EP2109877A2

EP2109877A2 - Method of forming a cold cathode fluorescent lamp, thick film electrode compositions used therein and lamps and lcd devices formed thereof

Info

Publication number: EP2109877A2
Application number: EP08713260A
Authority: EP
Inventors: Joel Slutsky; Brian D. Veeder; Andy Kao; Thomas Lin; Hsiu-Wei Wu; Tjong-Ren Chang; Shuang-Chang Yang; Wen-Chun Chiu; Jin-Yuh Lu
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 2007-01-23
Filing date: 2008-01-23
Publication date: 2009-10-21
Also published as: CN101636817A; WO2008091673A3; WO2008091673A2; KR101041243B1; JP2010517239A; KR20090102865A; TW200841377A

Abstract

The present invention relates to method(s) of fabricating a cold cathode fluorescence lamp (CCFL) utilizing thick film compositions. The CCFL of the present invention may be used in thin film transistor-liquid crystal display (TFT-LCD) applications and for providing a structure of the electrodes of used in TFT-LCD backlight units.

Description

TITLE

Method of Forming A Cold Cathode Fluorescent Lamp, Thick Film Electrode Compositions Used Therein and Lamps and LCD Devices Formed Thereof

FIELD OF THE INVENTION

The present invention claims priority from provisional application 60/802,912, filed on May 24, 2006. The present invention relates to method(s) of fabricating a cold cathode fluorescence lamp (CCFL) utilizing thick film compositions. The CCFL of the present invention may be used in thin film transistor-liquid crystal display (TFT-LCD) applications and to provide a structure of the electrodes of CCFL's used in TFT-LCD backlight units.

BACKGROUND OF THE INVENTION

Liquid crystal display devices comprise two pieces of polarized glass having a polarizing film side and a glass side. A special polymer that creates microscopic grooves (oriented in the same direction as the polarizing film) in the surface is rubbed on the non-polarizing film side of the glass. A coating of nematic liquid crystals is added to one of the filters.

The grooves cause the first layer of molecules of the liquid crystals to align with the filter's orientation. The second piece of glass is added with the polarizing film at a right angle to the first piece. Each successive layer of liquid crystal molecules gradually twists until the uppermost layer is at a 90 degree angle to the bottom, thus matching the orientation of the second polarized glass filter.

As light strikes the first filter, it is polarized. If the final layer of liquid crystal molecules is matched up with the second polarized glass filter, then the light will pass through. The light which passes through is controlled through the use of electric charges to the liquid crystal molecules.

Active-matrix LCDs depend on thin film transistors (TFT). Basically, TFTs are tiny switching transistors and capacitors arranged in a particular matrix on the glass substrate. These TFTs control which areas receive a charge and therefore, the image seen by the viewer. The light to the LCD device may be supplied through the use of a backlight unit (BLU). Two possible backlight unit types include cold cathode fluorescent lamps (CCFLs) and external electrode fluorescent lamps (EEFLs). In some embodiments, when compared with EEFL, CCFL has been used as a TFL-LCD BLU light source because of improved life/reliability performance and readied mass production line.

Fluorescent lamps are used to provide illumination in typical electrical devices for general lighting purposes because they are more efficient than incandescent bulbs in the production of light. A fluorescent lamp is a low pressure gas discharge source, in which light is produced predominately by fluorescent powders activated by ultraviolet energy generated by a mercury plasma forming an arc. The lamp, usually in the form of a tubular bulb with an electrode sealed into each end, contains mercury vapor at low pressure with a small amount of inert gas for starting. The inner walls of the bulb are coated with fluorescent powders commonly called phosphors. When the proper voltage is applied, the plasma-forming arc is produced by current flowing between the electrodes through the mercury vapor. This discharge generates some visible radiation. The ultraviolet light in turn excites the phosphors to emit light. In some fluorescent lamps, two electrodes are hermetically sealed into the bulb, one at each end. These electrodes are designed for operating as either "cold" or "hot" cathodes or electrodes. Electrodes for cold cathode (or glow) operation may consist of closed-end metal cylinders, coated on the inside with an emissive material.

