Basic PCB design knowledge

From http://blog.mcuol.com/User/luozhen0810/Article/9700_1.htm

Printed circuit boards (PCB) appear in almost every electronic device. If there are electronic parts in a device, they are all mounted on PCB of different sizes. In addition to fixing all kinds of small parts, the main function of PCB is to provide the electrical connection between the above parts. As electronic devices become more complex and more parts are required, the line and parts on the PCB head become more and more intensive. The standard PCB looks like this. The bare board (with no parts on the top) is also known as the printed circuit board Printed Wiring Board (PWB ).
The substrate of the Board itself is made of insulating material that is not easy to bend. On the surface, we can see that the fine line material is copper foil. The original copper foil is covered on the whole board, and it is etching in the middle of the manufacturing process, the remaining part becomes a small network line. These lines are referred to as conductor pattern and used to provide circuit connections for PCB parts.
In order to pin parts on the PCB, We weld their PINs directly to the wiring. On the most basic PCB (single panel), all parts are concentrated on one side, and all wires are concentrated on the other side. In this way, we need to punch holes in the board so that the PIN can pass through the board to the other side, so the pin of the parts is welded to the other side. In this case, the front and back sides of the PCB are called the component side and the soldside ).
If there are some parts on the PCB head, you can take them away or load them back after the preparation is complete, then the socket will be used for the part installation ). Because the socket is directly welded to the board, parts can be freely disassembled. The following shows the zif (zero insertion force, zero-dial plug-in force) socket, which allows the parts (CPU here) to be easily inserted into the socket or removed. The pin next to the socket can be fixed after you insert the part.
If we want to connect two PCB blocks, we usually use edge connectors commonly known as "golden finger ). The golden finger contains many bare copper pads, which are actually part of PCB wiring. During normal connection, we insert the golden finger on one PCB into the appropriate slot (usually called expansion slot) on the other PCB ). In a computer, such as a display card, sound card, or other similar interfaces, they are all connected to the motherboard through a golden finger.
The green or brown color on the PCB is the Solder Mask color. This layer is an insulating protective layer that can protect copper wires or prevent parts from being welded to an incorrect place. A silk screen is printed on the solder mask ). Generally, text and symbols (mostly white) are printed to indicate the position of each part on the board. The screen printing surface is also called the icon surface legend ).

Single-sided boards)
As we have just mentioned, on the most basic PCB, parts are concentrated on one side, and wires are concentrated on the other side. Because the wires only appear on one side, this PCB is called Single-sided ). Because the single panel has many strict restrictions on the Design Line (because only one side of the wiring can not cross and must bypass the independent path), only early circuits use such boards.

Double-sided boards)
Both sides of the circuit board are wired. However, to use the wires on both sides, you must have a proper circuit connection between the two sides. The bridge between these circuits is called a guide hole (). The guide hole is a small hole on the PCB that is filled with or coated with metal. It can be connected to both sides of the wire. Because the double panel is twice the size of a single panel, and the wiring can be staggered (and can be bypassed on the other side), it is more suitable for use in circuits that are more complex than a single panel.

multi-layer boards
to increase the area that can be cabled, more single-or double-sided Wiring Boards are used on the multi-layer board. The multi-layer board uses several dual-panels, and is attached to each layer with an insulating layer (pressed ). The layers of the Board represent several independent wiring layers. Generally, the layers are even and contain the outermost two layers. Most of the Boards are 4 to 8 layers, but technically they can achieve PCB boards of nearly 100 layers. Most large supercomputers use multi-layer boards. However, as these computers can already be replaced by clusters of many common computers, they are no longer used. Because the layers in the PCB are closely integrated, it is generally not easy to see the actual number, but if you carefully observe the motherboard, you may be able to see it.
If the guide hole (VIA) We just mentioned is applied on the dual panel, it must be worn through the whole board. However, if you only want to connect some of the lines, the guide hole may waste some line space on other layers. The Buried Vias and Blind Vias technologies can avoid this problem because they only penetrate several layers. A blind hole is used to connect several layers of internal PCB to the surface PCB without penetrating the whole board. The buried hole is connected only to the internal PCB, so it cannot be seen from the surface alone.
in Multi-board PCB, the whole layer is directly connected to the upper ground and power supply. Therefore, we classify each layer into signal layer, power layer, or ground layer ). If the components on the PCB need different power supplies, this type of PCB usually has more than two layers of power and wire layers.

