TWI442843B - Flexible printed circuit board - Google Patents

Description

Flexible circuit board

The present invention relates to a printed circuit board, and more particularly to a flexible circuit board.

A flexible circuit board is a printed circuit board made of a flexible insulating substrate, which has many advantages that a rigid printed circuit board does not have. For example, the flexible circuit board has a thin thickness and can be freely bent, wound, folded, and can be arranged arbitrarily according to the spatial layout requirements, and can be arbitrarily moved and expanded in a three-dimensional space, thereby achieving integration of component assembly and wire connection. The use of a flexible circuit board can greatly reduce the size of electronic products, and is suitable for the development of electronic products in the direction of high density, miniaturization, and high reliability. Therefore, flexible circuit boards have been widely used in aviation, military, mobile communications, laptop computers, computer peripherals, PDAs, digital cameras and other fields or products.

Because the thickness of the flexible circuit board is extremely thin, the distance between the transmission line on the upper circuit board and the lower ground plane is too close, so that the flexible circuit board cannot transmit the high-speed differential signal on the flexible circuit board, even if the general process can be achieved. The finest transmission line width, such as 4 mils (1 mil = 0.0254 mm), is also difficult to achieve the characteristic impedance requirements of high-speed signal transmission lines.

In view of the foregoing, it is necessary to provide a flexible circuit board that can transmit high speed signals.

A flexible circuit board includes an upper substrate and a lower substrate, wherein a differential pair is disposed on the upper substrate, the differential pair includes two transmission lines, and the lower substrate and the two The vertically opposite portion of the area between the strip transmission lines is provided with a grounded conductive material, the differential pair includes a plurality of segment groups, each segment group consisting of two segments symmetrically distributed on the two transmission lines, each of the transmission lines The line width of each two adjacent segments is different, wherein the line width of the two segments in each segment group and the width of the portion of the ground conductive material respectively opposite to each segment group are according to each segment A specific characteristic impedance of the group is correspondingly determined. Each adjacent two segment group is equivalent to a low-pass filter capacitor component and an inductor component, respectively, equivalent to each segment of the capacitor component segment group. The line width of the segment is greater than the line width of each segment in the segment group equivalent to the inductive component.

The foregoing flexible circuit board is provided with a ground conductive material only in a partial area on the lower substrate to avoid a problem of low transmission line impedance caused by the distance between the differential pair and the ground conductive material on the lower substrate being too low, according to each segment group. The specific characteristic impedance can correspondingly determine the line width of the two segments in each segment group and the width of the ground conductive material, so that each segment group of the differential pair can achieve the required characteristic impedance. The flexible board does not require additional cost, and only needs to adjust the existing wiring to achieve high-speed signal transmission.

1‧‧‧Soft circuit board

20‧‧‧Underlying substrate

40‧‧‧Differential pair

22‧‧‧Ground copper foil

Z1-Z5‧‧‧ segment group

L1, L2‧‧‧Inductance components

10‧‧‧Upper substrate

30‧‧‧Insulation medium

41, 42‧‧‧ transmission line

411-415, 421-425‧‧‧

44‧‧‧ equivalent circuit

C1, C2, C3‧‧‧ capacitor components

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view showing a preferred embodiment of a flexible circuit board of the present invention.

2 is a top plan view of the differential pair and the grounded copper foil of FIG.

FIG. 3 is an equivalent circuit diagram of the differential pair in FIG.

The present invention will be further described in detail below with reference to the accompanying drawings and preferred embodiments.

Referring to FIG. 1 and FIG. 2 together, a preferred embodiment of the flexible circuit board 1 of the present invention includes an upper substrate 10 and a lower substrate 20, and the upper substrate 10 and the lower substrate 20 are filled. A layer of insulating medium 30 is filled. A differential pair 40 is disposed on the upper substrate 10, and the differential pair 40 includes two transmission lines 41 and 42.

