U.S. patent application number 10/322113 was filed with the patent office on 2004-06-17 for bonding flexible film circuit and electronic circuit board.
This patent application is currently assigned to Xerox Corporation. Invention is credited to Brick, Jonathan R., Karandikar, Bhalchandra M., Schmachtenberg, Richard III.
Application Number | 20040112528 10/322113 |
Document ID | / |
Family ID | 32507224 |
Filed Date | 2004-06-17 |
United States Patent
Application |
20040112528 |
Kind Code |
A1 |
Karandikar, Bhalchandra M. ;
et al. |
June 17, 2004 |
Bonding flexible film circuit and electronic circuit board
Abstract
Two circuit bearing substrates, such as a rigid electronic
circuit board and a flexible circuit film, each having large arrays
or multiple arrays of surface contact pads or traces, are
mechanically and electrically joined without relocating the
substrates during the bonding process by using a bonding apparatus
with multiple working tools or bond shoes. The bonding apparatus
includes a fixture for holding the circuit bearing substrates with
a joining layer between them, and two or more independently
operated heated bond shoes that compress different portions of the
circuit bearing substrates with the joining layer, such as a layer
of anisotropic conductive film.
Inventors: |
Karandikar, Bhalchandra M.;
(Tigard, OR) ; Brick, Jonathan R.; (Tualatin,
OR) ; Schmachtenberg, Richard III; (Aloha,
OR) |
Correspondence
Address: |
Patent Documentation Center
Xerox Corporation
Xerox Square 20th Floor
100 Clinton Ave. S.
Rochester
NY
14644
US
|
Assignee: |
Xerox Corporation
|
Family ID: |
32507224 |
Appl. No.: |
10/322113 |
Filed: |
December 17, 2002 |
Current U.S.
Class: |
156/306.6 ;
156/583.1 |
Current CPC
Class: |
B29C 66/41 20130101;
B29C 66/91212 20130101; H05K 3/323 20130101; H01R 43/00 20130101;
B29C 66/81463 20130101; B29C 66/1122 20130101; B29C 66/81427
20130101; H05K 3/3494 20130101; H05K 3/361 20130101; B29C 66/8322
20130101; B29C 66/8167 20130101; B29C 66/91231 20130101; B29C
66/472 20130101; H01R 4/04 20130101; B29C 65/305 20130101; B29C
66/3452 20130101; B29C 65/18 20130101; H01R 12/62 20130101; B29C
66/346 20130101; B29C 66/8242 20130101; B29C 66/91421 20130101;
H05K 2203/0195 20130101 |
Class at
Publication: |
156/306.6 ;
156/583.1 |
International
Class: |
B30B 005/00 |
Claims
We claim:
1. A bonding apparatus for electrically joining a first circuit
bearing substrate having a first array of conductive surface traces
to a second circuit bearing substrate having a second array of
conductive surface traces, the bonding apparatus comprising: a
fixture having a fixture surface; first and second bond shoes; and
one or more tool presses connected to the first and second working
tools for selectively pressing the first bond shoe toward a first
portion of the fixture surface and for selectively pressing the
second bond shoe toward a second portion of the fixture
surface.
2. The bonding apparatus of claim 1, wherein: a first tool press is
connected to the first working tool; a second tool press is
connected to the second working tool; the first tool press has a
tacking mode of operation in which the first tool press presses the
first bond shoe toward the first portion of the fixture surface
with a tacking force; and the second tool press has a bonding mode
of operation in which the second tool press presses the second bond
shoe toward the second portion of the fixture surface with a
bonding force, which bonding force is greater than the tacking
force.
3. The bonding apparatus of claim 2, wherein: the first tool press
additionally has a bonding mode of operation in which the first
tool press presses the first bond shoe toward the first portion of
the fixture surface with the bonding force; and the second tool
press additionally has a tacking mode of operation in which the
second tool press presses the second bond shoe toward the second
portion of the fixture surface with the tacking force.
4. The bonding apparatus of claim 1, wherein the first and second
portions of the fixture surface do not overlap.
5. The bonding apparatus of claim 4, wherein the first and second
portions of the fixture surface are substantially adjacent one
another.
6. The bonding apparatus of claim 5, wherein: the first bond shoe
has a first face that opposes the first portion of the fixture
surface; the second bond shoe has a second face that opposes the
second portion of the fixture surface; and the first and second
bond shoe faces are separated from one another by no more than 0.25
inch.
