fpd mask washing

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Mask washing and drying Contents Contents......................................................................1 1. Introduction...............................................................1 2. Flowchart..................................................................2 3. Room conditions............................................................2 4. Specification incoming components/sub assemblies/Chemicals.................2 5. Process description process steps..........................................3 5.1 Degaussing..............................................................3 5.2 Showering...............................................................4 5.3 Ultra sonic mask cleaning (optional)....................................6 5.4 Draining................................................................9 5.5 Drying..................................................................9 5.6 Cooling down...........................................................11 6. Specification outgoing product............................................12 7. Auxiliary, tools and filter specifications................................12 8. Cleaning procedures.......................................................12 9. Waste treatment...........................................................13 10. Process control..........................................................13 11. Miscellaneous............................................................13 12. References...............................................................13 Introduction Aim of the mask cleaning is to remove particles from the mask that can block the mask holes or are a potential cause for loose particle reject. The principle of the cleaning process is intensively showering the mask to remove loose dirt, combined with ultra sonic treatment to remove adhered dirt. The mask is dried after cleaning. © LG.Philips Displays Page 1 of 18 COMPANY RESTRICTED LG.Philips Displays PPD Eindhoven TVZ-225-04-EK-D017 Final 2004-05-25 Screen processing Functional Process Description

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mask wash

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Contents

LG.Philips Displays

PPD

Eindhoven

TVZ-225-04-EK-D017

Final

2004-05-25

Screen processing

Functional Process Description

Mask washing and drying

Contents

1Contents

11. Introduction

22. Flowchart

23. Room conditions

24. Specification incoming components/sub assemblies/Chemicals

35. Process description process steps

35.1 Degaussing

45.2 Showering

65.3 Ultra sonic mask cleaning (optional)

95.4 Draining

95.5 Drying

115.6 Cooling down

126. Specification outgoing product

127. Auxiliary, tools and filter specifications

128. Cleaning procedures

139. Waste treatment

1310. Process control

1311. Miscellaneous

1312. References

Introduction

Aim of the mask cleaning is to remove particles from the mask that can block the mask holes or are a potential cause for loose particle reject.

The principle of the cleaning process is intensively showering the mask to remove loose dirt, combined with ultra sonic treatment to remove adhered dirt.

The mask is dried after cleaning.

Showering

Showering

US

washing

Empty

Empty

FlowchartRoom conditions

Item

Limits

Reason

Humidity

No requirement

High humidity caused by extensive water spraying

Temperature

No requirement

High temperature caused by mask drying tunnel

Dust class

100.000

Prevention of re-contamination

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. [1]

Specification incoming components/sub assemblies/Chemicals

Incoming components:

Incoming component is a mask from the mask department.

Sub assemblies:

The mask can be part of a married couple with a screen.

Chemicals used are:

De-ionized water: 1322 505 41701

For detailed information see the process steps where the chemicals are used and the 12 NC of the chemical.

Process description process steps1.1 Degaussing

Reason

The purpose is to neutralize the magnetic field in the mask.

Description of the process step

Masks from the mask assembly area may contain some magnetism. This magnetism can be difficult to remove once a tube is manufactured. Residual magnetism affects the landing performance. To degauss a mask, the mask is moved smoothly through a sufficiently strong alternating magnetic field.

Since masks already are hanging in a conveyor for mask cleaning, positioning the degaussing coil in this conveyor is a very cheap solution. An additional advantage might be that loose (magnetic) mask burrs are demagnetized as well and can be removed more easily during mask cleaning.

To de-magnetize a mask, the mask passes through the gap between 2 coils opposite to each other. The coils generate a magnetic field from and alternating current. The coils are positioned between 2 index positions, so the degaussing is executed during transportation, in order to prevent mask damage due to microphonic vibration.

Fig. 1. Degaussing unit

Relevant process parameters, typical values and tolerances, essentials and reasons

Process parameter

Typical value and tolerance

Reason

Power supply

Min. 3200 AW (Ampere turns)

Sufficient degaussing

Frequency

Continuously 50 or 60 Hz

Sufficient degaussing

Residual magnetic field

< 0.3 Gauss

Limit effect on landing

Description of chemicals and utilities

No chemicals are used for this process step.

Description of equipment requirements

Smooth movement of the mask is essential.

Coils must be positioned between 2 index positions.

Air gap between mask and degaussing coils must be minimal in order to get maximum magnetic field. A too small gap increases the risk for mask damage.

