U.S. patent application number 12/347076 was filed with the patent office on 2009-04-30 for lead free ltcc tape composition.
This patent application is currently assigned to E. I. DU PONT DE NEMOURS AND COMPANY. Invention is credited to Kenneth Warren Hang, Mark Frederick McCombs, Kumaran Manikantan Nair.
Application Number | 20090110939 12/347076 |
Document ID | / |
Family ID | 37734924 |
Filed Date | 2009-04-30 |
United States Patent
Application |
20090110939 |
Kind Code |
A1 |
Hang; Kenneth Warren ; et
al. |
April 30, 2009 |
LEAD FREE LTCC TAPE COMPOSITION
Abstract
A glass composition consisting essentially of, based on mole
percent, 46-56% B.sub.2O.sub.3, 0.5-8.5% P.sub.2O.sub.5, SiO.sub.2
and mixtures thereof, 20-50% CaO, 2-15% Ln.sub.2O.sub.3 where Ln is
selected from the group consisting of rare earth elements and
mixtures thereof; 0-6% M'.sub.2O where M' is selected from the
group consisting of alkali elements; and 0-10% Al.sub.2O.sub.3,
with the proviso that the composition is water millable.
Inventors: |
Hang; Kenneth Warren;
(Hillsborough, NC) ; Nair; Kumaran Manikantan;
(Head of the Harbor, NY) ; McCombs; Mark Frederick;
(Clayton, NC) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY;LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1122B, 4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Assignee: |
E. I. DU PONT DE NEMOURS AND
COMPANY
|
Family ID: |
37734924 |
Appl. No.: |
12/347076 |
Filed: |
December 31, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11543742 |
Oct 5, 2006 |
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12347076 |
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11824116 |
Jun 29, 2007 |
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11543742 |
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Current U.S.
Class: |
428/426 ; 501/32;
501/47; 501/78 |
Current CPC
Class: |
C03C 3/19 20130101; C03C
3/21 20130101; Y10T 29/49117 20150115; C03C 3/068 20130101; H05K
1/0306 20130101; C03C 3/253 20130101; Y10T 428/24926 20150115; H01L
31/00 20130101 |
Class at
Publication: |
428/426 ; 501/47;
501/78; 501/32 |
International
Class: |
C03C 3/068 20060101
C03C003/068; C03C 3/19 20060101 C03C003/19; C03C 14/00 20060101
C03C014/00; B32B 18/00 20060101 B32B018/00 |
Claims
1-15. (canceled)
16. A glass composition comprising, based on mole percent, 0.5-8.5%
of a glass former selected from the group consisting of
P.sub.2O.sub.5, SiO.sub.2 and mixtures thereof; 46-57.96%
B.sub.2O.sub.3, 20-50% CaO, 2-15% Ln.sub.2O.sub.3 where Ln is
selected from the group consisting of rare earth elements and
mixtures thereof; 0-6% M'.sub.2O where M' is selected from the
group consisting of alkali elements; and 0-10% Al.sub.2O.sub.3,
with the proviso that the composition is water millable.
17. The composition of claim 16 wherein Ln.sub.2O.sub.3 is
La.sub.2O.sub.3.
18. The composition of claim 16 wherein M'.sub.2O is selected from
the group consisting of Li.sub.2O, Na.sub.2O and mixtures
thereof.
19. The composition comprising, based on solids: (a) 25-100 weight
% glass composition of claim 16; (b) 0-75 weight % refractory
oxide; both dispersed in a solution of (c) organic polymeric
binder.
20. The composition of claim 19 further comprising a volatile
organic solvent.
21. The composition of claim 19 further comprising 0-5 weight %
Cu.sub.2O.
22. The composition of claim 19 wherein the refractory oxide is
selected from the group consisting of Al.sub.2O.sub.3,
.alpha.-quartz, CaZrO.sub.3, mullite, cordierite, fosterite,
zircon, zirconia, BaTiO.sub.3, CaTiO.sub.3, MgTiO.sub.3, amorphous
silica and mixtures thereof.
23. The composition of claim 19 wherein the refractory oxide is
Al.sub.2O.sub.3.
24. The composition of claim 16 wherein the CaO is partially
substituted by BaO, MgO or mixtures thereof.
25. A green tape formed by casting a layer of the dielectric
composition of claim 20 onto a flexible substrate forming a cast
layer, and heating the cast layer to remove the volatile organic
solvent forming a solvent-free layer.
26. The tape of claim 25 wherein the solvent-free layer is
separated from the substrate.
27. The tape of claim 25 wherein a conductor composition is
deposited on the tape.
28. An article comprising the tape of claim 27 wherein the tape is
processed to volatilize the organic polymeric binder and sinter the
glass composition.
