PCB Technical - What is PCB via hole?

Basic concepts of 1 PCB via

PCB via is one of the important components of multilayer PCB. The cost of drilling holes usually accounts for 30% to 40% of the cost of PCB board making. Simply put, every hole on a PCB can be called a PCB through hole. Functionally, PCB passes can be divided into two categories: one is used as electrical connection between layers; The second is to fix or locate the device. In terms of process, these PCBs are generally divided into three categories: blind via, buried via and through via. Blind holes are located on the top and bottom surfaces of the printed circuit board and have a certain depth for connection between the surface line and the underlying inner line. The depth of holes 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 printed circuit board. These two types of holes are located in the inner layer of the circuit board, which is completed by the through-hole forming process before lamination, and may overlap several inner layers during the process of PCB through-hole formation. The third is called a through hole, which passes through the entire circuit board and can be used to achieve internal interconnection or mount positioning holes as components. Because the holes are easier to achieve technically and cheaper, most printed circuit boards use them instead of the other two PCB holes. The PCB passes through holes mentioned below, without special instructions, are considered as passes through holes.

From the design point of view, a PCB pass is mainly composed of two parts, one is the drill hole in the middle, the other is the bonding pad area around the drill hole. The size of these two parts determines the size of PCB holes. Clearly, in high-speed, high-density PCB designs, designers always want the smaller the PCB passes through the hole, the better the wiring space on the board. In addition, the smaller the PCB passes through the hole, the smaller the parasitic capacitance of the PCB itself and the better it is for high-speed circuits. However, the smaller the hole size, the higher the cost, and the size of PCB holes can not be reduced indefinitely. It is limited by process technologies such as drilling and plating: the smaller the hole size, the longer it takes to drill, and the easier it is to deviate from the center position. And when the depth of the hole exceeds 6 times the diameter of the hole, it is not possible to ensure that the hole wall is evenly copper plated. For example, if a normal 6-layer PCB board has a thickness of 50 Mil (through hole depth).

Then, under normal conditions, the diameter of holes provided by PCB manufacturers can only reach 8Mil. With the development of laser drilling technology, the size of drilling holes can also be smaller and smaller. In general, PCB passes holes with a diameter less than or equal to 6Mils are called microholes. Microholes are often used in HDI (High Density Interconnect Structure) design. Microhole technology allows PCB holes to be punched directly onto the bonding pad (Via-in-pad), which greatly improves circuit performance and saves wiring space.

PCB holes in the transmission line show discontinuous impedance breakpoints, which cause signal reflection. In general, the equivalent impedance of PCB holes is about 12% lower than that of transmission lines. For example, the impedance of 50 ohm transmission lines through PCB holes will be reduced by 6 ohm (specific size of PCB holes, plate thickness is also related, not reduced). However, the reflection of PCB via due to discontinuous impedance is very slight, and its reflection coefficient is only (44-50)/(44+50)=0.06. The problems caused by PCB via are more concentrated on parasitic capacitance and inductance.

Parasitic capacitance and inductance of 2 PCB via hole

PCB passes have parasitic stray capacitance. If the diameter of the resistance zone of PCB passes on the layered layer is known to be D2, the diameter of PCB passes is D1, the thickness of PCB plate is T, and the dielectric constant of the plate base material is known to be D. ε, The parasitic capacitance of the PCB pore is approximately the same as C=1.41 ε TD1/(D2-D1)

The parasitic capacitance of the PCB through the hole will have a major impact on the circuit, which 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 50 Mil, if the diameter of the PCB through-hole pad is 20 Mil (drill hole diameter is 10 Mils) and the diameter of the resistance zone is 40 Mil, the parasitic capacitance of the PCB through-hole can be approximately calculated by the above formula:

C=1.41x4.4x0.050x0.020/(0.040-0.020)=0.31pF

The amount of rise time variation caused by this part of the capacitance is approximately:

T10-90=2.2C(Z0/2)=2.2x0.31x(50/2)=17.05ps

From these values, it can be seen that although the effect of the rise delay caused by the parasitic capacitance of a single PCB hole is not obvious, multiple PCB holes will be used for layer-to-layer switching if PCB holes are used multiple times in the wiring and should be carefully considered in design. In practical design, parasitic capacitance can be reduced by increasing the distance between PCB through hole and copper pad or by reducing the diameter of the pad.

Parasitic capacitance and inductance exist in PCB passes. In the design of high-speed digital circuit, the parasitic inductance of PCB passes often causes more harm than the parasitic capacitance. Its parasitic series inductance weakens the contribution of bypass capacitance and the filtering effectiveness of the entire power system. The following empirical formula can be used to calculate the parasitic inductance of a PCB through-hole approximation:

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

L refers to the inductance of PCB holes, h is the length of PCB holes, D is the diameter of the central hole. It can be seen from the formula that the diameter of PCB holes has little effect on inductance, while the length of PCB holes has little effect on inductance. Still with the example above, the inductance of the PCB via 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, the equivalent impedance is: XL=pi L/T10-90=3.19_. Such impedance can no longer be ignored when passing with high frequency current. It is important to note that the bypass capacitance needs to pass through two PCB holes when connecting the power layer and the layer, so that the parasitic inductance of the PCB holes can be multiplied.

3 How to use PCB to pass holes

From the above analysis of the parasitic characteristics of PCB holes, we can see that in high-speed PCB design, the seemingly simple PCB holes often have a large negative effect on circuit design. In order to reduce the adverse effects of the parasitic effect of PCB pore, it is possible to do this in the design:

A Choose a reasonable size of PCB via size considering both cost and signal quality. Consider using PCB passes of different sizes if necessary, such as larger sizes for power supply or ground wire PCB passes, to reduce impedance, and smaller PCB passes for signal routing. Of course, the cost will increase as the size of PCB holes decreases.

B From the two formulas discussed above, it can be concluded that using a thinner PCB plate is beneficial to reduce the two parasitic parameters of the PCB pore.

The signal wiring on the C PCB board should not be changed as much as possible, that is, unnecessary PCB holes should not be used.

D Power supply and ground pins should be punched close to the PCB through hole, the shorter the lead between the PCB through hole and the pin, the better. Multiple PCB passes can be considered in parallel to reduce the equivalent inductance.

E Places some grounded PCB passes near the PCB passes of the signal layer to provide a close circuit for the signal. You can even place some extra ground PCB holes on the PCB board.

F For high-speed PCB boards with high density, micro-PCB holes may be considered.