Patent Application
20100270580
Light emitting diodes (LEDs) are an important class of solid-state devices that convert electric energy to light. Improvements in these devices have resulted in their use in light fixtures designed to replace conventional incandescent and fluorescent light sources. The LEDs have significantly longer lifetimes and, in some cases, significantly higher efficiency for converting electric energy to light.
LEDs are particularly attractive as replacements for incandescent bulbs in flashlights and other battery powered devices. LEDs have significantly longer lifetimes than incandescent bulbs and light conversion efficiencies that are several times the efficiency that can be achieved with conventional incandescent bulbs. The increased light conversion efficiency extends the lifetime of the batteries used to power the devices, and hence, battery replacement is reduced. In addition, LED-based lights provide increased ruggedness relative to incandescent lighting, and hence, are better adapted to portable lighting applications.
In addition, LEDs have lifetimes that are greater than the lifetime of incandescent bulbs and fluorescent tubes. This feature is particularly attractive in applications in which the cost of replacing the light bulb is significant. For example, replacing a light bulb in a traffic signal can require a man-hour or more of labor as well as the interruption of the traffic at the intersection.
Unfortunately, the amount of light generated by a single LED is limited. Individual LEDs are limited to a few watts of power, and hence, even though the light output per watt is significantly greater that an incandescent light source, many applications of interest require multiple LEDs to provide sufficient light. For example, flashlights and lighting systems used with infrared cameras typically include an array of individually packaged LEDs. The cost of such systems is substantially increased by the need to accommodate the individually packaged components and the installation of those components.
Heat dissipation is also a significant problem with high power LED light sources. The conversion efficiency of electrical power to light in an LED decreases with increasing junction temperature within the LED. Hence, removing heat from the LEDs is a major factor in the design of any LED light source that generates significant amounts of heat. If the heat is not efficiently removed, the conversion efficiency, and hence, the amount of light that can be generated, is substantially reduced.
Most devices that utilize LEDs are based on some form of pre-packaged LED light source that is configured for attachment to a heat-dissipating surface in the final device. LED packaging solutions typically use either a lead-frame based packaging approach or a printed circuit board based packaging approach, often referred to as chip on a board (COB) packages. In both of these approaches, the heat from the packaged LED-containing die or dies is typically transferred to a heat dissipation surface that is used to transfer the heat to the ambient air, since the packaged LED has insufficient surface area to transfer the heat directly to the ambient environment without operating at a substantially elevated temperature.
In the lead-frame packaging approach the semiconductor chip is mounted to a lead frame based assembly that is encapsulated to provide a packaged part with leads that either exit the encapsulation or are flush with a surface of the package. Electrical interconnection to the final device is made via the external leads that are attached to the final product by a solder or mechanical process. In some lead-frame based packages, the electrical and thermal paths are combined, serving the dual purpose of providing the electrical power to the semiconductor chip and removing the heat generated to either the outside world or to a secondary heat sink. In other lead-frame based packages, the electrical and thermal paths are separated, leaving the lead frame material to serve the sole purpose of providing the electrical power to the semiconductor chip. In this approach a separate thermal path is designed into the package to remove the heat generated.
Typically the lead-frame based approach is facilitated using a molded packaging design where the lead frame material (usually a thin metal layer) is embedded in a packaging material (such as plastic, epoxy, or other moldable materials). In some approaches a secondary heat sink or thermal slug is either inserted into the molded lead frame during the assembly of the package or may be molded into the package during an insert molding process where the lead frame, body, and thermal slug or heat sink are molded together as a separate component to which the semiconductor chip is attached and encapsulated during the assembly process. While this lead frame based packaging approach is used for many semiconductor components today, typically the thermal resistance of such an assembly is high, on the order of 5-20 degrees C/W.
Even in lead frames in which the thermal paths are sufficient to avoid large temperature increases, the surface area of the thermal transfer surfaces are typically too small to allow direct heat dissipation to the ambient air. Hence, some form of secondary substrate is required to transfer heat. This substrate enables the user to connect to the electrical leads and to couple the thermal slug or heat sink of the lead frame based package to an external heat sink or to the overall system assembly, which may act as the heat sink for the lighting system.
Every additional thermal interface in the total integrated solution increases the thermal resistance of the heat-dissipation path, and hence, increases the temperature at which the LEDs must operate. Light output efficiency, output color and product reliability are all dependant on the junction temperature of the semiconductor chip. Hence, reducing the thermal resistance of the heat-dissipation path is an important consideration in light source design. In addition, the need for an additional substrate increases the cost of the lighting system. Hence, other forms of LED packaging have been sought.
In substrate-based packaging, the semiconductor chip containing the LED is mounted on a substrate material such as ceramic, printed circuit board, silicon, or other medium that can be mounted to a secondary substrate for electrical and thermal interconnection within the sub assembly or total lighting system. In the case of a COB type of package, the chip is mounted on a metal core printed circuit board that can be thermally mounted directly to a heat sink of a sub assembly or the total lighting system with a separate electrical interconnection method such as soldered leads or a separate electrical connector.
The first of these approaches has the same drawbacks of the lead frame based packaging approach. Although the substrate based package may reduce the thermal resistance compared to a lead frame based approach, the secondary substrate required to manage the electrical and thermal interconnections still adds a layer of thermal resistance in the lighting solution, increasing the junction temperature of the device resulting in lower efficiency, lower light output, and reduced reliability.