Description
FIELD OF THE INVENTION
[0001] The invention relates to glass, paste and tape
composition(s) suitable for application to the manufacture of
multilayer LTCC circuits. The tape exhibits process and materials
compatibility with conductors and passive electronic materials when
used to build high density, LTCC circuits. The non-toxic tape is
also suitable for use with Pb free solders and plated metal circuit
contact systems used in newer LTCC tape structures. The tape is
characterized as having low dielectric loss over frequencies up to
90 GHz or higher; it also excels in chemical durability,
hermeticity, mechanical strength and processing latitude.
TECHNICAL BACKGROUND OF THE INVENTION
[0002] An interconnect circuit board is a physical realization of
electronic circuits or subsystems made from a number of extremely
small circuit elements that are electrically and mechanically
interconnected. It is frequently desirable to combine these diverse
type electronic components in an arrangement so that they can be
physically isolated and mounted adjacent to one another in a single
compact package and electrically connected to each other and/or to
common connections extending from the package.
[0003] Complex electronic circuits generally require that the
circuit be constructed of several layers of conductors separated by
insulating dielectric layers. The conductive layers are
interconnected between levels by electrically conductive pathways,
called vias, through a dielectric layer. Such a multilayer
structure allows a circuit to be more compact.
[0004] The elimination of toxic materials from the chemical
constituency of LTCC tape materials is a desired goal to reduce the
environmental impact caused by public disposal of electronic
materials in worldwide community waste disposal systems. This tape
is designed to eliminate potentially toxic constituents. The tape
exhibits a uniform and low dielectric constant (6-8) with low
dielectric loss performance over a broad range of frequency up to
90 GHz or sometimes higher depending on the metal loading. The tape
is chemically resistant to acidic plating baths used for metal
addition to a base layer of printed conductive.
[0005] Typically, a LTCC tape is formed by casting a slurry of
inorganic solids, organic solids and a fugitive solvent on a
removable polymeric film. The slurry consists of glass powder(s)
and ceramic oxide filler materials and an organic based
resin-solvent system (medium) formulated and processed to a fluid
containing dispersed, suspended solids. The tape is made by coating
the surface of a removable polymeric film with the slurry so as to
form a uniform thickness and width of coating.
SUMMARY OF THE INVENTION
[0006] The present invention is a glass composition consisting
essentially of, based on mole percent, 46-56% B.sub.2O.sub.3,
0.5-8.5% P.sub.2O.sub.5, SiO.sub.2 and mixtures thereof, 20-50%
CaO, 2-15% Ln.sub.2O.sub.3 where Ln is selected from the group
consisting of rare earth elements and mixtures thereof; 0-6%
M'.sub.2O where M' is selected from the group consisting of alkali
elements; and 0-10% Al.sub.2O.sub.3, with the proviso that the
composition is water millable.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] The data in FIGS. 1, 2 and 3 were obtained by a fused silica
dilatometer on 2-inch length specimens.
[0008] FIG. 1 shows the thermal expansion behavior taken upon both
the heating and cooling of the glass #1 in a tape formulation.
[0009] FIG. 2 shows that the tape based upon substituting glass #6
in the tape formulation results in a relatively stable heat and
cool characteristic.
[0010] FIG. 3 shows the stability characteristics of tapes made
with a variety of glass compositions when the heating program of
the dilatometer is increased to approach a 1000.degree. C., while
heating at 3 C/min.
[0011] FIG. 4 shows the Thermo-Mechanical Analysis (TMA) properties
of the glass #1 tape, glass #2 tape and the glass #6 tape.
[0012] FIG. 5 shows that the TMA property of the glass alone.
[0013] FIG. 6 shows the effect on glass viscosity of the
substitution of P.sub.2O.sub.5 for B.sub.2O.sub.3 in the glass
composition.
[0014] FIG. 7 shows the resistivity of the Ag based conductor as a
function of the number of refires; the high level of stability is
achieved in FIG. 7, using the current invention tape based upon the
#6 glass.
DETAILED DESCRIPTION OF THE INVENTION
[0015] In working with U.S. Pat. No. 6,147,019 it was noted that
the dielectric tape had some continuing interaction with Ag bearing
conductors with repeated refires. The interaction might result in
the Ag based conductor increasing its resistivity following each
refire and the conductor resistance doubling after as little as 4
refires.
[0016] It has been observed that during the firing of an LTCC
circuit laminate, the glass softens and crystallization initiates.
As the temperature and/or time increases, more of the crystal
species grow from the glass melt; resulting in crystals surrounded
by a low viscosity "remnant glass". At the firing temperature, this
low viscosity "remnant glass" may react with the conductor
composition causing an increase in the conductor resistivity. In
extreme cases, the conductor lines dissipate within the fired film
causing shorting, lack of electrical connectivity, reliability
degradation, etc. This is particularly true for applications
requiring narrow lines and spaces between conductor lines.