Note

Optimal functioning of equipment and testing of effective degaussing is not the responsibility of the screen processing group.

1.2 Showering

Reason

The purpose is to remove loose dirt from the mask.

Description of the process step

The mask surface is rinsed from both sides simultaneously. In this way loose or slightly adhered dirt can be released and removed.

Typical dirt particles are:

Skin particles

Jelly particles

Hair

Fibers

Blackening particles

Corrosion particles from ovens

Welding splashes

Showering

Relevant process parameters, typical values and tolerances, essentials and reasons

Process parameter

Typical value and tolerance

Reason

Water spraying pressure (flow)

2-3 bar depending on nozzle type

Lower limit: sufficient removal of particles

Higher limit: Prevention of mask damage

Water spraying distribution

Complete covering of mask

Spraying time

> 120 sec

The pressure at inside spraying is often adjusted slightly higher (0.1-0.2 bar) than for outside spraying in order to reduce the risk for mask damage.

Description of chemicals and utilities

Recycled de-ionized water: No requirements on water temperature needed

De-ionized water: No requirements on water temperature needed

Note

1. In one occasion a bacteria killer was added. However, this is not a standard solution [2].

2. In the past, some agents to reduce the surface tension of the spraying water have been tested (e.g. Renex 688). No positive effects on washing results were observed. Advantage is that masks are easier to dry. Disadvantage is that these agents can promote algae growth and it can reduce the adhesion of mask coatings (Bi2O3).

Recycled de-ionized water:

De-ionized water is recycled in a circulation tank.

Attributes of the circulation tank are:

Upper and lower level control

Alarm level control to protect the circulation pump.

Circulation of water.

Overflow and drain.

For the last spraying position fresh and filtered de-ionized water is used which is drained into the circulation tank. The first position is drained to drain.

Description of equipment requirements

( 21 inch / 51 cm

> 21 inch / 51 cm

Type of nozzle internal

Number of nozzles internal

Pressure (orientation value), bar

SS 8W

1

2 - 3

SS 8W

2

2 - 3

Type of nozzle external

Number of nozzles external

Pressure (orientation value), bar

SS 8W

1

2 - 3

SS 8W

2

2 - 3

Numbering of showering cabinets,

1 for every 30 screens/hour

The spraying water will be collected in the lower mask corner and dirt may be sieved through the mask holes. Especially for CDT types, an increase of blocked mask holes can be observed in this corner. This problem can be reduced by minimizing the amount of spraying water, and optimizing the impact (distribution) of water droplets over the mask.

In production several types of showers are used [3]:

Gardening showers (Gardenas): these produce a lot of water. The spraying angel is small so showering the whole mask, especially under the mask skirt is difficult

An improvement was the introduction of a Lechler nozzle (type 502.448.17), which has a 130( spraying angle and produce much less water with a complete covering of the whole mask.

Further improvement was obtained by introducing a full cone nozzle of 120( (type SS 8W from spraying systems). The amount of water was comparable but the droplet size was bigger and therefore a larger impact was obtained. This type of nozzle is widely applied in the IPCs. This is the standard type of nozzle, which is used for new lines.

Notes

1. The cleaning tunnel consists out of an array of showering cabinets. The first and last position must be empty in order to prevent water splashes coming out of the tunnel. Also the top of the tunnel requires precautions to prevent water splashes to come out of the tunnel.

2. In case US-vibration is integrated, half of the spraying tunnel is placed before and half of the spraying tunnel is placed after the US equipment. Again, the first and last position of each tunnel section must be kept empty.

3. In some new lines a strong ultra violet source is introduced after the circulation pump system to prevent algae and bacteria growth, which can contaminate the mask.

Degaussing coils

Mask

1.3 Ultra sonic mask cleaning (optional)

Reason

Some particles have such a strong adhesion to the mask that they cannot be removed by showering. These particles are released from the mask surface by ultra sonic vibration.

Description of the process step

The masks are hanging vertically on a conveyor hook. After indexing the ultrasonic tank is lifted. After a few seconds of ultra sonic treatment in top position, the tank is moved down again.

Ultrasonic Mask washing

Ultra sonic sound is generated via vibration sources (transducers) immersed in water. A sound wave produces alternating patterns of compression and expansion, thus high and low pressure points. In low pressure points, vapor bubbles are formed throughout the liquid, even in hidden recesses and crevices. During the compression cycle these bubbles collapse violently (cavitations). The mask is exposed to cavitations for a few seconds to release attached particles.

