Patent Application
20120256813
The present disclosure relates to a light source and particularly a light source connection and a method for providing the same. More particularly, the disclosure relates to a light emitting device such as an organic light emitting diode panel and connection, as well as to low temperature materials and methods suitable for providing electrical connection to such a panel.
Organic light emitting diode (OLED) devices are generally known in the art. An OLED device typically includes one or more organic light emitting layer(s) disposed between electrodes. For example, a cathode, organic layer, and a light-transmissive anode formed on a substrate emit light when current is applied across the cathode and anode. As a result of the electric current, electrons are injected into the organic layer from the cathode and holes may be injected into the organic layer from the anode. The electrons and holes generally travel through the organic layer until they recombine at a luminescent center, typically an organic molecule or polymer. The recombination process results in the emission of a light photon usually in the ultraviolet or visible region of the electromagnetic spectrum.
The layers of an OLED are typically arranged so that the organic layers are disposed between the cathode and anode layers. As photons of light are generated and emitted, the photons move through the organic layer. Those that move toward the cathode, which generally comprises a metal, may be reflected back into the organic layer. Those photons that move through the organic layer to the light-transmissive anode, and finally to the substrate, however, may be emitted from the OLED in the form of light energy. Some cathode materials may be light transmissive, and in some embodiments light may be emitted from the cathode layer, and therefore from the OLED device in a multi-directional manner. Thus, the OLED device has at least a cathode, organic, and anode layers. Of course, additional, optional layers may or may not be included in the light source structure.
Cathodes generally comprise a material having a low work function such that a relatively small voltage causes the emission of electrons. Commonly used materials include metals, such as gold, gallium, indium, manganese, calcium, tin, lead, aluminum, silver, magnesium, lithium, strontium, barium, zinc, zirconium, samarium, europium, and mixtures or alloys of any two or more thereof. On the other hand, the anode layer is generally comprised of a material having a high work function value, and these materials are known for use in the anode layer because they are generally light transmissive. Suitable materials include, but are not limited to, transparent conductive oxides such as indium tin oxide (ITO), aluminum doped zinc oxide (AZO), fluorine doped tin oxide (FTO), indium doped zinc oxide, magnesium indium oxide, and nickel tungsten oxide; metals such as gold, aluminum, and nickel; conductive polymers such as poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS); and mixtures and combinations or alloys of any two or more thereof.
Preferably, these light emitting or OLED devices are generally flexible, i.e., are capable of being bent into a shape having a radius of curvature of less than about 10 cm. These light emitting devices are also preferably large-area, which means they have a dimension of an area greater than or equal to about 10 cm2, and in some instances are coupled together to form a generally flexible, generally planar OLED panel comprised of one or more OLED devices, which has a large surface area of light emission. Preferably, the panel is hermetically sealed since moisture and oxygen have an adverse impact on the OLED device.
The flexible nature of the OLED and the temperature tolerance level of the OLED panel combine to make providing a reliable electrical connection difficult. One concern relates to the material used to create the connection, which must be ductile, exhibit a suitable Young's modulus, and have an appropriate coefficient of thermal expansion with regard to a device that operates at temperatures below about 100° C., e.g. below about 80° C. Another concern arises with regard to the material used in the area where the electrical connection is made. While it is known to use silver epoxy or double-sided conductive tape in the area of the connection, both suffer from problems due to early delamination and poor electrical properties, thus shortening the useful life of the device. Structural and electrical deficiencies are, for example, seen in the double-sided conductive tape. In addition to the physical limitations of known materials, as just set forth, such materials, for example conductive epoxies, may eventually provide an acceptable connection but must be processed for extended periods of time, generally more than 4 hours, e.g. more than 10 hours, e.g. up to about 24 hours. This long manufacturing time is the result of the higher temperatures needed in order to render an acceptable connection, which is limited by the temperature parameters of the OLED, which are much lower than the higher temperatures at which such materials are generally processed. Therefore, it takes a longer period of time at a lower temperature to render a connection that is acceptable, but not necessarily up to the desired standard.