FIG. 1A represents a prior art cold cathode fluorescent lamp (CCFL). The lamp is in the form of a tubular bulb, typically a glass tube, 1 , with two electrodes, 4, extending from each end. The lamp contains fluorescent powders, 3, and a discharge gas, 2. Typical prior art CCFL electrodes achieve an electrical connection between the inverter and

CCFL by soldering a connection, 5, from each electrode to the inverter. In this prior art CCFL structure, before assembling the lamps to the BLU module, the electrical connection is made by soldering wires to the electrodes of the CCFL lamp. When assembling the CCFL lamp, these lamps, with wires hanging at both ends, are placed on the BLU panels. The electrical connection from the lamp to the BLU modules is then done by additionally soldering the wires to the inverters on the BLU module. This is very time and labor intensive. FIG. 1 B demonstrates the connection by soldering of the prior art CCFL. FIG. 1C represents a typical CCFL with multiple 1-to-2 inverters, 6.

In another CCFL example, a metal cap is affixed to the internal electrode and connected from the CCFL to the inverter. The metal cap is attached to the lamp by soldering. The lamps are then placed and clamped at the holder on the BLU modules to form an electrical connection. This method still requires soldering to attach the metal cap to the lamp. Despite the metal cap advances, a need still exists to improve CCFL processing in regard to ease of processing, ease of assembly and replacement of the lamp(s).

FIG. 2A illustrates a conventional external electrode fluorescent lamp (EEFL) in which metal capsules, 10, are bonded at the end of the glass tubes, 1 , and ferrodielectrics are applied to the inside of the metal capsules. This type of electrode is disclosed in U.S. Patent No. 2,624,858 to Greenlee. However, the bonded portions of the electrodes can be easily damaged since the coefficient of the thermal expansion of the glass tubes is different from that of the metal capsule.

FIG. 2B illustrates another type of electrode, which is disclosed in U.S. Patent No. 6,674,250 to Cho et al. The electrodes of Cho et al. are metal caps, 13, attached to the sealed glass tube by using conductive adhesives, 16. In the same disclosure, the electrodes can also be conductive tapes, 14, with adhesives wherein the tapes are attached to the glass tubes, as shown in FIG. 2C. FIG. 2D illustrates another type of electrode, which is disclosed in

U.S. Patent No. 6,914, 391 to Takeda et al. The electrodes disclosed in Takeda et al are aluminum foils, 15, attached to the sealed glass tube by using an electrically conductive silicone adhesive layer. The use of adhesives, as in the EEFLs mentioned above, has the disadvantage of creating weak bonds between the electrode and the glass tube of the EEFL device. The adhesives provide only mechanical bonding and the weak bonding of the electrodes may result in poor reliability performance. For example, gaps between the electrodes and the glass tubes may appear during thermal cycles due to the mismatching of thermal expansion coefficients between the metal caps (electrodes) and glass tubes. Gaps may also appear when the adhesives deteriorate in harsh environments. Gaps between the electrodes and the glass tubes can lead to EEFL failures because the high operating voltage of EEFL would not be uniformly applied to the glass tubes. Higher electrical resistance around the gaps leads to destructive damages to the glass tubes. Also, the higher stress around the gap can also intensify the separation and accelerate the failure of the device during the reliability testing.

The present invention addresses the above problems by providing a novel method of forming a CCFL and a method of forming an LCD device. SUMMARY OF THE INVENTION

The present invention provides a method of forming a cold cathode fluorescent lamp comprising the steps of: providing a conductive layer thick film composition comprising electrically functional particles and organic medium; providing a cylindrical glass tube having a first end, a second end, a first internal electrode, a second internal electrode, and an inner peripheral wall wherein a fluorescent substance is provided along said inner peripheral wall and wherein a discharge gas is injected into said glass tube and wherein said first internal electrode extends from inside said glass tube through said first end thus forming an internal and external portion of said first electrode and wherein said second internal electrode extends from inside said glass tube through said second end thus forming an internal and external portion of said second electrode, and wherein said glass tube, first electrode and second electrode are sealed to form a glass tube structure such that said fluorescent substance and discharge gas are contained within said glass tube structure; applying the conductive layer thick film composition onto said first end and said second end of said glass tube structure, thus forming a first conductive layer and a second conductive layer; and firing said glass tube and conductive layer thick film composition to form a cold cathode fluorescent lamp.

In one embodiment of the present invention the applying step above is selected from dip coating, screen printing, roll coating, and spray coating. In a further embodiment, the method of further comprises a step of drying said conductive layer thick film composition prior to said firing step. In still a further embodiment, the method further comprises the steps of providing a protective layer composition over one or both of said first conductive layer and said second conductive layer. In yet another embodiment, the conductive layer thick film composition of the present invention further comprises a glass frit.

