TWI439188B - Printed circuit board - Google Patents

Description

A printed circuit board

This invention relates to a printed circuit board.

In high-speed circuits, in order to eliminate the influence of the DC signal on the reference level of the receiving end, the capacitor is often used to isolate the DC signal from the high-speed signal and to couple the AC. A common method is to connect the capacitor directly to the signal line. The capacitor reference layer is a complete plane, and the return signal can be directly reflowed from the reference layer below the capacitor. Please refer to the time domain reflection waveform diagram of FIG. 1 , which is a signal simulation of a capacitor of 0402 package. The pad width is 20 mils during simulation, the width of the transmission line is 5 mils, and the abscissa indicates the signal in the waveform diagram. The transmission time, the ordinate represents the impedance value, wherein the impedance changes when the high-speed signal passes through the capacitor between 20.95 ns and 21.4 ns. This is because the width of the capacitor pad is much larger than the width of the transmission line, and the characteristic impedance is abruptly changed between the transmission line and the capacitor pad, so that the high-speed signal enters the low-impedance path from the high-impedance path. Due to the impedance mismatch, high-speed signals can cause reflections that affect signal integrity.

In view of the above, it is necessary to provide a printed circuit board that satisfies the impedance matching between the transmission line and the capacitor pad as much as possible, so as to avoid reflection phenomenon when transmitting high-speed signals, thereby improving the integrity of the high-speed signal.

A printed circuit board comprising a dielectric layer and a reference layer disposed under the dielectric layer, the dielectric layer being provided with a transmission line and a pad for soldering an electronic component connected to the transmission line, the reference layer corresponding to the electron The position of the component is provided with a hollowed out area with a smooth curved contour. The hollowed out area is centrally symmetrical, and its center corresponds to the center of the electronic component. The length of the hollowed out area perpendicular to the length of the two sections of the transmission line is : ,among them The width of the pad; The width of the two transmission lines; T is the thickness of the pad.

The printed circuit board increases the impedance of the pad by maximizing the impedance of the pad by increasing the impedance of the pad by adding a hollowed out area with a smooth outline of the peripheral profile on the reference layer corresponding to the pad, thereby greatly reducing the impedance of the printed circuit. The reflection phenomenon generated when the signal is transmitted on the board, thereby improving the integrity of the signal.

Referring to FIG. 2 and FIG. 3 together, a preferred embodiment of the printed circuit board 1 of the present invention includes a dielectric layer 10 and a reference layer 20 disposed under the dielectric layer 10. The dielectric layer 10 is provided with two sections of transmission lines 12, 13 and two pads 14 and 15 respectively connecting the two sections of transmission lines 12 and 13. The two pads 14 and 15 are used to solder an electronic component such as a capacitor 30. The reference layer 20 provides a return path for the high speed signals flowing through the two sections of transmission lines 12, 13 and the capacitor 30. A hollowed out area 22 is defined in the reference layer 20 at a position corresponding to the capacitor 30. The center of the hollowed out area 22 corresponds to the center of the capacitor 30, and the peripheral contour thereof has a smooth curve to avoid a reflection phenomenon. In this embodiment, The hollowed out area 22 is an elliptical hollowed out area.

In order to eliminate the influence of the DC signal on the reference level of the signal receiving end during high-speed signal transmission, the capacitor 30 is soldered to the two pads 14 and 15 for isolated DC and coupled to communicate. Since the reference layer 20 is provided with the hollowed-out area 22 at the position corresponding to the capacitor 30, the high-speed signal is transmitted back through the hollowed-out area of the hollowed-out area 22 when the high-speed signal is recirculated, thereby increasing the signal. The length of the return path (see the return path 24), the impedance of the pad 14 increases according to the characteristic principle of increasing the return path of the signal, and when it is increased to match the impedance of the transmission line 12, When the impedance of the pad 14 is equal to the impedance of the transmission line 12, the signal transmission does not reflect, so that the integrity of the high-speed signal can be maintained.

