Planar interconnect with electrical alignment indicator

In the manufacture of flat-panel video displays, an assembler must align long rows of closely-spaced electrical signal contacts on a planar electroluminescent panel with mating signal contacts on a flex circuit prior to clamping the panel and flex circuit together. The disclosed technique employs electrically conductive alignment contacts on both surfaces. The alignment contacts are formed via the same process that forms the signal contacts. The alignment contacts are connected to ohmmeters to form circuits that an assembler monitors to determine proper alignment of the surfaces. The use of electrical rather than visual alignment checking makes assembly easy and error-resistant.

FIELD OF THE INVENTION 
The present invention relates to the alignment and coupling of electrical 
signal contacts on a planar substrate, such as a glass electroluminescent 
panel, with signal contacts on another planar substrate, such as a flex 
circuit. 
DESCRIPTION OF THE RELATED ART 
It is a common practice in electronic systems to couple electrical circuits 
on a flexible substrate to electrical circuits on a rigid substrate. 
Signal traces on both the flexible substrate and the rigid substrate are 
terminated in contact pads on the surfaces; each contact pad on a surface 
is meant to be coupled to a mating contact pad on the other surface. 
Connection of the mating signal contacts can be accomplished in many ways, 
such as by bringing them into direct contact, or by interposing an elastic 
element having conductive traces. In the direct pressure contact method of 
coupling, the contact surface of the flexible substrate lies flat against 
the contact surface of the rigid substrate, such that corresponding signal 
contacts are directly touching each other. 
During assembly, before the flexible substrate and rigid substrate are 
fastened together, the flexible substrate signal contacts must be aligned 
with the rigid substrate signal contacts to ensure correct electrical 
connection. With high-density interconnect, in which the contacts are 
small and spaced very close together, the tolerance of this alignment is 
very small. Misalignment by an amount greater than the contact spacing 
results in incorrect connections and incorrect operation of the electronic 
circuits. Therefore, precise alignment of the signal contacts is critical. 
One alignment technique has been used that relies on interlocking features 
on a flex circuit and glass substrate. It is shown in U.S. Pat. No. 
4,289,364, issued Sep. 15, 1981 to Strom, et al., and is entitled "Plasma 
Display Panel Flexcircuit Connection". In this approach, the contact pads 
on the flex circuit are raised above the surface of the flexible polyimide 
material, and the contact pads on the panel are at the bottom of conductor 
pathways which are etched in the glass. The flex circuit contact pads fit 
within the walls of the pathways, and touch the panel contact pads at the 
bottom of the pathways. The flex circuit and panel are "interlocked" in 
much the same way that two forks can be interlocked by meshing their 
tines. 
While this connector achieves excellent alignment, its "interlocking" 
feature subjects the flex circuit contacts to shear stress perpendicular 
to the pathways as the temperature of the assembly changes during 
operation. This stress occurs because polyimide and glass expand and 
contract at different rates as temperature changes. The magnitude of the 
shear stress at points along the surface is a function of, among other 
factors, the width of the flex circuit, and thus is greater with a wide 
flex circuit that with a narrow one. The shear stress created on a wide 
flex circuit could be sufficient to damage either the flex circuit, its 
conductors, or the glass substrate. 
Another alignment technique is disclosed in U.S. Pat. No. 4,645,280, issued 
Feb. 24, 1985 to Gordon, et al., entitled "Solderless Connection Technique 
Between Data/Servo Flex Circuits and Magnetic Disc Heads". There, 
electrical contacts on a thin-film head are coupled to corresponding 
contacts on a flex circuit via a solderless connection. Alignment of the 
contacts is achieved by abutting active or dummy circuit traces against a 
pair of alignment pins or the sides of a single U-shaped alignment pin. 
A shortcoming of that alignment technique is the difficulty of determining 
when the traces and alignment pins actually abut. The difficulty arises 
from the small size of the assembly and the traces. An assembler must peer 
at tiny circuit features and judge their relative positions. This task is 
both time-consuming and error-prone, and therefore is an expensive 
manufacturing step. 