7. The bonding apparatus of claim 6, wherein: the first bond shoe
face has a length of less than 5 inches; and the second bond shoe
face has a length of less than 5 inches.
8. The bonding apparatus of claim 7, wherein the first and second
portions of the fixture surface have a combined length of at least
7 inches.
9. The bonding apparatus of claim 1, wherein: the first portion of
the fixture surface has a first surface contour; the first bond
shoe has a first face having the complement of the first surface
contour; the second portion of the fixture surface has a second
surface contour; the second bond shoe has a second face having the
complement of the second surface contour.
10. The bonding apparatus of claim 9, wherein the first and second
surface contours are planar.
11. The bonding apparatus of claim 10, wherein: the first bond shoe
face has a length of less than 5 inches; and the second bond shoe
face has a length of less than 5 inches.
12. The bonding apparatus of claim 1, wherein each of the bond
shoes includes a heater opening for receiving a heater element.
13. The bonding apparatus of claim 12, wherein each of the bond
shoes includes a sensor opening for receiving a temperature
sensor.
14. A method of electrically joining a first circuit bearing
substrate having a first array of conductive surface traces to a
second circuit bearing substrate having a second array of
conductive surface traces, the method comprising: positioning on a
fixture surface the first circuit bearing substrate with the first
array of conductive surface traces facing away from the fixture
surface; positioning the second circuit bearing substrate on the
first circuit bearing substrate, with the second array of
conductive surface traces facing the first array of conductive
surface traces; and compressing different sections of the arrays of
conductive surface traces between first and second working tools
and the fixture surface.
15. The method of claim 14, wherein compressing different sections
of the arrays of conductive surface traces between first and second
working tools and the fixture surface comprises: pressing the first
working tool against a section of the second circuit bearing
substrate; and pressing the second working tool against a different
section of the second circuit bearing substrate.
16. The method of claim 15, additionally comprising positioning a
joining material between the first and second circuit bearing
substrates.
17. The method of claim 16, wherein the step of positioning the
joining material is conducted before the step of positioning the
second circuit bearing substrate.
18. The method of claim 17, wherein the step of positioning the
joining material comprises positioning an anisotropic conductive
film between the first and second circuit bearing substrates.
19. The method of claim 18, additionally comprising heating the
first and second working tools during the compressing step.
20. The method of claim 15, additionally comprising heating the
first and second working tools during the compressing step.
21. A method of electrically and mechanically joining a flexible
circuit film having electrically conductive film surface traces to
an electronic circuit board having electrically conductive board
surface traces, the method comprising: positioning the electronic
circuit board on a fixture surface with the board surface traces
facing away from the fixture surface; positioning a joining layer
on the electronic circuit board; tacking the joining layer to the
electronic circuit board; positioning the flexible circuit film on
the joining layer, with the film surface traces facing and aligned
with the board surface traces; and heating first and second working
tools to a bonding temperature; pressing the heated first working
tool against one section of the flexible circuit film with a
bonding force; and pressing the heated second working tool against
a different section of the flexible circuit film with a bonding
force.
22. The method of claim 21, wherein the electronic circuit board
and flexible circuit film remain stationary between the step of
pressing the first working tool with the bonding force and the step
of pressing the second working tool with the bonding force.
23. The method of claim 22, wherein the step of pressing the first
working tool with the bonding force and the step of pressing the
second working tool with the bonding force overlap in time.
24. The method of claim 23, wherein the step of pressing the first
working tool with the bonding force and the step of pressing the
second working tool with the bonding force occur
simultaneously.
25. The method of claim 21, wherein tacking the joining layer to
the electronic circuit board comprises: heating the first and
second working tools to a tacking temperature; pressing the heated
first working tool against one section of the flexible circuit film
with a tacking force; and pressing the heated second working tool
against a different section of the flexible circuit film with a
tacking force.
Description
BACKGROUND
[0001] This invention relates to electrically and mechanically
joining a relatively flexible circuit-bearing element to a
relatively inflexible circuit-bearing element, each having multiple
arrays of closely spaced electrically conductive traces.
[0002] Electrical and electronic devices often formed by joining
together separate elements, each containing electronic circuitry.
The separate elements are electrically connected by providing each
element with electrical contact pads or traces, aligning the
contact pads or traces on one element with the corresponding
contact pads on the other element, and then bonding the elements
together in a manner that provides electrical conductivity between
the aligned contact pads or traces.
