U.S. patent application number 10/408149 was filed with the patent office on 2004-01-08 for method and composition for mounting an electronic component and device formed therewith.
Invention is credited to Davis, John G., Poole, Joseph D., Slesinger, Kris A., Weller, Michael C..
Application Number | 20040003882 10/408149 |
Document ID | / |
Family ID | 22079414 |
Filed Date | 2004-01-08 |
United States Patent
Application |
20040003882 |
Kind Code |
A1 |
Davis, John G. ; et
al. |
January 8, 2004 |
Method and composition for mounting an electronic component and
device formed therewith
Abstract
A method of mounting a component on a substrate includes
applying a conductive adhesive on a contact pad joined to a
substrate, aligning a component with the substrate such that at
least one lead of the component is juxtaposed with the conductive
adhesive, performing a partial cure of the conductive adhesive,
testing performance of the component, and performing a full cure of
the conductive adhesive. Another method includes the additional
steps of applying a tacky film to the substrate and juxtaposing the
component with the tacky film. When the testing in either
embodiment shows a defective or misaligned component, the component
may be replaced or repositioned by cold separation of the at least
one component lead from the partially cured conductive adhesive.
Optionally, additional conductive adhesive may be applied, when
needed, before replacement or repositioning of a component. A
composition for mounting a component on a circuit board includes a
conductive adhesive screened on a contact pad, the conductive
adhesive being partially cured, wherein the composition allows cold
separation of the component from the substrate and may be fully
cured by heat exposure. An apparatus is also provided including a
component, a substrate, and a means for temporarily mounting and
electrically connecting the component on the substrate.
Inventors: |
Davis, John G.; (Charlotte,
NC) ; Poole, Joseph D.; (Troutman, NC) ;
Slesinger, Kris A.; (Charlotte, NC) ; Weller, Michael
C.; (Harrisburg, NC) |
Correspondence
Address: |
SCHMEISER, OLSEN + WATTS
SUITE 201
3 LEAR JET
LATHAM
NY
12033
US
|
Family ID: |
22079414 |
Appl. No.: |
10/408149 |
Filed: |
April 4, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10408149 |
Apr 4, 2003 |
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09067940 |
Apr 28, 1998 |
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6589376 |
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Current U.S.
Class: |
156/64 ; 156/249;
156/299; 438/119 |
Current CPC
Class: |
H05K 2203/162 20130101;
H05K 2203/176 20130101; H05K 2203/0278 20130101; Y10T 156/1092
20150115; Y02P 70/613 20151101; H05K 2201/10734 20130101; Y10T
29/53183 20150115; H05K 3/321 20130101; Y02P 70/50 20151101; H05K
2203/1476 20130101; Y10T 29/53178 20150115; Y10T 29/49133 20150115;
H05K 2201/10689 20130101; H05K 2203/0191 20130101; H05K 3/305
20130101 |
Class at
Publication: |
156/64 ; 156/249;
156/299; 438/119 |
International
Class: |
H01L 021/44 |
Claims
1. A method for mounting a component on a substrate comprising the
steps of: a) applying a conductive adhesive on a contact pad joined
to a substrate; b) aligning a component with the substrate such
that at least one lead of the component is juxtaposed with the
conductive adhesive; c) performing a partial cure of the conductive
adhesive, such that an electrical and mechanical connection
suitable for testing is formed; d) testing the component; and e)
performing a full cure of the conductive adhesive, such that a
permanent connection is formed.
2. The method of claim 1, wherein the method additionally comprises
the step of applying a tacky film to the substrate and wherein the
step of aligning the component with the substrate additionally
comprises juxtaposing the component with the tacky film.
3. The method of claim 1, wherein the step of performing a partial
cure comprises heat curing the conductive adhesive at 50 to
105.degree. C. for 1 hour to 10 minutes.
4. The method of claim 1, wherein the step of performing a full
cure comprises heat curing the conductive adhesive at 50 to
200.degree. C. for 12 hours to 15 seconds.
5. The method of claim 1, additionally comprising the step of
replacing the component, if defective, with a new component and the
step of adjusting the position of or repositioning the component,
if misaligned, wherein the replacing and repositioning include cold
separation of the at least one component lead from the partially
cured conductive adhesive.
