PCB Technical - The effect of PCB through hole on signal transmission

Through hole is one of the important components of multilayer PCB boards,and the cost of drilling holes usually accounts for 30% to 40% of the PCB board manufacturing cost.Simply put, every hole on a PCB can be called a pass hole.

The effect of PCB through hole on signal transmission

Through hole (VIA) is an important part of multi-layer 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.

A parasitic capacitance through the hole

The through-hole itself has parasitic capacitance to the ground.If the diameter of the isolation hole on the paving layer is D2, the diameter of the through-hole 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 through-hole is approximately as follows: The parasitic capacitance of the C=1.41εTD1/ (d2-D1) hole mainly affects the circuit by prolonging the signal rise time and reducing the circuit speed. For example, for a PCB board with a thickness of 50Mil, if the inner diameter of the hole is 10Mil, the diameter of the pad is 20Mil, and the distance between the pad and the copper floor is 32Mil,we can approximate the parasitic capacitance of the hole by using the above formula: C=1.41x4.4x0.050x0.020/ (0.032-0.020) =0.517pF, the rise time variation caused by this part of capacitance is: T10-90=2.2C (Z0/2) =2.2x0.517x (55/2) =31.28ps. From these values, it is clear that although the effect of parasitic capacitance from a single hole on the rise delay is not obvious, designers should be careful if multiple holes are used for layer-to-layer switching.

Through the hole parasitic inductance

In the design of high-speed digital circuits,the parasitic inductance of the hole is often more harmful than 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 formula:L=5.08h [ln (4h/d) +1] where L refers to the through-hole inductance,it 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. Still using the above example, the inductance out of the hole 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 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.

The effect of PCB through hole on signal transmission

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.From the cost and signal quality of two aspects,choose a reasonable size of the hole. For example, for 6-10 layers of MEMORY module PCB design,it is better to choose 10/20mil (drilling/pad) through the hole,for some high-density small size board, you can also try to use 8/18mil through the hole.With current technology,it would be difficult to use smaller holes.For power supply or ground wire through holes can be considered to use a larger size to reduce impedance.

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

3.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 an increase in inductance. At the same time, the power and ground leads should be as thick as possible to reduce impedance.

4.The signal wiring on the PCB board should not change the layer as far as possible, that is to say, try not to use unnecessary holes.

5.Place some grounding holes near the holes of the signal layer change in order to provide a close loop for the signal. You can even put a lot of extra ground holes on the PCB. Of course, you need to be flexible in your design. The through-hole model discussed above is a situation where there are pads in each layer. Sometimes, we can reduce or even remove pads in some layers. Especially in the case of the hole density is very large, it may lead to the formation of a cut off circuit groove in the copper layer, to solve such a problem in addition to moving the location of the hole,we can also consider the hole in the copper layer to reduce the size of the pad.

In high-speed PCB design, full consideration and reasonable control of the impact of through-hole is the key to ensure stable and reliable circuit performance. Through continuous technological innovation and design optimization, we can achieve more efficient and reliable high-speed circuit design.