[0003] One combination of circuit elements is a rigid or inflexible
substrate, such as an electronic circuit board, bearing electronic
circuitry, joined to a circuit-bearing flexible substrate, such as
a flexible circuit film. The circuitry on each substrate contains
contact pads on the surface of the substrate. The flexible circuit
film and the rigid electronic circuit board are joined by a joining
layer between them. The joining layer is formed of a material, such
as anisotropic conductive film, that, under compression, is
electrically conductive in the direction of compression, but
remains substantially insulating in the directions orthogonal to
the direction of compression. Bonding the anisotropy conductive
film between the rigid electronic circuit board and the flexible
circuit film is performed by arranging the electronic circuit
board, the anisotropy conductive film, and the flexible circuit
film on a tabular fixture, and pressing a heated bond shoe onto the
assembled elements. An exemplary bonding apparatus is available
from DCI, Inc. of Lenexa, Kans., as Model No. 1093C Heat Seal
Press.
[0004] An existing bonding apparatus uses a single heated bond shoe
that is typically up to approximately 2-3 inches (5.0-7.5 cm) in
length, and 0.3 inches in width to bond circuit elements having an
array of contact pads up to the length of the bond shoe. However,
some electronic devices contain circuit elements having arrays of
contact pads in which the dimensions of the array exceed this bond
shoe length. For example, a large printhead for an ink jet printer
may have contact pads arranged in arrays that are up to 8-9 inches
(20-23 cm) in length. In some instances, the arrays of contact pads
may be quite dense (150 contact pads per inch) or the contact pads
may be arranged in multiple rows along this length. Each of these
circumstances requires that the bonding technique be highly
accurate, to ensure the corresponding contact pads on the circuit
elements are accurately connected electrically, and that
non-corresponding contact pads are not improperly connected.
[0005] For joining large arrays of contact pads, two techniques
have been available. One technique is to use a bond shoe that is as
long as the array of contact pads. This technique requires that the
face of the bond shoe be exceptionally flat, so that the bond shoe
applies pressure evenly along the entire array of contact pads. The
nature of the anisotropic conductive film is such that sections
subjected to different forces of compression have different
electrical conductivity. Variation in the face of the bond shoe may
cause variations in the performance of the resulting electrical
connections. The other technique is to use a standard sized bond
shoe to bond one section of the array, then relocate the assembly
of circuit elements on the fixture of the bonding apparatus, and
use the same bond shoe to bond an adjacent section of the array.
This technique requires accurately placing and the circuit bearing
elements multiple times on the fixture during the bonding process.
Each relocation of the circuit bearing elements creates a risk of
damaging the elements, or of inaccurately aligning the connections
between the elements. In addition, the repeated relocations of the
circuit elements slow production processes.
SUMMARY
[0006] A bonding apparatus for electrically joining a first circuit
bearing substrate, such as a rigid electronic circuit board having
a first array of conductive surface traces, to a second circuit
bearing substrate, such as a flexible circuit film having a second
array of conductive surface traces, allows for joining large arrays
of surface traces without relocating the circuit bearing substrates
by using multiple bond shoes to compress different portions of the
arrays. The apparatus includes a fixture having a fixture surface,
first and second bond shoes, and one or more tool presses connected
to the first and second working tools for selectively pressing the
first bond shoe toward a first portion of the fixture surface and
for selectively pressing the second bond shoe toward a second
portion of the fixture surface. In particular embodiments, a first
tool press is connected to the first working tool, and a second
tool press is connected to the second working tool. The first tool
presses each have a tacking mode of operation in which the first
tool press presses the first bond shoe toward the first portion of
the fixture surface with a tacking force, and a bonding mode of
operation in which the second tool press presses the second bond
shoe toward the second portion of the fixture surface with a
bonding force, which bonding force is greater than the tacking
force.
[0007] A method of electrically joining a first circuit bearing
substrate having a first array of conductive surface traces to a
second circuit bearing substrate having a second array of
conductive surface traces provides for compressing different
portions of the first and second circuit bearing substrates with
different working tools. The method includes positioning on a
fixture surface the first circuit bearing substrate, positioning
the second circuit bearing substrate, and compressing different
portions of the arrays of conductive surface traces between first
and second working tools and the fixture surface.