What is lacking in the industry is a mechanism for providing a long-lasting, yet flexible, connection that exhibits good electrical properties and is quickly processed even at the lower temperatures tolerated by the OLED panel. It is desired, therefore, that an efficient, less time-consuming, low temperature method, as well as suitable materials for use with the method, be provided for establishing an electrical connection with the light emitting panel of an OLED device, and also that the electrical connection maintain flexibility, be easily and accurately positioned and processed, establish good electrical continuity, and allow the device to maintain a thin profile.
A lighting assembly includes a light source having a first generally planar, light source including a perimeter edge. A backsheet is disposed in substantially parallel relation with the light source, and includes at least one electrical feedthrough region extending through the backsheet. In addition, each feedthrough region is substantially covered by a contact patch disposed in substantially planar relation between the backsheet and the light source. A generally planar, connector cable extends over the backsheet from a perimeter thereof and has connector pad(s) positioned thereon to associate with each feedthrough to the light source through the contact patch. A low temperature solder material is disposed between each connector pad and contact patch for establishing electrical connection with the light source, wherein one or more of the light source, connector cable, backsheet, or any portion thereof is constructed of one or more plastics.
In one embodiment, the front barrier sheet is adhesively sealed to the backsheet, thus hermetically encapsulating the light source within the lighting assembly.
In one embodiment, the light source is an OLED, and the connector cable is flexible.
The backsheet may include spaced apart, plural electrical feed-through regions, and consequently the connector cable includes spaced apart, plural electrically conductive pads for establishing electrical connection with the discrete light source portions of the panel. Each of the electrically conductive pads preferably includes an opening that aids in application of the low temperature solder for establishing electrical connection with the light source.
The backsheet includes a metallic patch that covers the feed-through region, providing an electrical pathway to the light source while maintaining the hermetic nature of the panel. This patch material in the area of the feed-through region(s) must also maintain the flexible nature of the device while providing a surface for a reliable and durable electrical connection. This material, which may for example be silver, copper, tin, nickel, gold or a combination thereof, must be compatible with the adhesive material within the hermetic package to prevent delamination and consequent failure due to oxygen and water vapor ingress, and it must also have properties that make it amenable to low temperature soldering techniques, i.e., it must have a high bond strength to the low temperature solder, exhibit low interface resistance, and have good oxidation resistance. The acceptable patch material must, therefore, be impermeable to oxygen and water vapor (e.g. having substantially no pin hole defects), provide good adhesion to the backsheet, be flexible, and in addition be burr-free to prevent possible shorting of the electrical device, which could occur if the patch edge(s) contact the inner metal foil in the backsheet through punctures in the outer layer.
Further provided is a material suitable for establishing and supporting an electrical connection between the adjacent surfaces of the connector cable and the patches on the backsheet. The material used to bond the two surfaces may be a low temperature solder having a melting point temperature below 200° C., e.g. below about 150° C., preferably below about 100° C. The solder may be a single material or an alloy of several suitable materials which are RoHS compliant, and must provide a strong bond between the connector cable and patch.
An associated method of assembling a light panel includes providing a substantially planar light source having a light emitting surface, a backsheet extending in substantially parallel relation therewith, and at least one electrical feed-through region in the backsheet located inwardly from a periphery of the substantially planar light source. Positioned over the electrical feed-through region is a flexible patch allowing an electrical pathway to the light panel as well as maintaining the hermetic seal of the entire device to prevent degradation from water and/or oxygen. Positioning a connector cable over the backsheet, including the patch, is a part of the assembly method so that a first portion of the connector cable extends outwardly of the light source periphery for connection with an associated drive circuit. The method further includes electrically connecting a second portion of the connector cable with the electrical feed-through region of the backsheet through the patch. The electrical connection is established between the connector cable and the patch using a low temperature solder composition that resists delamination, is oxidation resistant, and maintains the flexibility of the light panel.
The method includes insulating conductive traces along a length of the flexible connector cable, and providing plural, spaced apart conductive pads along a surface of the connector cable for establishing electrical contact with similarly spaced electrical feed-through regions in the backsheet, each such region be covered by a patch in accord herewith.
In one embodiment, the light panel is fully laminated, and the electrical connection then provided in a post-lamination step by soldering the conductive pads on the cable to the OLED through the patches provided on the backsheet in the feed-through region.
In another embodiment, the light panel is assembled, including providing solder in the feed-through regions of the backsheet, and during the hermetic encapsulation lamination process step, the solder is flowed to make the electrical connection from the patches to the connector cable.
