For hardware engineers, PCBs are encountered almost daily, being a fundamental aspect of their work. PCBs are the foundation of electronic devices and are often overlooked due to their basic nature.
This article will discuss PCBs and some commonly used terminologies in the PCB field.
1. What is a Printed Circuit Board?
The most common name is Printed Circuit Board. Before PCBs, circuits were constructed through a laborious point-to-point wiring process. As the insulation on wires began to age and crack, it often led to failures and short circuits at the connections.
As electronic products transitioned from vacuum tubes and relays to silicon and integrated circuits, the size and cost of electronic components began to decrease. Electronics became increasingly common in consumer products, and the pressure to reduce the size and manufacturing costs of electronic products led manufacturers to seek better solutions, which resulted in the birth of PCBs.
PCB stands for Printed Circuit Board, which is a board with lines and pads that connect various points together. In the image above, there are traces that electrically connect various connectors and components to each other. PCBs allow routing of signals and power between physical devices. Solder is the metal that establishes electrical connections between the PCB surface and electronic components. As a metal, solder can also act as a strong mechanical adhesive.
2. Structure of a PCB
PCBs are somewhat like a multi-layer cake, as they consist of alternating layers of different materials, which are laminated together through heating and adhesive to form a single object.
The substrate is usually glass fiber. Generally speaking, the most common designation for glass fiber is “FR4”. This solid core provides rigidity and thickness to the PCB, along with flexible PCBs based on high-temperature plastics (Kapton or equivalent materials).
Cheaper PCBs and perforated boards (as shown above) are made from other materials such as epoxy resin or phenolic plastic, which lack the durability of FR4 but are much cheaper. When you solder, you often smell a very distinctive odor; these types of substrates are typically used in low-end consumer electronics.
Phenolic resin has a lower thermal decomposition temperature, and if the soldering iron is left on the circuit board for too long, it can cause them to delaminate, smoke, and char.
The next layer is a thin copper foil, laminated to the circuit board through heating and adhesive. On a standard double-sided PCB, copper is applied to both sides of the substrate. In lower-cost electronic products, the PCB may only have copper on one side. When we refer to double-sided or 2-layer boards, we are referring to the number of copper layers (2) in the board. This can be as few as 1 layer or as many as 16 layers or more.
PCB Exposed Copper, No Solder Mask or Silk Screen
Copper thickness may vary and is specified by weight, in ounces per square foot. The vast majority of PCBs contain 1 ounce of copper per square foot, but some PCBs that handle very high power may use 2 or 3 ounces of copper. Each ounce per square foot corresponds to about 35 microns or 0.0014 inches of copper thickness.
The layer on top of the copper foil is called the solder mask layer. It is usually green (but can be other colors). The solder mask covers the copper layer to prevent copper traces from accidentally contacting other metals, solder, or conductive points. This layer helps users solder to the correct positions and prevents solder bridges.
In the example below, the green solder mask is applied to most of the PCB, covering fine traces but exposing silver rings and SMD pads for soldering.
The most common color for the solder mask is green, but there are many other colors as well. For example: blue, red, white.
The white silk screen layer is applied on top of the solder mask layer. The silk screen printing adds letters, numbers, and symbols to the PCB for assembly and indication, helping people better understand the circuit board. We often use silk screen labels to indicate the function of each pin or LED.
The most common color for silk screen is white, but any color of ink can be used. Black, gray, red, and even yellow silk screen colors are widely used, but generally, it is uncommon for a PCB to have multiple colors.
3. PCB Terminologies
The copper ring surrounding a plated through-hole in a PCB.
Design Rule Check. A software check of the design to ensure it contains no errors, such as improper trace contacts, traces that are too thin, or holes that are too small.
The intended location for drilled holes in the design, or where they actually drilled holes on the circuit board. Inaccurate drill hits caused by dull drill bits are a common manufacturing issue.
Metal pads exposed along the edge of the circuit board, used to establish connections between two circuit boards.
Common examples include computer expansion boards or memory boards and the edges of older cassette tape-based video games.
