U.S. patent application number 10/704738 was filed with the patent office on 2004-09-02 for device cover having a gapped adhesive preform thereon for covering a device on an electronic substrate.
Invention is credited to Chung, Kevin Kwong-Tai.
Application Number | 20040170825 10/704738 |
Document ID | / |
Family ID | 27376537 |
Filed Date | 2004-09-02 |
United States Patent
Application |
20040170825 |
Kind Code |
A1 |
Chung, Kevin Kwong-Tai |
September 2, 2004 |
Device cover having a gapped adhesive preform thereon for covering
a device on an electronic substrate
Abstract
A device cover for covering a device on an electronic substrate
according to the invention comprises a cover having a bonding
surface defining a closed bonding pattern, and a bonding pattern of
adhesive on the bonding surface of the cover. The bonding pattern
of adhesive has at least one gap therein, wherein the gap is
sufficiently small as to be filled by the adhesive when the
adhesive flows when the cover is adhered to the electronic
substrate.
Inventors: |
Chung, Kevin Kwong-Tai;
(Princeton Township, NJ) |
Correspondence
Address: |
DANN, DORFMAN, HERRELL & SKILLMAN
1601 MARKET STREET
SUITE 2400
PHILADELPHIA
PA
19103-2307
US
|
Family ID: |
27376537 |
Appl. No.: |
10/704738 |
Filed: |
November 10, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10704738 |
Nov 10, 2003 |
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10170533 |
Jun 13, 2002 |
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10170533 |
Jun 13, 2002 |
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09633792 |
Aug 7, 2000 |
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6432253 |
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09633792 |
Aug 7, 2000 |
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09232936 |
Jan 19, 1999 |
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6136128 |
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60090295 |
Jun 23, 1998 |
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60092170 |
Jul 9, 1998 |
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Current U.S.
Class: |
428/343 ;
257/E21.499 |
Current CPC
Class: |
H01L 2924/01006
20130101; B29C 65/76 20130101; B29C 65/4835 20130101; H01L
2924/16152 20130101; H01L 2924/01039 20130101; B29L 2031/3481
20130101; Y10T 428/24851 20150115; H01L 2924/19042 20130101; B29C
65/4815 20130101; B29C 65/50 20130101; B29C 65/5007 20130101; H01L
2924/07802 20130101; H01L 2924/01047 20130101; H01L 2924/01005
20130101; B29C 66/7392 20130101; H01L 2924/19043 20130101; B29C
65/526 20130101; H01L 2924/01078 20130101; Y10T 428/28 20150115;
B29C 66/112 20130101; H01L 21/4803 20130101; H01L 2924/14 20130101;
B29C 66/7394 20130101; H01L 2924/3025 20130101; H01L 21/50
20130101; H01L 2924/01082 20130101; B29C 65/52 20130101; B29C
66/73921 20130101; B29L 2031/3061 20130101; H01L 2924/01013
20130101; H01L 24/97 20130101; H01L 2924/19041 20130101; H01L
2924/3011 20130101; B29C 66/114 20130101; H01L 2924/01029 20130101;
H01L 2924/01079 20130101; H01L 2924/12042 20130101; H01L 2924/01033
20130101; B29C 66/53461 20130101; H01L 2924/07802 20130101; H01L
2924/12042 20130101; H01L 2924/00 20130101; H01L 2924/12042
20130101; H01L 2924/00 20130101 |
Class at
Publication: |
428/343 |
International
Class: |
B32B 007/12; B32B
015/04 |
Claims
What is claimed is:
1. A device cover for covering a device on an electronic substrate
comprising: a cover having a bonding surface defining a closed
bonding pattern; a bonding pattern of flowable adhesive on the
bonding surface of said cover, wherein said bonding pattern of
flowable adhesive is a flowable adhesive preform in the shape of
the closed bonding pattern, and wherein said bonding pattern of
flowable adhesive has at least one gap therein, wherein the gap in
said bonding pattern of flowable adhesive is sufficiently small as
to be filled by said flowable adhesive when said flowable adhesive
flows when the cover is adhered to the electronic substrate,
whereby a seal having a shape defined by the shape of the closed
bonding pattern is formed between the cover and the electronic
substrate by the flowed bonding pattern of flowable adhesive.
2. The device cover of claim 1 wherein said bonding pattern of
flowable adhesive is formed by a method selected from the group
consisting of mesh screening, stencil screening, contact screening,
mask screening and ink-jet printing.
3. The device cover of claim 1 wherein said bonding pattern of
flowable adhesive includes a dried flowable adhesive and/or a
B-staged flowable adhesive.
4. The device cover of claim 1 wherein said flowable adhesive is
selected from the group consisting of thermoplastic adhesives and
thermosetting adhesives.
5. The device cover of claim 1 wherein the at least one gap is
located at a corner of said bonding pattern of flowable adhesive
and/or along a side of said bonding pattern of flowable
adhesive.
6. The device cover of claim 1 wherein said cover is of a hollow
rectangular shape having an open end, the edges at the open end
thereof providing a bonding surface defining a rectangular bonding
pattern.
7. The device cover of claim 1 wherein said flowable adhesive has a
melting temperature at which its bond strength substantially
decreases, which melting temperature is in a range of about
90.degree. C. to about 180.degree. C., which is substantially less
than the melting temperature of solder.