6. The method of claim 2, wherein the tacky film is selected from
the group consisting essentially of pressure sensitive tape,
silicon-based adhesive tape, acrylic-based adhesive tape, partially
cured polymer resin, thermoset or thermoplastic adhesive, and
pressure sensitive, thermoset, or thermoplastic preform films.
7. The method of claim 4, wherein the step of performing a full
cure comprises heat curing the conductive adhesive at 125 to
150.degree. C. for approximately 1.5 hours.
8. The method of claim 5, wherein additional conductive adhesive is
applied on the contact pad before replacing the new component on
the substrate and before repositioning the misaligned component to
its correct position.
9. A method for mounting a component on a substrate comprising the
steps of: a) applying a conductive adhesive on a contact pad joined
to a substrate; b) applying a tacky film to the substrate; c)
aligning a component with the substrate such that the component is
juxtaposed with the tacky film and at least one lead of the
component is juxtaposed with the conductive adhesive; d) performing
a partial cure of the conductive adhesive, such that an electrical
and mechanical connection suitable for testing is formed; e)
testing the component; and f) performing a full cure of the
conductive adhesive, such that a permanent connection is
formed.
10. The method of claim 9, wherein the step of performing a partial
cure comprises heat curing the conductive adhesive at 50 to
105.degree. C. for 1 hour to 10 minutes.
11. The method of claim 9, wherein the step of performing a full
cure comprises heat curing the conductive adhesive at 50 to
200.degree. C. for 12 hours to 15 seconds.
12. The method of claim 9, additionally comprising the step of
replacing the component, if defective, with a new component and the
step of adjusting the position of or repositioning the component,
if misaligned, wherein the replacing and repositioning include cold
separation of the component from the tacky film and the at least
one component lead from the partially cured conductive
adhesive.
13. The method of claim 9, wherein the tacky film is selected from
the group consisting essentially of pressure sensitive tape,
silicon-based adhesive tape, acrylic-based adhesive tape, partially
cured polymer resin, thermoset or thermoplastic adhesive, and
pressure sensitive, thermoset, or thermoplastic preform films.
14. The method of claim 11, wherein the step of performing a full
cure comprises heat curing the conductive adhesive at 125 to
150.degree. C. for approximately 1.5 hours.
15. The method of claim 12, wherein additional conductive adhesive
is applied on the contact pad before replacing the new component on
the substrate and before repositioning the misaligned component to
its correct position.
16. A composition for mounting a component on a substrate
comprising a conductive adhesive applied on a contact pad, the
conductive adhesive being partially cured, wherein the composition
allows cold separation of the component from the substrate and may
selectively be fully cured to permanently mount the component.
17. The composition of claim 16, wherein the conductive adhesive
may be fully cured by heat exposure.
18. The composition of claim 16, wherein the conductive adhesive
may be partially cured by heat exposure.
19. The composition of claim 16, wherein the composition also
allows cold remounting of a separated component on the
substrate.
20. An apparatus comprising: a) a component; b) a substrate; and b)
a means for temporarily mounting and electrically connecting the
component on the substrate.
21. The apparatus of claim 20, wherein the component is selected
from the group consisting of a ball-grid array (BGA), a flip-chip,
components with array-type connections, a quad flat package (QFP),
a J-lead component, components suitable for tape automated bonding
(TAB), and components with peripheral-type connections.
22. The apparatus of claim 20, wherein the means for temporarily
mounting and electrically connecting the component is also a means
for selectively mounting the component permanently on the
substrate.
23. The apparatus of claim 22, wherein the means for temporarily
mounting and electrically connecting the component and selectively
mounting the component permanently on the substrate comprise a
conductive adhesive applied on a contact pad, the conductive
adhesive being partially cured, wherein the conductive adhesive
allows cold separation of the component from the substrate and may
be fully cured.
24. The apparatus of claim 23, wherein the conductive adhesive is
partially and fully cured by heat exposure.
25. The apparatus of claim 23, wherein the means for temporarily
mounting and electrically connecting the component and selectively
mounting the component permanently on the substrate comprise, in
combination, a conductive adhesive applied on a contact pad, the
conductive adhesive being partially cured, and a tacky film applied
to the substrate, wherein the conductive adhesive and tacky film
allow cold separation of the component from the substrate and the
conductive adhesive may be fully cured by heat exposure.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field
[0002] This invention relates to the field of mounting an
electronic component on a substrate, particularly on circuit
boards. More specifically the invention relates to a method and
composition for mounting an electronic component on a substrate and
the device formed by using such a method and composition.
