Contents
LG.Philips Displays
PPD
Eindhoven
TVZ-225-04-EK-D023
Final
2004-05-26
Screen processing
Functional Process Description
Precoating
Contents
1Contents
11. Introduction
32. Flowchart
43. Room conditions
44. Specification incoming components/sub
assemblies/Chemicals
45. Process description process steps
45.1 High pressure spraying (optional)
75.2 Application of pre-coat liquid
115.3 Drying and heating
145.4 Cooling down (for traditional pre-coated screens)
155.5 Selective heating
165.6 Hanger drip removing.
176. Specification outgoing product
177. Auxiliary, tools and filter specifications
188. Cleaning procedures
189. Waste treatment
1810. Process control
1911. Miscellaneous
1912. References
1. Introduction
Aim of the pre-coat process is to apply a pre-coat layer, which
provides phosphor adhesion. The pre-coat solution is sprayed in the
screen and dried after equalizing.
Adhesion model
The following model explains the function of (short) pre-coat
[1,2,3,4]:
After screen washing, the glass surface contains both OH and O-
groups. Macromolecules of poly-vinyl-alcohol 40-88 (PVAl) are very
long structures. Some of the OH-groups from the PVAl molecules can
form hydrogen bridges (or maybe even a chemical bond) with the OH
groups of the glass surface. Other OH groups of the PVAl molecule
are available to adhere to phosphor particles, photoresist and
graphite.
For a neutral environment (pH=7), which is used for traditional
pre-coat, the glass surface has more O- groups. Heating to 45 50 (C
is required to get sufficient adhesion of PVAl to the glass
surface. Before matricizing, the screen must be cooled to 30(C.
For short pre-coating, the PVAl solution is acidified with HNO3
to pH=2.0. At low pH, O- groups are restored to OH groups and more
hydrogen bridges can be formed. In this situation no heating is
required to get sufficient adhesion.
Adhesion model.
Line concepts
Many different versions of pre-coat lines are operational in
IPCs of LG.Philips Displays, due to historical evolution of product
and process design in screen processing.
For small and curved screens, the pre-coat solution can be
sprayed directly into a stationary screen. Due to the curvature of
the inside screen, the surplus of pre-coat liquid will be easily
drained and an acceptable uniform layer is formed. In this
situation, the pre-coat process is integrated with the screen wash
process. Like screen washing, the screens are transported in a
hanger system and moved from one position to another.
As CRTs evolve to large and flat screen types, equalizing of the
pre-coat liquid becomes a problem in stationary positions. Large
droplets at the inside of the screen will be formed after spraying.
A local thicker pre-coat layer will result at these locations. The
pre-coat layer will be partly dissolved during resist application
in the matrix process and cause matrix uniformity problems. A
rotational process was introduced in order to achieve uniform
pre-coat layers. A disadvantage is that screens have to be taken
out of the hanger system for rotational equalizing and must be
replaced for drying and heating.
For traditional pre-coating, the screen must be heated up till
45 50 (C in order to assure sufficient phosphor adhesion. After
heating, the screen must be cooled below 30 (C. Heating and
especially cooling requires long transport belts. The short
pre-coating process was introduced in order to omit heating and
cooling of screens, saving factory floor space and investment
costs.
For short pre-coating, the liquid must always be applied by a
rotational process in order to achieve good matrix uniformity.
Because heating and cooling can be omitted, the short pre-coat
process can be disconnected from the screen wash line and placed
directly in front of the matrix line. Screens are transported from
one position to another by the same pick and place system as the
matrix line.
Examples of line concepts
The precoat layer can resist the complete matrix process
including heat treatment and nitric acid spraying. The precoat
layer is only removed by HF treatment like in the rewash
process.
Pre-coat alternativesMany competitors dont use pre-coating for
matricized tube types. An addition of silane to the matrix
photoresist can provide a similar function as PVAl pre-coating. The
benefits of this option are less floor space, lower capital
investments for new lines and lower operational costs.
This option was intensively studied in the past for Philips
production lines [5,6]. The main problem was an increased green
haze level due to photoresist residues in the corners and often a
slightly lower phosphor adhesion was observed. The conclusion of
those studies was that silane based processing is still lacking
technical feasibility within Philips.
