Flexible optical circuit appliques - Patent Review 6427034

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BACKGROUND OF THE INVENTION

The present invention relates to flexible optical circuits. In particular, the present invention relates to flexible optical circuits having a backing layer, an adhesive coating and a release liner for attachment to a substrate such as an electronic printed circuit board.

The design of electronic circuits requires interconnections between devices for proper operation. With increased sophistication and operation speeds, design of functional interconnections requires careful engineering. The fastest data processing circuits and emerging technologies require large numbers of interconnects capable of carrying extremely high speed digital signals. Due to the increasing push for higher and higher speeds, engineers are facing fundamental limits in designing electronic interconnects.

In an attempt to handle higher speeds, interconnection technology has turned to optical interconnects for next generation circuits. Optical circuits have bandwidth capabilities orders of magnitude beyond electrical circuits, and are inherently immune to electrical interference. In some known designs, discrete fiber optic cables and fiber bundles are used to interconnect devices. Known standard fiber optic connection technology employed to connect optical fibers to devices are adequate for small numbers of interconnections. However, as optical circuit density grows, the physical bulk of cables and connectors make this approach unwieldy, especially for compact designs.

Attempts have been made to incorporate optical interconnects onto the surface of electronic circuit boards and substrates by constructing wave guides using optical polymers coated to the surface. An example of this is found in U.S. Pat. No. 5,521,992 to Chun, et al. The technology of the '992 patent requires highly specialized tooling to generate each custom optical circuit thus standard circuit boards cannot be used. For simple circuits, tooling costs may be prohibitive. Waveguide fabrication is also difficult due to the small geometry of the guide regions, and optical quality of finished wave guides is poor due to limitations in optical polymer chemistry.

Flexible optical interconnect circuit packs are also known in the art. An example of this is found in U.S. Pat. No. 5,204,925 to Bonanni, et al. The known optical interconnect circuits have optical fibers bonded between two flexible substrates and have one or more optical connectors connected along the edges of the circuit pack. The connectors are then connected to one or more optical devices. These known devices are not adapted to bond to a substrate or circuit board.

The concept of using high bond strength pressure sensitive adhesive coated laminating films is not new. However, there are certain problems associated with the known adhesive coated films. For instance, it is often difficult to obtain accurate positioning during film use. Improper placement, static charge, and accidental contact can all contribute to misalignment and immediate bonding to the surface in undesired positions. Because of the immediate aggressive bonding, if alignment is off, the film is often destroyed or seriously damaged attempting to remove the film for repositioning.

One known solution to the above problem is to use a less aggressive adhesive so the user may remove or reposition the film in case of misalignment. This can result in poor long term adhesion. Another known solution is to use a partially cured adhesive material followed by a final curing process. This not only results in additional process step, but may be impractical for many applications. Yet another known solution is to prepare a surface wetted with a material that interferes with adhesion and then removing the wetting agent after final alignment is achieved. This approach can be messy and adds process steps.

There is a continuing need for flexible optical circuits capable of being applied to new circuit board designs without changing board design and fabrication techniques. There is also a continued need for a laminating film that allows for repositioning of the optical circuits to achieve proper alignment.

SUMMARY OF THE INVENTION

The present invention provides a flexible optical circuit applique that can be mounted on a circuit board without modifying the circuit board substrate or the electronic circuits. The optical circuit applique of the present invention is also repositionable. A method of manufacturing flexible optical circuit appliques is also provided.

A preformed fiber optic applique is provided having a backing layer. The backing layer or film, has an adhesive coating applied thereon. At least one optical fiber is routed and bonded to the adhesive layer providing a continuous optical signal path from one end to another. A releasable liner is releasably attached to the adhesive layer and positioned to cover the backing layer including the at least one optical fiber.

In one embodiment of the present invention, microstructures are provided on the backing layer. The microstructures are crushable structures that prevent the adhesive coating from immediately adhering to a substrate. This allows the film to be repositioned until proper alignment has been achieved. Upon the application of appropriate force, the microstructures will crush allowing the adhesive coating to bond the film to the substrate.

