The invention relates to a method of manufacturing a display device
comprising an air-tight envelope, in which a glass face plate and at least one
further glass part are joined to form said air-tight envelope, said envelope being
heated and evacuated at a later stage.
Display devices of the type mentioned in the opening paragraph are
used, inter alia, in television receivers and computer monitors.
A display device of the type mentioned in the opening paragraph is
known. The known display device comprises an air-tight envelope with a display
window. In the case of a cathode ray tube (CRT), the envelope also comprises a
cone portion and a neck which accommodates an electron gun for generating (one
or more) electron beams. These electron beams are focused on a phosphor layer on
the inner surface of the display window. In the case of a plasma display panel
(PDD), the air-tight envelope comprises a faceplate, which serves as the display
window, and a rear plate, said plates being connected to each other by means of
connecting parts. A plasma display panel contains an ionizable gas in which a plasma
discharge is generated, and electroluminescent or photo-luminescent phosphors being
used to produce an image.
The known display device has a number of shortcomings, in particular
the occurrence of product failure during the manufacture of the display device,
which product failure is caused by fracture as a result of, for example, implosion
of the display device during the evacuation of the envelope.
It is an object of the invention to provide a method of selecting
the glass parts mentioned in the opening paragraph in an early stage of the manufacturing
process of the display device, so that the risk that the above-mentioned problem
occurs is reduced.
To achieve this, the method in accordance with the invention is characterized
in that, prior to joining the face plate and the at least further glass part, a
thermo shock is induced in the face plate, the face plate being rejected if the
thermo shock is found to induce the growth of cracks, the face plate being joined
to the at least further glass part if the thermo shock is found to not induce the
growth of cracks. According to a first embodiment, for inducing the thermo shock
the face plate or further glass part is warmed up to a first temperature during
a first time period, whereafter, commencing while said face plate or further glass
part is still warmed-up, said face plate or further glass part is immersed for
a second time period in a fluid that when said immersion commences is at a second
temperature lower than the first temperature. The fluid may be a gas or, preferable,
Since glass is a brittle material, it is sensitive to surface damage
and stress-related phenomena. Surface damage is generally difficult to detect by
people who are not skilled in the art, and adverse effects of (surface) stresses
in glass may not give rise to problems until late in the manufacturing process.
In addition, it is not clear how and which surface damage as well as which types
of stress in the part contribute substantially to product failure during the further
assembly of the air-tight envelope and the display device. Product failure is caused,
in particular, by implosion of the envelope of the display device when this is
evacuated (for the first time). In said evacuation process the envelope is also
brought to a relatively high temperature (300-400 °C). Such implosions are often
initiated by said surface damage or too high a surface stress. When the air-tight
envelope of the display device is evacuated for the first time, the display device
already is in an advanced stage of assembly, so that an implosion during evacuation
and warm-up implies a loss of production.
By subjecting the glass part to a thermo shock test in accordance
with the invention, any defects, such as surface defects and stresses at the surface
and in the interior of the glass part become visible. The method in accordance
with the invention enables said surface damage and stresses to be detected at an
early stage, so that such parts can be excluded from the further manufacturing
process of the display device. If, for example, in the case of a cathode ray tube,
a display window is subjected to the method in accordance with the invention, it
can be determined, before the display window is provided with a phosphor pattern
and a shadow mask, and before the display window is fritted to the cone portion
of the envelope of the display device, whether surface damage on or stresses in
the display window will lead to product failure at a later stage of the manufacturing
process (for example during evacuation of the envelope). A fluid which can particularly
suitably be used for immersing the glass part is the liquid medium water.
Factors involved in the initiation of surface damage of and stresses
in glass parts of display devices are, in particular, scratches made in the manufacture
of the glass parts and during positioning and handling the parts on a conveyor
belt. Another important factor, in particular, for display windows of CRTs having
a raised edge via which the display window is connected to the cone portion, and
which edge is generally provided with connecting points for connecting a selection
electrode or shadow mask, is the degree of compressive stress present in the raised
edge of the display window. In general, the method in accordance with the invention
does not make a distinction between surface damage and (internal) stresses of
the glass part. The resistance to quenching generally is a combination of surface
roughness and internal stress of the glass part. The term "quenching" of the glass
part is to be taken to mean, in this application, a thermal shock caused by suddenly
cooling the part ("thermoshock treatment"), for example by immersing in water.
Said thermoshock treatment in accordance with the method of the invention
causes cracks to grow at the outside surface of the glass part. These cracks are
generally caused by surface damage or they develop in a region where the stress
is relatively high. Quenching of the glass part causes the outside surface to be
subject to tensile stress, while the material in the interior of the glass part
is subject to compressive stress; as a result, cracks do not grow through the glass
(i.e. cracks do not propagate in the interior of the glass). This has the advantage
that no portions of the part become detached or severed, which would lead to contamination
of the set-up for carrying out the method.
