JPH09107210A - Micro strip line transmission line - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a micro strip line transmission circuit which can match the impedance of a pad part wiring to that of a signal line. SOLUTION: A metallic case 2 forming a part of the ground face is provided with a recessed part 3, and the space between a pad part 14 for mounting of chip parts and the ground face is widened by h2 to reduce the electrostatic capacity between this pad part 14 and the ground face. That is, the electrostatic capacity is reduced by the extent by which the electrostatic capacity is increased by making the width of the pad part 14 wider than that of a micro strip line 12. Thus, the impedance between the pad part 14 and the ground face is matched to that between the micro strip line 12 and a pattern 1 for ground potential.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a microstripline transmission line, and more particularly to a microstripline transmission line on which an ultrahigh speed circuit module used in a large capacity optical communication system or the like is mounted.

[0002]

2. Description of the Related Art FIG. 4 is a longitudinal (AA) sectional view of a signal line of a conventional microstrip line transmission line, and FIG.
FIG. 3 is a plan view of the transmission circuit.

A conventional microstrip line transmission line on which an ultrahigh-speed circuit module is mounted includes a dielectric substrate 11, a microstrip line 12 provided on the surface of the dielectric substrate 11, and a dielectric substrate 11, as shown in FIG. On the back surface of the body substrate 11, there is a ground potential pattern 13 provided on the surface facing the microstrip line 12.

The microstrip line 12 has a width Wa at the tip thereof.
Chip component mounting pad portion 14 having a wider width Wb
And the chip part mounting pad portions 14 of the two microstrip lines 12 are configured to face each other with a constant interval. A chip component 15 is mounted on the two chip component mounting pad portions 14.

Further, the dielectric substrate 11 is fixed with the ground potential pattern 13 side facing the metal case 16. A conductive adhesive 17 such as soldering is used for fixing,
It may be tightened with screws. In short, the dielectric substrate 11
That is, the ground potential pattern 13 and the metal case 16 are set to the same potential (ground potential in this case).

Hereinafter, alumina (A
The case of using 1 ₂ O ₃ ) will be described as an example.

When a microstrip line is formed on the alumina substrate 11 by double-sided printing, a wiring pattern 12 having a constant width Wa is formed on the front surface, and a solid ground pattern 1 is formed on the rear surface.
The combination of 3 makes it possible to design a microstrip line having a constant characteristic impedance.

The characteristic impedance of the microstrip line is determined with the relative permittivity, thickness and wiring width of the substrate 11 as the main parameters. Since the value of the relative permittivity of the substrate 11 is substantially determined by the material, for example, 50
The wiring width Wa, which is a characteristic impedance of Ω, is
Will be determined by the thickness of the. It is known that the relative permittivity of alumina is about 10, and the wiring width Wa at which the characteristic impedance is 50Ω is close to the substrate thickness.

Alumina having a thickness of 0.635 mm is standard, and this thickness is used except when the strength of the substrate 11 is a problem. Therefore, the microstrip line having a characteristic impedance of 50Ω has a width Wa of about 0.6 m for a thickness of the substrate 11 of 0.635 mm.
It will be decided to be m.

When elements such as resistors and capacitors are inserted in the microstrip line, chip components such as chip resistors and chip capacitors are surface-mounted. The shape of the chip part 15 is 1.0m in length with the current smallest size product.
m × width 0.5mm is available, but depending on the rated power and constant, it is a size product above it, length 1.6mm × width 0.8m
m must be used.

By the way, when the chip component 15 is soldered, in order to form the fillet 19 on the electrode 18, the pad portion 14 on which the component 15 is mounted is designed to be larger than the chip size by the fillet 19. In the example of the chip components 15 of the two sizes described above, the fillet 19 minutes is 0.
If the width is 2 mm, the width of the pad portion 14 is 0.9 mm with respect to the chip width of 0.5 mm (0.2 mm × 2 = 2 because the fillets 19 are formed on both side surfaces of the chip portion 14).
0.4 mm is added. ), And 1.2 mm for a chip width of 0.8 mm.

Therefore, as shown in FIG. 5, the wiring width Wb of the chip component mounting pad portion 14 is different from the wiring width Wa of the microstrip line of 50Ω (Wa <Wb).

[0013]

In the conventional microstrip line transmission circuit, since the wiring width of the pad portion for mounting the chip component is wider than the wiring width of the microstrip line having a characteristic impedance of 50Ω, the pad portion and the ground are grounded. The capacitance between the patterns becomes larger than the capacitance between the 50Ω signal line and the ground pattern, which causes impedance mismatch in the pad wiring and deteriorates the waveform of the propagated data signal.

A technique for matching the characteristic impedance of this type of line is disclosed in Japanese Examined Patent Publication No. 1-58882, Japanese Unexamined Patent Publication No. 63-144603, and Japanese Unexamined Patent Publication No. 64-5.
No. 102, Japanese Patent Laid-Open No. 1-97003, etc., but none of them have the same purpose and configuration as the present invention.

Therefore, an object of the present invention is to provide a microstrip line transmission circuit capable of matching the impedance of the pad wiring with the impedance of the signal line.