8. A device cover for a device on an electronic substrate, said
device cover having a gapped adhesive preform disposed on a bonding
surface thereof and adapted to flow for attaching the cover to the
electronic substrate comprising: a cover having a bonding surface
adapted for receiving adhesive and defining a closed bonding
pattern; an adhesive preform of flowable adhesive in the shape of
the bonding pattern disposed on the bonding surface of said cover,
wherein said flowable adhesive preform has at least one gap
therein, wherein the at least one gap is sufficiently small as to
be filled by said flowable adhesive when said flowable adhesive
flows for attaching the cover to the electronic substrate, whereby
a seal having a shape defined by the shape of the bonding pattern
is formed between the cover and the electronic substrate by the
flowed bonding pattern of flowable adhesive.
9. The device cover of claim 8 wherein said flowable adhesive
preform is formed by a method selected from the group consisting of
mesh screening, stencil screening, contact screening, mask
screening and ink-jet printing.
10. The device cover of claim 8 wherein said flowable adhesive
preform includes a dried flowable adhesive and/or a B-staged
flowable adhesive.
11. The device cover of claim 8 wherein said flowable adhesive is
selected from the group consisting of thermoplastic adhesives and
thermosetting adhesives.
12. The device cover of claim 8 wherein the at least one gap is
located at a corner of said adhesive preform and/or along a side of
said adhesive preform.
13. The device cover of claim 8 wherein said cover is of a hollow
rectangular shape having an open end, the edges at the open end
thereof providing a bonding surface defining a rectangular bonding
pattern.
14. The device cover of claim 8 wherein said flowable adhesive has
a melting temperature at which its bond strength substantially
decreases, which melting temperature is in a range of about
90.degree. C. to about 180.degree. C., which is substantially less
than the melting temperature of solder.
15. A hollow device cover for covering a device on an electronic
substrate, said hollow device cover having a gapped adhesive
preform disposed on a bonding surface thereof and adapted to flow
for attaching the hollow device cover to the electronic substrate,
said hollow device cover comprising: a hollow cover having an open
end defining a bonding surface adapted for receiving adhesive and
defining a closed bonding pattern; a preform of melt-flowable
adhesive formed in the shape of the bonding pattern and disposed on
the bonding surface of said hollow cover, wherein said
melt-flowable adhesive preform has two or more gaps therein,
wherein each of the two or more gaps is sufficiently small as to be
filled by said melt-flowable adhesive when said melt-flowable
adhesive flows for attaching the cover to the electronic substrate,
whereby a seal having a shape defined by the closed bonding pattern
is formed between the hollow cover and the electronic substrate by
the flowed bonding pattern of flowable adhesive.
16. The hollow device cover of claim 15 wherein said preform of
melt-flowable adhesive is formed by a method selected from the
group consisting of mesh screening, stencil screening, contact
screening, mask screening and ink-jet printing.
17. The hollow device cover of claim 15 wherein said melt-flowable
adhesive includes a dried thermoplastic adhesive, a B-staged
thermoplastic adhesive, a dried thermosetting adhesive, and/or a
B-staged thermosetting adhesive.
18. The hollow device cover of claim 15 wherein the two or more
gaps of said preform are at one or more corners of the bonding
pattern, along one or more sides of the bonding pattern and/or at
one or more corners and along one or more sides of the bonding
pattern.
19. The hollow device cover of claim 15 wherein said hollow cover
is of a hollow rectangular shape defining a rectangular closed
bonding pattern.
20. The hollow device cover of claim 15 wherein said melt-flowable
adhesive has a melt-flow temperature at which its bond strength
substantially decreases, which melt-flow temperature is in a range
of about 90.degree. C. to about 180.degree. C., which is
substantially less than the melting temperature of solder.
Description
[0001] This Application is a division of U.S. patent application
Ser. No. 10/170,533 filed Jun. 13, 2002, which is a division of
U.S. patent application Ser. No. 09/633,792 filed Aug. 7, 2000, now
U.S. Pat. No. 6,432,253, which is a division of U.S. patent
application Ser. No. 09/232,936 filed Jan. 19, 1999, now U.S. Pat.
No. 6,136,128, which claims the benefit of U.S. Provisional
Application Serial No. 60/090,295 filed Jun. 23, 1998 and of U.S.
Provisional Application Serial No. 60/092,170 filed Jul. 9,
1998.
[0002] The present invention relates to a device cover having
thereon a flowable adhesive preform having a gap therein for
covering a device on an electronic substrate.
[0003] Many approaches have been tried for packaging electronic
devices for protection against external hazards, such as handling
and other mechanical damage, environmental factors, chemical
attacks, and other potentially adverse elements. Depending on both
the functional and aesthetic requirements, these electronic devices
are typically packaged in several levels of packaging. The
outermost level is most likely a housing or enclosure for the
equipment of which such devices are a part.
[0004] Generally, a useful electronic device, such as electronic
circuit or integrated circuit, is packaged within a small package
or module providing the first of at least several levels of
protection. Electronic devices such as semiconductor devices are
often protected by solid organic encapsulation. When several of
these packaged electronic devices are put together as a functional
unit, such as in an electronic circuit module or on a printed
circuit board or other substrate, they are often protected with an
exterior lid, cover or other enclosure to form a protective
housing. These exterior lids or covers may be attached with
adhesive, solder, or by mechanical fasteners, such as screws, bolts
and clips.
[0005] In some applications, an electronic device at the
semiconductor device level may not be able to reliably be encased
in a solid encapsulant because of the adverse influence of stresses
induced in the device owing to direct contact with the encapsulant.