A method of fabricating the fiber optic appliques of the present invention is also disclosed. The method includes the steps of providing a supply of backing layer and applying an adhesive coating to the backing layer. At least one fiber is then provided and placed on the adhesive coating. Pressure is then applied to the at least one fiber to secure it to the backing layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of an electrical circuit board and a flexible optical circuit applique according to the present invention.

FIG. 2a illustrates a preferred method of fabricating an optical circuit applique according to the present invention.

FIG. 2b illustrates an alternative method of fabricating an optical circuit applique according to the present invention.

FIGS. 3a, b, c and d illustrate alternative embodiments of a continuous web with fibers available for connectorization.

FIGS. 4a, b and c illustrate standard sections of optical circuit appliques according to the present invention.

FIGS. 5a and b are top and side perspective views of the flexible optical circuit having microreplicated structures according to the present invention.

FIGS. 6a and b are top and side perspective views of the flexible optical circuits of FIGS. 5a and b having optical fibers placed thereon.

FIGS. 7a, b, c and d illustrate steps in a method for fabricating a flexible optical circuit and terminating the optical connectors at sockets according to the present invention.

FIG. 8 is a side perspective view of a first alternative embodiment of the present invention.

FIG. 9 is a side perspective view of a second alternative embodiment of the present invention.

FIG. 10 is a side perspective view of a third alternative embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a flexible optical circuit applique having a flexible backing with fibers bonded thereto covered by a releasable liner. The optical appliques are intended to provide an easy means for adding optical circuits to electronic circuit boards or other substrates by laminating. The flexible optical circuits of the present invention can also be incorporated into new board designs without changing standard board design and fabrication techniques. The optical appliques of the present invention are also repositionable on a substrate prior to bonding.

FIG. 1 is a perspective view of a preferred embodiment of a flexible optical circuit applique 10 according to the present invention. Also illustrated is a printed circuit board 12 to which applique 10 is to be mounted. Applique 10 comprises a durable backing material 14 that is coated with a temporarily repositionable adhesive 16. Applique 10 also includes optical fibers 18 bonded to backing 14 via adhesive 16. The particular backing material used depends upon the particular application. For instance, if high durability is desired, a polyester-type backing would be used. If the applique is to be mounted on a curved substrate, a flexible backing material would be chosen. Examples of backing materials include vinyl, polyurethane, polyamide, and polyester.

The printed circuit board 12, an electronic circuit board, includes electronic devices 11. Applique means are provided for optical connectorization of the devices. The applique 10 further includes at least one connector 50 (see FIGS. 7b-7d). The optical fibers 18 are terminated into the connector 50 and the electronic devices 11 couple to the connector 50. The optical fibers 18 are routed along desired optical circuit paths between connectors 50.

Adhesive 16 of the present invention is any temporarily repositionable adhesive. By temporarily repositionable, it is meant that the bond between the adhesive and the substrate is such that the material having the adhesive may be removed from the substrate without damaging either for a period of time or until the occurrence of a specified event which then permanently adheres the backing to the substrate. In the preferred embodiment, adhesive 16 is preferably a pressure sensitive adhesive. Pressure sensitive adhesives are known in the industry as a standard class of materials. They are adhesives, which in dry form (substantially solvent free except for residual solvent) are aggressively and permanently tacky at room temperature (approximately 15.degree. to 25.degree. C.) and firmly adhere to a variety of similar surfaces upon mere contact without the need for more than manual pressure. The adhesives require no activation by water, solvent, or heat in order to exert a strong adhesive holding force towards such materials as paper, cellophane, glass, plastic, wood, and metals. The adhesives have a sufficiently cohesive holding and elastic nature such that, despite their aggressive tackiness, it can be handled with fingers and removed from smooth surfaces without leaving a substantial residue. For a more detailed discussion of pressure sensitive adhesives, reference should be made to U.S. Pat. Nos. 5,296,277, 5,362,516 and 5,449,540, which are assigned to the Assignee of the present invention and the disclosures of which are herein incorporated by reference.

If adhesive 16 has low tack, e.g. is only slightly sticky to the touch and has low adhesion to certain types of surfaces, a release liner may not be required to cover the adhesive layer, and the backing material 14 may act as a release liner when the film is rolled.

