If you polled a reasonably sized group of drivers about the most annoying habits of their contemporaries during those daily rush hour dashes, it is a safe bet that many would point to failing to use turn signals. Perhaps, those motorists figure signal lights are unnecessary. After all, they know when they are going to change lanes or make a turn. Of course, this misses the entire point of not only signal lights, but horns and tail lights as well: to inform others of your intent so that you will be on the same page and avoid negative contingencies.
Being on the same page with your contract manufacturer (CM) is imperative for an accurate embodiment of your design intent to be created without suffering through unnecessary quote delays, redesigns and/or board respins. As we established in Part 1 of the PCB terminology series, precision in communication among all stakeholders in your board’s development is the key. This leads to the need for the addition of PCB design terminology to your foundational database, which is done in this article.
List of Important PCB Design Terminology
Typically, there are two types of PCB drill holes that may be bored during fabrication. The first, non-plated through holes (NPTHs), are used for mounting and installation purposes. The other type is plated through holes (PTHs) for current-carrying vias. The annular ring is a copper area around the drill hole that serves to ensure a good connection for vias and provide spacing for solder mask application.
Bill of Materials (BOM)
A bill of materials or BOM is a standard means of listing all of the items required to build or construct a structure. For the PCB construction process, the BOM is actually an electronics component list that provides information essential for the accurate placing of components on your board.
The board thickness is the total height of the board; not including components. It is well-known that the size (in the horizontal plane) of circuit boards has been decreasing to accommodate the demand for smaller electronic devices and products. The ability to route more signals, which allows for greater complexity and functionality, is the primary determinant for the number of layers in the stackup of multilayer PCBs, and thus the use of increased board thickness.
With more and more applications employing Internet of Things (IoT) devices, which typically require high speed signal transmission, controlling impedance on your board’s transmission lines is one of the most important considerations for signal integrity. PCB impedance control is accomplished by matching trace and material impedances to optimize signal fidelity and minimize interference.
For PCBs, copper weight is the term used to describe the amount of copper on an external or internal layer of the board’s stackup. This is not the total weight of copper on the actual layer surface; instead, it is defined as the amount of copper needed to cover a 1ft x 1ft area at a particular copper thickness, which is directly proportional to the weight (weight = thickness÷1.37) and readily measurable.
Clearance is defined as the shortest distance between two points on a PCBA (for example, a component to the edge of the board, through the air). However,
Creepage is similar to clearance in that it is a distance measurement; however, for creepage the distance is the shortest path along the board surface. The importance of both for the assembly and reliable operation of your boards cannot be overstated, as evidenced by industry guidelines for design and manufacturing, including creepage and clearance standards.
Cutouts, which includes slots, are designed removals of board area, typically used to adhere to installation requirements or as a PCB grounding technique to mitigate the possibility of unwanted EMI loops forming. Milled edges can also be considered cutouts, if used for a purpose other than cosmetic; such as to reduce board thickness. Admittedly, most boards exhibit the familiar rectangular shape; however, there are increasingly more applications that require a non-standard PCB form factor.
Design Rule Check (DRC)
A design rule check is a comparison of your design specifications with a set of guidelines and within specific limitations, some of which are standards and DFM parameter ranges, within which your design parameters should be selected. These restrictions that must be adhered to when creating the PCB layout are known as design rules.The creation of a PCBA, which undoubtedly reflects the uniqueness of the designer, nevertheless must adhere to these constraints to ensure manufacturability and regulatory compliance.
One of the most challenging design tasks can be creating the PCB layout pattern for a new or non-standard component package. This pattern, which is known as the footprint or footprint pad layout, defines the area in which the component should be placed during assembly and may include a reference indicator, polarity markings or a pin 1 indicator.
For circuit boards, good grounding technique is essential for effective signal routing. This is especially true for multisignal boards where many different signals may need to utilize a single centralized ground. This is most often accomplished by a copper pour that encompasses an entire layer and is known as a ground plane.
Manufacturer Part Number (MPN)
The manufacturer part number (MPN) is a unique identifier for each component type on your board. The MPN is not to be confused with the reference identifier, which identifies each specific component in the board’s BOM and its specific location on the board, as there may be several components with differing reference identifiers that have the same MPN. The traceability of the MPN is one of the primary means of ensuring supply chain integrity. For PCBA development, the selection of components is possibly the most important task, as successful operation rests upon component quality and reliability.
The location on the board that defines where a component pin and/or lead should be placed is its footprint. And where the footprint and the component are placed should be an early design decision. Each footprint contains pads that enable the component to be soldered to the board. For through-hole components, the pads are PTH vias that extend through the entire board’s thickness. For SMT, the pads may be solid copper shapes on the surface or under the component, such as for ball grid array (BGA) and chip-scale packaging (CSP) configurations.
PCB design file
The PCB design file(s) which are more accurately the PCB manufacturing file(s) should contain all information, specifications, and images needed by your contract manufacturer (CM) to fabricate and assemble your boards. The most common formats are the multifile Gerbers or CAD single file format, such as IPC 2581.
Pitch refers to the amount of space between the center of adjacent pads (or pins) for a component. As PCBAs have become smaller and denser, so have component packages and footprints. Consequently, the amount of space between the center of adjacent pads (or pins) or the pitch is a very important design parameter. For these small components, the pitch may determine what solder mask process can be used to avoid assembly problems, such as solder bridging, or whether solder masking should not be used for the component pads.
PCB trace routing is the process of specifying the copper traces that connect the board elements. These connections are typically based on the netlist created during schematic generation. One technique utilized to verify the success of these two processes is layout versus schematic (LVS) simulation.
In the PCB stackup, a signal layer is any layer that is designed to carry current. Although power and ground planes also carry current and are included in the number of PCB layers that define the stackup (for example, 2 or 4 layers), they are often not considered signal layers. The types of signals that the board traces carry also serve as one of the major taxonomies for board type classification.
The silkscreen, which contains pin 1 indicators, polarity symbols, reference indicators, component footprints, labels, and other identification, is an essential part of a complete manufacturing package. Functionally, it is possible to design and build some PCBAs without this top informational layer. However, for optimization of your board’s manufacturing and elimination of assembly problems, it should be included.
PCB traces are effectively small transmission lines that are etched from a layer of copper and their parameters (e.g. width, length, copper weight, and impedance) directly impact your board’s signal integrity. For some applications, such as high-speed design, trace design should be coordinated with material selection to ensure the best signal routing.
A via provides a means of passing current from one layer to another. Choosing the best PCB via option is very important to your board development as the option selected (for example, via-in-pad) may extend the manufacturing turnaround time and increase costs.
With the addition of the list of terms above, we are expanding and strengthening the essential database of terms to facilitate excellent communication and the best PCBA development.
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And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.
If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on PCB design terminology that your colleagues consider important, contact us.
To access the complete PCB Terminology series and expand your PCB knowledge, please refer to the following (upcoming) articles:
Part 1: PCB Terminology You Should Know
Part 3: PCB Manufacturing Terminology You Should Know
Part 4: PCB Fabrication Terminology You Should Know
Part 5: PCB Assembly Terminology You Should Know