In other applications, the use of the encapsulation may be too
costly. In still other applications, there may be a need for a lid
or cover that is electrically conductive so as to provide shielding
against electromagnetic interference (EMI) which may originate in
the covered device or which may originate externally and to which
the covered device may be susceptible. In this type of
EMI-resistant application, the lid must be electrically conductive
and must also be connected to the electrical ground of the
electronic device. This requirement cannot be easily met with
either an insulating organic encapsulant which does not provide
shielding or with a conductive encapsulant which is likely to
electrically short the electronic device or the conductors
connecting thereto. Even the use of an electrically conductive lid
that is soldered in place may be inconvenient or impractical
because of the adverse effects on the devices that result from the
high temperatures required for making soldering attachments. In
addition, if one needs to rework the soldered module, the
de-soldering operation may also cause overheating or other damage
or the inadvertent de-soldering of other electronic elements inside
of the package.
[0006] In fact, most of the electronic devices utilized in
aerospace, military and other high reliability applications make
use of a hermetically-sealed lid to prevent moisture and other
adverse elements from affecting or damaging the electronic
components employed therein. However, true hermetically-sealed
packages are very expensive to fabricate. Most high-reliability
hermetically-sealed packages employ either metal soldering or
brazing for lid attachment, especially for applications requiring
an electrically conductive housing for EMI protection. In those
applications where an insulating lid or cover must be employed,
high temperature glass seals are often utilized. In order to
prevent damage to the electronic devices from the high-temperature
processing necessary to form the glass seals, the packages and lids
must be heated up locally only along the rim of the package and
lid. As a result, the processing time is long and the work of
attaching the protective lids is delicate. In addition, the
materials employed in both the glass seal and lid must have
respective coefficients of thermal expansion (CTE) that are matched
to that of the electronic substrate or package to which they
attach. This additional requirement of matching the respective CTEs
of the substrate, sealing material, and lid, all increase the
difficulty of package design and the cost of the finished device.
In general, the cost of both the materials and the processing of
matched-CTE packages are prohibitive for commercial electronics
products for general use, such as consumer electronic products.
[0007] Electronic package lids and covers are used, however, to a
certain extent in commercial electronics products where required to
achieve necessary performance parameters. For example,
frequency-determining electronic devices that are susceptible to
frequency errors caused by stress-induced mechanical distortion or
that must mechanically change to function, such as piezo-electric
sound generators and frequency crystals employed in communication
equipment, cannot be simply encapsulated and so are protected by a
lid. These lids are generally attached with adhesive.
[0008] Conventionally, adhesive in the form of dispensable paste or
die-cut preforms is applied to the device or to the lid immediately
before or as part of the lid attachment bonding process. In certain
cases, for, example, when the number of lid attachments is high,
lids are pre-coated with adhesive or with die-cut adhesive preforms
that will flow and cure when applied under heat and pressure
conditions during the lid attachment process. However, the cost of
adhesive pre-coating and die cut adhesive preform application to
lids and covers is still quite high, in part due to the number of
steps required and the handling of individual lids and even
individual adhesive preforms. Adhesives in liquidous form are
typically dispensed with a programmable automatic dispenser or are
roller-coated onto the sealing areas of each lid, and are then
subsequently dried or B-staged at a temperature and for a time
substantially lower than the specified curing temperature and time
for the particular adhesive. The liquidous adhesive is thus changed
into a solid state either through solvent evaporation or chemical
cross-linking of the adhesive during this drying or B-staging.
[0009] U.S. Pat. No. 5,056,296 issued to Ross et al and entitled
"Iso-Thermal Seal Process for Electronic Devices" discloses an
apparatus and process wherein the apparatus heats the lid, the
package and the surrounding thermosetting adhesive so that they all
attain an isothermal condition, i.e. a uniform temperature, before
the lid is mated to the package in the bonding process. The Ross et
al patent describes the pre-sealing isothermal condition as
necessary to prevent differential air pressure between the inside
and outside of the package that can cause "blow-out"-induced
pinholes along the bond line provided by the sealing thermosetting
adhesive if the parts are brought together and then are heated.
Because of the time required to raise the temperature of the lid
and the package, perhaps several minutes to achieve uniform
temperature, the Ross et al process would appear able to achieve
significant quantity production only when applied in a batch
processing of lids, which often is impracticable. Moreover, because
of the long heating time, the Ross et al process would seem to
require a slower curing adhesive so as to avoid gelling or partial
curing of the pre-heated adhesive before attachment of the lid to
the package, thereby also extending the post-attachment curing time
of the adhesive and further reducing the ability to achieve
quantity production.
[0010] U.S. Pat. No. 5,427,642 issued to Akiguchi et al entitled
"Method for Mounting Electronic Parts on a Printed Circuit Board by
Use of an Adhesive Composition" describes an arrangement for
mounting components to a substrate so as not to form enclosed areas
in which air could be trapped during a soldering operation, and
does not relate to a device cover. Specifically, Akiguchi et al
relates to an adhesive composition for attaching electronic
components to a substrate prior to their being electrically
connected by soldering. The adhesive of Akiguchi et al is applied
to the substrate and then is partially cured by ultraviolet (UV)
light to harden the surface layer thereof. Placing the electronic
part then "dents" the adhesive composition. It appears that the
hardening of the surface layer before application of the electronic
part acts to decrease any flow that might otherwise occur. The gap
of Akiguchi et al must remain open for a subsequent soldering
operation.
[0011] U.S. Pat. No. 5,932,875 to R. Chung et al entitled "Single
Piece Integrated Package and Optical Lid" relates to an integrated
circuit package that has been attached to a PC board and an optical
lid that is thereafter placed to cover the previously attached
package R. Chung et al describes placing epoxy onto the flange of
the lid or onto the PC board (substrate) to which the lid is
placed, and does not describe or suggest a bonding pattern or
preform of adhesive on the lid wherein the bonding pattern or
preform of adhesive has at least one gap therein, and so it cannot
describe or suggest that the at least one gap is filled when the
adhesive flows.
