CN105592624A - High-density PCB high-efficiently suppressing edge radiation and method for suppressing edge radiation - Google Patents

Abstract

The invention discloses a high-density PCB board for efficiently suppressing edge radiation and a method for suppressing edge radiation. The main source of EMI in the PCB board is the power distribution network. The power/ground plane in the power distribution network forms a resonant cavity, which can cause serious electromagnetic radiation at the resonant frequency. The high-density PCB board and edge radiation suppression proposed by the present invention The method can effectively shield the electromagnetic radiation in the power supply/ground, and reduce the EMI to an optimal state. Among them, the stack design adopts embedded planar capacitor stacking. This embedded planar capacitor composed of thin dielectric and power ground plane can be regarded as an AC short circuit in the high frequency band, which can achieve a particularly good electromagnetic radiation suppression effect.

Description

High-density PCB board and edge radiation suppression method for efficiently suppressing edge radiation

technical field

The invention relates to the field of PCB boards, and more particularly to a high-density PCB board and an edge radiation suppression method for efficiently suppressing edge radiation.

Background technique

With the rapid development of semiconductor technology, the integration scale of electronic systems is getting bigger and bigger, the volume is getting smaller, the speed is getting faster, and the functions are getting more powerful. But it is precisely because many transistors are integrated into a single chip, the power consumption of processors and chips continues to increase, the supply voltage continues to decrease, the voltage noise margin also decreases, electromagnetic radiation continues to increase, and signal integrity problems It's getting tougher. The problem of electromagnetic interference (EMI) has become a huge challenge for the design of high-speed digital systems.

PCB edge radiation suppression is a part of high-speed PCBEMI/EMC (board-level EMI) design, and it is also the most important part. At this stage, some methods that are often used to suppress electromagnetic radiation include: 20-H rule, edge via guardrail, electromagnetic bandgap structure, decoupling wall of separated capacitors, etc. The 20-H guideline is only a rule of thumb, and the suppression achieved in many cases is not ideal. The edge via guardrail is to place some fence-like via guards on the edge of the PCB to shield electromagnetic radiation, which can have a good suppression effect. The electromagnetic bandgap structure can also have a better suppression effect, but the general mushroom-type EBG structure has poor low-frequency isolation and a narrow stop band. The split capacitor decoupling wall is simple in design, but it only performs well at low frequencies and does not perform well at high frequencies.

The solution provided by the present invention is mainly based on the stacking design of the PCB with the embedded planar decoupling capacitor and the design of the short circuit hole. Nowadays, the speed of single boards and systems is getting higher and higher, and the stacking of single board PCBs is becoming more and more important. The stacking of a single-board PCB is to reasonably stack the signal layer, power plane layer, and ground plane layer together under the requirements of both the mechanical process and the performance of the single board. Reasonable lamination can not only control the impedance of the signal transmission line, but also suppress the system noise on the board and suppress electromagnetic radiation. The proposed design method is mainly aimed at the stack design of embedded planar decoupling capacitors. Embedded planar capacitors need to use short-circuit holes to connect noise to the entire current loop of the PCB, so the connection method of the short-circuit holes also affects the effect of EMI.

Contents of the invention

In order to solve the above defects of the prior art, the present invention provides a high-density PCB board that efficiently suppresses edge radiation, can efficiently shield electromagnetic radiation in the power layer/earth layer, and reduce EMI to an optimal state.

For realizing above-mentioned purpose of the invention, the technical scheme that adopts is:

A high-density PCB board that efficiently suppresses edge radiation is characterized in that it adopts an asymmetrical laminated structure or a symmetrical laminated structure, wherein the asymmetrical laminated structure consists of several Distributed ground layer-power layer pair, wherein the ground layer-power layer pair is located on the top of the power layer; the symmetrical stack structure consists of several power layer-ground layer-power layer distributed sequentially from top to bottom A pair of configurations, wherein the ground layer is arranged between two power supply layers;

In the asymmetric laminated structure and the symmetrical laminated structure, a dielectric layer with a high dielectric constant is provided between the adjacent ground layer and the power supply layer, and the ground layer, the power supply layer and the dielectric layer are bonded together; the high dielectric constant The dielectric constant of the dielectric layer is greater than 3.7;

In the asymmetric stacked structure and the symmetrical stacked structure, all power planes/ground planes are connected through short-circuit vias.