[0003] 2. Background Art
[0004] An increasing need exists in the electronics and/or computer
industry to provide electronic packages of components and
substrates, wherein multiple components are joined to a single
substrate, such as a circuit board, particularly a printed circuit
board (PCB). In fabricating such packages, the components and
substrate are typically tested individually prior to assembly.
However, once the package is fully assembled, defective devices may
still be discovered in subsequent testing of the overall package
due to problems at the system level, that is, problems only
apparent when certain components and the substrate operate together
in their respective systems. Also, problems may become apparent due
to improper alignment or orientation of a component on the
substrate. In either case, the most common remedy of the defect is
rework, that is, removal and replacement of a component with a new
component or repositioning of the component.
[0005] The rework process is time consuming and can introduce new
defects into the package, other components, and/or the substrate.
Often, solder paste is used to attach components to a substrate.
The soldered connections that result from the soldering process are
conductive and durable, as desired, however, they are also
difficult to rework. A component attached with solder is generally
removed by using a soldering iron or hot gas removal tools. The
goal is to remove selectively and reattach only the desired
components without effecting the delicate components that neighbor
the desired components on the substrate. Nevertheless, whenever a
soldering iron, hot gas tools, or other heated tools are used it is
extremely difficult to avoid exposing neighboring components to the
heat as well. The heat exposure may damage a component, the
substrate, or a solder connection or otherwise introduce another
defect into the package. Accordingly, a removal process that does
not require heat is needed.
[0006] In some packages, adhesives are used to attach components to
the substrate. The adhesives are often either conductive or
nonconductive epoxies, wherein conductive epoxies are used in a
similar fashion to solder paste. That is, the conductive epoxy is
applied to the substrate, the electrical leads of a component are
juxtaposed with the epoxy, and then heat is usually required to
cure the epoxy and create a sufficiently strong mechanical bond and
sufficiently reliable electrical connection. When solder paste is
used, a similar process is followed except that the heat causes the
solder paste to flow into a molten state and harden after removal
of the heat. The ability to later reflow the solder allows
reworking of soldered components, however, existing technology does
not provide a process for reworking cured conductive epoxy.
Accordingly, even if subsequent testing showed that an epoxied
package was defective, current practices would not allow for the
replacement or repositioning of selected components. For this
reason, the inability to rework conductive epoxy is presently
considered a disadvantage of such technology. Accordingly, an
attachment process that also allows reworking is needed.
[0007] Attempts have been made to remedy the problems discussed
above, however, they have met only limited success. Adhesive tape
and conductive resin have been used for components with peripheral
leads, wherein the leads electrically connect to contact pads on
the substrate surrounding the perimeter of the component. The
adhesive tape is placed between the component and the substrate to
provide a mechanical connection and the conductive resin is placed
on the contact pads to provide an electrical connection with the
peripheral leads. The conductive resin does not bond the leads to
the pads, it simply provides a conduction path, thus, the component
may be replaced or repositioned by removing it from the adhesive
tape. Accordingly, the removal process does not require heat and
the attachment process allows reworking, but the electrical
connection has proven unreliable. Because the conductive resin does
not provide a mechanical bond of the leads to the contact pads, the
electrical connection may be easily compromised. The adhesive tape
might be sufficient to bond the component to the substrate, but it
is not sufficient to keep the leads electrically connected to the
contact pads. Accordingly, an attachment process that provides a
durable electrical connection is also needed.
[0008] Notably, the tape and resin process described above is
applicable to a limited number of component types. For example, it
would be impractical to use the process with a component having for
its leads an array of conductive bumps between the component and
the substrate, providing both an electrical and a mechanical
connection. The difficulty of applying adhesive tape amongst an
array of contact pads on a substrate is typically sufficient to
discourage use of the tape and resin process. Accordingly, an
attachment process applicable to a variety of components is also
needed.
[0009] Thus, it can be seen from the above discussion that it would
be an improvement in the art to provide a method for forming
durable connections between a variety of components and substrates,
wherein the connections do not require heat for reworking. In other
words, the ability to rework is needed while providing durable and
reliable connections, wherein the rework has little impact on the
substrate and neighboring components and connections.