In a further embodiment of the present invention, a cold cathode fluorescent lamp is formed by the method(s) of the present invention detailed above and described in detail below. In another embodiment, a liquid crystal display device is formed which comprises the cold cathode fluorescent lamp described above.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 A - an illustrative view of a conventional cold cathode fluorescent lamp.

FIG. 1 B - an illustrative view of a single conventional cold cathode fluorescent lamp with solder connection.

FIG. 1C - an illustrative view of conventional cold cathode fluorescent lamps with multiple 1-to-2 inverters and solder connection. FIGS. 2A -2D - an illustrative view of conventional external electrode fluorescent lamps.

FIGS. 2E-2G - an illustrative view of external electrode fluorescent lamps disclosed in U.S. Provisional Patent Application 60/802912.

FIG. 3A - 3E - illustrative views of the present invention cold cathode fluorescent lamps.

Drawings — Reference Numerals

1 - tubular bulb (glass tube) 2 - discharge gas

3 - fluorescent layer (typically phosphor)

4 - internal electrodes

5 - solder connection

6 - inverter 10 - bonded metal capsules

13 - metal cap

14 - conductive tapes with adhesive

15 - aluminum foil

16 - adhesive material 17 - thick film conductive paste

18 - protective layer

19 - EEFL holder with mechanical contact to electrode

20 - CCFL holder with mechanical contact to electrode

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2E represents an external electrode fluorescent lamp (EEFL) disclosed in U.S. Provisional Patent Application No. 60/802,912 to Lin et al. (Attorney Docket No. EL-0663), herein incorporated by reference. FIGS. 2F and 2G represent various embodiments of solder-free connections utilizing the EEFL described in Lin et al. While U.S.

Provisional Patent Application No. 60/802,912 is related to EEFL applications, the present invention relates to CCFL applications.

One advantage of the present invention is the excellent bonding strength of the thick film conductive layer to the internal electrode and the glass tubes 3 of the fluorescent lamps therefore better reliability performance of the internal electrodes can be achieved. During the firing process, glass frit in the electrode paste provides strong chemical and mechanical bonding of the conductive layer to the glass tubes. Compared to the examples in prior arts, strong, uniform, and closely bonding of the electrodes provides superior performance in reliability and electric characteristic.

Another advantage of good bonding of the electrodes is the electrical performance and increased reliability. Strong and uniform bonding of the electrodes provides very close contact of the electrodes to the glass tubes of the lamps hence lower electric resistance and higher conversion efficiency of the power applied to the lamps to the power to excite the fluorescent substance inside the glass tubes. The AC power for operating CCFL is usually in a range of 2OkHz to 10OkHz and the bonding at the interface of electrodes and glass tubes would likely affect the reliability performance more substantially in high electric frequency like that in CCFL.

A further advantage of this invention is the ease of adaptation to mass production. The processes in this invention, such as rolling, spraying, dipping etc., are typically easy process to carry out in the industry. Low cost of equipment investment is required and CCFL devices with high performance reproducibility can be fabricated. Physical and performance uniformity of the electrodes is easier to achieve when the conductive materials are in the form of pastes, as mentioned in this invention, than in the form of tapes, metal caps, or foils, as mentioned in the prior arts. Thus, high quality CCFL devices can be fabricated in mass. Furthermore, the present invention eliminates the need for a solder connection through the use of thick film paste. The paste is applied to the lamp and fired to form the solder-free electrode. The lamp is then placed and clamped at the holder on the BLU module to form the complete backlight unit.

Description of the Processes The fluorescent lamp fabrication processes in one embodiment of the present invention and structures of the internal electrodes of the fluorescent lamps are explained in detail. Those skilled in the art will understand that the description is merely one example of a fabrication process and other fabrication processes are known to those skilled in the art.

FIGS. 3A-3E show a fluorescent lamp according to exemplary embodiments of the present invention. Referring to FIGS. 3A-3E, the fluorescent lamp includes a cylindrical glass tube 1. The fluorescent substance 3 is provided along the inner peripheral wall of the glass tube 1. After the fluorescent substance is applied inside the glass tube 1 , a discharge gas 2 which comprises an inert gas, mercury (Hg), etc. mixed with one another, is injected into the glass tube 1 , then both ends of the glass tube 1 are sealed.