Furthermore, newer LTCC circuits require the use of tape having a
thickness on the order of 0.1 mm-0.3 mm and tape laminates of 20 or
more layers. Processing steps of such thick laminates require a
long heating profile of 30 hours or more. Such a long heating
profile increases the interaction between the low viscosity
"remnant glass" and conductor components resulting in increased
conductor property degradation. In order to reduce conductor
property degradation and improve the reliability of the circuit,
the viscosity of the "remnant glass" may be increased by adding
"glass network formers" such as SiO.sub.2 and/or P.sub.2O.sub.5.
These added "network formers" are expected to remain within the
"remnant glass" network and increase the viscosity of the "remnant
glass" at the firing temperature. As disclosed in U.S. Pat. No.
6,147,019 to Donohue incorporated herein by reference, the addition
of SiO.sub.2 to the glass network or to the tape formulation
results in high dielectric losses. For this reason, efforts to
raise the viscosity of the glass by incorporation of SiO.sub.2
appeared to be adverse to the desirable low loss characteristics of
the tape dielectric properties. In the present application, glasses
were prepared with lower amounts of SiO.sub.2 than shown in Donohue
to raise the glass, "remnant glass", viscosity. The addition of
P.sub.2O.sub.5 into network is expected to increase the high
temperature viscosity of the "remnant glass". The favorable results
of this plan could not be anticipated, as possible loss of desired
properties was an expected outcome due to the teachings of
Donohue.
[0017] The present invention is directed to a glass composition,
paste composition and an LTCC tape composition suitable for a
variety of circuit manufacturing needs. The materials are
characterized by their freedom from toxic metal oxides. The
materials are designed to process at the standard 850-875 degrees
C. found in current tape dielectric materials. The tape is designed
to co-fire with conductors and other passive electrical components
applied by screen printing or tape laminating. The properties of
the fired LTCC tape have very low dielectric loss characteristics
for applications pushing the current upper limits of circuit
operation frequency up to 90 GHz or higher.
[0018] The glass component of the LTCC tape is an important
determinant of many tape properties including the physical and
chemical compatibility with other circuit forming materials,
dielectric performance, chemical resistance and hermeticity. This
invention was unexpectedly discovered during an effort to improve
process latitude and conductor and passive component compatibility
properties of a related LTCC tape, originally described in U.S.
Pat. No. 6,147,019 invented by Paul C. Donohue and assigned to E.
I. du Pont de Nemours and Company. The unexpected discovery was
realized when properties of LTCC tape made as described in U.S.
Pat. No. 6,147,019 Table 1, glass composition #16 was compared to
tape made from modified glasses of the current invention. The goal
was to increase processing latitude and improve the compatibility
with Ag bearing conductor materials without sacrificing the
chemical durability, hermeticity or dielectric loss properties of
the fired tape.
[0019] For this reason, compositional modification of the glass
used in the tape dielectric was initiated to determine whether
improvement in conductor interaction properties could be achieved,
while maintaining the value of the tape dielectric performance. It
was hoped that the incorporation of other glass former additions
might raise the viscosity of the glass to decrease the firing
sensitivity and potentially improve Ag conductor interaction
issues. However, it was unknown whether adverse property changes
might develop as a consequence of the composition modifications.
So, many of the glasses tested were designed with P.sub.2O.sub.5
and SiO.sub.2 additions.
[0020] The glass(es) of the present invention are "ceramic-filled
devitrified glass composition(s)." Devitrified glasses are defined
herein as glasses that flow prior to crystallization (i.e., the
exotherm of crystallization follows the softening and flow of the
glass). Additionally, the glasses of the present invention form a
crystalline material upon heating (i.e., a crystallized glass is
present in the remnant glass). The chemistry of the crystals formed
upon heating are different to the chemistry of the parent glass
(prior to heating). Additionally, the ceramic filler present in the
glasses of the present invention may form crystals upon reaction
with the remnant glass. "Remnant glass" is herein defined as the
glass of the composition that has not crystallized. The glasses of
the present invention are present in the composition in the range
of 25 to 100 weight %, based on solids.
[0021] One embodiment of the present invention is a glass
composition consisting essentially of, based on mole percent,
46-56% B.sub.2O.sub.3, 0.5-8.5% P.sub.2O.sub.5, SiO.sub.2 and
mixtures thereof, 20-50% CaO, 2-15% Ln.sub.2O.sub.3 where Ln is
selected from the group consisting of rare earth elements and
mixtures thereof; 0-6% M'.sub.2O where M' is selected from the
group consisting of alkali elements; and 0-10% Al.sub.2O.sub.3,
with the proviso that the composition is water millable. A further
embodiment is the composition above wherein Ln.sub.2O.sub.3 is
La.sub.2O.sub.3. Still, a further embodiment is the composition
above wherein M'.sub.2O is selected from the group consisting of
Li.sub.2O, Na.sub.2O and mixtures thereof.