PVAl can also be replaced by silane in the traditional pre-coat
process. Heating and cooling of the screens can be skipped for this
option, similar to short pre-coating. Also for this option a
slightly reduced phosphor adhesion and increased green haze level
was observed.
After introduction of short pre-coating, all investigations on
silane-based processes were cancelled.
2. Flowchart
3. Room conditions
ItemLimitsReason/remark
HumidityNo requirementOrientation value 40 75 %
TemperatureNo requirementOrientation value 20 30 (C
Dust class100.000
Exhaust to scrubber is required for acid spraying position, acid
carts and settling tanks.
Special caution has to be taken for screen contamination with
copper and teflon. Copper reacts at flowcoating with blue phosphor
during fritting into green phosphor. Teflon is an emission killer
of the tube!
See also ref. [7]
4. Specification incoming components/sub
assemblies/Chemicals
Incoming components:
Incoming component is a clean screen from the screenwash
process. The inner screen surface must be wet.
Sub assemblies:
No requirements
Chemicals used are:
Precoat PVAl 0.02 - 0.1 %
Short precoat PVAl 0.2 %No document
1322 530 07701
De-ionized water1322 505 41701
For detailed information see the process steps where the
chemicals are used and the 12 NC of the chemical.
5. Process description process steps
5.1 High pressure spraying (optional)
Reason
Remove particles that are strongly adhered to the glass and not
completely removed by glass etching. Furthermore, acid residues
from small glass cracks can be effectively removed.
Description of the process step
After glass washing the inner glass surface still can have small
particles pressed in or adhered to the glass surface [8].
High-pressure water spraying is able to remove (part) of those
inclusions.
Small caves and cracks can contain some residues of fluorides or
acids from the screen washing process, which can also be removed
with high-pressure water spraying.
A T-shape arm equipped with an array of nozzles is moved back
and forwards in West - East or North - South direction underneath
the screen.
The pressure has to be above 50 bar. Preferred 70 bar.
High-pressure spraying arm.
The spraying profile of the nozzles is indicated in the figure
below. The maximum pressure is in the center of the profile. The
pressure drops towards the edge of the profile. When all nozzles
are aligned in the same direction, the nozzles will interfere with
each other. The spraying efficiency will be very low at these
intersections.
Spraying pattern for all nozzles in line.
To avoid interference between nozzles, all nozzles are tilted
over 30(. The distance between nozzles, angels, distance between
nozzles and screen etc are optimized in order to find an optimum
pressure distribution over the screen. These parameters are usually
found by experience and trial and error.
Spraying pattern high pressure spraying with tilted
nozzles.Relevant process parameters, typical values and tolerances,
essentials and reasons
Process parameterTypical value and toleranceReason
Spraying pressure> 50 bar Efficiency
Spraying timet.b.f.Efficiency
Nozzle angle30 (Efficiency
See also process control list in chapter 10.
Description of chemicals and utilities
Warm demin water (Chemical code CE) [23]
Water must be filtered to avoid damage of the screens.
Description of equipment requirements
A small de-ionized water storage tank serves as buffer between
the de-ionized water circulation system and the high-pressure pump.
The high-pressure pump feeds the developing position. The water is
not circulated to the buffer tank.
Buffer tank and high-pressure pump
Nozzle high pressure spraying: MEG-4003
Rubber sealing from quick-fit connections are not resistant
against 50 bar.
Therefore no quick-fit connections can be used.
The spraying arm must be secured.
High pressure developing arm
Type range: (NZ 29
Configuration elstein boxes6 x 500 W 9 x 650 W9 x 650 W
Number of heater boxes5 x 2 (130 / h)10 x 2 (160 / h)10 x 2 (160
/ h)
Configuration selective heating boxes6 x 500 W12 x 500 W12 x 500
W
Number of sel. heating boxes1 or 2 x 11 x 21 x 2
For supply of compressed air, a service unit with active carbon
and particle filter, pressure gauge and/or flow meter to control
the airflow is required.
5.3.2 Drying of short pre-coat
Reason
Create a dry short pre-coat layer with good adhesion to the
glass surface.
Description of the process step
This process is used for drying of short pre-coat layers.
