Through hole (via) is one of the important components of multi-layer PCB board, and the cost of drilling usually accounts for 30% to 40% of the cost of PCB board. Simply put, every hole on the PCB can be called a pass hole.

In terms of function, pores 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.

Blind hole

Located on the top and bottom surfaces of the printed circuit board, with a certain depth for the connection of the surface line and the inner line below, the depth of the hole usually does not exceed a certain ratio (aperture).

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.

through-hole

This hole runs through the entire circuit board and can be used for internal interconnection or as an installation positioning hole 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. The size of these two parts determines the size of the hole.

Obviously, in high-speed, high-density PCB design, it is always hoped that the smaller the hole, the better, the more wiring space can be left on the template, 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.

With the development of laser drilling technology, the size of the drilling can also be smaller and smaller, and the diameter of the hole is generally less than or equal to 6Mil, we are called microholes. Microholes are often used in HDI (high density interconnect structure) design, and microhole technology can allow the hole to be directly drilled on the pad (Via-in-pad), which greatly improves circuit performance and saves wiring space.

The through-hole appears as a breakpoint of impedance discontinuity on the transmission line, causing a reflection of the signal. Generally, the equivalent impedance of the hole is about 12% lower than the transmission line, for example, the impedance of a 50 ohms transmission line will be reduced by 6 ohms when it passes through the hole (specifically and the size of the hole and the plate thickness are also related, not reduced).

However, the reflection caused by the impedance discontinuity through the hole is actually very small, and its reflection coefficient is only:

(44-50)/(44+50) =0.06

The problems arising from the hole are more concentrated on the effects of parasitic capacitance and inductance.

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:

C=1.41εTD1/ (D2-D1)

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 hole with an inner diameter of 10Mil and a pad diameter of 20Mil is used, and the distance between the pad and the copper floor area is 32Mil, then 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

The amount of rise time change caused by this part of 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, if the hole is used several times in the line to switch between layers, EDA365 electronic forum reminds the designer to carefully consider.

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 use the following formula to simply calculate the parasitic inductance of a through-hole approximation:

L=5.08h [ln (4h/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 hole has little influence on the inductance, while the length of the hole has little influence on the inductance.

Still 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 size is:

XL = = 3.19 Ω PI L/T10-90

Such impedance can not be ignored in the presence of 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.