Abstract:
A plasma display panel is provided with a front substrate ( 22 ); a rear substrate ( 23 ) arranged to face the front substrate; and an exhaust tube ( 21 ). A discharge space is formed by sealing the circumference of the front substrate with the circumference of the rear substrate. The exhaust tube is connected to the rear substrate for exhausting the discharge space and filling the discharge space with a discharge gas. The exhaust tube is formed of a lead-free glass, and the ratio of its thickness to its outer diameter is 0.2 or more.

Description:
[0001]    This application is a U.S. National Phase application of PCT International Application PCT/JP2007/055534. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a plasma display panel (hereinafter abbreviated as PDP) which is a flat plate type display apparatus used in a large-size television receiver or a public display and a production method thereof. Particularly, the present invention relates to an evacuation pipe which introduces a discharge gas while being provided in a PDP to evacuate discharge spaces, and a method of producing a PDP including the evacuation pipe. 
       BACKGROUND ART 
       [0003]    A PDP can realize high definition and a large screen. Therefore, a 65-inch class television receiver, a large public display apparatus, and the like are being commercialized, and an over 100-inch product has also been realized. 
         [0004]    Basically, a PDP includes a front plate and a rear plate. The front plate includes a glass substrate, display electrodes, a dielectric layer, and a protective layer. Sodium borosilicate glass produced by a float process is used as the glass substrate. The display electrodes include transparent electrodes and metal bus electrodes, which are formed in a stripe shape on a surface of the glass substrate. The dielectric layer is formed so as to cover the display electrodes, and acts as a capacitor. The protective layer is made of magnesium oxide (MgO), for example, and the protective layer is formed on the dielectric layer. 
         [0005]    On the other hand, the rear plate includes a glass substrate, address electrodes (or data electrodes), an underlying dielectric layer, barrier ribs, and fluorescent layers. The glass substrate is provided with a small hole in order to evacuate air and introduce a discharge gas. The address electrodes are formed in a stripe shape on a surface of the glass substrate. The underlying dielectric layer covers the address electrodes. The barrier ribs are formed on the underlying dielectric layer. Each of the fluorescent layers which produces fluorescence of red, green, and blue colors respectively is formed between the barrier ribs. 
         [0006]    The front plate and the rear plate are arranged such that electrode forming surface sides thereof faces each other, and peripheries thereof are sealed with a sealing material. An outlet is provided in the glass substrate of the rear plate, and the evacuation pipe (or tip pipe) for evacuating air and introducing a discharge gas is coupled to the outlet and sealed with a sealing material. The evacuation pipe is provided in order to evacuate discharge spaces partitioned by the barrier ribs through the small hole and introduce a discharge gas into the discharge spaces after evacuation. The space in the evacuation pipe is sealed in an airtight manner by locally heating and melting (tipping off) a proper portion of the evacuation pipe. In the completed PDP, when a video signal voltage is selectively applied to the display electrodes, discharge is generated in the discharge spaces, and an ultraviolet ray generated by the discharge excites the red, green, and blue fluorescent layers to emit light in the red, green, and blue colors. Thus, the PDP displays a color image. 
         [0007]    The discharge spaces formed by the front plate, the rear plate, and the barrier ribs are evacuated, and the discharge gas is introduced into the discharge spaces. However, because each of the barrier rib has an extremely low profile, conductance is extremely small in evacuating air and introducing a gas. Therefore, it is desirable that an inner diameter of the evacuation pipe is enlarged as much as possible. 
         [0008]    Generally, low-melting glass mainly containing a lead oxide is used as the dielectric layer and the sealing material. Furthermore, recent examples using a non-lead material, known as “lead-free material” or “leadless material”, containing no lead component, are disclosed for environmental concerns (for example, see Patent Documents 1, 2, and 3). Borosilicate glass containing lead is used in the conventional evacuation pipe because of a relatively low softening point and excellent sealing workability. However, there is a shift to use borosilicate glass containing no lead for environmental concerns. 
