PCB Technical - In high-speed PCB design, how to avoid negative effects caused by through holes

First, the basic concept of the hole

Through hole (VIA) is an important part of multilayer PCB, and 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 pass hole. In terms of function, the hole can be divided into two categories: one is used for the electrical connection between layers; The other is used for device fixation or positioning. In terms of the process, these through-holes are generally divided into three categories, namely 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 connecting 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 in the inner layer of the printed circuit board that do not extend to the surface of the printed circuit board. The 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, called through-holes, runs through the entire circuit board and can be used for internal interconnections or as mounting and locating holes for components. Because the through hole is easier to implement in the process, the cost is lower, so most printed circuit boards are used it, rather than the other two kinds of through hole. The following through holes, without special explanation, shall be 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 through-hole. Obviously, in the design of high-speed, high-density PCB, the designer always wants the hole as small as possible, this sample can leave more wiring space, in addition, the smaller the hole, its own parasitic capacitance is smaller, more suitable for high-speed circuit. But the hole size decreases at the same time brings the cost increase, and the size of the hole can not be reduced without limit, it is limited by drilling (drill) and plating (plating) and other technology: the smaller the hole, the longer it takes to drill, the easier it is to deviate from the center; When the depth of the hole is more than 6 times the diameter of the hole, it is impossible to guarantee the uniform copper plating of the hole wall. For example, if the thickness (through-hole depth) of a normal 6-layer PCB board is 50Mil, then the PCB manufacturer can provide a hole diameter of 8Mil under normal conditions. With the development of laser drilling technology, the size of drilling can also be smaller and smaller. Generally, the diameter of the hole is less than or equal to 6Mils, we call it micro hole. Microholes are often used in HDI (high density Interconnect structure) design. Microhole technology allows the hole to be hit directly on the pad (VIA-in-pad), which greatly improves circuit performance and saves wiring space.

The through-hole on the transmission line is a break point of impedance discontinuity, which will cause the reflection of the signal. Generally, the equivalent impedance of the through-hole is about 12% lower than that of the transmission line. For example, the impedance of the 50ohm transmission line will decrease by 6 ohm when it passes through the through-hole (the specific is related to the size of the through-hole and the plate thickness, not decreased). However, the reflection caused by the discontinuity of impedance through the hole is actually very small, and its reflection coefficient is only :(44-50)/(44+50)=0.06. The problems caused by the hole are more focused on the influence of parasitic capacitance and inductance.

Parasitic capacitance and inductance through the hole

The parasitic stray capacitance exists in the hole itself. If the diameter of the welding resistance zone of the hole on the laying layer is D2, the diameter of the welding pad is D1, the thickness of the PCB board is T, and the dielectric constant of the substrate is ε, the parasitic capacitance of the hole is approximately C=1.41εTD1/(D2-D1).

The main effect of parasitic capacitance on the circuit is to prolong the signal rise time and reduce the circuit speed. For example, for a PCB board with a thickness of 50Mil, if the diameter of the through-hole pad is 20Mil (the diameter of the borehole is 10Mils) and the diameter of the solder block is 40Mil, we can approximate the parasitic capacitance of the through-hole by the formula above: C=1.41x4.4x0.050x0.020/(0.040-0.020)=0.31pF The rise time change caused by the capacitor is roughly: 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 parasitic capacitance of a single hole in ascending delay and slowing down is not obvious, if multiple holes are used for layer-to-layer switching in wiring, multiple holes will be used and should be carefully considered in design. In practical design, parasitic capacitance can be reduced by increasing the distance between the hole and the copper laying zone (anti-pad) or by reducing the diameter of the pad.

In the design of high-speed digital circuit, the parasitic inductance of the through-hole is more harmful than that of the parasitic capacitance. Its parasitic series inductance will weaken the contribution of bypass capacitance and reduce the filtering effectiveness of the entire power system. We can simply calculate the parasitic inductance of a through-hole approximation using the following empirical 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 hole. It can be seen from the equation that the diameter of the hole has little effect on the inductance, but the length of the hole has an effect on the inductance. Using the above example again, the inductance out of the hole can be calculated as L=5.08x0.050[ln(4x050/0.010)+1]= 1.015nh. If the signal rise time is 1ns, then the equivalent impedance size is XL=πL/T10-90=3.19 ω. This impedance cannot be ignored in the presence of high frequency current. In particular, the bypass capacitor has to pass through two holes to connect the supply layer to the formation, thus doubling the parasitic inductance of the hole.

Three, how to use the hole

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

1. Considering the cost and signal quality, a reasonable hole size is selected. If necessary, consider using different sizes of holes. For example, for power or ground cables, consider using larger sizes to reduce impedance, and for signal wiring, use smaller holes. Of course, as the hole size decreases, the corresponding cost will increase.

2. The two formulas discussed above show that the use of thinner PCB boards helps to reduce the two parasitic parameters of the perforations.

3. The signal wiring on the PCB board should not change layers as far as possible, that is to say, do not use unnecessary holes as far as possible.

4. The pins of the power supply and the ground should be drilled in the nearest hole, and the lead between the hole and the pins should be as short as possible. Multiple through-holes can be considered in parallel to reduce equivalent inductance.

5. Some ground holes are placed near the signal layering holes to provide a close loop for the signal. You can even put some extra ground holes in the PCB.

6. For high-speed PCB board with higher density, micro-holes can be considered.