Referring to FIG. 4 and FIG. 1 together, in order to improve the integrity of the high-speed signal, the impedance of the transmission line 12 and the pad 14 should be kept as close as possible. According to the impedance of the microstrip line

Calculation formula: ,because , which is , ie ,among them For the impedance of pad 14, For the impedance of the transmission line, H1 assumes the distance from the bottom of the pad 14 to the underlying reference layer; H is the distance between the two transmission lines 12 and 13 to the underlying reference layer 20, ie the thickness of the dielectric layer 10; T is the pad Thickness of 14; The width of the two pads 14 and 15 as a whole; Is the width of the transmission line 12. H1 is directly related to the size of the hollowed out area 22, assuming ,among them As a coefficient, The length of the elliptical hollowed out region 22 along the minor axis perpendicular to the direction of the transmission lines 12 and 13. This gives you: Obtained through simulation So get: Hypothesis ,among them The length of the major axis of the elliptical hollowed out region 22 along the direction of the transmission lines 12 and 13. Obtained through simulation . Through length and The size of the elliptical hollowed out area 22 can be determined.

Please refer to FIG. 5 , which is a time domain reflection waveform diagram of the printed circuit board of the present invention. The high frequency signal simulation is performed by using a capacitor of 0402 package as an example. The width of the two pads 14 and 15 as a whole is 20 mils during simulation. The widths of the two transmission lines 12 and 13 are 5 mils, and the length W1 of the hollow area 22 perpendicular to the transmission line 12 and 13 and the length of the long axis along the two transmission lines 12 and 13 are 30 respectively. Mill and 51 mil, and the abscissa in the waveform diagram represents the signal transmission time T (ns), and the ordinate represents the impedance value Z0 (ohm), wherein the high-speed signal passes through the capacitor 30 between time 20.95 ns and 21.4 ns. The impedance change at the time. As can be seen from FIG. 5, the characteristic impedance curve 40 has a variation of less than 1 ohm between the transmission line and the capacitor pad. Compared with FIG. 1, FIG. 5 shows that the high-speed signal on the printed circuit board of the present invention passes through the capacitor 30. No mutation occurs, but only a small change, indicating that the impedance of the pad 14 closely matches the impedance of the two transmission lines 12 and 13, thereby greatly reducing the reflection phenomenon, thereby improving the signal integrity.

In summary, the present invention complies with the requirements of the invention patent and submits a patent application according to law. The above description is only the preferred embodiment of the present invention, and equivalent modifications or variations made by those skilled in the art will be included in the following claims.

1‧‧‧Printed circuit board

10‧‧‧ dielectric layer

12, 13‧‧‧ transmission line

14, 15‧‧‧ pads

20‧‧‧ reference layer

22‧‧‧Knockout area

24‧‧‧Return path

30‧‧‧ Capacitance

Figure 1 is a diagram showing the time domain reflection waveform of a conventional printed circuit board.

2 is a perspective view of a preferred embodiment of a printed circuit board of the present invention.

3 is a top plan view of a reference layer of a preferred embodiment of the printed circuit board of the present invention.

4 is a schematic longitudinal cross-sectional view showing a preferred embodiment of the printed circuit board of the present invention.

Figure 5 is a time domain reflectance waveform diagram of a preferred embodiment of the printed circuit board of the present invention.

1‧‧‧Printed circuit board

10‧‧‧ dielectric layer

12, 13‧‧‧ transmission line

14, 15‧‧‧ pads

20‧‧‧ reference layer

22‧‧‧Knockout area

24‧‧‧Return path

30‧‧‧ Capacitance

Claims (5)

A printed circuit board comprising a dielectric layer and a reference layer disposed under the dielectric layer, the dielectric layer being provided with a transmission line and a pad for soldering an electronic component connected to the transmission line, wherein the reference layer corresponds to The position of the electronic component is provided with a hollowed out area with a smooth curved contour. The hollowed out area is centrally symmetrical. The center of the hollowed out area corresponds to the center of the electronic component, and the hollowed out area is perpendicular to the direction of the two sections of the transmission line. The length of the longest distance is: ,among them The width of the pad; The width of the two transmission lines; T is the thickness of the pad. The printed circuit board of claim 1, wherein the length of the hollowed out area along the longest distance of the transmission line is: ,among them . The printed circuit board of claim 2, wherein the hollowed out area is elliptical. The length of the short axis of the hollowed out area perpendicular to the direction of the transmission line, The length of the long axis of the hollowed out area along the direction of the transmission line. The printed circuit board of claim 1, wherein the electronic component is a capacitor. The printed circuit board of claim 1, wherein the transmission line is a high speed serial signal line.