A light cured adhesive could be used with a transparent backing layer without departing from the spirit or scope of the invention. Such an adhesive would allow the film to be repositioned on a substrate until a light source, typically an ultra violet light, is applied through the transparent film which would then activate the adhesive to secure the film to the substrate. Nonpressure sensitive adhesives, heat curable adhesives, and thermally activated adhesives such as hot melt glue or solvent activated adhesives could also be used if desired, without departing from the spirit or scope of the invention. They are, however, less preferred embodiments because of the additional application steps and greater tendency to entrap air during application.

Optical fibers 18 of the present invention are preferably 80 to 100 micron diameter glass (silica) fibers with a special fiber coating described in U.S. Pat. No. 5,381,504. Standard glass optical fibers have 125 micron outside diameters. By using glass fibers with diameters of 80 to 100 microns, the present invention can obtain bend radiuses as small as 0.25 inches (0.64 cm) while staying below a bending stress of 100 K psi. At a bend radius of 0.25 inches (0.64 cm), a fiber having a diameter of 125 microns has a bending stress of approximately 110 K psi and a fiber having a diameter of 200 microns has a bending stress of approximately 175 K psi. In the preferred embodiment of the present invention, bend radii of less than 0.3 inches (0.76 cm) are achieved at bend stresses of less than 100 K psi.

FIG. 2a illustrates a preferred method of fabricating an applique according to the present invention. A supply roll 20 of applique backing layer or film, 14 is provided, as is a takeup roll 22. Film 14 is precoated with an adhesive layer 16 and covered with a releasable liner 17. A second takeup roller 19 is provided to take away releasable liner 17 as illustrated. A rotating drum 24 is provided to move backing film 14 from supply roll 20 to takeup roll 22. Multiple spools 28 of fibers 18 are provided and are suspended above backing film 14. A laminating roll spacing guide 30 is provided to space fibers 18 as desired and to press the fibers onto backing film 14 with sufficient force to activate pressure sensitive adhesive 16 to bond fibers 18 in place. A second supply roller 32 is provided carrying a second releasable liner 34 which releasably adheres to pressure sensitive adhesive 16. It should be noted that film 14 may be a continuous and solid film or it may have holes formed therethrough for connectorization with optical circuit components, as will be described in greater detail below.

FIG. 2b illustrates an alternative method of fabricating an applique according to the present invention. FIG. 2b is similar to FIG. 2a and thus like elements are correspondingly identified. Supply roll 20 of applique backing film 14 is provided as is takeup roll 22. Rotating drum 24 is provided to move backing film 14 from supply roll 20 to takeup roll 22. A pressure sensitive adhesive coating device 26 is provided to apply a coating of pressure sensitive adhesive 16 to backing film 14 as the backing film comes off of supply roll 20. Multiple spools 28 of fibers 18 are provided and are suspended above backing film 14. Laminating roll spacing guide 30 is provided to space fibers 18 as desired and to press the fibers onto backing film 14 with sufficient force to activate pressure sensitive adhesive 16 to bond fibers 18 in place. Second supply roller 32 is provided carrying release liner 34 which releasably adheres to pressure sensitive adhesive 16. One alternative method of optically connecting a plurality of optical components on a substrate includes the steps of providing a backing layer, providing at least one optical fiber coated with an adhesive, placing the optical fiber on the backing layer in a desired pattern and releasably securing the fiber to the backing layer. The fiber and the backing layer are placed on a substrate in a desired position and the fiber is fixedly adhered to the substrate. The backing layer may be removed while leaving the fiber adhered to the substrate.

Fabricating appliques in a continuous web process, as illustrated in FIGS. 2a and 2b is a very cost effective way to fabricate the appliques. Depending upon the length of appliques desired, large numbers of standard appliques can easily be made from a single roll of backing film 14.

FIGS. 3a-3d illustrate alternative ways to make fibers 18 available for connectorization in a continuous web process similar to those described in FIGS. 2a and 2b above. It should be noted that the examples given below are given only as exemplary ways to avail the fibers for connectorization, and the examples given are not meant to be an exhaustive list. The illustrations in FIGS. 3a-3d use some of the same components as used in FIGS. 1 and 2, therefore, like elements will be correspondingly identified. In FIG. 3a, film 14 is coated with adhesive layer 16. Fibers 18 are bonded to adhesive layer 16 as previously described. A release strip 35 is provided across the width of film