Liquid crystal cells – elements and systems – Nominal manufacturing methods or post manufacturing...
Reexamination Certificate
1999-03-30
2002-12-24
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Nominal manufacturing methods or post manufacturing...
C349S149000, C349S150000, C349S151000, C349S152000
Reexamination Certificate
active
06498636
ABSTRACT:
TECHNICAL FIELD
The present invention relates, generally, to liquid crystal display assemblies and, more particularly, relates to miniature liquid crystal display assemblies constructed to reduce residual stresses.
BACKGROUND ART
In the recent past, substantial research and development resources have been directed toward small scale Liquid Crystal Display (LCD) and light valve technologies. These high information content, miniature LCD assemblies enable enhanced availability of graphics, data and video information for employment in high resolution projection displays, such as a reflective LCD projectors, SXGA formats (1,280×1,024 pixel resolution) and even HDTV formats (above 1,000 line resolution), or the like.
Reflective LCD projectors, in particular, are highly desirable since they offer the brightness of traditional three-lamp front-projection systems in combination with the high resolution of an LCD panel. At the heart of these optical engines is reflective liquid crystal on crystalline silicon light valve technology which, when combined with sophisticated optical architecture and the appropriate electronic interface, enables very high resolution, high brightness, large screen displays.
One problem associated with both transmissive and reflective-type LCD panels assemblies is bowing or warpage of individual panels caused by residual stresses acting upon the components during operation. This is particularly noticeable in reflective-type LCD panels which have increased flatness requirements due to the nature of the reflective surface of the die. For example, thermal expansion characteristics, as well as lattice mismatching, can generate significant stresses in the underlying substrate material (the silicon), therein causing significant bowing of the mirrored surface. The bowing, which translates to a non-planarity of the surface, causes both (1) a non-uniform thickness of the liquid crystal layer between the bowed reflective surface and the planar transmissive top layer, and (2) variations in the path length of the reflected light from different parts of the element, and of the array. These effects compromise the electro-optic properties of the elements and/or array.
Typically, a primary source of these residual stresses originate from the different materials and composites of the LCD panel having different coefficients of expansion. This is best shown in
FIGS. 1 and 2
which illustrate a flex circuit device
18
electrically coupling a conventional small scale LCD assembly
20
to an electronic interface (not shown). The LCD assembly
20
includes a die
21
having a pixel array
22
which is generally composed of rows and columns of electrically conductive pathways each forming an individual pixel (not shown). Each pixel can be individually changed to an “on” condition by selecting the appropriate row and column of pixel array
22
. Positioned around or concentrated on one end of the pixel array are a plurality of die bond pads
23
which are internally connected to the pixel array
22
to enable operational control thereof. Selection of the appropriate pixel is controlled by control circuitry, either included within the die
21
or external to the die
21
. In either configuration, external control signals may be used to control the functions of the die
21
.
A transparent glass plate
24
is typically placed over the die
21
and the pixel array
22
, such that a portion of the glass plate
24
overhangs the die
21
. The glass plate
24
is usually affixed to die
21
through an adhesive seal
25
which together cooperate to define a sealed volume encompassing the pixel array
22
. This sealed volume is then commonly filled with a solution
26
of Polymer Dispersed Liquid Crystals (PDLC). To facilitate grounding or the application of a charge across the face of the glass plate
24
, a conductive coating
37
may be deposited over the undersurface
28
thereof.
The die
21
is typically rigidly or semi-rigidly mounted to a substrate
27
for mounting support and heat conductive dissipation for the die. A conductive adhesive
29
(FIG.
2
), such as a conductive epoxy or a thermally conductive non-adhesive grease, is generally applied to the undersurface of the die
21
to affix the die directly to the top surface of the substrate
27
. Accordingly, a heat conductive pathway is created directly between the die and the substrate to dissipate heat generated by the die.
The flex circuit
18
includes a plurality of flex circuit bond pads
30
which are typically wire bonded to the die bond pads
23
through bonding wires
31
. More recently, a distal ringed coupling portion
32
of flex circuit
18
is adhesively or fixedly mounted to the top surface of substrate
27
for support thereof. Finally, a glob coating
33
is applied to die
21
, substrate
27
and the distal end of flex circuit
18
. The glob coating
33
(
FIG. 3
) further normally encapsulates the bonding wires
31
and the die and flex circuit bonding pads
23
and
30
without obscuring a view of the pixel array
22
through the glass plate
24
.
As previously indicated, one important aspect in the proper operation of these small scale LCD or light valve assemblies is the maintenance of proper distance uniformity (preferably about 2-4&mgr;m) between the pixel array and the undersurface
28
of the glass plate. Variances in the separation of the glass plates may often times cause the pixel array to function improperly or cause operational failure.
Conventional rigid display device constructions, for example, often warp during operation since the substrate
27
, the glass plate
24
and the silicon die
21
are all composed of materials or composites having different coefficients of expansion. The individual components of the LCD assembly, therefore, often expand at different degrees and rates. Further, depending in part upon the construction processes, such as the adhesive curing techniques, significant residual stresses may be induced upon the cell. Eventually, in severe instances, the glass plate
24
may delaminate from the die
21
. At a minimum, these internal stresses cause optical defects such as variations in color uniformity and fringes, and variations in the cell gap thickness may cause optical shadows.
One particular cause of residual stresses is the formation of an electrical connection between the transparent glass plate
24
and a circuit
36
of the flex circuit
18
which may be employed for applying a charge or the like of the glass plate. A conductive coating
37
of indium tin oxide is typically deposited over the undersurface
28
of the glass plate
24
to facilitate electrical connection therewith. As best viewed in
FIG. 3
, an indium solder bump or tab
38
is pre-bonded to the indium coating
37
to enhance electrical coupling therewith. A conductive kovar slug
40
is positioned between the solder tab
38
and the circuit
36
which in turn is bonded therebetween using a conductive epoxy
41
or the like.
While this approach sufficiently electrically couples the transparent glass
24
to the flex circuit
18
, the electrical connection is substantially rigid in nature. Hence, the epoxies
41
, the kovar slug
40
and/or the indium solder tab
38
expand or contract during operation thereof, residual stresses are imparted upon the transparent glass plate by the rigid connection to the circuit
36
, which in turn is fixedly mounted to the rigid substrate
27
. For example, upon curing of the epoxy
41
, shrinkage occurs which imparts a residual stress upon the glass
24
.
Accordingly, there is a need to provide a LCD assembly which minimizes residual stress induced upon the cell.
DISCLOSURE OF INVENTION
The present invention provides a connection assembly adapted to electrically couple a transparent plate of a liquid crystal display device to an operating subsystem. The connection assembly includes an elongated flexible tape member having an elongated metallic circuit. This metallic circuit includes a main circuit portion fixedly mounted to the tape member, and a lead terminal.
Mathew Ranjan J.
Murray G. Cade
Beyer Weaver & Thomas LLP
Chung David
National Semiconductor Corporation
Parker Kenneth
LandOfFree
Apparatus and method for substantially stress-free... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Apparatus and method for substantially stress-free..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Apparatus and method for substantially stress-free... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2991989