Led lead frame assembly

[0002] 1. Field of the Invention

[0003] This invention pertains to LED lead frame assemblies. More particularly, the invention relates to an improved LED lead frame structure having a thermally conducting, electrically insulating material that enhances both the thermal conduction and structural integrity of the assembly while allowing for the presence of a UV-resistant soft encapsulated material and an integral ESD device.

[0004] 2. Discussion of the Art

[0005] A standard 5 mm light emitting diode (LED) package generally includes a hard plastic encapsulant material, which supplies a high degree of mechanical stability to the lead frame structure. The encapsulant allows for the manipulation and bending of the lead frame leads for solder configuration.

[0006] Various polymers have successfully been used over the years by different manufacturers as the plastic encapsulant, particularly in connection with the packaging of red and green LED products. These polymers are not generally subject to ultraviolet damage such as color centers and color shift. Suitable polymers used in the past include cycloalaphatics, PMMA, epoxies and polyurethanes.

[0007] With the invention of white LEDs and the use of gallium nitride material therein, ultraviolet light at 360-420 nanometers presents new obstacles in connection with the packaging of white light LEDs semiconductor products. For example, gallium nitride and other semiconductor materials, such as silicone carbide, emit ultraviolet light, which degrades many of the above-mentioned polymers typically used in the prior art LED assembly processes. Further, electrostatic discharge (ESD), generally associated with the handling of any LED structure including white light LEDs, induces electrical energies that damage or destroy the structures. In addition, higher power devices are presently being developed for the LED industry thereby necessitating packaging that is capable of withstanding the higher power output.

[0008] In view of its UV resistance, it is desired to use silicone as an encapsulant material in the assembly package. But the softness of silicone presents structural, mechanical, and assembly problems. More particularly, its softness does not supply a high degree of stability to the conventional LED lead frame assembly package typically used in high volume manufacturing, where lead tie bars are removed.

[0009] Thus, there exists a need for improved packaging for an LED lead frame assembly that optimizes the thermal performance of the package, allows for the presence of an encapsulant material that is UV-resistant, includes integral ESD protection, and supplies a high degree of structural and mechanical integrity to the LED lead frame assembly.

[0010] A new and improved semiconductor device packaging assembly is provided. Briefly, in accordance with one embodiment of the present invention, the packaging assembly employs a light-emitting semiconductor and input terminals connected to the light-emitting semiconductor for energizing the light-emitting semiconductor to emit light. Thermally conductive material in the assembly forms a thermally conductive path for dissipation of power out of the input terminals when the light-emitting semiconductor is energized.

[0011] A principal advantage of the present invention is that an LED lead frame assembly is disclosed that optimizes the thermal and mechanical performance of the package.

[0012] Another advantage of the present invention is that a thermally optimized lead frame assembly for LEDs is provided that enables integral ESD protection.

[0013] Still another advantage of the present invention is that an LED lead frame assembly is disclosed that provides a mechanical structure, which in turn allows for the use of soft encapsulant materials in the frame assembly.

[0014] Still another advantage of the present invention is that thermal performance of an LED lead frame package is optimized by thermal conductive bonding of the leads.

[0015] Another advantage of the present invention is that an integral ESD device is disclosed that can be used in connection with both generic semiconductor and capacitor manufacturing and packaging structures.

[0016] A further advantage of the present invention is that a low cost assembly structure, which uses a paradigm change, for an improved lead frame packaging assemblies is disclosed.

[0017] Another advantage of the present invention is that a method of structural enhancement of a LED package when using a soft encapsulant material such as silicone is disclosed.

[0018] Still another advantage of the present invention is that a lead frame assembly structure is disclosed, which can be used in the mass production of LEDs, laser diode devices, and other semiconductor and capacitor technologies.