DRAWINGS
[0008] FIG. 1 is a frontal view of a circuit element bonding
apparatus in an open position.
[0009] FIG. 2 is a frontal view of the bonding apparatus of FIG. 1
in a closed position.
[0010] FIG. 3 is a side view of a bond shoe portion of the bonding
apparatus of FIGS. 1 and 2.
[0011] FIG. 4 is a perspective view of two bond shoes, with a
portion of the bonding apparatus removed.
DETAILED DESCRIPTION
[0012] FIGS. 1 and 2 show a bonding apparatus 10 for bonding a
circuit-bearing flexible substrate, such as a flexible circuit film
12, to a circuit bearing inflexible substrate, such as an
electronic circuit board 14. FIG. 1 shows the bonding apparatus in
an "open" position. FIG. 2 shows the bonding apparatus in a
"closed" position, in which it applies vertical force to the
flexible circuit film 12 and the electronic circuit board 14.
[0013] The flexible circuit film 12 contains electronic circuitry
(not shown), with a plurality of film contact pads or film surface
traces 16 on one surface of the flexible circuit film providing
electrical contact to portions of that electronic circuitry. The
film surface traces 16 are electrically conductive. The flexible
circuit film is formed of an organic polymer film, and is familiar
to those skilled in the art. The electronic circuit board 14 also
contains electronic circuitry (not shown). The electronic circuitry
in the electronic circuit board may be formed on multiple
interconnected layers of the electronic circuit board, as is
familiar to those skilled in the art. Board contact pads or board
surface traces 18 on at least one surface of the electronic circuit
board provide electrical contact to portions of the electronic
circuitry contained in the electronic circuit board. The board
surface traces 18 are also electrically conductive. Each array of
board surface traces and film surface traces may have at least
approximately 20 traces per centimeter (50 traces per inch), and
may have 40-100 traces per centimeter (100-250 traces per inch).
Each array may have a length of up to approximately 10 cm (4
inches). In an example, the arrays of board surface traces are
separated from one another by a space. The space between the arrays
of surface traces can be sufficiently small that the arrays are
immediately adjacent one another.
[0014] In the example illustrated, the film surface traces 16 and
the board surface traces 18 are to be electrically joined so that
electrical signals can flow between the board surface traces and
the corresponding film surface traces. The board surface traces 18
are arranged in elongate arrays, and the film surface traces 16 are
arranged in corresponding elongate arrays. Generally, the number
and shapes of the board surface traces match the number and shapes
of the film surface traces. The array dimensions of an array of
film surface traces 16 closely or exactly matches the array
dimensions of the corresponding array of board surface traces 18.
In an example, one array of board surface traces has the same
number of traces and the same array dimensions as an adjacent array
of board surface traces. However, adjacent arrays of traces may
have different numbers and shapes of traces, and have different
array dimensions.
[0015] In the bonding apparatus of FIG. 1, the electronic circuit
board 14 is secured to a fixture 20 with the surface containing the
board surface traces 18 exposed (facing away from the fixture
surface). The electronic circuit board 14 may be secured to the
fixture with removable clamps (not shown) or other attachment
mechanisms that are familiar to those skilled in the art. The
flexible film is arranged with the exposed film surface traces 16
opposed to and facing the corresponding board surface traces.
[0016] A joining layer 22 is positioned between the electronic
circuit board and the flexible film circuit. The joining layer 22,
after bonding, electrically connects the film surface traces 16 and
the board surface traces 18 to provide electrical interconnection
between the circuits of the electronic circuit board and the
circuits of the flexible circuit film. The joining layer 22 is not
necessarily adhesive in nature, although an anisotropic conductive
adhesive in the form of an anisotropic conductive film, may be used
for the joining layer 22. Such anisotropic conductive film is
sometimes called "z-axis tape." The anisotropic conductive film may
be formed of an epoxy resin-based adhesive containing embedded
metal particles that allow the material to become electrically
conductive in a direction in which the material is compressed, but
remain substantially insulating in the directions orthogonal to the
direction of compression. Thus, in the case of an anisotropic
conductive film as the joining layer 22, compressing the joining
layer 22 in the vertical direction (z-axis), across the thickness
of the film, causes the joining layer to become electrically
conductive in the vertical direction, while remaining electrically
substantially insulating in the horizontal directions (x-y plane),
along the length and width of the film. Exemplary anisotropic
conductive films include Hitachi AC2052P45, AC2054P35, and
AP2101P35 available from Hitachi Chemical Company of Tokyo,
Japan.
