Seal Width: The next display feature requiring design consideration is the seal area. The seal provides a number of attributes essential to the proper operation of the display. First, it is the mechanical link between the upper and lower glass plates. While the vibrational aspects of the display may be such that the seal does not get over-stressed, the interface is critical to the environmental stability of the display. In most applications, a seal width of 1 mm is adequate.
Second, the seal protects the liquid-crystal material from external environmental effects. In applications with extended operation in humid conditions, the seal width must be increased.
Third, the seal keeps the two plates of glass together at a precise spacing. This is critical to the proper operation of the display, as the internal cell gap must be maintained across the entire surface of the display (Fig. 3, again).
Contact Ledge: The interconnection contact ledge connects the LCD glass to the external driver circuitry. The width of the contact ledge determines the mechanical integrity of the interconnection medium in the glass. This means that the ledge must be of sufficient width to ensure stable contact with the interconnecting circuitry.
Various materials are available to connect between the electronic driver circuitry and the LCD glass assembly, including elastomeric, heat seal, and various forms of tape-automated-bonding (TAB) connectors. For elastomeric connectors, this ledge width should be at least 3 mm; for heat-seal connectors, flex, or TAB, the ledge width should be at least 2.5 mm wide. The contact pitch for the elastomeric connectors should be no less than 0.5 mm. For heat-seal connectors, the contact pitch should be no less than 0.25 mm, and for flex and TAB circuits, the pitch should be no less than 0.12 mm. TAB circuits can reliably connect pitches below 0.07 mm.
Manufacturers of interconnection materials usually specify the required ledge width and contact pitch to be used with their products. In most cases, these dimensions should not be compromised. This will preserve the integrity of the interconnection method and keep reliability as high as possible. Interconnection techniques will be discussed in more detail below.
Glass Thickness
Now that the basic building blocks of the display have been evaluated and selected, the display glass assembly can be accurately modeled. At this point, the mechanical designer must evaluate the environment in which the display must perform.
Standard glass thicknesses of 0.55, 0.7, and 1.1 mm exist to satisfy the requirements of various display applications. The structural integrity of the display is directly related to the thickness of the glass used. Of course, if structural support can be given to the glass, then the glass thickness can be reduced. Additionally, the surrounding environment must be considered. Also, the designer must take into account the possibility of damage to the display module caused by shock or vibration.
Note: As the glass gets thinner, the cost increases. Glass and glass-coating cost is a significant factor, and as the glass becomes slimmer, the yield is reduced, causing costs to rise.
Polarizer Selection
The environmental characteristics of an LCD are profoundly impacted by the selection of polarizer material. Many grades and types of polarizers are available today to produce a wide variety of looks and environmental capabilities.
In most hand-held display applications, high-quality, environmentally stable polarizers are used. When designing the display module, the operational/storage environment of the display will dictate the polarizer to be used. Cost must become secondary to performance. For example, a polarizer used in an office product will not see the same temperature extremes as those encountered in a cellular-phone environment. The polarization efficiency can be increased in the office environment, reducing the cost.
With this trade-off in mind, the system designer must be aware that improper polarizer selection can unnecessarily increase the cost or degrade the optical performance of the display.
Wiring And Interconnections
A number of technologies are available to provide electrical drive signals to the display. They include pc boards, heat-seal connectors, flexible circuit-board material, and TAB technology. The first of these, the pc board, is the most cost-effective method of providing signals to the display (Fig. 4). Whenever possible, a pc board, preferably using a thin material to reduce weight, is used as the base material for the interconnection of all electronic components. The pc board can be designed to accommodate a number of driver types and interconnection styles, and can be used as the structural support and mechanical fixturing for the display module.
Pc-Board Trace Resolution: To produce the most cost-effective module, the circuitry required to properly drive the display must be placed entirely on the pc-board substrate. Thus, the smallest components are used, as well as the most efficient interconnection methods between the driver and the output pins.
To this end, the trace resolution of the pc board should be kept to the minimum possible widths. In most high-volume applications, trace and space widths are kept to approximately 0.125 mm. Although further reduction in trace and space widths can be realized by some suppliers, the highest degree of manufacturability comes from pc boards that do not violate this basic size constraint. In most applications, this trace and space width will be adequate to attach most of the common LCD drivers to the pc board. Higher pin densities require more-advanced and expensive interconnection techniques.
The Chip-On-Board Process
The enabling technology for low-cost electronic modules is chip on board (COB). This technology, used in various forms since the mid 1970s, attaches the bare silicon die directly to the pc board. The die is then wire bonded to the board to create the interconnections between the driver and the display. After wire bonding, the die and wire bonds are protected with an epoxy encapsulation. As a rule, standard pitch die consisting of between 100 to 150 outputs can be easily wire bonded to the pc-board substrate.
The COB packaging method eliminates the requirement for a physical package around the die. The elimination of the package translates directly into cost savings to the module. Additionally, the use of COB allows a smaller interface circuit board to be used--again translating directly into cost savings on the module, and size reduction of the end item.
Heat-Seal Interconnection
When using a heat-seal interconnection from the pc board to the LCD glass assembly, a critical design factor is that the pitch of the glass must equal the pitch of the pc board. Slight variations in pitch can be accommodated on either the glass or the pc board. However, neither the pc board nor the glass can be too wide because the overall module width must be maintained between components.
Both the glass and the pc board must have enough interconnection length to attach an interface connector. In most cases, the interface connector consists of graphite traces on a polyester carrier, which is bonded to both the glass and pc board with an anisotropic conductive adhesive.
Various manufacturers of this material are available. They have generated the data necessary to ensure that the assembly parameters of bonding time, pressure, and temperature produce an environmentally sound bond that will last for the service life of the product.
The heat-seal conductors can be fabricated in pitches that are slightly smaller than the pitches realizable on a pc board. Both the pc board and the glass must have sufficient interconnection trace length to allow the heat-seal connector to be bonded properly to both substrates.
Reprints