McKiddy discloses a technique for detecting misregistration in U.S. Pat. 
No. 3,859,711, issued Jan. 14, 1981. That technique, which employs plated 
through-holes to form a detection circuit, addresses the problem of 
incorrect alignment of printed circuit board layers. Using that technique, 
unacceptable misalignment can be detected after the printed circuit board 
has been assembled. It cannot be used to align the layers before 
lamination of the layers, because the plated through-holes are formed 
after lamination. Therefore the use of that technique is limited to 
post-assembly testing of the printed circuit boards; it does not aid the 
assembler in aligning the layers to begin with. 
For the interconnection of planar substrates with high-density signal 
contacts, an alignment technique is needed that has very small tolerances 
and does not subject either substrate to life-reducing shear stresses. 
Additionally, for ease of assembly and low manufacturing cost, a technique 
that can be used to aid assembly and post-assembly testing, and that does 
not require intense visual scrutiny, is essential. 
SUMMARY OF THE INVENTION 
The present invention advances the art of planar signal interconnection by 
incorporating electrical alignment indication into the planar interconnect 
to provide a method for precisely and easily aligning corresponding 
electrical signal contacts on mating surfaces. In accordance with the 
principles of the invention, two sets of electrically conductive alignment 
traces are formed on the surfaces of both the flexible and rigid 
substrates. The alignment traces can be formed during the same process 
steps that form the signal contacts, so their relative placement is 
tightly controlled. Each set of traces comprises electrical trace material 
on one surface (called "boundary trace") that bounds an insulative target 
area, and trace material on the other surface (called "indicator trace") 
that fits within the target area when the surfaces are together. The two 
sets of alignment traces are shaped and located such that, when the 
surfaces are brought together and the mating signal contacts are precisely 
aligned, each indicator trace is within its corresponding target area but 
does not touch the corresponding boundary trace material. In this aligned 
position, each boundary and indicator trace form an electrical open 
circuit. When the signal contact misalignment exceeds a tolerance, at 
least one indicator trace forms a short circuit with its corresponding 
boundary trace. The presence of the short circuit, which indicates 
misalignment, can be detected by a device such as an ohmmeter. 
During assembly, an assembler simply monitors a pair of ohmmeters while 
lining up the two substrates. The assembler receives a clear indication of 
alignment when the meters indicate that both sets of alignment traces form 
open circuits. In addition to being easily assembled, the combination of 
the flexible and rigid substrates has improved maintainability. A service 
technician can easily determine whether the surfaces are misaligned by 
checking for shorts between the alignment traces in each set. If either 
set forms a short circuit, the technician can realign the surfaces by 
using the disclosed technique.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
A particular application of the present invention to planar interconnect is 
in the coupling of flex circuits to a glass panel in a flat-panel video 
display of the type used in computer systems. FIG. 1 shows the components 
of a flat-panel display, including a glass electroluminescent panel 1, 
four row-driver flex circuits 2, and four column-driver flex circuits 3. 
The electroluminescent panel 1 is a flat, glass panel coated with an 
electroluminescent material. Over the electroluminescent material runs a 
grid of fine electrical traces (not shown) that direct electrical energy 
from drivers on the flex circuits 2 and 3 to specific points on the panel 
1 for illumination. The flex circuits 2 and 3 contact the panel 1 at its 
edges, and are folded in order to rest flat upon a stiffener 4 and to make 
electrical contact with a controller module 5. The stiffener 4 is attached 
to the panel 1 by adhesive tape 6. Controller module 5 contains circuitry 
that receives an input video signal and creates row- and column-drive 
signals that create a display image that the video signal represents. 
In operation, the flex circuits 2 and 3 have drivers that receive the 
relatively weak drive signals from the controller module 5 and provide 
strong drive signals to the electroluminescent panel 1. The signals from 
the row flex circuits 2 and column flex circuits 3 cooperate to control 
the illumination of picture elements, or "pixels", which are arranged in a 
grid on the panel 1. 
