U.S. patent application number 09/136201 was filed with the patent office on 2001-10-11 for dimensionally stable core for use in high density chip packages and a method of fabricating same.
Invention is credited to FISCHER, PAUL J., GORRELL, ROBIN E., SYLVESTER, MARK F..
Application Number | 20010029065 09/136201 |
Document ID | / |
Family ID | 25003954 |
Filed Date | 2001-10-11 |
United States Patent
Application |
20010029065 |
Kind Code |
A1 |
FISCHER, PAUL J. ; et
al. |
October 11, 2001 |
DIMENSIONALLY STABLE CORE FOR USE IN HIGH DENSITY CHIP PACKAGES AND
A METHOD OF FABRICATING SAME
Abstract
A dimensionally stable core for use in high density chip
packages is provided. The stable core is a metal core, preferably
copper, having clearances formed therein. Dielectric layers are
provided concurrently on top and bottom surfaces of the metal core.
Metal cap layers are provided concurrently on top surfaces of the
dielectric layers. Blind or through vias are then drilled through
the metal cap layers and extend into the dielectric layers and
clearances formed in the metal core. If an isolated metal core is
provided then the vias do not extend through the clearances in the
copper core. The stable core reduces material movement of the
substrate and achieves uniform shrinkage from substrate to
substrate during lamination processing of the chip packages. This
allows each substrate to perform the same. Additionally, a
plurality of chip packages having the dimensionally stable core can
be bonded together to obtain a high density chip package.
Inventors: |
FISCHER, PAUL J.; (EAU
CLAIRE, WI) ; GORRELL, ROBIN E.; (EAU CLAIRE, WI)
; SYLVESTER, MARK F.; (EAU CLAIRE, WI) |
Correspondence
Address: |
VICTOR M GENCO
W L GORE AND ASSOCIATES
551 PAPER MILL ROAD
NEWARK
DE
197149206
|
Family ID: |
25003954 |
Appl. No.: |
09/136201 |
Filed: |
August 19, 1998 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09136201 |
Aug 19, 1998 |
|
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08747169 |
Nov 8, 1996 |
|
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5847327 |
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Current U.S.
Class: |
438/115 ;
257/758; 257/773; 257/774; 257/E23.067; 428/209; 438/106;
442/232 |
Current CPC
Class: |
H05K 3/429 20130101;
H05K 2201/09536 20130101; H05K 2201/0116 20130101; Y10T 442/3415
20150401; H01L 23/49827 20130101; H05K 3/4641 20130101; H01L
2924/0002 20130101; H05K 3/4608 20130101; H05K 3/4614 20130101;
Y10T 428/24917 20150115; Y10T 428/249958 20150401; H05K 2201/015
20130101; H05K 3/445 20130101; H01L 2924/0002 20130101; H05K 3/323
20130101; H01L 2924/00 20130101; H05K 3/427 20130101 |
Class at
Publication: |
438/115 ;
438/106; 257/758; 257/773; 257/774; 428/209; 442/232 |
International
Class: |
H01L 021/44; H01L
021/48; H01L 021/50; H01L 029/40 |
Claims
1. A method for forming a dimensionally stable core for a chip
package, said method comprising the steps of: a) forming a metal
core with clearances therein; b) placing a dielectric layer
concurrently on top and bottom surfaces of the metal core, said
dielectric layer having top and bottom surfaces, respectively; and
c) placing a metal cap layer concurrently on the top surface of a
top dielectric layer and the bottom surface of a bottom dielectric
layer.
2. The method according to claim 1, further comprising the steps
of: d) laminating the metal core, the dielectric layers and the
metal cap layer on each side of the metal core.
3. The method according to claim 1, further comprising the steps
of: e) drilling vias through the dielectric layer the metal cap
layer and the metal core.
4. The method according to claim 1, further comprising the steps
of: f) metallizing the vias by forming a metal layer on the sides
of the vias and extending onto the metal cap layer and the
dielectric layer.
5. The method according to claim 1, further comprising the steps
of: g) repeating steps b) through f) from 2 to 100 times.
6. A method according to claim 1, wherein in the metal core is
copper.
7. A method according to claim 1, wherein the metal cap layer is a
copper cap layer.
8. A method according to claim 1, wherein in said step b) the
dielectric is a thermosetting prepreg.
9. A method according to claim 8, wherein the thermosetting prepreg
is a non-woven material containing a cyanate ester resin in a
polytetrafluoroethylene matrix.
10. A method according to claim 9, wherein in said step b) the
dielectric layer is a sheet having a sufficient thickness to
completely fill the clearances in the copper core.
11. A method according to claim 5, wherein said laminating step d)
comprises the steps of: i) applying pressure to the dielectric and
metal cap layers at approximately 300-350 psi; ii) applying
temperature at a ramp rate of 5-7.degree. C. per minute until
reaching a temperature of 177.degree. C.; iii) holding the
temperature of 177.degree. C. for approximately 30 minutes; iv)
raising the temperature to 220.degree. C.-225.degree. C. and
holding at this temperature for approximately 60 minutes; and v)
slowly cooling the chip package while maintaining the pressure.
12. A method according to claim 11, wherein said step e) comprises
drilling through vias and blind vias.
13. A method according to claim 12, wherein in said step e), the
vias are drilled using a laser.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of application Ser. No.
08/747,169 filed Nov. 8, 1996.
FIELD OF THE INVENTION
[0002] The present invention is directed to a dimensionally stable
core for high density chip packages which reduces material movement
variability during the processing and the manufacture of the chip
packages. The stable core, which provides the stability to the high
density chip package, is preferably formed from a metal and
includes concurrently formed dielectric and metal layers. Vias are
subsequently formed to permit the manufacture of a high density
chip package.
BACKGROUND OF THE INVENTION
[0003] High density chip package substrates require many vias which
connect the input/output (I/O) of an integrated circuit chip to
routing layers or power/ground layers buried within the chip
package substrate. Conventional mechanical drilling processes which
have been used in the past for the formation of vias are being
replaced with laser via drilling processes. Laser via drilling
processes offer the potential to form vias much more rapidly, and
at less cost, than conventional mechanical drilling processes. To
enable this rapid rate of via production, it is advantageous to use
dielectrics which are thinner than those which have been used in
the past. The thin dielectrics allow the formation of micro vias,
i.e., vias <100 microns in diameter.
[0004] To effectively metallize these micro vias after they have
been formed, the aspect ratio (the via's height divided by its
diameter) must be maintained at a predetermined value. For example,
for blind vias, the aspect ratio must be approximately one to
insure that conventional metallization methods will be adequate to
deposit a sufficient amount of metal onto the wall surface of the
micro blind via.
[0005] As should be understood, it is disadvantageous to form micro
blind vias in a dielectric material that contains a woven glass
reinforcement. More particularly, if a woven glass reinforced
dielectric is used, the laser power required to ablate the woven
glass is as high as the laser power required to ablate a buried
copper pad on the layer to which the blind via is to provide
connection. Accordingly, the laser will penetrate the copper pad
and continue into layers below, thus extending the via. After
metallization, the unintentionally extended via will short to
subsequent layers, causing the entire circuit to be scrapped.
[0006] In order to solve the shortcomings associated with forming
micro blind vias in a dielectric material containing woven glass,
dielectric material manufacturers have introduced thin,
unreinforced prepregs to replace conventional glass reinforced
prepregs. However, the use of such thin, unreinforced prepregs has
caused other difficulties in the production of high density printed
circuit boards and chip package substrates. More particularly, the
elimination of woven glass from the dielectric material has reduced
the dimensional stability of a chip package substrate. When
thermosetting resins are used as a component of the dielectric
material, these resins commonly induce shrinkage in a multilayer
circuit board, or chip package substrate, upon lamination and
curing of the resins. The shrinkage is limited to the order of a
few thousand parts per million at most, and is not in itself a
manufacturing problem, as the shrinkage can be compensated for when
the artwork for the substrate is produced. However, variability in
the degree of shrinkage exists, even for one unchanging circuit
design. This variability in the degree of shrinkage can lead to
reduced yields in high density printed circuit boards and chip
packaging substrates. For example, if one of the internal layers in
a multilayer circuit board is a ground or power layer consisting of
a plane of copper, and if vias have to pass through clearances in
this layer, the shrinkage variability induced during lamination can
result in a variable location of the ground clearances from panel
to panel. When vias are drilled in these circuits, some percentage
of these vias may miss the clearances in the ground or power layer
and strike the copper. After metallization, these vias would be
shorted to the ground or power layer, resulting in a scrapped
circuit.
[0007] There exists a need for a dimensionally stable core to
facilitate the manufacture of high density chip packages having
dielectric materials unsupported by woven glass.
SUMMARY OF THE INVENTION
[0008] The present invention includes a method of manufacturing
high density chip package substrates by processing circuit layers
concurrently on both sides of a metal core, which is preferably
made of copper. The copper core improves the dimensional stability
of thin, unreinforced dielectric materials used in the substrate.
As a result of the metal core, material movement is reduced during
lamination, which facilitates the processing and manufacture of the
high density chip package. The metal core also provides buried
capacitance which helps reduce simultaneous switching noise on the
chip.