         [0009]    A locally heating sealing method with a fixed gas burner or a current-carrying heater is adopted in sealing the evacuation pipe of the PDP in an airtight manner. Conventionally, the locally heating sealing method is widely used in producing lamp products such as an electric bulb, a fluorescent light, and a CRT. In the locally heating sealing method, a portion to be closed and sealed of the fixed evacuation pipe is locally heated, melted, and cut by the fixed gas burner or the current-carrying heater (for example, see Patent Document 4). As described above, in the case where the relatively low-melting glass pipe containing lead is used in the evacuation pipe of the PDP and the thick evacuation pipe is used, the glass pipe is generally sealed with electric heating sealing. In the case where the thin evacuation pipe is used, the glass pipe is generally sealed with the fixed gas burner. 
         [0010]      FIGS. 5A to 5C  are sectional views explaining an evacuation pipe sealing procedure of a conventional PDP. As shown in  FIG. 5A , portion to be sealed  70  of evacuation pipe  71  is heated by flame  73  of gas burner  72  while the inside of the panel is evacuated through evacuation pipe  71 . Portion to be sealed  70  is heated and softened during the evacuation, and portion to be sealed  70  is constricted and extended by a force in a direction of an arrow C caused by elastic portion  74  such as a spring provided in evacuation head  75  and a negative pressure in evacuation pipe  71 . Then, as shown in  FIG. 5B , glass walls softened at portion to be sealed  70  are melted, and the glass walls are fused at connection portion  76  by the action of additional surface tension. As shown in  FIG. 5C , the force in the direction C further acts to cut evacuation pipe  71  at connection portion  76 , and sealed portion  77  is formed to complete the sealing of evacuation pipe  71 . 
         [0011]    At that time, the negative pressure during the constriction and the surface tension during the fusion do not act in an axially symmetric manner with respect to a pipe axis of evacuation pipe  71 . Therefore, deformation is partially generated to easily cause an uneven thickness of sealed portion  77 . For example, as shown in  FIG. 5D , recess  79  in which the partial thickness is extremely thin is sometimes generated, and thick and biased reservoir  78  is sometimes generated. As such, sealed portion  77  cut in a state where axial symmetry is disturbed may be formed. When sealed portion  77  is formed in the biased thickness, strain may remain. When the strain remains, leakage is generated or breakage occurs due to the generation of a crack in the thin portion of sealed portion  77  during the subsequent production process or handing of the product. 
         [0012]    However, in the case where evacuation pipe  71  is made of relatively soft glass containing lead, the above problems are never generated irrespective of a pipe diameter, and reliability concerning the sealing does not become such a large problem. On the other hand, in the case where the sealing work is performed with gas burner  72  using the evacuation pipe made of the borosilicate glass containing no lead, the above-mentioned reservoir  78  or thin recess  79  is generated to disturb the good sealing. In addition, the crack is generated in sealed portion  77  and neighborhood of sealed portion  77  by generation of the strain, which causes decrease in reliability such as a shortened product lifetime. 
         [0013]    Because the borosilicate glass containing no lead has a high softening point, the electric heating sealing by the current-carrying heater heating is frequently used when the local heating sealing is performed. The electric heating sealing has an advantage in that handling is easy during mass production and automation is easily achieved because a heating temperature can be relatively accurately controlled. However, when compared with the method in which gas burner  72  is used, the current-carrying heater which is the heating portion is enlarged. Additionally, because a time necessary for heating and cooling is lengthened, a production tact time is hardly shortened. 
         [0014]    Patent Document 1: Unexamined Japanese Patent Publication No. 2002-053342 
         [0015]    Patent Document 2: Unexamined Japanese Patent Publication No. 09-050769 
         [0016]    Patent Document 3: Unexamined Japanese Patent Publication No. 2003-095697 
         [0017]    Patent Document 4: Unexamined Japanese Patent Publication No. 2001-351528 
       DISCLOSURE OF THE INVENTION 
       [0018]    The present invention provides a PDP in which the decrease in reliability caused by sealing troubles such as a crack or leakage is not generated when an evacuation pipe made of lead-free hard glass having a small coefficient of thermal expansion is sealed with a gas burner. 
         [0019]    The PDP according to the present invention includes a front plate and a rear plate which is arranged to face the front plate, and peripheries of the front plate and the rear plate are sealed and bonded to form a discharge space. In addition, an evacuation pipe to evacuate the discharge space and charge the discharge space with a discharge gas is provided. The evacuation pipe is made of lead-free glass, and a ratio of a thickness of the evacuation pipe to an outer diameter of the evacuation pipe is at least 0.2. 