[0019] FIG. 1 (PRIOR ART) illustrates a cross-section of a prior art LED lead frame package assembly;

[0020] FIG. 2 (PRIOR ART) illustrates a cross-section of a prior art LED lead frame package assembly, including a tie bar;

[0021] FIG. 3 illustrates a cross-section of the LED lead frame package assembly of the present invention;

[0022] FIG. 4 illustrates the lead frame package assembly of FIG. 3, including a thermal molding mechanical support ring;

[0023] FIG. 5 illustrates a cross-section of the lead frame package assembly of the present invention, including ESD material;

[0024] FIG. 6 illustrates the LED lead frame package assembly of FIGS. 4-5;

[0025] FIG. 7 illustrates the LED lead frame package assembly of FIG. 6, including encapsulation with a polymer;

[0026] FIGS. 8A-8D illustrate a first process for adding the thermal molding mechanical support ring to the packaging assembly;

[0027] FIGS. 9A-9B illustrate an alternate process for adding the thermal molding mechanical support ring to the packaging assembly;

[0028] FIG. 10 illustrates a top plan view of a multi-layer flip chip frame structure, including a polymer film insulator layer;

[0029] FIG. 11 illustrates a top plan view of a multi-layer flip chip frame structure, including a polymer glue insulator layer;

[0030] FIG. 12 illustrates a top plan view of a multi-layer flip chip frame structure, including a thermally enhanced bonding layer;

[0031] FIG. 13 illustrates a top plan view of a multi-layer flip chip frame structure of FIG. 10, including an attached flip chip;

[0032] FIG. 14 illustrates a side view of a multi-layer flip chip frame structure, including an attached flip chip;

[0033] FIG. 15 illustrates a top plan view of a multi-layer flip chip frame structure into a cup geometric structure;

[0034] FIG. 16 illustrates a cutaway side view of a multi-layer flip chip frame structure; and

[0035] FIG. 17 illustrates a cutaway side view of a multi-layer flip chip frame structure.

[0036] Referring now to the drawings, which illustrate a preferred embodiment of the invention only and are not intended to limit same, FIGS. 1 and 2 (PRIOR ART) shows a conventional LED lead frame package assembly 10. As is clearly illustrated in FIG. 1, the LED package assembly 10 includes a lead frame having first and second terminals, or die attach lead 12 and isolated lead 14, by which electrical power is supplied to the package assembly 10. The leads 12, 14 are held together during processing by a tie bar structure 15 (shown in FIG. 2) which is cut away after final encapsulation. The die attach lead 12 has a recessed reflector area 16 in which is disposed an LED 18. The LED 18 is connected by a wire bond 20 to the die attach lead 12 and by a wire bond 22 to the isolated lead 14. The LED 18 optionally may include a layer of fluorescent or phosphorescent material 24 disposed over it depending upon the nature of the LED 18. The entire assembly 10 is embedded in a transparent encapsulation epoxy resin 26.

[0037] In operation, the LED package assemble 10 of FIG. 1 (PRIOR ART) has power applied to the lead frame at lead frame at 12 or 14, depending on which part of the LED is the p-junction and which part is the n-junction. Thermal modeling of the prior art LED package assemble of FIG. 1 illustrates that the die attach lead 12 is the primary thermal sinking means.

[0038] The package assembly 10 additionally includes a thermal gap 30 designed into the LED lead frame assembly 10 that electrically separates the interconnect wire bond areas. The consequence of this electrical separation is an intrinsic thermal bias that causes the heat to track on the die attach lead 12 preferentially over the isolated lead 14. This results in poor thermal optimization, particularly with the higher power devices used in connection with current technology. By optimizing the thermal cooling path, as demonstrated in FIG. 3, it is possible to extract higher power out of the prior art LED package assembly 10.

[0039] In the LED lead frame package assembly 30 of FIG. 3, an electrically insulating but thermally conductive path is created in the lead frame package assembly of FIG. 1 (PRIOR ART) by use of a thermally conductive material 40. The material 40 may be positioned in or, in the alternative, may encase the thermal gap 32 or create a bridge in the gap 32. The present invention contemplates that the thermally conductive material 40 may be used in connection with the LED lead frame package assembly 30 in a number of ways to create an electrically insulating but thermally conductive path. All are considered to be within the scope of this invention. The path greatly increases the thermal conduction of the package by using both leads 12, 14 to remove heat generated by the LED device. The thermally conducting, electrically insulating material 40 may be injection, or transfer molded and is preferably Plaskon SMT-B-1, a product of Amoco Electronic Materials (Plaskon Division), which is a highly-filled fast curing epoxide having good thermal and electrical insulation properties. Many other companies, including Dexter Hysol, make similar materials. In the preferred embodiment, the thermally conducting, electrically insulating material 40 is added in the largest amount possible that will not alter fixed dimensions of the packaging. Further, the thermally conducting, electrically insulating material 40 should not cover the LED 18 or any area that the LED 18 radiates from.

