PCB Blog - What is pcb and the pcb design process

What is pcb?PCB,full name printed circuit board,is the cornerstone and bloodline of building modern electronic products, plays an indispensable role.From the daily wear of electronic watches, students can not do without the calculator,to the office of the indispensable computer,and then connected to the world of communication electronic equipment,and even a symbol of the country's military power of the military weapons system,these seemingly different,different functions of the electronic equipment,in their internal structure,are invariably embedded in a sophisticated and complex pcb board.

Printed circuit boards can be divided into three main categories:Flexible Printed Circuits (FPC),rigid circuit boards,and Rigid-Flexible PCBs (Rigid-Flex PCB).

FPC, or Flexible Printed Circuit, is widely recognised for its high-density wiring, light weight,ultra-thinness and excellent bending properties.For example,one of our current products uses FPC as the wiring, which is valued for its light weight and easy bending characteristics.

Rigid-Flex PCBs (Rigid-Flex printed circuit boards) are made by combining a flexible pcb with a rigid circuit board through a series of processes such as lamination.The advantage of this electronic board is that it combines the respective advantages of FPC and PCB board,however, its disadvantages are equally obvious:complex production process,relatively low yield,production difficulties,and relatively long production cycle.

Classification of pcb

Classification according to the number of layers and copper foil distribution

According to the number of layers of copper foil,circuit boards can be divided into single-sided boards, double-sided boards and multilayer boards.

Single-Layer Boards: Wires are provided on one side only.Since the wiring is confined to one side only, there are many design constraints. These boards were more common in early circuits, but are now mainly used in products with simple structures and strict cost requirements.

Double-Layer Boards: Wiring is provided on both sides of the board. Thanks to the double-sided wiring, it is possible to solve the problems caused by staggered wiring in a single panel, thus achieving a more optimal layout. Double-sided boards are often used in relatively simple circuits, chip pin distribution is not dense occasions.

Multi-layer boards (Multi-Layer Boards): In order to expand the wiring area, multi-layer boards using multiple layers of single- or double-sided boards, and bonded through the insulating material. Commonly used multilayer boards are usually between 4 and 8 layers.

Classification by substrate

Common classifications by substrate include: phenolic paper laminates, epoxy paper laminates, polyester glass mat laminates, and epoxy glass cloth laminates.

Based on the type of through holes,pcb circuit board can be classified as:

Through Hole: This type of hole completely penetrates the entire electronic circuit board, with each end located in the outermost layer of the board.

Blind Hole: A blind hole starts on one of the outer surfaces of the electronic circuit board,but does not penetrate the entire board, but ends somewhere on the inner layer.

Buried Via: A buried via is located completely inside the electronic circuit board, with neither end touching the outer layer of the board, and is only used to connect different layers inside the board.

PCB boards can be classified according to whether the through-hole contains copper or not, and are divided into two types of holes: copper-plated holes and copper-free holes.

Plated Through Hole (PTH): This type of through hole is plated with copper and serves as a conductor, and is a common type of hole used for pcb routing.

Non-Plated Through Hole (NPTH): NPTH means that there is no copper connection inside the hole, which is usually used to fix the position of the pcb circuit board.

In practice, most circuit boards contain both copper and copper-free holes.

Classification by Function

Impedance boards have stable impedance characteristics, which gives them an advantage in high-speed signal transmission and high-frequency applications.

Microwave pcb, on the other hand, stand out for their excellent microwave performance, which makes them particularly suitable for RF and microwave communication applications.

Flex PCB, with their excellent bending properties and fatigue resistance, are ideal for equipment that is subject to bending and stretching stresses.

Classification by surface treatment

Since copper is prone to oxidisation in its natural environment,which may lead to poor soldering results, a protective coating is usually applied to the copper surface. The following are a few common types of surface treatments:

Leaded (HASL: Hot Air Level Soldering):The copper surface is tinned in a leaded environment using hot air levelling technology.

Lead Free Soldering (HASL/LF:Hot Air Level Soldering Lead Free):The same hot air levelling technology is used, but in a lead-free environment.

Immersion Gold (ENIG: Electroless Nickel/Immersion Gold):A layer of electroless nickel is deposited on the surface of the copper, which is then dip-plated with gold.

Immersion Tin/Chemical Sn:A layer of tin is chemically deposited on the surface of copper.

Immersion Silver/Chemical Ag:A layer of silver is chemically deposited on the copper surface.

