Custom VFD Glass Design Guidance
Vacuum Fluorescent Display (VFD) technology with its inherent flexibility can be utilized very effectively in unique display formats for specific customer applications. VFD is the preferred medium in many consumer product display applications and customization allows the display to be targeted to specific applications and/or markets by using instantly recognizable icons or images. Cost savings are inherent in a custom design because the user does not specify and therefore does not pay for display functions they will not use. The following criteria need consideration when designing your VFD.
1. VFD Operation
The VFD is composed of three basic electrodes; the Cathode (Filaments), Anodes (Phosphor) and Grids under a high vacuum condition in a glass envelope.
The Cathode consists of fine tungsten wires which are coated by alkaline earth metal oxides which emit electrons.
The Grids are a thin metal mesh which control and diffuse electrons emitted from the Cathode.
The Anodes are conductive electrodes on which the phosphor is printed to indicate characters, icons or symbols.
Electrons emitted from the Cathode are accelerated with positive potential applied to both Grid and Anode, which upon collision with the Anode excites the phosphor to emit light. The desired illuminated patterns can be achieved by controlling the positive or negative potentials on each Grid and Anode. This voltage can be as low as 10VDC.
Fig.1 Basic VFD Structure
|1.||Glass Substrate (Anode Plate)||10.||Getter|
|2.||Conductive Layer||11.||Face Glass (Cover Glass)|
|3.||Anode (Base)||12.||Spacer Glass|
|4.||Insulation Layer||13.||Evacuation Tube|
|5.||Phosphor (Display Pattern)||14.||NESA (or ITO) coating|
|6.||Conductive Paste||15.||Lead Pin|
|7.||Grid Mesh||16.||Mold Resin|
|8.||Conductive Frit Glass||17.||Solder|
|9.||Filament (cathode)||18.||Frit Glass|
2. VFD Construction
Itron VFD has several methods of construction. The basic model is that of the frame type construction.
The other variants are specific to the product type and are described in more detail in the relevant application notes associated with CIG (Chip in Glass Driver), Active Matrix and Rib Grid
The Grid Rim, Filament Support and Lead Pins are provided on a single metal frame. The ends of the Grid Rim are extended to the outside of the envelope, and are formed as lead pins for Grid. The Anode leads are extended into the envelope to connect with the pads which are placed on the glass substrate. Both ends of the filament are welded to the Filament Support and Anchor with the appropriate tension.
The Frame is assembled with the Face Glass and Glass Substrate (Anode Plate). The Lead Pins are tinned and formed into a suitable shape for PC Board assembly.
FRAME-Types require press formed metal dies for construction. They offer good production yield and high reliability against various environmental conditions.
A hybrid of this construction mounts the grids directly on the glass substrate which allows complex grid patterns.
Fig. 2 Frame Type Construction
3. Package Size, Display Area and Viewing Angle Relationship
Certain mechanical constraints imposed by the drive conditions, construction and materials need understanding before finalizing your initial ideas.
The preferred combinations of package width (W), display area and viewing angle are referred to in Table 1.
After the package width is selected, standard packages can be selected from Table 2.
The length of the display area can be calculated as: P1=L-2xS
The values may vary depending on the package length or construction of the VFD. Please consult us for details.
Full custom packages are possible but it is generally more economical to use standard packages where possible.
Fig.3 Package Dimensions
Fig.4 Viewing Angle
|Package Length (L)|
The table shows the standard packages size, suitable and efficient for the automated assembly process.
Since package sizes other than from the above table are available, please contact us.
4. Position and Dimension of Evacuation Tube
The standard VFD has an evacuation tube which is sealed once the vacuum is established. It’s location can be varied to suit the space constraints of the design. An rear mounted metal ‘tipless’ seal can be utilised at extra cost where space does not permit a tube to be used.
Fig.5 Evacuation Tube Position
|Custom||T1,T3 or T4|
|D: Tube Diameter F: Tube Length H: Dimension from the center of the tube to the substrate edge.|
5. Pattern Design Rules
The standard printed phosphor and grid structure dimensions are limited by various technology constraints.