The very large PVAl molecules will be clustered at low
temperature (see fig. A) or stretched at higher temperature (see
fig B) [11].
PVAl molecules at different temperatures.
To realize good phosphor adhesion, stretched PVAl molecules are
preferred in order to have more hydroxyl groups available for
adhesion to the glass surface and for cross linking with the
PVAl-ADC resist from flowcoating (see fig. C). As long as the
screen is wet, situation A will gradually change to situation B.
Conversion from clusters to stretched molecules take some time, so
drying speed must be relatively slow.
The drying process is carried out in a pick and place system.
Screens are heated from the inside. The heater capacity is adjusted
in such way that the screen is spontaneously dried with minimal
heating up of the screen. It is preferred that drying takes place
on one process position. Little experience is available for the
sensitivity of interrupting the drying front. Indications from CDT
show that it is not very critical.
Drying position
The exact adhesion model for short pre-coating is not completely
understood. It is generally expected that stretching of the PVAl
molecules takes some time and are essential for good phosphor
adhesion. Air blowing during drying can give an enormous increase
of the drying speed and it has been shown that it has a significant
effect on the minimum phosphor adhesion [16,17]. In new large/jumbo
lines the time between pre-coat spraying and start drying is
elongated by an extra dummy position between these two process
steps in order to have more time available for stretching of the
PVAl molecules. Although experiments have shown that the time
between spraying and drying has no or little effect on phosphor
adhesion [17], there is too little confidence to omit the dummy
position.
In the past, the dummy position was combined with a soak mat to
remove pre-coat droplets from the seal edge. However, it was shown
that this function was not required.
Air bubbles or foam formation can lead to pre-coat faults in the
matrix pattern.
Dirt from the environment will also give rejects.
Relevant process parameters, typical values and tolerances,
essentials and reasons
Process parameterTypical value and toleranceReason
Heating powerDepending on type and process speedPre-coat layer
must be dry before the screen is loaded on the matrix line.
Heating timeDepending on type and process speedPre-coat layer
must be dry before the screen is loaded on the matrix line.
End temperature30 34 (CTo meet matrix loading temperature
requirement
See also process control list in chapter 10.
Description of chemicals and utilities
None
Description of equipment requirements
Time between pre-coat spraying and start drying must be more
than t.b.f. sec.
Heater box configuration CDT Heater box configuration
Large/Jumbo
Short pre-coat dryingScreen diagonal
< 21 (CDT)21 29> 29
Configuration elstein boxes11 x 500 W 14 x 250 W +
28 x 125W14 x 250 W +
28 x 125W
Number of boxes3 x 1 (130 / h)4 x 1 (150 / h)4 x 1 (150 / h)
5.4 Cooling down (for traditional pre-coated screens)
Reason
Cool down the screen to meet the matrix loading temperature.
Description of the process step
After traditional pre-coat drying and heating, the screens are
45 50 (C and must be cooled down to meet the matrix loading
temperature specification.
Cooling down takes place in:
The same hanger system as used for washing, pre-coat spraying
and drying, or
A separate power and free transport system. The screen and mask
is placed on carriers.
To enable better control of the temperature at loading the
matrix line, cooling down is preferably a few degrees lower than
the specified matrix loading temperature. Cooling down requires a
long time depending on environmental temperature and glass
thickness (type). Cooling down times can vary between 30 and 90
minutes.
Relevant process parameters, typical values and tolerances,
essentials and reasons
Process parameterTypical value and toleranceReason
Cooling time20 50 minTo meet matrix loading temp.
End temperature< 30 (CBelow matrix loading temperature
See also process control list in chapter 10.
Description of chemicals and utilities
None
Description of equipment requirements
End temperature must be < 30 (C.
Cooling of screensScreen diagonal
< 21 (CDT)21 29> 29
Cooling time [min]20 3030 - 4030 - 50
Cooling down is done by transporting the screen in a hanger
system. 5.5 Selective heating
Reason
Heat up the screen to the temperature specified for loading the
matrix line.
Description of the process step
The selective heating process is positioned in the same hanger
system as used for washing, drying, heating and cooling of the
screens (generally CPT types), or on a power and free transport
system as used for cooling of the screens (some CDT types).
