Printed circuit boards (PCBs) are found in almost every type of electronic device. If there are electronic parts in a certain kind of equipment, then they are also set in the PCB of different sizes. In addition to fixing a variety of small parts, PCB's main function is to provide the top of the various parts of the mutual electrical connection. With the electronic equipment is more and more complex, the need for more and more parts, PCB on the head of the line and parts are also more and more dense. The standard PCB looks like this. A bare board (with no parts on it) is also often called a "Printed Wiring Board (PWB)".
The substrate of the board itself is made of a material that is insulating and resistant to bending. The small lines visible on the surface are made of copper foil, which originally covered the entire board, but were partially etched away during the manufacturing process, leaving the remaining portion as a mesh of small lines. These lines are called conductors (conductor pattern) or wiring, and are used to provide circuit connections to parts on the PCB.
To secure parts to the PCB, we solder their pins directly to the wiring. On the most basic PCB (single panel), the parts are concentrated on one side and the wires are concentrated on the other side. In this case we need to punch holes in the board so that the pins can go through the board to the other side, so the pins of the parts are soldered to the other side. Because of this, the front and back sides of the PCB are called the Component Side and Solder Side, respectively.
If there are certain parts on the PCB that need to be taken off or put back on after the production is complete, then the part will be installed using the socket (Socket). Since the socket is soldered directly to the board, the part can be removed at will. Below you can see a ZIF (Zero Insertion Force) socket, which allows the part (in this case the CPU) to be easily inserted into the socket or removed. A retaining bar next to the socket holds the part in place once you've inserted it.
To connect two PCBs to each other, we generally use an edge connector, commonly known as a "gold finger". The edge connector contains a number of exposed copper pads, which are in fact part of the PCB's wiring. Usually, when connecting, we insert the edge connector on one piece of PCB into the appropriate slot on another piece of PCB (generally called an expansion slot). In computers, the connectors for graphics cards, sound cards, and other similar interface cards are used to connect to the motherboard.
The green or brown color of the PCB is the color of the solder mask. This layer is an insulating shield that protects the copper wires and prevents parts from being soldered in the wrong place. On top of the solder mask, a silk screen is printed. Often, text and symbols (mostly white) are printed on this screen to indicate the location of each part on the board. The silk screen is also known as the legend.
Single-Sided Boards
We just mentioned that on the most basic PCBs, the parts are clustered on one side and the wires on the other. Because the wires are only on one side, we call this PCB a single-sided board. Because single-side PCBs have many strict limitations in designing the wiring (because there is only one side, the wires can't cross each other but must go around a separate path), only early circuits used these boards.
Double-Sided Boards
These boards have wiring on both sides. However, to use the wires on both sides, there must be a proper circuit connection between the two sides. This "bridge" between circuits is called a via. A via is a small hole in the PCB filled or coated with metal that connects the wires on both sides. Because a double-sided board is twice as large as a single-sided board, and because the wiring can be interleaved with each other (it can be wrapped around to the other side), it's better suited for more complex circuits than single-sided boards.
Multi-Layer Boards
In order to increase the amount of area that can be routed, multi-layer boards use more single- or double-sided wiring boards. Multi-layer boards use several double-sided boards and are glued (pressed together) with an insulating layer placed between each layer. The number of layers on the board indicates how many separate wiring layers there are, usually the number of layers is even and includes the two outermost layers. Most motherboards are constructed with four to eight layers, although technically it is possible to do PCBs with nearly 100 layers. Most large supercomputers use fairly multilayer motherboards, although because such computers have been able to be replaced by clusters of many ordinary computers, ultra-multilayer boards have fallen out of use. Because the layers in a PCB are tightly packed together, it's generally not too easy to tell the actual number, though you might be able to if you look closely at the motherboard.
We've just mentioned vias, which, if applied to a double-sided board, must be punched through the entire board. However, in a multilayer board, if you only want to connect some of the wires, then the vias may waste some of the wire space in the other layers. Buried vias and Blind vias avoid this problem because they only penetrate a few of the layers. Blind vias connect several layers of internal PCBs to the surface PCBs without penetrating the entire board. Buried vias, on the other hand, connect only the internal PCBs, so they are not visible from the surface alone.
