Patent Application
20060006791
Light-emitting diodes(LEDs) have significant advantages over incandescent and fluorescent light sources both in terms of life time and light output per unit of electricity consumed. Hence, there is a significant incentive to move this technology from simple status indicators on electronic devices to more complex displays that require light sources having geometries that are significantly different from the simple point sources utilized in status indicators.
Consider a seven-segment display of the type utilized to display numbers on control panels and the like. The display may be viewed as 7 elongated segments that emit light when turned on. By turning on these segments in various combinations, the numbers from 0 to 9 as well as some other characters can be displayed. While linear incandescent sources are easily constructed by utilizing an extended filament in the light source, LEDs are typically limited to point sources. Hence, to implement such a display with an LED, the LED must be mounted in an optical housing that converts the LED point light source into a bar-shaped light source.
Extended LED displays are typically implemented by mounting the LED in the bottom of a cavity or well that has a region with a cross-section having the desired shape. The LED is positioned such that light from the LED illuminates the region in question. The well is then filled with a transparent epoxy. A light diffuser can be incorporated in the epoxy or placed between the viewer and the region in question to spread the light so as to form a more uniformly illuminated display segment.
The epoxy encapsulating material imposes design limitations that can substantially increase the cost of a display, or alternatively, limit the lifetime of the display. The epoxy must be compatible with the LED die and the material in which the cavity is formed. In addition, the epoxy must withstand the operating temperatures imposed both by the LED itself and the environment in which the final light source is to operate.
In addition, the epoxy encapsulation process is a lengthy process that increases the manufacturing costs. To assure that the final encapsulation is clear and free from bubbles, the process requires that the components be degassed in vacuum, that the underlying printed circuit board have sufficient holes to allow any entrapped air or bubbles to be removed, and that the epoxy curing be done in an environment that assures that the epoxy is properly cured. If the epoxy is under cured, thermal instabilities in the material that cause in-field failures can occur. If the epoxy is over cured, the material can become very brittle and may crack under thermal cycling either during the testing phase of the manufacturing process or during the utilization of the device in the field. In addition, the inexpensive epoxies that are commonly used are moisture-sensitive materials that have hygroscopic characteristics. The material absorbs moisture over time. Material that is under cured is particularly vulnerable to this type of problem. The absorbed moisture can result in optical defects that appear during processing steps that subject the displays to high temperatures such as wave soldering. In addition, the moisture can cause long-term aging effects that limit the lifetime of the displays.
Furthermore, the epoxy materials can expand or contract significantly during the curing process. This can lead to warping of the display unless materials that resist the warping are utilized. These stronger materials increase the cost of the displays. It should also be noted that these epoxies have different thermal coefficients of expansion than the surrounding materials, and hence, can lead to warping or epoxy separation during the operation of the displays if high power LEDs are utilized in the display.
Finally, it should be noted that the epoxy encapsulation process is irreversible. A typical display includes a large number of LEDs and light shaping elements. All of the LEDs are encapsulated at the same time. If one of the LED display elements has a defect such as entrapped bubbles or a defective LED die, the display cannot be repaired by replacing the defective component, and hence, the entire display must be discarded. The failure probability increases with the number of LEDs, and hence, the yield of devices on the fabrication line can be significantly reduced for displays having large numbers of display segments.
The present invention includes a display having a cover element that is fastened to a base element. The base element includes a substrate having a die mounted thereon, the die includes a semiconductor light source. A transparent protective layer covers the die. The cover element includes an opaque layer having a top surface and a bottom surface. The opaque layer includes an opening extending from the bottom surface to the top surface. The opening is positioned to allow the die and the protective layer to protrude through the opening in the bottom surface. The cover element further includes a transparent window covering the opening above the die and the protective layer. The transparent window defines a pattern that is illuminated by the light source and visible from above the top surface of the cover element. A fastener affixes the base element to the cover element such that the die protrudes through the opening in the bottom surface. In one embodiment, the opaque layer includes a molded plastic element of a first plastic having a first melting point, and the transparent window includes a molded plastic element of a second plastic having a second melting point, the second melting point is less than the first melting point. In one embodiment, the opaque layer and the transparent window are connected to one another by a protrusion that extends from one of the opaque layer and the transparent window into the other of the opaque layer and the transparent window. In one embodiment, the fastener includes a latch that is affixed to either the cover element or the base element, the latch engaging the other of the base element or the cover element. In one embodiment, the transparent window includes an optical element for imaging the light source. In one embodiment, the transparent protective layer includes a pliable material. In one embodiment, the transparent window includes a light guide that contacts the pliable material.