[0012] Thus, there is a need for adhesive preform lids and covers
that avoid the "blow-out" problem and provide a cost-effective
solution for protecting devices such as sensitive electronic
components. It is also desirable that such cover lend itself to
automated processing and that the adhesive of the preform be melt
flowable so as to be removable at a temperature and an applied
force that will not damage either the electronic components inside
the package and/or on the substrate to which they are attached for
covering the electronic components therein and/or thereon.
[0013] There is also a need for lids and covers that provide
shielding against EMI and that can be attached at a temperature
substantially below the general soldering temperature of about
220.degree. C., i.e. at a temperature that will not disturb or
damage soldered connections, e.g., a melting temperature in a range
of about 90.degree. C. to about 180.degree. C. It is also desirable
that the adhesive employed therein is electrically conductive and
bonds essentially instantly upon reaching the bonding temperature,
and that the lids or covers so attached be removable at a
temperature below the general soldering temperature so as to
eliminate the possibility of thermally-induced damage to or
misalignment of components inside the package.
[0014] To this end, a device cover for covering a device on an
electronic substrate according to the invention comprises a cover
having a bonding surface defining a closed bonding pattern, and a
bonding pattern of adhesive on the bonding surface of the cover.
The bonding pattern of adhesive has at least one gap therein,
wherein the gap is sufficiently small as to be filled by the
adhesive when the adhesive flows when the cover is adhered to the
electronic substrate.
BRIEF DESCRIPTION OF THE DRAWING
[0015] The detailed description of the preferred embodiments of the
present invention will be more easily and better understood when
read in conjunction with the FIGURES of the Drawing which
include:
[0016] FIG. 1 is a cut-away perspective view of an electronic
device including an adhesively attached cover;
[0017] FIG. 2 is a plan view of a plurality of adhesive preforms on
a release substrate;
[0018] FIG. 3 is a side cross-sectional view of the adhesive
preforms and release substrate of FIG. 2 taken along line I-I;
[0019] FIG. 4 is a side cross-sectional view of the adhesive
preforms and release substrate of FIG. 3 with a plurality of lids
or covers thereon; and
[0020] FIG. 5 is a perspective view of a portion of an electronic
device having a plurality of lids or covers thereon.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0021] FIG. 1 is a cut-away perspective view of an electronic
device 10 including an electronic substrate 20 upon which are
mounted one or more electronic components 22, such as semiconductor
chips, integrated circuits, transistors, diodes, resistors,
capacitors, inductors, and combinations thereof. The electronic
devices are connected in circuit by electrical conductors (not
visible in FIG. 1) formed on or within substrate 20, as is known to
those having ordinary skill in the art. Electrical leads 24, 26
extending outwardly from substrate 20 as in a "flat-pack"
arrangement, for example, provide conductive connections between
the electrical conductors and components 22 of electronic device 10
and the apparatus to which electronic device 10 is
incorporated.
[0022] Because electronic components 22 commonly include very fine
features that are delicate and susceptible to damaged by mechanical
and electrical means, and/or are susceptible to contamination by
moisture or other foreign matter, a protective lid or cover 40 is
attached over and protecting electronic components 22. Protective
cover 40 is attached to substrate 20 by a continuous line of
adhesive 30 that joins the edges 42 of cover 40 to the surface of
substrate 20 completely around the periphery thereof. Edges 42 of
cover 40 are a bonding surface that define a bonding pattern,
wherein the adhesive 30 is deposited in a pattern substantially
corresponding in size and shape to that bonding pattern.
[0023] Where cover 40 is a protective cover only, it may be formed
of stamped or cast or molded epoxy, liquid-crystal polymer or other
suitable plastic, and adhesive 30 may be a non-electrically
conductive thermoplastic adhesive, such as types TP7150, TP7090,
TP7750 and TP7260 or a non-electrically conductive thermosetting
adhesive, such as types ESP7675, ESP7670 and ESP7450, all of which
are available from AI Technology, Inc. located in Princeton, N.J.
Surfaces of plastic covers to which adhesive is to be applied are
preferably prepared for improved adhesion, such as by oxidizing the
bond surfaces by flame or corona treatment. Covers typically range
in size from about 100 mils.times.100 mils, which could be employed
to protect an individual transistor or diode or a small integrated
circuit, to about one or two inches by one or two inches, which
could be employed to protect a large integrated circuit such as a
micro-processor.
[0024] Where cover 40 is for providing electrostatic and/or
electromagnetic shielding of the electronic components it encloses,
cover 40 may be formed of a metal, such as copper, aluminum, steel,
stainless steel and alloys thereof, with and without protective
plating. Alternatively, cover 40 may be formed of a non-conductive
material as above and plated with an electrically-conductive
coating, such as copper, silver, gold or combinations thereof, or
may be filled with conductive particles such as copper, silver,
gold, aluminum and/or carbon particles. In the case where such
electrostatic and/or electromagnetic shielding is also provided,
adhesive 30 may be an-electrically conductive thermoplastic
adhesive, such as types TP8090 (filled with silver particles),
TP8093 (filled with silver-plated copper particles) and TP8150
(filled with silver particles) or an electrically conductive
thermosetting adhesive, such as types ESP8680 (filled with silver
particles), ESP8450 (filled with silver particles) and ESP8453
(filled with silver-plated copper particles), all also available
from AI Technology, Inc. Adhesives of the foregoing types are
considered flexible adhesives in that they have a modulus of
elasticity that is less than about 200,000 psi over the specified
and/or operating temperature range of the electronic devices with
which covers 40 are intended to be employed, and also will elongate
by at least 10% before fracturing. For example, type ESP8450
adhesive has a modulus of elasticity between about 200,000 psi and
20,000 psi over the temperature range of about -55.degree. C. to
+150.degree. C. It is noted that covers 40 formed with the
foregoing materials and employing adhesive preforms 30 of the
foregoing exemplary materials will be resistant to the passage of
moisture and chemical cleaners and solvents commonly employed in
the manufacture of electronic devices, such as isopropyl alcohol,
volatile methylsiloxane, terpenes and other solvents. The adhesive
preforms 30 will exhibit volume resistivity in a range of about 100
million ohm-cm to about 0.1 ohm-cm, depending upon the adhesive
material and the fillers therein, if any, and so will tend to
dissipate electrostatic potential.