In multilayer PCBs, the entire layer is directly connected to ground and power. This is why we categorize each layer as either a Signal, Power or Ground layer. Often PCBs have more than two power and wire layers if the parts on the PCB require different power supplies.
Parts Packaging Technology
Through Hole Technology
Placing parts on one side of the board and soldering pins on the other side is called "Through Hole Technology (THT)" packaging. These parts take up a lot of space and require a hole to be drilled for each pin. So their pins actually take up space on both sides and the solder joints are larger. On the other hand, THT parts have a better PCB connection than SMT (Surface Mounted Technology) parts, which we'll talk about later. Sockets such as wires, and similar interfaces need to be able to withstand pressure, so they are usually THT packages.
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Surface Mounted Technology (SMT)
With Surface Mounted Technology (SMT) parts, the pins are soldered on the same side as the part. This technology eliminates the need to drill holes in the PCB for each pin. Surface mounted parts can even be soldered on both sides. SMT is also smaller than THT parts. PCBs using SMT technology are much denser than PCBs using THT parts, and SMT packaged parts are also cheaper than THT. So it's not surprising that most of today's PCBs are SMT. Because the solder joints and the pins of the parts are very small, it is really very difficult to solder them manually. But if you consider that current assemblies are fully automated, this problem will only occur when fixing parts, I guess.
Design process
In the PCB design, in fact, before the formal wiring, but also after a long step, the following is the main design process:
System specifications
First of all, we must first plan out the electronic device's system specifications. This includes system functionality, cost constraints, size, operating conditions, and so on.
System Block Diagram
The next step must be to create a functional block diagram of the system. The relationships between the blocks must also be labeled.
Splitting the system into PCBs
Splitting the system into PCBs not only reduces the size of the system, but also allows the system to be upgraded and have the ability to exchange parts. A system function block diagram provides the basis for this. For example, a computer can be divided into a motherboard, a graphics card, a sound card, a floppy disk drive, a power supply, and so on.
Deciding on the packaging method to be used and the size of each PCB
When the technology and number of circuits to be used on each PCB have been decided, the next step is to decide on the size of the board. If the design is too large, then the packaging technology has to be changed or re-divided. When choosing a technology, the quality and speed of the circuit diagram should also be taken into account.
Draw an overview of the circuitry of all PCBs
The overview should show the details of the interconnections between the components. All PCBs in the system must be traced out, and today most use CAD (Computer Aided Design). Here is an example of a design using CircuitMakerTM.
Overview of the PCB circuit
Preliminary design simulation
In order to make sure that the designed circuit diagram will work properly, it must first be simulated using computer software. This software can read the design and show how the circuit works in a number of ways. This is much more efficient than actually making a sample PCB and measuring it manually.
Placing parts on a PCB
The way parts are placed is determined by how they are connected to each other. They must be connected to the path in the most efficient way possible. Efficient routing means that the shorter the traces are and the fewer layers they pass through (which also reduces the number of vias), the better, but we'll come back to that when it comes to the actual routing. Here's what the bus looks like routed on a PCB. Placement is important in order to have perfect wiring for each part.
Testing wiring possibilities and correct operation at high speeds
Some of today's computer software can check to see if the parts are placed in a way that allows them to be connected correctly, or if they work correctly at high speeds. This step is called arranging the parts, but we won't get too deep into that. If there are problems with the circuit design, you can also rearrange the parts before exporting the circuit in the field.
Exporting the wiring on the PCB
The connections in the schematic will now be made in the field to look like wiring. This step is usually fully automated, but it is usually necessary to change some parts manually. Below is the wire template for the 2-layer board. The red and blue lines represent the component and solder layers of the PCB. The white text and the squares represent the markings on the printed side of the stencil. The red dots and circles represent the drilled holes and guide vias. On the far right we can see the solder side of the PCB with gold fingers. The final image of the PCB is often referred to as the artwork.