In the above scheme, the embedded planar capacitance composed of the dielectric layer, the power layer, and the ground layer can be regarded as an AC short circuit in the high frequency band due to the large distributed capacitance. Therefore, the power layer and the ground layer can be regarded as a short circuit with infinitely small impedance. Such an embedded planar capacitor can be regarded as a plane, and in this case, the embedded planar capacitor can be regarded as a shorting hole to ground.

Preferably, the thickness of the dielectric layer is 0.4 mm.

Preferably, the dielectric layer has a thickness of 0.01 mm and a dielectric constant of 20.

Preferably, in the asymmetric stacked structure or the symmetrical stacked structure, the power layer and the ground layer have the same thickness.

Preferably, the radius of the short-circuit via hole is 0.15 mm.

Simultaneously, the present invention also provides a kind of edge radiation suppression method, and its specific scheme is as follows:

A method for suppressing edge radiation, which suppresses edge radiation by selecting a stacked structure of a high-density PCB board and designing short-circuit vias, comprising the following steps:

S1. Determine whether the EMI interference is low-frequency interference or high-frequency interference. If it is low-frequency interference, use a symmetrical laminated structure for the high-density PCB board. If it is high-frequency interference, use an asymmetrical laminated structure for the high-density PCB board. layer structure;

S2. Calculate the period of the short-circuit via. If the period of the short-circuit via is greater than 20H, the high-density PCB board adopts an asymmetrical laminated structure, and the short-circuit via only connects all the ground layers; if the short-circuit via If the period is less than 20H, the high-density PCB board adopts a symmetrical laminated structure, and the short-circuit via holes connect all ground layers and all power layers;

S3. If the stacked structure determined in S1 and S2 conflicts, adopt the stacked structure determined in step S1.

Compared with prior art, the beneficial effect of the present invention is:

(1) Embedded planar decoupling capacitors can provide fast switching speeds, thereby improving the power transfer performance of high-speed circuits.

(2) Effectively reduce the power of high-frequency noise, improve signal integrity performance, and reduce EMI.

(3) It has excellent performance and can achieve a good suppression effect.

(4) The suppression band can be very wide and a high corner frequency can be obtained.

Description of drawings

Figure 1 is a schematic diagram of the experimental prototype board.

FIG. 2 is a schematic diagram of the structure of an embedded planar decoupling capacitor.

FIG. 3 is a schematic diagram of an asymmetric laminated structure.

Fig. 4 is a schematic diagram of a symmetrical laminated structure.

FIG. 5 is a schematic diagram of a G short circuit via.

FIG. 6 is a schematic diagram of a P short circuit via.

FIG. 7 is a schematic diagram of a P/G short circuit via.

FIG. 8 is a schematic diagram of a grounding short circuit hole.

detailed description

The accompanying drawings are for illustrative purposes only and cannot be construed as limiting the patent;

The present invention will be further elaborated below in conjunction with the accompanying drawings and embodiments.

Example 1

1. Experimental board prototype selection of the present invention.

The present invention takes the three-layer PCB board with the most common size as the experimental board prototype, the size is 80mm×120mm, the dielectric material is the most commonly used FR4 with a dielectric constant of 4.4, the thickness of the dielectric layer is 0.4mm, and the power supply/ The thickness of the ground plane layer is 0.03mm. The excitation source is the lumped port excitation, which is located on the lower right side of the PCB, as shown in Figure 1.

