Abstract:
Proposed is a lighting device ( 100 ), comprising LEDs ( 130 ) mounted on a transparent substrate ( 110 ), provided with a transparent electrically conductive layer ( 120 ) and a contact pad ( 140 ). The contact pad has a second part ( 142 ), extending away from a first part ( 141 ), for further reducing the current density in the conductive layer ( 120 ). This is advantageous for making the lighting device robust to large power dissipation, especially under high current testing conditions. Moreover, as the voltage drop over transparent conductive layer is reduced, the efficiency of the lighting device is increased.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a lighting device comprising LEDs mounted on a transparent substrate. In particular, the invention relates to the contact interface of an LED to a glass substrate coated with a transparent electrode. 
       BACKGROUND OF THE INVENTION 
       [0002]    A lighting device of the kind set forth is known from U.S. Pat. No. 6,270,236. That document discloses a lighting device including a transparent carrier panel provided with transparent electrically conductive structures (e.g. an ITO layer) on one of the panel surfaces. On the carrier panel an LED—with terminals on both sides of a light emitting side—is mounted in such a way that the terminals are in contact with the conductive structures via a conductive adhesive (e.g. a soldering bump.) In particular, the conductive structure may have an additional electrically conductive contact pad in the form of a metal coating in the area of contact with the terminals of the LED. The advantage of this approach is disclosed to be an increase in contact stability. 
         [0003]    However, experience shows that the increased stability provided by the metal contact pads in the direct vicinity of the LED terminals is limited. Due to the low conductivity of the ITO layer (R□˜15 Ohm) the lighting device is prone to failure. Especially under high current testing and over lifetime, the device fails as the electrical contact of the LED with the contact pad/ITO layer is lost due to very local heat dissipation. 
       SUMMARY OF THE INVENTION 
       [0004]    It is an object of the present invention to provide a lighting device, comprising an LED and a substrate provided with a conductive layer, which is robust to large power dissipation, especially under high current conditions. This object is achieved with a lighting device according to the invention as defined in claim  1 . A lighting device comprising a carrier plate, a structured electrically conductive layer, provided on a first surface of the carrier plate and having a sheet resistance R tcl , a LED having a terminal, a contact pad, provided on the layer, with a sheet resistance R cp , the ratio R tcl /R cp  being larger than 1, the contact pad having a first part, the LED terminal being coupled to the electrically conductive layer via the first part, characterized in that the contact pad further has a second part extending away from the LED for reducing the current density in the conductive layer. 
         [0005]    The invention provides a lighting device that realises a significant reduction of the power dissipation in the transparent conductive layer. For it is largely there where the highest power is dissipated. The power dissipation results from an increased current density in the conductive layer directly outside the metallic contact-pad. By extending the contact-pad substantially beyond the terminal of the LED, the current density increase in the layer is considerably moderated. Hence, the lighting device according to the invention is robust to power dissipation, especially under high current conditions. Moreover, as the voltage drop over transparent conductive layer is reduced, the efficiency of the lighting device is increased. 
         [0006]    In an embodiment of the present invention, the second part of the contact pad has a first edge with a length L cp , a second edge with a width W cp , L cp  and W cp  satisfying the relation L cp ≦W cp *R tcl /R cp . 
         [0007]    According to an embodiment of the present invention, the second part of the contact pad is free from sharp angles. This advantageously prevents increased current density levels, which are typically found near sharp angles. 
         [0008]    In an embodiment of the lighting device according to the invention the electrically conductive layer is structured by a separating groove, and the second part of the contact pad extending away from the LED perpendicular to the groove. 
         [0009]    In an embodiment of the invention the width W cp  of the second part is smaller than the size of the contact pad&#39;s first part to which the second part is connected. This advantageously optimizes the transparency of the lighting device  100 . In an embodiment the contact pad comprises a metal. 
         [0010]    In an embodiment the electrically conductive layer comprises a metal-oxide, which is chosen from the group consisting of InO x :Sn, SnO x :F, SnO x :Sb, ZnO x :Ga, ZnO x :B, ZnO x :F, ZnO x :Al and Ag/TiO x . 
         [0011]    These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Further details, features and advantages of the invention are disclosed in the following description of exemplary and preferred embodiments in connection with the drawings. 
           [0013]      FIG. 1  shows a prior art lighting device in side view. 
           [0014]      FIG. 2  shows a lighting device according to the invention in top view. 
           [0015]      FIG. 3  schematically shows the current density in a prior art lighting device (left hand side) and in a device according to the invention (right hand side.) 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0016]      FIG. 1  shows a lighting device  100  according to the prior art. It comprises a transparent substrate  110 , usually made of glass or a plastic like PVC. This glass panel  110  is provided with a transparent electrically conductive layer or coating  120  on one of its surfaces. Typically the layer  120  is made from an Indium-Tin Oxide coating. The layer  120  is structured, usually through laser ablation, by separation grooves  125 . Across the groove  125 , an LED  130  is mounted. The LED terminals  131  are coupled to the conductive layer  120  by means of a conductive adhesive  150 , e.g. glue or solder. In order to increase the contact stability of the LED, an additional metal coating can be applied on top of the ITO layer  120  to form a contact pad  140  for the solder bumps  150 . Connecting the separated parts of the layer  120  to a power source (not shown) will light up the LED. As indicated in  FIG. 1 , the LED  130  emits light in a downward direction. That is to say, through the transparent substrate  110 . 