Every design must meet a set of regulations, such as minimum clearance between circuits, minimum line width, and other similar practical constraints. These regulations vary according to the speed of the circuit, the strength of the signal being transmitted, the sensitivity of the circuit to power consumption and noise, and the quality of the materials and manufacturing equipment. If the current strength increases, then the wire thickness must also increase. In order to reduce the cost of PCBs, it is important to be aware of whether these regulations are still met while reducing the number of layers. If more than 2 layers are required, then a power layer and a ground layer are often used to prevent the transmission of signals on the signal layer from being affected and to act as a shield for the signal layer.
Behind-the-wire circuit testing
In order to make sure that the wiring is functioning correctly behind the wires, it must pass a final test. This test also checks that there are no incorrect connections and that all inlines follow the generalized diagram.
Creating a Fabrication Profile
Because there are so many CAD tools available for designing PCBs, manufacturers must have a profile that meets the standard in order to fabricate boards. There are several standard specifications, but the most commonly used is the Gerber files specification. A set of Gerber files includes a plan view of each signal, power, and ground layer, a plan view of the soldermask and stencil printing surface, and designated files for drilling and pick-and-place.
Electromagnetic Compatibility Issues
Electronic equipment that is not designed to meet EMC (Electromagnetic Compatibility) specifications is likely to emit electromagnetic energy and interfere with nearby appliances.EMC sets maximum limits for electromagnetic interference (EMI), electromagnetic fields (EMF), and radio frequency interference (RFI). This regulation ensures the proper functioning of the appliance in relation to other appliances in the vicinity.EMC has strict limits on the amount of energy that can be scattered or transmitted from one device to another and is designed to reduce the magnetization of external EMF, EMI, RFI, etc. In other words, the purpose of this regulation is to ensure the proper functioning of the appliance in relation to other appliances in the vicinity. In other words, the purpose of this regulation is to prevent electromagnetic energy from entering or being emitted by the device. This is a very difficult problem to solve, and it is usually solved by using a power and ground layer, or by placing the PCB in a metal box. The power and ground layer prevents the signal layer from being disturbed, and the metal box is similarly useful. We won't go too far into these issues.
The maximum speed of the circuitry depends on how it complies with EMC regulations. Internal EMI, like current dissipation between conductors, increases with frequency. If the difference in current between the two is too large, then the distance between them must be stretched. This also tells us how to avoid high voltages and minimize the current consumption of the circuit. The latency of the wiring is also important, so naturally the shorter the length the better. So a small PCB with good cabling will be better suited to operate at high speeds than a large PCB.
Manufacturing process
The PCB manufacturing process begins with a "substrate" made of Glass Epoxy or similar material
Imaging (molding/conductor fabrication)
The first step in the fabrication process is to create the wiring for the interconnections between the parts. We use a subtractive transfer to represent the working negative on a metal conductor. This technique involves laying a thin layer of copper foil over the entire surface and eliminating the excess. Additive Pattern transfer is another lesser-used method, which involves applying copper wire only where it is needed, but we won't go into that here.
The PCB substrate is covered with copper foil on both sides if it's being made as a double-sided board, or glued together in the next step if it's being made as a multilayer board.
The next flowchart shows how the wires are soldered to the substrate.
Positive photoresist, which is made from a photoconductor, dissolves under illumination (negative photoresist breaks down if not illuminated). There are a number of ways to treat photoresist on copper surfaces, but the most common way is to heat it up and roll it over the surface containing the photoresist (called dry film photoresist). It can also be sprayed on in a liquid form, though the dry-film style offers higher resolution and allows for the creation of finer wires.
The photoresist is simply a template for the PCB layer in fabrication. Before the photoresist on the PCB is exposed to UV light, the mask covering it prevents some areas of the photoresist from being exposed (assuming a positive photoresist is used). These areas covered by the photoresist will become wiring.
After the photoresist is developed, the rest of the bare copper is etched. The etching process can be done by immersing the board in an etching solvent or by spraying the solvent on the board. Generally used as etching solvents are Ferric Chloride (Ferric Chloride), Alkaline Ammonia (Alkaline Ammonia), Sulfuric Acid + Hydrogen Peroxide (Sulfuric Acid + Hydrogen Peroxide), and Cupric Chloride (Cupric Chloride). After etching, the remaining photoresist is removed. This is called the Stripping process.
