The Chinese name of PCB is printed circuit board, also known as printed circuit board and printed circuit board. It is an important electronic component, a support body for electronic components, and a provider of electrical connections for electronic components. Because it is made using electronic printing, it is called a "printed" circuit board. As the PCB size requirements become smaller and higher, the device density requirements become higher and higher, and the PCB design becomes more and more difficult. How to achieve a high PCB routing rate and shorten the design time, let's talk about the design skills of PCB planning, layout and routing.
Careful analysis of the design and careful setup of the tool software should be done before starting routing, which will make the design more compliant.
1. Determine the number of layers of the PCB
Board size and routing layers need to be determined early in the design. The number of wiring layers and the stack-up method will directly affect the wiring and impedance of the traces. The size of the board helps determine the stack-up and trace width to achieve the desired design effect. At present, the cost difference between multilayer boards is small, and it is better to use more circuit layers and evenly distribute the copper at the beginning of the design.
2. Design Rules and Limitations
To successfully complete routing tasks, routing tools need to work within the correct rules and constraints. To classify all signal lines with special requirements, each signal class should have a priority, and the higher the priority, the stricter the rules. Rules related to trace width, maximum number of vias, parallelism, interaction between signal lines, and layer limitations have a large impact on the performance of routing tools.
3. Layout of components
During optimal assembly, design for manufacturability (DFM) rules impose constraints on component placement. If the assembly department allows the components to move, the circuit can be properly optimized for easier automatic routing. The rules and constraints you define affect the layout design. The automatic routing tool only considers one signal at a time. By setting the constraints of routing and setting the layers that can route the signal lines, the routing tool can complete the routing as the designer envisioned.
For example, for the layout of the power lines:
a. In the PCB layout, the power supply decoupling circuit should be designed near each related circuit, not placed in the power supply part, otherwise it will not only affect the bypass effect, but also flow pulsating current on the power line and ground line, causing harassment;
b. For the direction of the power supply inside the circuit, the power supply should be adopted from the last stage to the previous stage, and the power supply filter capacitor of this part should be arranged near the last stage;
c. For some main current channels, such as disconnecting or measuring current during debugging and testing, current gaps should be arranged on the printed conductors during layout.
In addition, pay attention to the layout of the regulated power supply on a separate printed board as much as possible. When the power supply and the circuit share the printed board, in the layout, the mixed arrangement of the regulated power supply and the circuit components or the grounding of the power supply and the circuit should be avoided. Because this kind of wiring is not only prone to interference, but also cannot disconnect the load during maintenance, and only part of the printed wires can be cut, thereby damaging the printed board.
Although the changes in the surface treatment process of PCBs are not very big at present, it seems to be a relatively distant thing, but it should be noted that long-term slow changes will lead to huge changes. Under the circumstance that the voice of environmental protection is getting higher and higher, the surface treatment process of PCB will definitely undergo great changes in the future.
The most basic purpose of surface treatment is to ensure good solderability or electrical properties. Since copper in nature tends to exist in the form of oxides in the air and is unlikely to remain as raw copper for long periods of time, additional copper treatments are required. Although in subsequent assembly, strong flux can be used to remove most copper oxides, but strong flux itself is not easy to remove, so the industry generally does not use strong flux.
There are many PCB surface treatment processes, the common ones are hot air leveling, organic coating, electroless nickel plating/immersion gold, immersion silver and immersion tin, which will be introduced one by one below.
1. Hot air leveling (spray tin)
Hot air leveling, also known as hot air solder leveling (commonly known as spray tin), is a process of coating molten tin (lead) solder on the surface of the PCB and leveling (blowing) it with heated compressed air to form a layer that resists copper oxidation. It can also provide a coating with good solderability. Solder and copper form a copper-tin intermetallic compound at the junction during hot air leveling. PCBs are submerged in molten solder during hot air leveling; the air knife blows the liquid solder before the solder solidifies; the air knife can minimize the meniscus of the solder on the copper surface and prevent solder bridging.
2. Organic Solderability Preservative (OSP)
OSP is a process for surface treatment of printed circuit board (PCB) copper foil that meets the requirements of the RoHS directive. OSP is the abbreviation of Organic Solderability PreservaTIves, which is translated as organic solder protection film in Chinese, also known as copper protection agent, also known as Preflux in English. Simply put, OSP is to chemically grow an organic film on the clean bare copper surface.
This layer of film has anti-oxidation, thermal shock resistance, moisture resistance, and is used to protect the copper surface from rusting (oxidation or vulcanization, etc.) in the normal environment; but in the subsequent high temperature welding, this protective film must be very It is easily and quickly removed by the flux, so that the exposed clean copper surface can be combined with the molten solder to form a strong solder joint in a very short time.
3. The whole board is nickel-plated with gold
Nickel-plated gold is to first coat a layer of nickel and then a layer of gold on the surface conductor of the PCB. Nickel plating is mainly to prevent the diffusion between gold and copper. Now there are two types of electroplated nickel gold: soft gold plating (pure gold, the gold surface does not look bright) and hard gold plating (the surface is smooth and hard, wear-resistant, contains other elements such as cobalt, and the gold surface looks brighter). Soft gold is mainly used for gold wire in chip packaging; hard gold is mainly used for electrical interconnection in non-soldering places.
4. Immersion Gold
Immersion gold is a thick layer of nickel-gold alloy with good electrical properties wrapped on the copper surface, which can protect the PCB for a long time; in addition, it also has environmental tolerance that other surface treatment processes do not have. In addition, immersion gold can also prevent the dissolution of copper, which will benefit lead-free assembly.
5. Immersion tin
Since all current solders are based on tin, the tin layer can be matched to any type of solder. The immersion tin process can form a flat copper-tin intermetallic compound, which makes the immersion tin have the same good solderability as the hot air leveling without the headache of the flatness problem of the hot air leveling; the immersion tin board cannot be stored for too long , the assembly must be carried out according to the order of immersion tin.
6. Shen silver
The immersion silver process is between organic coating and electroless nickel / immersion gold, the process is relatively simple and fast; even when exposed to heat, humidity and pollution, silver can still maintain good solderability, but will lose its luster . Immersion silver does not have the good physical strength of electroless nickel/immersion gold because there is no nickel under the silver layer.
7. Electroless nickel palladium gold
Compared with immersion gold, chemical nickel palladium gold has an additional layer of palladium between nickel and gold. Palladium can prevent corrosion caused by replacement reaction and be fully prepared for immersion gold. Gold covers the palladium tightly, providing a good contact surface.
8. Electroplating hard gold
In order to improve the wear resistance of the product and increase the number of insertion and removal, hard gold is plated.
As the requirements of users are getting higher and higher, the environmental requirements are getting stricter, and there are more and more surface treatment processes. In the end, it seems a bit dazzling and confusing to choose the kind of surface treatment process that has development prospects and greater versatility. . Where the PCB surface treatment process will go in the future cannot be accurately predicted now. In any case, satisfying user requirements and protecting the environment must come first!
English
Chinese