PCB Chinese name is called printed circuit board, also known as printed circuit board, printed circuit board, is an important electronic component, is the support body of electronic components, is the electrical connection provider of electronic components. Because it is produced by electronic printing, it is called a "printed" circuit board. As PCB size requirements become smaller and smaller, device density requirements become higher and higher, PCB design is becoming more and more difficult. How to achieve PCB high distribution flux and shorten the design time, in this author to talk about PCB planning, layout and routing design skills.

    Before starting the wiring, the design should be carefully analyzed and the tool software should be carefully set up, which will make the design more consistent with the requirements.
1 Determine the number of PCB layers
    The size of the board and the number of wiring layers need to be determined at the beginning of the design. The number of wiring layers and the STack-up mode will directly affect the wiring and impedance of the printed line. The size of the board helps determine the stacking pattern and the width of the printed line to achieve the desired design effect. At present, the cost difference between multi-layer boards is very small, and it is best to use more circuit layers and make the copper evenly distributed at the beginning of the design.

2 Design rules and restrictions
    To successfully complete the wiring task, the wiring tool needs to work under the correct rules and restrictions. 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 the width of the printed line, the maximum number of holes, parallelism, the interaction between the signal lines, and layer limitations, these rules have a great impact on the performance of the wiring tool.

3 Component layout
    During optimal assembly, design for manufacturability (DFM) rules restrict component layout. If the assembly department allows the components to move, the circuit can be properly optimized for automatic routing. The rules and constraints you define affect the layout design. The automatic wiring tool only considers one signal at a time. By setting the constraints of the wiring and setting the layer of the deploable signal line, the wiring tool can complete the wiring as envisaged by the designer.
    For example, for the layout of power cables:
    (1) In the PCB layout, the power decoupling circuit should be designed near the relevant circuits, rather than placed in the power supply part, otherwise it will affect the bypass effect, and will flow pulsating current on the power line and ground wire, causing interference;
    (2) For the power supply direction inside the circuit, power supply should be taken from the final stage to the forward stage, and the power filter capacitor of this part is arranged near the final stage;
    ③ For some main current channels, such as to disconnect or measure the current during debugging and detection, the current notch should be arranged on the printed wire during layout.
    In addition, pay attention to the layout of the regulated power supply, as far as possible arranged on a separate printed board. When the power supply and circuit are combined with the printed board, in the layout, the mixed layout of the regulated power supply and circuit components should be avoided or the power supply and circuit are combined with the ground wire. Because this wiring is not only easy to cause interference, but also can not disconnect the load during maintenance, then only part of the printed wire can be cut, thus damaging the printed board.
    Although at present, the PCB surface treatment process changes are not very large, it seems to be a relatively distant thing, but it should be noted that long-term slow changes will lead to great changes. In the case of increasing calls for environmental protection, the surface treatment process of PCB will definitely change dramatically in the future.
    The most basic purpose of surface treatment is to ensure good solderability or electrical properties. Since natural copper tends to exist in the form of oxides in the air, it is unlikely to remain as original copper for a long time, so other treatments are needed for copper. Although a strong flux can be used to remove most copper oxides in subsequent assembly, the 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 is hot air smoothing, organic coating, electroless nickel plating/gold dipping, silver dipping and tin dipping these five processes, will be introduced one by one below.
1, hot air smoothing (spray tin)
    Hot air smoothing, also known as hot air solder smoothing (commonly known as spray tin), it is the process of coating molten tin (lead) solder on the surface of the PCB and heating compressed air to smooth (blow), so that it forms a coating layer that is resistant to copper oxidation and can provide good solderability. During hot air conditioning, a copper-tin metal intermetallic compound is formed at the joint of solder and copper. PCB for hot air finishing usually to sink in the molten solder; The air knife blows out the liquid solder before it solidifies; The air knife minimizes the meniscus of solder on the copper surface and prevents solder bridging.

2, organic weldability protectant (OSP)
    OSP is a process for printed circuit board (PCB) copper foil surface treatment in accordance with the requirements of the RoHS directive. OSP is short for Organic Solderability PreservaTIves, also known as organic solderability preservatives, also known as Preflux in English. Simply put, OSP is a chemically grown organic skin film on a clean, bare copper surface.
    This film has anti-oxidation, heat shock, moisture resistance, to protect the copper surface in the normal environment no longer rust (oxidation or vulcanization, etc.); However, in the subsequent welding high temperature, this protective film must be easily removed by the flux quickly, so that the exposed clean copper surface can be immediately combined with the molten solder in a very short time to become a solid solder joint.

3, the whole plate plated nickel gold
    Plate nickel plating is the PCB surface conductor first plated with a layer of nickel and then plated with a layer of gold, nickel plating is mainly to prevent the diffusion between gold and copper. There are two types of electroplated nickel gold: soft gold plating (pure gold, gold surface does not look bright) and hard gold plating (smooth and hard surface, wear-resistant, containing other elements such as cobalt, gold surface looks brighter). Soft gold is mainly used for chip packaging gold wire; Hard gold is mainly used in non-welded electrical interconnections.

4. Sink gold
    Sinking gold is wrapped in a thick, electrically good nickel gold alloy 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, sinking gold can also prevent the dissolution of copper, which will benefit lead-free assembly.

5. Sinking tin
    Since all current solders are tin-based, the tin layer can be matched to any type of solder. The process of sinking tin can form flat copper-tin metal intermetallic compounds, which makes the sinking tin have the same good solderability as the hot air leveling without the headache flat problem of hot air leveling; The tin plate cannot be stored for too long, and the assembly must be carried out according to the order of the tin sinking.

6. Sinking silver
    Silver sinking process is between organic coating and electroless nickel/gold plating, the process is relatively simple and fast; Even when exposed to heat, humidity and pollution, silver is still able to maintain good weldability, but will lose its luster. Silver plating does not have the good physical strength of electroless nickel plating/gold plating because there is no nickel underneath the silver layer.

7, chemical nickel palladium
    Compared with the precipitation of gold, there is an extra layer of palladium between nickel and gold, and palladium can prevent the corrosion phenomenon caused by the replacement reaction and make full preparation for the precipitation of gold. Gold is closely coated with palladium, providing a good contact surface.

8. Electroplating hard gold
    In order to improve the wear resistance of the product, increase the number of insertion and removal and electroplating hard gold.
    With more and more high user requirements, more and more stringent environmental requirements, more and more surface treatment processes, in the end, the choice of the kind of development prospects, more versatile surface treatment process, it seems a little dazzling, confusing. Where the PCB surface treatment process will go in the future can not be accurately predicted now. In any case, meeting user requirements and protecting the environment must be done first!