[0022] The composition of glass(es) of the present invention is
shown in Table 1 below:
TABLE-US-00001 TABLE 1 Glass Powder Compositions Density Item #
SiO2 Al2O3 ZrO2 B2O3 CaO La2O3 Na2O Li2O GeO2 P2O5 g/cc Weight % 1
42.16 16.98 39.47 0.94 0.45 3.52 2 37.19 16.37 38.05 0.90 0.44 7.05
3.48 3 2.65 34.20 15.65 36.38 0.87 0.42 9.83 3.12 4 4.18 37.95
15.28 35.52 0.84 0.41 5.82 3.36 5 3.02 36.72 16.16 37.56 0.89 0.43
5.22 3.52 6 36.91 16.34 37.97 0.90 0.44 7.44 3.46 7 36.89 16.24
37.73 0.90 0.43 2.57 5.24 3.53 8 36.69 16.31 37.91 0.90 0.43 7.76
3.33 9 6.26 35.39 15.84 36.80 0.88 0.42 4.41 3.57 10 1.42 36.14
16.17 37.58 0.89 0.43 7.37 3.49 11 39.66 16.98 39.47 0.94 0.45 2.50
3.45 12 1.10 41.06 16.98 39.47 0.94 0.45 3.54 13 1.61 40.55 16.98
39.47 0.94 0.45 3.57 14 2.14 40.02 16.98 39.47 0.94 0.45 3.60 15
1.10 38.56 16.98 39.47 0.94 0.45 2.50 3.42 16 40.96 16.98 39.47
0.94 0.45 1.2 3.52 17 42.88 16.07 37.36 0.89 0.43 2.37 3.44 18 2.68
39.48 16.98 39.47 0.94 0.45 3.52 19 3.21 38.95 16.98 39.47 0.94
0.45 3.51 20 2.14 33.02 16.98 39.47 0.94 0.45 7.00 3.55 Mole % 1
57.14 28.57 11.43 1.43 1.43 2 52.28 28.57 11.43 1.43 1.43 4.86 3
2.58 48.87 27.77 11.11 1.39 1.39 6.89 4 3.96 52.62 26.30 10.52 1.32
1.32 3.96 5 2.40 51.66 28.23 11.29 1.41 1.41 3.60 6 52.00 28.57
11.43 1.43 1.43 5.14 7 51.66 28.23 11.29 1.41 1.41 2.40 3.60 8
51.77 28.57 11.43 1.43 1.43 5.37 9 5.01 50.14 27.86 11.14 1.39 1.39
3.07 10 1.14 51.14 28.41 11.36 1.42 1.42 5.11 11 54.70 29.08 11.63
1.45 1.45 1.69 12 1.72 55.52 28.51 11.40 1.43 1.42 13 2.52 54.77
28.47 11.39 1.43 1.42 14 3.34 54.00 28.44 11.38 1.42 1.42 15 1.75
53.06 29.00 11.60 1.45 1.45 1.69 16 55.98 28.81 11.52 1.45 1.44
0.80 17 57.96 26.98 10.79 1.35 1.35 1.57 18 4.18 53.21 28.41 11.36
1.42 1.42 19 5.00 52.43 28.38 11.35 1.42 1.42 20 3.52 46.80 29.88
11.95 1.49 1.49 4.87
[0023] Composition #1 of Table 1 is a reference composition from
U.S. Pat. No. 6,147,019 (#16) and is not part of the current
invention.
[0024] The glasses in Table 1 are normally used only in the context
of a formulation to make a tape dielectric. A typical formulation
of the tape solids used in comparative testing consists of the
following:
TABLE-US-00002 Glass Powder 48 volume % Alumina 52 volume %
[0025] The thermal expansion behavior taken upon both the heating
and cooling of the glass #1 in a tape formulation is shown in the
FIG. 1. The data in FIGS. 1, 2 and 3 were obtained by a fused
silica dilatometer on 2-inch length specimens. The dilatometer was
calibrated by regression fitting to the expansion over the thermal
range tested for a sapphire standard. The data for FIGS. 1 and 2
were taken upon heating at 4.5.degree. C./min. to 865.degree. C.
soaking for 3 min. then cooling at 3.degree. C./min. to 250.degree.
C.
[0026] In FIG. 1, the heating and cooling paths are separated in a
way that indicates the yield that is still possible when fired to
850.degree. C. in a belt furnace then re-measured in a fused silica
dilatometer to 865.degree. C. This indicates that the composite of
crystal and residual glass is yielding under the loading applied
during the measurement (0.05 newton). Typically, if the crystal
development has become dominant, the yield in the microstructure
would not occur. For this reason, it is expected that the residual
glassy phase is still controlling the yield characteristic in glass
#1 tape. The presence of a low viscosity residual glass is
potentially a reason for reaction with buried Ag conductor lines
causing the observed conductivity changes with refires and/or
circuit opens in the case of conductor lines and spaces of small
dimensions (<0.15 mm).