For a hanger transport system, the heater boxes with short wave
infrared heaters are placed on both sides of the screen. On the
first position the screen temperature is measured in the center.
The radiators are activated for a period depending on the
difference between the measured temperature and the target
temperature. A calibration curve for the relation between heating
time and screen temperature increase has to be prepared
[18,19,20,24].
Selective heating in conveyor belt
For a power and free transport system, the heater boxes are
positioned above the screen. Assumption is that the screen has a
homogeneous incoming temperature distribution. In this system,
separate calibration curves can be applied for the center and the
corner to control the temperature distribution.
Selective heating in power
and free belt
Matrix window width distribution and spread is influenced by the
performance of the selective heating.
Relevant process parameters, typical values and tolerances,
essentials and reasons
Process parameterTypical value and toleranceReason
Heating timeDepending on incoming screen temperature and target
temperature (calibration curve)Little temperature variation between
screens
Heater distributionDepending on local situationOptimize MWW
distribution
See also process control list in chapter 10.
Description of chemicals and utilities
None
Description of equipment requirements
See section drying and heating of traditional pre-coating for an
overview of line configuration.
The screen temperature is measured with calibrated IR sensors
(kT-15) before heating.
5.6 Hanger drip removing.
Reason
Remove droplets from the hanger and seal edge of the screen.
Description of the process step
After pre-coating or screen washing, the hangers and possibly
the seal edge of the screens are still wet. Even after hanger
washing, remaining droplets are very aggressive and can seriously
damage equipment and floor. PVAl can make the floor very slippery,
which can create dangerous situations.
Droplets are removed by lifting a module with absorption
material against the hanger (and if desired also against the
screen) for a specified time. Droplets are removed and the module
is lowered.
Unit must be connected to the acid drain.
Risk for plashes when the screen is in the hanger during drip
removing.
Drip remover unit
Relevant process parameters, typical values and tolerances,
essentials and reasons
Process parameterTypical value and toleranceReason
Soaking time> 5 secEffective removal of droplets
See also process control list in chapter 10.
Description of chemicals and utilities
None
Description of equipment requirements
None
6. Specification outgoing product
Outgoing product is a screen with a dry precoat layer with good
adhesion to the glass surface.
Screen temperature is 30 34 + 1 (C and must be uniformly
distributed.
7. Auxiliary, tools and filter specifications
Auxiliary materials and tools:
PurposeTool
Rotational speedRevolution counter
Process times.Stop watch
Temperature measurementsThermometers up to 100 (C
Portable IR meter
Screen/mask storageScreen/mask transport carts
Small adjustments / repairsWrenches, hammer, flexible ruler,
screw drivers, etc
Rinsing cleaningKitchen-sink with water / drain
Towels,
Sponges,
Buckets
Screen handlingManual vacuum chucks
ProtectionRubber gloves
Safety glasses
Ear plugs
Sampling, measuring and adjustmentSpraying bottles 500 ml
See also auxiliary list is ref [21].
Filter specifications:
For filtration of demi water and pre-coat solution, a pore size
of 6 (m is required. Size of the filter is depending on the
required water flow.
Compressed air must be filtered over an active carbon filter to
remove oil and over an absolute particle filter with a pore size of
0.2 (m.
An example of the filters that are used in the pre-coat area of
large/jumbo lines:
PositionHousingSupplierCartridgeSupplier
Service racks airAG0006 A/EPallAK 03/10Pall
Service racks airn.a.TNFZ 01 H03 H03Parker
Service racks
liquidNP10-DO-DV-EParkerPAB 050-10FA-DO or
MCY 1001 U6-40 ZH13Parker
Pall
See also ref [22] for more information regarding process
filters.
8. Cleaning procedures
None
9. Waste treatment
Pre-coat liquid must be drained in a special organic drain and
treated according local regulations.
10. Process control
This process control list gives an example of the items that
have to be controlled for the pre-coating process during normal
production. An advice is given about the frequency of control. The
list can be used as a starting point for defining a process control
list dedicated for a certain line or as help for updating existing
checklists. It is important that a process control list is made
specifically for a certain line and type, using the process
description, this list and the local situation as input.
