fpd precoating

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.


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


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



None


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.


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.



Cooling time20 50 minTo meet matrix loading temp.

End temperature< 30 (CBelow matrix loading temperature



None


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.


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.



Heating timeDepending on incoming screen temperature and target temperature (calibration curve)Little temperature variation between screens

Heater distributionDepending on local situationOptimize MWW distribution



None


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.


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



Soaking time> 5 secEffective removal of droplets



None


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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fpd precoating

Documents

precoat process

precoat solution

application of precoat

surplus of precoat liquid

function of short precoat

uniform precoat layers

screen wash process

local thicker precoat