The electrodes of the fluorescent lamps are respectively formed at the opposite ends of the glass tube 1. The structure of the electrode 4 is coated with a thick film conductive layer 17 and an optional protective layer 18, partially or completely, covering the conductive layer 17. The portion of the electrodes 4 extending outside the glass tube 1 may be completely covered by the conductive layer 17 or may extend beyond the conductive layer 17.

The conductive layer 17 is a thick film paste which comprises binder materials and metals selected from the group comprising: Al, Ag, Cu, Pd, Pt and mixtures thereof. The metals chosen in this invention give the conductive layer 17 very low electrical resistance. An electrical sheet resistance of less than 100 m ohms/sq at 25μm may be achieved. In one embodiment, the electrical sheet resistance range is in the range of 1 to 10 m ohms/sq @ 25μm. In a further embodiment, the sheet resistance is 3 m ohms/sq (S) 25μm. The binder composition provides the conductive layer 17 strong adhesion to the glass tube, 1 , and electrode material. Typically, the method of application of the thick film paste is screen printed or dip coated. However, other methods well known to those skilled in the art are possible. Applicable thick film paste compositions useful in the present invention are described in detail below.

I. Thick Film Paste Conductive Layer

A. Electrically Functional Particles In conductor applications, the functional phase is comprised of electrically functional conductor powder(s). The electrically functional powders in a given thick film composition may comprise a single type of powder, mixtures of powders, alloys or compounds of several elements. Electrically functional conductive powders that may be used in this invention comprise, but are not limited to gold, silver, nickel, aluminum, palladium, molybdenum, tungsten, tantalum, tin, indium, ruthenium, cobalt, tantalum, gallium, zinc, magnesium, lead, antimony, conductive carbon, platinum, copper, and mixtures thereof.

The metal particles may be coated or not coated with organic materials. In particular, the metal particles may be coated with a surfactant. In one embodiment, the surfactant is selected from stearic acid, palmitic acid, a salt of stearate, a salt of palmitate and mixtures thereof. The counter-ion can be, but is not limited to, hydrogen, ammonium, sodium, potassium and mixtures thereof.

A metal powder(s) of virtually any shape, including spherical particles and flakes (rods, cones, and plates) may be used in practicing the invention. In an embodiment, metal powders are gold, silver, palladium, platinum, copper and combinations thereof. In a further embodiment, the particles may be spherical.

In a further embodiment, the present invention relates to dispersions. The dispersions may include compositions, particles, flakes, or a combination thereof. The metal powder(s) may be nano-sized powders. Furthermore, the electrically functional particles may be coated with a surfactant. The surfactant may help to create desirable dispersion properties. Typical particle sizes of the electrically functional particles are less than approximately 10 microns. It is understood the particle size will vary dependant upon the application method and desired properties of the thick film composition. In one embodiment, an average particle size (D₅₀ ) of 2.0-3.5 microns is used. In a further embodiment, the D₉₀ is approximately 9 microns. Additionally, in one embodiment, the surface area to weight ratio is in the range of 0.7 - 1.4 m2/g. B. Organic Medium

The inorganic components are typically mixed with an organic medium by mechanical mixing to form viscous compositions called "pastes", having suitable consistency and rheology for the applicable coating method, including but not limited to screen printing and dip coating. A wide variety of inert viscous materials can be used as organic medium. The organic medium must be one in which the inorganic components are dispersible with an adequate degree of stability. The rheological properties of the medium must be such that they lend good application properties to the composition, including: stable dispersion of solids, appropriate viscosity and thixotropy for screen printing, appropriate wettability of the substrate and the paste solids, a good drying rate, and good firing properties. The organic vehicle used in the thick film composition of the present invention is preferably a nonaqueous inert liquid. Use can be made of any of various organic vehicles, which may or may not contain thickeners, stabilizers and/or other common additives. The organic medium is typically a solution of polymer(s) in solvent(s). Additionally, a small amount of additives, such as surfactants, may be a part of the organic medium. The most frequently used polymer for this purpose is ethyl cellulose. Other examples of polymers useful in the present invention include ethylhydroxyethyl cellulose, wood rosin, mixtures of ethyl cellulose and phenolic resins, varnish resins, and polymethacrylates of lower alcohols can also be used. The most widely used solvents found in thick film compositions are ester alcohols and terpenes such as alpha- or beta-terpineol or mixtures thereof with other solvents such as pine oil, kerosene, dibutylphthalate, butyl carbitol, butyl carbitol acetate, hexylene glycol and high boiling alcohols and alcohol esters. In addition, volatile liquids for promoting rapid hardening after application on the substrate can be included in the vehicle. Various combinations of these and other solvents are formulated to obtain the viscosity and volatility requirements desired. The polymer present in the organic medium is in the range of 0.2 wt. % to 8.0 wt. % of the total composition, and any range contained therein. The thick film conductive composition of the present invention may be adjusted to a predetermined, screen-printable viscosity with the organic medium. In one embodiment, the thick film conductive composition comprises silver.