Oxidation Protection (OSP:Organic Solderability Preservatives/Entek/Copper passivated):Coating an organic protective film on the copper surface to prevent oxidation.

Gold plating/flash gold: Plating a layer of gold on the surface of copper by electroplating.

Carbon oil:A layer of carbon oil coated on the surface of a pcb circuit board for specific applications.

Peelable solder mask:A peelable solder mask used to protect certain areas of a PCB board from soldering.

Plating Gold Finger/Edged Contact/Connecting Finger:A layer of gold plated on the edges or specific areas of a pcb circuit board to enhance conductivity and corrosion resistance. According to IPC-6012 CLASS 2 standard, the minimum gold thickness of gold finger is 0.80UM (30U") and the minimum nickel thickness (NI) is 2UM.

Functions of PCB board

Electrical Interconnection

The copper traces on a PCB board are carefully laid out to connect various electronic components, such as resistors, capacitors and integrated circuits, together to build a complete circuit system. This type of connection not only simplifies the circuit design process, but also significantly improves circuit reliability.

Component Support

PCBs provide a solid support platform for electronic components,ensuring that they are arranged in a tight and orderly manner. Through soldering and other processes, components can be firmly fixed on the PCB, thus enhancing the structural stability and portability of the equipment.

Circuit Protection

The insulating material used on the PCB boards effectively shields the circuits from electromagnetic interference and protects them from external factors such as moisture and dust. This protection is critical to prolonging the life of electronic devices, especially in harsh industrial environments.

Thermal Performance

In high-performance, power-hungry electronics, where heat dissipation is a particular problem, the metal layers on the PCB board, especially the copper foils, are carefully designed to act as efficient heat sinks. They quickly absorb and disperse the heat generated by the circuitry and prevent components from being damaged by overheating.The heat dissipation efficiency of PCB boards can be further improved by increasing the thickness of copper foils, optimising the layout and other design means. 

In addition, innovative heat dissipation structures such as heat sinks and heat sinks are also widely used in PCB board design. These designs enhance the heat dissipation effect while maintaining the compactness and aesthetics of the PCB boards, providing a solid technical support for building high-performance and long-life electronic devices.

Space optimisation and compact structure

The design of PCB boards not only concerns the realisation of circuit functions,but also profoundly affects production costs, production efficiency and subsequent maintenance and replacement. In the choice of manufacturing process, advanced technologies such as the hole-in-disc process are widely used in PCB board manufacturing.These processes allow for a more flexible layout of components within the limited PCB board space, maximising the use of space.The use of resin plug holes + electroplated caps to achieve the hole-in-dish process not only optimises the layout and wiring, but also avoids problems such as tin leakage, making the internal structure of electronic devices more compact and beautiful.

Commonly used raw materials for pcb board

1.The original material of the pcb circuit board is the copper-clad substrate, referred to as the substrate. A substrate is essentially a resin sheet with copper foil laminated on both sides.Among many manufacturers, FR-4 boards have become the first choice in the field of high-grade electronic products such as computers and communication equipment due to their excellent performance. 

For FR-4 sheet,the industry has three core requirements: the first is flame resistance, that is, the board must be able to encounter high temperatures to maintain non-combustible, only softening; followed by the Tg point (glass transition temperature),which reflects the stability of the material at high temperatures; and then is the dielectric constant, a parameter that is directly related to the circuit board's signal transmission efficiency and quality. In short, FR-4 boards used in pcb circuit board need to have excellent flame resistance, to be able to maintain the form at a specific temperature without burning, and at the same time have a suitable Tg point and low dielectric constant, in order to meet the stringent requirements of modern electronic products for high performance and high stability.

Copper cladding boards are commonly used in the following categories:

FR-1 - phenolic cotton paper (commonly known as bakelite, higher economy than FR-2)

FR-2 - phenolic cotton paper

FR-3 - cotton paper, epoxy resin

FR-4 - glass cloth, epoxy resin (Shenzhen Qinji electronic commonly used substrates)

FR-5 ── Glass cloth, epoxy resin

FR-6 ──Raw glass, polyester

CEM-1 ──Cotton paper, epoxy resin (flame retardant)

CEM-2 ──Cotton paper, epoxy resin (non-flame retardant)

CEM-3 ──Glass cloth, epoxy resin

CEM-4 ──Glass cloth, epoxy resin

CEM-5 ──Glass cloth, polyester

AIN ──Aluminium nitride

SIC ──Silicon carbide

G-10 ──Glass cloth, epoxy resin

Copper Clad Laminate (CCL) is a material that can be classified in a variety of ways. Depending on the insulation material, it can be divided into three main types: paper, glass cloth and synthetic fibre. Depending on the type of binder resin used, copper-clad laminates can be further subdivided into phenolic, epoxy, polyester and PTFE types. In addition, from the point of view of application, copper-clad laminates can also be distinguished into two categories: general-purpose and special-purpose, to meet the specific needs of different industries and products.

