High speed PCB via design techniques

    In high-speed PCB board design, via design is an important factor, which consists of holes, pad areas around holes, and POWER layer isolation areas. It is usually divided into three categories: blind holes, buried holes, and through holes. In the process of PCB board design, some precautions in high-speed PCB board via design are summarized by analyzing the parasitic capacitance and parasitic inductance of vias.  

    At present, the design of high-speed PCB boards is widely used in fields such as communication, computer, and graphics and image processing. All high-tech value-added electronic product designs are pursuing characteristics such as low power consumption, low electromagnetic radiation, high reliability, miniaturization, and lightweight. In order to achieve these goals, through-hole design is an important factor in high-speed PCB board design.

    1. Via

    Through holes are an important factor in multi-layer PCB board design, and a through hole is mainly composed of three parts: holes; The second is the pad area around the hole; The third is the POWER layer isolation zone. The process of through-hole is to chemically deposit a layer of metal on the cylindrical surface of the through-hole wall to connect the copper foils that need to be connected in the middle layers. The upper and lower sides of the through-hole are made into ordinary pad shapes, which can be directly connected to the lines on the upper and lower sides or not connected. Through holes can serve as electrical connections, fasteners, or xxx components.

    Through holes are generally divided into three categories: blind holes, buried holes, and through holes.

    Blind hole refers to a hole located on the top and bottom surfaces of a printed circuit board, with a certain depth, used for connecting the surface circuit and the inner layer circuit below. The depth of the hole usually does not exceed a certain ratio to the aperture.

    Buried hole refers to a connection hole located in the inner layer of a printed circuit board, which does not extend to the surface of the board.

    Both blind holes and buried holes are located in the inner layer of the circuit board, and are completed using through-hole forming technology before lamination. During the through-hole formation process, several inner layers may overlap and be made. Through holes, which pass through the entire circuit board and can be used for internal interconnection or as installation positioning holes for components. Due to the easier implementation and lower cost of through holes in the manufacturing process, they are generally used in printed circuit boards.  

    2. Parasitic capacitance of via

    The via itself has parasitic capacitance to ground. If the diameter of the isolation hole on the ground layer of the via is D2, the diameter of the via pad is D1, the thickness of the PCB is T, and the dielectric constant of the board substrate is E, then the parasitic capacitance of the via is approximately:

    C=1.41 ETD1/(D2-D1)

The parasitic capacitance of the via will mainly affect the circuit by prolonging the rise time of the signal and reducing the speed of the circuit. The smaller the capacitance value, the smaller the impact.

    3. Parasitic inductance of via

    The via itself has parasitic inductance, and in the design of high-speed digital circuits, the harm caused by parasitic inductance of the via is often greater than the impact of parasitic capacitance. The parasitic series inductance of the via will weaken the effect of the bypass capacitor and weaken the filtering efficiency of the entire power system. If L refers to the inductance of the via, h is the length of the via, d is the diameter of the central borehole, and the parasitic inductance of the via is approximately:

    L=5.08h［ln（4h/d） 1］

    From the equation, it can be seen that the diameter of the via has a relatively small impact on the inductance, while the length of the via affects the inductance.

    4. Non through-hole technology

    Non through holes include blind holes and buried holes. In non through-hole technology, the application of blind holes and buried holes can greatly reduce the size and quality of PCBs, reduce the number of layers, improve electromagnetic compatibility, increase electronic product features, reduce costs, and also make design work simpler and faster.

    In traditional PCB design and processing, through holes can bring many problems. Firstly, they occupy a large amount of effective space, and secondly, a large number of through holes densely packed in one place also pose a huge obstacle to the routing of the inner layers of multi-layer PCBs. These through holes occupy the space required for routing, and they densely pass through the surfaces of the power and ground layers, which can also damage the impedance characteristics of the power and ground layers and cause them to fail. And conventional mechanical drilling will require 20 times the amount of work compared to non drilling techniques.  

    In PCB design, although the size of solder pads and vias has gradually decreased, if the thickness of the board layer does not decrease proportionally, it will lead to an increase in the aspect ratio of the through holes, which will reduce reliability. With the maturity of advanced laser drilling technology and plasma dry etching technology, the application of non penetrating small blind holes and buried holes has become possible. If the diameter of these non penetrating guide holes is 0.3mm, the parasitic parameters they bring are about 1/10 of the original conventional holes, which improves the reliability of PCBs. Due to the use of non through-hole technology, there will be fewer large vias on the PCB, which can provide more space for wiring.

    The remaining space can be used for large-area shielding purposes to improve EMI/RFI performance. At the same time, more remaining space can also be used for partial shielding of devices and critical network cables in the inner layer, giving them electrical performance. By using non through holes, it is more convenient to fan out device pins, making high-density pin devices (such as BGA packaged devices) easy to wire, shortening the length of connections, and meeting the timing requirements of high-speed circuits.

    5. Through hole selection in ordinary PCB

    In ordinary PCB design, the parasitic capacitance and parasitic inductance of vias have a relatively small impact on PCB design. For 1-4 layer PCB design, 0.36mm/0.61mm/1.02mm is generally selected, and vias (drilling/pad/POWER isolation area) are better. Some special signal lines (such as power lines, ground lines, clock lines, etc.) can use 0.41mm/0.81mm/1.32mm vias, or other sizes of vias can be selected according to actual requirements.