FIG. 2 is a side view of one of the column flex circuits 3 of FIG. 1 in 
contact with the panel 1 of FIG. 1. The column flex circuit 3 has two rows 
of electrical signal contacts 10 that are aligned and in contact with two 
corresponding rows of electrical signal contacts 11 on the panel 1. The 
panel and flex circuit may be held together in this position by retaining 
pressure beams clamped together with C-clamp fasteners, as in U.S. Pat. 
No. 4,997,389, issued Mar. 5, 1991 to Doumani, et. al., and entitled 
"Planar Connector System With Zero Insertion Force and Distributed 
Clamping". The '389 patent is assigned to Digital Equipment Corporation, 
and is incorporated herein by reference. All of the row flex circuits 2 
and column flex circuits 3 of FIG. 1 are aligned and held in place in this 
fashion. 
FIG. 3 shows one column connection site with two rows of rectangular signal 
contacts 11 along an edge of the panel 1. The signal contacts 11 are for 
connection with the signal contacts 10 on the flex circuit 3 of FIG. 1. 
Each signal contact 11 is the termination of an electrical signal trace 20 
on the surface of the panel 1. The panel signal traces 20 form the columns 
of the pixel grid. There are also row signal traces on the panel that form 
the rows of the pixel grid; these are not shown in FIG. 3. 
At opposite ends of the rows of panel signal contacts 11 are first and 
second boundary traces 21 and 22 connected respectively to first and 
second boundary test points 23 and 24 by electrical traces 25 and 26. The 
test points 23 and 24 are landing pads of electrical trace, to which the 
leads of an ohmmeter or other continuity-measuring device may be attached. 
The signal traces 20, boundary traces 21 and 22, signal contacts 11, and 
test points 23 and 24 are made of a conductive metal, such as plated 
copper, and formed upon the surface of the panel 1 by a process that is 
known to the art. In the high-density interconnection scheme of the 
present embodiment, the connection site is about 7 inches wide, and there 
are two rows of panel signal contacts with 256 equally spaced contacts per 
row. Those signal traces 20 that extend from the row of signal contacts 11 
closer to the edge of the panel 1 are interleaved with the other row of 
panel signal contacts 11. 
The boundary traces 21 and 22 are split rings having gaps 27. The boundary 
traces 21 and 22 surround first and second target areas 28 and 29. In the 
embodiment shown in FIG. 3, both target areas 28 and 29 have diameter D; 
their diameters may differ in other embodiments. Although in FIG. 3 the 
boundary traces 21 and 22 are shown at the ends of the rows of signal 
contacts 11, one skilled in the art will realize that they may be located 
in other positions on the panel 1 without sacrificing alignment accuracy; 
in practice they are widely spaced apart from each other in order to 
minimize the effect of small positioning errors that occur during 
assembly. Wide spacing exists when they are at opposite ends of the rows 
of panel signal contacts 11, as shown in FIG. 3. Boundary traces 21 and 22 
should be large enough for an assembler to see easily and small enough to 
fit between adjacent connection sites on the panel 1; additionally, it is 
not necessary that they be the same size. In this embodiment, they measure 
approximately 0.25" in outside diameter. 
FIG. 4 is a surface view of the flex circuit 3 with two rows of 
rectangular, parallel flex circuit signal contacts 10. Flex circuit 3 
contains driver circuitry, not shown, on the surface of a flexible 
polyimide substrate, made of a material such as "Kapton".TM., manufactured 
by duPont. Each flex circuit signal contact 10 is an extension of a flex 
circuit signal trace 30 that carries signals from the driver circuitry to 
one of the flex circuit signal contacts 10. There are first and second 
indicator traces 31 and 32 at opposite ends of the rows of flex circuit 
signal contacts 10. In this embodiment, indicator traces 31 and 32 are 
shaped as disks of diameter d, to conform to the shape of boundary traces 
21 and 22 of FIG. 3. Other shapes of boundary traces 21, 22, and indicator 
traces 31, 32 are possible, as long as indicator traces 31 and 32 fit 
within boundary traces 21 and 22 when the flex circuit 3 is aligned and 
coupled with the panel 1. 