[0025] Covers 40 with preformed adhesive 30 applied thereto may be
made by the following method which is described in relation to
FIGS. 2, 3 and 4. A release substrate 32 such as a sheet of steel
coated with a layer of poly-tetra-fluoro-ethylene, such as
Teflon.RTM., available from E.I. duPont de Nemoirs located in
Wilmington, Del., is obtained and a set of at least two relational
alignment holes 34, 36 are made therein, as by punching, die
cutting or laser cutting. Release substrate 32 may also employ
polypropylene plate, and, if a mechanically self supporting release
substrate is desired, it may be made of a self-supporting sheet of
low surface energy (e.g., surface energy less than 30 dyne/cm)
material such as poly-tetra-fluoro-ethylene or may be made of
aluminum, stainless steel, steel or other metal and coated with
such low surface energy material. The relational alignment holes
34, 36 are located in known predetermined relationship to each
other, as may be seen in the plan view of FIG. 2.
[0026] A flexible adhesive is deposited on release substrate 32 to
form a pattern of a plurality of adhesive preforms 30 conforming
substantially to the bonding pattern defined by edges 42 of cover
40, in positions determined by the relational alignment holes 34',
36' in the screen, stencil or mask employed to deposit the flexible
adhesive, which relational alignment holes 34', 36' are in the same
known predetermined relationship to the pattern of adhesive
preforms 30 as are the corresponding relational alignment holes 34,
36 in release substrate 32. Deposition of flexible adhesive may be
accomplished by mesh screening, stencil screening, contact
screening, mask screening ink-jet printing or other suitable
method. Flexible adhesive preforms 30 are formed of a deposition of
flexible adhesive that may be electrically insulating or
electrically conductive, or may be of a thermoplastic or
thermosetting adhesive type, as set forth above. Each adhesive
preform 30 has a shape that corresponds to the bonding pattern
defined by the shape of the edges 42 of the cover or lid 40 that is
to be attached to an electronic substrate. For example, if the
cover 40 is in the form of a hollow rectangular solid, as is
illustrated in FIG. 1, adhesive preform 30 is in the shape of a
rectangle as is illustrated in FIG. 2, and if the cover 40 is in
the form of a hollow cylinder (not illustrated), adhesive preform
30 is in the shape of a circle.
[0027] FIG. 3 is a side cross-sectional view of the release
substrate 32 of FIG. 2 taken along section line I-I with the
plurality of adhesive preforms 40 thereon. Each adhesive preform 30
is relatively thin because it need only contain sufficient adhesive
to form a bond between a cover 40 and a substrate 20 when they are
pressed together in assembling an electronic device. Release
substrate 32 with the pattern of wet adhesive preforms 30 thereon
is ready to receive covers 40 on the respective wet preforms
30.
[0028] A guide plate 50 has a pattern of receptacles 52 therein
corresponding to the pattern of adhesive preforms 30 on release
substrate 32. Each receptacle 52 is adapted for releasably
receiving a cover 40 therein. Preferably, guide plate 50 also has a
set of relational alignment holes 34', 36' therethrough located to
correspond to the set of relational alignment holes 34, 36 in
release substrate 32 and in the same known relationship to the
pattern of receptacles 52 as are relational alignment holes 34, 36
to the pattern of adhesive preforms 30. Guide plate 50 is placed
over release substrate 32 so that the receptacles 52 in the guide
plate 50 are in direct correspondence in shape and size to the
adhesive preforms 30, preferably passing an alignment pin through
each of the corresponding pairs of respective relational alignment
holes 34, 36 and 34', 36'. Also preferably, receptacles 52 may be
several thousandths of an inch larger than the size of covers 40 to
allow easy placement thereof. The covers 40 are then placed
directly on top of the wet adhesive preforms 30 through receptacles
52 in guide plate 50. After all of the covers 40 have been placed
on adhesive preforms 30, the guide plate 50 is removed, producing
the result shown in FIG. 4. Release substrate 32 with the covers 40
on the adhesive preforms 30 is dried or B-staged, for example, in a
belt oven or a box oven, for a time sufficient to remove solvent
from the adhesive and/or for some chemical cross-linking of the
adhesive to occur, whereby the wet adhesive preforms 30 become
solid adhesive preforms 30, each one attached to a respective one
of the covers 40.
[0029] Covers 40 with dried adhesive preforms 30 thereon may then
be released from the release liner 32 and are ready to be used, for
example, in attachment onto a substrate of an electronic or other
functional device. Alternatively, covers 40 with adhesive preforms
30 thereon may be packaged in either tape-and-reel or waffle
packaging for ease of transportation and storage for later use, for
example, with conventional "pick-and-place" apparatus.
[0030] Alternatively, release substrate 32 may be employed with
conventional "pick-and-place" apparatus in two different ways.