[0027] In contrast to the thermal expansion behavior of the glass
#1 tape, the tape based upon substituting glass #6 in the tape
formulation (FIG. 2) results in a relatively stable heat and cool
characteristic. This would suggest that the fired glass #6 tape has
achieved a more refractory state than that of the glass #1 tape.
The result is that less yield in the tape is possible upon
re-heating the fired tape to 865.degree. C. and cooling. This
likely indicates that the residual glass is more refractory or that
the crystal development has become more dominant in the
microstructure of the fired tape body. The most likely result is
that the remnant glass is more refractory.
[0028] If the heating program of the dilatometer is increased to
approach a 1000.degree. C., while heating at 3 C/min., the
stability characteristics of tapes made with varied glass
compositions can be seen in FIG. 3.
[0029] The tape made with glass #6 tape shows good stability to
temperatures in excess of 950.degree. C. Whereas, the glass #1 tape
shows a significant volume expansion characteristic at temperatures
above 850 C and deforms somewhat before 950.degree. C. The glass
#10 tape is located between the glass #1 tape and glass #6 tape
expansion behavior. These characteristic differences suggest that
the remnant glass is in a smaller proportion to the crystal or that
the composite viscosity of the glass is higher for the glass #6
tape as shown in the increased refractory performance of the
thermal expansion data. The improvement in refractory performance
would be expected to improve the compatibility with conductor
materials. This improved refractory performance for the
substitution of P.sub.2O.sub.5 for B.sub.2O.sub.3 in the glass #6
tape unexpectedly shows a reduction on the dielectric constant
(E.sub.r) and dielectric loss (tangent delta) as can be seen in
Table 2 below.
[0030] U.S. Pat. No. 6,147,019 to Donohue discloses that the
substitution of SiO.sub.2 for B.sub.2O.sub.3 is adverse to the tape
dielectric loss for even small additions (column 2, line 5). So,
the current invention that substitutes glass formers including
P.sub.2O.sub.5, GeO.sub.2, and SiO.sub.2 is not taught in the prior
art and is unexpected to have benefit. In fact, Donohue teaches
away from the present invention.
[0031] The measurement of dielectric constant, E.sub.r and
dielectric loss (tangent delta) has been performed for selected
samples of tape made from the glasses indicated in Table 2. These
measurements were performed using a (non-metallized) split cavity
method in a range of frequency from 3.3 GHz to 16 GHz. A reference
to the measurement method is given in "Full-Wave Analysis of a
Split-Cylinder Resonator for Nondestructive Permittivity
Measurements" by Michael Janezic published in IEEE Transactions on
Microwave Theory and Techniques, Vol 47, No. 10, October 1999. Data
for two frequencies are provided in Table 2. The data, (E.sub.r and
loss), for all measured samples shows a very slight increase with
frequency. It is also apparent from the data in Table 2 that
improvements (Lower E.sub.r and Loss) in the attained LTCC tape
properties are yielded from the substitution for B.sub.2O.sub.3 of
either P.sub.2O.sub.5 or SiO.sub.2 as in glasses 6, 14, and 15. It
is also important to note that some variation in attained
dielectric properties is expected due to small experimental errors
in the measurements.
TABLE-US-00003 TABLE 2 Dielectric Properties of Fired Tape Samples
3.3 GHz Tan 8.3 GHz Tan Glass # E.sub.r Delta E.sub.r Delta 1 7.40
0.00130 7.57 0.00100 2 7.08 0.00110 7.13 0.00098 3 4 7.44 0.00120 5
7.30 0.00110 7.34 0.00130 6 7.28 0.0008 7.31 0.0009 7 8 9 10 11 12
13 14 6.90 0.00065 6.92 0.00075 15 7.10 0.00048 7.12 0.00059 16 17
18 19 20 6.7 0.0011 Note: Data for glass #20 taken at 9.4 GHz
[0032] The Thermo-Mechanical Analysis (TMA) properties of the glass
#1 tape, glass #2 tape and the glass #6 tape are shown in FIG. 4.
The TMA measurements were performed using a TA Instruments Inc. TMA
using a heating program of room temperature to 850.degree. C. at
10.degree. C./min. The temperature of 850.degree. C. was held for 5
minutes, then power off for all samples. The glass #1 tape is seen
to initiate sintering earlier than the glass #6 or glass #2 tape;
it is also seen to develop a critical volume of crystal more
quickly than glass #6 or glass #2 tape resulting in less
dimensional change (tape shrinkage). This is shown by the
attenuation of deformation under the loading. The glass #2 tape
shows the influence of the abrupt glass volume expansion (between
780.degree. C.-810.degree. C.) that is seen in the TCE
characteristic of glass #2. The glass #6 tape does not have the
same volume expansion characteristic for the glass and the tape
shows continuous deformation until the composite viscosity increase
rigidifies the tape.