In yet another embodiment, the backsheet is pre-assembled, including soldering of the cable at the feed-through regions through the patch material, and then the light panel including the pre-assembled backsheet is assembled and laminated.
The electrically connecting step includes tilling a feed-through region with a conductive material to establish the electrical contact between the conducting pad and the patches in the feed-through region.
The filling step includes introducing a conductive bonding material such as a low temperature solder between the conductive pads and the patch, and optionally thereafter covering the conductive pad and feed-through region with an electrical insulator.
A primary benefit is the ability to provide an effective, reliable electrical pathway for the panel from a region external of the panel while maintaining flexibility. Yet another benefit is found in the thin profile maintained by the lighting assembly when using a flat flex connector cable, along with the flexibility of the low temperature solder and, if used, the insulator sheet that allow for conformable lighting solutions.
Openings in the conductive pad of the flat flex cable simplify manufacturing, allowing the flat flex cable to be initially positioned, and then a bonding material applied to insure positional accuracy.
The use of low temperature solder bonding material and a compatible patch material provide for processing at temperatures suitable for use with the temperature sensitive OLED panel, while still establishing a reliable, flexible connection between the lighting assembly and an external drive circuit.
In addition to the foregoing, this method provides an option to construct a lighting assembly in a single lamination process, thus providing a more economical assembly, as well as providing for ease in uniformly illuminating a large area, interconnecting multiple devices, and maintaining flexibility of the panel.
Still further, the use of a low temperature solder as disclosed herein for electrically connecting the patch and the connector cable allows for efficient, fast processing at lower temperatures, in keeping with the temperature sensitive processing limits of the OLED, as compared to other known epoxy or tape adhesive arrangements and materials.
Still other benefits and advantages of the present disclosure will become apparent upon reading and understanding the following detailed description of the preferred embodiments.
For purposes of the following description, since the particular details of a generally planar, flexible light source or OLED device are known to those skilled in the art and previously referenced in the Background of the present application, further description herein is deemed unnecessary. Those details required for the present disclosure are provided below and illustrated in the accompanying drawings. As used herein, the term “lighting assembly” refers to any assembly of all or some of the components and materials described herein, including at least a light source, which may be an OLED device or a panel including at least one hermetically encapsulated OLED device, and a low temperature mechanism, for example a low temperature bonding material, in conjunction with a connector cable, for providing electrical power to the assembly. The terms “OLED” and “light source”, and variations thereof, may be used interchangeably herein. Though the invention may be described herein with respect to a flexible light assembly, one skilled in the art would understand that the various features and attributes of the disclosure are equally applicable to other lighting solutions. In the Figures provided, like elements of the lighting assembly are denoted with the same reference number in order to provide continuity and understanding.
More particularly, and with initial reference to
With reference to
With reference to
Between the backsheet 110 and the OLED 102 is patch 106 corresponding to each feed through region 120. Patch 106 may be shown in the Figures in phantom. The patch is comprised of a material that exhibits high bond strength to the low temperature solder contemplated for use in certain embodiments. As such, though aluminum is conventionally used in conjunction with silver epoxy or double-sided conductive tape adhesives, it is not well suited for use herein given that the low temperature solder best suited to use with temperature-limited OLEDs does not adhere well to an aluminum surface. A more suitable patch material is, for example, silver, tin, or copper. In one embodiment, the patch comprises tin coated copper. The copper may be coated with the tin by any known method. For example, it has been discovered that in general a hot coated tin dipping method provides a patch exhibiting the requisite strength and other desirable characteristics. As noted, given the temperature sensitive nature of the OLED device, the patch should be comprised of a material compatible with the low temperature solder preferred for use in the electrical connection herein.
One concern with regard to the foregoing relates to the material used to create the connection, which must be ductile, exhibit a suitable Young's modulus, and have an appropriate coefficient of thermal expansion with regard to a device that operates at temperatures below about 100° C., e.g. below about 80° C.