         [0017]    The lighting devices so manufactured are very suitable for decorative applications. Furthermore, they can be applied as glass doors, in showcases, as wainscots, or even as a wall itself. The conductive layer  120  and LEDs  130  can be structured and positioned to make (multi-colour) figurative forms light up in an otherwise (almost) complete transparent panel. Alternatively, textual messages can be displayed. For instance, the name of a shop can be brought to light up when the lighting device is used as the shop&#39;s window. 
         [0018]    In order to safeguard against more expensive fall out later on the production line, the electronic structure on the glass panel  120  is usually tested under stress conditions. Experience shows many lighting devices  100  fail due to the high currents used under such testing condition and/or over the lifetime of the device. Failure of a device  100  results from thermal hot spots, causing a loss of the electrical connection between the LED  130  and the conductive layer  120 . Energy dissipation as a result of current crowding inside the layer  120  with its finite sheet resistance R tcl  gives rise to the hot spot. The situation deteriorates even further due to the positive temperature dependence of the sheet resistance R tcl . 
         [0019]    The invention provides a solution for this technical problem. According to the invention the contact pads  140  are designed to have two parts ( FIG. 2 ). A first part  141  corresponding to the contact pad known from the prior art, and a second part  142  extending away from the LED  130 . This measure creates a larger borderline between the relatively high conductive metallic contact pad  140  and the relatively low conductive transparent layer  120 . As a result, the current density  210 , 220  in the transparent conductive layer directly outside the metallic contact pad  140  is considerably reduced in an embodiment of the invention in operation compared to the prior art.  FIG. 3  depicts this graphically, with the prior art situation on the left hand side and the invention on the right hand side. Preferably, the second part  142  of the contact pad  140  is free of (sharp) angles and has a rounded borderline with the conductive layer  120 . A rounded borderline prevents increased current density  210 , 220  levels typically found near (sharp) angled corners. 
         [0020]    Usually, the transparent coating  120  is made from ITO. However, many metal-oxide materials are known to be applicable in lighting devices of the kind set forth. They are usually chosen for their specific characteristic combination of light transparency and electrical conductivity. Examples of such metal-oxides are: InO x :Sn, SnO x :F, SnO x :Sb, ZnO x :Ga, ZnO x :B, ZnO x :F, ZnO x :Al and Ag/TiO x . Taking ITO as a general example, a 350 nm thick transparent electrically conductive layer  120  typically has a sheet resistance R tcl ˜15 Ω/□. To increase the stability of the lighting device, the contact pad  140  should have a much lower resistance than the conductive layer  120 . Metallic coatings are very suitable for this purpose, as for instance a 200 nm thick silver layer can have a sheet resistance as low as R cp ˜0.2 Ω/□. 
         [0021]    As shown in  FIG. 2 , the second part  142  of the contact pad  140  has a longer edge  145  with a length L cp  and a shorter edge  146  with a width W cp . Note that the second part  142  may consist of several extensions. It has been found that the extension of the contact pad  140  is only beneficial to the lighting device&#39;s  100  increase in robustness for a length over width ratio L cp /W cp &lt;R tcl /R cp . This can be understood, as otherwise the resistance of the contact pad  140  would be larger than the resistance of the conductive layer  120  of area L cp   33  W cp . Power dissipation reductions of over 55% have been realised by extending the contact pad  140  in line with the invention. Hence, the invention contributes substantially to the robustness of the lighting device  100 . 
         [0022]    To protect the LEDs  130  mounted on the glass substrate  110  from the environment, it is preferred to sandwich them using a second glass panel (not shown.) For constructive stability, a transparent foil such as a polyvynilbutyral (PVB), usually fills the space between the sandwiching glass plates  110 . With the metal contact pad  140  being non-transparent, it is preferable to assemble the contact pad below the also non-transparent LED  130  as much as possible. As a consequence, it is also preferable to apply top lit LEDs  130  instead of the down lit LEDs  130  disclosed in the prior art. The advantage of this approach lies in the maximization of the optical transparency of the lighting device  100 . 
         [0023]    To further optimize the transparency, it is beneficial to construct the extension  142  of the contact pad  140  to maximize the contact pad—conductive layer borderline. This is favourably realized by extending perpendicular to the separation groove  125 , forming the extension  142   b  in  FIG. 2 . In line with the optimization of the transparency of the lighting device  100 , the second part&#39;s  142  width W cp    146  beneficially is smaller than the size of the contact pad&#39;s first part  141  to which the second part  142  is connected, while maintaining the relation L cp ≦W cp *R tcl /R cp . 
         [0024]    Although the invention has been elucidated with reference to the embodiments described above, it will be evident that other embodiments may be alternatively used to achieve the same object. The scope of the invention is therefore not limited to the embodiments described above, but can also be applied to any other application device where LEDs are mounted on a transparent substrate and electrically addressed through transparent conductive layers.