Also shown in FIG. 4 are first and second indicator test points 33 and 34 
connected respectively to indicator traces 31 and 32 by first and second 
electrical traces 35 and 36. The electrical traces 35 and 36 fit within 
the gaps 27 in the boundary traces 21 and 22 on the panel 1, so that the 
traces 35 and 36 do not touch their respective boundary traces 21 and 22 
when the flex circuit 3 and panel 1 are aligned. The indicator test points 
33 and 34 are landing pads of electrical trace material. 
FIG. 5 shows the dimensions and spacing of the flex circuit signal contacts 
10, and of the panel signal contacts 11. The spacing RS between rows is 
0.05". The signal contacts 10, 11 have height SCH of 0.12", width SCW of 
0.018", and laterally spacing SCS of 0.008". The panel signal traces 20, 
30 have width TW of 0.003". Those panel signal traces 20, 30 that extend 
from the lower row of panel signal contacts 10, 11 are interleaved with 
the upper row of panel signal contacts 10, 11. The spacing TCS between 
those panel signal traces 20, 30 and adjacent upper-row signal contacts 
10, 11 is 0.0025", which is equal to one-half the difference between the 
contact spacing and the trace width. The spacing TCS is the tolerance in 
the alignment between the flex circuit 3 and the panel 1, because 
mis-alignment exceeding this value could result in a short circuit between 
a flex circuit signal contact 10 and an adjacent panel signal trace 20, or 
between a panel signal contact 11 and an adjacent flex circuit signal 
trace 30. 
Indicator traces 31 and 32 of FIG. 4 are located in positions that 
correspond to the positions of the target areas 28 and 29 of FIG. 3, such 
that the first and second indicator traces 31 and 32 fit within the first 
and second boundary traces 21 and 22, respectively, when the flex circuit 
3 and panel 1 are placed together with their respective signal contacts 10 
and 11 in aligned contact. 
FIG. 6 illustrates the flex circuit 3 superimposed on the panel 1 in 
precise alignment, indicating the relative positions of the target and 
indicator traces 21, 22, 31, and 32. The flex circuit signal contacts 10 
are aligned with the panel signal contacts 11 and are in direct contact 
with them. The indicator traces 31 and 32 are aligned with target areas 28 
and 29, respectively, and are concentric with boundary traces 21 and 22, 
respectively. The diameter d of the indicator traces 31, 32 is less than 
the inside diameter D of the boundary traces 21, 22. Further, if the 
maximum lateral tolerance to maintain signal contact alignment is 
.DELTA.1, then d, D, and .DELTA.1 must obey this relationship: 
EQU 1/2(D-d)&lt;.DELTA.1 (1) 
In the embodiment shown in FIGS. 1 through 6, the maximum lateral alignment 
tolerance .DELTA.1 is equal to the trace-to-contact spacing TCS of 
0.0025", as described above in reference to FIG. 5. Therefore, the 
diameters D and d are constrained by: 
EQU D-d&lt;0.005" (2) 
Equation (2) says that the difference between the diameters d and D cannot 
exceed 0.005" in this embodiment. What is important is the relative values 
of D and d, not their absolute values. D and d may be any reasonable 
values that satisfy equation (2). Also, if the diameters of the first and 
second target areas 28, 29 are different, then equation (2) must be 
satisfied for both the first boundary and indicator traces 21, 31 and the 
second boundary and indicator traces 22, 32. 
The preceding is a description of the placement of the alignment traces 
upon the electroluminescent panel 1 and the flex circuit 3. During 
manufacturing, an assembler uses the alignment traces to align the panel 1 
and flex circuit 3 prior to clamping them together. The alignment process 
is described below. 
First, the assembler connects an ohmmeter (or similar circuit that can 
detect an open circuit and a short circuit) to the first indicator test 
point 33 and its corresponding boundary test point 23. He then connects 
another ohmmeter to the second indicator test point 34 and second boundary 
test point 24. 