         [0020]    According to the above configuration, the glass thickness of the sealed portion of the evacuation pipe can evenly be formed, and the strong sealed portion can be formed with no residual stress caused by thermal strain in the sealed portion. Therefore, a highly reliable PDP without any leakage or a crack in the sealed portion can be realized. Because the evacuation pipe made of the borosilicate glass containing no lead is used, a totally lead-free PDP can be realized to eliminate the adverse influence on the environment. The enlargement can be prevented by sealing with the gas burner, and the time necessary to heat and cool the sealed portion can be shortened to decrease a man-hour of the sealing process. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a partially enlarged exploded perspective view showing a structure of a PDP in an embodiment of the present invention. 
           [0022]      FIG. 2A  is a plan view showing a state in which a front plate and a rear plate of the PDP in the embodiment of the present invention are sealed and bonded. 
           [0023]      FIG. 2B  is a sectional view taken on a line  2 B- 2 B of  FIG. 2A . 
           [0024]      FIG. 3A  is a sectional view showing a state in which an evacuation head is attached to an evacuation pipe of the PDP in the embodiment of the present invention. 
           [0025]      FIG. 3B  is a sectional view taken on a line  3 B- 3 B of  FIG. 3A . 
           [0026]      FIG. 4A  is a sectional view explaining a procedure sealing the evacuation pipe of the PDP in the embodiment of the present invention. 
           [0027]      FIG. 4B  is a sectional view explaining a procedure sealing the evacuation pipe of the PDP subsequent to  FIG. 4A . 
           [0028]      FIG. 4C  is a sectional view explaining a procedure sealing the evacuation pipe of the PDP subsequent to  FIG. 4B . 
           [0029]      FIG. 5A  is a sectional view explaining a procedure sealing an evacuation pipe of a conventional PDP. 
           [0030]      FIG. 5B  is a sectional view explaining a procedure sealing the evacuation pipe subsequent to  FIG. 5A . 
           [0031]      FIG. 5C  is a sectional view explaining a procedure sealing the evacuation pipe subsequent to  FIG. 5B . 
           [0032]      FIG. 5D  is an enlarged sectional view showing a sealed portion of the evacuation pipe shown in  FIG. 5C . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0033]      FIG. 1  is an exploded perspective view showing a partially enlarged plasma display panel (PDP) in an embodiment of the present invention, and  FIGS. 2A and 2B  are a plan view and a sectional view showing a state in which a front plate and a rear plate of the PDP shown in  FIG. 1  are sealed and bonded. 
         [0034]    PDP  20  includes front plate  22 , rear plate  23 , and evacuation pipe  21 . Front plate  22  and rear plate  23  are arranged to face each other. Peripheries of front plate  22  and rear plate  23  are sealed and bonded, and discharge spaces  14  are formed by front plate  22 , rear plate  23 , and barrier ribs  11  formed on rear plate  23 . Evacuation pipe  21  is used when discharge spaces  14  are evacuated to introduce the discharge gas into discharge spaces  14 . 
         [0035]    In front plate  22 , pairs of scan electrode  2  for sequentially displaying and sustain electrode  3  for inputting a discharge sustaining signal are formed in a stripe shape on transparent front glass substrate  1 . Display electrode  4  includes scan electrode  2  and sustain electrode  3 , and plural pairs of scan electrode  2  and sustain electrode  3  are formed. Scan electrode  2  and sustain electrode  3  include transparent electrodes  2 A and  3 A made of an indium tin oxide and auxiliary electrodes (or metal bus electrodes)  2 B and  3 B made of conductive materials such as sliver respectively. Light shielding layer  5  which becomes a black matrix can be formed between the sets of sustain electrode  3  and scan electrode  2  to enhance contrast of a display surface, if needed. Dielectric layer  6  made of low-melting glass is formed so as to cover display electrodes  4 . Protective layer  7  made of MgO is formed on dielectric layer  6 . Front plate  22  is formed as described above. 
