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PCB Circuit Board Through-hole Basic Knowledge

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Through-hole (via) is a single-sided PCB, double-sided circuit boards, multilayer circuit boards, one of the important parts of the drilling costs usually accounted for the PCB circuit board board production costs of 20% to 30%. From the role of the hole can be divided into the electrical connection between the layers and used as a device fixed or positioning of the two categories. From the process on the hole is generally divided into three categories, namely, blind holes (blindvia), buried holes (buriedvia) and through-hole (throughvia). Blind vias are located on the top and bottom surfaces of the printed wiring board, with a certain depth, for the surface layer of the line and the following inner layer of the line connection, the depth of the hole is usually no more than a certain ratio (aperture diameter). Buried holes are connection holes located in the inner layer of the printed wiring board, which do not extend to the surface of the PCB.

The above two types of holes are located in the inner layer of the PCB circuit board, lamination before the use of through-hole molding process to complete, in the process of through-hole formation may also overlap well several inner layers. The third type is called through-hole, this hole through the entire PCB circuit board, can be used to achieve internal interconnections or as a component of the mounting and positioning holes. Because the through-hole in the process is easier to realize, lower cost, so the vast majority of printed circuit boards use it, rather than the other two types of holes. From the design point of view, a through-hole is mainly composed of the middle of the drill hole (drillhole) and the pad area around the drill hole, the size of these two parts determine the size of the hole. The smaller the hole, the smaller its own parasitic capacitance, suitable for high-speed circuits. But the reduction of hole size at the same time brings an increase in cost, and by drilling (drill) and plating (plating) and other process technology limitations.

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PCB Circuit Board Through-hole:

First, About the parasitic capacitance of the perforations

Parasitic capacitance of the over-hole over-hole itself exists on the parasitic ground stray capacitance, the parasitic capacitance of the over-hole will cause the main effect of the circuit is to prolong the rise time of the signal, reducing the speed of the circuit. Although the parasitic capacitance of a single over-hole caused by the rise of the delay in the effect of slowing down is not very obvious, but if the alignment of the multiple use of over-holes for layer switching, the designer should be carefully considered.

Second, On the use of vias

A. PCB board signal alignment as far as possible not to change the layer, that is, try not to use unnecessary holes.

B. For higher density of high-speed PCB circuit boards, you can consider the use of micro-perforation.

C. The two formulas discussed above can be derived from the use of thinner PCB boards is conducive to reducing the two parasitic parameters of the vias.

D. Power and ground pins should be played near the hole, the shorter the lead between the hole and the pin, the better. Consider playing multiple vias in parallel to reduce the equivalent inductance.

E. Place some grounded vias near the vias of the signal transitions to provide the closest loop for the signals. You can even place some extra grounded vias on the PCB.

F. From the cost and signal quality considerations, choose a reasonable size of the over-hole size. If necessary, you can consider using different sizes of vias, for example, for power or ground vias, you can consider using larger sizes to reduce impedance, while for signal routing, you can use smaller vias. Of course, with the reduced size of the over-hole, the corresponding cost will increase.

Third, About the parasitic inductance of the vias

PCB circuit boardvia the existence of parasitic capacitance at the same time there is also a parasitic inductance, in the design of high-speed digital circuits, the parasitic inductance of the hole is often greater than the parasitic capacitance of the harm brought about by the impact. Its parasitic series inductance weakens the contribution of the bypass capacitors and diminishes the filtering utility of the entire power system. The diameter of the via has less impact on the inductance, while the biggest impact on the inductance is the length of the PCB via.The impedance generated by the PCB via can no longer be ignored when there is a high frequency current passing through it, and it is especially important to note that the bypass capacitor needs to pass through two bypasses when connecting the power supply layer and the ground layer, which multiplies the parasitic inductance of the bypass hole.

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