A preferred embodiment of the method in accordance with the invention
is characterized in that the temperature difference between the first and the second
temperature ranges between 25° and 85°, and is preferably approximately 50°.
An important criterion for a good selection test is that the method
yields a reliable distinction between usable and non-usable glass parts. A "non-usable"
part is to be taken to mean, in this application, that there is a relatively great
risk that such a part, which forms part of the air-tight envelope of a display
device, will be subject to implosion during evacuation and warm-up of the envelope;
conversely, a "usable" part runs a relatively small risk of implosion during evacuation
and warm-up. In addition, care must be taken that, in the long run, the method
does not adversely affect the glass part, for example, because the treatment causes
the quality of the part to deteriorate, which may not give rise to problems until
later in the life of the display device. If the temperature difference between the
first and the second temperature is too large, i.e. T2 - T1
> 85°, the risk of crack growth as a result of the thermoshock treatment is
increased, which leads to a relatively high failure percentage of the glass parts,
which is undesirable. In general, the failure probability increases substantially
with temperature. If the temperature difference between the first and the second
temperature is too small, i.e. T2 - T1 < 25°, crack growth
occurs only exceptionally, so that the selection treatment has (almost) no power
of discernment. Experiments have shown that, between said differences in temperature
(25° ≤ T2-T1 ≤ 85°), a noticeably different response
to the thermoshock treatment occurs. Experiments have further shown that the method
in accordance with the invention has a great power of discernment as to the further
processability of the part at a temperature difference between the first and the
second temperature of approximately 50° (T2-T1 ≈ 50°).
A suitable value for the first temperature ranges between 50 and 100°C,
and is preferably approximately 65°C. In the case of a temperature difference of,
preferably, approximately 50° (T2-T1 ≈ 50°), this results
in a value for the second temperature of approximately 15°C (T2 ≈
A display window which cracks as a result of the thermo shock test
can be added without further treatment (as so-called cullet) to the glass mixture
in the melting furnace from which display windows or cone portions are made. If
the display window is already provided with a phosphor pattern and/or, during removing
the frit connection between the display window and the cone portion, residues of
materials (phosphor, cone glass or fritted glass) remain in or on the display window,
the composition of the glass mixture in the melting furnace is adversely affected.
A preferred embodiment of the method in accordance with the invention
is characterized in that the fluid comprises a liquid having a coefficient of thermal
conduction (λ) above 0.4 W m-1 K-1. A liquid having
a relatively high coefficient of thermal conduction allows an effective heat transfer
of the second temperature to the glass part, if said part originates from an environment
having a higher first temperature. The higher the coefficient of thermal conduction,
the more effective the thermoshock treatment is. Water is a particularly suitable
Preferably, the fluid comprises a liquid such that the product of
the specific mass (ρ) and the specific heat (cp) is
greater than ρ x cp = 2 x 106 J m-3
K-1. Water is a particularly suitable liquid.
By using the method in accordance with the invention, the risk of
fracture or implosion of the display device during the manufacture of the display
device is reduced, which has a favorable effect on the reduction of the failure
percentage and hence on the cost price.
These and other aspects of the invention will be apparent from and
elucidated with reference to the embodiments described hereinafter.
In the drawings:
- Fig. 1A is a cut-away view of a display device comprising a cathode ray tube;
- Fig. 1B is a cross-sectional view of a display window of the display device
shown in Fig. 1A, and
- Figs. 2A and 2B schematically show an example of the method in accordance with
The figures are purely schematic and not drawn to scale. Particularly
for clarity, some dimensions are exaggerated strongly. In the Figures, like reference
numerals refer to like parts, whenever possible.
Fig. 1A schematically shows a cut-away view of a display device comprising
a cathode ray tube (CRT) 1 having a glass envelope 2 including a display window
3, a cone portion 4 an a neck 5. The neck accommodates an electron gun 6 for generating
one or more electron beams. This (these) electron beam(s) is (are) focused on a
phosphor layer 7 on the inner surface of the display window 3. The electron beam(s)
is (are) deflected across the display window 3 in two mutually perpendicular directions
by means of a deflection coil system 8.
