Designing and implementing a liquid-crystal display (LCD) module for small, handheld applications requires particular attention to issues such as size, viewability, weight, cost, ruggedness, and tolerance of temperature extremes. While available technologies have greatly advanced the LCD's performance in each of these areas, the advances themselves create problems. To avoid these "gotchas" and ensure a smoother design flow, it is helpful to break down the module design stages and examine them in the context of these recently introduced advances. The stages can be loosely defined as the glass design and layout, the driver and interconnection technology, and the backlight as both an optical and mechanical component.
To create a cost-effective module design, the issue foremost in the designer's mind should be how to create a glass layout that will use the least amount of glass area to implement the desired display format (Fig. 1). In addition to the active viewing area of the glass, the other physical constraints of interconnection ledge width, seal width, and the inactive area between the active viewing area and the environmental seal need to be minimized. This must be done without compromising the mechanical/environmental integrity of the display.
Pixel Size: At the heart of the design are the size of each pixel and the number of pixels that are required to present a suitable display to the user. Today's glass manufacturer can fabricate displays with pixel sizes that are smaller than can be suitably viewed by a human. When pixel size is below a threshold of approximately 0.25 mm, the usefulness of the display in the handheld environment begins to degrade. At issue is character size, especially in a single-pixel format. Below this 0.25-mm threshold, multiple pixels must be used to create alphanumeric characters, leading to an increase in hardware and software complexity.
Generally, the larger the pixel, the more readable the information on the display. As a result, in the small, handheld environment, pixel sizes are usually greater than 0.30 mm. This pixel size allows enhanced character readability, and leads to quicker recognition of characters by the user. Thus, the user experiences less fatigue, especially under conditions of high usage.
To provide some aspect of differentiation, some manufacturers have adopted an asymmetric pixel shape for their characters' dot-matrix display formats. They have opted for a pixel that is taller in the vertical dimension than the square pixel. This pixel format is due to the aspect ratio required by the display. Again, a larger display makes it easier for the user. Therefore, if any dimension can be increased, without compromising the quality of the display, it will further aid the user in recognizing the presented information. In these instances, it makes sense to increase only the vertical size of the pixel. Note, however, that the format will make it unsuitable for use as a graphics display, due to the asymmetry of the pixel.
Active Viewing Area: The next step in the design process is to determine the number of pixels necessary for proper display of the desired information. In small, handheld environments, the trend is to put as much information on the display as possible, while keeping it from appearing crowded. In most cases, the display size is limited to no less than two lines of 10 characters, and no more than eight lines of 20 characters, within the graphics area. Icons can either be included in the display, or generated in the graphics area. Of course, in the graphics mode, the character formats are user programmable, thus individual character format is a function of the application.
For the application of eight lines of approximately 14 characters, the total number of pixels in the graphics display will be at least 100 pixels horizontally by 64 pixels vertically. Standard LCD drivers are available which will drive formats of 102 by 65 pixels.
A viewable graphics area of 47.9 by 30.5 mm results when using the square-pixel format and a pixel size, trace, and space of 0.47 mm. The active area of the display is shown in Figure 1.
Note that the active area is much smaller than the actual glass area. This is to accommodate the area needed on the ends of the glass to route the row traces. In small, handheld display applications, only one ledge can be used to interconnect the row and column lines. In our example, 32 row lines must be routed on each edge of the glass. As the number of rows increases, the number of crossover connections between the glass plates must increase, leading to lower reliability and higher cost.
Once the pixel format is chosen, the entire display format should be modeled. When modeling a display format, it is imperative for it to be scaled to actual size, to assess the readability of the information presented. Mylar film, the same material used in the pc-board industry, should be used to evaluate the display formats. CAD packages, such as AutoCAD, are more than adequate for producing a mylar model (Fig. 2).
Display format modeling will save headaches in the future, especially from the marketing and sales, as well as the human aspect of the product. Many projects have failed after the prototypes have been delivered because the utility of the display was compromised due to inadequate pixel and/or display size, or unacceptable information content. It is obvious that, at this end point, time and money have been expended for no useful purpose.