The ratio of organic medium in the thick film composition to the inorganic components in the dispersion is dependent on the method of applying the paste and the kind of organic medium used, and it can vary. Usually, the dispersion will contain 40 - 90 wt % of inorganic components and 10 -60 wt % of organic medium (vehicle) in order to obtain good wetting.

C. Optional Glass Frit

Typical glass frit compositions (glass compositions) of the present invention are listed in Table 1 below. The glass frit of the present invention is optional. It is important to note that the compositions listed in Table 1 are not limiting, as it is expected that one skilled in glass chemistry could make minor substitutions of additional ingredients and not substantially change the desired properties of the glass composition of this invention. For example, those skilled in the art understand that useful glass frit compositions may be modified to optimize abrasion resistance, solderability, plating, as well as other properties. The glass compositions in weight percent total glass composition are shown in Table 1. Preferred glass compositions found in the examples comprise the following oxide constituents in the compositional range of: Siθ2 4-8, AI2O3 2-3, B2O3 8-25, CaO 0-1 , ZnO 10-40, Bi2θ3 30-70, Snθ2 0-3 in weight percent total glass composition. The more preferred composition of glass being: Siθ2 7, AI2O3 2, B2O3 8, CaO 1 , ZnO 12, BΪ2O3 70 in weight percent total glass composition. Several embodiments of the present invention comprise a Pb-free glass composition. When glasses are used in the thick film composition of the present invention, this may lead to a more compatible thermal coefficient of expansion (TCE) match between the substrate and the composition upon processing. A particularly beneficial embodiment is one in which the thick film composition comprises a Pb-free glass.

Table 1 : Glass Composition(s) in Weight Percent Total Glass

Composition

Glass ID

No.

SiO₂ AI₂O₃ B₂O₃ Ca ZnO Bi₂O₃ SnO

O 2

Glass I 4.00 2.50 21.00 40.00 30.00 2 .50

Glass Il 4.00 3.00 24.00 31.00 35.00 3 .00

Glass III 7.11 2.13 8.38 0.53 12.03 69.82

Glass frits useful in the present invention include ASF1100 and

ASF1100B, which are commercially available from Asahi Glass Company.

An average particle size of the glass frit (glass composition) of the present invention is in the range of 0.5-5.0 μm in practical applications while an average particle size in the range of 2.5 -3.5 μm is preferred. The softening point of the glass frit (Ts: second transition point of DTA) should be in the range of 300-600⁰C. When present in the conductive layer thick film composition, the amount of glass frit in the total composition is in the range of 0.5 to 10 weight percent of the total composition. In one embodiment, the glass composition is present in the amount of 1 to 3 weight percent total composition. In a further embodiment, the glass composition in present in the range of 4 to 5 weight percent total composition.

The glasses described herein are produced by conventional glass making techniques. The glasses were prepared in 500-1000 gram quantities. Typically, the ingredients are weighed then mixed in the desired proportions and heated in a bottom-loading furnace to form a melt in platinum alloy crucibles. As well known in the art, heating is conducted to a peak temperature (1000 -1200⁰C) and for a time such that the melt becomes entirely liquid and homogeneous. The molten glass was quenched between counter rotating stainless steel rollers to form a 10-20 mil thick platelet of glass. The resulting glass platelet was then milled to form a powder with its 50% volume distribution set between 1-3 microns.