2. Embedded planar capacitor stack design that suppresses edge radiation.

For such prototyping boards, electromagnetic radiation is generated for various reasons. The current path is interrupted at the edge of the plane, the impedance changes suddenly, the signal is emitted and rings, and the spectrum peaks at the ringing frequency, which intensifies the radiation. The power/ground plane pair constitutes a flat panel resonant cavity, which is excited by the current or noise between the planes to cause resonance, resulting in serious electromagnetic radiation at the edge of the PCB. The embedded planar capacitor stack design proposed by the present invention can achieve a good electromagnetic radiation suppression effect, and the specific steps are as follows:

(1) Each power layer and ground layer of the prototype board is designed as an embedded planar capacitor structure, that is, a power/ground pair separated by a very thin, high dielectric constant medium. The thickness of the new dielectric layer is usually 0.01 mm with a dielectric constant of 20.

(2) There are two stacks of embedded planar decoupling capacitors, one is the asymmetric G-P-G-P-G-P stack design, as shown in Figure 3, and the other is the symmetrical P-G-P-P-G-P stack design, as shown in Figure 4 .

For Figure 3, the decoupling effect of the top plane capacitor is relatively poor at low frequencies, and the current is more likely to flow through the P short circuit path and the subsequent bottom plane capacitor to flow and form a loop. At high frequencies, the decoupling effect of the top plane capacitor is greatly improved, and the current Select the top plane capacitor and G short circuit to flow through the hole and form a loop. The analysis of Figure 4 is similar, except that the short-circuit hole through which the current passes at low frequencies is the G short-circuit hole, and the short-circuit hole through which the current passes at high frequencies is the P short-circuit hole. Observing Figure 3 and Figure 4, it can be seen that the path of the short-circuit hole is different for the asymmetric and symmetrical stacks. At low frequencies, the length of the G-short-circuit hole in Figure 4 is shorter than that of the P-short-circuit hole in Figure 3, and the corresponding parasitic The inductance and the impedance it brings are also small, so the noise decoupling performance is better, and the electromagnetic radiation generated will be smaller. At high frequencies, the length of the P short-circuit via in Figure 4 is longer than that of the G short-circuit via in Figure 3, so the denoising performance is relatively poor, and the electromagnetic radiation it brings is greater than that in Figure 3.

Therefore, the EMI suppression at low frequencies adopts a symmetrical stack design, and the EMI suppression at high frequencies adopts an asymmetric stack structure.

3. Design of short-circuit via wall.

Around the edge of the PCB board, as long as the upper and lower embedded plane decoupling capacitors are connected with short-circuit holes, the effective management of power supply noise can be realized, thereby eliminating noise between planes. The main design steps of the short-circuit via wall are as follows:

(1) Determination of the dimensions of vias, pads, and anti-pads.

The connection of via holes is a non-ideal interconnection, which will cause problems such as impedance mutation and reflection. The pad is the fusion part of the via and the trace at the junction, and the purpose is to match the impedance of the junction as much as possible. The via hole will form an anti-pad through the plane that is not necessary to connect, and the radius is slightly larger than the via, commonly known as the sand hole. In consideration of cost and signal quality, a reasonable via hole size is selected, and the radius of the via hole selected in the present invention is 0.15 mm. The sand outlet hole is equivalent to a tiny slot on the plane, which will destroy the continuity of the plane, increase the loop inductance of the plane, and deteriorate the electromagnetic integrity. Therefore, in the redesign, try to use small sand outlet holes and make the sand outlet The distance between the holes should be as large as possible.

(2) Determination of the via cycle.

The period of the short-circuit via can be calculated according to the following formula:

Where f _c represents the corner frequency of the short-circuit via, C ₀ is the speed of light in vacuum, k is a correction coefficient with a value of 0.13, p is the distribution period of the short-circuit hole, r is the radius of the short-circuit hole, and εr is the dielectric constant of the medium.

Each short-circuit hole decoupling wall design with period and radius corresponds to a radiation suppression zone. The smaller the period, the larger the radius and the wider the suppression zone. We call the upper limit frequency of this suppression zone the corner frequency of the short-circuit hole. In actual design, if the required corner frequency and the radius of the short-circuit hole are known, the period of the short-circuit via can be calculated according to the above formula.