[0033] The TMA property of the glass alone is shown in FIG. 5. The
glass #1 is seen to onset sintering and flow at lower temperatures
than the glasses of the current invention. Both glass #2 and glass
#6 exhibit less shrinkage before dimensional stability is achieved
due to the development of a crystallizable phase. The crystalline
phase development is estimated to be in excess of 50 volume % by
comparison with other crystallizable glasses characterized by TMA
and X-ray methods to estimate crystalline content.
[0034] As can be seen from the glass viscosity data provided in the
FIG. 6, the substitution of P.sub.2O.sub.5 for B.sub.2O.sub.3 in
the glass composition raises the glass viscosity. The glass
viscosity has been measured by the parallel plate method in the
viscosity region between log(viscosity)=5 to 10. The parallel plate
method used is the type 1 described by G. J. Dienes and H. F. Klemm
(J. Applied Physics, 17(6) p 458, 1946). The measurement data has
been fitted and extrapolated with some risk to accuracy beyond the
measurement region. This increase in viscosity is expected due to
the formation of refractory BPO.sub.4 structural groups as
P.sub.2O.sub.5 is added to a predominantly borate glass.
[0035] Other glass former and intermediate oxides are known to
increase glass viscosity. Substitution of GeO.sub.2, SiO.sub.2 and
ZrO.sub.2 are also represented in the compositions listed in Table
1. However, not all viscosity enhancing additions produce desired
improvements in the stability properties of tape made from the
modified glasses. To those skilled in the art, the addition of
other glass former oxides such as GeO.sub.2, or SiO.sub.2 may be
used in combination or singly to modify the viscosity properties of
a high borate glasses with similar benefits to tape stability
properties as P.sub.2O.sub.5. The low viscosity property of high
borate glasses is known to make the control of glass and tape
properties more difficult to control in a manufacturing operation.
For this reason, it would appear obvious to apply the teaching of
this patent to other high borate glasses to improve the firing
stability, conductor compatibility and dielectric properties of
other LTCC dielectrics. The addition of P.sub.2O.sub.5 in the range
of 0.5-8.5 mole % for B.sub.2O.sub.3 in the glass composition has
the unexpected result of enhancement of tape dielectric properties.
The Ag based conductor interaction with the tape dielectric is also
significantly improved as will be described in a later section on
conductor compatibility testing.
[0036] The compositions of Table 1 that have SiO.sub.2 additions
have shown significant improvement in the compatibility with Ag
based conductor lines. The tendency to interact in proximity to Ag
conductor lines is suppressed in the tape compositions tested that
were made from glasses 12, 13, 14, 15, 18 and 19. This result was
not taught in Donohue. The dielectric loss properties reported in
Table 2 unexpectedly shows that the addition of SiO.sub.2 in
composition 14 substantially reduces the electrical loss
characteristics of the tape dielectric. The composition 15 shows
the largest reduction in dielectric loss, as compared with
composition 1. This reduced loss result is also likely but not
measured for compositions 12 and 13. The low addition levels of
SiO.sub.2 addition to glass shown in this case was not reported in
Donohue. The addition of SiO.sub.2 was indicated as not beneficial
to dielectric loss.
[0037] The compositional range demonstrated in this testing is as
follows: B.sub.2O.sub.3 46-58 mole %, CaO 28-29 mole %,
La.sub.2O.sub.3 10-12 mole %, Na.sub.2O 1.3-1.5 mole %, Li.sub.2O
1.3-1.5 mole %, P.sub.2O.sub.5 0.-5.5 mole %, SiO.sub.2 0-5.5 mole
%. In order to gain the performance advantages shown in the
experimental data over the reference composition 1, a minimum of
0.5 mole % of SiO.sub.2, P.sub.2O.sub.5, or both must be
substituted for B.sub.2O.sub.3. Although testing has been conducted
on glass compositions formulated to make LTCC tape that contains
primarily one additional glass former, such as SiO.sub.2,
P.sub.2O.sub.5, GeO.sub.2 etc. at a time, more than one glass
former maybe used to modify this predominately borate glass by
those skilled in the art. It is expected that equivalent benefits
to the manufacture of similar low loss LTCC tapes based upon high
borate glasses would be expected by using the teachings of this
case.
Glass Preparation Procedures
[0038] The glasses were melted in platinum crucibles at a
temperature in the range of 1350-1450.degree. C. The batch
materials were oxide forms with the exception of lithium carbonate,
sodium carbonate and calcium carbonate. The phosphorous pentoxide
was added in the form of a pre-reacted phosphate compound, such as
Ca.sub.2P.sub.2O.sub.7, Na.sub.3P.sub.3O.sub.9, LiPO.sub.3, or
BPO.sub.4. The glass was melted for 0.5-1 hour, stirred, and
quenched. The glass may be quenched in water or by metal roller.
