example of the latter application is the cable connected to the carriage in an inkjet printer. Power electronic applications require low-thermal resisivity substrates, with thick copper track to carry high currents. The main technologies are ceramic-based substrates (Direct Bonded Copper) and metal-based substrates (Insulated Metal Substrate).
The vast majority of printed circuit boards are made by adhering a layer of copper over the entire substrate, sometimes on both sides, (creating a blank PCB then removing unwanted copper after applying a temporary mask (e.g. by etching in ferric chloride), leaving only the desired copper traces. A few PCBs are made by adding traces to the bare substrate usually by a complex process of multiple electroplating.
Some PCBs have trace layers inside the PCB and are called multi-layer PCBs. These are formed by bonding together (using high pressure in a press) separately etched thin boards.
Holes, or vias, through a PCB are typically drilled with tiny drill bits made of solid tungsten carbide. The drilling is performed by automated drilling machines with placement controlled by a drill tape or drill file. These computer-generated files are also called numerically controlled drill (NCD) files or “Excellon files”. The drill file describes the location and size of each drilled hole.
When very small vias are required, drilling with mechanical bits is costly because of high rates of wear and breakage. In this case, the vias may be evaporated by lasers. Laser-drilled vias typically have an inferior surface finish inside the hole. These holes are called micro vias.?
It is also possible with controlled-depth drilling, laser drilling, or by pre-drilling the individual sheets of the PCB before lamination, to produce holes that connect only some of the copper layers, rather than passing through the entire board. These holes are called blind vias when they connect an internal copper layer to an outer layer, or buried vias when they connect two or more internal copper layers.?
The walls of the holes, for boards with 2 or more layers, are plated with copper to form plated-through holes that electrically connect the conducting layers of the PCB.?
Solder plating and solder resist
The pads and lands to which components will be mounted are typically plated, because the bare copper is not readily solderable. Traditionally, any exposed copper was plated with solder. This solder was traditionally a tin-lead alloy, however new solder compounds are now used to achieve compliance with the RoHS directive in the EU, which restricts the use of lead. Edge connectors, made on the sides of some boards, are often gold plated. Gold plating is also sometimes applied on the whole boards.
Areas that should not be soldered to may be covered with a polymer solder resist coating. The solder resist prevents short circuits between nearby component leads.
Line art and text may be printed onto the outer surfaces of a PCB by silk screening. When space permits the silk screen text can indicate component designators, switch setting requirements, test points, and other features helpful in assembling, testing, and servicing the circuit board. In one sided PCBs, silk screen is also known as the ‘red print’.
In through-hole construction, component leads may be inserted in holes and electrically and mechanically fixed to the board with a molten metal solder.In surface-mount construction, the components are simply soldered to pads or lands on the outer surfaces of the PCB. Often through-hole and surface-mount construction must be combined in a single PCB because some required components are available only in surface-mount packages, while others are available only in through-hole packages.
Test and validation
Unpopulated boards may be subjected to a bare-board test where each circuit connection as defined in a netlist is verified as correct on the finished board. For high-volume production, a Bed of nails tester or fixture is used to make contact with copper lands or holes on one or both sides of the board to facilitate testing. A computer will instruct the electrical test unit to send a small amount of current through each contact point on the bed-of-nails as required, and verify that such current can be seen on the other appropriate contact points. For small- or medium-volume boards, flying-probe testers use moving test?heads to make contact with the copper lands or holes to verify the electrical connectivity of the board under test.
Protection and packaging
PCBs intended for extreme environments often have a conformal coat, which is applied by dipping or spraying after the components have been soldered. The coat prevents corrosion and leakage currents or shorting due to condensation. The earliest conformal coats were wax. Modern conformal coats are usually dips of dilute solutions of silicone rubber, polyurethane, acrylic, or epoxy. Some are engineering plastics sputtered onto the PCB in a vacuum chamber. Mass-production PCBs have small pads for automated test equipment to make temporary connections. Sometimes the pads must be isolated with resistors.