Viewing Area-To-Seal Distance: The next area of consideration is the distance between the active pixel area and the inside edge of the LCD seal (Fig. 3). This distance is usually defined to be approximately 1 mm minimum. However, the actual distance is determined by the bezel opening in the user's housing.
The edge of the bezel opening should be within this area. To properly hide the seal area, the bezel should be placed as closely to the glass surface as possible, while retaining mechanical integrity.
The factor that defines the active area-to-seal distance is the thickness of the glass itself. The thicker the glass, the greater the parallax between the glass surfaces. This presents the end user with the opportunity to view the seal through the upper glass layer. Seeing the edge seal is highly objectionable in most display applications, therefore the LCD module designer must plan accordingly to properly obscure the seal material. As the thickness of the glass decreases, the minimum distance required to obscure the seal is reduced, allowing a smaller distance between the edge of the active area and the seal edge.
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.
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.
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.
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.
Tape Automated Bonding
TAB technology is capable of the finest interconnection pitches. This method eliminates the need for wire bonding as a method of attaching the driver die to the Kapton/copper substrate material.
A process called inner-lead bonding is used to directly connect the pads on the driver dice to the copper traces of the TAB package. After inner-lead bonding has been completed, the driver dice is encapsulated in an epoxy material to maintain the environmental stability of the package. Inner-lead bonding pitches of 0.07 mm are routinely used for this application.
In general, the pitch used to accomplish the inner-lead bonding can also be used in other locations of the TAB circuit. The output portion of the TAB package usually consists of an arrangement of fingers that will make contact with the matching pattern on the interconnection ledge of the glass. Current technology allows this pitch to be as low as 70 µm. In practice, making the attachment pitch larger can enhance the yield of the TAB-to-glass interconnection.
Usually, high-density pitches are necessary to create one-third of a color pixel--either the red, green, or blue portion. The pitch required for color display applications is approximately three times smaller than the pitch that necessary to make the monochrome counterpart.
Chip-on-glass (COG) interconnection technology is quickly becoming a recognized industry alternative to attaching the driver die to the liquid-crystal display (Fig. 5). For a COG implementation, each pad on the driver die is patterned with a gold interconnection "bump" to create a path of conduction from the glass substrate. These bumps are deposited onto the die, and allow a coplanar, conductive offset interconnection path from the driver dice to the glass surface.
Currently, the methods of attaching the dice to the glass involve an anisotropic adhesive, comprising a matrix of conductive gold spheres. The spheres are spaced apart from one another in such a manner that they are only conductive in the vertical axis. However, from a manufacturing standpoint, this attachment process is in its adolescence. High-volume assembly equipment is now becoming available to allow efficient, high-yielding processes to be realized.
COG is an appropriate interconnect technology for many LCDs, as the pitch of the interconnect pads on the dice translates directly to the input pitch of the glass. Current glass-fabrication technology makes it relatively easy to align the dice with the glass traces.
The benefits of COG are many, and include fewer interconnection process steps to produce the assembly. Rather than three steps, as is the case for a pc-board implementation, there is only one step required for COG. Thus, yield is significantly enhanced. However, there are negatives as well. For instance, the glass package size must be increased slightly to accommodate the chip and the fan in of the row and column leads.
LCD Driver Architecture
Having discussed the interconnection aspects of the driver die to the glass, it is time to discuss the electrical functionality of the LCD driver itself. In most handheld display modules, the entire electrical functionality of the chip is contained on a single silicon substrate, including the row drivers, column drivers, controller, and voltage generator.
One of the most important criteria of the LCD driver is its ability to support the required LCD voltage. The quest for lower battery voltage is very much alive in the portable display industry, and has been responsible for the considerable reductions in battery weight and/or the increase in battery life.