The underlying substrate 20 is covered with a grounded copper foil, and the problem of low transmission line impedance caused by avoiding the vertical distance between the two transmission lines 41 and 42 of the differential pair 40 being too close to the grounded copper foil is only in the lower layer. A ground copper foil 22 is disposed on a region of the substrate 20 that is perpendicular to a region between the transmission lines 41 and 42. The length of the ground copper foil 22 is equal to the length of the differential pair 40. Of course, the width of the ground copper foil 22 may be appropriately increased or decreased according to actual needs, as long as the transmission line caused by the vertical distance between the two transmission lines 41, 42 of the differential pair 40 and the ground copper foil can be avoided. The problem of low impedance is enough. In other embodiments, the grounded copper foil 22 can also be other grounded conductive materials.

The differential pair 40 includes a plurality of segment groups disposed between the signal input end of the differential pair 40 and the signal output end, and each segment group includes a segment symmetrically disposed on the transmission line 41 and A segment on the transmission line 42, the two segments in each segment group are the same size and shape. The difference pair 40 can be equivalent to a low-pass filter. The number of the segment groups is determined by the specification requirements of the designed low-pass filter. In this embodiment, five segment groups are taken as an example for description.

The transmission line 41 comprises segments 411-415, the transmission line 42 comprising segments 421-425, which form a segment group Z1 of the differential pair 40, the segments 412, 422 constituting the differential pair 40 a segment group Z2, the segments 413, 423 forming a segment group Z3 of the differential pair 40, the segments 414, 424 forming a segment group Z4 of the differential pair 40, the segments 415, 425 forming the segment A segment group Z5 of the differential pair 40.

According to the electrical characteristics of the transmission line, when the line width of the transmission line is sufficiently narrow, it has an inductance characteristic; when the line width of the transmission line is sufficiently wide, it has a characteristic of capacitance. Because every The line width of each adjacent two segments on a transmission line 41, 42 is different, so that the differential pair 40 having the segment group Z1-Z5 can be equivalent to a low-pass filter.

Referring to FIG. 3 together, the line length of the two segments in each of the segment groups Z1-Z5 is determined by a prototype of the low-pass filter, that is, the equivalent circuit 44 of the differential pair 40. This embodiment The equivalent circuit 44 of the differential pair 40 includes three capacitive components C1-C3 and two inductive components L1 and L2 connected to each other. In the differential pair 40, the segment groups Z1, Z3, and Z5 are equivalent to the capacitor components C1, C2, and C3, respectively, and the segment groups Z2 and Z4 are equivalent to the inductor components L1 and L2, respectively. The line width of each adjacent two segments on the transmission lines 41, 42 is different. Wherein, the line length of two segments in each segment group Z1, Z3, and Z5 is according to a formula Corresponding to determine, the line length of two segments in each segment group Z2, Z4 according to the formula Correspondingly, wherein C is the capacitance value of the equivalent capacitive component C1, C2 or C3 corresponding to the segment group Z1, Z3 or Z5, and L is the equivalent inductance component L1 of the segment group Z2 or Z4 or The inductance value of L2, Z ₀ is the specific characteristic impedance of the corresponding segment group, f is the cutoff frequency of the low-pass filter transmission signal, λ _g is the wavelength of the signal at the cutoff frequency, and l is the corresponding segment group The line length of each segment in which the values of f and λ _g are fixed values, so it can be determined according to the values of the equivalent capacitance components C1-C3 corresponding to each segment group Z1, Z3, Z5. Determining the line length of the segments in each of the segment groups Z1, Z3, Z5, and determining the segment group Z2 according to the values of the equivalent inductance components L1, L2 corresponding to each segment group Z2, Z4 , the line length of the segment in Z4.