The patch 106 preferably exhibits certain interface properties, such as providing a durable electrical connection to the solder, low electrical interface resistance, and oxidation resistance. Given those general requirements, tin plated copper is a good candidate material. In addition, the patch material should be substantially free of pinhole defects. Certain metals tend to exhibit pinhole defects when provided in the thin film form required for use herein. The patch material must further exhibit good adhesion to the thermoplastic and pressure sensitive adhesives used in adjacent layers of the hermetically sealed panel, be flexible, and have burr-free edges, so as to reduce the potential for electrical shorting failures. Edges of the patch 106 may be rendered burr-free for example by laser, chemical etching, or any other suitable method.
In the preferred embodiment of
Once temporarily positioned in place as illustrated in
In addition, the solder material exhibits a suitable Young's modulus, and a coefficient of thermal expansion (CTE) that is compatible with the other materials at the connection interface. Young's modulus, which is a ratio of stress to strain, or a measure of the stiffness of a material, is generally represented in terms of pressure, i.e. GPa. Suitable solder materials exhibit a Young's modulus consistent with a high degree of flexibility, e.g. 2-150 GPa, e.g. 5-50 GPa.
Also important is the CTE of the solder material and its compatibility with the other materials in the region of the electrical connection. The materials used in the different components of the system will expand and contract in response to heat generated during processing or during use over the life of the device in accord with the CTE of each. Therefore, the patch material and solder, as well as the pads of the flat flex cable, should have comparable CTE values. A thermal mismatch between adjacent materials may quickly degrade the electrical connection, by causing, for example, cracking or delamination in response to unequal stretching and/or compression caused by reaction of each different material to heat. For example, a CTE match within 150%, e.g. within 50%, e.g 20% or less is desirable. TABLE 2 below provides Young's modulus and CTE data for patch and solder material components in accord with the disclosure.
22 × 10−6
The low temperature solder material 160 substantially tills the cavity 144 and is in electrical contact with patch 106 that is, in turn, in electrical contact with a conductive portion of the OLED device 102. Portions of the cavity 144 may be lined with the insulating material 162. In one embodiment, the low temperature solder 160 is introduced from the outwardly facing surface 146 of the flat flexible cable, through the openings 140 in the conductive pad 130 and into the cavity 144 (
As shown in
As has been noted, it is important that the materials used have comparable and compatible CTE's and exhibit acceptable flexible, ductile behavior. By using the system and device as structured in accord with the foregoing, the electrical connection can be precisely located, and yet the final assembly is effectively hermetically sealed from the external environment. Electrical continuity is created through the back of the OLED panel to the rest of the system, i.e., through the backsheet and the patch, without compromising the remainder of the structure. The end connector is then a simple, one-stage connection that can be used to connect all of the individual OLED devices that comprise the panel to the rest of the electrical system. By individually addressing electrical feed-through regions 120, individual OLED devices can be individually addressed. For example, a device may need to be tuned and thus one OLED device treated differently than another OLED device in the lighting assembly.
The lighting assembly disclosed herein may be assembled in accord with the following procedural example to ensure that a quality electrical connection is established. It is understood, however, that this example is not intended to be limiting with regard to the assembly or system, but rather provides one manner of assembling the disclosed system using the low temperature solder and patch materials, in conjunction with a flat flex cable, to establish a path for electrical connection of the lighting assembly. Other methods or processes known to those skilled in the art may be employed.
This Example may be understood best with reference to
Prior to actually soldering the materials to create an electrical path, all materials were carefully cleaned. Any dirt or grease that gets onto the materials, even from human contact, will cause reduce adherence of the solder to the contact patch and consequently degrade strength and durability causing connection problems in the resulting device. In order to avoid such problems, the materials were first cleaned with 2-propanol, i.e. both sides of the connection pads, and the exposed contact patch regions, being careful not to get excess alcohol on the backsheet of the hermetic OLED panel as this may cause delamination. It is also important to maintain an oxidation-free soldering tip, as any residual oxidation may work its way into the solder joint and degrade the same. Therefore, the solder iron tip was cleaned continually throughout the following processing.