Firstly, release substrate 32 with wet adhesive preforms 30 thereon
as shown in FIG. 3 may be transferred to a pick-and-place
apparatus, such as a model ECM 93 pick-and-place machine available
from Manncorp located in Huntingdon Valley, Pa., which then picks
up individual covers 40 and places one on each of the adhesive
preforms 30 on release substrate 32, thereby also producing the
result shown in FIG. 4. Release substrate 32 containing the wet
adhesive preforms 30 is then processed as described above.
Secondly, release substrate 32 with covers 40 attached thereto by
dried adhesive preforms 30 as shown in FIG. 4 may be transferred to
a pick-and-place apparatus, such as the Manncorp model ECM 93,
which apparatus then picks up each cover 40 with dried adhesive
preform 30 attached thereto and places it in the predetermined
location on the substrate of an electronic or other functional
device. In either of the foregoing ways of utilizing release
substrate 32 with pick-and-place apparatus, release substrate 32
may be positioned on such pick-and-place apparatus by employing the
relational alignment holes 34, 36 therein, whereby the location of
each adhesive preform 30 and/or of each cover 40, as the case may
be, on the pick-and-place apparatus is determined precisely.
[0031] In the perspective view of FIG. 5 is shown a plurality of
non-conductive lids or covers 40 and a plurality of electrically
conductive lids or covers 40' attached to an electronic substrate
20' such as a printed circuit wiring board. Each cover 40, 40'
covers and protects one or more components that are attached to
printed wiring board 20', for example, by adhesives, soldering,
wire bonding or other known arrangement. Respective ones of covers
40, 40' are attached to printed wiring board by an insulating
adhesive preform 30 or by an electrically-conductive adhesive
preform 30' that was formed on covers 40 in the manner described
herein above.
EXAMPLE 1
[0032] Example 1 involves a lid 40 for protecting semiconductor
devices 22 inside a small module 10' for communication equipment,
such as portable electronic pagers and mobile or cellular
telephones. Semiconductor devices 22 are attached onto a functional
board 20' that is a printed wiring circuit board 20' made of
standard FR4 substrate material. Interconnections between circuit
board 20' and devices 22 may be made, for example, either by
conventional wire-bonding or by conventional "flip-chip" bonding.
The electronic modules are typically arranged in a panel of
multiple repeated circuitry. Lids 40 with pre-applied adhesive
preforms 30 thereon are placed on top of the circuit board 20'
substrate and are bonded thereto with heat and pressure for a
specific period of time determined by the adhesive. In this
example, a B-stageable insulating epoxy adhesive type LESP7670
available from AI Technology, Inc. is employed for lid sealing. The
LESP7670 adhesive paste is first deposited onto the release
substrate base 32 in the form of a pattern of repetitive units of
rectangular preforms 30 located in known predetermined relationship
with respect to a set of relational alignment holes 34, 36 as shown
in FIG. 2 that have preferably been made outside the area useful
for depositing adhesive preforms 30. Typically, adhesive preforms
30 have a thickness of about 75 to 150 microns. Although deposition
methods including screen-printing, stencil-printing, and contact
and impact deposition methods have been found useful, stenciling is
preferred in this example. Use of the relational alignment holes
34, 36 is particularly advantageous when adhesive preforms 30 are
to be deposited on many release substrates 32 that are to be used
to facilitate high-volume assembly-line-like deposition of
adhesive. Release substrate 32 with the wet adhesive preforms 30 is
then transferred to another station where a guide plate 50 is
placed over release substrate 32 and is aligned therewith by a
corresponding set of relational alignment holes 34', 36' in guide
plate 50, as described above. Lids 40 are then placed through the
receptacle holes 52 and directly on the wet adhesive preforms 30.
After all the lids 40 have been so placed, guide plate 50 is
removed. Release substrate 32 with lids 40 attached thereon by
adhesive preforms 30 is then placed in a belt oven or box oven
heated to a temperature of about 60-80.degree. C. for a time, such
as about 30-60 minutes, sufficient to remove solvent from adhesive
preforms 30 and to permit partial chemical cross-linking thereof,
so that wet adhesive preforms 30 become solid adhesive preforms
attached to lids 40. Lids 40 with dry adhesive preforms 30 attached
thereto are released from release liner 32 and are ready for
attachment onto circuit board 20' by pick-and-place equipment. Lids
40 with adhesive preform 30 attached thereto are pressed against
electronic circuit board substrate 20' (as shown in FIG. 5) at a
temperature of about 150-180.degree. C. for about three to ten
minutes with about 10 psi applied pressure, which is sufficient to
produce adequate flow of adhesive preform 30, during the bonding
process of lids 40 to circuit board 20'. Type LESP7670 epoxy
adhesive may be used without additional curing. Lids 40 may be
easily removed without damaging circuit board 20' by concentrating
the stress upon the adhesive preform 30, as by pulling the lid,
twisting the lid, or prying the lid, and may be facilitated by
heating the adhesive preform to a temperature sufficient to reduce
its bonding strength.
EXAMPLE 2
[0033] Example 2 is an alternative employing the same adhesive
deposition method and adhesive material as in Example 1, however,
instead of using guide plate 50 to facilitate precision placement
of lids 40 on the wet adhesive preforms 30, standard pick-and-place
equipment conventionally employed for precisely mounting components
by surface mounting technology (SMT) is employed. Suitable SMT
pick-and-place equipment is commercially available from Mydata
Automation located in Peabody, Mass., from Universal Instrument
located in Binghamton, N.Y., from Zevatech Inc. located in
Morrisville, N.C., and from Manncorp, and can place components
(i.e. lids 40) onto circuit boards with a positional inaccuracy of
one one-thousandth of an inch or less and at a rate greater than
one lid per second. In fact, positioning lids 40 within two
one-thousandths of an inch is more than adequate accuracy for most
applications. Once release substrate 32 is fully populated with
lids 40, it is heated for B-staging adhesive preforms 30. The fact
that the wet adhesive preform 30 populated release substrate 32 can
be handled in much the same way as is a conventional printed
circuit board deposited with solder paste and the lids can be
handled as components, greatly facilitates automating process of
applying adhesive preforms 30 to covers 40, thereby to increase the
production rate and uniformity of adhesively preformed covers,
while reducing the production cost thereof. Advantageously, the
cover of the present invention is compatible with conventional
automated assembly equipment that users thereof may already have
and so may elect to employ.
