What is PCB through hole
1, Basic concepts of PCB through hole
PCB Though hole (via) is an important part of multilayer PCB.Drilling typically accounts for 30 to 40 percent of the cost of PCB fabrication.Simply put, each hole on a PCB can be called a PCB through hole.From the perspective of function, the through hole can be divided into two types: one is used as the electrical connection between the layers;Second, it is used for fixing or positioning components.As for the production process, they are usually divided into three categories: blind via, buried via and through via.
The blind hole is located on the top and bottom surface of the printed circuit board and has a certain depth for connection between the surface line and the inner line below. The hole depth usually does not exceed a certain ratio (aperture).Buried hole is a connection hole located in the inner layer of the printed circuit board, which does not extend to the surface of the circuit board.Both of the above two types of holes are located in the inner layer of the circuit board, which is completed by through hole forming process before laminating, and several inner layers may be overlapped in the process of PCB through hole forming.The third type is called a through hole, which passes through the entire circuit board and can be used to achieve internal interconnection or as a positioning hole for component installation.Because the through-hole is easier to achieve in the process, the cost is lower, so most of the printed circuit board are using it, without the other two kinds of through-hole.The following PCB through holes, without special instructions, are considered as PCB through holes.
From the design point of view, a PCB through hole is mainly composed of two parts: the drill hole in the middle and the welding disc area around the drill hole.The size of these two parts determines the size of the PCB through hole.Obviously, in high-speed and high-density PCB design, designers always hope that the smaller the hole is,this template can be left with more wiring space, In addition, the smaller the hole is, the smaller its parasitic capacitance will be, which is more suitable for high-speed circuits.But the hole size decrease with the increase of the cost, and the size of the hole cannot be reduced limitlessly and it is drilling (drill) and the limitation of the plating (plating), and other technology: the smaller the hole, the longer time drilling, also the easier it is off center location;And when the depth of the hole is more than 6 times the diameter of the hole, there is no guarantee that the hole wall can be uniformly copper plated.For example, if a normal 6-layer PCB has a thickness (through-hole depth) of 50Mil.
So PCB manufacturer can provide the minimum diameter of drilling hole can reach 8Mil under normal conditions, .With the development of laser drilling technology, the size of the drilling hole can also be smaller and smaller. Generally, the diameter of the hole is less than or equal to 6Mils, which is called microhole.Micropores are often used in HDI (high density interconnection) designs. Micropores allow Via-in-pad to be punched directly into the pad, greatly improving circuit performance and saving wiring space.
The through-hole is a break point in the transmission line where the impedance is discontinuous, causing a reflection of the signal.In general, the equivalent impedance of the transmission line through the hole is about 12% lower than that of the transmission line. For example, the impedance of the transmission line with 50 ohms will decrease by 6 ohms when passing through the hole (it depends on the size of the hole and the thickness of the plate, but it is not an absolute decrease).However, the reflection caused by impedance discontinuity through the hole is actually very small, and its reflection coefficient is only (44-50)/(44+50) =0.06. The problem caused by the hole is more concentrated on the influence of parasitic capacitance and inductance.
2.Parasitic capacitance and inductance for PCB through hole
There is stray capacitance in the hole itself.If the diameter of the solder block on the layer is D2, the diameter of the solder pad is D1, the thickness of the PCB plate is T, and the dielectric constant of the substrate is on the die groups, then the parasitic capacitance on the die groups is approximately: C=1.41 / (D2-D1)
The main effect of the parasitic capacitance passing 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 the diameter of the through-hole pad used is 20Mil (drilling diameter is 10Mils) and the diameter of the soldering mask area is 40Mil, then we can approximately calculate the parasitic capacitance outside the hole through the above formula:
C=1.41×4.4×0.050×0.020/ (0.040-0.020) =0.31pF
The rise time variation caused by this part of capacitance is roughly as follows:
T10-90=2.2C (z0/2) =2.2×0.31x (50/2) =17.05ps
It can be seen from these values that although the effect of slow rising delay caused by the parasitic capacitance of a single pass hole is not obvious, if multiple passes are used to switch between layers, multiple passes will be used, which should be considered carefully in design.In practical design, parasitic capacitance can be reduced by increasing the distance between the PCB through hole and the copper laying area (anti-pad) or reducing the diameter of the pad.
There are parasitic inductance and parasitic capacitance through the hole. In the design of high-speed digital circuit, the harm of parasitic inductance through the hole is often greater than the influence of parasitic capacitance.Its parasitic series inductance will weaken the contribution of the bypass capacitor and the filtering effect of the whole power system.We can use the following empirical formula to simply calculate the parasitic inductance of a via hole approximation:
L=5.08h[ln (4h/d) +1]
Where L refers to the inductance through the hole, h is the length of the hole, and d is the diameter of the central hole.It can be seen from the equation that the diameter of the through hole has little influence on the inductance, while the biggest influence on the inductance is the length of the through hole.Still using the above example, the inductance out of the hole can be calculated as:
L=5.08×0.050[ln (4×0.050/0.010) +1]=1.015nH
If the rise time of a signal is 1 ns, then the equivalent impedance of size: XL = = 3.19 Ω PI L/T10-90.Such impedance cannot be ignored when there is a high-frequency current passing through. In particular, the bypass capacitor needs to pass through two holes when connecting the power layer and the ground layer, so that the parasitic inductance of the hole will be multiplied.
3.How to use the hole
Through the above analysis of the parasitic characteristics of the through-hole, we can see that in high-speed PCB design, seemingly simple through-hole often brings great negative effects to the circuit design.In order to reduce the adverse effect brought by the parasitic effect of the hole, the following can be done as far as possible in the design:
A, Choosing the reasonable size of the through-hole from the cost and signal quality.If necessary, consider using holes of different sizes. For example, for power supply or ground wire, consider using a larger size to reduce impedance, while for signal wiring, use a smaller size.Of course, as the size of the hole decreases, the corresponding cost will increase.
B,The two formulas discussed above suggest that the use of thinner PCB boards can reduce the two parasitic parameters of the PCB through hole.
C, The signal routing on the PCB board should not change layer as much as possible, that is to say, unnecessary holes should not be used
D, Power and ground pin to the nearest hole, the shorter between hole and pin lead the better.Multiple PCB through hole can be considered in parallel to reduce the equivalent inductance.
E, Put some ground holes near the holes in the signal layer to provide the nearest circuit for the signal.Additional grounding holes can even be placed on the PCB.
F, For high density high speed PCB board, it can be considered to use micro through hole.
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