[0017] The bonding apparatus 10 includes two or more working tools,
such as bond shoes 24, each of which corresponds to one of the
corresponding arrays of film surface traces and board surface
traces. The shoes 24 are substantially identical, except that
certain dimensions may differ. The distal end of each shoe 24 has a
working surface 26 that has lateral dimensions that equal or
slightly exceed the dimensions of its corresponding arrays of film
surface traces and board surface traces. In an example in which the
multiple arrays of traces all have the same array dimensions, the
working surfaces 26 of each shoe have the same dimensions. The face
of the working surface 26 is substantially flat, so that when the
working surface is pressed against the flexible circuit film (see
FIG. 2), the working surface can apply uniform pressure across the
entire array of film surface traces and board surface traces. The
working surface 26 (and preferably the other parts of the shoe 24)
is formed of a very hard thermally conductive material. The
material of the working surface 26 is sufficiently hard to be able
to withstand high pressures (up to at least 150 pounds per square
inch) without deforming. In addition, the material of the working
surface has substantially uniform thermal expansion characteristics
in the vertical dimension (toward the face of the working surface)
so that the face of the working surface remains substantially flat
as the temperature of the working surface changes. For example, the
working surface 26 (as well as other portions of the shoe 24) may
be formed of carbon steel plated with Nickel. The face of the
working surface 26 is generally less than 13 cm (5 inches) in
length, is preferably up to approximately 11 cm (4.2 inches) in
length, but may be smaller, such as 2.5-7.5 cm (1-3 inches) in
length. The face of the working surface 26 may be greater than 11
cm (4.2 inches) in length, although the greater the length of the
working surface face, the greater the difficulty in maintaining the
flatness of the face. The working surface face may have a width of
approximately 0.8-1.0 cm (0.3-0.4 inch). The dimensions of the face
of the working surface of each shoe closely match the dimensions of
the corresponding arrays of film surface traces and board surface
traces. The face of the working surface may have the same
dimensions as the cross-section of the upper portion of the bond
shoe, or the face may have different dimensions. the bond shoes may
be configured so that the faces of the working surfaces of adjacent
shoes have little or virtually no gap between them. Such a
configuration allow adjacent shoes to bond an essentially
continuous array of surface traces 16, 18. In an example, two
adjacent bond shoes, each having a 10 cm (4 inch) working surface
face, positioned immediately adjacent one another, can bond a
single continuous array of surface traces up to 20 cm (8 inches) in
length. The bond shoes may be positioned so that the separation
between the faces of the working surfaces of adjacent bond shoes is
0.1-0.6 cm (0.05-0.25 inch).
[0018] Each shoe 24 (see FIGS. 3 and 4) includes a cylindrical
heater opening 30 extending along the length of the shoe. The
heater opening provides space for a heater (not shown) to be
inserted into the shoe. Upon activation, the heater heats the body
of the shoe, including the working surface 26. An exemplary heater
is an elongate electrical resistance heater. The heater may be
similar to a 3 inch, 400 watt, 120 volt heater Part No. 48-0099-00
available from DCI Incorporated of Lenexa, Kans. (though the length
of the heater should match the length of the shoe 24). A heater
inserted through the heater opening 30 heats the shoe along the
entire length of the shoe. Preferably, the heater heats the shoe,
and particularly the working surface 26, evenly along the length of
the shoe. The ends of adjacent shoes may be extremely close to one
another. Therefore, external connections to the heaters of adjacent
shoes may be attached to the heaters at opposite ends of the shoes.
In some implementations, other access mechanisms through the shoe
may provide connections to a heater installed in the bond shoe. In
addition, those skilled in the art will recognize that other shapes
of heaters can be accommodated in shoe heater openings of different
shapes.
[0019] Each shoe also includes a sensor opening 32. The sensor
opening also extends along the length of the shoe, substantially
parallel to the heater opening. The sensor opening receives a
temperature sensor, such as an elongate thermocouple. An exemplary
thermocouple is Part No. 47-003700 from DCI Incorporated of Lenexa,
Kans. The temperature sensor in the sensor opening allows
monitoring of the temperature of the shoe. In an example, the
sensor opening 32 is near the working surface 26 so that the
temperature sensor can particularly sense the temperature of the
shoe near the working surface. Those skilled in the art will
recognize that other shapes and installations of temperature
sensors can be used.