EXAMPLE 3
[0034] Example 3 utilizes the same processes for pre-applying
adhesive preforms 30 onto protective lids 40 and for bonding lids
40 to circuit board 20', however, the lid 40 in this Example 3 has
a wider bonding edge, for example, because the material of lid 40
is thicker or the edges thereof are flared to increase the bonding
area. As a result, lid 40 may be attached with an adhesive preform
30 having a lower bonding strength and yet provide the same
mechanical protection. To that end, a B-stageable flexible epoxy
paste type LESP7450 also available from AI Technology, Inc. is
employed. Type LESP7450 has an intrinsic bond strength of
approximately 2000 psi at ambient temperature, which is less than
about 30% of the bond strength of typical high-strength lid seal
adhesives, and is flexible (i.e. has a modulus of elasticity of
less than about 200,000 psi) over substantially more than half of
its specified operating and storage temperature range, for example,
a temperature range of -55.degree. C. and 150.degree. C. The bond
strength of type LESP7450 adhesive drops to approximately 300 psi
at temperatures at or above about 90.degree. C., i.e. a temperature
substantially lower than the melting temperature of solder, thereby
to allow easier removal of lid 40 by applying torque, prying or
other concentration of stress. Ease of removal is a desirable
feature, especially for larger lids and lids with larger bonding
areas.
EXAMPLE 4
[0035] Example 4 employs an electrically conductive B-stageable
flexible thermoplastic adhesive paste, type LTP8090 available from
AI Technology, Inc., in conjunction with conductive covers to
provide EMI shielding. Specifically, cover 40' is a metallic shell
formed of a magnetic stainless steel sheet having a thickness of
approximately 150 microns. Small openings are provided on the top
of cover 40' to allow viewing of the interior thereof, for example,
for inspection, and to permit air flow for cooling the electronic
components enclosed by cover 40'. These openings are small as
compared to the wavelength of the electromagnetic radiation of
interest and thus prevent EMI from leaking into and out of the
cover 40', for example, where cover 40' is employed in a handset of
mobile cellular telephone. Openings smaller than about 5 mm, for
example, will not pass electromagnetic signals at frequencies less
than about 50 GHz. Type LTP8090 conductive adhesive paste is
deposited onto a release substrate 32 in a preform shape to
coincide with the bonding area shape of cover 40' which are placed
onto the wet adhesive preforms 30' with a guide plate 50 as in
Example 1. Covers 40' with the wet adhesive preforms 30' thereon
are then B-staged to form dry preforms 30' attached to covers 40'
which are then attached onto the electronic module 20' at a
temperature of about 150-180.degree. C. with about 10 psi pressure.
it is noted that adhesive preform 30' and cover 40' form a Faraday
electrostatic shield against EMI leakage. Because type LTP8090
adhesive is a thermoplastic resin having sharp or well-defined
melting temperature of about 110.degree. C., covers 40' can be
easily removed once the temperature of the bonding areas is raised
above that melting temperature. As a result, electronic devices
including such covers may be easily reworked at temperatures well
below the melting point of solder and the maximum temperature that
semiconductor and other electronic components can withstand,
thereby avoiding degradation of or damage to such electronic
components.
[0036] In Examples 1-4 above, the adhesive preforms are generally
preferred to be slightly wider than are the edges of lids 40, 40'
that serve as bonding areas, so that the preforms attach to lids
40, 40' with sufficient bonding area before they are attached to an
electronic device 10, 10'. However, where lids 40, 40' have wide
bonding edge areas, and particularly where adhesive preform 30, 30'
is an electrically conductive adhesive, it may be important to
confine the area and volume of adhesive in adhesive preforms 30,
30' on lids 40, 40' to avoid unwanted electrical connections,
bridges and short circuits by adhesive preforms 30, 30', such as to
electronic components and conductors located close to lids 40, 40'.
It is noted that even insulating adhesives can form a high
resistance (e.g., multi-megohm) path that will disturb certain
high-impedance circuits. In some cases, it may be advantageous to
substantially displace adhesive preforms 30, 30' toward the outside
edges of the lids 40, 40' not only to avoid potential electrical
bridging and other contamination problems, but also to avoid
adhesive flowing into the interior of the space covered by lids 40,
40' that can not be inspected. It is also noted that the
temperature at which attachment and removal of the lids 40, 40' of
Examples 1-4 is performed is substantially lower than the
temperature of about 220.degree. C. at which soldering is
performed, thereby reducing the likelihood that high temperature
will disturb, damage or degrade the electronic devices proximate to
such covers.
[0037] Conventional isothermal curing or similar curing of
thermosetting adhesive preforms 30, 30' is generally undesirable
because the time that the lids 40, 40' and adhesive preforms 30,
30' attached thereto are heated may be too long unless great care
is exercised. If the time of pre-attachment heating to a
temperature at or near the adhesive curing temperature is too long,
the adhesive may gel too much or may partially cure and so not have
sufficient strength to properly bond to substrate 20, 20'.