         [0036]    Data electrodes (or address electrodes)  10  which input display data signals are formed in the stripe shape on rear glass substrate  8  which is arranged so as to face front glass substrate  1 . Data electrode  10  is covered with underlying dielectric layer  9 . Barrier ribs  11  are made on underlying dielectric layer  9  so as to be arranged in the stripe shape in parallel with data electrode  10 . On side faces between barrier ribs  11  and on the surface of underlying dielectric layer  9 , fluorescent layer  12 R which emits the red light, fluorescent layer  12 G which emits the green light, and fluorescent layer  12 B which emits the blue light are formed, so as to constitute rear plate  23 . Fluorescent layers  12 R,  12 G, and  12 B are separately and sequentially formed in discharge spaces (or discharge cells)  14  partitioned by barrier ribs  11 . Rear plate  23  is formed as described above. 
         [0037]    Front plate  22  and rear plate  23  are arranged to face each other across fine discharge spaces  14  such that display electrodes  4  and data electrodes  10  are orthogonal to each other. After the peripheries of front plate  22  and rear plate  23  are sealed and vacuum evacuated with a predetermined pressure, discharge spaces  14  are filled at a predetermined pressure with a mixed rare gas, such as neon (Ne) and xenon (Xe), which is the discharge gas. Discharge is generated to emit an ultraviolet ray in the sealed rare gas by applying a voltage pulse of a predetermined signal to sustain electrodes  3 , scan electrodes  2 , and data electrodes  10 . Fluorescent layers  12 B,  12 G, and  12 R are excited by the ultraviolet ray to emit visible light. Thus, PDP  20  displays information. 
         [0038]    Next, production method for the PDP will be briefly described. Transparent electrodes  2 A and  3 A which constitute scan electrode  2  and sustain electrode  3  respectively are formed on front glass substrate  1 . Then, auxiliary electrodes  2 B and  3 B and light shielding layer  5  are formed. Next, dielectric layer  6  having a predetermined thickness is formed so as to cover transparent electrodes  2 A and  3 A, auxiliary electrodes  2 B and  3 B, and light shielding layer  5  by a screen printing method or the like. Protective layer  7  having a predetermined thickness is formed on dielectric layer  6  by a film formation process such as a vacuum evaporation method, so as to complete front plate  22 . 
         [0039]    On the other hand, data electrodes  10  are formed in the stripe shape on rear glass substrate  8  by the screen printing method, a photolithography method, or the like. Underlying dielectric layer  9  is formed by the screen printing method or the like so as to cover data electrodes  10 . Then, barrier ribs  11  are formed, e.g., in the stripe shape by the screen printing method, die coating method, photolithography method, or the like. Fluorescent layers  12 R,  12 G, and  12 B are formed in a groove between adjacent barrier ribs  11 , so as to complete rear plate  23 . 
         [0040]    Then, as shown in  FIGS. 2A and 2B , the peripheries of front plate  22  and rear plate  23  are sealed and bonded by sealing material  31 . At this time, front plate  22  and rear plate  23  are arranged to face each other such that display electrodes  4  and address electrodes  10  are orthogonal to each other. Rear plate  23 E is previously provided with evacuation hole  30  at a predetermined position therein. Evacuation pipe  21  is sealed and bonded with sealing material  32  so as to cover evacuation hole  30 . Sealing material  32  is applied in the periphery of the expanded end portion of evacuation pipe  21 . Sealing materials  31  and  32  are formed by low-melting glass frit, for example. 
         [0041]    Then, discharge spaces  14  are evacuated to high vacuum (for example, 1.1×10 −4  Pa) through evacuation pipe  21 . Then, discharge spaces  14  are charged with the discharge gas including neon, xenon, and the like through evacuation pipe  21  at a predetermined pressure (for example, pressure of 5.3×10 4  Pa to 8.0×10 4  Pa in the case of Ne—Xe mixed gas). The evacuation pipe  21  is sealed and cut, thereafter. Thus, PDP  20  is completed. 
         [0042]    Procedures of sealing evacuation pipe  21  will be described with reference to  FIGS. 3A to 4C .  FIG. 3A  is a sectional view showing a state in which an evacuation head is attached to the evacuation pipe of the PDP in the embodiment of the present invention,  FIG. 3B  is a sectional view taken on a line  3 B- 3 B of  FIG. 3A , and  FIGS. 4A to 4C  are sectional views explaining procedures sealing the evacuation pipe of the PDP in the embodiment of the present invention. 
