# Patent

## Title
Flexible printed circuits capable of transmitting electrical and optical signals

## Abstract

A flexible printed circuit capable of transmitting electrical and optical signals is disclosed. The flexible printed circuit includes a set of optical waveguides for transmitting optical signals and a set of conductors for transmitting electrical signals. Each of a subset of the optical waveguides is enclosed by a respectively one of the conductors. The optical waveguide is made of glass or plastic. The conductors are formed within a first building block constructed by a first dielectric layer and a first substrate layer, and a second building block constructed by a second dielectric layer and a second substrate layer.

## Background
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to print circuits in general. More particularly, the present invention relates to flexible printed circuits. Still more particularly, the present invention relates to a method for manufacturing a flexible printed circuit capable of transmitting electrical and optical signals.
2. Description of Related Art
Printed circuit boards are commonly used to interconnect electronic components to each other. A multilayer printed circuit board typically includes several layers of copper traces embedded within a epoxy board. The copper traces are designed and lay out to provide proper electrical connections for various electronic components mounted on the multilayer printed circuit board. Electronic components can be mounted on a multilayer printed circuit board using solder joint or surface mount technology.
Conventional printed circuit boards are typically rigid in nature and may not be suitable for all applications. Thus, for some applications, flexible printed circuits are used to reduce packaging costs without affecting electrical performance.
As for high-speed electronic applications, optical devices are used instead of electronic components. As such, optical fibers are needed to interconnect optical devices. However, conventional printed circuit boards include only interconnects that are only capable of transmitting electrical signals.
Consequently, it would be desirable to provide an improved flexible printed circuit board that is capable of transmitting more than electrical signals.
SUMMARY OF THE INVENTION
In accordance with a preferred embodiment of the present invention, a flexible printed circuit includes a set of optical waveguides for transmitting optical signals and a set of conductors for transmitting electrical signals. Each of a subset of the optical waveguides is enclosed by a respectively one of the conductors. The optical waveguide is made of glass or plastic. The conductors are formed within a first building block constructed by a first dielectric layer and a first substrate layer, and a second building block constructed by a second dielectric layer and a second substrate layer.
All features and advantages of the present invention will become apparent in the following detailed written description.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention itself, as well as a preferred mode of use, further objects, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:
FIGS. 1a-1h are cross-sectional views of a method for manufacturing a flexible printed circuit capable of transmitting electrical and optical signals, in accordance with a preferred embodiment of the present invention; and
FIG. 2 is a cross-sectional view of a flexible printed circuit capable of transmitting electrical and optical signals, in accordance with a preferred embodiment of the present invention.



DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
Referring now to the drawings and in particular to FIGS. 1a-1h, there are depicted cross-sectional views of a method for manufacturing a flexible printed circuit capable of transmitting electrical and optical signals, in accordance with a preferred embodiment of the present invention. Initially, a first flexible printed circuit building block is assembled. The first flexible printed circuit building block includes a substrate layer 10 and a dielectric layer 11, as shown in FIG. 1a. Substrate layer 10 is preferably made of copper, but substrate layer 10 can also be made of other types of conductive metals. Dielectric layer 11 is preferably made of polymide; however, dielectric layer 11 can be any one or a combination of polyester, teflon or glass weaves.
After an adhesive layer 12a has been placed on top of dielectric layer 11, a set of optical waveguides 20 is placed on top of adhesive layer 12a, as depicted in FIG. 1b. Optical waveguides 20 are preferably made of glass or plastic having a diameter of similar to that of their copper counterparts.
Next, a second flexible printed circuit building block, which includes a dielectric layer 13 and a substrate layer 14, is prepared by adding an adhesive layer 12b adjacent to dielectric layer 13, as depicted in FIG. 1b. Substrate layer 14 is preferably made of copper, but substrate layer 14 can also be made of other types of conductive metals. Dielectric layer 13 is preferably made of polymide; however, dielectric layer 13 can be any one or a combination of polyester, teflon or glass weaves.
The second flexible printed circuit building block is then placed on top of the first flexible printed circuit building block with adhesive layer 12a in direct contact with adhesive layer 12b. As a result, optical waveguides 20 are embedded within an adhesive layer 12 formed by the combination of adhesive layers 12a and 12b, as shown in block 1c. 
Subsequently, portions of substrate layer 14 are removed using any etching method that is well-known in the relevant art, as depicted in FIG. 1d. The etching is preferably performed with an appropriate mask to allow a desired portion of substrate layer 14 to be removed. The remaining portions of substrate layer 14 provide the necessary electrical interconnects for a specific application.
Laser ablation is then used to construct multiple channels 21 that pass through various layers, as shown in FIG. 1e. Preferably, channels 21 pass though layers 14-11 and stop within substrate layer 10 (i.e., did not pass through substrate layer 10).
Next, the walls of channels 21 are plated by metal coatings 22 via any of the well-known plating methods, such as seeding and plating, as depicted in block If. Metal coatings 22 are preferably made of copper, but metal coatings 22 can also be made of other types of conductive metals.
A third flexible printed circuit building block, which includes a dielectric layer 16 and a substrate layer 17, is placed on top of substrate layer 14 via an adhesive layer 15, as shown in FIG. 1g. As such, dielectric layer 16 is bound to substrate layer 14 by adhesive layer 15. Substrate layer 17 is preferably made of copper, but substrate layer 17 can also be made of other types of conductive metals. Dielectric layer 16 is preferably made of polymide; however, dielectric layer 16 can be any one or a combination of polyester, teflon or glass weaves.
Next, substrate layer 10 is selectively etched to provide openings 23. As a result, each of waveguides 20 is enclosed by an individual conductor (i.e., substrate layer 10, substrate layer 14 and metal coatings 22), as depicted in FIG. 1h. 
Finally, a fourth flexible printed circuit building block, which includes a dielectric layer 26 and a substrate layer 27, is adhered to substrate layer 10 via an adhesive layer 25, as shown in FIG. 2. Dielectric layer 26 is bound to substrate layer 10 via adhesive layer 25. Substrate layer 27 is preferably made of copper, but substrate layer 27 can also be made of other types of conductive metals. Dielectric layer 26 is preferably made of polymide; however, dielectric layer 26 can be any one or a combination of polyester, teflon or glass weaves.
In the flexible printed circuit shown in FIG. 2, optical waveguides 20 are used to transmit optical signals and conductors (i.e., substrate layer 10, substrate layer 14 and metal coatings 22) are used to transmit electrical signals.
As has been described, the present invention provides a method for manufacturing a flexible printed circuit capable of transmitting electrical and optical signals.
While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.

## Claims

1. A method for manufacturing a flexible printed circuit, said method comprising:
providing a plurality of optical waveguides for transmitting optical signals;
constructing a first building block with a first dielectric layer adhered to a first substrate layer;
constructing a second building block with a second dielectric layer adhered to a second substrate layer;
forming a plurality of conductors for transmitting electrical signals by binding said first building block and said second building block via a first adhesive layer in which said plurality of optical waveguides are embedded;
forming channels through said first and second building blocks to isolate at least one of said plurality of optical waveguides; and
plating said channels with metal coatings to enclose said one of said plurality of optical waveguides by connecting said first substrate layer from said first building block to said second substrate layer from said second building block.

2. The method of claim 1, wherein said optical waveguides are made of glass.

3. The method of claim 1, wherein said optical waveguides are made of plastic.

4. The method of claim 3, wherein said metal coatings are made of copper.

5. The method of claim 1, wherein said first and second substrate layers are made of copper.

6. The method of claim 1, wherein said first and second dielectric layers are selected from a group consisting of polymide, polyester, teflon and glass weaves.

7. The method of claim 3, wherein said method further includes
constructing a third building block with a third dielectric layer adhered to a third substrate layer; and
constructing a fourth building block with a fourth dielectric layer adhered to a fourth substrate layer.

8. The method of claim 7, wherein said method further includes
binding said third building block to said second building block via a second adhesive layer; and
binding said fourth building block to said first building block via a third adhesive layer.

9. The method of claim 7, wherein said third and fourth substrate layers are made of copper.

10. The method of claim 7, wherein said third and fourth dielectric layers are selected from a group consisting of polymide, polyester, teflon and glass weaves.