[0020] Each shoe 24 is independently driven by an associated tool
press 34 that presses the shoe downward, toward the surface of the
fixture, so that the working surface 26 of the shoe can apply force
to the flexible circuit film 12 (FIGS. 1 and 2). An exemplary tool
press is an air cylinder. The air cylinder uses pneumatic pressure
to extend and retract one or more shafts 36, 37. In one example, a
1.5 inch (3.8 cm) three-stage air cylinder is used. The shoe 24 is
attached to the tool press 34 by an attachment device, such as a
clamp 38. The clamp 38 is secured to the ends of the shafts 36, 37,
and mechanically linked to the shoe so that longitudinal movement
of the shafts produces movement of the shoe toward or away from the
surface of the fixture 20. The mechanical linkage between the shoe
and the clamp 38 is adjustable so that the position of the shoe can
be adjusted to ensure the working surface 26 of the shoe makes even
contact with the flexible circuit film. An exemplary clamp 38 is
U-shaped, with an internal width between its legs approximately
equal to the width of the shoe 24. The clamp includes a cylindrical
pivot pin 40 that extends between the legs of the clamp. The pivot
pin 40 fits through a cylindrical pivot pin opening 42 across the
width of the shoe 24 so that the shoe has a limited range of pivot
movement. This pivot movement permits the apparatus operator to
adjust the angle of the working surface of the shoe relative to the
plane of the surface of the fixture 20. Set screws 44 extend
through the clamp 38 to press against a lateral side of the shoe 24
to hold the position of the shoe.
[0021] A press controller (not shown) controls the operation of the
tool press 34 and the heater in the heater opening 30. The press
controller receives temperature information from the temperature
sensor in the sensor opening 32. The press controller may be a
special purpose controller, or a programmed general purpose
controller, such as a microprocessor. A single press controller may
control multiple of the tool presses, each driving its own shoe 24,
or each tool press 34 may have its own independent press
controller. One press controller is a DCI 9000. Controller (Part
No. 01-0138-00) from DCI Incorporated of Lenexa, Kans.
[0022] The bonding apparatus 10 is usable to bond the flexible
circuit film 12 to the electronic circuit board 14 in a manner that
provides electrical conductivity between arrays of film surface
traces or contact pads 16 and corresponding arrays of board surface
traces or contact pads 18. The bonding apparatus permits bonding
multiple arrays of surface traces either simultaneously or in rapid
succession, without repositioning either the flexible circuit film
and the electronic circuit board, or the shoes of the bonding
apparatus.
[0023] The positions of the shoes 24 are adjusted in the clamp 38
by pivoting each shoe about the pivot pin 40 until that the face of
the working surface 26 is parallel the upper surface of the fixture
20. The position of each shoe is fixed with the set screws 44. The
electronic circuit board 14 is positioned and secured to the
fixture 20 so that the arrays of board surface traces are directly
below the working surfaces 26 of the corresponding shoes 24.
[0024] The anisotropic conductive film 22, which is stored at a
substantially lowered temperature, such as -40.degree. C.
(-40.degree. F.), is brought from storage. The temperature of the
anisotropic conductive film 22 is gradually raised to a standard
room temperature of approximately 20-22.degree. C. (68-72.degree.
F.), and the anisotropic conductive film is placed over the board
surface traces 18 on the electronic circuit board 14. The
anisotropic conductive film 22 is lightly attached, or "tacked," to
the electronic circuit board 14 over each array of board surface
traces 18. The tacking operation includes heating each bond shoe to
a tacking temperature, and pressing each shoe 24 against the
anisotropic conductive film with a tacking force. Typically, the
tacking temperature and the tacking force are less than the
temperature and force required to securely attach the anisotropic
conductive film to the electronic circuit board. The tacking
temperature, the tacking force, and the tacking time are functions
of the material of the inflexible substrate and the material of the
joining layer. In an example in which the inflexible substrate is a
circuit board of conventional circuit board material, and the
joining layer is Hitachi AC2052P45 anisotropic conductive film,
each shoe 24 has a tacking temperature of 72.degree. C.