Drilling and Plating
If you are making a multilayer PCB with buried or blind holes, each layer must be drilled and plated before bonding. Without this step, there is no way to connect them to each other.
After the holes have been drilled by the machine according to the drilling requirements, the inside of the hole must be plated (Plated-Through-Hole technology, PTH). Plated-Through-Hole technology (PTH) is a metal treatment on the inside of the hole, which allows the internal layers of wiring to be connected to each other. Before plating is started, it is necessary to remove debris from the hole. This is because the resin epoxy produces some chemical changes when heated, and it will cover the internal PCB layer, so it must be removed first. Both the removal and plating actions will be done in the chemical process.
Multilayer PCB lamination
Multilayer boards are created by laminating individual layers. Pressing involves adding insulation between the layers and gluing them to each other. If there are vias through several layers, the process must be repeated for each layer. The wiring on the outer faces of the multilayer board is usually handled after the multilayer board is pressed together.
Processing the soldermask, stencil print surface, and gold finger plating
The soldermask is then applied to the outermost layer of the wiring so that the wiring does not come into contact with the outside of the plating. A screen print is applied to mark the location of the parts. It should not be applied to any wires or gold fingers, as this may reduce solderability or the stability of the current connection. The gold fingers are usually gold plated to ensure a high quality current connection when plugging into expansion slots.
Testing
To test a PCB for shorts or breaks, either optical or electronic testing can be used. Optical testing uses scanning to find defects in each layer, while electronic testing typically uses a Flying-Probe to check all connections. Electronic testing is more accurate at finding shorts or breaks, but optical testing makes it easier to detect problems with incorrect gaps between conductors.
Mounting and Soldering Parts
The final step is to mount and solder the parts. Both THT and SMT parts are mounted on the PCB using machines and equipment.
THT parts are usually soldered using a method called Wave Soldering. This allows all the parts to be soldered to the PCB in one pass, first cutting the pins close to the board and bending them slightly to allow the parts to be held in place. The PCB is then moved to a wave of fluxed water so that the underside is exposed to the flux, which removes any oxides from the metal on the underside. After heating the PCB, this time it is moved to the melted solder, and after contact with the bottom, the soldering is complete.
Automatic soldering of SMT parts is called reflow soldering (Over Reflow Soldering). Inside the paste containing flux and solder solder in the parts mounted on the PCB after the first treatment, after the PCB is heated and then processed again. After the PCB cools, the soldering is complete, and the PCB is ready for final testing.
Ways to save on manufacturing costs
In order to keep the cost of the PCB as low as possible, there are a number of factors that have to be taken into account:
The size of the board is naturally a priority. The smaller the board, the lower the cost. Some PCB sizes have become standardized, so if you follow them, the cost will come down. the CustomPCB website has some information on standard sizes.
Using SMT will save you money compared to THT because the parts on the PCB will be denser (and smaller).
On the other hand, if the board is very dense, then the wiring has to be finer and the equipment used has to be relatively high grade. The materials used must also be more advanced, and more care must be taken in the design of the wires to avoid problems such as power dissipation that can affect the circuitry. The cost of these issues can be more than offset by the reduction in PCB size.
The more layers, the higher the cost, although PCBs with fewer layers usually result in an increase in size.
Drilling takes time, so the fewer vias the better.
Buried vias are more expensive than guide vias through all the layers. This is because buried holes have to be drilled before they are joined.
The size of the holes on a board is determined by the diameter of the part's pins. If the board has parts with different types of pins, then because the machine can't use the same drill to drill all the holes, it's more time consuming and represents a higher manufacturing cost.
Electronic testing using flying probe methods is usually more expensive than optical methods. Optical testing is generally sufficient to ensure that there are no errors on the PCB.
In short, manufacturers are getting more sophisticated with their equipment. Understanding how PCBs are made is useful because when we compare motherboards, boards of the same performance can have different costs and varying levels of stability, and this allows us to compare the capabilities of the various vendors.