The manner in which the present invention provides its advantages can be more easily understood with reference to
Refer now to
The second component is a cover element that includes an opaque substrate 41 that includes a hole 43 that defines the maximum size of the segment. A transparent window 42 covers the hole 43 in opaque substrate 41 and is sealed to opaque substrate 41 by virtue of the manufacturing methodology discussed below. In the embodiment shown in
The shape of the display segment can be defined by hole 43 or by a pattern that is added to the top surface of window 42. If the clear window is partially covered by an opaque layer as shown at 45, the segment will take on the shape defined by the clear portion of the pattern created by the opaque layer. Such embodiments have the advantage of allowing the final segment shape to be determined after the individual light sources have been constructed by depositing an opaque layer using lithographic methods to the completed light source. Hence, one light source can be utilized for a variety of displays.
Alternatively, hole 43 can be constructed with a cross-section that provides the desired shape. If the placement of the segments is unique to the display in question, little is gained by adding the display shape after the underlying base element has been assembled, since the underlying base element cannot be used for other displays.
The present invention separates the shape defining functions of the cover element from the die protecting functions. Since the die is sealed by layer 34, the opaque substrate and segment window 42 do not need to be hermetically sealed over the die to protect the die. Furthermore, the substrate and window can be removed to access the die in the event the die must be replaced. In addition, the warping problems discussed above are eliminated.
The cover element is preferably constructed via a two-step molding process. Substrate 41 is molded in the first step from a plastic that is opaque and that has a relatively high melting point. In the second molding step, the transparent window 42 is molded into substrate 41 using a plastic with a significantly lower melting temperature. This lower melting temperature allows the window to be molded into substrate 41 without distorting the pre-molded substrate. Substrate 41 can be rendered opaque by using an appropriate plastic or by incorporating a material such as TiO2 in the plastic to absorb any light that enters substrate 41. Any suitable plastic can be utilized for substrate 41. For example, substrate 41 can be constructed from polycarbonate, ABS, polycarbonate and acrylonitrile/butadiene/styrene, polybutylene terephthalate, liquid crystal polymer, Polyphtalamide or other plastics having suitable melting temperatures. The choice of material will, in general, depend on the particular design and application.
The window shown at 42 can likewise be constructed of any plastic that has a suitable melting temperature and which is transparent to the light from the LED. Once again polycarbonate or ABS plastics can be utilized. The plastic used for the windows may include a diffusing material, a coloring agent, phosphor particles for converting a portion of the LED light to another wavelength, etc.
Embodiments of the present invention that utilize a window that also includes optical elements for imaging the light from the LED can also be constructed. Since the window is molded in a separate fabrication operation from a material that is different from the opaque portion of the cover element, the present invention can utilize a wide variety of optical elements. For example, the window can include a collimating lens or a plurality of lenses over different portions of the window. In addition, optical elements based on stamped diffraction gratings can be incorporated in the windows.
Refer now to
In the embodiment shown in
Since the present invention utilizes a cover element that is fabricated in a manner that does not subject the die to the fabrication process, the cover element can be fabricated using temperatures, molding conditions, and solvents that could damage the die. In contrast, the prior art epoxy-based encapsulation methods are limited to conditions and chemicals that are compatible with the die.
In addition, the cover elements can be molded in sheets having a large number of separate cover elements that are then separated after the molding operations into the individual cover elements. As a result, significant economies of scale can be achieved through mass production techniques. In contrast, the prior art methodology is limited by the need to individually dispense epoxy in precise quantities under carefully controlled conditions. The cost of this prior art encapsulation procedure substantially increases the cost of the resulting displays.
The above-described embodiments of the present invention utilize only a single die within each segment of the display. However, embodiments having multiple dies can also be constructed. Such embodiments provide more uniform light output across the segment in the display. In addition, display segments having arbitrary colors can be fabricated utilizing conventional RGB LEDs as the light source.
The embodiments of the present invention described above utilize a mechanical clip mechanism for securing the cover element to the printed circuit board having the LEDs that illuminate the various display segments and features. However, other methods of attaching the cover element to the printed circuit board can be utilized. For example, the cover element can be bonded to the printed circuit board utilizing a glue layer or can be attached using screws or other forms of fasteners.
The above-described embodiments of the present invention have utilized a source layer that includes a printed circuit board to which the dies are connected. However, other substrates can be used for mounting the dies. In principle, any substrate to which the cover element can be affixed and which acts as a mounting platform for the dies can be utilized.
The exemplary embodiments of the present invention described above have been directed to displays that utilize display segments that are simple rectangles. However, many other shapes of display segments can be utilized. Any shape that can be illuminated by placing one or more LED under the transparent window can be utilized. For example, display segments that include words or logos can be constructed in a manner analogous to that described above for the simple rectangular shapes. Hence, as used herein, the term display segment includes any geometric pattern that acts as a light source.
The above-described embodiments of the present invention have utilized an arrangement in which the transparent window is molded into the cover after the cover is formed. In this case, the window must be made of a plastic having a lower melting point than that of the cover. However, embodiments in which the window is formed first, and then the cover is molded around the window can also be practiced. In this case, the window needs to have the higher melting point.
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.