II. Optional Protective Layer of Electrode

As detailed in various embodiments of the present invention detailed in FIGS. 3B-3E, a protective layer, 18, is shown which covers, at least partially, the conductive layer, 17. The protective layer, 18 may cover the entire conductive layer or just a portion. Additionally, in some embodiments, the electrode extends into the protective layer and the protective layer covers the electrode. The protective layer of the electrodes 18 is made of metals with low reactivity such as Sn in order to protect the conductive layer 17 from reacting with the elements of the environment, such as moisture and reactive gas. The protective layer is purely optional.

Different methods of applying conductive layer 17 onto the glass tube 1 may be utilized. The portion of the electrode, 4, extending outside the glass tube, 1 , may be completely covered by the conductive layer, 17, or may extend beyond the conductive layer, 17. In some embodiments, the electrode may extend beyond the conductive layer, even when there is no protective layer over it, as long as the electrode is connected to the conductive layer.

In some embodiments, the electrode extends beyond the conductive layer and into the protective layer. The protective layer may partially or entirely cover the electrode and conductive layer. The electrode materials, including metal powders and binders (as detailed above), are well mixed together to form electrode pastes. The conductive layer 17 is made from a thick film conductive paste. Thick film conductive pastes of different viscosities can be applied onto the glass tubes and electrodes by different coating processes, such as rolling, spraying, dipping processes, etc.

In one embodiment, the thick film conductive paste is applied by a roll coating process wherein the glass tubes approach the paste container or tank, the paste is transferred to the glass tube, and the glass tube then departs from the thick film conductive paste tank, leaving a coating of thick film conductive paste in the desired location of the glass tube. Throughout the rolling process, the glass tubes are spinning by the axis penetrating both ends and the glass tubes aligned in a small angle to the surface of the thick film conductive pastes in the tank.

In another embodiment, the CCFL is formed by applying the conductive paste via a spraying process done by ejecting the thick film pastes through a nozzle into the air to form droplets and the droplets of the pastes accumulate on the ends of the glass tubes. It's preferred that the glass tubes spin during the process for better coating uniformity.

A dipping process may also be used for the conductive layer thick film application by dipping the glass tubes into the conductive pastes and pulling away from the surface of the pastes in a tank. The alignment of the glass tubes is not limited to being perpendicular to the surface of the pastes and spinning of the glass tubes can be adopted during the dipping process.

The subsequent process typically includes drying, firing, and cooling of the glass tubes. In some embodiments, a specific drying step is not necessary, dependent upon the conditions of the process. The process of drying, firing, and cooling can be done in the sense of batches or continuous process.

In one embodiment, the drying process is defined and performed by heating the glass tubes and the conductive layer to 50-180 degree C for certain amount of time. The heating of the glass tubes can be done in a drying oven, by radiation, circulation of a heated atmosphere or both combined. The low boiling-point organic solvents in the electrode pastes on the glass tubes are driven away during drying process and the glass tubes are then ready to go through the firing process because the conductive layer is less susceptible to physical deformation after being dried.

In one embodiment, the firing process is defined and done by heating the glass tubes and conductive layers to a temperature in the range of approximately 300 to 600 degrees C. The glass tubes can be heated by radiation, circulation of a heated atmosphere, or both combined in a firing furnace. During the firing step, typically heat-resistant carriers, e.g. quartz tubes, are used for uniform heating and mechanical support of the glass tubes 1. The composition of the heated atmosphere can be modified and controlled for different types of the conductive pastes and different targeted performance of the electrode. In continuous firing process, the glass tubes can be aligned perpendicular to the moving direction of the carrier in order to have uniform heating of the glass tubes. The object of the firing process is to achieve low electrical resistance (a low sheet resistance of <100 m ohms/sq @ 25μm may be achieved) of the conductive layer and high bonding strength of the conductive layer to the glass tubes. During the firing process, all organic materials in the conductive layer thick film are burned out. Typically, the firing step takes place in the temperature range of 300 to 600 degrees C. After firing, only metals and glass frit (when added to the thick film composition) are left in the conductive layer.

After the firing process, the glass tubes are slowly cooled down. The cooling process provides a tempered decreasing temperature gradient for the glass tubes. Moderate cooling rate is typical in order to slowly release the thermal stress at the interface between the glass tubes and the conductive layer during the cooling process. The glass tubes may cool adequately in ambient conditions, in some embodiments.