2.Copper Foil

After a specific process, the remaining part of the copper foil is able to construct the wires required for the circuit on the substrate. The production process of copper foil mainly includes calendering and electrolysis.

3.Semi-cured sheet (PP)

In the manufacturing process of electronic circuit boards, semi-cured sheet (PP) is an indispensable key material, which is mainly responsible for the bonding task between layers. In short, a semi-cured sheet is a thin sheet of substrate at stage B. Its characteristics are determined by its thickness and the amount of resin (adhesive) it contains.

4.Dry Film (Light Sensitive Materials)

Dry film, also known simply as photosensitive dry film, is composed of a special resin-like substance at its core, which undergoes a photochemical reaction when exposed to a specific spectrum. In practice, dry film is usually made up of three layers: the light-sensitive layer is cleverly sandwiched between two protective plastic films. Based on the unique chemical properties of the photosensitive substances, dry films can be divided into two main categories: photopolymeric and photodegradable. Photopolymerised dry film undergoes a transition from water-soluble to water-insoluble and hardens when exposed to a specific spectrum of light, while photodecomposed dry film reacts in the opposite way.

5.Solder Resist Ink

Solder Resist Ink, essentially a professional solder resist, acts as a liquid photographic material and has no affinity for liquid solder. Similar to photosensitive dry film, solder resist ink hardens when exposed to a specific spectrum of light. When used, solder resist ink needs to be mixed well with a hardener. What we commonly refer to as ink, also known as soldermask, gives our common printed circuit boards their rich colours.

6.Image Negative (Film Sheet)

The function of the image negative here is similar to that of the negative film in photography, which uses light-sensitive materials to capture and record images. When the customer transmits the completed design to the circuit board factory, the workstation in the CAM centre outputs the circuit diagram as an image negative using a light plotter rather than a normal printer. The image negative plays a crucial role in the production of circuit boards because all patterns or lines that need to be realised on the substrate by image transfer technology must first be converted into a negative.

PCB Design Process

The PCB design process is a systematic approach to creating functionally stable and reliable printed circuit boards, where every step is critical, from the initial conceptualisation to the final manufacturing of the finished product. The process consists of a number of core elements, each of which plays a decisive role in the success of the project.

The PCB design process begins with the conceptual design phase, where the main task is to define the overall requirements and specifications of the electronic product. During this phase, the electrical engineering team, mechanical engineering team, and other relevant stakeholders work closely together to define the functional features, dimensions, and performance criteria required for the PCB.

Once the conceptual design phase is complete,the schematic design phase follows.In this phase,engineers use schematic capture tools to accurately depict the electrical connections and components of the circuit.The schematic is the cornerstone of the PCB layout,providing a clear guide to the design of the electronic circuit board and demonstrating the electrical functionality of the circuit.

Once the schematic design is complete,the PCB layout phase begins. In this step, engineers build the physical form of the circuit by carefully arranging and routing components on the board. During the layout process, they must strictly adhere to various design guidelines, such as minimum line widths, spacing, and the size of vias, to ensure the board's manufacturability and operational reliability.

Throughout the PCB design process, the synergy between the various teams plays an important role. The electrical engineering team works in close co-operation with the mechanical engineering team to ensure that the PCBs are perfectly adapted to the required housing and meet all thermal and structural requirements. At the same time, the manufacturing team provides important guidance on Design for Manufacturing (DFM), which is designed to minimise problems during the production process and increase productivity.

Once the PCB layout has been completed,the design is then subjected to a rigorous verification and validation process that includes Design Rule Check (DRC),Electrical Rule Check (ERC) and Signal Integrity Simulation.These processes are critical to identifying and resolving potential problems before going into production.The tail end of the PCB design process is then the generation of manufacturing files,specifically such as Gerber files and drill files, which are necessary for the PCB manufacturer to produce the physical board.The manufactured printed circuit boards are then subjected to a rigorous testing and assembly process before final integration into the electronics.