Even though the trend toward decreasing battery voltage continues rapidly, the rate of voltage reduction occurring in the available liquid-crystal-fluid materials is not keeping pace. Although the physics of today's fluids changes such that we can create new displays with lower voltage, there is a physical limit that cannot be surmounted once the battery voltage goes too low. This means that higher voltages must be created and supplied by the driver chip, especially with a display module that has wide temperature-range requirements and/or high multiplex rates. As a result, voltage multiplication must be contained within the chip. In some cases--especially in higher-multiplexed display formats--voltage quadrupling or quintupling is necessary for proper operation of the display.
For instance, let's take the example of a 1/32 multiplex display operating from a battery with 1.8 V. The operational temperature of the module must range from -20° to 70°C. This range, with a commonly available fluid, can be realized with approximately 8.5 V. A voltage quintupler is necessary to generate the required LCD driver voltage. With a 2.7-V-minimum voltage supply, a voltage quadrupler is necessary to properly drive the LCD fluid.
As the state of the art in submicron photolithography continues to generate ever smaller structures, more functionality can be placed onto an LCD driver chip. Gone are the days when a separate controller and separate row- and column-driver chips were necessary to realize a display module. In fact, the chip is becoming pad limited before 100% of the silicon functionality is used.
Only the number of outputs that are put into the driver chip limits the functionality of the chip. In many cases, this number is now over 300 outputs per die for a TAB or COG implementation
The typical small. handheld backlight consists of a light pipe, diffuser material, and an LED light source. Generally made of polycarbonate material, the light pipe serves two functions in the display. First, it efficiently disperses the light from the LED sources to illuminate the display. Secondly, it provides mechanical structure for the display glass and pc board (Fig. 6).
The light from the LED sources must be efficiently dispersed throughout the light pipe to provide uniform illumination at the surface of the light pipe. Several techniques are employed to diffuse the light sources so that hot spots and non-uniform area illumination is kept to a minimum. Microstructures are usually molded into the light pipe to diffuse the light into a uniform pattern. Additionally, materials can be placed into the light pipe to diffuse the light as it encounters these particles in the light pipe itself.
In some applications, a diffusing material is placed on the rear surface of the light pipe. The diffusing material acts to redirect the light upward to the surface of the light pipe, where a non-uniform surface treatment further diffuses the incoming light rays.
When the diffusing material is placed on the surface of the light pipe, between the pipe and the display, the incoming light rays are reduced in intensity and scattered at the surface of the diffuser material. In practice, both methods of diffusing the LED light are employed, and application specific as to their relative performance.
Light Pipes Doing Triple Duty: In hand-held display modules, weight is a key consideration. Thus, any dual or triple functionality that can be realized from existing mechanical components is mandated. With this in mind, the light pipe serves to take on a few more functions than just light distribution.
First, the light pipe serves to anchor the glass (Fig. 6, again). Features can be molded into the light pipe, which will capture the glass and hold it in place. This is especially useful in the vibration environment. The mass of the display glass is usually insufficient to deform or break the features clamping the glass to the light pipe. The light pipe provides the necessary support to keep the glass in place during vibration and shock.
The light pipe also is used to anchor the pc board. Using a heat-seal connector type of electronic module, the pc board can be wrapped around to the other side of light pipe. Features are molded into the light pipe, which hold the pc board in place. As with the glass, the pc board is held in place during shock and vibration.
Finally, the light pipe is used to hold the entire display module in place within the housing. Tabs, rings, holes, or other features can be molded into the light pipe to allow alignment of the module into the housing. These features can also serve as positive connection points between the housing and the display module. Special consideration must be given, however, to these points as mounting features.
While the mounting features on the light pipe may be strong enough to hold the display module in place during vibration or shock, they may not be strong enough to prohibit deflection of the display module when impacted by other mechanical components within, or external to, the housing.
Experience shows that the housing must be designed to prevent deformation of, and deflection into, the display module. The most common occurrence is glass breakage due to direct impact of a housing feature on the display module. The display module/housing interface must be adequately designed and modeled to prevent high-mass components from coming into contact with the display module. As an additional precaution, the mounting features of the display module, most notably ring or pin structures, must be modeled to prevent stress fractures from occurring over time. This stress leads to premature fatigue of the display module.