Accordingly, it is desirable that the attachment bonding process
employed with the adhesive selected for adhesive preforms 30, 30'
be improved over that of the prior art.
[0038] In an improved cover attachment process, substrate 20, 20'
is preheated to a substantially higher temperature than are lids
40, 40'. For example, electronic circuit substrate 20, 20' may be
heated to about 150-200.degree. C., i.e. a temperature sufficiently
high to tack thermosetting adhesive preforms 30, 30', while lids
40, 40' with thermosetting adhesive preforms 30, 30' attached
thereto are maintained at ambient temperature or an elevated
temperature less than about 80.degree. C. Lids 40, 40' with
pre-applied thermosetting adhesive preforms 30, 30' attached
thereto may be placed onto the preheated electronic circuit
substrate 20, 20' by a standard pick-and-place apparatus and, upon
placement, lids 40, 40' having adhesive preforms 30, 30' are heated
by and become tacked to substrate 20, 20'. Then substrate 20, 20'
may be placed in a heating belt oven for about an additional 3-5
minutes at a temperature slightly below that of the substrate 20,
20' preheat station. For example, substrate 20, 20' may be
preheated to about 175.degree. C. and may be cured subsequent to
lid 40, 40' attachment in a belt-oven for an additional three
minutes at about 150.degree. C.
[0039] In the case of thermoplastic adhesive preforms 30, 30',
post-attachment curing is not necessary and the only temperature
requirement on the process for attaching lid 40, 40' to substrate
20, 20' is that the thermoplastic adhesive preform 30, 30' be
heated to the melt-flow temperature of the thermoplastic adhesive.
The necessary heat can be provided by preheating lids 40, 40' or by
the transfer of heat from the preheated substrate 20, 20' to lids
40, 40'. It is preferred to preheat lids 40, 40' to a temperature
substantially above the melt-flow temperature of the thermoplastic
adhesive preforms 30, 30' and to then press lids 40, 40' against
the warm substrate 20, 20' that may be at a temperature about
50-100.degree. C. below the temperature of lids 40, 40'. The
temperature differential causes rapid cooling of the thermoplastic
adhesive preforms 30, 30' immediately following pressing of lids
40, 40' against substrate 20, 20', thereby promoting rapid setting
of the thermoplastic adhesive.
[0040] Thus, lids or covers 40, 40' are attached to an electronic
circuit substrate 20, 20' at a high rate, for example, one per
second, and by employing automated assembly equipment of a kind
presently available in most modern manufacturing facilities. This
result is obtained with thermoplastic and thermosetting adhesives,
and with electronic circuit modules, flip-chip modules, and printed
wiring circuit boards whether receiving one or a large number of
covers or lids attached thereto. The lids with adhesive preforms
attached thereto applied in the foregoing manner may be of the same
or different size and shape, may be of the same or different
material, and may provide physical protection and/or electrostatic
or electromagnetic protection.
[0041] In addition, adhesive preforms and lids according to the
present invention advantageously may be employed to avoid the
so-called "blow-out" problem caused by gas trapped in the interior
of a lid or cover that, when heated during the lid attachment
process, ruptures the adhesive attachment between the lid and the
package, thereby causing a failure in the adhesive seal 30 between
the cover 40 and the substrate 20. To this end, preforms 30, 30'
are formed having one or more gaps therein, as shown in FIG. 2,
through which gas may bleed or flow. For example, adhesive preform
30a has one gap 31 in one side thereof, whereas adhesive preform
30b has two gaps 31, one in each of two opposing sides thereof.
Adhesive preform 30c has four gaps 31, one in each of the four
sides thereof. Similarly, adhesive preforms 30d, 30e and 30f have
gaps 31 in one, two and four corners thereof, respectively. Each
gap is narrow, being sufficient to permit entrapped gas molecules
to pass, but is narrow enough to be closed by the flowing of the
adhesive 30 when cover 40 is attached to a substrate 20 by heating
and pressing against substrate 20. For example, in a square
adhesive preform 30 formed of type ESP7450 adhesive that is about
0.35 inch long on each side, wherein the adhesive preform sides are
about 40 mils wide and 6 mils thick, each of the four gaps is about
5 mils across. Segmented adhesive preforms 30a, 30b, 30c, 30d, 30e,
30f are easily fabricated and applied to covers 40, 40' by
employing the method described herein because such preforms are
deposited by accurate processes on a release substrate 32 and
covers 40, 40' are attached thereto while the preforms are still
attached to the release substrate 32. Thereafter, the covers 40,
40' with adhesive preform 30a, 30b, 30c, 30d, 30e, 30f attached is
easily handled by pick-and-place equipment. To attempt to form such
gapped adhesive preform 30a, 30b, 30c, 30d, 30e, 30f by
conventional methods which require handling of the preform would be
extremely difficult, if not impossible, due to the small size and
delicacy of the preform alone.
[0042] While the present invention has been described in terms of
the foregoing exemplary embodiments, variations within the scope
and spirit of the present invention as defined by the claims
following will be apparent to those skilled in the art. For
example, the adhesives of which preforms 30, 30' are formed may be
filled with certain materials to tailor their characteristics to a
particular application. Thermal conduction of the adhesive may be
increased by the addition of particles of a high-thermal
conductivity material, such as alumina (Al.sub.2O.sub.3), aluminum
nitride (AlN), boron nitride (BN), silicon carbide (SiC), or
diamond, which fillers may also be employed to modify the
coefficient of thermal expansion thereof. The coefficient of
thermal expansion thereof may also be reduced by the addition of
particles of glass silicates, for example.
* * * * *