Leadframe-Based Optical Assembly

1. Field of the Invention

The present invention relates to an optical assembly and, more particularly, to a leadframe-based optical assembly.

2. Description of the Prior Art

When packaged for use as indicator devices, visible light-emitting diodes (LEDs), such as GaP or GaAsP LEDs, are often attached to a leadframe to provide the required electrical connections, and encapsulated in a plastic or other transparent material to provide mechanical protection. See, for example, U.S. Pat. No. 4,129,682 issued to W. P. Stewart et al. on Dec. 12, 1978. Since a visible LED is used primarily as an indicator bulb (in key telephone sets, for example) the direct attachment thereto of an optical signal path, such as an optical fiber or waveguide, is not required.

Alternatively, for optical communication applications, a semiconductor optical device (i.e., LED or photodiode) must be both electrically connected to an associated transmitter or receiver circuit and optically coupled to an associated optical data signal path. The packaged optical assembly generally comprises a separate optical submount for holding the optical device and some sort of arrangement for inserting an optical fiber through the package and aligning the fiber to the optical device. In general, the electrical leads to the optical device are physically attached to the optical submount and exit through the package to associated external circuitry. See, for example, U.S. Pat. No. 4,296,998 issued to W. H. Dufft on Oct. 27, 1981.

The optical assembly, as described above, is often a relatively expensive item, since each assembly must be individually packaged. In most cases, the packing requires a number of individual manual operations including, but not limited to, mounting the optical device, attaching the electrical leads to the optical device, inserting the optical fiber, aligning the optical fiber to the device and attaching the aligned fiber to the package. The use of such a large number of manual operations necessitates that the package dimensions be sufficiently large to allow for manipulation of the various piece parts by the assembler.

In light of the above, there exists a need for reducing the cost, size and complexity of such packaged optical assemblies.

The need remaining in the prior art is addressed by the present invention which relates to an optical assembly and, more particularly, to a leadframe-based optical assembly.

In one embodiment of the present invention, an optical device is attached to a leadframe section (many devices being simultaneously attached along the length of the complete leadframe) such that the active region of the device is aligned with an aperture formed in one contact lead of the leadframe section. A fiber section, such as a fiber-holding ferrule, is attached to the opposite side of the leadframe section in the vicinity of the aperture and aligned to the active region of the associated optical device. An exemplary ferrule may include a flanged end portion to facilitate attachment of the fiber section to the leadframe. Each optical device, associated leadframe section and ferrule may then be encapsulated (with a portion of the ferrule and the electrical leads extending beyond the encapsulant) to form the final optical assembly. The semiconductor optical device may comprise, for example, a surface-emitting LED (to form a transmitter assembly), or a PIN or avalanche photodiode (to form a receiver assembly).

Thermal dissipation may be provided in accordance with an additional aspect of the present invention, where the leadframe section is designed to incorporate a separate thermal dissipation feature for transporting heat away from the optical device. The feature may be attached to the optical device by a thermally conductive adhesive.

Improved coupling efficiency, less restrictive alignment tolerances, and flexibility in design may be achieved in association with the various embodiments of the present invention by the utilization of a lensed optical device, lensed optical fiber, a separate lens element or any combination thereof.

An additional advantage of the leadframe-based design of the present invention is that the package may be formed to include relatively thin, flexible leads to accommodate variations in both the angle and position of the incoming fiber attachment. In accordance with one embodiment of the present invention, flexibility is achieved by tapering the width of the leads.

Other and further advantages of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.

Referring now to the drawings, where like numerals represent like parts in several views:

FIG. 1 illustrates an exemplary leadframe which may be used in assembling the optical assembly of the present invention;

FIG. 2 illustrates an exemplary single leadframe section and associated optical device;

FIG. 3 is a side view of an exemplary leadframe section, illustrating in particular a lensed surface-emitting LED and lensed fiber attachment;

FIG. 4 illustrates an alternative leadframe arrangement including a thermal dissipation feature;

FIG. 5 illustrates yet another leadframe arrangement including an alternative thermal dissipation feature;

FIG. 6 illustrates an alternative leadframe-base assembly wherein an optical fiber, instead of a fiber ferrule, is directly attached to the leadframe;

FIG. 7 illustrates yet another leadframe-based optical assembly including a fiber receptacle directly mounted on the leadframe; and

FIG. 8 illustrates various optical assemblies formed in accordance with the present invention, attached by the exposed electrical leads to an associated printed circuit board.