[0009] The dimensionally stable core of the present invention
includes a metal core having at least one clearance etched
therethrough, a dielectric layer on both sides of the metal core
and filling the clearance, a metal cap layer on both sides of the
dielectric layer, and drilled vias in the metal core through the
metal cap layer. When the metal core is to be electrically
isolated, vias are formed to extend through the metal cap layer,
the dielectric layer and the clearance in the metal core, thus
isolating the metal core. A metal layer is deposited on the wall
surfaces of such vias. The dielectric layer and metal cap layer can
be placed concurrently on top and bottom surfaces of the metal
core. Additional dielectric layers and metal cap layers can again
be concurrently laminated on top and bottom surfaces of the cap
layers in a similar fashion. Additional vias, where necessary, are
formed. Repeatedly adding dielectric and metal caps layers, and
vias provides a dimensionally stable, high density chip
package.
[0010] The metal core and the metal layers can be copper, or any
other suitable metal, such that the coefficient of thermal
expansion (CTE) of the chip package matches the CTE of the printed
circuit board. The dielectric can be any unsupported prepreg.
Further, the vias can be blind vias or through vias. The vias are
drilled using a laser which ablates material as it drills.
[0011] A method for fabricating a dimensionally stable core
according to the present invention includes providing a metal core,
such as copper, placing dielectric layers on top and bottom
surfaces of the metal core, placing metal cap layers, such as
copper, on top and bottom surfaces of the dielectric layer and
laminating the dielectric and metal cap layers together and to the
metal core. The method further includes drilling vias through the
metal cap layers, dielectric layers and the metal core, or
clearances in the metal core, and then repeating the steps to form
a multi-layer high density chip package.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The foregoing summary, as well as the following detailed
description of a preferred embodiment of the invention, will be
better understood when read in conjunction with the appended
drawings. For purposes of illustrating the invention, there is
shown in the drawings an embodiment which is presently preferred.
It should be understood, however, that the invention is not limited
to the precise arrangement and instrumentality shown. In the
drawings:
[0013] FIG. 1 is a side view of a metal core prior to
processing;
[0014] FIG. 2 is a side view of a metal core having a patterned
photoresist thereon prior to etching;
[0015] FIG. 3 is a side view of the metal core in FIG. 1 having a
clearance etched through;
[0016] FIG. 4 is a side view of a substrate after a dielectric and
metal "cap" layers have been laminated to the metal core;
[0017] FIG. 5 is a side view of the substrate according to the
present invention after vias have been laser drilled and
metallized, and circuit traces have been formed;
[0018] FIG. 6 is a side view of the substrate in FIG. 4 after
additional dielectric and metal "cap" layers are added;
[0019] FIG. 7 is a side view of the finished chip package according
to the present invention, after blind vias have been drilled;
[0020] FIG. 8 is a side view of a substrate according to a
preferred embodiment of the present invention;
[0021] FIG. 9 is a scanning electronmicrograph of an expanded
polytetrafluoroethylene (ePTFE) material that can be used as the
dielectric layer in the present invention;
[0022] FIG. 10 is a scanning electronmicrograph of an expanded PTFE
material that forms the matrix that contains the adhesive filler to
form one dielectric material in the present invention;
[0023] FIG. 11 shows two substrates laminated together using a
conductive Z-axis material;
[0024] FIGS. 12-14 schematically show the formation of a conductive
Z-axis material containing irregularly shaped conductive
pathways;
[0025] FIGS. 15-18 are scanning electronmicrographs showing the
node fibril structure of some ePTFE matrix materials used to form a
conductive Z-axis material; and
[0026] FIG. 19 shows two substrates containing blind vias laminated
together with a conductive Z-axis material.
[0027] FIG. 20 illustrates a high density chip package in
accordance with the teachings of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0028] The present invention is directed to a dimensionally stable
core for high density chip packages. The stable core is a metal
core, preferably copper, that contains clearances therein.
Dielectric layers are placed on top and bottom surfaces of the
metal core. Metal cap layers are placed on top and bottom surfaces
of the dielectric layers, respectively. The dielectric layers and
metal cap layers can be placed concurrently on both sides of the
package. Blind or through vias are then drilled through the metal
cap layers and into and through the dielectric layers and
clearances in the metal core. A laser is used to drill the vias by
ablating material where the vias are to be formed. If the metal
core is to be a power or ground plane, the vias are formed directly
through the metal core. If the metal core is an isolated metal
core, then the vias extend through the clearances in the metal
core, i.e., the metal core does not have vias making direct contact
therewith.
[0029] The stable core reduces material movement of the substrate
and achieves uniform shrinkage from substrate to substrate during
lamination processing of the chip packages. This allows each
substrate to perform the same. Additionally, a plurality of
dimensionally stable core chip packages can be bonded together to
obtain a high density chip package.
[0030] FIG. 20 illustrates a high density chip package 1 in
accordance with the teachings of the present invention. High
density chip package 1 includes a substrate 2 which mechanically
mounts a high density integrated circuit chip 3.
[0031] As seen in FIG. 1, the metal core 10 is a metal sheet, such
as a copper sheet, and has opposed rough surfaces 12 and 14 on both
sides of the metal. The metal sheet has been treated by a process
known as a "double treat" process. The rough surfaces allow better
adhesion of the dielectric layers. Other metals can be used instead
of copper as long as the coefficient of thermal expansion (CTE) of
the metal core is closely matched to the CTE of the printed circuit
board to which the chip package will eventually be attached. For
example, CIC, a lamination of copper-INVAR 36-copper, can be used
in place of a solid copper core. The nickel alloy that is used can
contain from 30 wt % to 42 wt. % nickel. Metal core 10 must be of
sufficient thickness to impart dimensional stability to the
finished substrate, e.g., between 0.0005 inches and 0.0028 inches,
preferably 0.0014 inches. A thickness in this range provides a
lower flexural modulus to the chip package.
[0032] As seen in FIG. 2, a photoresist 22 is applied to both sides
of the metal core 10 and is imaged with a pattern of clearances 20
where vias are to be formed, using a conventional method. The
imaged areas are chemically etched, using conventional processes,
to remove the copper and form clearances 20 (FIG. 3). The
clearances 20 are wider in diameter than vias that will be formed
later. The dimensions of the vias are dependent on the type of
dielectric that will be placed on the metal core. The photoresist
is then removed using conventional processes.
[0033] As seen in FIG. 4, the metal core 10, including at least one
clearance 20, includes dielectric layers 30 and 32. Preferably, the
dielectric layers 30 and 32 are formed concurrently on the surfaces
12 and 14 of metal core 10. The CTE of the dielectric layers 30 and
32 can be as high as 40 ppm/.degree. C. The dielectric layers 30
and 32 must be of sufficient thickness to fill the clearances 20.
The thickness of the dielectric layers 30 and 32 is at least 5
microns or larger, and is typically between 25 microns and 50
microns. The thickness is chosen so that there is no loss of
dimensional stability of the substrate. The dielectric thickness
should not be significantly greater than the thickness of the metal
core 10, or dimensional stability of the substrate will
decrease.
[0034] Metal core 10 includes metal cap layers 40 and 42 on
surfaces 34 and 36 of dielectric layers 30 and 32, respectively.
Metal cap layers 40 and 42 are concurrently placed on surfaces 34
and 36 of the dielectric layers, and are formed from copper. The
caps layers 40 and 42 can be formed from any suitable conductive
metal. The metal core 10, dielectric layers 30 and 32, and the
metal cap layers 40 and 42 are laminated together using any
suitable lamination process.
[0035] In one embodiment of the present invention layers 30 and 32
were formed from SPEEDBOARD C.RTM., manufactured by W. L. Gore and
Associates. The dielectric layers 30 and 32 were laminated to the
core 10 at a pressure of 300-350 psi, and a temperature ramp rate
of 5-7.degree. C. per minute up to a temperature of 177.degree. C.
The temperature was held at 177.degree. C. for 30 minutes. The
temperature was then raised to 220-225.degree. C. and held for 60
minutes. The lamination was slowly cooled for several hours while
the substrate was under pressure.
[0036] As shown in FIG. 5, the laminated dielectric layers 30 and
32, and the metal cap layers 40 and 42 are processed to contain at
least one via 50 which is drilled through the metal cap layers 40
and 42 and dielectric layers 30 and 32.
[0037] FIG. 5 shows via 50 as a through via, which can be drilled
using any known process, such as, lasing, for example. For through
via 50, the laser must be capable of penetrating dielectric layers
30 and 32 and the metal core 10, when not passing through clearance
20. The through via 50 can have an aspect ratio varying from 1:1 to
20:1, or higher. When blind vias are formed, the laser has to
penetrate the dielectric layers 30 or 32, but not the metal core
10. A typical aspect ratio of 1:1 should be maintained. However,
the aspect ratio is not limited to 1:1.
[0038] In accordance with the present invention, to obtain uniform
laser via formation with suitable aspect ratios, a pulse repetition
rate ranges from 1 KHz to 10 MHZ, which varies the pulse width of
the laser. Residue remaining in the vias is then ablated by again
adjusting the pulse intensity. Lasers capable of producing energy
densities in the range of 0.5 to 20 J/cm.sup.2, and those capable
of making in situ and virtually instantaneous changes in the energy
density by varying the pulse repetition rate from 1 KHz to 10 MHZ,
are employed.
[0039] In one embodiment of the present invention, such as that of
FIG. 7, blind vias were formed with lasers that are solid state
pulsed lasers such as a pulsed Nd:YAG laser. The fundamental output
from the Nd:YAG laser is at a wavelength of 1064 nm. This
wavelength is in the infrared portion of the electromagnetic
spectrum. By installing beta-barium borate (BBO) crystals in the
optical path, harmonic generation facilitates the output of light
at 355 nm (third harmonic) and 266 nm (fourth harmonic), which fall
in the ultraviolet range. The 355 nm and 266 nm wavelengths are
particularly well suited for drilling vias in the laminated
substrates of the present invention.