[0016]

In order to solve the above problems, the present invention provides a dielectric substrate, a signal line provided on the front surface of the dielectric substrate, and at least the back surface of the dielectric substrate. A microstrip line transmission line composed of a signal line and a grounding member provided on a surface facing the signal line, wherein the signal line has a wide line width portion and a narrow line portion, and the wide signal line width portion. The distance between the ground member and the ground member is larger than the distance between the narrow portion of the signal line width and the ground member.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION A gap between a wide signal line portion and a grounding member is made wider than a gap between a narrow signal line width portion and a grounding member, so that a static signal between the wide signal line portion and the grounding member is reduced. The electric capacity can be reduced. Therefore, the impedance between the wide signal line width portion and the ground member can be matched with the impedance between the narrow signal line width portion and the ground member.

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a longitudinal direction (A-) of a signal line of a microstrip line transmission line according to the present invention.
2A is a sectional view, and FIG. 2 is a plan view of the same transmission line.
The same components as those of the conventional example are designated by the same reference numerals and the description thereof will be omitted.

The microstrip line transmission line according to the present invention is different from the conventional microstrip line transmission line in that the ground potential pattern 1 and the metal case 2 are used.
The shape is changed, and the second dielectric 4 is added.

That is, the chip part mounting pad portion 14
The ground pattern of the portion facing to is removed to form the ground potential pattern 1, and the concave portion 3 is formed in the metal case according to the dimension of the deleted portion.
And Also, the ground potential pattern 1 and the metal case 2
Are fixed by a conductive adhesive 17, and both are at the same potential (ground potential in this case).

The recess 3 of the metal case 2 is filled with air as the second dielectric 4. However, this is not limited to air, and other dielectrics can be used as long as the conditions described below are satisfied.

The characteristic impedance of the microstrip line is inversely proportional to the capacitance per unit length with respect to ground. To qualitatively explain the principle of the present invention, the fact that the capacitance with respect to ground increases (the characteristic impedance becomes a low value conversely) as the wiring width Wb of the chip component mounting pad portion 14 becomes wider is determined. The capacitance is reduced to the impedance of 50 by reducing the capacitance by separating the ground.
The characteristic impedance is kept constant according to the capacitance value in the case of Ω.

The depth h2 (m of the recess 3 of the metal case 2 is
Derive the formula that gives m). In the following equation, h1
Is the thickness (mm) of the dielectric substrate 11, εr1 is the relative permittivity of the dielectric substrate 11, εr2 is the relative permittivity of air, Wa is the wiring width (mm) of the microstrip line 12, and Wb is for mounting chip components. The wiring width (mm) of the pad portion 14 is set.

Since the electrostatic capacitance between the microstrip line 12 having a characteristic impedance of 50Ω and the ground is equal to the electrostatic capacitance between the chip component mounting pad portion 14 and the ground, εr1. (Wa / h1) = ( (H1 / (εr1 ・ Wb)
+ H2 / (εr2 · Wb) ⁻¹ . Since Wa, Wb, h1, εr1 and εr2 are known parameters, h2 can be uniquely obtained. When the above equation is rearranged for h2, h2 = (εr2 / εr1) · h1 · ((Wb / Wa) −
1)

Further, according to FIG. 2, the width of the concave portion 3 of the metal case 2 is the wiring width Wb of the chip component mounting pad portion 14.
With the same size as, the electric force lines leaking from the chip component mounting pad portion 14 are terminated at a thickness h1 other than the recessed portion 3 and the capacitance value is increased. It is preferable to make it larger than the wiring width Wb of 14.

[0026]

According to the present invention, the distance between the wide signal line portion and the ground member is made wider than the distance between the narrow signal line portion and the ground member. The capacitance between them can be reduced. Therefore, the impedance between the wide signal line width portion and the ground member can be matched with the impedance between the narrow signal line width portion and the ground member.

[Brief description of the drawings]

FIG. 1 is a longitudinal (AA) cross-sectional view of a signal line of a microstrip line transmission line according to the present invention.

FIG. 2 is a cross-sectional view (BB) in a direction perpendicular to the longitudinal direction of the transmission line.

FIG. 3 is a plan view of the transmission line.

FIG. 4 is a longitudinal (AA) cross-sectional view of a signal line of a conventional microstrip line transmission line.

FIG. 5 is a plan view of the transmission circuit.

[Explanation of symbols]

 1 Ground potential pattern 2 Metal case 3 Second dielectric 11 Dielectric substrate 12 Microstrip line 14 Chip component mounting pad

Claims (5)

[Claims] 1. A dielectric substrate, a signal line provided on a front surface of the dielectric substrate, and a grounding member provided on at least a surface of the rear surface of the dielectric substrate facing the signal line. A microstrip line transmission line, wherein the signal line has a wide line width portion and a narrow line portion,
The distance between the wide portion of the signal line and the ground member,
A microstrip line transmission line, characterized in that it is wider than a space between the narrow portion of the signal line and the ground member. 2. The signal line and the ground so that the capacitance between the wide portion of the signal line and the ground member is equal to the capacitance between the narrow portion of the signal line and the ground member. The microstrip line transmission line according to claim 1, wherein a distance from the member is set. 3. The microstrip line transmission line according to claim 1, wherein a second dielectric body is sandwiched between the wide portion of the signal line and the ground member together with the dielectric substrate. . 4. The microstrip line transmission line according to claim 3, wherein the second dielectric is air. 5. The microstrip line transmission line according to claim 1, wherein the wide portion of the signal line forms a pad portion for mounting a chip component.