ProcessItemSpecAdvised control frequency
UtilitiesDemi water pressure 30 (C
Compressed air pressure
- process air
- control air
Vacuum6.0 + 0.5 bar
6.0 + 0.5 bar
6.0 + 0.5 bar
t.b.f.1x / shift
1x / shift
1x / shift
1x / shift
HP developingWater pressure60-70 bar1x / shift
Stationary pre-coat sprayingPVAl flow rate
PVAl temperature1000 + 100 l/h
20 - 30 + 2 (C1x / shift
1x / shift
Rotating pre-coat sprayingPVAl flow rate
PVAl temperature
pH (short pre-coat)350 + 50 l/h
30 + 2 (C
2.0 + 0.11x / shift
1x / shift
1x / shift
Drying and heating traditional pre-coatingTop temperature
Air flow rater45 50 (C
15 + 0.5 Nm3/h1x / shift
1x / shift
Cooling down of traditional pre-coated screensEnd
temperature< 30 (C1x / shift
Selective heatingEnd temperature
Calibration of IR cameras30 + 1 (CContinuous
1x / year
11. Miscellaneous
None
12. References
1. P. HENRIETTEPrecoat onderzoekMvdE/L5661-3,1980-09-12
2. E. KEETELS, H.J.M. PENNERSShort precoating TVT. TF/DA
reportTVR-298-01-EK/D005
3. H.J.M. PENNERSMask washing, screen washing and precoating
course LG PhilipsTVZ-298-03-HP-D024, 2003-05-14
4. H.J.M. PENNERSInventory of the washing and precoating of
screen panels for matricizing/flowcoatingTVR-34-90-hp/D108,
1990-16-02
5. H.J.M. PENNERS, I.M.M. DURLINGERDiscussions about
PVA-precoating and alternativesTVR-657-99-HP/D114, 1999-09-20
6. H.J.W. LENDERINKOmit precoatingTVR-298-01-JWL/D030,
2001-04-06
7. NIESSEN F.Room conditions for new linesTVR-217-03-FN-D006,
2003-10-07
8. B.SMITHigh pressure cleaningTVR-lr 88/53-0012, 1988-14001
9. H.J.M. PENNERSRotating precoating: process optimization in
DaponTVR-57-95-HP/D0160, 1995-06-07
10. H.J.M. PENNERSInfluence of the PVAl precoat layer onto the
matrix window sizeTVR-34-90-HP/D446, 1990-06-12
11. H.J.M. PENNERSShortening the PVAl precoat process for
matricized screensTVR-47-91-HP/D0238, 1991-10-16
12. E. KEETELSIF report short precoatingTVR-298-02-EK-D036,
2002-04-10
13. H.J.M. PENNERSShort precoating using acidified PVAl solution
of pH 2.2TVR-57-96-HP-0301, 1996-12-20
14. H.J.M. PENNERSShort PVAl precoating conditions and its
influence on the minimal adhesion level at
flowcoatingTVR-657-99-HP-D0001, 1999-01-04
15. H.J.M. PENNERSTechnical feasibility of short precoating
TVTsTVR-698-00-HP/053, 2000-05-10
16. B. v RIETAlternative short precoat procedure in
TTCTVR-298-03-BvR-C067, 2003-05-13
17. B. v RIETProcess window short precoatingTVR-298-03-BvR-D109,
2003-08-05
18. I. DURLINGERSelective heating fcline 51FS Hua Fei design
justificationTVR-57-96-ID/D0153, 1996-07-02
19. E. KEETELSScreen conditioning with feed forward
controlTVR-654-97-EK/D054, 1997-04-04
20. M. BUSIOFunctional process specification flowcoating L/J
linesTVR-298-03-MB-D097, 2003-08-04
21. H. PENNERS
Auxiliary list for mask washing, screen washing, matricizing and
rewashing.TVZ-298-01-HP/D002A, 2001-01-04
22. J. NILLISSENFiltration Management, FTSRTVR-657-99-JN/D0139,
1999-07-15
23. PTE, Standardisation mechanical constructionsUtility and
chemical standards00-04-15
24. I. DURLINGERSelective screen heating, excel software
toolTVR-253-01-ID/D053, 2001-06-14
AMENDMENT RECORD
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