A key component in the electronics industry is called printed circuit board (PCB). This is a too basic component that makes it difficult for many people to explain what a PCB is. This article will explain in detail the structure of the PCB and some of the terms commonly used in the field of PCB.
In the next few pages, we will discuss the composition of the PCB, including some terminology, a brief assembly method, and an introduction to the PCB design process.
What’s a PCB?
Printed circuit board (PCB) is one of the most common names, and can also be called “printed wiring boards” or “printed wiring cards”. Before the PCB appeared, the circuit was made up of point-to-point wiring. The reliability of this method is very low, because as the circuit ages, the breakage of the line can cause an open or short circuit of the line node.
Winding technology is a major advancement in circuit technology that enhances the durability and replaceability of the line by winding small diameter wires around the posts at the joint.
As the electronics industry moves from vacuum tubes and relays to silicon semiconductors and integrated circuits, the size and price of electronic components are also declining. Electronic products are appearing more and more frequently in the consumer sector, prompting manufacturers to look for smaller and more cost-effective solutions. So, the PCB was born.
The PCB looks like a multi-layered cake or lasagna – a layer of different materials in the production, pressed together by heat and adhesive.
The substrate of the PCB is generally glass fiber. In most cases, the fiberglass substrate of a PCB is generally referred to as “FR4″. The solid material “FR4″ gives the hardness and thickness of the PCB. In addition to the FR4 substrate, there are flexible circuit boards produced on flexible high-temperature plastics (polyimide or the like) and the like.
You may find PCBs of different thicknesses; however, the thickness of SparkFun’s products is mostly 1.6mm (0.063”). Some products also use other thicknesses, such as LilyPad and Arudino Pro Micro boards with a thickness of 0.8mm.
Cheap PCBs and hole plates are made of materials such as epoxy or phenol, which lack the durability of FR4, but are much cheaper. When soldering things on such boards, you will smell a lot of odor. This type of substrate is often used in very low-end consumer products. Phenolic substances have a low thermal decomposition temperature, and the long welding time causes decomposition and carbonization, and gives off an unpleasant taste.
The following is a very thin copper foil layer that is pressed into the substrate by heat and adhesive during production. On the double-sided board, the copper foil is pressed to the front and back sides of the substrate. In some low-cost applications, copper foil may only be pressed on one side of the substrate. When we refer to “double-panel” or “two-layer board”, it means that there are two layers of copper foil on our lasagna. Of course, in different PCB designs, the number of copper foil layers may be as small as one layer or more than 16 layers.
The thickness of the copper layer is relatively large and is measured in weight. It is generally expressed by the uniform weight of one square foot of copper (oz). Most PCBs have a copper thickness of 1 oz, but some high-power PCBs may use 2 oz or 3 oz of copper. Convert ounces (oz) per square foot, which is about 35um or 1.4mil copper.
Above the copper layer is a solder mask. This layer makes the PCB look green (or SparkFun’s red). The solder mask covers the traces on the copper layer to prevent traces on the PCB from coming into contact with other metals, solder or other conductive objects. The presence of a solder mask allows you to solder in the right place and prevent solder bridges.
Above the solder mask, there is a white silk screen layer. Letters, numbers, and symbols are printed on the silkscreen of the PCB to facilitate assembly and guide you to better understand the design of the board. We often use the silkscreen symbol to indicate the function of certain pins or LEDs.
The most common color of the silk screen layer is white. Similarly, the silk screen layer can be made into almost any color. Black, gray, red and even yellow silk screens are not uncommon. However, it is rare to see a variety of silkscreen color on a single board.
In general, solder masks are green, but almost all colors can be used for solder masking.
Now that you know the structure of the PCB, let’s take a look at the PCB-related terminology.
Hole ring — A copper ring on a metallized hole in the PCB.
DRC — Design rule checking. A program that checks if the design contains errors, such as shorted traces, too thin traces, or too small holes.
Drill Hole Hit — Used to indicate the deviation of the required drilling position and the actual drilling position in the design. Incorrect drilling centers caused by blunt drill bits are a common problem in PCB manufacturing.
(Gold) Finger — A bare metal pad on the side of the board that is typically used to connect two boards. Such as the edge of the computer’s expansion module, the memory stick and the old game card.
Stamp Holes – In addition to V-Cut, there is an alternative method of board design. By forming a weak joint between some continuous holes, it is easy to separate the board from the imposition.
Pad — A portion of the metal exposed on the surface of the PCB that is used to solder the device.
Jigsaw — A large board made up of many small, sizable boards. Automated circuit board production equipment often has problems when producing small boards, and combining several small boards can speed up production.
Stencil – A thin metal formwork (also available as plastic) that is placed on the PCB to allow solder to pass through certain areas during assembly.
Pick-and-place – A machine or process that places components on a board.
Plane — A continuous piece of copper on the board. It is generally defined by boundaries, not by paths. Also known as “Copper Copper”
Metalized via — A hole in the PCB that contains the hole ring and the plated hole wall. The metallized via may be the connection point of a plug-in, the layer of the signal, or a mounting hole.
The plated hole walls allow the wires on both sides of the PCB to be joined together.
Reflow soldering — Thaw the solder so that the pads (SMD) and device pins are connected together.
Silkscreen — letters, numbers, symbols, or graphics on a PCB. Basically there is only one color on each board and the resolution is relatively low.
Slotting — refers to any hole on the PCB that is not circular. Slotting can be electroplated or not. Since slotting requires additional cutting time, it sometimes increases the cost of the board.
Solder Paste Layer — A layer of solder paste of a certain thickness formed on the pads of the surface mount device through a stencil before placing the components on the PCB. During the reflow process, the solder paste melts and establishes a reliable electrical and mechanical connection between the pad and the device pins.
Soldering Furnace – A furnace for soldering inserts. Generally, there is a small amount of molten solder inside, and the board is quickly passed through, so that the exposed pins can be soldered.
Solder Mask — To prevent short circuits, corrosion, and other problems, the copper is covered with a protective film. The protective film is usually green or it may be other colors (SparkFun Red, Arduino Blue, or Apple Black). Generally referred to as “resistance soldering.”
Lianxi — The two connected pins on the device are incorrectly connected by a small drop of solder.
Surface Mounting — A method of assembly where the device simply needs to be placed on the board without the need to route the device pins through the vias on the board.
Thermal pad — refers to a short trace of the connection pad to the plane. If the pad is not properly designed for heat dissipation, it is difficult to heat the pad to a sufficient soldering temperature during soldering. Improper thermal pad design will make the pad more sticky and reflow soldering time is relatively long.
Trace – A generally continuous path of copper on a board.
V-score — An incomplete cut of the board that breaks the board through this line.
Via – A hole in the board that is typically used to switch signals from one layer to another. The plug hole refers to the over-hole covering the solder mask to prevent it from being soldered. Connector or device pin via, because plugging is not required because soldering is required.
Wave soldering — a method of soldering plug-in devices. The board is passed at a constant speed through a molten solder furnace that produces a stable peak, and the solder peaks solder the device pins and exposed pads together.
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Post time: Aug-07-2019