Please consult the application note on fine pattern Rib Grid and Active Matrix VFDs for details of alternate strategies.
Minimum Width of Each Segment.
0.18mm MIN for Standard Green
0.20mm MIN for other colors
Minimum Gap between Each Segment.
- Same color segments placed on the same anode base.
- Different color segments placed on the same anode base.
- Same color segments placed on the separate anode base.
- Different color segments placed on the separate anode base.
Lighting up large areas of the display or flashing certain segments may decrease the luminous uniformity of the display. Restrictions on minimum distance between anodes as follows
- Different anodes close together
- Line formed anode and another anode close together
Standard Grid Mesh Separation
Please consult us for the size
of the overlapped area.
Standard Grid Mesh Separation
This figure may vary based
on the pattern.
6. Package Thickness and Standard Dimensions of Lead Pins
Table 4 relates package width to package thickness and options for lead pin construction. The standard lead pitches are 2.54mm and 2.0mm. If you need any different lead lengths, please consult us.
|Straight Leads||Right Angle Leads||
Fig.6 Standard Lead Pin
|25.0||6.2, 6.7, 8.0|
Table 5 shows the options for welded wire lead assembly where 0.5mm diameter wire pins are welded to frame leads when longer pin length is needed.
|Straight Leads||Right Angle Leads||
Fig.7 Welded Wire Lead Pin
7. Drive Scheme and Power Supply
Noritake Itron VFD are available with or without integrated Chip In Glass drive circuits. If the CPU controlling the display does not provide VFD driver outputs, we recommend selecting a CIG drive scheme when the number of grid and anode lead pins exceeds 40. The CIG driver allows direct static drive at 12VDC for up to 144 segments without the need to multiplex the display and provides a direct micro-controller clock serial interface. Please consult the application notes on VFD Technology and Chip In Glass VFD for further details.
The Cathode (Filaments) are normally driven by an AC supply of 2Vac to 9Vac which can be generated by a transformer or transistor bridge circuit to provide a current of 22mA to 200mA depending on display size. The anode and grid voltage require a supply of 12V for static drive and 24V to 70V with a current capability of 10 to 60mA for multiplex drive schemes dependent upon the number of grids scanned. Please advise the voltages that are available in your system.
8. Selecting Phosphor Colours
The standard phosphor colour is green which can be complemented by a range of other phosphor colours shown in Table 6 to provide a bright multi-colour display. The names of the phosphors were changed in January 2000.
If by definition, Green (Blue Green) is rated 100%, each of the colors when driven under the same conditions would be rated as shown in the brightness ratio of Table 6. Although there are different brightness characteristics between phosphors, this is not always readily apparent because a sensitivity of human eyes to brightness varies with color wavelength. Therefore, legibility may not be affected. In a multicolor display specific brightness balance can be achieved by controlling voltages and/or duty factor of each color.
|Name of Color||CIE color coordinate (TYP)||Brightness Ratio|
|New Name||Previous Name||X||Y||%|
|Light Blue (lt.B)||Neo Blue||0.18||0.19||10|
|Light Greenish Blue-N (lt.G.B-N)||Sky Blue(Blue)||0.18||0.27||20|
|Bluish Green (B.G)||Sky Blue 2||0.20||0.40||45|
|Green (G)||Blue Green||0.24||0.41||100|
|Vivid Green (vv.G)||Emerald
|Yellowish Green (Ysh.G)||Neo Green||0.28||0.62||20|
|Yellow Green (Y.G)||Lemon||0.38||0.57||40|
|Greenish Yellow (Gsh.Y)||Yellow||0.47||0.51||30|
|Yellowish Orange (Ysh.O)||Amber||0.53||0.47||30|
|Reddish Orange (Rsh.O)||Red||0.64||0.36||10|
Fig 8 CIE Chromatocity Diagram
Fig 9 Spectrum of Colour Phosphor Material
Note: The peak wave length of each color may vary and the figures
in the above are for reference only.
9. Environmental Considerations
TThe operating temperature range for standard VFD with frame construction is -40C to +85C.
Please submit the expected storage temperature and humidity range and vibration data.