Wave transmission lines and networks – Long lines – Strip type
Reexamination Certificate
1999-08-12
2001-08-21
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Long lines
Strip type
C174S254000
Reexamination Certificate
active
06278345
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of The Invention
The present invention relates to a flexible printed circuit and the method for making the same, especially to a printed circuit which can compensate the original characteristic impedance of the conductor trace thereof for obtaining a new value of characteristic impedance which is in an allowable range of a standard value.
2. The Prior Art
Flexible printed circuit (FPC) has been used in many fields for years such as the consumer products including the walk-man, the cellular phone, and the computer peripheral products including the printer, the disk drive. A conventional flexible printed circuit is shown in
FIG. 7
, wherein an adhesive layer
2
is formed in a flexible insulative board
1
and a pattern of conductor traces
3
are then formed on the adhesive layer
2
via a photoresist technique. This FPC works well only in low frequency but it may cause malfunction in high frequency. This is because the conductor trace
3
of the FPC functions as a transmission line other than a simple conductor line. The characteristic impedance of the conductor trace has to be considered in a high frequency environment. Normally, the characteristic impedance of a transmission line is determined by the line width, the line thickness, the dielectric constant of the insulative material, the distance thereof to the grounding potential.
Conventionally, to design an FPC having a desired constant characteristic impedance along the length thereof, an ideal grounding plane
4
such as copper foil shown in
FIG. 8
is provided at an opposite side of the pattern of conductor traces
3
of FIG.
7
. The characteristic impedance distribution is controlled by the physical structure of the transmission line such as the width and the thickness along the lengthwise direction thereof and the distance between the transmission line and the grounding plane. The physical dimension of the transmission line may have to be repeatedly adjusted along the lengthwise direction thereof according to repeated testing for maintaining the characteristic impedance thereof along the line within an allowable tolerance to a standard value. However, the job for repeatedly testing and adjusting the physical dimension of the transmission line is cumbersome, expensive, and may cause errors. It is even more difficult to adjust the distance between the transmission line and the grounding plane for obtaining a constant characteristic impedance along the line because this method needs to change the thickness of the insulative board repeatedly by sections.
It is requisite to provide a new structure of an FPC having a constant characteristic impedance value along a length thereof.
SUMMARY OF THE INVENTION
The primary purpose of the present invention is to provide an FPC having a proper characteristic impedance distribution within an acceptable tolerance to a standard value.
The second purpose of the present invention is to provide a method for making an FPC having a characteristic impedance distribution within an acceptable tolerance to a standard value.
In accordance with one aspect of the present invention, a flexible printed circuit comprises a flexible insulative board having a first surface and a second surface opposite thereto. At least one conductive transmission line is formed on the first surface and(comprises same length of a plurality of tested sections. Each tested section has an original characteristic impedance and the original characteristic impedance of each tested section is beyond a tolerance to a standard value. A compensation region is defined at the second surface and separated into a plurality of compensation units having identical area. Each compensation unit is vertically related to a specific tested section of the at least one transmission line and having a conductive region and a non-conductive region. A final characteristic impedance of the tested section is obtained within an acceptable tolerance to the standard value due to compensation of the compensation unit.
In accordance with another aspect of the present invention, a method for making a flexible printed circuit comprising steps of: (a)forming a first conductive layer on a first surface of a flexible insulative board; (b)forming at least one transmission line on the first surface by removal of a portion of the first conductive layer; (c)defining the at least one transmission line into same length of a plurality of tested sections, each tested section having an original characteristic impedance, wherein the original characteristic impedance of each tested sections are beyond a tolerance to a standard value; (d) defining a compensation region at a second surface of the flexible insulative board opposite the first surface and separating the compensation region into a plurality of compensation units having identical area, each compensation unit vertically corresponding to a specific tested section of the at least one transmission line and having a conductive region and a non-conductive region; and (e)determining a specific ratio of the conductive region area to the compensation unit area based an offset between the original characteristic impedance and the standard value so that the compensation unit can compensate the original characteristic impedance of the tested section of the transmission line opposite the compensation unit and resulting in a final characteristic impedance of the tested section of the at least one transmission line in a tolerance to the standard value.
REFERENCES:
patent: 5479138 (1995-12-01), Kuroda et al.
patent: 5539360 (1996-07-01), Vannatta et al.
Transmission line with windowed ground plane, Sep. 1, 1990, IBM Technical Disclosure Bulletin, vol. No. 33 pp. 152-153.
Chung Wei Te
Glenn Kimberly E
Hon Hai - Precision Ind. Co., Ltd.
Pascal Robert
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