[0040] In general, the pulse length, energy density and number of
pulses can be varied depending on the type of via being formed and
the type of materials used in the laminated substrate. When
drilling blind vias in a laminated substrate made of alternating
layers of copper and a dielectric comprised of adhesive, filler and
expanded polytetrafluoroethylene (ePTFE), and using a 266 nm laser,
the energy density is 1.5 J/cm.sup.2, the energy per pulse is 10 J,
and the power density is 20 megawatts per cm.sup.2. At the 355 nm
wavelength, blind vias are formed by setting the energy density at
3.5 J/cm.sup.2, the energy per pulse is 30 J, and the power density
is 35 megawatts per cm.sup.2. The post-pulse step for the blind
vias using a 355 nm laser requires adjusting the conditions such
that the energy density is 11 J/cm.sup.2, the energy per pulse is
100 J, and the power density is 200 megawatts per cm.sup.2.
[0041] When forming blind vias in the adhesive-filler-ePTFE
dielectric material using the 355 nm laser, the energy density is 7
J/cm.sup.2, the energy per pulse is 65 J, and the power density is
100 megawatts per cm.sup.2. A post-pulse step would require
adjusting these parameters such that the energy density is 11
J/cm.sup.2, the energy per pulse is 100 J, and the power density is
200 megawatts per cm.sup.2.
[0042] As shown in FIG. 5, through via 50 is metallized by forming
a metal layer 60 on the surface of the via 50. The metal layer 60
includes, for example, a layer of electroless copper formed on the
surface of the via 50 and a layer of electrolytic copper formed on
the layer of electroless copper using conventional processes. The
metal layer 60 extends onto the metal cap layers 40, 42 and the
dielectric layers 30, 32. Traces (not shown), i.e., signal wires,
are then etched on the metal cap layers 40, 42 and the metal layers
60 by techniques known in the art. That is, for inner layer traces,
i.e., traces that are not on a final layer, the top most metal cap
layers 40, 42, and metal layer 60 are imaged with a photoresist.
The photoresist is patterned and then developed so that the
photoresist defines the signal traces in the area beneath the
photoresist. The metal between the patterned photoresist is then
etched and any remaining photoresist is stripped away leaving
traces around the vias. This is referred to as a subtractive
process.
[0043] For final layer traces, i.e., traces on a final layer, the
top most metal cap layers 40, 42 and the metal layer 60 are imaged
with a photoresist. The photoresist is then patterned and developed
so that the photoresist is removed where the signal traces are to
be formed. Additional metal, such as copper, is plated where the
photoresist was removed. Then an etch resistant metal, such as
nickel, gold, tin or solder, is plated on the additional metal, the
photoresist is stripped off and the metal cap layers 40, 42 under
the photoresist is etched off. Areas that are to remain are plated
with the metal, and areas that are to be etched away, are exposed.
This is referred to as a semi-additive process.
[0044] Preferably, as much of the metal layer 60 as possible should
be left on the dielectric layers 30 and 32 to further aid in
producing a dimensionally stable substrate. If the metal layer 60
is a ground or power plane, a large amount of metal, such as
copper, would remain by operation of the process employed.
[0045] FIG. 6 shows another embodiment of the present invention
wherein metal cap layers 40 and 42, and metal layer 60 are not a
final layer. In this embodiment, additional dielectric layers 70
and 72 are respectively placed on surfaces 62 and 64 of metal layer
60. Additional dielectric layers 70 and 72 fill the via 50 and
extend over the metal layer 60, the dielectric layers 30 and 32 and
the remaining cap layers 40 and 42. The additional dielectric
layers 70 and 72 encapsulate the traces.
[0046] Additional metal cap layers 80 and 82, such as copper, are
placed on the surfaces 74 and 76 of dielectric layers 70 and 72,
respectively. The additional dielectric layers 70 and 72, as well
as, additional metal cap layers 80 and 82 can be placed
concurrently on both sides of the package. The dielectric layers 70
and 72 and metal cap layers 80 and 82 are then laminated together
as described hereinabove.
[0047] FIG. 7 shows an embodiment where blind vias are added to the
substrate of FIG. 6. Blind vias 90 and 92 are formed in metal cap
layers 80 and 82 respectively, as well as, dielectric layers 70 and
72, by using laser drilling techniques as described hereinabove.
The vias 90 and 92 are shown as blind vias, but could be through
vias such that they extend completely through the substrate,
connecting any layer to any other layer in the substrate. The vias
90 and 92 are then metallized by forming a metal layer 100 using
suitable plating processes. The blind vias 90 and 92 have pads 100
which can be further processed to have metal traces (not shown)
using techniques known in the art. It is possible to continue
adding dielectric and cap layer, concurrently or otherwise, as set
forth above, to form seven, nine, or more layers.
[0048] It should be understood that a preferred chip package and
method for making the preferred chip package have been explained.
However, for example, the dielectric and metal layers can be placed
one layer at a time, rather than concurrently, on the
substrate.
[0049] In another embodiment of the present invention, the
substrates, such as those depicted in FIGS. 5 or 7, or other
containing three, five, seven, or more layers, as set forth above,
can be bonded together to form a very dense chip package, or a very
dense printed circuit board. The individual substrate layers are
bonded together using a conductive Z-axis material.
[0050] For a chip package having a copper metal core with
dielectric and metal cap layers concurrently formed thereon as set
forth above, the chip packages have a pressure applied thereto of
approximately 300-350 psi and a temperature ramp rate of
approximately 5-7.degree. C. per minute up to a temperature of
approximately 177.degree. C. The temperature is held at 177.degree.
C. for approximately 30 minutes. The temperature is then raised to
approximately 220.degree. C.-225.degree. C. for approximately 60
minutes. The chip package is then slowly cooled under pressure of
approximately 300-350 psi which has been maintained through out the
bonding process. Cooling occurs in a few hours.
[0051] FIG. 8 shows a chip package according to the present
invention having an electrically isolated copper core 110 and a
prepreg sheet 120. The prepreg sheet 120 is produced by W. L. Gore
& Associates under the name SPEEDBOARD.RTM.. Such a prepreg
sheet has a dielectric constant of about 2.6-2.7 at 1 MHz-10 GHz, a
glass transition temperature (Tg) of 220.degree. C., a flow of less
than 4%, and a cure temperature of 177.degree. C. to 225.degree. C.
Prepreg sheet 120 has a thickness ranging from about 25 microns to
about 175 microns.
[0052] The substrate of FIG. 8 has been evaluated and compared to a
system using a conventional polyimide core. Optical registration
targets on the chip package utilizing the substrate of FIG. 8 were
measured after the final lamination steps and compared with a chip
package having a substrate containing a conventional polyimide core
to determine the dimensional variability of each chip package. Post
lamination shrinkage standard deviation was found to be on the
order of 200 parts per million for the conventional polyimide core
construction. The post lamination shrinkage standard deviation was
found to be on the order of only 25 parts per million for the chip
package made in accordance with the teachings of the present
invention.
[0053] For a high density chip package, such a substantial decrease
in the post lamination shrinkage standard deviation can
significantly decrease scrap due to shorted and open vias. It also
allows construction of substrates to be carried out on larger sized
panels, as the effect of a high post lamination shrinkage standard
deviation is magnified at the edges of the printed circuit
board.
[0054] In the above examples, a particular dielectric material was
used to form the packages. However, any suitable dielectric
material can be used in the present invention, such as, but not
limited to, polyimides and polyimide laminates, epoxy resins, epoxy
resins in combination with other resin material, organic materials,
alone or any of the above combined with fillers. Preferred
dielectric materials include a fluoropolymer matrix, where the
fluoropolymer can be polytetrafluoroethylene (PTFE), expanded
polytetrafluoroethylene (ePTFE), or copolymers or blends. Suitable
fluoropolymers include, but are not limited to,
polytetrafluoroethylene or expanded polytetrafluoroethylene, with
or without an adhesive filler mixture.
[0055] Preferred materials include SPEEDBOARD.RTM. bond plies
available from W. L. Gore and Associates, such as, SPEEDBOARD.RTM.
C which is a prepreg of non-woven material containing a cyanate
ester resin in a polytetrafluoroethylene matrix. SPEEDBOARD.RTM. C
has a dielectric constant, (Dk) of 2.6-2.7 at 1 MHz-10 GHz, a loss
tangent of 0.004 at 1 MHz-10 GHz, a dielectric strength greater
than 1000 V/mil, a glass transition (T.sub.g) of 220.degree. C., a
resin content of 66-68 wt. % and is available in a variety of
thicknesses.
[0056] Another suitable dielectric is the expanded PTFE matrix,
shown in FIG. 9, that includes a mixture of at least two of a
cyanate ester compound, an epoxy compound, a bis-triazine compound
and a poly (bis-maleimide) resin. For example, a varnish solution
was made by mixing 5.95 pounds of M-30 (Ciba Geigy), 4.07 pounds of
RSL 1462 (Shell Resins, Inc.), 4.57 pounds of 2, 4,
6-tribromophenyl-terminated tetrabromobisphenol A carbonate
oligomer (BC-58) (Great Lakes Inc.), 136 g bisphenol A (Aldrich
Company), 23.4 g Irganox 1010, 18.1 g of a 10% solution of Mn
HEX-CEM in mineral spirits, and 8.40 kg MEK. The varnish solution
was further diluted into two separate baths--20% (w/w) and 53.8%
(w/w). The two varnish solutions were poured into separate
impregnation baths, and an e-PTFE web was successively passed
through each impregnation bath one immediately after the other. The
varnish was constantly agitated so as to insure uniformity. The
impregnated web was then immediately passed through a heated oven
to remove all or nearly all the solvent and partially cure the
adhesives, and was collected on a roll. The ePTFE web can be any
desired thickness, such as 25 microns, 40 microns, for example. A
25 microns thick material has a mass of approximately 0.9 g and a
weight per area of approximately 11.2 to 13.8 g/m.sup.2.