DISCLOSURE OF INVENTION
[0010] According to the present invention, a method is provided for
mounting a component on a substrate by: applying a conductive
adhesive on a contact pad joined to a substrate; aligning a
component with the substrate such that at least one lead of the
component is juxtaposed with the conductive adhesive; performing a
partial cure of the conductive adhesive, such that an electrical
and mechanical connection suitable for testing is formed; testing
performance of the component; and performing a full cure of the
conductive adhesive, such that a permanent connection is formed.
The method may include additional steps, for example, applying a
tacky film to the substrate and juxtaposing the component with the
tacky film. Also, for example, when the testing shows a defective
or misaligned component, the component may be adjusted in its
position or replaced or repositioned by cold separation of at least
one component lead from the partially cured conductive adhesive.
Further, additional conductive adhesive may be applied, when
needed, before replacement or repositioning of a component.
[0011] A composition is also provided for mounting a component on a
substrate including a conductive adhesive applied on a contact pad,
the conductive adhesive being partially cured, wherein the
composition allows cold separation of the component from the
substrate and may selectively be fully cured to permanently mount
the component. By way of example, the conductive adhesive may also
be partially cured and/or fully cured by heat exposure.
[0012] An apparatus is also provided including a component, a
substrate, and a means for temporarily mounting and electrically
connecting the component on the substrate. In one example, the
temporary mounting means may also be a means for selectively
mounting the component permanently on the substrate. A partially
cured conductive adhesive is one example of both a temporary
mounting means and a selectively permanent mounting means. Tacky
film may also be used in combination with the conductive
adhesive.
[0013] The foregoing and other features and advantages of the
present invention will be apparent from the following more
particular description of preferred embodiments of the invention,
as illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
[0014] Preferred embodiments of the present invention will
hereinafter be described in conjunction with the appended drawings,
where like designations denote like elements, and:
[0015] FIG. 1 is flow diagram showing a method according to the
present invention;
[0016] FIG. 2 is a continuation of the flow diagram in FIG. 1;
[0017] FIG. 3 is a partial side view of an electronic package with
a component having array connections that are mounted to a circuit
board according to a preferred embodiment of the present invention;
and
[0018] FIG. 4 is a partial side view of an electronic package with
a component having peripheral leads that are mounted to a circuit
board according to a preferred embodiment of the present
invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0019] According to a preferred embodiment of the present
invention, a method is provided for mounting a component on a
substrate by applying a conductive adhesive on a contact pad joined
to a substrate, aligning a component with the substrate such that
at least one lead of the component is juxtaposed with the
conductive adhesive, performing a partial cure of the conductive
adhesive, testing performance of the component, and performing a
full cure of the conductive adhesive. In another preferred
embodiment, the method includes the additional steps of applying a
tacky film to the substrate and juxtaposing the component with the
tacky film. Preferably, when the testing in either embodiment shows
a defective or misaligned component, the component is replaced or
repositioned by cold separation of the at least one component lead
from the partially cured conductive adhesive. Most preferably,
additional conductive adhesive is applied, when needed, before
replacement or repositioning of a component.
[0020] According to a preferred embodiment of the present
invention, a composition is also provided for mounting a component
on a substrate including a conductive adhesive applied on a contact
pad, the conductive adhesive being partially cured, wherein the
composition allows cold separation of the component from the
substrate and may selectively be fully cured to permanently mount
the component. Preferably, the conductive adhesive may also be
partially cured and/or fully cured by heat exposure.
[0021] According to a preferred embodiment of the present
invention, an apparatus is also provided including a component, a
substrate, and a means for temporarily mounting and electrically
connecting the component on the substrate. Preferably, the
temporary mounting means may also be a means for selectively
mounting the component permanently on the substrate. Most
preferably, a partially cured conductive adhesive is the temporary
mounting means and selectively permanent mounting means. In another
preferred embodiment, tacky film is used in combination with the
conductive adhesive.
