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[PCB] have holes on the impact of signal transmission

Date:2008-12-05
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[PCB] have holes on the impact of signal transmission

Through-hole (via) multi-layer PCB is an important part of one of the drilling costs usually account for the cost of the PCB-board 30% to 40%. Simply put, to, PCB's on every hole can be called-off hole.

From the role, the through-hole can be divided into two categories: One is used for electrical connections between layers; The second is used as a device or a fixed location.

From the process, these holes have been generally fall into three categories, namely, blind holes (blind via), buried in holes (buried via) and through-hole (through via). Blind hole at the top of the printed circuit board and the bottom surface of a certain depth to the surface and line the following lines to connect the inner layer, the depth of the hole is usually no more than a certain percentage (Aperture). Buried hole refers to the printed circuit board is located in the inner hole of the connection, it does not extend to the surface of the board. Both types of holes are located in the inner layer circuit boards, the use of laminated before the through-hole forming process, in the process of forming through-hole may do a good job in a few overlapping layer. The third is called through-hole, the hole through the entire circuit board can be used to achieve the internal components Interconnection, or as a location to install the hole. As the through-hole in the process easier to achieve, low cost, so the vast majority of printed circuit boards are used, rather than have the other two holes. Via said the following, there is no special note, are considered as a through-hole.

From the design point of view, had a hole by the two main parts, one in the middle of drilling (drill hole), second drilling pad around the area. These two parts of the size of the decision over the size of the hole. It is clear that in the high-speed, high-density PCB design, designers always want to have, the better the smaller the hole, this board can leave more room for wiring, in addition, the smaller the hole, its own parasitic capacitance Also, the smaller, more suitable for high-speed circuit. Reduce the size of the hole, but at the same time brought the increase in the cost and size of the holes have been impossible to reduce the limit, which by drilling (drill) and plating (plating) process technology, and other restrictions: the smaller the hole, drilling Kong takes longer, the easier it is off-center location; and the depth of the hole when the drill diameter of more than 6 times, the hole on the wall can not guarantee uniformity of copper. For example, is now a normal 6 layer PCB thickness of the plate (through-hole depth) for about 50Mil, so PCB manufacturers to provide only the smallest diameter drill hole to reach 8Mil.

Second, the parasitic capacitance of the through-hole

Via its own way on the existence of the parasitic capacitance, if the hole in the shop have been known to stratum segregation hole with a diameter of D2, through-hole pad with a diameter of the D1, PCB thickness of the plate for the T, substrate sheet for the dielectric constant ε, the hole is too similar to the size of parasitic capacitance: C = 1.41εTD1 / (D2-D1) Via parasitic capacitance of the circuit will cause the main impact is to extend the signal rise time, reducing the speed of the circuit. For example, to a thickness of the PCB board 50Mil, if used for the diameter of 10Mil, pad with a diameter of the hole 20Mil, copper pad on the floor and away from the area for 32Mil, we can calculate the approximate formula of the above had hole The parasitic capacitance is more or less: C = 1.41x4.4x0.050x0.020 / (0.032-0.020) = 0.517pF, this part of the increase in capacitance caused by changes in the amount of time: T10-90 = 2.2C (Z0 / 2) = 2.2 x0.517x (55 / 2) = 31.28ps. From these values can be seen, although a single through-hole caused by the parasitic capacitance of the slow increase in the effectiveness of the extension is not obvious, but if you take the line on many occasions in the hole used for switching between layers, the designer should consider carefully.

Third, the hole over the parasitic inductance

Similarly, the hole over the existence of the parasitic capacitance at the same time there is a parasitic inductance in high-speed digital circuit design, the hole over the parasitic inductance is often greater than the harmful effects of the parasitic capacitance. It's parasitic inductance in series will weaken the contribution to the bypass capacitor, weakening the entire power system, the effectiveness of the filter. We can use the following simple formula to calculate a through-hole similar to the parasitic inductance: L = 5.08h [ln (4h / d) +1] which means the L-off hole inductors, h is over the length of the hole, d is the center The drill diameter. From the style can be seen through-hole diameter of the impact of smaller inductors, and the greatest impact on the inductance is over the length of the hole. We will use the example above, you can calculate the inductance for the through-hole: L = 5.08x0.050 [ln (4x0.050/0.010) +1] = 1.015nH. If the signal rise time is 1ns, then the equivalent impedance of its size: XL = πL/T10-90 = 3.19Ω. Such resistance in the adoption of high-frequency current does not have been able to ignore, especially bearing in mind that bypass capacitor connected in the power level and time of the formation by the two through-hole, the hole had such a parasitic inductance will be doubled.

Fourth, high-speed PCB design of the through-hole

In the face of on-off hole through the parasitic characteristics of the analysis, we can see that in the high-speed PCB design, it is in the hole too often to the circuit design of a great negative effect. In order to reduce the parasitic effect over the hole caused by the adverse effects of the design can be as much as possible:

1. From the cost and quality of the signal two considering choosing a reasonable size of the through-hole. For example, 6-10 layers of memory module PCB design, selection of 10/20Mil (drilling / pad) of the better-off hole, for a number of high-density small size of the board, you can try to use the cross 8/18Mil Hole. At present, technical conditions, it is difficult to use the smaller size of the hole over. The power of the ground or through-hole may consider the use of larger size in order to reduce the resistance.

2. The two discussed above formula can be drawn, the use of thinner will help reduce PCB board through-hole two parasitic parameters.

3.PCB on-board signal alignment as far as possible not-for-layer, that is to say as far as possible not to use unnecessary through-hole.

4. Power and the nearest to the pin holes played, and through-hole pin between the lead as short as possible, because they will lead to an increase in inductance. At the same time, power and lead to the rough as much as possible in order to reduce the resistance.

5. Signals for over-the-hole placed in the vicinity of some of the ground over the hole in order to provide the latest signal of the loop. Even in the PCB board to place some of the large number of extra holes have been grounded. Of course, also need to design flexible. Have been discussed in front of the hole is a model of each pad are, and some time, we can reduce some of the layers of the pad or even removed. Especially over a very large hole density, could lead to the shop to form a layer of copper loop cut off the slots off to address such issues in addition to moving the location of the through-hole, we can consider that the shop had holes in the copper layer Reduce the size of the pad.

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