Important PCB Terminology for Beginners
For a customer, especially for those starting out in the electronic industry, it is imperative to talk the same language as the contract manufacturer. This ensures the possibility to create an accurate embodiment of the design intent without going through painful quote delays, board respins, and redesign efforts. It requires all stakeholders to communicate precisely for a successful board development. Therefore, Rush PCB Inc recommends customers to add PCB design terminology to their foundation database. Here is a list of important PCB terminology.
Vias help to pass current from one layer to another. For a proper board development, it is essential to choose the best PCB via options, as this may affect the turnaround time and increase costs during manufacturing.
Traces on a PCB are small transmission lines that the fabricator etches from a layer of copper. The board’s signal integrity depends on the parameters of the PCB trace—length, width, impedance, and copper weight. Some applications, such as those with high-speed designs, require coordination between trace design and material selection to ensure the best signal routing and integrity.
Silkscreen on a board represents an image layer depicting polarity symbols, component orientation indicators, reference indicators, labels, component footprints, and other identifications. A silkscreen is a constituent step in completing the manufacturing of the board. Although the absence of this top informational layer does not affect the functionality of a PCB, designers and fabricators must include it for eliminating assembly problems, and for optimizing the board’s manufacturing process.
Signal layer is a part of the PCB stackup. Any layer can be a signal layer, provided it has traces carrying signal currents. A signal layer is different from ground or power layers, although they too carry the signal currents and are a part of the board layers that form the stackup. The signal layer, along with the ground or power layers around it, define the trace impedance, and contribute to the signal integrity.
Proper functioning of a circuit board often depends on effective signal routing, including the use of ground and power planes. Multilayer boards mostly make use of ground and power planes to improve signal integrity and reduce crosstalk. Designers typically accomplish this by a copper pour encompassing an entire layer in the form of a ground plane connected to the electrical ground or a power plane connected to one of the supply lines.
Routing or trace routing is the process of specifying the path the copper traces will take when connecting board components or elements. The netlist, created by the designer during schematic capture, defines the identity of the traces. By running the process of layout versus schematic simulation it is possible to cross-verify the routing success of the traces to the netlist.
Pitch is the length of the distance between the centers of adjacent pins or pads for a component. With PCBAs becoming increasingly smaller and denser, component packages and their footprints have followed suite, and so has the space between adjacent pins and pads of components, making the pitch an important design parameter. For components with high density of pins, the pitch determines the solder mask process necessary to avoid assembly problems, including solder bridging.
An accurately created PCB layout pattern is essential during the design phase. This includes the actual footprint or footprint pad layout for each component package in the circuit. This pattern defines the placement of each component during assembly, and may include a polarity marking or the reference indicator for a specific pin.
PCB Manufacturing Files
PCB manufacturing files are the design documents that contain all information, specifications, and images the contract manufacturer requires for fabricating and assembling the boards. The most common formats are Gerber, carrying multiple files, or the IPC 2581, which is a single CAD file.
A pad on a board defines the location where the pin or lead of a component will lie, with the combination of pads forming the footprint of the component. What footprint the component will use is an early design decision by the designer, as each footprint has a number of pads that will enable soldering the component to the board. For SMT components, the pads are solid copper shapes on the surface of the board. For through-hole components, pads have PTH vias extending through the entire thickness of the board. Although pads are visible for most components even after assembly, some packages like BGAs and CSP may hide them under their body.
Each component on a board has two identifiers or part numbers. One of them is the component number the manufacturer has assigned to their component. The other is the reference identifier for identifying each component in the Bill of Materials of the board and its specific location on the board. While a board may contain several components with the same manufacturer’s part number, each of them will have a unique reference identifier locating their position on the board. The manufacturer’s part number offers traceability as the primary means of ensuring supply chain integrity. It is also necessary for selecting components during the design phase, as a proper selection defines the successful functioning, quality, and reliability of the board.