Since the characteristic impedance of the differential pair 40 changes with the horizontal spacing between the transmission lines 41, 42 or the horizontal spacing between the transmission lines 41, 42 and the grounded copper foil 22, each can be adjusted The horizontal spacing of the two segments in the segment groups Z1-Z5, or simultaneously adjusting the horizontal spacing between the two segments in each segment group Z1-Z5 and the grounding copper foil 22, and by simulation of the simulation software, To meet the requirements of each segment group Z1-Z5 for its specific characteristic impedance Z ₀ to achieve high-speed signal transmission. In this embodiment, the horizontal spacing of the two segments in each segment group Z1-Z5 can be adjusted by adjusting the line width of each segment, and two segments in each segment group Z1-Z5 are grounded. The horizontal spacing between the copper foils 22 can be adjusted by adjusting the width of the portion of the grounded copper foil 22 opposite to each of the segment groups Z1-Z5, such that the grounded copper foil 22 also has a plurality of segments, and the grounded copper foil Each two adjacent segments of 22 have different widths.

The flexible circuit board 1 is provided with a ground conductive material only on a portion of the lower substrate 20 that is perpendicular to a region between the two transmission lines 41 and 42 of the differential pair 40, thereby avoiding ground conduction on the differential pair 40 and the lower substrate 20. The problem that the transmission line impedance is low due to the too close distance of the material, the line length of the two segments in each of the segment groups Z1-Z5 is determined by the equivalent circuit 44 of the low-pass filter, and the simulation of the simulation software can be performed. To set the line width of the two segments in each of the segment groups Z1-Z5 or to simultaneously set the width of the portion of the ground copper foil 22 opposite each of the segment groups Z1-Z5 to achieve each of the differential pairs 40 The characteristic impedance required for a segment group Z1-Z5. The flexible circuit board 1 of the present invention does not need to add extra cost, and can adjust the existing wiring mode to have the function of a low-pass filter and realize high-speed signal transmission.

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‧‧‧Soft circuit board

41, 42‧‧‧ transmission line

411-415, 421-425‧‧‧

30‧‧‧Insulation medium

22‧‧‧Ground copper foil

Z1-Z5‧‧‧ segment group

Claims (5)

A flexible circuit board comprising an upper substrate and a lower substrate, wherein the upper substrate is provided with a differential pair, the differential pair comprises two transmission lines, and the vertically opposite portion of the region between the lower substrate and the two transmission lines Having a grounded conductive material, the differential pair comprises a plurality of segment groups, each segment group consisting of two segments symmetrically distributed on the two transmission lines, each two adjacent segments having different line widths on each transmission line, The line width of the two segments in each segment group and the width of the portion of the ground conductive material respectively opposite to each segment group are determined according to a specific characteristic impedance of each segment group. Each adjacent two segment group is equivalent to a low-pass filter capacitor component and an inductor component, respectively, and the equivalent line width of each segment in the segment group of the capacitor component is greater than the equivalent of the inductor component. The line width of each segment in the segment group. The flexible circuit board of claim 1, wherein the grounded conductive material is a grounded copper foil. The flexible circuit board of claim 2, wherein the upper substrate and the lower substrate are filled with an insulating medium. The flexible circuit board of claim 1, wherein each equivalent is a line length of two segments in the segment group of the capacitor component according to a formula Correspondingly, the line length of each of the two segments in the segmentation group equivalent to the inductance component is determined according to the formula Correspondingly, wherein C is the capacitance value of the capacitor component corresponding to each segment group, L is the inductance value of the inductance component corresponding to each segment group, and Z ₀ is a specific characteristic impedance of the corresponding segment group, f is the cutoff frequency of the low pass filter transmission signal, λ _g is the wavelength of the signal at the cutoff frequency, l is the line length of each segment in the corresponding segment group, and the values of f and λ _g are Value, by adjusting the line width of the two segments in each segment group and the width of the portion of the ground conductive material respectively opposite to each segment group, the characteristic impedance of the corresponding segment group can be adjusted to its specific Characteristic impedance. The flexible circuit board of claim 1, wherein the length of the grounded conductive material is equal to the length of the differential pair.