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    Cutting edge! PCBA board planning, layout and wiring design skills

    Author:xiaozhi-zhan Views:26 Release time:2019-05-30 11:01:40

    No matter how long you have been in the PCBA industry, even senior engineers have some unknown skills in PCBA board design. Today, GENITEC Automation Extension Machine shares with you, if you change it, you can't help it.



    The Chinese name of PCB is printed circuit board.


    Also known as printed circuit board, printed circuit board, is an important electronic component, is the support of electronic components.


    It is a supplier of electrical connection for electronic components.


    Because it is made by electronic printing, it is called "printed" circuit board.


    As PCB size requirements become smaller and smaller, device density requirements become higher and higher, and PCB design becomes more and more difficult.


    How to achieve high PCB routing rate and shorten the design time, we will analyze the design skills of PCB planning, layout and routing.


    Before starting wiring, the design should be carefully analyzed and the tools and software should be carefully set up, which will make the design more in line with the requirements.


    1 Determine the number of PCB layers


    The size of the circuit board and the number of layers of wiring need to be determined at the beginning of the design. The number of layers and the way of STack-up will directly affect the wiring and impedance of printed lines.


    The size of the board is helpful to determine the overlay mode 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. It is better to use more circuit layers and distribute copper evenly at the beginning of the design.


    2 Design Rules and Limitations


    To successfully complete the routing task, routing tools need to work under the correct rules and constraints.




    To classify all special signal lines, each signal class should have priority. The higher the priority, the stricter the rules.


    Rules refer to the width of printed lines, the maximum number of holes, parallelism, the interaction between signal lines and layer limitations. These rules have a great impact on the performance of wiring tools.



    Layout of 3 Components


    In the process of optimizing assembly, the rules of DFM will restrict the layout of components.


    If the assembly department allows the components to move, the circuit can be optimized appropriately and the automatic wiring is more convenient.



    The rules and constraints defined will affect layout design.


    The automatic wiring tool only considers one signal at a time. By setting the constraints and layers of the wiring, the wiring tool can complete the wiring as the designer envisages.


    For example, for the layout of power lines:


    The power decoupling circuit should be designed in the PCB layout near the relevant circuits, rather than in the power part, otherwise it will not only affect the bypass effect, but also flow through the pulsating current on the power line and ground line, causing interference.


    (2) For the direction of power supply in the circuit, the power supply should be from the last stage to the front stage, and the power filter capacitor of this part should be arranged near the last stage.



    (3) For some main current channels, such as disconnecting or measuring current during debugging and testing, current gaps should be arranged on the printed guidance line in the layout.


    In addition, attention should be paid to the layout of voltage regulator power supply, as far as possible in a separate printed circuit board. When power supply and circuit are used in PCB, in the layout, it should be avoided to mix regulated power supply and circuit components or to make power supply and circuit co-located wiring. Because this kind of wiring is not only prone to interference, but also unable to disconnect the load during maintenance. At that time, only a part of the PCB can be cut, thereby damaging the PCB.




    Although at present, PCB surface treatment process changes are not very big, as if it is still a relatively distant thing, but it should be noted that long-term slow changes will lead to tremendous changes. With the increasing voice of environmental protection, the surface treatment technology of PCB is bound to change dramatically in the future.


    The 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, it is unlikely to remain as raw copper for a long time, so other treatments are needed for copper.


    Although in the subsequent assembly, strong flux can be used to remove most of the copper oxide, but the strong flux itself is not easy to remove, so the industry generally does not use strong flux.


    Nowadays, there are many PCB surface treatment processes, such as hot air leveling, organic coating, electroless nickel plating/gold impregnation, silver impregnation and tin impregnation. GENITEC automatic splitter will be introduced one by one.



    1. Hot air leveling (tin spraying)

    Hot air leveling, also known as hot air solder leveling (commonly known as tin spraying), is a process of coating molten tin (lead) solder on PCB surface and heating compressed air leveling (blowing) to form a coating which not only resists copper oxidation, but also provides good solderability.



    Copper-tin intermetallic compound is formed at the joint of solder and copper during hot air rectification. PCB should sink into molten solder during hot air rectification; air knife can blow liquid solder before solidification; air knife can minimize the meniscus of solder on copper surface and prevent solder bridging.



    2. Organic solderability protector (OSP)


    OSP is a process for surface treatment of printed circuit board (PCB) copper foil which meets the requirements of RoHS instructions.


    OSP is the abbreviation of Organic Solderability PreservaTIves, which is translated into organic soldering coating, also known as copper protection agent, also known as Preflux in English.


    Simply put, OSP is to grow a layer of organic skin film on a clean bare copper surface by chemical method.


    This film has anti-oxidation, heat shock resistance and moisture resistance to protect the copper surface from rusting (oxidation or sulfuration, etc.) in normal environment.


    However, in the subsequent high temperature welding, the protective film must be easily removed by flux, so that the exposed clean copper surface can be bonded with molten tin in a very short time to form a solid solder joint.




    3. Full Plate Nickel-Gold Plating


    Nickel-gold plating on PCB is to deposit a layer of nickel on PCB surface and then a layer of gold. Nickel plating mainly prevents the diffusion between gold and copper.


    There are two kinds of nickel-gold plating: soft gold (pure gold, the surface of gold looks not bright) and hard gold (smooth and hard surface, wear-resistant, containing other elements such as cobalt, the surface of gold looks brighter). Soft gold is mainly used to make gold wire for chip packaging, while hard gold is mainly used for electrical interconnection in non-welded parts.


    4. Sink gold.


    Deposited gold is coated with a thick nickel-gold alloy with good electrical properties on the copper surface, which can protect PCB for a long time. In addition, it also has environmental tolerance that other surface treatment processes do not have. In addition, gold deposition can also prevent the dissolution of copper, which will benefit lead-free assembly.



    5. Tin deposit


    Since all solders are tin-based at present, the tin layer can match any type of solder.


    The tin deposition process can form a flat copper-tin intermetallic compound, which makes the tin deposition as solderable as hot air leveling without the headache of hot air leveling.


    The tin plate can not be stored for too long, and must be assembled according to the sequence of tin deposition.



    6. Silver immersion.


    Silver deposition process is between organic coating and electroless nickel/gold plating, which is relatively simple and fast. Silver can maintain good solderability even when exposed to hot, wet and polluted environment, but will lose luster.


    Silver deposits do not have the good physical strength of electroless nickel plating/gold deposits because there is no nickel under the silver layer.



    7. Chemical Nickel, Palladium and Gold


    Compared with gold deposit, chemical nickel, palladium and gold deposit has an additional layer of palladium. Palladium can prevent the corrosion caused by replacement reaction and make full preparations for gold deposit. Gold is tightly covered with palladium, providing a good contact surface.



    8. Electroplating Hard Gold


    In order to improve the wear resistance of the product and increase the number of insertions, hard gold was plated.


    With the increasing requirements of users, stricter environmental requirements and more and more surface treatment processes, it is necessary to choose the surface treatment process with more development prospects and more versatility.


    At present, it seems a little dazzling and confusing. It is impossible to predict exactly where PCB surface treatment technology will go in the future. Anyway, satisfying users'requirements and protecting the environment must be done first!


    Previous: Solutions to Common Problems in PCB Printing
    Next: Effect of Temperature Curve on Cleanness and Electrical Performance-Pursuing Reliability

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