fpd mask washing
DESCRIPTION
mask washTRANSCRIPT
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
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Reference
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Cooling time [min]
Temperature [C]
Iron
Invar
Degaussing
Drying
Draining
Showering
US
vibration
(optional)
Showering
Cooling
down
Clean and dry
mask
Dirty mask
Showering
Showering
USwashing
Empty
Empty