All electronic products must use the Printed Circuit Board (PCB) to fix the integrated circuit (IC) and other electronic components, and all the integrated circuits and electronic components with different functions are connected with thin copper wires to provide a stable working environment so that electronic signals can circulate between different electronic components.
What is a printed circuit board?
A printed circuit board (PCB), also known as a printed wire board (PWB), is a base plate used for assembling electronic components, and is a very important support element for electronic components. Layers of metal copper foil circuits on the board are designed to connect relevant components to achieve effective operation of the electric circuit.
The various components of early electronic products were connected by wires to form a complete path. Later, to simplify the procedures of electronic product manufacturing and reduce costs, a printing method was developed to make circuits, and the copper foil on the substrate was used to replace the original wire connection, thereby improving production efficiency. The various components are mainly connected through the metal copper foil circuits on the board through the design of the various layers to connect and conduct the relevant components.
The traditional circuit board construction method uses printed resists to make the circuits and graphics of the circuit, so it is called a printed circuit board. Because the size of electronic products continues to be miniaturized and refined, most of the current circuit boards are covered with etching resists (wet film or dry film), and after exposure and development, the unnecessary copper foil is etched away to make circuit boards.
What is the substrate of PCB?
Substrates are generally classified based on the insulation, material composition, or flame resistance characteristics of the substrate. Common raw materials are glass fiber, and various types of plastic boards. PCB substrate manufacturers generally use an insulating prepreg composed of glass fiber non-woven material and epoxy resin and then press the copper foil to form a copper foil substrate for use.
What is PCB surface treatment?
Because the copper surface of the PCB is easily oxidized in the environment, it is necessary to reprocess and cover a layer of coating on the exposed area that is not covered by the solder mask to protect the area from oxidation. In response to various subsequent processing needs, various surface treatment methods with different materials, prices, and different degrees of protection have been developed.
Common PCB surface treatments include the following items: bare copper board, spray tin board, lead-free spray tin board, chemical gold board, electroplated gold, chemical silver board, OSP board.
What are the types and applications of PCB structures?
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Single-Layer PCB:
There are copper foil wires on only one side of the circuit board, and no copper foil wires at all on the other side. The circuit of early electronic products was simple, only one side was needed for connection and conduction, and parts could be placed on the other side without copper foil.
With glass fiber covering a single-sided copper foil as the bottom plate, integrated circuits (IC) and other electronic components are concentrated on one side, and copper wires are concentrated on the other side. The number of copper wires that can be made on a single panel is small, and only early circuit boards will be used.
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Double-Layer PCB:
There are copper foil wires on both sides of the circuit board, and the paths of the front (Top layer) and the back (Bottom layer) can be connected via holes. Since both sides can be wired, the usable area is twice that of a single panel, which is more suitable for products with complex circuits. In the design, the parts are placed on the front side, while the backside is the welding surface of the part feet.
Glass fiber covered with double-sided copper foil is used as the bottom plate, and copper wires are made on the front and back surfaces of the bottom plate. "Vias" are drilled to let the copper wires pass through the plate from the front to the back, to connect the front and back copper wires. Double-Layer PCBs are used in circuits that are more complex than single-sided boards.
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Multi-Layer PCB:
Multi-Layer PCBs are made by using multiple etched double-sided boards, stacking an insulating layer (Prepreg) between the boards, laying copper foil on both sides of the outermost layers, and pressing them together. Since multiple double-sided panels are used for pressing, the number of layers is usually an even number. The copper foil layer pressed inside can be conductive, a signal layer, a power layer, or a ground layer. In theory, the multilayer board can reach more than 50 layers, but practical applications are currently about 30 layers.
At present, the motherboards used in computers are mostly eight-layer boards due to too many components. Small electronic products, such as mobile phones, tablet computers, etc., due to their small size generally require at least eight layers of boards. For electronic components with smaller the product size, usually more layers of PCB are required.
The PCB manufacturing process depends on the structure:
- Single panel: Engineering → Cutting → Drilling → Laminating → Copper Corrosion → Solder Mask → Text → Surface Treatment → Forming → Electrical Testing → Quality Inspection
- Double panel: Engineering → Cutting Board → Drilling → PTH → Primary Copper → Lamination → Exposure and Development → Secondary Copper-Tin Lead → Film Removal → Copper Corrosion → Tin And Lead Stripping → Solder Mask → Text → Surface Treatment → Molding → Electrical Measurement → Product Check
- Multilayer board: Engineering → Paneling → Inner Laminate Film → Inner Layer Copper Etching → Inner Layer Film Removal → Pressing → Drilling → PTH → Primary Copper → Lamination → Exposure And Development → Secondary Copper-Tin Lead → Film Removal → Copper Etching → Strip Tin And Lead → Solder Mask → Text → Surface Treatment → Molding → Electrical Test → Quality Inspection
PCB manufacturing depends on the process:
- Dry process:
Panel cutting, laminating, exposure, pressing, drilling, forming
- Wet process:
Brushing, inner layer development, inner layer etching, inner layer film removal, black/brown oxidation, scum removal, plated through holes, full board copper plating, outer layer development, circuit copper plating, tin-lead plating, outer layer film removal, outer layer Etching, peeling tin and lead, solder mask printing, text printing, surface treatment quality.