DRC or Design Rule Check
A design rule check compares the design specifications against a set of guidelines with specific limitations. Standards or DFM parameter ranges may define the basis of these guidelines. Designers must adhere to these restrictions, or design rules, when creating PCB layouts. The constraints ensure the regulatory compliance and the manufacturability for a PCB assembly.
Cutouts are removal of board area by design, and may include slots. Sometimes, these may be necessary for installation purposes. In some cases, cutouts are a part of a PCB’s grounding technique for breaking up EMI loop formation. Cutouts may also be milled edges on PCBs, sometimes necessary for reducing board thickness. Although most boards are of a familiar rectangular shape, many applications also use non-standard PCB form factors.
Creepage between two entities on the board is the shortest path distance measured along the board surface. Design and manufacturing guidelines exist for creepage standards for assembly and reliable operation of the board.
Clearance between two entities on a board is the shortest distance between them. This may include the distance in air, such as the clearance across a slot. Another example is the distance of a component from the edge of the board.
Copper weight for PCBs refers to the amount of copper on an internal or external layer of the board’s stackup. Rather than the total weight of copper on the layer surface, copper weight is the amount of copper in ounces covering a 1ft x 1ft of board area. The copper thickness is directly proportional to its weight.
With more applications involving high-speed signal transmission, controlled impedance on the transmission line of a board is important for maintaining signal integrity. By matching material impedance and trace geometry, it is possible to optimize signal fidelity, thereby minimizing interference.
A board’s thickness is the total height of the board, excluding any components on it. The form of electronic circuit boards has been reducing, mainly for accommodating smaller devices and products. For routing greater number of signals, allowing greater functionality and complexity, designers are having to increase the number of layers in the stackup of multilayer boards, thereby increasing the thickness of boards.
Bill of Materials
A BOM or bill of materials is a list of all items necessary to build a printed circuit board. The list provides information on the component part number and reference identifier of all components that will go on to the PCBA.
In general, a printed circuit board may have two types of drill holes. The holes, made during fabrication, may be NPTH or non-plated through holes, or PTH or plated through holes. NPTH types are those necessary for mounting and installation purposes. PTH types are vias that are a part of the circuit and will carry current. An annular ring is the copper area around the hole, providing spacing for solder mask application and a proper connection for vias or PTH.
Prior to automation of most of the manufacturing process, the industry called electronic circuit boards as printed wiring boards. The common name nowadays is the printed circuit board or PCB. This nomenclature includes two type of boards, those undergoing the fabrication process, and those that are undergoing assembly. More appropriately, a PCB represents a circuit board without mounted components that a manufacturer has fabricated. The assembled PCB is more of a circuit card, such as a mother board of a computer.
Multilayer PCBs are boards with stackup containing more than one signal layer. The stackup defines the various layers in the board.
Double-sided PCBs are circuit boards with two signal layers. They may have SMCs mounted on both sides, THCs mounted on side and SMCs on the other, or only THCs mounted on one side. The second side serves to enhance the routing of signals, with vias making electrical connection from one side of the board to the other.
Single-Sided PCBs are circuit boards with only a single signal layer. The circuit with copper footprint and pads is present only on the bottom layer for THCs and only on the top layer for SMCs. This is the simplest type of board for manufacturing.
SMC or Surface Mount Components
These are tiny electronic components available in various packages. The most important aspect of SMCs is their complete absence of long connecting leads. Rather, they have shot pins or end caps that sit on the copper pad for soldering. SMCs do not require any mounting holes, and that makes them suitable for dense packing on a PCB. Soldering boards with SMCs requires passing them though reflow ovens.
THC or Through-Hole Components
These are electronic components also available in various packages with long connecting leads. THCs require holes in PCBs through which their leads will pass for soldering on the bottom side. Soldering THC boards require them to pass through a wave soldering machine.
According to Rush PCB Inc, using common terminology is beneficial to both the customer and the contract manufacturer. It reduces ambiguity, misunderstandings, and confusion, thereby promoting a smooth manufacturing process.