         [0043]    The locally heating sealing method in which the fixed gas burner or the current-carrying heater is used is adopted in sealing evacuation pipe  21 . In the locally heating sealing method, as shown in  FIG. 3A , portion to be sealed  21 A of fixed evacuation pipe  21  is sequentially heated, melted, and cut. The electric heating sealing in which the current-carrying heater is used has the advantages in that handling is easy during mass production and automation is easily achieved because the heating temperature can be relatively accurately controlled. However, compared with the method in which the fixed gas burner is used, the current-carrying heater which is the heating portion is enlarged, and the time necessary for heating and cooling is lengthened. Therefore, the production tact time is hardly shortened. Accordingly, evacuation pipe  21  is sealed with fixed gas burner  43  in the embodiment of the present invention. 
         [0044]    As shown in  FIGS. 3A and 3B , evacuation pipe  21  is arranged so as to cover evacuation hole  30  made at a predetermined position in rear plate  23 . End portion  21 E of evacuation pipe  21  has a enlarged funnel shape, and another end portion  21 F is formed in a straight pipe shape having an outer diameter of about 5.0 mm. Evacuation pipe  21  is made of the borosilicate glass which contains no lead component and has a relatively small thermal conductivity. It is not true that the borosilicate glass does not contain lead at all, and an analysis shows that the borosilicate glass contains a trace amount of lead at a PPM level. However, in the definition of the EC-RoHS directive in Europe, it can be assumed that the borosilicate glass contains no lead when the content is not more than 1000 PPM. Therefore, expressions such as “contains no lead”, “non-lead”, and “lead-free” are used for glass having such compositions in the embodiment of the present invention. 
         [0045]    Sealed and bonded PDP  20  is arranged on a panel fixing base (not shown) such that straight-pipe shaped end portion  21 F of evacuation pipe  21  located on the side attached to an evacuation device (not shown) is orientated downward. Evacuation head  41  of the evacuation device is attached to end portion  21 F, the inside of PDP  20  is evacuated in a furnace at a predetermined temperature, and the discharge gas is charged. Then, fixed gas burner  43  is arranged to heat an outer periphery of portion to be sealed  21 A. Evacuation head  41  has applying portion  42  including a spring or the like such that force is applied to evacuation pipe  21  downward, i.e., toward the direction shown by the arrow C in  FIG. 3A . It is preferable that gas burner  43  has a configuration in which plural flames  44  are horizontally formed in a plane perpendicular to evacuation pipe  21  as shown in  FIG. 3B . 
         [0046]    As shown in  FIG. 4A , evacuation pipe  21  is softened when the outer periphery of portion to be sealed  21 A of evacuation pipe  21  is heated to a predetermined temperature by flame  44 . Then, portion to be sealed  21 A is vertically extended because of the decreased pressure in evacuation pipe  21  communicated with discharge spaces  14  shown in  FIG. 1  and the force of applying portion  42 , which forms shrinking portion  21 B. When shrinking portion  21 B is continuously heated by flame  44 , the inner surfaces of evacuation pipe  21  come into contact with each other to form melting connection portion  21 C as shown in  FIG. 4B , and the glass becomes an evenly melted state. At this time, the fire of gas burner  43  is strengthened and the force of applying portion  42  in the direction C is decreased, and glass viscosity is thus decreased in melting connection portion  21 C. Sequentially, the force of applying portion  42  is increased, so that melting connection portion  21 C is extended and thinned, and finally cut. As a result, as shown in  FIG. 4C , sealed portion  21 D is formed which has a curved end portion and a substantially even glass thickness, and sealing of evacuation pipe  21  is thus completed. 
         [0047]    Formation of sealed portion  21 D which has the curved end portion and the substantially even glass thickness as shown in  FIG. 4C  is attributed to the following reason. That is, the thickness of evacuation pipe  21  is not extremely thin, and melting connection portion  21 C has a sufficient length. The glass of sealed portion  21 D extended and thinned during cutting becomes immediately massed together by the heat of flame  44  of gas burner  43  whose fire is strengthened. It is thought that a volume of the glass melted at the low viscosity and the surface tension of the melted portion contribute to the formation of such sealed portion  21 D. In other words, the thickness of evacuation pipe  21  is not extremely thin, and melting connection portion  21 C has a sufficient length, so that the melted glass of sealed portion  21 D has a proper volume. Therefore, the melted glass of sealed portion  21 D is not sucked irrespective of the negative pressure in evacuation pipe  21 . As with the sealing with the electric-current heater, it is thought that the cooling process is controlled by a sufficient heat capacity of sealed portion  21 D. Therefore, it is assumed that sealed portion  21 D which has the curved end portion and the substantially even glass thickness is formed as shown in  FIG. 4C . Although longer time is required in this method compared with the conventional sealing method in which the gas burner is used, the fire of gas burner  43  and the force of applying portion  42  are easily controlled. Therefore, the longer time does not become a large problem. 