[0057] Another embodiment of the present invention is defined by
dielectric layers 30, 32, 70 and 72 which are formed from a porous
matrix system that is imbibed or impregnated with an
adhesive-filler mixture. FIG. 10 shows the node-fibril
infrastructure of an ePTFE matrix which exemplifies one embodiment
of such a porous matrix system.
[0058] In general, in the present invention, the porous matrix is a
non-woven substrate that is imbibed with high quantities of filler
and a thermoplastic or thermoset adhesive, as a result of the
initial void volume of the substrate. Such a porous matrix is then
heated to partially cure the adhesive and form a B-stage composite.
Substrates include fluoropolymers, such as the porous expanded
polytetrafluoroethylene material of U.S. Pat. Nos. 3,953,566 and
4,482,516, each of which is incorporated herein by reference.
Desirably, the mean flow pore size (MFPS) should be between about 2
to 5 times, or above, that of the largest particulate, with a MFPS
of greater than about 2.4 times that of the largest particulate
being particularly preferred. However, it is also within the scope
of the present invention that suitable composites can be prepared
by selecting a ratio of the mean flow pore size to average particle
size of greater than 1.4. Acceptable composites can also be
prepared when the ratio of the minimum pore size to average
particle size is at least above 0.8, or the ratio of the minimum
pore size to the maximum particle size is at least above 0.4. Such
ratio may be performed with a Microtrak.RTM. Model FRA Particle
Analyzer.
[0059] Alternatively, another mechanism for gauging relative pore
and particle sizes may be calculated as the smallest pore size
being not less than about 1.4 times the largest particle size.
[0060] In addition to the expanded fluoropolymer substrates
described hereinabove, porous expanded polyolefins, such as ultra
high molecular weight (UHMW) polyethylene, expanded polypropylene,
or polytetrafluoroethylene prepared by paste extrusion and
incorporating sacrificial fillers, porous inorganic or organic
foams, microporous cellulose acetate, can also be used.
[0061] The porous substrate has an initial void volume of at least
30%, preferably at least 50%, and most preferably at least 70%, and
facilitates the impregnation of thermoset or thermoplastic adhesive
resin and particulate filler paste in the voids while providing a
flexible reinforcement to prevent brittleness of the overall
composite and settling of the particles.
[0062] The filler comprises a collection of particles which, when
analyzed by a Microtrak.RTM. Model FRA Partical Analyzer device,
displays a maximum particle size, a minimum particle size and an
average particle size by way of a histogram.
[0063] Suitable fillers to be incorporated into the adhesive
include, but are not limited to, BaTiO.sub.3, SiO.sub.2,
Al.sub.2O.sub.3, ZnO, ZrO.sub.2, TiO.sub.2, precipitated and
sol-gel ceramics, such as silica, titania and alumina,
non-conductive carbon (carbon black) and mixtures thereof.
Especially preferred fillers are SiO.sub.2, ZrO.sub.2, TiO.sub.2
alone or in combination with non-conductive carbon. Most preferred
fillers include filler made by the vapor metal combustion process
taught in U.S. Pat. No. 4,705,762, such as, but not limited to
silicon, titanium and aluminum to produced silica, titania, and
alumina particles that are solid in nature, i.e., not a hollow
sphere, with a uniform surface curvature and a high degree of
sphericity.
[0064] The fillers may be treated by well-known techniques that
render the filler hydrophobic by silylating agents and/or agents
reactive to the adhesive matrix, such as by using coupling agents.
Suitable coupling agents include, silanes, titanates, zirconates,
and aluminates. Suitable silylating agents may include, but are not
limited to, functional silylating agents, silazanes, silanols,
siloxanes. Suitable silazanes, include, but are not limited to,
hexamethyldisilazane (Huls H730) and hexamethylcyclotrisilazane,
silylamides such as bis(trimethylsilyl)acetam- ide (Huls B2500),
silylureas such as trimethylsilylurea, and silylmidazoles such as
trimethylsilylimidazole.
[0065] Titanate coupling agents are exemplified by the tetra alkyl
type, monoalkoxy type, coordinate type, chelate type, quaternary
salt type, neoalkoxy type, cycloheteroatom type. Preferred
titanates include, tetra alkyl titanates, Tyzor.RTM. TOT
(tetrakis(2-ethyl-hexyl) titanate), Tyzor.RTM. TPT (tetraisopropyl
titanate), chelated titanates, Tyzor.RTM. GBA (titanium
acetylacetylacetonate), Tyzor.RTM. DC (titanium
ethylacetacetonate), Tyzor.RTM. CLA (proprietary to DuPont),
Monoalkoxy (Ken-React.RTM. KR TTS), Ken-React.RTM., KR-55 tetra
(2,2 diallyloxymethyl)butyl, di(ditridecyl)phosphito titanate,
LICA.RTM. 38 neopentyl(diallyl)oxy, tri(dioctyl)pyro-phosphato
titanate.
[0066] Suitable zirconates include any of the zirconates detailed
at page 22 in the Kenrich catalog, in particular KZ 55-tetra (2,2
diallyloxymethyl)butyl, di(ditridecyl)phosphito zirconate,
NZ-01-neopentyl(diallyl)oxy, trineodecanoyl zirconate,
NZ-09-neopentyl(diallyl)oxy, and tri(dodecyl)benzene-sulfonyl
zirconate.
[0067] The aluminates that can be used in the present invention
include, but are not limited to Kenrich.RTM.,
diisobutyl(oleyl)acetoacetylaluminat- e (KA 301),
diisopropyl(oleyl)acetoacetyl aluminate (KA 322) and KA 489.
[0068] In addition to the above, certain polymers, such as
cross-linked vinylic polymers, e.g., divinylbenzene, divinyl
pyridine or a sizing of any of the disclosed thermosetting matrix
adhesives that are first applied at very high dilution (0.1 up to
1.0% solution in MEK) can be used. Also, certain organic peroxides,
such as, dicumylperoxide can be reacted with the fillers.
[0069] The adhesive itself may be a thermoset or thermoplastic and
can include polyglycidyl ether, polycyanurate, polyisocyanate,
bis-triazine resins, poly (bis-maleimide), norbornene-terminated
polyimide, polynorbornene, acetylene-terminated polyimide,
polybutadiene and functionalized copolymers thereof, cyclic
olefinic polycyclobutene, polysiloxanes, poly sisqualoxane,
functionalized polyphenylene ether, polyacrylate, novolak polymers
and copolymers, fluoropolymers and copolymers, melamine polymers
and copolymers, poly(bis phenycyclobutane), and blends or
prepolymers thereof. It should be understood that the
aforementioned adhesives may themselves be blended together or
blended with other polymers or additives, so as to impact flame
retardancy or enhanced toughness.
[0070] As used herein, mean flow pore size and minimum pore size
were determined using the Coulter.RTM. Porometer II (Coulter
Electronics Ltd., Luton UK) which reports the value directly.
Average particle size and largest particle size were determined
using a Microtrak.RTM. light scattering particle size analyzer
Model No. FRA (Microtrak Division of Leeds & Northup, North
Wales, Pa., USA). The average particle size (APS) is defined as the
value at which 50% of the particles are larger. The largest
particle size (LPS) is defined as the largest detectable particle
on a Microtrak.RTM. histogram. Alternatively, the largest particle
size is defined as the minimum point when the Microtrak.RTM. Model
FRA Particle Analyzer determines that 100% of the particulate has
passed.
[0071] In general, the method for preparing the adhesive-filler
dielectric involves: (a) expanding a polytetrafluoroethylene sheet
by stretching a lubricated extruded preform to a microstructure
sufficient to allow small particles and adhesives to free flow into
the void or pore volume; (b) forming a paste from polymeric, e.g.,
thermoset or thermoplastic material and a filler; and (c) imbibing
by dipping, coating, or pressure feeding, the adhesive-filler paste
into the highly porous scaffold, such as expanded
polytetrafluoroethylene.
[0072] Table 1 shows the effect of the relationship of the
substrate mean flow pore size (MFPS) and particulate size. When the
ratio of the mean flow pore size (MFPS) to largest particulate is
1.4 or less, poor results are observed. In this case, a homogeneous
composite is not observed, and most of the particulate filler does
not uniformly penetrate the microporous substrate. When the ratio
of the MFPS to largest particulate is greater than about 2.0, then
a uniform composite is obtained. It is also observed that the
larger the ratio of MFPS to largest particulate, the greater the
relative case it is to imbibe a homogeneous dispersion into the
microporous substrate.