[0040] In the alternative, a highly thermally conductive epoxy may be applied by machine in a dot-like format and heat cured, thereby eliminating the need for a molding process. This process is preferred in a retrofit application and is illustrated in FIGS. 9A and 9B. In the retrofit application, the preferred thermally conductive epoxy blend is P280 made by Bacon Industries, Inc.

[0041] Other suitable thermally conducting, electrically insulating materials 40 include glass, silicone, SiC, mineral substances, anodizing, oxidized metals such as oxides of tungsten and oxides of molybdenum, crystalline materials such as silicone, epoxy matrix, resistive materials such as RH, semiconductor devices, diamond or diamond matrix, ESD materials with good thermal properties, or Z-axis thermal material. These embodiments may be planer or may be 3-D to increase the area thermal path, and may be used with or without glue as is known and appreciated by those skilled in the art.

[0042] In the embodiment of FIG. 4, a thermal molding mechanical support ring 42 is shown in connection with the lead frame package assembly 30. As illustrated in FIG. 1, the prior art 5 mm LED lamps are typically held in place using a similar support ring 17 that serves as a mechanical stop. But contrary to the prior art support ring 17, the support ring 42 of the present invention is formed of thermally conducting, electrically insulating material 40 and thus also removes heat generated by the LED device. The ring 42 may be used alone or in combination with the thermally conducting, electrically insulating path created in FIG. 3 to remove heat. In addition to its function as a heat sink, the support ring 42 is designed so that it also serves as a mounting structure thus allowing for the use of a soft encapsulant material in the LED package assembly 30.

[0043] FIGS. 8A-8D and 9A-9B illustrate alternate processes for the addition of the thermal molding mechanical support ring 42 to the packaging assembly 30. Referring to FIG. 8A, a cross-sectional view of the lead frame package assembly with attached support ring 42, leads 12, 14 are positioned in a notched high thermal material frame 54 filled with a thermally conducting, electrically insulating material 40, preferably a thermal epoxy, to form the thermal molding mechanical support ring 42. FIG. 8B is a top plan view of the notched high thermal material frame 54 of FIG. 8A taken along line 1—1. The leads 12, 14 are positioned in the notch area 55 of the frame 54 as best illustrated in FIG. 8C, a top plan view of FIG. 8A taken along line 1—1, prior to filling of the notch area 55 with a thermally conducting, electrically insulating material 40. As shown in FIG. 8D, a top plan view of FIG. 8A taken along line 1—1, the notch area 55 is then filled with a thermally conducting, electrically insulating material 40.

[0044] FIG. 9A illustrates a cross-sectional view of an assembly 30 formed with a support ring 42 using an alternate process. With reference to FIG. 9B, a top plan view of the assembly 30 of FIG. 9A taken along lines 2—2, the thermally conducting, electrically insulating material 40, preferably an epoxy, is applied over the lead frame leads 12, 14 which are positioned against a release layer 56. The release layer 56 is used to set the epoxy and is later removed.

[0045] FIG. 5 includes an ESD material 50 inserted in or bridging the thermal gap 32. The ESD material 50 is inserted in a pre-shaped format in any optional shape. The ESD material 50 may function as a thermal enhancement material in addition to an electrostatic discharge material. The preferred ESD material 50 is a MOV material manufactured by Harris Semiconductor and many other companies. The MOV material is designed to provide adequate voltage discharge protection depending upon the application.

[0046] FIG. 6 illustrates the lead frame package assembly 30 of the present invention, including ESD material 50, thermally conducting, electrically insulating material 40 creating a thermally conductive path, and thermal molded support ring 42 for mounting and additional power dissipation.