Stamping holes are an alternative method to separate the board from the panel using v-scores. Many holes are concentrated together, forming a weak point that can easily break the board afterward.
The exposed metal part on the surface of the circuit board, used for soldering components.
PTH (Plated Through Hole) Pad
SMD (Surface Mount Device) Pad
A larger circuit board composed of many smaller circuit boards that will be separated before use.
Automated circuit board handling equipment often has issues when handling smaller circuit boards, and by grouping multiple circuit boards together, processing speed can be significantly increased.
A thin metal (sometimes plastic) template on the circuit board that allows solder paste to be deposited in specific areas during assembly.
The machine or process that places components on the circuit board.
Continuous copper blocks on the circuit board, defined by boundaries rather than paths, usually also referred to as “fills”.
Parts of the PCB that are not routed but are grounded
A hole on the circuit board that has an annular ring and is plated through the circuit board. It may be a connection point for through-hole components, a via for signal passage, or a mounting hole.
Insert PTH resistors into the PCB, ready for soldering
Insert PTH resistors into the PCB, ready for soldering. The legs of the resistors go through the holes. Plated holes may have traces connected to them on the front and back of the PCB.
Spring contacts are used for temporary connections for testing or programming purposes.
Popular spring contacts with pointed ends
Melting solder to form joints between pads and component leads.
Letters, numbers, symbols, and images on the circuit board. Usually only one color is available, and the resolution is typically low.
Silk screen identifies this LED as the power LED
Any non-circular hole on the board, slots may or may not be plated. Slots sometimes increase the cost of the circuit board because they require additional cutting time.
Complex slots showing many stamping holes
Note: The corners of slots cannot be made perfectly square because they are cut with round milling cutters.
A small solder ball suspended in a gel medium, applied to the surface mount pads on the PCB with the help of a solder paste stencil before placing components.
During reflow soldering, the solder in the solder paste melts, forming electrical and mechanical joints between the pads and components.
Solder paste on the PCB shortly before placing components
A paste used for quickly hand-soldering circuit boards with through-hole components. Typically contains a small amount of melted solder, allowing the circuit board to be quickly dipped into it, leaving solder spots on all exposed pads.
A layer of protective material covering the metal to prevent short circuits, corrosion, and other issues. Usually green, but other colors (SparkFun red, Arduino blue, or Apple black) are also possible. Sometimes referred to as “resist”.
The solder mask layer covers signal traces but leaves pads to be soldered exposed
A small amount of solder connecting two adjacent pins on a component on the circuit board, according to the design, solder bridges can be used to connect two pads or pins together, which may also lead to unwanted short circuits.
20. Surface Mount Technology
A construction method that allows components to be simply mounted on the board without leads passing through holes on the board. This is the primary assembly method used today, allowing for quick and easy assembly of circuit boards.
A small trace connecting pads to planes; if pads do not have thermal vias, it can be difficult to get pads to a sufficiently high temperature to form good solder joints. When you try to solder, poorly thermally managed pads feel “sticky”, and reflow takes an unusually long time.
Pads with two small traces (thermal vias) connecting the pins to the ground plane. On the right, a via without thermal connection connects it completely to the ground plane
Shadow lines, grid lines, or copper dots left in areas of the circuit board without planes or traces. Reduces the difficulty of etching, as it takes less time to remove unwanted copper from the slots.
A continuous path of copper on the circuit board.
Small traces connecting the reset pad to other areas on the board. Larger, thicker traces connect to the 5V power pin
Partial cuts through the board that allow the board to be easily broken along a line.
Holes on the circuit board used to pass signals from one layer to another. Tented vias are covered with solder mask to prevent them from being soldered to. Vias connecting connectors and components are usually uncovered (not masked) to allow for easy soldering.
Front and back of the same PCB showing a tented via
Front and back of the same PCB showing a tented via. This via carries signals from the front of the PCB through the middle of the board to the back.
A soldering method for circuit boards with through-hole components, where the circuit board is passed through a wave of molten solder, adhering to exposed pads and component leads.
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