Once the materials are properly prepared, the Weller soldering station is preheated. For the 61° C. solder, the soldering station was used at 120° C., and for the 79° C. solder the station was set to 160° C. A small amount of room temperature flux was applied to the contact patch and preheated using the soldering iron until it was more liquid. In order to achieve a good connection between the solder and the contact patch, the patch should be at or about the same temperature as the solder. It is also important throughout the following process to continually clear any oxidative or material build-up from the tip of the solder iron. When the patch was at or about the same temperature as the solder, the solder was applied to the patch using the solder iron tip. If the solder does not adhere immediately, continued application of heat from the solder iron may be used. Unbonded solder will have the appearance of a ball, while bonded solder will appear more like a drop of liquid/water. Enough solder should be applied to cover the exposed patch region. Because the solder hardens almost immediately, it was important to immediately place the connection pad of the flat flex cable over the solder joint. Continued application of heat from the solder iron tip was used to keep the solder from completely hardening prematurely. After placement of the cable connection pad over the solder joint, heat was continued to the top surface of the connection pad, causing the solder to flow through the holes in the connection pad (see
The foregoing processing was used to create two electrical connection samples, one using the 61° C. solder (Sample A), applied at 120° C., and one using the 79° C. solder (Sample B), applied at 160° C. In addition, two more samples were prepared, one using a silver epoxy adhesive (Sample C) and one using double-sided Z-tape as the adhesive mechanism (Sample D). Samples C and D were both prepared at room temperature in accord with conventional processing for such materials, i.e. the various components were assembled, with the silver epoxy or Z-tape applied to bond the electrical connection materials. Samples A and B were prepared using tin coated copper patch material given that the low temperature solder did not adhere well to plain aluminum patch material, which might be conventionally used in order to retain the flexible nature of the over-all device. Moreover, it was found that the processing used to prepare the patch material had an affect on the quality of the connection. Vapor deposited tin coated copper had a rougher surface that did not allow for quality bonding. Therefore, it was determined that hot tin dipped copper had a smoother surface and thus allowed for a much better bonding surface. This material, obtained from All Foils, Inc., was cut to the desired dimension (30 mm×60 mm) and then finished to remove any defects from the edges of the material, rendering the patches burr-free.
The contact patches, processed in accord with the foregoing, were laminated to a backsheet made from Tolas TPC-0814B lidding foil, which is a multi-layer barrier material available commercially from Oliver-Tolas Healthcare Packaging. In addition, insulator rings were attached to the backsheet in the region of the electrical feed through and connection. Next, the flat flex cable having connection pads, as described herein, was attached using material A, B, C and D, generating four samples.
Each sample was next attached to a metal plate using double sided tape, and the flat flex cable was extended using additional tape. In order to test the ability of the connection material to resist stress and strain, each sample was loaded into a Chatillon TCM201 motorized force tester. The samples were tested in a shear pull method and also in a 180° peel test method. Both tests were conducted at a pull speed of 1 inch/minute.
The following Tables 3-6 provide the data obtained during these tests. As can be seen, the average force at which the two soldered samples, A and B, failed was significantly higher than that at which the silver epoxy and Z-tape adhesives failed, for both test methods. This indicates that the low temperature solder adhesive system will provide greatly enhanced performance with regard to durability and flexibility of the OLED device without experiencing failure due to delamination or other structural failure.
In addition to the foregoing, it has been determined that the optional addition of a thermocouple on the back side of the contact patch helps to maintain an OLED temperature below the 120° C. limit of the panel. With the thermocouple in place, sample in accord with A and B above were tested and it was determined that the patch in sample B (79° C. solder) experienced a maximum temperature of 85° C., and the patch in sample A (61° C. solder) experienced a maximum temperature of 74° C. In both samples, no visible defects were detected.
The thin profile of the flat flexible cable 122, along with the flexibility of the patch 106 and the low temperature solder 160, and optionally the insulator cap 164/rings 112/and or sheet 114 where used, still allows the OLED panel to be adapted for use in conformable lighting solutions. The corresponding openings in the connector pad of the flat flex cable and the patch assist in manufacture/assembly of the OLED panel. In one embodiment, the low temperature soldering process is completed post lamination, i.e. the various layers of the OLED device, which may for example be a dual layer encapsulated device, are laminated, followed by low temperature processing to flow the low temperature solder, which is present on the patch, through the holes in the connection pad(s) of the flat flex cable to the feed through openings to create the requisite electrical connection. Alternatively, the entire OLED package, shown in an exploded view in
The embodiment of
The disclosure has been described with respect to preferred embodiments. Obviously, modifications and alterations may be contemplated by one skilled in the art, and the subject disclosure should not be limited to the particular examples described above but instead through the following claims.