1TABLE 1 Substrate Particle Pore Size Size MFPS Pore.sub.Min
Pore.sub.Min Sam- Min MFPS Avg Max .div. .div. .div. Re- ple
(.mu.m) (.mu.m) (.mu.m) (.mu.m) Part.sub.Avg Part.sub.Max
Part.sub.Avg sult A 4 7 5 10 1.4 0.4 0.8 Poor B 4 5 5 10 1.0 0.4
0.8 Poor C -- 58 5 10 12.4 N/A -- Good D 18 32 6 10 5.3 1.8 3.0
Good E 18 32 1 1 32.0 18.0 18 Good F 17 24 6 10 4.0 1.7 2.8 Good G
0.2 0.4 0.5 1.6 0.8 0.125 0.4 Poor H -- 60 18 30 3.3 -- -- Good I
14 11 0.5 1.6 22.0 8.8 28 Good J 14 29 4 8 7.3 1.8 3.5 Good K 14 29
5 10 5.8 1.4 2.8 Good
EXAMPLE 1
[0073] A fine dispersion was prepared by mixing 281.6 g TiO.sub.2
(TI Pure R-900, Du Pont Company) into a 20% (w/w) solution of a
flame retarded dicyanamide/2-methylimidazole catalyzed bisphenol-A
based polyglycidyl ether (Nelco N-4002-5, Nelco Corp.) in MEK. The
dispersion was constantly agitated so as to insure uniformity. A
swatch of expanded PTFE was then dipped into the resin mixture. The
web was dried at 165.degree. C. for 1 minute under tension to
afford a flexible composite. The partially-cured adhesive composite
thus produced comprised of 57 weight percent TiO.sub.2, 13 weight
percent PTFE and 30 weight percent epoxy adhesive. Several plies of
the adhesive sheet were laid up between copper foil and pressed at
600 psi in a vacuum-assisted hydraulic press at temperature of
225.degree. C. for 90 minute then cooled under pressure. This
resulted in a copper laminate having dielectric constant of 19.0,
and withstood a 30 sec. solder shock at 280.degree. C. at an
average ply thickness of 100 mm (0.0039"(3.9 mil)) dielectric
laminate thickness.
EXAMPLE 2
[0074] A fine dispersion was prepared by mixing 386 g SiO.sub.2
(HW-11-89, Harbison Walker Corp.) which was pretreated with
phenyltrimethoxysilane (04330, Huls/Petrarch) into a manganese
catalyzed solution of 200 g bismaleimide triazine resin (BT206OBJ,
Mitsubishi Gas Chemical) and 388 g MEK. The dispersion was
constantly agitated so as to insure uniformity. A swatch of 0.0002"
thick expanded PTFE was then dipped into the resin mixture,
removed, and then dried at 165.degree. C. for 1 minute under
tension to afford a flexible composite. Several plies of this
prepreg were laid up between copper foil and pressed at 250 psi in
a vacuum-assisted hydraulic press at temperature of 225.degree. C.
for 90 minute then cooled under pressure. This resulting dielectric
thus produced comprised of 53 weight percent SiO.sub.2, 5 weight
percent PTFE and 42 weight percent adhesive, displayed good
adhesion to copper, dielectric constant (at 10 GHz) of 3.3 and
dissipation factor (at 10 GHz) of 0.005.
EXAMPLE 3
[0075] A fine dispersion was prepared by mixing 483 g SiO.sub.2
(HW-11-89) into a manganese-catalyzed solution of 274.7 g
bismaleimide triazine resin (BT2060BJ, Mitsubishi Gas Chemical) and
485 g MEK. The dispersion was constantly agitated so as to insure
uniformity. A swatch of 0.0002" thick expanded PTFE was then dipped
into the resin mixture, removed, and then dried at 165.degree. C.
for 1 minute under tension to afford a flexible composite. Several
plies of this prepreg were laid up between copper foil and pressed
at 250 psi in a vacuum-assisted hydraulic press at temperature of
225.degree. C. for 90 minutes then cooled under pressure. The
resulting dielectric thus produced comprised of 57 weight percent
SiO.sub.2, 4 weight percent PTFE and 39 weight percent adhesive,
displayed good adhesion to copper, dielectric constant (at 10 GHz)
of 3.2 and dissipation factor (at 10 GHz) of 0.005.
EXAMPLE 4
[0076] A fine dispersion was prepared by mixing 15.44 kg TiO.sub.2
powder (TI Pure R-900, DuPont Company) into a manganese-catalyzed
solution of 3.30 kg bismaleimide triazine resin (BT206OBH,
Mitsubishi Gas Chemical) and 15.38 kg MEK. The dispersion was
constantly agitated so as to insure uniformity. A swatch of 0.0004"
TiO.sub.2-filled expanded PTFE (filled according to the teachings
of Mortimer U.S. Pat. No. 4,985,296, except to 40% loading of
TiO.sub.2 and the membrane was not compressed at the end) was then
dipped into the resin mixture, removed, and then dried at
165.degree. C. for 1 minute under tension to afford a flexible
composite. The partially cured adhesive composite thus produced
comprised of 70 weight percent TiO.sub.2, 9 weight percent PTFE and
21 weight percent adhesive. Several plies of this prepreg were laid
up between copper foil and pressed at 500 psi in a vacuum-assisted
hydraulic press at temperature of 220.degree. C. for 90 minutes
then cooled under pressure. This resulting dielectric displayed
good adhesion to copper, dielectric constant of 10.0 and
dissipation factor of 0.008.
EXAMPLE 5
[0077] A fine dispersion was prepared by mixing 7.35 kg SiO.sub.2
(ADMATECHS SO-E2, Tatsumori LTD) with 7.35 kg MEK and 73.5 g of
coupling agent, i.e., 3-glycidyloxypropyltri-methoxysilane
(Dynasylan GLYMO (Petrach Systems)). SO-E2 is described by the
manufacture as having highly spherical silica having a particle
diameter of 0.4 to 0.6 mm, a specific surface area of 4-8
m.sup.2/g, and a bulk density of 0.2-0.4 g/cc (loose).
[0078] To this dispersion was added 932 g of a 50% (w/w) solution
of a cyanated phenolic resin, Primaset PT-30 (Lonza Corp.) in
methylethylketone (MEK), 896 g of a 50% (w/w) solution of RSL 1462
(Shell Resins, Inc.(CAS #25068-38-6)) in MEK, 380 g of a 50% (w/w)
solution of BC-58 (Great Lakes, Inc.) in MEK, 54 g of 50% solution
of bisphenol A (Aldrich Company) in MEK, 12.6 g Irganox 1010 (Ciba
Geigy), 3.1 g of a 0.6% solution of Manganese 2-ethylhexanoate (Mn
HEX-CEM (OMG Ltd.)), and 2.40 kg MEK. This dispersion was subjected
to ultrasonic agitation through a Misonics continuous flow cell for
about 20 minutes at a rate of about 1-3 gal./minute. The fine
dispersion thus obtained was further diluted to an overall bath
concentration of 11.9% solids (w/w).
[0079] The fine dispersion was poured into an impregnation bath. A
expanded polytetrafluoroethylene web having the node fibril
structure of FIG. 10, and the following properties:
2 Frazier 20.55 Coverage 9 g/m.sup.2 Ball Burst 3.2 lbs. Thickness
6.5 mil. Mean Flow Pore Size 9.0 microns
[0080] The Frazier number relates to the air permeability of the
material being assayed. Air permeability is measured by clamping
the web in a gasketed fixture which is provided in circular area of
approximately 6 square inches for air flow measurement. The
upstream side was connected to a flow meter in line with a source
of dry compressed air. The downstream side of the sample fixture
was open to the atmosphere. Testing is accomplished by applying a
pressure of 0.5 inches of water to the upstream side of the sample
and recording the flow rate of the air passing through the in-line
flowmeter (a ball-float rotameter that was connected to a flow
meter).
[0081] The Ball Burst Strength is a test that measures the relative
strength of samples by determining the maximum at break. The web is
challenged with a 1 inch diameter ball while being clamped between
two plates. The Chatillon, Force Gauge Ball/Burst Test was used.
The media is placed taut in the measuring device and pressure is
fixed by raising the web into contact with the ball of the burst
probe. Pressure at break is recorded.
[0082] The web described above was passed through a constantly
agitated impregnation bath at a speed at or about 3 ft./minute to
insure uniformity. The impregnated web was immediately passed
through a heated oven to remove all or nearly all the solvent, and
was collected on a roll.
[0083] Several plies of this prepreg were laid up between copper
foil and pressed at 200 psi in a vacuum-assisted hydraulic press at
temperature of 220.degree. C. for 90 minutes and then cooled under
pressure. This resulting dielectric displayed good adhesion to
copper, dielectric constant (10 GHz) of 3.0 and dissipation factor
of 0.0085 (10 GHz).
[0084] The physical properties of the particulate filler used in
Example 4 and Example 7 are compared below.
3 Tatsumori Property (ADMATECHS) Harbison Walker Manufacture
Technique Vapor Metal Amorphous Fused Silica Combustion Designation
Silica SO-E2 HW-11-89 Median Particle Size 0.5 micron 5 micron
Shape Spherical Irregular, jagged Surface Area 6-10 m.sup.2/g 10
m.sup.2/g Bulk Density 0.47 g/cc 1.12 g/cc Specific Density 2.26
g/cc 2.16 g/cc
EXAMPLE 6
[0085] An ePTFE matrix containing an impregnated adhesive filler
mixture, based on SiO.sub.2 prepared from the vapor combustion of
molten silicon was prepared as follows. Two precursor mixtures were
initially prepared. One being in the form of a slurry containing a
silane treated silica similar to that of Example 5 and the other an
uncatalyzed blend of the resin and other components.