[0022] Referring to FIGS. 1 and 2, the flow diagram displays a
method 100 for mounting a component on a substrate. Method 100
includes step 105 of applying a conductive adhesive on a contact
pad joined to a substrate. Any method of applying conductive
adhesive known to those skilled in the art is suitable, however,
screen printing is preferred. Contact pads of various
configurations are typically provided on substrates for forming
electrical, and often mechanical, connections to components. For
example, an array of contact pads may be provided for a component
having for its leads an array of conductive bumps. Also, contact
pads may be provided to surround the perimeter of a component
having peripheral leads. Other configurations of contact pads may
also be provided. The substrate may be of almost any type, but
preferably is a circuit board, particularly a printed circuit
board. Also, although step 105 and the discussion below describe
applying conductive adhesive on a contact pad, it is also
understood that the adhesive may alternatively be applied to
component leads. The characteristics of a suitable conductive
adhesive and a few exemplary adhesives are described below.
[0023] Step 110 includes positioning a component on the substrate
by aligning a component with the substrate such that at least one
lead of the component is juxtaposed with the conductive adhesive.
The component may be of almost any type, including those presently
mounted on substrates with the use of solder, adhesives, tape, etc.
The leads of the component are the conductive devices typically
used to electrically connect the component to the substrate. In the
present invention, the leads also mechanically connect the
component to the substrate, in particular, to the contact pads,
although other devices may be provided that also mechanically
connect the component to the substrate.
[0024] For example, another preferred embodiment of method 100
includes an additional step (not shown) of applying a tacky film to
the substrate and juxtaposing the component with the tacky film.
The tacky film could alternatively be applied to the component. The
tacky film increases the strength of the mechanical connection,
otherwise provided by the conductive adhesive applied in step 105,
but should still allow replacement or repositioning of the
component when necessary. Any method of applying tacky film known
to those skilled in the art is suitable and will depend upon the
type of tacky film used. The tacky film is preferably pressure
sensitive tape, including tapes with silicon or acrylic-based
adhesive. Some types of tacky film may also be preformed, wherein
they are cut and/or stamped to have a predetermined two-dimensional
or three-dimensional shape so that they may be easily applied to
the appropriate surface.
[0025] One example of a suitable acrylic-based adhesive tape is
SCOTCH brand 467MP or 468MP Roll Laminating Adhesive available from
3M Corp. in Minneapolis, Minn. Alternatively, the tacky film may be
other materials, such as a layer of silicon or acrylic-based
adhesive applied directly to the substrate, for example by screen
printing. A few examples of other suitable adhesives are the
partially curable nonconductive polymer resin coatings of U.S. Pat.
No. 3,955,024 entitled "PRINTED CIRCUIT BOARD" to Goldman et al.
issued on May 4, 1976 which is hereby incorporated by reference for
its pertinent and supportive teachings. Thermoset materials may be
used for the tacky film, wherein application of heat forms the
mechanical connection between the component and the substrate.
However, it should be remembered that the mechanical connection
should be temporary, allowing replacement or repositioning of the
component when necessary. Thermoplastic materials may also be used,
wherein application of heat allows replacement or repositioning of
the component. Caution should be exercised with using thermoplastic
materials, since the heat required for removal should be less than
the heat required for removal of solder-connected components.
[0026] Generally, it is preferable that the tacky film be
non-conductive to prevent electrical shorts between contact pads or
between component leads that the film might touch. However, if
there is little possibility of electrical shorts, then conductive
tacky film could be used, even though not necessary. The purpose of
the tacky film is simply to provide additional strength to the
mechanical connection between the component and substrate.
Application of the tacky film may be useful only for limited types
of components. For example, tacky film may be easily applied to
form a mechanical connection between a substrate and a component
with peripheral leads. However, it may be difficult to apply tacky
film to the substrate or component when array connections are
used.
[0027] Next in FIG. 1, step 115 includes performing a partial cure
of the applied adhesive. Partially curing the conductive adhesive
allows for reworking without heat, yet forms a temporary electrical
and mechanical connection suitable for testing that may be
selectively transformed into a permanent connection. Preferably the
conductive adhesive is partially cured by heat exposure, although,
it is conceivable that other curing mechanisms could be provided.
For example, the partial cure could be completed by radiation
exposure, such as visible light illumination, or chemical exposure,
such as catalysts or gases that partially cure the conductive
adhesive. It is even conceivable that the applied adhesive
partially cures at ambient conditions without special processing
and later processing completes the full cure described below.
