How to design holes in PCB wiring?

What is a hole?

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

Blind holes are located on the top and bottom surfaces of the PCB and are of a certain depth. They are used to connect the surface circuit to the inner circuit below. The depth of the holes usually does not exceed a certain ratio (aperture). Buried holes are connection holes located in the inner layer of the PCB that do not extend to the surface of the PCB. Both types of holes are located in the inner layer of the circuit board, which is completed by a through-hole molding process prior to lamination. Several inner layers may be overlapped during the through-hole formation process.

The third type, called through-holes, runs through the entire circuit board and can be used for internal interconnection or as installation positioning holes for components. Because the through hole in the process is easier to achieve, the cost is lower, so most of the printed circuit board is used in it, rather than the other two through holes. The following mentioned through holes, without special instructions, are considered as through holes.

The composition of the pores

From the design point of view, a through hole is mainly composed of two parts, one is the middle of the drilling hole, the other is the welding pad area around the drilling hole. The size of these two parts determines the size of the hole. Obviously, in the design of high speed, high density PCB, the designer always wants the hole as small as possible, the template can leave more wiring space, in addition, the hole is smaller, its own parasitic capacitance is smaller, more suitable for high speed circuit.

However, the decrease of hole size also brings the increase of cost, and the size of the hole can not be reduced indefinitely, it is limited by drilling and electroplating technology: the smaller the hole, the longer the drilling time, and the more easy to deviate from the center position; When the hole depth is more than 6 times the diameter of the hole, it is impossible to guarantee the uniform copper plating on the hole wall. For example, the thickness (through hole depth) of a normal 6-layer PCB board is about 50Mil, so the minimum drilling diameter provided by PCB manufacturers can only reach 8Mil.

Parasitic properties of pores

1. Parasitic capacitance

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 plate through the hole is D1, the thickness of the PCB board is T, and the dielectric constant of the substrate is ε, then the parasitic capacitance through the hole is approximately as follows:

C = 1.41 epsilon TD1 / (- D1, D2)

The main effect of the parasitic capacitance 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 board with a thickness of 50Mil, if a through-hole with an inner diameter of 10Mil, a diameter of 20Mil of the pad is used, and the distance between the pad and the copper floor area is 32Mil, then we can approximate the parasitic capacitance of the through-hole through the above formula:

C = 1.41 x4.4 x0.050 x0.020 / (0.032 0.020)

= 0.517 pF

The variation of rising time caused by this part of capacitance is:

T10-90 = 2.2 C (Z0) / 2 = 2.2 x0.517 x (55/2)

Ps = 31.28

It can be seen from these values that although the effect of the parasitic capacitance of a single hole is not obvious, the designer should be careful if multiple holes are used to switch between layers in the route.

2. Parasitic inductance

In the design of high-speed digital circuit, the harm caused by the parasitic inductance is often greater than the influence of the parasitic capacitance. Its parasitic series inductance will weaken the contribution of the bypass capacitance and reduce the filtering effectiveness of the whole power system. We can use the following formula to simply calculate the parasitic inductance of a through-hole approximation:

L = 5.08 h [ln (4 h/d) + 1)

Where L refers to the inductance of the hole, h is the length of the hole, and d is the diameter of the central hole.

It can be seen from the formula that the diameter of the through hole has little influence on the inductance, while the length of the through hole has the biggest influence on the inductance. Using the above example again, the inductance of the hole can be calculated as:

L = 5.08 x0.050 [ln (4 x0. 050/0.010) + 1)

= 1.015 nH

If the rise time of the signal is 1ns, its equivalent impedance is:

XL = = 3.19 Ω PI L/T10-90

Such impedance can no longer be ignored in the presence of high frequency current. In particular, note that the bypass capacitor needs to pass through two holes when connecting the power layer to the formation, so that the parasitic inductance of the holes will be multiplied.

The technique of designing through holes

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

1, from the 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) pass hole, for some high-density small size boards, you can also try to use 8/18Mil pass hole.

With current technology, it is difficult to use smaller holes. For the power supply or ground wire hole can be considered to use a larger size, to reduce the impedance.

2. From the two formulas discussed above, it can be concluded that the use of thinner PCB board is beneficial to reduce the two parasitic parameters through the hole.

3. The signal routing 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 drilled nearby. The shorter the lead between the pins and the holes, the better, because they will lead to the increase of inductance. At the same time, power and ground leads should be as thick as possible to reduce impedance.

5. Place some grounded holes near the holes of the signal changing layer in order to provide the nearest loop for the signal. You can even put a lot of extra ground holes on the PCB board. Of course, you need to be flexible in your design. The perforated model discussed above is the case that each layer has pads, and sometimes, we can reduce or even remove the pads of some layers.

6. Especially in the case of very large pore density, it may lead to the formation of a cut off circuit 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 solder pad in the copper layer.