(160.degree. F.). In an example in which the inflexible substrate
is glass, the tacking temperature is higher. The tool press applies
sufficient force to the shoe to press the anisotropic conductive
film 22 against the surface of the electronic circuit board with a
pressure of approximately 80-85 psi, such as 82.1 psi, for a
tacking time of approximately 15 seconds. The tacking time may
include time for the heater to raise the temperature of the working
surface 26 to the appropriate tacking temperature. In one example,
a total tacking time includes 10 seconds of temperature rise time,
and 5 seconds at the tacking temperature.
[0025] After the joining layer of anisotropic conductive film is
tacked to the electronic circuit board, the controller causes the
tool press to raise the shoe from the joining layer. The
anisotropic conductive film and the electronic circuit board are
allowed to cool to approximately room temperature.
[0026] The flexible circuit film 12 is positioned on the
anisotropic conductive film joining layer 22 so that the arrays of
film surface traces are aligned with the corresponding arrays of
board surface traces 18. Those skilled in the art recognize that
such alignment can be provided by providing either the fixture 20
or the electronic circuit board with alignment pins (not shown),
and providing the flexible circuit film with corresponding
alignment openings (not shown) that mate with the fixture or board
alignment pins.
[0027] After the electronic circuit board 14 and the flexible
circuit film 12 are positioned, with the joining layer of
anisotropic conductive film 22 between them, the anisotropic
conductive film is bonded to both the electronic circuit board and
the flexible circuit film. The anisotropic conductive film is
bonded between the electronic circuit board and the flexible
circuit film so that the anisotropic conductive film is
significantly compressed between each board surface trace and the
corresponding film surface trace. Such compression of the
anisotropic conductive film causes the anisotropic conductive film
to become conductive between each board surface trace and its
corresponding film surface trace, but remain substantially
insulating between adjacent surface traces.
[0028] The tool press 34 causes the working surface 26 of the bond
shoe 24 to apply to the upper surface of the flexible circuit film
12 a bonding force at a bonding temperature to bond the anisotropic
conductive film between the flexible circuit film and the
electronic circuit board. The heater in the heater opening 30
raises the temperature of the shoe 24 (including the working
surface 26) to the bonding temperature. The bonding temperature is
a function of the materials of the anisotropic conductive film, the
flexible circuit film, and the electronic circuit board. In an
example using Hitachi AC2052P35 anisotropic conductive film,
heating each bond show to approximately 300-350.degree. C.
(575-650.degree. F.) takes about 10 seconds. Such a bond shoe
temperature during the bonding process (with the bond shoe pressing
against the flexible circuit film, anisotropic conductive film, and
electronic circuit board) produces a bondline temperature at the
anisotropic conductive film of approximately 145-180.degree. C.
(290-360.degree. F.). The tool 34 presses the bond shoe toward the
surface of the fixture. The force applied is sufficient that the
bond shoe presses the assembled flexible circuit film, anisotropic
conductive film, and electronic circuit board with a bonding
pressure of 140-150 psi. The tool continues to apply this force for
approximately 20 seconds after the bond shoe has reached its
bonding temperature.
[0029] In an example, the bonding apparatus lowers and heats two or
more bond shoes simultaneously onto different sections of the
flexible circuit film for each of the tacking and bonding steps. In
another example, the two or more bond shoes are operated
sequentially, but without moving the circuit elements 12, 14 on the
fixture 20, and without moving the fixture. The combined arrays of
board surface traces 18 may be 7-9 inches or more in length. The
fixture surface may be at least 7-9 inches long to accommodate an
electronic circuit board having such an array of board surface
traces. The ability to bond multiple sections of surface traces
without adjusting the position of either the fixture or the bond
shoe between sections essentially eliminates misalignment,
buckling, or warping. In another example, the fixture 20 is
translatable longitudinally relative to the bond shoes. One bond
shoe 24 is used for tacking the anisotropic conductive film to the
electronic circuit board, and the second bond shoe is used for
bonding the anisotropic conductive film to the flexible circuit
film and the electronic circuit board. In that arrangement, the one
bond shoe can be performing a tacking operation on one array of
surface traces while the adjacent bond shoe is performing a bonding
operation on an adjacent array of surface traces. Such simultaneous
operation allows for a higher net throughput for bonding than is
possible if a single bond shoe performs both tacking and the
bonding operations for each array of surface traces.
[0030] This arrangement is exemplary only. Other types of bond
shoes and electronic circuits can be used, such as differently
shaped bond shoes or heating elements or a composition of a
flexible circuit and a glass electronic circuit (LCD).
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