         [0048]    However, even if evacuation pipe  21  made of the borosilicate glass containing no lead is sealed by the above-described method, not all sealed portions  21 D are formed in the shape shown in  FIG. 4C . When some samples are actually observed, sometimes sealed portion  21 D is formed as shown in  FIG. 4C , and sometimes sealed portion  77  is formed as shown in  FIG. 5D . In sealed portion  21 D shown in  FIG. 4C , the glass has a substantially even thickness and a curved shape. In sealed portion  77  shown in  FIG. 5D , the glass has reservoir  78  or thin recess  79 . 
         [0049]    When these PDPs  20  are subjected to a heating and cooling repetition test, there is generated no problem in PDPs  20  in which sealed portion  21 D is formed, whereas the leakage defect or breakage due to the crack is frequently generated in PDPs  20  in which sealed portion  77  is formed. This shows that the little strain remains in sealed portion  21 D while the strain caused by the residual stress still remains in sealed portion  77 . 
         [0050]    A close investigation of measurement data between the outer diameter and the thickness of evacuation pipe  21  of PDP  20  in which evacuation pipe  21  is sealed by the above method shows that acceptable products are distinguished from defective products depending on the thickness of evacuation pipe  21 . The thicknesses of evacuation pipes  21  having nominal outer diameters of 5.0 mm are distributed in a range of 0.9 mm to 1.4 mm. In the case where the thickness is at least 1.0 mm (inner diameter is at most 3.0 mm), sealed portion  21 D is formed, and there is no defect in the heating and cooling repetition test. However, in the case where the thickness is less than 1.0 mm (inner diameter is more than 3.0 mm), sometimes sealed portion  77  shown in  FIG. 5D  is formed. In such cases, some products include the defect such as the leakage in the heating and cooling repetition test. 
         [0051]    Therefore, the evacuation pipes are prepared from eight types of lead-free borosilicate glass which have the nominal outer diameter of 5.0 mm and the thicknesses of 0.8 mm, 0.9 mm, 1.0 mm, 1.1 mm, 1.2 mm, 1.3 mm, 1.4 mm, and 1.5 mm, and the PDP samples in which the evacuation pipes are sealed by the above procedure are produced. An appearance inspection of the sealed portion and the heating and cooling repetition test are performed to these samples. In the sealed portions in which the six types of evacuation pipes having the thicknesses of 1.0 mm or more are used, the sealing was performed in a shape that the curved glass had the substantially even thickness as shown in  FIG. 4C , and there occurs no problem in the heating and cooling repetition test. On the other hand, in PDP in which the remaining two types of evacuation pipes having the thicknesses of 0.8 mm and 0.9 mm which are less than 1.0 mm are used, as evacuation pipe is thinned, the number of PDPs having sealed portion  77  of the uneven thickness shown in  FIG. 5D  is increased. Furthermore, as the thickness is decreased, the generation of the defect such as leakage and a crack tends to be remarkably increased. 
         [0052]    As the result mentioned above, it is preferable that the thickness of the evacuation pipe be at least 1.0 mm, when the sealing is performed with the gas burner by the above method using the evacuation pipe, which has the nominal outer diameter of 5.0 mm and is made of the borosilicate glass containing no lead. When the evacuation pipe having such dimensions is used, the sealed portion is formed in the curved shape having the even thickness, and the defect such as the leakage and the crack is not generated in the sealed portion. 
         [0053]    However, in the evacuation pipe having the nominal outer diameter of 5.0 mm, when the thickness exceeds 1.5 mm, the inner diameter of the evacuation pipe becomes less than 2.0 mm which is the diameter of evacuation hole  30 . In such dimensional configuration, the evacuation conductance is decreased to lengthen the evacuation time. Therefore, the thickness of evacuation pipe  21  is preferably set such that the inner diameter of evacuation pipe  21  is not smaller than the diameter of evacuation hole  30 . 
