Printed circuit board (PCB) are found in almost every kind of electronic device. If there are electronic components in a piece of equipment, they are also embedded in various sizes of PCB. In addition to fixing various small parts, the main function of the PCB is to provide electrical connections between the components. As electronic equipment becomes more and more complex, more and more parts are needed, and the wiring and parts on the PCB become more and more dense. A standard PCB looks something like this. Bare Board (without parts on it) is also often referred to as “Printed Wiring Board (PWB).

The substrate of the board itself is made of a material that is insulated and resistant to bending. The small line material that can be seen on the surface is copper foil. Originally, the copper foil is covered on the whole board, and the middle part is etched away in the manufacturing process, and the remaining part becomes a network of small lines. These lines are called conductors or conductors and are used to provide electrical connections to parts on the PCB.

To secure parts to the PCB, we solder their pins directly to the wiring. On a basic PCB, the parts are concentrated on one side and the wires are concentrated on the other. So we need to make holes in the board so that the pins can go through the board to the other side, so the pins of the parts are welded to the other side. Because of this, the front and back sides of a PCB are called Component Side and Solder Side respectively.

If there are parts on the PCB that can be removed or put back on after manufacture, Socket will be used to install the parts. Because the socket is directly welded to the board, the parts can be arbitrarily disassembled. A ZIF (Zero InserTIon Force) plug allows parts to be inserted and removed easily. The lever next to the socket can hold the parts in place after you insert them.

To connect two PCBS to each other, an edge connector is commonly used. The gold finger contains a number of bare copper pads that are actually part of the PCB wiring. Normally, to connect, we insert the gold finger on one PCB into the appropriate Slot (commonly called expansion Slot) on the other PCB. In computers, display cards, sound cards, and similar interface cards are connected to the motherboard by means of a gold finger.

The green or brown color on the PCB is the color of the solder mask. This layer is an insulating shield that protects the copper wire and prevents parts from being welded to the wrong place. Another silk screen will be printed on the solder resistance layer. It is usually printed with words and symbols (mostly white) to indicate the position of the parts on the board. Screen printing surface is also known as icon surface

Legend).

Single-sided Boards

As we mentioned, on a basic PCB, the parts are concentrated on one side and the wires are concentrated on the other. Because the wire appears on only one side, we call this TYPE of PCB single-sided. Because single panels had many strict restrictions on the design of the circuit (because there was only one side, the wiring could not cross and had to take a separate path), only early circuits used such boards.

Double-sided Boards

The circuit board has wiring on both sides. But in order to use both wires, there must be proper electrical connections between the two sides. This “bridge” between circuits is called a guide hole (VIA). Guide holes are small holes in the PCB filled or coated with metal that can be connected to wires on both sides. Because a dual panel has twice the area of a single panel, and because the wiring can be interlaced (it can be wound around to the other side), it is better for more complex circuits than a single panel.

Multi-layer Boards

In order to increase the area that can be wired, more single – or double-sided wiring boards are used. The multilayer board uses several double panels, and a layer of insulation is placed between each panel and glued (pressed). The number of layers of the board represents several independent wiring layers, usually an even number of layers, including the outermost two layers. Most motherboards are built with four to eight layers, but it is technically possible to build up to 100 layers of PCBS. Most large supercomputers use quite a few layers of motherboards, but they have fallen out of use as they can be replaced by clusters of ordinary computers. Because the layers in a PCB are so tightly integrated, it’s not always easy to see the actual number, but if you look closely at the motherboard, you might be able to.

The guide hole (VIA) we just mentioned, if applied to a double panel, must be through the entire board

But in a multilayer, if you only want to connect some of the lines, the guide holes may waste some of the line space in the other layers. Buried vias and Blind vias avoid this problem because they only penetrate a few layers. Blind holes connect several layers of internal PCBS to surface PCBS without penetrating the entire board. Buried holes are only connected to the internal PCB, so light is not visible from the surface.

In a multilayer PCB, the entire layer is directly connected to the ground wire and the power supply. So we classify the layers as Signal, Power or Ground. If the parts on the PCB require different power supplies, they usually have more than two power and wire layers.

Part packaging technology

Through Hole Technology

The technique of placing parts on one side of the board and welding the pins to the other side is called “Through Hole Technology (THT)” encapsulation. This part takes up a lot of space and one hole is drilled for each pin. So their joints actually take up space on both sides, and the solder joints are relatively large. On the other hand, THT parts are better connected to PCB than Surface Mounted Technology (SMT) parts, which we will talk about later. Sockets like wired sockets and similar interfaces need to be pressure-tolerant, so they are usually THT packages.

Surface Mounted Technology

For Surface Mounted Technology (SMT) parts, the pin is welded on the same side with the parts. This technique does not drill holes in the PCB for each pin.

Surface adhesive parts can even be welded on both sides.

SMT also has smaller parts than THT. Compared to PCB with THT parts, PCB with SMT technology is much denser. SMT package parts are also less expensive than THT’s. So it’s no surprise that most of today’s PCBS are SMT.