Component mounting and welding technology
After the printed circuit board is completed, the integrated circuit (IC) and other electronic components must be connected and fixed onto the printed circuit board.
- Through Hole Technology (THT):
The electronic components are placed on the front of the circuit board, and the pins are soldered on the back. Wave soldering is usually used. The pins are first cut to be close to the board and bent slightly to allow the components to be fixed. Then the circuit board is moved to allow the bottom of the board to contact a flux to remove oxide from the bottom pins. The circuit board is then heated and moved over the melted solder. When the solder cools, the connection is completed. This method will take up a lot of space, as it is necessary to drill a hole for each pin. As the pins occupy space on both sides of the circuit board, the solder joints are relatively large, but the fixing effect is better.
- Surface Mounted Technology (SMT):
Electronic components and pins are placed on the front of the circuit board. Over reflow soldering is usually used. Solder paste containing flux and solder is first printed on the circuit board where it is ready to be connected to the electronic component pins. Then the electronic components are placed on the circuit board so that the pins are in contact with the solder paste on the circuit board. Heat is then applied to the circuit board to melt the solder paste and bond with the electronic component pins. After cooling, the electronic component pins will be fixed on the circuit board. Electronic components can be more densely attached to the circuit board, reducing the total size of the printed circuit board, so most current electronic products use SMT technology to replace THT technology.
Special PCB materials and uses
In the PCB industry, PCBs can be classified into rigid PCBs, flexible PCBs (FPCBs), and Rigid-flex PCBs (RFPCBs) based on their properties. Take the internal parts of a computer as an example. The hardboard is the main substrate of the computer's main board and so on. It is called a printed circuit board or PCB, and most of it refers to a hardboard. The soft board is the board used to connect the components. For example, in the mobile phone, the antenna soft board is generally needed to connect the antenna to the mainboard. Generally, the soft board is required to connect because it can be bent, concave, and thin. When the space is less and the components are far away, flexible boards are often used to connect.
- Flexible Printed Circuit (FPC):
The flexible board is integrated with a flexible plastic base film, copper foil, and adhesive. It is free to bend and is flexible, thin and light, with high precision. It can have multi-layer circuits and may use SMT to attach chips to the board. It is generally called a flexible printed circuit board, abbreviated as a flexible board, soft board, or soft film.
Like other substrates, FPCs are constantly pursuing higher line density and layer counts to improve FPC performance and reduce transmission power consumption. However, the manufacturing process of FPCs is very complex, and the technical capabilities required for electronic components manufacturing is relatively high.
The application of soft boards is very wide. Soft boards are most commonly used in communication products such as smartphones. The use of smartphone soft boards accounts for about 40% of the overall soft boards. In addition, softboards are used in laptops, automotive electronics, medical, military, and wearable devices. In the pursuit of lightness, thinness, and smallness in modern products, soft boards have become very important. The applications of soft boards in terminal equipment include the following: antenna soft boards, backlight module soft boards, camera lens soft boards, touch screen soft boards, Touch ID soft boards, sim card soft boards, laptop screen connection soft boards, automobiles Image sensing soft boards, automotive light sets soft boards, etc., this shows the importance of soft boards and their application scope.
Soft board materials used include polyimide (PI), Modified PI (MPI), and liquid crystal polymer (LCP). PI is now rarely used because of its poor performance. Now the main materials used for soft boards are MPI and LCP. Compared with LCP, MPI is relatively cheaper and its performance has greatly improved in recent years, threating to replace LCP. Some manufacturers have replaced LCP soft boards with MPI soft boards to reduce cost. The biggest consideration factor in the use of these three types of soft boards is transmission loss. In the case of low-frequency transmission, there is no significant difference in the loss of the three. However, as the frequency increases, the loss in PI gradually becomes larger, and the loss in MPI is even greater. With higher frequencies, the advantages of LCP become more obvious. With the development of the 5G era, the transmission frequency will be greatly increased to above 24GHz, so there will be higher requirements for the performance and yield of the soft board.
- Rigid-Flex Printed Circuit Board (RFPCB):
RF PCB is called a soft and hard composite board. In general, in a PCB, between two hard boards, the soft board is pressed together in series to form a printed circuit board, forming a Rigid-Flex PCB. Because of the current combination of HDI technology and the development trend of high-frequency signals, the use of rigid-flex boards is also more extensive.
The traditional method of combining a soft board and hardboard is usually to use a connector or a hot-bar soldering (HotBar) process and use a soft board to connect the two hard boards. The rigid-flex board transmits signals in a combination of rigid board→soft board→hardboard. The transmission distance is shortened and the speed is increased, which can effectively improve the reliability. An RFPCB can effectively save space on the circuit board and eliminate the need for connectors or HotBar, simplifying product assembly. Although the price is relatively high, it is extremely versatile and can be tailored for applications in many industries. Because of their high reliability, these circuits are designed to be used in demanding fields such as aerospace, medical, and military applications. They are often used in smartphone boards, photovoltaic panels, battery modules, wearable devices, and high-end storage devices.