Accordingly, the nature of any partial cure will depend primarily
on the composition of the conductive adhesive used since multiple
compositions presently available or to be developed later may be
suitable. Exemplary compositions and their corresponding processing
conditions for a partial cure are described below.
[0028] In step 120, the electronic package assembled from the
component and substrate is performance tested. Testing may include
any testing techniques known to those skilled in the art or later
developed. It is likely that individual components and circuit
boards were tested for manufacturing defects soon after their
fabrication before the start of method 100. Accordingly, the basic
objective of testing in step 120 is primarily to detect system
level problems and connection defects. System level problems are
primarily problems only apparent when certain components and the
substrate operate together in their respective systems. Connection
defects are primarily from misalignment or inadequate attachment of
components with contact pads. Nevertheless, testing in step 120 may
also be used to identify defects in individual components or
substrates incurred after fabrication, such as from mishandling, or
other defects not identified in previous testing.
[0029] If the electronic package passes the performance testing of
step 120, then step 125 is executed to perform a full cure of the
conductive adhesive. The full cure is similar to the partial cure
in that its process conditions depend primarily upon the
composition used and examples are described below. However, the
objectives of the full cure are different. For example, one
objective is to complete the formation of an electrical and
mechanical connection between the component and substrate,
rendering the connection permanent. That is, once the conductive
adhesive is fully cured, the connection generally cannot be altered
except, perhaps, by the application of heat as with soldered
components.
[0030] If the electronic package does NOT pass the performance
testing of step 120, then rework processing preferably occurs,
although a manufacturer may opt not to rework as shown in step 130.
The option exists in method 100 for no rework to occur when
circumstances dictate that as the preferred option. FIG. 2 displays
the preferred steps 135 through 155 of method 100 for a rework
process. Alternatively, a rework process need not include all of
steps 135 through 155, but instead may only include only steps 135
and 145. In step 135 the component is removed or adjusted in
position as allowed by virtue of the partial cure in step 115. This
is possible, as described above, because partial curing forms a
temporary electrical and mechanical connection having sufficient
strength and conductive properties. Specifically, the position of
the component on the substrate does not change when the substrate
is moved during testing, yet, the connection is conductive and the
component may be selectively removed or adjusted in position when
needed. In conventional processes, a soldering iron, hot gas tools,
other heated tools, or additional process steps are generally
required to remove or adjust the component. However, according to
the present invention, no such tools or additional steps are
required, rather the removal or adjustment occurs simply by
grasping the component and pulling it off the substrate or
adjusting its position, respectively.
[0031] Once the component is removed or adjusted in position, the
remaining adhesive is checked in step 140 to ensure that it is
adequate, since portions of the adhesive may be dislodged or
compromised during the removal or adjustment step 135. If the
remaining adhesive is adequate, then a removed component may be
replaced with a new component or the removed component repositioned
as shown in step 145. If the component was only adjusted in
position and not fully removed, then step 145 is not necessary.
Next, method 100 resumes with performance testing in step 120 of
the reworked electronic package. Although not shown, an alternative
method is also conceivable wherein the method resumes after step
145 with full cure of the adhesive in step 125 without repeating
performance testing in step 120.
[0032] If the remaining adhesive is not found adequate in step 140,
then additional conductive adhesive may be locally applied as in
step 150 prior to replacement or repositioning in step 155. Any
method of reapplying conductive adhesive known to those skilled in
the art is suitable, however, screen printing is preferred. When
reapplication of adhesive occurs in step 150, method 100 preferably
resumes with step 115 of performing a partial cure. Although not
shown, alternative methods are also conceivable wherein the method
resumes with performance testing in step 120 or full cure of the
adhesive in step 125, skipping the partial cure or performance
testing, respectively. Accordingly, the present invention provides
a method for forming durable connections between a variety of
components and substrates, wherein the connections do not require
heat for reworking.
[0033] Referring to FIG. 3, the side view displays part of an
electronic package 300 including a component 310 and a substrate
320. Component 310 has leads 330 arranged in an array and substrate
320 has contact pads 340 arranged in a corresponding array to match
up with array leads 330. Leads 330 are temporarily joined to
corresponding contact pads 340 by a partially cured conductive
adhesive 350. Partially cured conductive adhesive 350 allows for
reworking of package 300 without heat, yet forms an electrical and
mechanical connection suitable for testing that may be selectively
transformed into a permanent connection. That is, a connection
suitable to allow placing electronic package 300 in service.