[0047] FIG. 7 shows the embodiment of FIG. 6, encapsulated with a soft material 52, preferably silicone in a range of 20-80 durometers, more preferably 50 durometers. Preferably, the silicone is a crosslinked silicone polymer, e.g., polydimethylsiloxane or any other UV-resistant silicone polymers and copolymers. The silicone may be of many varieties made by General Electric of Waterford, N.Y. and many others, including Dow Corning.

[0048] FIG. 10 illustrates a top view of a multi-layer flip chip frame structure 60, including two layers of frame metal 62 and a polymer film insulator layer 64 formed of an epoxy structure that once cured cannot be melted. Suitable examples of the polymer film insulator layer 64 include , for example, KAPTON, a registered trademark of DuPont. The frame metal 62 varies greatly in thickness depending upon the application used. The layers making up frame metal 62 and polymer film insulator layer 64 may be pre-formed and then laminated or glued using any method known by those skilled in the art. The insulating layer 64 can subsequently be patterned by chemical etching, laser patterning by ablation, or plasma or reactive ion etching (RIE). In the alternative, the frame structure 60 may be formed in panels or rolled layers and chemically etched or electro-etched. The insulating layer 64 would subsequently be patterned using any of the methods previously noted.

[0049] FIG. 11 illustrates a top view of the multi-layer flip chip frame structure 60 of FIG. 10, with the exception that a polymer glue insulator layer 66 is used in place of the polymer film insulator layer 64. The glue insulator layer 66 can be dissolved with a solvent and is of the type manufactured by Ulton of Pittsfield, Mass., or ULTEM, a trademark of General Electric. In the top view of a multi-layer flip chip frame structure shown in FIG. 12, a thermally enhanced bonding layer 68 is substituted for the insulator layers 64, 66 of FIGS. 10 and 11. The thermally enhanced bonding layer 68 preferably includes diamond in an amount sufficient to allow for thermal conductivity across the boundary.

[0050] Polymer film insulator layer 64, polymer glue insulator layer 66, and thermally enhanced bonding layer 68 create an insulating, thermally conductive path in the flip chip frame structure 60. The path greatly increases the thermal conduction of the flip chip frame structure 60 by using the frame metal 62 to remove heat generated by any attached LED flip chip structure.

[0051] FIG. 13 illustrates a top view of the multi-layer flip chip frame structure of FIG. 10 with the flip chip 70 attached, including a polymer film insulator layer 64. The preferred flip chip 70 is disclosed in detail in copending application, Ser. No. 09/542,037, entitled “Flip Chip LED Apparatus,” to Wojnarowski, Robert J., et al., FIG. 14 is a side view of a multi-layer flip chip frame structure 60, including the flip chip 70 attached. The embodiment of FIG. 14 includes a patterned thermally enhanced bonding layer 68 such as that shown in FIG. 12. Suitable examples of the patterned thermally-enhanced bonding layer 68 include thermally-enhanced KAPTON (DuPont), TEFLON (DuPont), or ULTEM (General Electric). The shape of the frame metal 62 is not critical and is selected based upon the desired application.

[0052] FIG. 15 is a top view of a multi-layer flip chip frame structure 60 with flip chip 70 attached, into a cup geometric structure 72 including a dimple 74. The dimple 74 is formed by an etching or punching process.

[0053] FIG. 16 illustrates a cutaway side view of a multi-layer flip chip frame structure 60 with an attached micro BGA flip chip structure 70. This embodiment includes a patterned thermally enhanced bonding layer 68.

[0054] FIG. 17 illustrates a cutaway side view of a multi-layer flip chip frame structure 60, including an attached micro BGA chip 70 and micro lens 76. The chip 70 also includes micro BGA 78. The preferred microlens 76 is disclosed in copending application, Ser. No. 09/542,037 entitled “Flip Chip LED Apparatus.”

[0055] The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon a reading and understanding of this specification. For example, other suitable thermally conducting, electrically insulating materials or ESD materials can be used without departing from the principles of the present invention. In addition, the basic principals of the invention are applicable to the packaging of semiconductors in general. The invention is intended to include all such modifications and alterations in so far as they come within the scope of the appended claims and the equivalents thereof.