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  • Back Drilling Process of PCB

    1. What is the back drilling?

            Back drilling is a special kind of deep hole drilling. In the production of multi-layer boards, such as 12-layer boards, we need to connect the first layer to the ninth layer. Usually, we drill a through hole (a single drill) and then sink copper.In this way, the first floor is directly connected to the 12th floor. In fact, we only need the first floor to connect to the 9th floor, and the 10th floor to the 12th floor because there is no line connection, like a pillar.This pillar affects the path of the signal and can cause signal integrity problems in communication signals.So drill the redundant column (STUB in the industry) from the reverse side (secondary drill).So called back drill, but generally not drill so clean, because the subsequent process will electrolysis off a little copper, and the drill tip itself is pointed.Therefore, the PCB manufacturer will leave a small point. The STUB length of this STUB is called B value, which is generally in the range of 50-150um.

    2.The advantages of back drilling

        1) reduce noise interference

        2) improve signal integrity

        3) local plate thickness decreases

        4) reduce the use of buried blind holes and reduce the difficulty of PCB production.

    3.  The usage of back drilling

    Back to drill the drill didn’t have any connection or the effect of hole section, avoid to cause the reflection of high-speed signal transmission, scattering, delay, etc., brings to the signal “distortion” research has shown that the main factors influencing the signal system signal integrity design, plate material, in addition to the factors such as transmission lines, connectors, chip packages, guide hole has a large effect on the signal integrity.

    4. Working principle of back drilling

    When the drill needle is drilling, the micro current generated when the drill needle contacts the copper foil on the surface of the base plate will induce the height position of the plate, and then the drill will be carried out according to the set drilling depth, and the drill will be stopped when the drilling depth is reached.

    5.Back drilling production process

             1) provide a PCB with a tooling hole. Use the tooling hole to position the PCB and drill a hole;

             2) electroplating the PCB after drilling a hole, and seal the hole with dry film before electroplating;

             3) make outer layer graphics on electroplated PCB;

             4) conduct pattern electroplating on the PCB after forming the outer pattern, and conduct dry film sealing of the positioning hole before pattern electroplating;

            5) use the positioning hole used by one drill to position the back drill, and use the drill cutter to back drill the electroplating hole that needs to be back drilled;

            6) wash back drilling after back drilling to remove residual cuttings in back drilling.

    6. Technical characteristics of back drilling plate

         1) Rigid board ( most)

         2) Usually it’s 8 – 50 layers

         3) Board thickness : over 2.5mm

         4) Thickness diameter is relatively large

         5) Size of board is relatively large

         6) Minimum hole diameter of first drill is > = 0.3mm

         7) Outer circuit less, more square design for the compression hole

         8) The back hole is usually 0.2mm larger than the hole that needs to be drilled

         9) The depth tolerance is +/- 0.05mm

         10) If the back drill requires drilling to the M layer, the thickness of the medium between the M layer and the m-1 (the next layer of the M layer) shall be a minimum of 0.17mm

     7.The main application of back drilling plate

            Communication equipment, large server, medical electronics, military, aerospace and other fields. As military and aerospace are sensitive industries, The domestic backplane is usually provided by the research institute, research and development center of military and aerospace systems or PCB manufacturers with strong military and aerospace background.In China, the demand for backplane mainly comes from the communication industry, and now the communication equipment manufacturing field is gradually developing.