Referring to FIG. 1, an exemplary leadframe 10 is illustrated which includes a large number of severable leadframe sections for use with a number of separate optical assemblies. It is an advantage of the present invention that the use of leadframe 10 allows for batch processing of optical assemblies such as transmitters (LED-based) or receivers (photodiode-based). Similar to integrated circuit processing, a plurality of optical devices and optical fiber sections may be attached to a single leadframe 10 so as to allow for batch fabrication. Leadframe 10, may then be severed along dotted lines 11 into a plurality of leadframe sections, each section associated with a separate optical assembly. The remaining figures illustrate a single leadframe section and the attachments thereto. It is to be understood that the illustration of a single leadframe section is only for the sake of discussion and in general a larger number of optical assemblies may simultaneously be formed.

A single leadframe section 12 is particularly illustrated in FIG. 2. Section 12 includes a first contact lead 14 and a second contact lead 16. First lead 14 includes an aperture 18. An optical device 22 (i.e., g surface-emitting LED or PIN photodiode), illustrated in phantom in FIG. 2, is attached to first lead 14 such that active region 23 of device 22 is positioned over aperture 18. Aperture 18 is formed to be sufficiently large so that substantial alignment thereto of active region 23 is relatively straightforward. Aperture 18 is limited in the extreme by the requirement for sufficient electrical contact between first lead 14 and bottom surface 24 (shown in FIG. 3) of device 22. Second contact lead 16 is disposed as shown in FIG. 2 so as to be electrically isolated from first contact lead 14. The remaining electrical contact to device 22 is provided by wire bond(s) 26 between top surface 28 and second lead 16. The arrangement of FIG. 2 may then be encased within an encapsulant 30 (for example, a plastic material) to form the final packaged optical assembly. As indicated, end portions of leads 14,16 must extend beyond the outer boundary of encapsulant 30 to provide the electrical connections between the optical assembly and the associated electronic circuitry (not shown).

FIG. 3 illustrates, in a cut-away side view, an exemplary leadframe arrangement, including an attached optical fiber segment. As shown, optical device 22 is positioned with its active region 23 aligned to aperture 18 of leadframe 14, as discussed above. A bonding material 25, such as gold/tin, gold/germanium, indium, or lead/tin solder (or a conductive adhesive), is used to electrically connect bottom surface 24 of device 22 to first contact lead 14. In this particular embodiment, optical device 22 includes a lensed surface 32 to increase the coupling efficiency between the optical fiber and the device. The fiber attachment comprises a fiber section 34 encased within a fiber ferrule 36. It is to be understood that either multimode or single mode fiber may be used in the formation of fiber section 34. Ferrule 36 may include a flanged end portion 38 to facilitate attachment to leadframe 14. Such a flanged arrangement, however, is merely an alternative and various other fiber ferrule designs may be utilized. For the particular arrangement as illustrated in FIG. 3, end region 35 of fiber 34 is lensed to further improve the coupling (and the alignment tolerances) between fiber 34 and device 22. Encapsulant 30, illustrated in phantom in this view, is shown as encasing a portion of fiber ferrule 36 as well as leadframe section 12 and optical device 22. It is to be understood that such an encapsulant may be formed so as to encompass only the leadframe and optical device, leaving the fiber ferrule portion fully exposed. Additionally, it is to be understood that the assembly sequence used to form the inventive leadframe-based assembly is discretionary in that either the semiconductor optical device or fiber section may be first attached to the leadframe. Indeed, the assembly process may be simplified if the fiber (having a relatively large core region with respect to the frame aperture) is attached first, since active alignment means may then be used to position the semiconductor optical device and provide maximum coupling efficiency therebetween. Alternatively, a fiber which comprises a smaller outer diameter than the aperture may be positioned within the aperture such that its endface is approximately flush with the surface of contact 14. Visual coupling means can then be used to position the semiconductor optical device and provide coupling therebetween.

An alternative leadframe section 40, including an additional feature 42 for providing improved thermal dissipation is illustrated in FIG. 4. Similar to the arrangement of FIG. 2, leadframe section 40 includes first lead 14, with an aperture 18, for attachment thereto of an optical device 22. Second lead 16 is then attached via wirebond 26 to top surface 28 of device 22. Thermal dissipation feature 42 is located in relatively close proximity to optical device 22, remaining electrically isolated form both leads 14 and 16. A thermally conductive (and electrically insulative) material 44 is then used to attach device 22 to feature 42. The external lead portions 46,48 of feature 42 may then be connected to a conventional heat sink surface (not shown) on the integrated circuit board (or package wall) to which the final optical assembly is attached. The final structure, as shown in FIG. 4 may then be covered with an encapsulant 49. Alternatively, thermally conductive material 44 may be utilized as the final encapsulant for the structure.

