2019年11月11日星期一

Through hole design skills

  • Through hole design skills

Via hole is one of the important components of multi-layer PCB.The cost of drilling is usually 30%-40% of the cost of PCB board.To put it simply,each hole in the PCB can be called a via hole .In terms of function,the vias can be divided into two types:one is used as an electrical connection between the layers;the other is used for fixing or positioning the components.In terms of process,the vias generally divided into three types:blind via、buried via and through via.
The blind via are located on the top and bottom surface of the printed circuits board,and have a depth for the connection of the surface circuit to the underlying inner circuit.The depth of the holes usually does not exceed a certain ratio(aperture) .The buried hole is a connection hole located in the inner layer of PCB,which does not extend to the surface of circuit board.The above two types of holes are located in the inner layer of circuit board,and are made by a through-hole forming process before lamination.The third type is called a through hole,and the hole goes through the entire circuit board and can be used to implement internal interconnection or as a mounting hole for the component.Since the through via is easier to implement in the process and lower in cost,it is used in most printed circuit boards.
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First, from a design point of view, a via is mainly composed of two parts, one is a drill hole in the middle, and the other is a pad area around the drilling hole. The size of these two parts determines the size of the vias. Obviously, in high-speed, high-density PCB design, the designer always hopes that the the via are small , so that more wiring space can be left on the board.In addition, the smaller the via is, the smaller its own parasitic capacitance would be, and making it more suitable for high speed circuits.However, the reduction in the size of the hole also brings about an increase in cost, and the size of the via hole can not be reduced indefinitely. It is limited by the process techniques such as drilling and plating: The smaller the hole, the longer it takes to drill holes,the easier it is to deviate from the center position; and when the depth of the hole exceeds 12 times the diameter of the hole, there is no guarantee that the hole wall can be uniformly plated with copper.
Second, While parasitic capacitance exists in the through hole,parasitic inductance also exists. In the design of the high-speed digital circuit, the parasitic inductance of the via hole is often more harmful than the parasitic capacitance. Its parasitic series inductance weakens the contribution of the bypass capacitor and reduces the filtering effectiveness of the entire power system.
Third, in high-speed PCB design, seemingly simple vias often have a large negative effect on the design of the circuit. In order to reduce the adverse effects of the parasitic effects of vias, it is possible to do as much as possible in the design:
1. Consider the cost and signal quality, and choose a reasonable size of the via size. For vias for power supply or ground wire, consider using larger sizes to reduce impedance.
2. The thin PCB board is beneficial to reduce the two parasitic parameters of the via.
3. The signal traces on the PCB should not be changed as much as possible, that is, try not to use unnecessary vias.
4. Place some grounded vias near the vias of the signal-changing layer to provide the most recent return circuit for the signal. It is even possible to place a large number of surplus grounding holes on the PCB. Of course, you need to be flexible in design. Sometimes we can reduce or even remove pads from certain layers. Especially in the case of very large via density, it may lead to a break in the copper layer to form a partition circuit. To solve such a problem, we can also consider reducing the size of the pad with through holes in the copper layer.

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