(3) Selection of short-circuit hole connection method

There are three connection methods for short-circuit holes, G-short-circuit vias, P-short-circuit vias, and P/G short-circuit vias, corresponding to Figures 5, 6, and 7, respectively. Under normal circumstances, the effect of connecting only the P short-circuit vias on the power layer and the G short-circuit vias only on the ground layer is similar, but the effect of connecting both the power layer P and the ground layer G with short-circuit vias at the same time is not good, so In general, P/G short-circuit via connection is selected.

If the period of the short-circuit via is too large, an asymmetric stack and a single G short-circuit via should be used. If the period of the short-circuit via is small enough, an asymmetric stack plus a mixed P/G short-circuit design should be used.

4. High frequency equivalent design

The embedded planar capacitance composed of a thin dielectric and the power ground plane pair, as shown in Figure 2, can be regarded as an AC short circuit in the high frequency band due to the large distributed capacitance, so the power ground plane pair can be regarded as an infinitely small impedance short circuit. Such an embedded planar capacitor can be regarded as a plane. In this case, the embedded planar capacitor can be regarded as a ground short-circuit hole, as shown in FIG. 8 .

Apparently, the above-mentioned embodiments of the present invention are only examples for clearly illustrating the present invention, rather than limiting the implementation of the present invention. For those of ordinary skill in the art, other changes or changes in different forms can be made on the basis of the above description. It is not necessary and impossible to exhaustively list all the implementation manners here. All modifications, equivalent replacements and improvements made within the spirit and principles of the present invention shall be included within the protection scope of the claims of the present invention.

Claims (6)

1. A high-density PCB board that efficiently suppresses edge radiation is characterized in that: an asymmetrical laminated structure or a symmetrical laminated structure is used, wherein the asymmetrical laminated structure consists of several from top to bottom The ground layer-power layer pair distributed in sequence below, wherein the ground layer-power layer pair is located on the top of the power layer; the symmetrical laminated structure consists of several power layer-ground layers- A pair of power supply layers, wherein the ground layer is arranged between two power supply layers; In the asymmetric laminated structure and the symmetrical laminated structure, a dielectric layer with a high dielectric constant is provided between the adjacent ground layer and the power supply layer, and the ground layer, the power supply layer and the dielectric layer are bonded together; the high dielectric constant The dielectric constant of the dielectric layer is greater than 3.7; In the asymmetric stacked structure and the symmetrical stacked structure, all power planes/ground planes are connected through short-circuit vias. 2. The high-density PCB board for efficiently suppressing edge radiation according to claim 1, characterized in that: the thickness of the dielectric layer is 0.4mm. 3. The high-density PCB board capable of efficiently suppressing edge radiation according to claim 2, wherein the dielectric layer has a thickness of 0.01 mm and a dielectric constant of 20. 4. The high-density PCB board capable of efficiently suppressing edge radiation according to claim 1, characterized in that: in the asymmetric laminated structure or the symmetrical laminated structure, the thickness of the power supply layer and the ground layer are the same. 5. The high-density PCB board capable of efficiently suppressing edge radiation according to claim 1, characterized in that: the radius of the short-circuit via hole is 0.15 mm. 6. A method for suppressing edge radiation, characterized in that: suppressing edge radiation by selecting the laminated structure of the high-density PCB board described in any one of claims 1 to 5 and designing short-circuit vias, comprising the following steps : S1. Determine whether the EMI interference is low-frequency interference or high-frequency interference. If it is low-frequency interference, use a symmetrical laminated structure for the high-density PCB board. If it is high-frequency interference, use an asymmetrical laminated structure for the high-density PCB board. layer structure; S2. Calculate the period of the short-circuit via. If the period of the short-circuit via is greater than 20H, the high-density PCB board adopts an asymmetric laminated structure, and the short-circuit via only connects all the ground layers; if the short-circuit via If the period is less than 20H, the high-density PCB board adopts a symmetrical laminated structure, and the short-circuit via holes connect all ground layers and all power layers; S3. If the stacked structure determined in S1 and S2 conflicts, adopt the stacked structure determined in step S1.