In the particular embodiment illustrated in FIG. 4, leads 46,48 are disposed on the opposite side of leadframe section 12 from electrical leads 14,16. For some applications, it may be desirous to have all leads positioned on the same side of the package. FIG. 5 illustrates an alternative leadframe 50 with a thermal dissipation feature 52 designed so as to exit the final package on the same side as electrical leads 14,16. As shown, thermal dissipation feature 52 is located in close proximity to optical device 22 and attached thereto using a thermally conductive (electrically nonconductive) epoxy 54, such as that described above in association with FIG. 4. Feature 52 may be used, alternatively, as a ground plane to provide EMI shielding for the optical device.

An alternative leadframe-based assembly of the present invention is illustrated in FIG. 6. In this embodiment, a fiber 34 is directly attached to first lead 14, without the use of a surrounding ferrule. As shown in FIG. 6, endface 37 of fiber 34 may protrude through aperture 18 and may even be brought into physical contact with device 22 (as long as damage is avoided). In this example, with the lensed surface 32 of device 22. In order to facilitate the attachment of fiber 34 to first contact lead 14, fiber 34 may include an outer coating 60 of a suitable material, such as a metal or polyimide material.

FIG. 7 illustrates yet another embodiment of the present invention where a fiber receptacle 62 is attached as shown to first contact lead 14. Receptacle may be of a plastic, metallic, or other suitable material. As shown in FIG. 7, receptacle 62 includes an opening 64 which is substantially aligned with aperture 18 of first contact lead 14. Receptacle 62 may include a recessed portion 66 to allow for attachment thereto of a ferrule, such as ferrule 36 of FIG. 3. Alternatively, receptacle 62 may be formed to include a central bore 68 (as shown in phantom in FIG. 7) to allow for direct placement of an optical fiber. Advantageously, the use of receptacle 62 provides for simplified mechanical alignment of the included optical fiber to the active region of the associated optical device.

FIG. 8 illustrates an exemplary system 70 utilizing leadframe-based optical assemblies of the present invention. As shown, system 70 includes a printed circuit board 72 with a plurality of different leadframe-based optical assemblies attached thereto. In some instances, it may be desirous to allow the attached optical assembly a degree of motion (e.g., 1-10 mil) with respect to printed circuit board 72. That is, to allow the assembly to "float" with respect to the plane established by top surface 73 of printed circuit board 72. The motion of the optical assembly may then be used to accommodate for variations in the angle and position of the incoming fiber attachment (not shown). Accordingly, a first leadframe-based assembly 74 (similar in design to that illustrated in FIG. 2), may be formed utilizing relatively thin and flexible leads 14,16, with leads 14,16 being attached to top surface 73 of printed circuit board 72. As shown in FIG. 2, leads 14,16 may be designed to incorporate such flexibility by forming tapered leads. In particular, the leads may be tapered from a width of approximately 20 mil at the site of the optical device attachment to a width of approximately 10 mil at the attachement to printed circuit board 72. The thickness of the leads may also be tapered. In general, the leads are tapered to provide the desired flexibility without sacrificing the integrity of the electrical attachment to printed circuit board 72. Thus, fiber ferrule 36 of optical assembly 74, illustrated as being disposed along the z-direction of printed circuit board 72, will be able to accommodate motions, as indicated by the arrows in FIG. 8, to facilitate the attachment of a communication optical fiber (not shown). A second leadframe-based assembly 76, similar in design to that illustrated in FIG. 4, is also illustrated in FIG. 8. In this case, assembly 76 is mounted to provide attachment for leads 14,16 as well as thermal dissipation leads 46,48. In this particular arrangement, fiber ferrule 36 is illustrating as exiting circuit board 64 in the y-direction such that a communication fiber may then be attached thereto. Again, the utilization of flexibility leads provides for some degree of motion of ferrule 36, as illustrated in FIG. 8. For most applications, it may be desired for the fibers to exit the circuit board in the same direction. The various options are being illustrated here solely for the sake of discussion.