         [0054]    Then, the case in which sealing is performed by the above method using the evacuation pipe which has the nominal outer diameter different from 5.0 mm and is made of the lead-free borosilicate glass will be described. 
         [0055]    The evacuation pipes having the four types of nominal outer diameters of 3.5 mm, 4.0 mm, 6.0 mm, and 7.0 mm are prepared. The samples in which these evacuation pipes are sealed by the above procedure are produced, and the appearance inspection of the sealed portion and the heating and cooling repetition test are performed. As described above, in the case of the evacuation pipe having the nominal outer diameter of 5.0 mm, the sealed portion is formed in the curved shape having substantially even thickness as shown in  FIG. 4C  when the evacuation pipe having the thickness of 1.0 mm or more is used. The defect such as the leakage and the crack is not generated through the heating and cooling repetition test in the sealed portion. Even in the four types of evacuation pipes having the nominal outer diameters different from 5.0 mm, it is found that the glass pipes have the thickness as the boundary. 
         [0056]    That is, according to the thickness measurement values of the evacuation pipes, the thickness boundary values are 0.7 mm, 0.8 mm, 1.2 mm and 1.4 mm in the evacuation pipes having the nominal outer diameters of 3.5 mm, 4.0 mm, 6.0 mm, and 7.0 mm, respectively. As is clear from the results, a ratio of the thickness to the outer diameter of the evacuation pipe is 0.2, and the ratio is kept constant irrespective of the nominal outer diameter. Accordingly, it is necessary that the ratio of the thickness of the evacuation pipe to the outer diameter of the evacuation pipe be at least 0.2 irrespective of the numerical values of the thickness and outer diameter of the evacuation pipe. In consideration of the evacuation conductance, it is preferable that the thickness of the evacuation pipe be set such that the inner diameter of the evacuation pipe is not lower than the diameter of the evacuation hole to which the evacuation pipe is connected. 
         [0057]    As described above, the ratio of the thickness to the outer diameter of the evacuation pipe made of the lead-free borosilicate glass is set to 0.2 in the PDP according to the embodiment of the present invention. Thus, because the thickness is set to a relatively thick value, the glass thickness of the sealed portion can evenly be formed even if the sealing is performed with the fixed gas burner. As a result, the strong sealed portion having no residual stress caused by the thermal strain can be formed to realize a high-reliability PDP in which leakage or a crack is not generated in the sealed portion. Because of the use of the evacuation pipe made of the lead-free borosilicate glass, a totally lead free PDP can be realized, and the load on the environment can be eliminated. Additionally, because the sealing can be performed with the gas burner, the apparatus is not enlarged unlike the electric heating sealing, and the time necessary to heat and cool the sealed portion can be shortened to decrease the sealing man-hour. As a result, manufacturing cost of the PDP can be reduced to provide the PDP at a moderate price. 
         [0058]    As described above, the evacuation pipe made of the lead-free, hard borosilicate glass having the small coefficient of thermal expansion is sealed with the gas burner in the PDP and the production method thereof according to the present invention. Even in such cases, there is no decrease in reliability such as the crack and the leakage which is associated with the trouble with the sealed portion. Additionally, the production tact time is shortened to decrease the man-hour, which allows the high-quality PDP to be produced at a moderate price. As long as the borosilicate glass contains no lead is used, the same effect is obtained when the borosilicate glass is used in the evacuation pipe  21 . Particularly, among various kinds of the lead-free glass, a temperature-viscosity curve of the borosilicate glass is close to that of the glass material containing lead. Therefore, conditions of the gas burner and the like can be set similar to those of the glass material containing lead. 
       INDUSTRIAL APPLICABILITY 
       [0059]    In the present invention, the ratio of the thickness to the outer diameter of the evacuation pipe made of the lead-free glass is set to 0.2, namely the thickness is set to a relatively thick value. Therefore, even if the sealing is performed with the fixed gas burner, the glass thickness of the sealed portion can evenly be formed, and the defect such as the leakage and the crack is not generated in the sealed portion. Thus, the configuration and the production method for producing the high-reliability PDP which adapts to the environment are suitable to the large-screen display device and the like.