Since particles in the tank can deteriorate the cavitations process, intake of particles must be prevented. Therefore part of the showering tunnel is placed before the US equipment. US vibration is very effective to release particles form the mask, but not effective to remove them from the mask. Therefore mask showering is also required after US vibration. See also chapter showering.

To promote vapor bubble formation the water temperature should be high. To increase the implosion force of the bubbles, the water temperature should be low. Optimal results will be obtained between 60 and 80 (C. To limit the thermal shock during immerging the mask in the water, 60 (C has been selected. For more details see also [3,4]

Cavitation killing parameters.

Dissolved gasses. During cavitation, the dissolved gas (nitrogen, oxygen) will escape. The created gas bubbles reduce the implosion force.

Particles in the liquid form many cavitation cores. So less energy available on location were cavitation is required.

Movement of the water, either by pumping and or by immersing the object to be cleaned.

Distance between transducers and the object to be cleaned.

Masks must not be (re)-contaminated when lifting the mask out of the tank. Therefore particles must be effectively removed by continuously circulation and filtration of the water. Due to the shape of the tank, particles are collected at the bottom and removed.

Also the water surface must be kept clean from floating particles. This is done by spraying water just below the water surface at three sides of the tank via a pipe with holes. This system is called a skimmer. Floating particles are removed by means of an overflow.

Showering

Showering

US

washing

Empty

Empty

Skimmer

Note:

Other versions of US equipment consisted out of a large tank with US transducers. The masks are dragged trough the tank for several cycles. Critical item of this equipment is that the water surface is very large and difficult to keep clean which increased the risk of recontamination. Some IPCs have skipped this process after some time in operation because it didnt give an improvement on blocked mask holes.

Relevant process parameters, typical values and tolerances, essentials and reasons

Process parameter

Typical value and tolerance

Reason

Vibration time

> 4 s

Vibration energy

25 30 W/l

Cavitation power

Water temperature

60 + 5 (C

Cavitation efficiency

For other process control parameters see process control list is chapter 10.

Description of chemicals and utilities

De-ionized water. No temperature specification of the incoming water is required.

The water temp. is controlled by the US equipment.

The equipment can be directly connected to a de-ionized water supply system.

Description of equipment requirements

The lifting speed must be less than 0.23 m/s. Higher speeds gives risks for mask dents or the mask may be lifted from the conveyor hook.

For supply of de-ionized water a service unit with filter, pressure gauge and/or flow meter to control the water flow is required.

1.4 Draining

Reason

Drain most of the water from the mask to limit transport of water into the drying tunnel.

Description of the process step

After showering, the mask is very wet. The excess of water can be drained by transporting the mask for a specified time without water spraying, before entering the drying tunnel.

Relevant process parameters, typical values and tolerances, essentials and reasons

Process parameter

Typical value and tolerance

Reason

Draining time

Approx. 1 min

Description of chemicals and utilities

None

Description of equipment requirements

None

1.5 Drying

Reason

Dry the mask, the mask ring, the mask components, the spaces and gaps.

Description of the process step

Masks are transported vertically through a drying tunnel.

Transducers

Pump

Skimmer

Temp control

Mask drying tunnel

A drying tunnel consists out of a number of positions. In each position, the air is circulated continuously. The air is sucked at the bottom of the tunnel into a compartment, which divides the airflow in two parts. Each flow is respectively heated up, filtered and blown from the side against the mask.

Note

Because of natural loss of circulated air through the tunnel openings it does not lead to excessive moisture build up in the tunnel

Side view

Top view

Water flow

direction

Over flow

Over flow

The lower mask corner is most difficult to dry. Often some water droplets are collected in the mask ring or mask suspension system. In some cases extra air blowing at this corner is required for complete drying of the mask.

An extra blowing pipe is placed in the 2nd or 3rd position of the drying tunnel. Two nozzles are focused on both side of the lower mask corner like indicated in the figure.

When masks are not completely dry, masks holes will be blocked during exposure and will give trio blocked rejects. Also splashes may come into the screen in MIM or MEM.

Additional air blowing of lower mask corner.

Relevant process parameters, typical values and tolerances, essentials and reasons

Process parameter

Typical value and tolerance

Reason

Air temperature

130 + 10 (C (set point)

Fast water evaporation

Drying time

3 min.

Air flow

Not measured

Description of chemicals and utilities

None

Description of equipment requirements

For drying a dwell time of about 3 minutes is required.