In one embodiment of the present invention, glass frit is not included in the thick film paste conductive layer. The electrode paste in this alternative embodiment will comprise the function metals detailed above, such as Al, Cu, Ag, Au and mixtures thereof, and organic medium, such as solvents and resins. In one embodiment of this glass-free embodiment, the firing temperature is in the range of 80 to 300 degrees C. In a further glass-free embodiment, the firing temperature is in the range of 300 to 600 degrees C. In one embodiment, the electrically functional 5 particles are nano-sized particles. In some embodiments, the thick film composition comprises a polymer and is thus, a polymer thick film composition. Such polymer thick film compositions may be cured. This curing typically allows for a lower firing temperature and less energy use. The advantages of this alternative glass-free embodimento include lower machinery cost, lower materials cost, and higher throughput of the process. The disadvantages of the alternative embodiment will be lower bonding strength and a slightly worse electrical performance. Both glass-containing and glass-free embodiments share the advantages of easy adoption to mass production. 5 The optional protective layer, 18 is applied to the conductive layer 17 after the cooling process. Coating the conductive layer with less reactive metal layers, such as Sn, Ni, and Zn, can provide the protective layer 18. Different coating processes, such as soldering, electrical plating, chemical plating etc., can be adopted for the protective layer 18. o Length of the thick film paste layer (i.e. coverage of the glass tube) needs to be optimized. Thick film coverage of the CCFL affects the electrical performance of the lamps significantly. Lamps with longer coverage have larger contact area with the glass tubes, hence have lower electric resistance. For example, to obtain a typical tube current of 4mA in 5 the lamp having a reduced length lamp of 10mm long, voltage as high as

1 .7 times the voltage required for lamps with 20mm long electrodes must be applied. The higher operation voltage of the lamps with shorter electrodes leads to issues such as ozone generation around the electrodes, the need for specially made insulating materials in the 0 backlight module, and reaching inverter output voltage limit. Higher luminance of the lamp requires higher operation current. In order to operate the lamps in high electric current without high operation voltage, the solution of increasing the electrode length has been widely adopted. The drawback of this solution is that the actual illumination area of the lamp would be smaller with longer electrodes. Therefore, optimization of electrode length and lamp luminance should be considered.

Claims

CLAIMS What is claimed is:

1. A method of forming a cold cathode fluorescent lamp comprising the steps of: providing a conductive layer thick film composition comprising electrically functional particles and organic medium; providing a cylindrical glass tube having a first end, a second end, a first internal electrode, a second internal electrode, and an inner peripheral wall wherein a fluorescent substance is provided along said inner peripheral wall and wherein a discharge gas is injected into said glass tube and wherein said first internal electrode extends from inside said glass tube through said first end thus forming an internal and external portion of said first electrode and wherein said second internal electrode extends from inside said glass tube through said second end thus forming an internal and external portion of said second electrode, and wherein said glass tube, first electrode and second electrode are sealed to form a glass tube structure such that said fluorescent substance and discharge gas are contained within said glass tube structure; applying the conductive layer thick film composition onto said first end and said second end of said glass tube structure, thus forming a first conductive layer and a second conductive layer; and firing said glass tube and conductive layer thick film composition to form a cold cathode fluorescent lamp.

2. The method of claim 1 wherein said first conductive layer completely covers said external portion of said first electrode.

3. The method of claim 1 wherein a protective layer is applied over one or more of said first and second conductive layers.

4. The method of claim 3 wherein said protective layer is a lead-free layer.

5. The method of claim 1 wherein said applying step is selected from the group comprising dip coating, screen printing, roll coating, and spray coating.

6. The method of claim 1 further comprising a step of drying said conductive layer thick film composition prior to said firing step.

7. The method of claim 1 wherein said conductive layer thick film composition further comprises a glass frit.

8. The method of claim 1 wherein said firing step take place in the temperature range of 300 to 600 degrees C.

9. The method of claim 1 wherein said firing step takes place in the temperature range of 80 to 300 degrees C.

10. The method of claim 7 wherein said glass frit composition is a lead- free glass frit composition.

11. The method of claim 7 wherein said glass frit composition comprises: Siθ2 4-8, AI2O3 2-3, B2O3 8-25, CaO 0-1 , ZnO 10-40, Bi2θ3 30-70, Snθ2 0-3, in weight percent total glass frit composition.

12. A cold cathode fluorescent lamp formed by the method of claim 1.

13. A liquid crystal display device comprising the cold cathode fluorescent lamp of claim 12.