[0086] Mixture I
[0087] The silica slurry was a 50/50 blend of the SO-E2 silica of
Example 5 in MEK, where the silica contains a coated of silane
which is equal to 1% of the silica weight. To a five gallon
container, 17.5 pounds of MEK and 79 grams of silane were added and
the two components mixed to ensure uniform dispersion of the silane
in the MEK. Then, 17.5 pounds of the silica of Example 5 were
added. Two five gallon containers of the MEK-silica-silane mixture
were added to a reaction vessel, and the contents, i.e., the
slurry, was recirculated through an ultrasonic disperser for
approximately one hour to break up any silica agglomerates that may
be present. The sonication was completed and the contents of the
reaction vessel were heated to approximately 80.degree. C. for
approximately one hour, while the contents were continuously mixed.
The reacted mixture was then transferred into a ten gallon
container.
[0088] Mixture II
[0089] The desired resin blend product was an MEK based mixture
containing an uncatalyzed resin blend (the adhesive) contains
approximately 60% solids, where the solid portion was an exact
mixture of 41.2% PT-30 cyanated phenolic resin, 39.5% RSL 1462
epoxy resin, 16.7% BC58 flame retardant, 1.5% Irganox 1010
stabilizer, and 1% bisphenol A co-catalyst, all percentages by
weight.
[0090] Into a ten gallon container, 14.8 pounds of PT-30 and 15-20
pounds of MEK were added and stirred vigorously to completely
solvate the PT-30. Then 6 pounds of BC58 were measured and added to
the MEK/PT-30 solution and vigorously agitated to solvate the BC58.
The stabilizer, 244.5 grams of Irganox 1010 and bisphenol A, 163
grams were added. The ten gallon container was reweighed and 14.22
pounds of RSL 1462 were added. Additional MEK was added to bring
the mixture weight to 60 pounds. The contents were then vigorously
agitated for approximately 1 to 2 hours, or as long is necessary to
completely dissolve the solid components.
[0091] The desired product is a mixture of the silica treated with
a silane, the uncatalyzed resin blend, and MEK in which 68% by
weight of the solids are silica, and the total solids are between
5% and 50% by weight of the mixture. The exact solids concentration
varies from run to run, and depends in part on the membrane to be
impregnated. The catalyst level was 10 ppm relative to the sum of
the PT-30 and RSL1462.
[0092] The solid contents of mixtures I and II were determined to
verify the accuracy of the precursors and compensate for any
solvent flash that had occurred. Then mixture I was added to a ten
gallon container to provide 12 pounds of solids, e.g., 515 solids
content, 23.48 pounds of mixture I. Then mixture II was added to
the container to provide 5.64 pounds of solids, e.g., 59.6% solids,
9.46 pounds of mixture II. The, 3.45 grams of the manganese
catalyst solution (0.6% in mineral spirits) was added to the
mixture of mixture I and mixture II and blended thoroughly to form
a high solids content mixture.
[0093] The bath mixture for impregnating an ePTFE matrix, 28%
solids mixture, was prepared by adding sufficient MEK to the high
solids content mixture to a total weight of 63 pounds.
[0094] Thereafter, an ePTFE matrix was impregnated with this bath
mixture to form a dielectric material.
EXAMPLE 7
[0095] A fine dispersion was prepared by mixing 26.8 grams Furnace
Black (Special Schwarz 100, Degussa Corp., Ridgefield Park, N.J.)
and 79 grams of coupling agent,
3-glycidyloxypropyl-trimethoxysilane (Petrach Systems)). The
dispersion was subjected to ultrasonic agitation for 1 minute, then
added to a stirring dispersion of 17.5 pounds SiO.sub.2 (SO-E2) in
17.5 pounds MEK which had previously been ultrasonically agitated.
The final dispersion was heated with constant overhead mixing for 1
hour at reflux, then allowed to cool to room temperature.
[0096] Separately, an adhesive varnish was prepared by adding the
following: 3413 grams of a 57.5% (w/w) mixture of Primaset PT-30 in
MEK, 2456 grams of a 76.8% (w/w) mixture of RSL 1462 in MEK, 1495
grams of a 53.2% (w/w) solution of BC58 (Great Lakes, Inc.) in MEK,
200 grams of 23.9% (w/w) solution of bisphenol A (Aldrich Company)
in MEK, 71.5 grams Irganox 1010, 3.21 grams of a 0.6% (w/w)
solution of Mu HEX-CEM (OMG Ltd.) in mineral spirits, and 2.40 kg
MEK.
[0097] In a separate container, 3739 grams of the dispersion
described above was added, along with 0.0233 grams of Furnace Black
(Special Schwarz 100, Degussa Corp., Ridgefield Park, N.J.), 1328
of the adhesive varnish described above and 38.3 pounds MEK. This
mixture was poured into an impregnation bath, and an ePTFE web was
passed through the impregnation bath at a speed at or about 3
ft/minute This dispersion was constantly agitated to insure
uniformity. The impregnated web was immediately passed through a
heated oven to remove all or nearly all the solvent, and was
collected on a roll.
[0098] Several plies of this prepreg were laid up between copper
foil and pressed at 200 psi in a vacuum-assisted hydraulic press at
temperatures of 200.degree. C. for 90 minutes then cooled under
pressure. This resulting dielectric displayed good adhesion to
copper.
EXAMPLE 8
[0099] An adhesive varnish was prepared by adding the following:
3413 grams of a 57.5% (w/w) solution of Primaset PT-30 (P.N.
P-88-1591)) in MEK, 2456 grams of a 76.8% (w/w) solution of RSL
1462 in MEK, 1495 grams of a 53.2% (w/w) solution of BC58 (Great
Lakes, Inc.) in MEK, 200 grams of 23.9% (w/w) solution of bisphenol
A (Aldrich Company) in MEK, 71.5 grams Irganox 1010, 3.21 grams of
a 0.6% (w/w) solution of Mn HEX-CEM in mineral spirits, and 2.40 kg
MEK.
[0100] In a separate container, 1328 grams of the adhesive varnish
described above, 42.3 pounds MEK, 6.40 grams of Furnace Black
(Special Schwarz 100, Degussa Corp., Ridgefield, N.J.) and 1860.9
grams SiO.sub.2 (SO-E2). This mixture was poured into an
impregnation bath, and an ePTFE web was passed through the
impregnation bath at a speed at or about 3 ft/minute. The
dispersion was constantly agitated so as to insure uniformity. The
impregnated web is immediately passed through a heated oven to
remove all or nearly all the solvent, and is collected on a
roll.
[0101] Several plies of this prepreg were laid up between copper
foil and pressed at 200 psi in a vacuum-assisted hydraulic press at
temperature of 220.degree. C. for 90 minutes then cooled under
pressure. This resulting dielectric displayed good adhesion to
copper.
[0102] As set forth above, the present invention provides a
dimensionally stable core for chip packages. This core is a metal
core such as copper or INVAR plated with copper, for example.
Copper is one of the preferable materials since it is easier to
match the CTE of the chip packages to that of the printed circuit
board to which it is bonded. This allows the package to be easier
to handle because the metal provides needed stability to the
package. Concurrently placing layers on top and bottom surfaces of
the chip package and concurrently laminating the layers allows the
fabrication process of the chip packages to be more efficient. In
addition, the amounts of metal on each layer on each side of the
substrate are balanced so that the package is easily attached to a
chip. An additional advantage of a metal core includes providing
buried capacitance which helps reduce simultaneous switching noise
on the chip.
[0103] As shown in FIG. 11, multilayer substrates using a
conductive Z-axis material, such as that taught in U.S. Pat. No.
5,498,467, which is incorporated herein by reference, may be used.
Thus, multiple layers are bonded together using such a conductive
Z-axis material to obtain a very dense printed circuit board. One
hundred or more layers could conceivably be achieved. In FIG. 11,
substrates 140 and 150 are identical to one another, and are
similar in structure to the dimensionally stable substrate of FIG.
5. Substrates 140 and 150 are laminate together with conductive
Z-axis material 160 which is described in greater detail below.
FIG. 19 shows the substrates 290 and 300, which are similar to that
of FIG. 8, laminated together with conductive Z-axis material 160,
which is described below in reference to FIGS. 12-14.
[0104] The multilayer packages that are exemplified in FIG. 11, are
prepared by using a conductive Z-axis material, such as that
described in the '467 patent discussed above, having irregularly
shaped conductive pathways that extend through the thickness of the
Z-axis material. The substrate for forming the conductive Z-axis
material is a planar, open cell, porous member having continuous
pores from one side to the other. The porous planar member must
have an internal morphology in which the material defining the
pores forms an irregular path through the Z-axis direction within a
vertically defined cross section through the Z-axis plane, as shown
in FIGS. 12-14.
[0105] Suitable materials for the conductive Z-axis member have a
thickness on the order of 5.times.10.sup.-6 m and 5.times.10.sup.-4
m (5 and 500 microns), and include woven or non-woven fabric, such
as a nylon, glass fiber or polyester fabric or cotton, or the like.
The member can also be a porous polymeric film or membrane, that is
flexible, such as porous polyolefins, e.g., porous polyethylene,
porous polypropylene, porous fluoropolymers, or open cell, porous
polyurethanes. Additionally, open cell, porous inorganic materials,
such as thin porous ceramic plates that have continuous pores from
one side to the other can be used.
[0106] Porous fluoropolymers include, but are not limited to,
porous polytetrafluoroethylene (PTFE), porous expanded
polytetrafluoroethylene (ePTFE), porous copolymers of
polytetrafluoroethylene and polyesters or polystyrenes, copolymers
of tetrafluoroethylene and fluorinated ethylene-propylene (FEP) or
perfluoroalkoxy-tetrafluoroethylene (PFA) with a C.sub.1-C.sub.4
alkoxy group. Preferred porous materials include expanded
polypropylene, porous polyethylene and porous
polytetrafluoroethylene. Most preferably, the material is expanded
polytetrafluoroethylene having a microstructure of nodes
interconnected with fibrils, a void volume of about 20 to 90%, such
as the material prepared in accordance with the teachings of U.S.