The glass was then ball milled in water to a 5-7 micron powder. The
glass slurry was screened through a 325-mesh screen. The slurry was
dried then milled again to a final size of about 1-3 micron D50.
The dried glass powder was then ready to be used in the tape
formulation to make a tape.
[0039] Ceramic fillers (refractory oxide(s)) such as
Al.sub.2O.sub.3, ZrO.sub.2, SiO.sub.2, TiO.sub.2 or mixtures
thereof may be added to the castable dielectric composition in an
amount of 0-75 weight %, based on solids. Depending on the type of
filler, different crystalline phases are expected to form after
firing. The filler can control dielectric constant and loss over
the frequency range. For example, the addition of BaTiO.sub.3 can
increase the dielectric constant significantly.
[0040] Al.sub.2O.sub.3 is the preferred ceramic filler since it
reacts with the glass to form an Al-containing crystalline phases.
Al.sub.2O.sub.3 is very effective in providing high mechanical
strength and inertness against detrimental chemical reactions.
Another function of the ceramic filler is rheological control of
the entire system during firing. The ceramic particles limit flow
of the glass by acting as a physical barrier. They also inhibit
sintering of the glass and thus facilitate better burnout of the
organics. Other fillers, .alpha.-quartz, CaZrO.sub.3, mullite,
cordierite, forsterite, zircon, zirconia, BaTiO.sub.3, CaTiO.sub.3,
MgTiO.sub.3, amorphous silica or mixtures thereof may be used to
modify tape performance and characteristics. The amount of filler,
type of filler and physical characteristics of the filler will
influence the shrinkage of the fired green tape. Tape shrinkage
maybe adjusted to controlled levels by the use of a multi-modal
particle size distribution optimized to reduce shrinkage by
increasing filler packing density.
[0041] The slurry and/or tape composition may further comprise 0-5
weight % Cu.sub.2O, based on solids.
[0042] In the formulation of tape compositions, the amount of glass
relative to the amount of ceramic material is important. A filler
range of 40-55% by weight is considered desirable in that the
sufficient densification is achieved. If the filler concentration
exceeds 60% by wt., the fired structure is not sufficiently
densified and is too porous. Within the desirable glass to filler
ratio, it will be apparent that, during firing, the filler phase
will become saturated with liquid glass.
[0043] For the purpose of obtaining higher densification of the
composition upon firing, it is important that the inorganic solids
have small particle sizes. In particular, substantially all of the
particles should not exceed 15 .mu.m and preferably not exceed 10
.mu.m. Subject to these maximum size limitations, it is preferred
that at least 50% of the particles, both glass and ceramic filler,
be greater than 1 .mu.m and less than 6 .mu.m.
[0044] The organic medium in which the glass and ceramic inorganic
solids are dispersed is comprised of an organic polymeric binder
which is dissolved in a volatile organic solvent and, optionally,
other dissolved materials such as plasticizers, release agents,
dispersing agents, stripping agents, antifoaming agents,
stabilizing agents and wetting agents.
[0045] To obtain better binding efficiency, it is preferred to use
at least 5% wt. polymer binder for 90% wt. solids (which includes
glass and ceramic filler), based on total composition. However, it
is more preferred to use no more than 30% wt. polymer binder and
other low volatility modifiers such as plasticizer and a minimum of
70% inorganic solids. Within these limits, it is desirable to use
the least possible amount of binder and other low volatility
organic modifiers, in order to reduce the amount of organics which
must be removed by pyrolysis, and to obtain better particle packing
which facilitates full densification upon firing.
[0046] In the past, various polymeric materials have been employed
as the binder for green tapes, e.g., poly(vinyl butyral),
poly(vinyl acetate), poly(vinyl alcohol), cellulosic polymers such
as methyl cellulose, ethyl cellulose, hydroxyethyl cellulose,
methylhydroxyethyl cellulose, atactic polypropylene, polyethylene,
silicon polymers such as poly(methyl siloxane), poly(methylphenyl
siloxane), polystyrene, butadiene/styrene copolymer, polystyrene,
poly(vinyl pyrollidone), polyamides, high molecular weight
polyethers, copolymers of ethylene oxide and propylene oxide,
polyacrylamides, and various acrylic polymers such as sodium
polyacrylate, poly(lower alkyl acrylates), poly(lower alkyl
methacrylates) and various copolymers and multipolymers of lower
alkyl acrylates and methacrylates. Copolymers of ethyl methacrylate
and methyl acrylate and terpolymers of ethyl acrylate, methyl
methacrylate and methacrylic acid have been previously used as
binders for slip casting materials.
