The role of through-hole (VIA) in PCB design

Through hole (via) is one of the important components of multi-layer PCB, and the cost of drilling usually accounts for 30% to 40% of the PCB board cost. Simply put, every hole on the PCB can be called a pass hole. From the function point of view, the hole can be divided into two categories: one is used as an electrical connection between the layers; The second is used as the fixing or positioning of the device. If you look at the process, these holes are generally divided into three categories, namely blind via, buried via and through via.

1, the composition of the hole

Blind holes are located on the top and bottom surfaces of the printed circuit board and have a certain depth for the connection of the surface line and the inner line below, and the depth of the holes usually does not exceed a certain ratio (aperture).

The buried hole refers to the connection hole located in the inner layer of the printed circuit board, which does not extend to the surface of the board. The above two types of holes are located in the inner layer of the circuit board, which is completed by the through hole molding process before lamination, and several inner layers may be overlapped during the formation of the through hole.

The third type is called through holes, which pass through the entire circuit board and can be used to achieve internal interconnection or as installation positioning holes for components. Because the through hole is easier to achieve in the process and the cost is lower, the vast majority of printed circuit boards use it, rather than the other two through holes. The following holes, without special instructions, are considered as through holes. From a design point of view, a through hole is mainly composed of two parts, one is the drill hole in the middle, and the other is the pad area around the drill hole, see the figure below. The size of these two parts determines the size of the hole. Obviously, in high-speed, high-density PCB design, the designer always hopes that the smaller the hole, the better, the template can leave more wiring space, in addition, the smaller the hole, its own parasitic capacitance is smaller, more suitable for high-speed circuits. But the reduction in hole size also brings an increase in cost, and the size of the hole can't be reduced indefinitely, it is limited by technology such as drilling and plating: the smaller the hole, the longer it takes to drill, and the easier it is to off-center; When the depth of the hole is more than 6 times the diameter of the hole, it is impossible to ensure that the hole wall can be uniformly plated with copper. For example, the thickness (through hole depth) of a normal 6-layer PCB board is about 50Mil, so the minimum drilling diameter that PCB manufacturers can provide can only reach 8Mil.

2. Parasitic capacitance through the hole

There is a parasitic capacitance to the ground through the hole itself. If the diameter of the isolation hole through the hole on the paving layer is D2, the diameter of the welding pad through the hole is D1, the thickness of the PCB board is T, and the dielectric constant of the substrate is ε, the parasitic capacitance through the hole is approximately: The main effect of the parasitic capacitance of C=1.41εTD1/(D2-D1) through the hole on the circuit is to prolong the rise time of the signal and reduce the speed of the circuit. For example, for a PCB with a thickness of 50Mil, if you use a hole with an inner diameter of 10Mil and a pad diameter of 20Mil, The distance between the pad and the copper area on the floor is 32Mil, so we can approximate the parasitic capacitance of the hole through the above formula :C=1.41x4.4x0.050x0.020/(0.032-0.020)=0.517pF, and the rise time change caused by this part of the capacitance Is :T10-90=2.2C(Z0/2)=2.2x0.517x(55/2)=31.28ps. From these values, it can be seen that although the utility of the rise delay caused by the parasitic capacitance of a single hole is not very obvious, the designer should still carefully consider if the hole is used multiple times in the line to switch between layers.

3. Parasitic inductance through the hole

Similarly, the presence of parasitic capacitance through the hole also exists parasitic inductance, in the design of high-speed digital circuits, the harm caused by parasitic inductance through the hole is often greater than the impact of parasitic capacitance. Its parasitic series inductance will weaken the contribution of the bypass capacitor and weaken the filtering effectiveness of the whole power system. We can simply calculate the parasitic inductance of a through-hole approximation using the following formula: L=5.08h[ln(4h/d) 1] where L refers to the inductance of the through-hole,h is the length of the through-hole, and d is the diameter of the central borehole. It can be seen from the formula that the diameter of the hole has little influence on the inductance, while the length of the hole has the greatest influence on the inductance. Using the above example, the out-of-hole inductance can be calculated as :L=5.08x0.050[ln(4x0.050/0.010) 1]=1.015nH. If the rise time of the signal is 1ns, then its equivalent impedance is :XL=πL/T10-90=3.19Ω. Such impedance can not be ignored when there is a high frequency current through, in particular, note that the bypass capacitor needs to pass through two holes when connecting the power layer and the formation, so that the parasitic inductance of the hole will be multiplied.

4, high-speed PCB hole design

Through the above analysis of the parasitic characteristics of the hole, we can see that in high-speed PCB design, seemingly simple holes often bring great negative effects to the design of the circuit. In order to reduce the adverse effects caused by the parasitic effect of the hole, the design can be as far as possible:

1, from the two aspects of cost and signal quality, choose a reasonable size of the hole size. For example, for the design of 6-10 layers of memory module PCB, it is better to choose 10/20Mil(drill/pad) through holes, and for some high-density small-size boards, you can also try to use 8/18Mil through holes. Under current technical conditions, it is difficult to use a smaller size of the hole. For the through-hole of the power supply or ground wire, a larger size can be considered to reduce the impedance.

2, the two formulas discussed above can be concluded that the use of a thinner PCB board is conducive to reducing the two parameters of the hole.

3, the signal wiring on the PCB board should not be changed as far as possible, that is to say, try not to use unnecessary holes.

4. The pins of the power supply and the ground should be punched in the nearest hole, and the shorter the lead between the hole and the pin, the better, because they will lead to an increase in inductance. At the same time, the power and ground leads should be as thick as possible to reduce impedance.

5. Place some grounded holes near the holes of the signal change layer to provide the nearest loop for the signal. You can even place a large number of excess ground holes on the PCB board. Of course, you need to be flexible in your design. The through hole model discussed earlier is the case that each layer has a pad, and sometimes, we can reduce or even remove the pad for some layers. Especially in the case of very large hole density, it may lead to the formation of a fault slot in the copper layer. To solve this problem, in addition to moving the position of the hole, we can also consider reducing the size of the pad in the copper layer.