Because the solder joints and pins of parts are very small, it is very difficult to weld them manually. However, given that current assembly is fully automated, this problem will only occur when repairing parts.

The design process

In PCB design, there are actually very long steps to go through before formal wiring. The following is the main design process:

The system specifications

First of all, the system specifications of the electronic equipment should be planned. It covers system functionality, cost constraints, size, operation and so on.

System function block diagram

The next step is to create a functional block diagram of the system. The relationship between the squares must also be marked.

Divide the system into several PCBS

Dividing the system into several PCBS not only reduces the size, but also gives the system the ability to upgrade and swap parts. The system function block diagram provides the basis for our segmentation. Computers, for example, can be divided into motherboards, display cards, sound cards, floppy disk drives, power supplies, and so on.

Determine the packaging method to be used and the size of each PCB

Once the technology and the number of circuits used for each PCB has been determined, the next step is to determine the size of the board. If the design is too large, then packaging technology will have to change, or re-split the action. The quality and speed of the circuit diagram should also be taken into consideration when selecting the technology.

Draw schematic circuit diagrams of all PCB’s

The details of the interconnections between the parts should be shown in the sketch. PCB in all systems must be described, and most of them use CAD (Computer Aided Design) at present. Here is an example of a CircuitMakerTM design.

Schematic diagram of PCB circuit

Preliminary design of simulation operation

To ensure that the designed circuit diagram works, it must first be simulated using computer software. Such software can read blueprints and show how the circuit works in many ways. This is much more efficient than actually making a sample PCB and then measuring it manually.

Place the parts on the PCB

The way parts are placed depends on how they are connected to each other. They must be connected to the path in the most efficient way. Efficient wiring means the shortest possible wiring and fewer layers (which also reduces the number of guide holes), but we’ll come back to this in actual wiring. Here is what the bus looks like on a PCB. Placement is important in order for each part to have perfect wiring.

Test wiring possibilities with correct operation at high speed

Some of today’s computer software can check whether the placement of each component can be correctly connected, or check whether it can operate correctly at high speed. This step is called arranging parts, but we won’t go too far into this. If there is a problem with the circuit design, parts can also be rearranged before the circuit is exported in the field.

Export circuit on PCB

The connections in the sketch will now look like wiring in the field. This step is usually fully automated, although manual changes are usually required. Below is the wire template for 2 laminates. The red and blue lines represent the PCB parts layer and the welding layer respectively. The white text and squares represent the markings on the screen printing surface. The red dots and circles represent drilling and guiding holes. On the far right we can see the gold finger on the welding surface of the PCB. The final composition of this PCB is often referred to as the working Artwork.

Each design must conform to a set of rules, such as minimum reserved gaps between lines, minimum line widths, and other similar practical limitations. These specifications vary according to the speed of the circuit, the strength of the signal to be transmitted, the sensitivity of the circuit to power consumption and noise, and the quality of the material and manufacturing equipment. If the strength of the current increases, the thickness of the wire must also increase. In order to reduce PCB costs, while reducing the number of layers, it is also necessary to pay attention to whether these regulations are still met. If more than 2 layers are needed, the power layer and ground layer are usually used to avoid the transmission signal on the signal layer is affected, and can be used as a shield of the signal layer.

Wire after circuit test

In order to be sure that the line is working properly behind the wire, it must pass the final test. This test also checks for incorrect connections, and all connections follow the schematic diagram.

Establish and file

Because there are currently many CAD tools for designing PCBS, manufacturers must have a profile that meets the standards before they can manufacture boards. There are several standard specifications, but the most common is the Gerber Files specification. A set of Gerber files includes a plan of each signal, power and ground layer, a plan of the solder resistance layer and the screen printing surface, and specified files of drilling and displacing.

Electromagnetic compatibility problem

Electronic devices that are not designed to EMC specifications are likely to emit electromagnetic energy and interfere with nearby appliances. EMC imposes maximum limits on electromagnetic interference (EMI), electromagnetic field (EMF) and radio frequency interference (RFI). This regulation can ensure the normal operation of the appliance and other nearby appliances. EMC imposes strict limits on the amount of energy that can be scattered or transmitted from one device to another, and is designed to reduce susceptibility to external EMF, EMI, RFI, and so on. In other words, the purpose of this regulation is to prevent electromagnetic energy from entering or emanating from the device. This is a very difficult problem to solve, and is usually solved by using power and grounding layers, or putting PCBS into metal boxes. The power and ground layers protect the signal layer from interference, and the metal box works equally well. We won’t go too far into these issues.

The maximum speed of the circuit depends on EMC compliance. Internal EMI, such as current loss between conductors, increases as the frequency rises. If the current difference between the two is too large, make sure to lengthen the distance between them. This also tells us how to avoid high voltage and minimize the current consumption of the circuit. The rate of delay in wiring is also important, so the shorter the length, the better. So a small PCB with good wiring will work better at high speeds than a large PCB.