Part Packaging Technology
Plug-in encapsulation technology (through hole technology)
Place parts on one side of the board and weld the pin on the other side. This technology is called the "through hole technology (tht)" encapsulation. This kind of parts will take up a lot of space and drill a hole for each pin. Therefore, their connectors occupy space on both sides, and the solder joints are relatively large. On the other hand, tht parts, compared with surface mounted technology (surface mounted technology) parts, have a better structure than PCB connections. We will discuss this later. Such as a plug-in, and similar interfaces must be able to withstand pressure, so they are generally tht packages.

Surface Mounted Technology)
Parts of surface mounted technology (SMT) are welded to the same surface as the parts. This technology does not need to be welded for each pin, but is drilled into the PCB.
Surface adhesive parts can even be welded on both sides.
SMT is also smaller than tht parts. Compared with the PCB using tht parts, the PCB using SMT Technology is much more intensive. SMT encapsulated parts are also cheaper than tht. Therefore, most of today's PCB is SMT, not surprising.
Because solder joints and parts have very small joints, it is very difficult to use manual welding. However, if the current Assembly is fully automated, this problem will only occur when the parts are repaired.

Design Process
In the PCB design, before the formal wiring, it takes a long time to complete. The following is the main design process:

• System Specification
  First, you must plan the system specifications of the electronic device. Includes system functions, cost restrictions, sizes, operating conditions, and so on.
• System functional area block diagram
  Next, you must create a functional block chart for the system. The relationship between blocks must also be identified.
• Split the system into several PCB
  Splitting the system into several PCB not only reduces the size, but also enables the system to have the ability to upgrade and swap parts. The System Function Square diagram provides a basis for separation. For example, a computer can be divided into a motherboard, a display card, a sound card, a floppy disk drive, and a power supply.
  Determine the encapsulation method and the size of each PCB.
  When the technology and number of circuits used by each PCB are determined, the next step is to determine the size of the Board. If the design is too large, the encapsulation technology must be changed or split again. When selecting technology, you must also consider the quality and speed of the line chart.
• Circuit overview of all PCB
  The details of the connection between parts are displayed in the overview. The PCB in all systems must be illustrated. Most of today's systems use CAD (Computer Aided Design, computer aided design.
• Initial simulation operation
  To ensure that the designed circuit diagram works properly, it must be simulated with computer software. This type of software can read design drawings and display circuit operations in many ways. This is much more efficient than making a sample PCB and then using manual measurement.
• Place parts on PCB
  The way parts are placed depends on how they are connected. They must be connected to the path in the most efficient way. The so-called efficient cabling means that the shorter the cabling and the smaller the number of passing layers (which also reduces the number of guide holes), the better, but we will mention this issue when wiring. The following figure shows how the bus is cabled on the PCB. In order to make all parts have perfect wiring, it is very important to place the cables.
• Test wiring possibilities, and operate properly at high speed
  In today's computer software, you can check whether the positions of each part can be correctly connected, or check whether it works properly at high speed. This step is called arranging parts, but we will not study it too deeply. If there is a problem with the circuit design, you can reschedule the part location before exporting the line.
• Export PCB line
  The connections in the overview diagram will now be structured as cabling. This step is generally fully automated, but it is generally necessary to manually change some parts. The following is a 2-Layer Steel Wire template. The red and blue lines represent the parts and welding layers of the PCB. The white text and square represent the logo of the printed area on the net. The red points and circles represent the drill holes and guide holes. On the rightmost side, we can see that the solder surface on the PCB has a golden finger. The final diagram of this PCB is usually called an artwork ).
  Each design must comply with a set of rules, such as the minimum reserved gap between lines, the minimum line width, and other similar actual restrictions. These regulations vary with 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 material quality and manufacturing equipment. If the current strength increases, the thickness of the wire must also increase. In order to reduce the cost of PCB, while reducing the number of layers, you must also pay attention to whether these regulations are still in line. If two layers of construction are required, the power supply layer and ground layer are usually used to avoid the impact of transmission signals on the signal layer, and can be used as a shield for the signal layer.
• Wire rear circuit test
  To ensure that the line works properly after the wire, it must pass the final detection. This check can also check whether there is an incorrect connection, and all the connections are taken according to the overview.
• Create an archive
  Because there are currently many CAD tools for designing PCB, the manufacturer must have compliant files to make the board. There are several standard specifications, but the most common ones are Gerber files. A set of Gerber files includes a plan for each signal, power supply, and ground layer, a plan for solder mask and a screen printing surface, and specified files such as drilling and picking.
• Electromagnetic compatibility problems
  Electronic devices without EMC (Electromagnetic compatibility) specifications may emit electromagnetic energy and interfere with nearby electrical appliances. EMC imposes maximum limits on electromagnetic interference (EMI), electromagnetic field (EMF), and RF interference (RFI. This regulation ensures the normal operation of the appliance and other nearby appliances. EMC imposes strict limits on the energy of one device, scattering, or transmitting to another, and reduces magnetic susceptibility to external EMF, EMI, RFI, etc. In other words, the purpose of this provision is to prevent electromagnetic energy from entering or being emitted by the device. This is actually a very difficult problem to solve. Generally, the power supply and ground layer are used, or the PCB is put into a metal box to solve these problems. The power supply and ground layer can prevent interference on the signal layer, and the utility of the metal box is similar. We will not go into depth on these issues.
  The maximum speed of the circuit depends on how EMC works. The internal EMI, such as the current loss between conductors, will increase as the frequency increases. If the current gap between the two is too large, the distance between the two must be extended. This also tells us how to avoid high voltage and minimize the current consumption of the circuit. The delay rate of cabling is also very important, so the shorter the length, the better. Therefore, small PCB with good wiring will be more suitable for high-speed operation than large PCB.