The assembler then places the flex circuit 3 upon the panel 1 such that the 
flex circuit signal contacts 10 face corresponding panel signal contacts 
11, and the indicator traces 31 and 32 are approximately concentric with 
their corresponding boundary traces 21 and 22. This is an initial rough 
position. If the flex circuit 3 is made from a clear material, as in this 
embodiment, the assembler can see when both indicator traces 31, 32 are 
approximately concentric with boundary traces 21, 22. If the flex circuit 
3 is opaque, this initial placement could be accomplished indirectly, for 
example by employing registration marks on the panel 1 to which the 
corners of the flex circuit 3 can be aligned. 
From this initial position, the assembler then moves the flex circuit 3 
until both ohmmeters indicate an open circuit. In this position, the 
indicator traces 31 and 32 are entirely within their corresponding 
boundary traces 21 and 22; all of the flex circuit signal contacts 10 are 
in direct contact with the corresponding panel signal contacts 11; and the 
flex circuit 3 is aligned with the panel 1 within tolerance. The assembler 
then clamps the assembly together to maintain this position during 
operation. These alignment and clamping steps are repeated for all row 
flex circuits 2 and column flex circuits 3. 
FIGS. 7, 8 and 9 show an alternative embodiment of the present invention. 
The difference between the embodiment of FIGS. 7, 8 and 9 and that of 
FIGS. 3, 4 and 5 is that the second boundary trace 22' is formed as a pair 
of rectangular bars, and the second indicator trace 32' is also a 
rectangular bar that fits within a rectangular second target area 29'. 
This embodiment has the advantage that the combination of the second 
boundary trace 22' and second indicator trace 32' only constrains 
alignment in the direction perpendicular to the line between the first 
boundary trace 21' and the second boundary trace 22'. Therefore, this 
embodiment is more tolerant of variation in the placement of the traces 
22' and 32' relative to traces 21' and 31', respectively. 
In FIG. 7, the second boundary trace 22' consists of a pair of parallel 
rectangular bars defining a second target area 29' of width H. In FIG. 8, 
the second indicator trace 32' is a rectangular bar, of width h, that fits 
within the second target area 29'. FIG. 9 shows the flex circuit 3' and 
the panel 1' in proper alignment, with first and second indicator traces 
31' and 32' aligned with first and second target areas 28' and 29', 
respectively. 
In the embodiment of FIGS. 7, 8 and 9, the dimensions H and h must satisfy 
the following equation so that signal contacts 10' from one row on the 
flex circuit 3' do not inadvertently touch signal contacts 11' in the 
other row on the panel 1': 
EQU 1/2(H-h)&lt;.DELTA.w (3) 
.DELTA.w in equation (3) represents the spacing between the rows of 
contacts 10', which is 0.05" in this embodiment. Therefore, the dimensions 
H and h are constrained by: 
EQU H-h&lt;0.1" (4) 
Equation (4) says that the difference between the dimensions h and H cannot 
exceed 0.1" in this embodiment. What is important is the relative values 
of H and h, not their absolute values. H and h may be any reasonable 
values that satisfy equation (4). 
The electroluminescent panel 1, 1' and flex circuit 3, 3' may be 
disconnected and reconnected non-destructively throughout the life of the 
assembly, because of the solderless, zero-insertion-force connector. A 
technician can easily reconnect the assembly using the alignment method 
described. Additionally, a technician can easily detect if the assembly is 
misaligned by checking for shorts between each boundary test point 23, 24 
and its corresponding indicator test point 33, 34. 
Although the embodiments that have been described consist of direct contact 
connections between a glass panel and a flex circuit, the invention is 
applicable to interconnections between any generally rigid substrate (such 
as a circuit board, glass display panel, integrated circuit package, etc.) 
and a generally flexible substrate (such as flexible cable, TAB tape, 
etc.). Various changes in the details, geometry, and arrangement of the 
components may be made by those skilled in the art within the principle 
and scope of the invention as expressed in the following claims.