The COB approach introduces problems related to the electrical connection to the electronic driver or power supply. Most products today using the COB approach either use solder pads on the substrate for soldering wires or include a connector mounted to the top surface of the substrate. Solder pads on the substrate, require the user to solder wires from the driver circuit to the assembly, which can be a difficult, expensive task and risks damage to the LED array. In general such an assembly approach is not deemed high volume assembly capable. If connectors are used, the size of the overall solution is typically increased, which results in a more expensive and larger or bulkier solution. Additionally, the cost of the overall system is increased by the cost of the connector on the LED assembly and the mating connector in the final light source.
The present invention includes a light source and method for making the same. The light source includes a base member and a lead structure. The lead structure is attached to the base member such that the lead structure extends beyond the base member and has an opening for accessing a surface of the base member. A die containing a light emitting semiconductor device is bonded to the surface of the base member. The die is electrically connected to the lead structure and overlaid with a transparent material. In one aspect of the invention, an electrically insulating layer is bonded between the lead structure and the base member, the electrically insulating layer having an opening for accessing the surface of the base member. The electrically insulating layer can be an adhesive for bonding the lead structure to the base member. In another aspect of the invention, the lead structure includes a rigid connector for coupling power to the die. In a still further aspect of the invention, the base member includes a feature for aligning the base member with an external device.
The manner in which the present invention provides its advantages can be more easily understood with reference to
Light source 20 is limited by the underlying printed circuit board structure used to construct the light source. For example, the thickness of the metal layer that is patterned to provide the traces 24A-24C limits the thickness of the connection structures used to access the dies from the external circuits. In addition, the connection structures overlie the underlying insulation layer 23, and hence, extending the connection structures beyond insulation layer 23 to form a rigid connector or pin is difficult. Hence, connections are made by soldering directly to pads 24C and 24D or by providing a separate connector that is bonded to these pads.
Similarly, the thickness of layer 22 places limitations on the design parameters of light source 20. For example, the thickness of layer 22 limits the heat transfer area that is effectively available for removing heat from die 27 to an area that is immediately under die 27 even when layer 22 is in continuous contact with a larger heat dissipating surface. In addition, it is often useful to provide features on the base structure that provide mechanical connections to the final structure in the finished device that utilizes light source 20. To provide the structures, a separate structure must be connected to layer 22 or the structure must be limited to structures that can be etched into a metal layer of thickness allowed in such printed circuit board structures.
Refer now to
The dies are connected to the external circuitry by a set of leads that are part of a lead structure 33. The electrical leads provide connection from the dies to the driver or power system in the sub assembly or the lighting system. The electrical leads may be constructed of a flexible material (such as a flexible circuit embedded in polyamide or another similar material) or may be constructed from a more rigid material, similar to those found in a lead frame based packaging approach.
Since the thickness of the leads is not dictated by printed circuit board considerations, the leads can be shaped to provide a connector that extends beyond the boundaries of base member 31. The electrical leads can be configured in many different geometries, and may either be flat or produced (either prior to, during, or after assembly) into a formed lead which may be suitable for a surface mount, through hole, hot bar, glue, laser weld or other electrically conductive assembly methods to connect to the power source or electronic driver in the sub assembly or lighting system.
The electrical leads may be produced as a separate stamped or formed component and then be bonded to the base member during the assembly of the base member or the leads could be bonded during the assembly of the light source. The electrical leads may be bare metal or may be plated with a variety of plating metals including gold, silver, nickel, or others, or a combination of these or other metals. They may also be pre plated with a solder or other materials to aid in assembly into the sub assembly or lighting system. The assembly of the electrical leads to the base member may be made via soldering, gluing, co-firing, or other process either during the assembly of the base member or at a later assembly step in the construction of the LED light source.
An optional insulating layer 32 can be introduced between lead structure 33 and base member 31 to prevent shorts when base member 31 is constructed from an electrical conductor. The insulating layer could be constructed from an adhesive material that acts to bond lead structure 33 to base member 31 in addition to providing electrical insulation.
The dies 34 are bonded to leads in lead structure 33 by wire bonds or other suitable conductors. An exemplary wire bond connecting a die to a lead in lead structure 33 is shown at 39. The wire bonds can also connect the dies to one another as shown at 38. In embodiments in which base member 31 is a conductor and acts as a common electrode for the dies, one of the power connections can be made through base member 31.
In general, the dies and the wire bonds must be protected from the external environment both in terms of physical stresses and moisture. A ring 35 which acts as a mold for a protective layer 36 can be bonded to lead structure 33 and then filled with a transparent medium. Various additives such as phosphors and light scattering particles can be suspended in the transparent medium depending on the particular application. The layer of transparent medium can also be used to bond ring 35, lead structure 33 and base member 31 to one another. The protective layer can be constructed from silicon or epoxy materials that are transparent to the light that is to leave layer 36. This protective layer may also be introduced through over molding.
As noted above, lead structure 33 is not constrained by the processes used to construct printed circuit boards, and hence, can have a thickness that enables the leads to be bent to provide various mounting configurations for the light source 30. Refer now to
Lead structure 33 can also be configured to provide a connection that bonds the light source to a printed circuit board from the bottom surface of the printed circuit board such that the light source is visible through an opening in the printed circuit board. Refer now to
In addition, lead structure 33 can be configured to form a connector that mates with a complimentary connector in the final assembly that utilizes the light source. Refer now to
In the above-described embodiments, the light sources were described in terms of the construction of an individual light source. However, light sources according to the present invention can be constructed in groups to further reduce the cost of each light source. Refer now to
Refer now to
Referring to
The above-described embodiments of the present invention have been provided to illustrate various aspects of the invention. However, it is to be understood that different aspects of the present invention that are shown in different specific embodiments can be combined to provide other embodiments of the present invention. In addition, various modifications to the present invention will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Accordingly, the present invention is to be limited solely by the scope of the following claims.