Manufacturing Process
The PCB Manufacturing process starts with a "Substrate" made of glass epoxy resin (Glass Epoxy) or similar materials.
Image (forming/wire manufacturing)
The first step is to establish cabling between parts. We use the subtractive transfer method to present the work film on the metal conductor. This technique is to pad the entire surface with a thin copper foil and remove the excess parts. Additive pattern transfer is another method that few people use. This is a method that only applies copper wires where needed, but we will not talk about it here.
If a dual-panel is created, both sides of the PCB Substrate will be covered with copper foil. If a multi-panel is created, the boards will be pasted together in the next step.
The following flowchart describes how to weld a wire to a substrate.
Positive Photoresist is made of a photosensitive agent that will dissolve under illumination (negative photoretardant will be decomposed if it is not illuminated ). There are many ways to deal with copper surface light resistance agent, but the most common way is to heat it and scroll on the surface containing the light resistance agent (called dry film light resistance agent ). It can also be sprayed in a liquid way, but the Dry Film Type provides a relatively high resolution, you can also make a relatively fine wire.
The hood is just a template for the PCB layer in manufacturing. The mask on the PCB must be covered before UV exposure to prevent the blocking agent from being exposed in some areas (assuming a positive blocking agent is used ). The places covered by the light resistance agent will change to wiring.
Other bare copper parts to be etching after the light resistance agent is developed. The etching process may immerse the board in the etching solvent or spray the solvent on the board. Generally used as etching solvents, ferric chloride, alkaline ammonia, sulfuric acid + hydrogen peroxide, and cupric chloride. After etching, remove the remaining photomasks. This is called stripping) Program .
Drilling and electroplating
If the multi-layer PCB is made and contains buried or blind holes, drilling and electroplating must be performed on each layer of board before bonding. If you do not perform this step, you cannot connect to each other.
After drilling is performed by machine equipment according to drilling requirements, the holes must be plated (plating through hole technology, plated-through-hole technology, PTH ). After the metal processing is performed inside the kongbi, the lines of each layer can be connected to each other. Before plating, clear the debris in the hole. This is because resin epoxy may produce some chemical changes after heating, and it will cover the internal PCB layer, so clear it first. Both cleanup and electroplating operations are completed in the chemical process.
Multilayer PCB combination
Each monolithic layer must be pressed to create a multi-layer board. The clamping action includes adding insulation layer between layers and sticking each other. If there are guide holes passing through several layers, each layer must be processed repeatedly. Wiring on the outer sides of a Multi-board is usually handled after the multi-board is pressed.
Process Solder Mask, screen printing surface and Gold finger Plating
The solder mask will be covered in the outer layer of the wiring, so that the wiring will not be exposed to the plating part. The Netscape printing area is printed on it to indicate the position of each part. It cannot be covered in any wiring or Gold finger. Otherwise, it may reduce the stability of solderability or current connection. The golden finger is usually plated with gold, so that high-quality current connections can be ensured when an expansion slot is inserted.