Because the first and last positions are not very effective use the following rule:

The number of drying positions: 0.04 * process speed + 2

(Process speed in pcs/hour)

Optional: Extra air blowing in 2nd or 3rd position towards the lower mask corner.

1.6 Cooling down

Reason

Cool down the mask in order to achieve a stable mask position in the screen during exposure.

Description of the process step

During cooling, mask cools down much faster than the mask ring with mask suspension system. Therefore the mask ring determines the required cooling down time.

Mask should be cooled down to temperatures below 30 (C.

Relevant process parameters, typical values and tolerances, essentials and reasons

The most relevant process parameter is cooling time.

Filter

Filter

motor

Example of cooling curve 17 Invar and 15 iron mask ring. Ambient temp = 22 (C.

Description of chemicals and utilities

None

Description of equipment requirements

The minimum length of the mask transport from mask washing to matrix exposure is determined by the cooling down time. Special attention is required to prevent recontamination of the mask during cooling down.

Specification outgoing product

The outgoing product is a clean and dry mask.

Auxiliary, tools and filter specifications

Auxiliary materials and tools

Purpose

Tool

Process times.

Stop watch

Screen/mask storage

Screen/mask transport carts

Small adjustments / repairs

Wrenches, hammer, flexible ruler, screw drivers, etc

Rinsing cleaning

Kitchen-sink with water / drain

Towels,

Sponges,

Buckets

Protection

Ear plugs

See also auxiliary list is ref [5].

Filter specifications

For filtration of demi water a pore size of 6 (m is required. Size of the filter is depending on the required water flow.

An example of the filters that are used in the mask wash area of large/jumbo lines:

Position

Housing

Supplier

Cartridge

Supplier

Circulation tank

FH620-50-SG

Parker

PM 050-20AN-DO

Parker

Circulation tank

UV 750 BA

A&C eng.

UV-lamps Q3-30

A&C eng.

US tank

T910065-246

?

??

Last spraying pos.

NP10-DO-DV-E

Parker

PAB 050-10FA-DO or

MCY 1001 U6-40 ZH13

Parker

Pall

Drying tunnel

n.a.

VARICEL HT900 10-1224-12

AAF

See also ref [6] for more information regarding process filters.

Cleaning procedures

Circulation tank and US vibration position

Drain circulation tank.

Wash inside tank. Remove deposits from the wall with cloth and commercial detergent.

Rinse the tank with de-ionised water.

Close the drain and refill with de-ionised water.

Spraying positions

Check function of spraying nozzles. When some nozzles are blocked, open, clean and re-install the nozzles.

Clean the inside of the tunnel with water and a cloth.

Waste treatment

All water can be drained directly into the normal city sewer.

Process control

This process control list gives an overview of the items that have to be controlled in the mask washing area 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.

Process

Item

Spec

Advised control frequency

Utilities

Demi water pressure 30 (C

Compressed air pressure

- process air- control air

6.0 + 0.5 bar

6.0 + 0.5 bar

6.0 + 0.5 bar

1x / shift

1x / shift

1x / shift

Degaussing

Power

3200 AW

1x / shift

Water spraying

Water pressure spraying nozzles

2 + 0.5 bar

1x / shift

US vibration

US generator ok?

Water flow skimmer

Water flow central

Water flow bottom

Water temp

25 30 W/l

800 + 50 l/h

1200 + 100 l/h

1500 + 100 l/h

60 + 5 (C

4x / year

1x / shift

1x / shift

1x / shift

1x / shift

Mask drying

Air temperature drying tunnel

130 + 5 (C

1x / shift

Miscellaneous

None

References

2. NIESSEN F.Room conditions for new linesTVR-217-03-FN-D006, 2003-10-07

3. FOREAU P. Introduction of Ucarcide 250 in the photo resist. 1997-09-22.

4. PENNERS H.J.M. Status report on washing and drying of CMT masks, progress within the 90% direct yield project in Lebring; TVR-657-97-HP-D0096; 1999-09-10

5. PENNERS H.J.M. Design justification of the cmt prototype for ultrasonic mask washing. Proposal for a second generation; TVR-657-97-HP-D0127; 1997-10-07

6. H. PENNERSAuxiliary list for mask washing, screen washing, matricizing and rewashing.TVZ-298-01-HP/D002A, 2001-01-04

7. J.A. NillissenFiltration managementTVR-657-99-JN/D0139, 199-07-15

AMENDMENT RECORD

Revision

Reference

Pages

Description

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First Issue

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Clean and dry

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Dirty mask

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