Pat. No. 3,953,566, which has been incorporated herein by
reference.
[0107] In general, the conductive Z-axis member will have a
thickness of between about 5 and 500 microns, preferably between
about 5 and 125 microns, but thickness is not a critical factor so
long as ultra-violet light will penetrate the sample.
[0108] Referring to FIGS. 12-14, material 170 for forming the
conductive Z-axis member is expanded polytetrafluoroethylene,
having pores 180 which are defined as the space between nodes 190
interconnected with fibrils 200. The internal structure of nodes
190 interconnected with fibrils 200 is of a material density that
results in an irregular continuous path through the Z-axis within a
vertically defined cross section 220, from one side 230 of the
planar member to the other side 240 (see FIG. 14).
[0109] As taught in the '467 patent, the selectively conductive
member 250 (FIG. 14) is prepared by making areas 220, through the
Z-axis direction, receptive to the deposition of a metal salt,
which metal salt on exposure to radiant energy is converted to
nonconductive metal nuclei which then act to catalyze the
deposition of a conductive metal from an electroless metal
deposition solution. The pores 180 of a porous member 170 are first
wetted with a wetting agent, such as an alcohol, or organic aqueous
surfactant. Methanol, propanol, tetrafluoroethylene/vinyl alcohol
copolymers or the like may be used. The wetting agent acts to make
the material of the member receptive to conductive metals such as
nickel or copper. Particularly preferred is copper.
[0110] A radiation sensitive metal salt composition is a liquid
radiation sensitive composition comprising a solution of a light
sensitive reducing agent, a metal salt, a source of halide ions,
and a second reducing agent. The radiation sensitive solution
contains water, the metal salt, a light sensitive reducing agent, a
second reducing agent, and optionally (for hard to wet surfaces) a
surfactant. The metal salt includes, but is not limited to, copper
acetate, copper formate, copper bromide, copper sulfate, copper
chloride, nickel chloride, nickel sulfate, nickel bromide, ferrous
bearing compounds, such as, ferrous sulfate, ferrous chloride, and
noble metals such as palladium, platinum, silver, gold and
rhodium.
[0111] Suitable light-sensitive reducing agents are aromatic diazo
compounds, iron salts, e.g., ferrous or ferric oxalate, ferric
ammonium sulfate, dichromates e.g., ammonium dichromate,
anthraquinone disulfonic acids or salts thereof, glycine
(especially active under humid surface conditions), L-ascorbic
acid, azide compounds, and the like, as well as metal accelerators,
e.g., tin compounds, e.g., stannous chloride or compounds of
silver, palladium, gold, mercury, cobalt, nickel, zinc, iron, etc.,
the latter group optionally being added in amounts of 1 mg to 2
grams per liter.
[0112] The second reducing agents, include, but are not limited to,
polyhydroxy alcohols, such as glycerol, ethylene glycol,
pentaerythritol, mesoerythritol, 1,3-propanediol, sorbitol,
mannitol, propylene glycol, 1,2-butanediol, pinacol, sucrose,
dextrin, and compounds such as triethanolamine, propylene oxide,
polyethylene glycols, lactose, starch, ethylene oxide and gelatin.
Compounds which are also useful as secondary reducing agents are
aldehydes, such as formaldehyde, benzaldehyde, acetaldehyde,
n-butyraldehyde, polyamides, such as nylon, albumin and gelatin;
leuco bases of triphenyl methane dyes, such as
4-dimethylaminotriphenylmethane,
4',4',4"-tri-di-methylamino-triphenylmet- hane; leuco bases of
xanthene dyes, such as 3,6-bis dimethylamino xanthene and 3,6-bis
dimethylamino-9-(2-carboxyethyl) xanthene; polyethers, such as
ethylene glycol diethyl ether, diethylene glycol, diethyl ether
tetraethylene glycol dimethyl ether, and the like.
[0113] A second reducing agent that is also a humectant,
exemplified by sorbitol, is used in one embodiment as a constituent
of the treating solution, for the humectant, apparently by reason
of a moisture conditioning effect on the "dry" coating prior to
developing. It provides substantial aid in maintaining density of
the metal coating on the internal material of the member during a
developing step in which any unconverted radiation-sensitive
composition in the coating is washed off of the base.
[0114] Among the suitable surfactants are polyethenoxy nonionic
ethers, such as TRITON X-100, manufactured by Rohm & Haas Co.,
and nonionic surfactants based on the reaction between nonyl phenol
and glycidol, such as Surfactants 6G and 10G manufactured by Olin
Mathieson Company.
[0115] This treating solution, i.e., the radiation sensitive
composition, contains an acidifying agent in the form of an acid
salt for adjusting the pH of the aqueous solution to usually
between about 2.0 and 4.0 (preferably 2.5 to 3.8) and a small
quantity of halide ions (iodide, bromide or chloride ions), so that
a combination of additives provides a surprising effect in
substantially intensifying the density of the coating that is
formed subsequently by exposure of the treated planar material to
radiant energy. Adjusting the acidity does not always require
introducing an agent for that purpose alone, because the adjustment
may be accomplished wholly or partially by means of an acidic
substance that has other functions also, as exemplified by a
light-sensitive reducing agent of an acidic nature (e.g., ascorbic
acid, glycerin, etc.) or by some additives for introducing halide
ions (e.g., hydrochloric acid). Similarly, some or all of the
halide ions may be introduced as components of the reducible metal
salt (e.g., cupric chloride).
[0116] Among the many suitable acidic substances which may be
employed in controlling or adjusting the pH of the sensitizing
solution are fluoroboric acid, citric acid, lactic acid, phosphoric
acid, sulfuric acid, acetic acid, formic acid, boric acid,
hydrochloric acid, nitric acid and the like. A wide variety of
bromide, chloride and iodide salt and other halide-generating water
soluble compounds may be utilized to provide part or all of the
desired halide ion content of the treating solution. These may
include, inter alia, salts of metals in general and these halogens
as exemplified by cupric bromide, nickel chloride, cobalt chloride,
cupric chloride, sodium iodide, potassium iodide, lithium chloride,
magnesium iodide, magnesium bromide, sodium bromide, potassium
bromide, and the like. Bromide salts are preferred, as they produce
a higher degree of sensitivity (i.e., darker and denser deposits)
on the substrate than the corresponding chloride in at least
certain instances.
[0117] The halide ions constitute only a minor proportion of the
solute and may typically range from about 0.045 to 1.6%, preferably
about 0.13 to 0.45%, based on the total weight of dissolved solids.
The amount of halogen may be stated otherwise as between about 0.9
and 25 milliequivalents of halogen per liter of the sensitizing
solution, preferably about 2.5 to 9 milliequivalents, e.g., 0.3-1.0
gm/l for cupric bromide. Increasing the proportions of the halide
ions is usually undesirable as such increases appear to gradually
diminish the sensitizing effect of the treatment below what is
obtainable with the optimum amount. Also, the proportion of these
halide ions expressed as equivalents is less than that of the
cupric or other reducible non-noble metal cations in the treating
solution. For instance, the ratio of equivalents of such metal ions
to halide ions is usually in the range of at least 2:1 and
preferably about 4:1 to 100:1.
[0118] The radiation sensitive composition is applied to the
material 170 to thoroughly wet the material defining the pores
whereby the porous member is subjected to the radiation sensitive
composition for a time sufficient for the composition to permeate
or penetrate through the pores 180 of the material 170 and form a
coating on the pore interior along the material defining the pores
from one side of the porous planar material to the other.
Thereafter the coating porous member is dried by air drying or oven
baking at below 50.degree. C. At this stage, to preserve the
light-sensitive nature of the treating compositions, the material
should be processed under yellow light conditions. The member
should also be kept at a temperature less than 70.degree. F. and at
no greater than 60% relative humidity because of possible
absorption of water by the material of the member which can
adversely affect the process.
[0119] The surface of one side of the coated porous member is
masked with an opaque cover 260 (FIG. 12) in selected areas so that
subsequent radiation will not strike the covered area. The masking
260 can result in dot shaped conductive areas of any desired,
shape, size, array or alternating bands or strips of conductive
areas through the Z-axis direction, separated by alternating bands
of nonconductive areas. The dots are conventionally circulator but
could have other geometrical configurations, as squares,
rectangles, etc. The size of the dot can be as small as a 0.0001
inches and as large as 0.025 inches, preferably 0.001 inches, 0.002
inches, 0.003 inches, 0.004 inches, 0.005 inches, 0.008 inches,
0.009 inches or fraction thereof where the pitch, defined as the
distance between the centers of adjacent dots is preferably at
least twice the dimension of the dots, e.g., 1 mil dot, 2 mil
pitch.
[0120] The masked member 170 is exposed to radiation, such as
light, electron beams, x-ray, and the like, preferably ultraviolet
radiation, for a time and at a power sufficient to reduce the
metallic cations in the metal salt to metal nuclei throughout the
thickness of the member. The member 170 is then unmasked and washed
with an acidic or alkaline washing solution to wash off the
radiation sensitive composition that had been protected by the
opaque cover. The acidic or alkaline washing (or fixing) solution
does not affect the areas where the radiation had reduced the metal
cations to metal nuclei, if the solution is not left in contact
with the areas for more than a few minutes, e.g., 5 minutes or
less.