Fig. 1B is a cross-sectional view of a display window 3 of the display
device 1 shown in Fig. 1A. The display window comprises a curved or substantially
flat part 11, a raised edge 13, 13' by means of which the display window 3 is connected,
during assembly, to the cone portion 4 of the air-tight envelope 2 of the display
device (see Fig. 1A). This raised edge 13, 13' generally comprises connecting points
15, 15' for a so-called shadow mask or selection electrode. For this reason, protrusions
14, 14' are provided at suitable locations on the inside of the raised edge 13,
Figs. 2A and 2B schematically show an example of the method in accordance
with the invention. In Fig. 2A, a glass part is warmed up to a first temperature
T1. In the example of Fig. 2A, a display window 3, which forms part
of a display device, is immersed in a warming-up vessel 21 containing a fluid 22,
for example water, at a temperature T1. Water has the advantage that
it has a high coefficient of thermal conduction (λ ≈ 0.6 W m-1K-1),
which leads to a rapid warm up of the glass part. The warm-up in a water bath 21
having a suitable temperature leads to a uniform and homogeneous warm-up of the
part. The shape of the part determines the time period t1 which the part
needs to reach a uniform temperature T1. To render the production of
said parts as economical as possible, it is desirable that the residence time in
the warming-up vessel 21 is as short as possible. In the case of a display window
3 having a relatively large surface area relative to the generally small thickness
of the glass, the desired warming-up time t1 is at least 2 minutes and
preferably 5 minutes. Alternative ways of warming up the glass part include: irradiating
the part using heat-emitting (infrared) radiators, or introducing the glass part
into a suitable furnace. The temperature T1 preferably ranges between
50°C ≤ T1 ≤ 100°C, and is, in particular T1 ≥ 65°C,
said temperatures being suitable if water is used as the warming-up medium.
In Fig. 2B, a glass part is cooled down to the second temperature
T2. In the example of Fig. 2B, a display window 3, which forms part
of a display device, is immersed in a cooling vessel 31 which contains a fluid
32, for example water. The temperature difference between the first and the second
temperature preferably ranges from 25° ≤ T1 - T2 ≤
85°, and, in particular, T1 - T2 ≈ 50°, which yields a
suitable temperature for the cooling vessel if water is used as the cooling medium.
Water has the advantage that it has a high coefficient of thermal conduction (λ
≈ 0.6 W m-1 K-1). Moreover, the product of the specific
mass (ρ) and the specific heat (cp) of water: ρxcp,
gives a high value of 4.2x106 J m-3 K-1, which
results in a desirable, rapid cooling of the glass part. To produce said parts
as economically as possible, it is desirable that the residence time in the cooling
vessel 31 is as short as possible. For a display window 3 having a relatively large
surface area relative to a generally small thickness of the glass, a cooling time
t2 of at least 5 seconds, preferably 10 seconds, is sufficient.
At a given moment, after the glass part has been warmed up at least
substantially uniformly to a temperature T1, it is transferred from
the warming up vessel 21 to the cooling vessel 32 having a temperature T2.
In Fig. 2, this transfer operation is symbolically indicated by arrow 25. The transfer
of the glass part to a colder environment causes the glass part to be cooled-down
suddenly, which is also referred to as quenching. Such a thermoshock treatment
gives rise to crack formation in the glass part, which process is initiated at
a location where the surface is damaged and/or at locations where relatively large
(surface) (tensile) stresses occur in the glass part. Such a treatment of glass
parts, in particular of display windows which form part of the air-tight envelope
of display devices, enables a good selection to be made at an early stage between
usable and non-usable display windows.
Experiments have shown that the method in accordance with the invention
yields a good selection of glass parts. Minor surface damage at arbitrary locations
gives rise to crack growth which starts already at the location of the damaged
spot at relatively low thermal stress levels. In the case of display windows, it
has further been found that, in the absence of surface damage, crack growth generally
starts at the location of the raised edge of the display window. Crack growth is
often initiated by a relatively low compressive stress in this so-called seal edge.
The thermoshock treatment does not distinguish between surface roughness and internal
stress, so that the thermoshock treatment generally is indicative of a combined
effect of both phenomena.
In general, the invention relates to a method of manufacturing a display
device comprising an air-tight envelope and at least a glass part (3) which forms
part of said air-tight envelope. The method is characterized in that the glass
part is warmed up, during a first time period, at a first temperature (T1),
whereafter the glass part is immersed, during a second time period, in a fluid
at a second temperature (T2), said second temperature being lower than
the first temperature (T2 < T1). Preferably, 25° ≤ T1-T2
≤ 85°, and, in particular, T1-T2 ≈ 50°. Preferably,
50°C ≤ T1 ≤ 100°C, and, in particular, T1 ≈ 65°C.
Preferably, the glass part is a display window or a cone portion of a display window,
and the fluid is water.
The invention can be summarized as follows:
A method for manufacturing a display comprises a thermoshock test
for parts such as the faceplate (3).
The faceplate is first placed in a fluid at a high temperature, whereafter
it is quickly transferred to a second fluid (both fluids could be the same, e.g.
water) at a substantially lower temperature. The sudden drop in temperature induces
a thermoshock effect in the part, which causes flaws such as cracks and stress
to become visible. The appearance of such flaws is used to distinguish usable parts
from flawed parts. The method makes it possible to remove flawd parts from the
production line at an early stage, thus reducing the percentage of displays that
do not pass the final inspection or have a reduced life expectancy.