Exemplary components suitable for the preferred embodiment shown in
FIG. 3 include a ball-grid array (BGA), flip-chip, and other
components with array-type connections.
[0034] Referring to FIG. 4, the side view displays part of an
electronic package 400 including a component 410 and a substrate
420. Component 410 has peripheral leads 430 and substrate 420 has
contact pads 440 arranged in a corresponding pattern about the
perimeter of component 410 to match up with peripheral leads 430.
Leads 430 are temporarily joined to corresponding contact pads 440
by a partially cured conductive adhesive 450. Partially cured
conductive adhesive 450 allows for reworking of package 400 without
heat, yet forms an electrical and mechanical connection suitable
for testing that may be selectively transformed into a permanent
connection. FIG. 4 also displays tacky film 460 positioned between
component 410 and substrate 420 as described in method 100 above.
Exemplary components suitable for the preferred embodiment shown in
FIG. 4 include a quad flat package (QFP), J-lead components,
components suitable for tape automated bonding (TAB), and other
components with peripheral-type connections.
[0035] The present invention also provides a composition for
mounting component 310 or 410 on substrate 320 or 420, the
composition including partially cured conductive adhesive 350 or
450 applied on contact pad 340 or 440, wherein the composition
allows rework as described in method 100 above and may selectively
be fully cured to permanently mount component 310 or 410.
Preferably, conductive adhesive 350 or 450 is fully cured by heat
exposure, and most preferably, it is also partially cured by heat
exposure. Examples of specific compositions and curing processes
are described below.
EXAMPLE 1
[0036] EPO-TEK E2101 and EPO-TEK H20E-PFC (two-component silver
filled epoxies available from Epoxy Technology, Inc. in Billerica,
Mass.) may be used in the method, apparatus, and composition
described above. Partial curing of the epoxies is performed at a
bond line temperature of 70.degree. C. (Celsius) .+-.5.degree. C.
for 30 min (minutes) .+-.5 min. Full curing of E2101 is performed
at a bond line temperature of 175.degree. C. for 15 min or
150.degree. C. for 60 min. Full curing of H20E-PFC is performed at
a minimum bond line temperature of 175.degree. C. for 45 sec
(seconds), 150.degree. C. for 5 min, 120.degree. C. for 15 min,
80.degree. C. for 90 min, or 50.degree. C. for 12 hours. Full
curing may also be otherwise according to the manufacturer's
specifications.
EXAMPLE 2
[0037] EPO-TEK E3114-PFC and EPO-TEK E3116 (one-component silver
filled epoxies available from Epoxy Technology, Inc. in Billerica,
Mass.) may be used in the method, apparatus, and composition
described above. Partial curing of the epoxies is performed at a
bond line temperature of 100.degree. C..+-.5.degree. C. for 20 min
.+-.5 min. Full curing of E3114-PFC is performed at a minimum bond
line temperature of 200.degree. C. for 30 sec, 175.degree. C. for
45 sec, 160.degree. C. for 5 min, or 150.degree. C. for 15 min.
Full curing of E3116 is performed at a minimum bond line
temperature of 200.degree. C. for 15 sec, 175.degree. C. for 45
sec, 160.degree. C. for 5 min, or 150.degree. C. for 15 min. Full
curing may also be otherwise according to the manufacturer's
specifications.
EXAMPLE 3
[0038] The conductive adhesive of U.S. Pat. No. 5,372,750 entitled
"ELECTRICALLY CONDUCTIVE SCREEN PRINTABLE COMPOSITION AND METHOD OF
MAKING SAME" to Loma issued on Dec. 13, 1994 may also be partially
cured according to the present invention.
[0039] While the invention has been particularly shown and
described with reference to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
spirit and scope of the invention. Accordingly, unless otherwise
specified, any dimensions or positions of the apparatus indicated
in the drawings or herein are given as an example of possible
dimensions and positions and not as a limitation. Similarly, unless
otherwise specified, any sequence of steps of the method indicated
in the drawings or herein are given as an example of a possible
sequence and not as a limitation. For example, it will be
understood that, while various of the conductors (connections) are
shown in the drawing as single lines, they are not so shown in a
limiting sense, and may comprise plural conductor (connections), as
is understood in the art.
* * * * *