[0121] Specifically, the treated member 170 is selectively masked
with a metallic mask, diazo or silver halide film, as shown in FIG.
12. The masked member 170 is then photo imaged with either a
non-collimated or collimated ultraviolet light source of less than
500 manometers wavelength. The catalyst, the nonconductive metal
nuclei, itself requires a minimum of 200 millijoules radiant energy
to establish a stable photo image.
[0122] The UV light energy is strong enough to penetrate through
the thickness of the porous member. Thus, in subsequent plating
operations, the conductive metal plates continuously through the
Z-axis and provides electrical continuity in the Z-axis. If
desired, the UV light energy can be applied to both sides of the
planar member.
[0123] After a 5 minute normalization period, the catalyzed
material is then washed for a short period of 30-90 seconds in a
sulfuric acid solution, e.g., a solution consisting of 8% sulfuric
acid by weight and 92% deionized water by weight or an alkaline
solution consisting of 40 g/l of ethylene diamine tetraacetic acid,
100 ml/l of formaldehyde, adjusted at a pH of greater than 10 with
sodium hydroxide. The purpose of this washing step is to eliminate
the unexposed catalyst from the material while retaining the
photo-reduced image.
[0124] The washed material containing the selective image is next
stabilized with a reactive metal cation replacement solution. A
convenient solution is:
4 COMPONENT AMOUNT (gms) 2,6 di-sodium anthraquinone di-sulfonic
salt 30 2,7 di-sodium anthraquinone di-sulfonic salt 30 sorbitol
220 220 cupric acetate 15 15 cupric bromide 0.5 0.5 olin G-10
surfactant 2 2 fluoroboric acid pH 3.5-3.8
[0125] The image undergoes a replacement reaction of the copper
with more stable cation, e.g., palladium. A more stable system is
desired because of tendency of the copper to oxidize at such thin
layer amounts and because of the ability of the palladium to
initiate the reduction reaction in the electroless both more
rapidly. The member is kept in this solution at least 30 seconds,
and is subsequently washed in D.I. water for about 1 minute.
[0126] The catalyzed member is selectively electrolessly plated
with one or more conductive metals to a deposition thickness of
about 50-60 micro inches. Such metals include copper, nickel, gold,
silver, platinum, cobalt, palladium, rhodium, aluminum and
chromium. During the time in the electrolyses baths, the member is
agitated with a rocking motion to promote diffusion of the metal to
the innermost region of the substrate. Plating is carried out by
first rinsing in deionized water, then dipping in an agitated
electroless copper bath for a time sufficient to deposit copper in
the material over the palladium and through the substrate
thickness.
[0127] Thus, within selected areas through the material in the
Z-axis direction, the material nodes 190 and fibrils 200 are at
least partially covered with a conductive metal layer 230, having a
Z-axis portion 290 and upper and lower contact pads 270 and 280.
The conductive metal forms a continuous path of conductivity 290
through the selected areas between upper and lower pads 270 and
280.
[0128] In the following examples, the catalytic treating solution
used was prepared by adding to one liter of D.I. water:
5 COMPONENT AMOUNT (gms) 2,6 di-sodium anthraquinone di-sulfonic
salt 30 2,7 di-sodium anthraquinone di-sulfonic salt 30 sorbitol
220 220 cupric acetate 15 15 cupric bromide 0.5 0.5 olin G-10
surfactant 2 2 fluoroboric acid pH 3.5-3.8
[0129] The fixing solution used was 8% sulfuric acid by weight, 92%
distilled water by weight. A stabilizing solution is also used, and
contains the following components.
6 STABILIZING SOLUTION 0.25 g/l palladium chloride 8% sulfuric acid
by weight 92% distilled water by weight
EXAMPLE 9
[0130] As taught in the '467 patent, a stretched porous
polytetrafluoroethylene membrane obtained from W. L. Gore &
Associates was treated with a wetting agent by immersing it in a
solution of 75% methanol, 25% ethanol and of 1 weight % copolymer
of tetrafluoroethylene and vinyl alcohol at room temperature for
about 30 seconds.
[0131] The wetted membrane was then dipped into the catalytic
treating solution for 60 seconds and was then dried in an oven at
50.degree. C. for 3 minutes. One surface of the membrane was then
masked with dots of a diazo film of 6 mil diameters and 6 mil pitch
(center to center).
[0132] The membrane was then exposed to a collimated UV light
source at 1600 millijoule for about 2 minutes. After a 5 minute
normalization period, the UV treated membrane was then washed for
30 seconds in the fixing solution to eliminate unexposed catalytic
treating solution. The selectively imaged membrane was then
stabilized by dipping into the stabilizing solution for one minute
and then washing in distilled water for one minute.
[0133] The stabilized membrane was then dipped into a copper
plating bath composition (Shipleys 3) on a per liter of D.I. water
basis.
7 PLATING SOLUTION 30 grams of ethylenediamine tetra acetic acid 6
to 8 grams sodium hydroxide 5 to 7 grams copper II sulfate 2 to 3
grams formaldehyde 2 grams of a given surfactant
[0134] The membrane was agitated in the bath using an agitation bar
for 71/2 minutes to promote diffusion of copper throughout the
pores of the membrane in the catalyzed portion throughout in the
Z-axis.
[0135] The passageways of the conductive Z-axis material can be
filled with an adhesive to function as a connector interface
between two other conductive materials. Suitable adhesives include
epoxy resin, acrylic resin, urethane resin, silicone resin,
polyimide resin, cyanate ester resin, or the like. The adhesive is
conveniently imbibed into the pores by immersing the member in a
solution of the adhesive. For an epoxy resin, a suitable solvent is
methylethylketone. Once the adhesive is imbibed or impregnated into
the conductive Z-axis material, in order to provide bonding
capability, it is baked at 160.degree. C. to form a B-stage
conductive Z-axis member.
EXAMPLE 10
[0136] A 6 mil (150 micrometer) thick, stretched, porous
polytetrafluoroethylene membrane was wetted by subjecting it to 2
propanol by dipping for 1 minute. It was then dipped into the
catalytic treating solution for one minute, dried, masked and
subjected to UV light as in Example 9. It was then subjected to the
fixing and to the stabilizing solution as in Example 9 and
subsequently plated with copper as in Example 9.
EXAMPLE 11
[0137] A 2 mil thick stretched porous polytetrafluoro-ethylene
membrane was prepared as in Example 9 except that the masking
strips were 3 mil pad with a 10 mil pitch.
EXAMPLES 12-14
[0138] The procedure of Example 9 was followed for membranes formed
from porous polyethylene, porous poly-propylene and open cell,
porous polyurethanes to produce a conductive Z-axis material having
irregularly shaped conductive pathways that extend along the
Z-axis.
EXAMPLE 15
[0139] A membrane formed from a stretched porous
polytetrafluoroethylene membrane having the node-fibril structure
shown in FIG. 15 (1000.times. magnification) is 76 .mu.m thick with
a density of 0.22 gm/cm.sup.3 and an air volume of 70% at
25.degree. C., and is available from W. L. Gore & Associates,
was prepared as in Example 9 to form a conductive Z-axis membrane,
except that the masking strips were 2 mil pad with a 5 mil
pitch.
EXAMPLE 16
[0140] A stretched porous polytetrafluoroethylene membrane with the
node-fibril structure shown in FIG. 16 (1500.times. magnification)
that is 40 .mu.m thick, with a density 0.4 gm/cm.sup.3 and an air
volume of 20% at 25.degree. C., available from W. L. Gore &
Associates, was prepared as in Example 9 to form a conductive
Z-axis membrane, except that the masking strips were 8 mils with a
15 mil pitch.
EXAMPLE 17
[0141] A stretched porous polytetrafluoroethylene membrane with the
node-fibril structure of FIG. 17 (1000.times. magnification) that
is 100 .mu.m thick, with a density of 0.35 gm/cm.sup.3 and an air
volume of 30% at 25.degree. C., available from W. L. Gore &
Associates, was prepared as in Example 9 to form a conductive
Z-axis membrane, except that the masking strips were 8 mils with a
pitch of 15 mils.
EXAMPLE 18
[0142] A stretched porous polytetrafluoroethylene membrane with the
node-fibril structure of FIG. 18 (1000.times. magnification) is 150
.mu.m thick, with a density of 0.20 gm/cm.sup.3 and an air volume
of 70% at 25.degree. C., and which is available from W. L. Gore
& Associates, was prepared as in Example 1 to form a conductive
Z-axis membrane, except that the masking strips were 8 mils with a
15 mil pitch.
[0143] The conductive Z-axis materials of Examples 10-18 containing
conductive pathways described above can be filled with an adhesive
to be a connector interface between two other conductive materials.
Suitable adhesives include epoxy resin, acrylic resin, urethane
resin, silicone resin, polyimide resin, cyanate ester resin, or the
like. The adhesive is conveniently imbibed into the pores by
immersing the member in a solution of the adhesive. For an epoxy
resin, a suitable solvent is methylethylketone. As with Example 9,
the Z-axis material is impregnated with epoxy adhesive in order to
provide bonding capability and baked at 160.degree. C. to provide a
B-stage Z-axis composite.
[0144] While particular embodiments of the present invention have
been illustrated and described herein, the present invention is not
limited to such illustrations and descriptions. It is apparent that
changes and modifications may be incorporated and embodied as part
of the present invention within the scope of the following
claims.
* * * * *