Test
Optical or electronic testing can be used to test whether a PCB has a short circuit or an open circuit. Optical Scanning is used to identify defects in each layer. In electronic testing, flying-probe is often used to check all connections. Electronic testing is more accurate in the search for short or open circuit, but optical testing can more easily detect the problem of incorrect gaps between conductors.
Installation and welding of Parts
The last step is to install and weld the parts. Both tht and SMT parts are installed and placed on the PCB using machine equipment.
Tht parts are usually welded by wave soldering. This allows all parts to be welded to the PCB at a time. First, cut the pin close to the board and bend it slightly so that the parts can be fixed. Then move the PCB to the water wave of the assisting solvent, so that the bottom is exposed to the assisting solvent, so that the oxides on the bottom metal can be removed. After the PCB is heated, this time it is moved to the melted solder, and the welding is completed after it is in contact with the bottom.
The automatic Soldering of SMT parts is called over reflow soldering ). Paste solder which contains solvent and solder. It is first processed after the parts are installed on the PCB and then heated by the PCB. After the PCB is cooled down, the welding is completed. The next step is to prepare for the final test of the PCB.
Methods To save manufacturing costs
To lower the cost of PCB, the better, there are many factors that must be taken into consideration:
The size of the board is naturally a key point. The smaller the board size, the lower the cost. Part of the PCB size has become the standard, as long as the size is used, the cost will naturally decrease. The custompcb website has some information about the standard size.
Using SMT will save more money than tht, because the parts on the PCB will be more intensive (and smaller ).
On the other hand, if the parts on the board are dense, the wiring must be finer, and the devices used must be higher. At the same time, the materials used must be more advanced. In the wire design, You must be more careful to avoid problems that may affect the circuit, such as power consumption. These problems cause more cost savings than reducing the PCB size.
The higher the number of layers, the higher the cost, but the smaller the number of layers of PCB will usually increase the size.
Drilling takes time, so the fewer the guide holes, the better.
The buried hole is more expensive than the guide hole that runs through all layers. Because the buried hole must be drilled before the joint.
The size of the upper hole of the board is determined by the diameter of the part pin. If there are different types of Pin parts on the board, because the machine cannot use the same drill bit to drill all holes, the relative time consumption also means that the manufacturing cost is relatively higher.
Electronic Tests Using flying-needle probes are typically more expensive than optical ones. In general, optical testing is sufficient to ensure that there are no errors on the PCB.
All in all, the effort of the manufacturer on the device is becoming more and more complex. It is useful to understand the PCB Manufacturing Process, because when we compare the motherboard, the cost of boards with the same performance may vary, and the stability may vary, this allows us to compare the capabilities of various vendors.
Good engineers can look at the motherboard design to know the quality of the design. You may think you are not that strong, but the next time you get the motherboard or display card, you may want to first appreciate the beauty of PCB design.