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 Technology
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.
Driver Functionality
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
Backlighting
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.
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