[0047] U.S. Pat. No. 4,536,535 to Usala, issued Aug. 20, 1985, has
disclosed an organic binder which is a mixture of compatible
multipolymers of 0-100% wt. C.sub.1-8 alkyl methacrylate, 100-0%
wt. C.sub.1-8 alkyl acrylate and 0-5% wt. ethylenically unsaturated
carboxylic acid of amine. Because the above polymers can be used in
minimum quantity with a maximum quantity of dielectric solids, they
are preferably selected to produce the dielectric compositions of
this invention. For this reason, the disclosure of the
above-referred Usala application is incorporated by reference
herein.
[0048] Frequently, the polymeric binder will also contain a small
amount, relative to the binder polymer, of a plasticizer that
serves to lower the glass transition temperature (Tg) of the binder
polymer. The choice of plasticizers, of course, is determined
primarily by the polymer that needs to be modified. Among the
plasticizers which have been used in various binder systems are
diethyl phthalate, dibutyl phthalate, dioctyl phthalate, butyl
benzyl phthalate, alkyl phosphates, polyalkylene glycols, glycerol,
poly(ethylene oxides), hydroxyethylated alkyl phenol,
dialkyldithiophosphonate and poly(isobutylene). Of these, butyl
benzyl phthalate is most frequently used in acrylic polymer systems
because it can be used effectively in relatively small
concentrations.
[0049] The solvent component of the casting solution is chosen so
as to obtain complete dissolution of the polymer and sufficiently
high volatility to enable the solvent to be evaporated from the
dispersion by the application of relatively low levels of heat at
atmospheric pressure. In addition, the solvent must boil well below
the boiling point or the decomposition temperature of any other
additives contained in the organic medium. Thus, solvents having
atmospheric boiling points below 150.degree. C. are used most
frequently. Such solvents include acetone, xylene, methanol,
ethanol, isopropanol, methyl ethyl ketone, ethyl acetate,
1,1,1-trichloroethane, tetrachloroethylene, amyl acetate,
2,2,4-triethyl pentanediol-1,3-monoisobutyrate, toluene, methylene
chloride and fluorocarbons. Individual solvents mentioned above may
not completely dissolve the binder polymers. Yet, when blended with
other solvent(s), they function satisfactorily. This is well within
the skill of those in the art. A particularly preferred solvent is
ethyl acetate since it avoids the use of environmentally hazardous
chlorocarbons.
[0050] In addition to the solvent and polymer, a plasticizer is
used to prevent tape cracking and provide wider latitude of
as-coated tape handling ability such as blanking, printing, and
lamination. A preferred plasticizer is BENZOFLEX.RTM. 400
manufactured by Rohm and Haas Co., which is a polypropylene glycol
dibenzoate.
APPLICATION
[0051] A green tape is formed by casting a thin layer of a slurry
dispersion of the glass, ceramic filler, polymeric binder and
solvent(s) as described above onto a flexible substrate, heating
the cast layer to remove the volatile solvent. This forms a
solvent-free tape layer. The tape is then blanked into sheets or
collected in a roll form. The green tape is typically used as a
dielectric or insulating material for multilayer electronic
circuits. A sheet of green tape is blanked with registration holes
in each corner to a size somewhat larger than the actual dimensions
of the circuit. To connect various layers of the multilayer
circuit, via holes are formed in the green tape. This is typically
done by mechanical punching. However, a sharply focused laser or
other method(s) can be used to volatilize and form via holes in the
green tape. Typical via hole sizes range from 0.1 to 6.4 mm. The
interconnections between layers are formed by filling the via holes
with a thick film conductive ink. This ink is usually applied by
standard screen printing techniques. Each layer of circuitry is
completed by screen printing conductor tracks. Also, resistor inks
or high dielectric constant inks can be printed on selected
layer(s) to form resistive or capacitive circuit elements.
Furthermore, specially formulated high dielectric constant green
tapes similar to those used in the multilayer capacitor industry
can be incorporated as part of the multilayer circuitry.
[0052] After each layer of the circuit is completed, the individual
layers are collated and laminated. A confined uniaxial or isostatic
pressing die is used to insure precise alignment between layers.
The laminate assemblies are trimmed with a hot stage cutter. Firing
is typically carried out in a standard thick film conveyor belt
furnace or in a box furnace with a programmed heating cycle. This
method will, also, allow top and/or bottom conductors to be
co-fired as part of the constrained sintered structure without the
need for using a conventional release tape as the top and bottom
layer, and the removal, and cleaning of the release tape after
firing.
[0053] The dielectric properties of the fired tape (or film) of the
present invention depend on the quantity and/or quality of total
crystals and glasses present. The low temperature co-fired ceramic
(LTCC) device dielectric properties also depend on the conductor
used. The interaction of conductor with the dielectric tape may, in
some embodiments, alter the chemistry of the dielectric portion of
the device. By adjusting the heating profile and/or changing the
quality and/or quantity of the filler in the tape and/or chemistry
of the conductor, one skilled in the art could accomplish varying
dielectric constant and/or dielectric loss values.
[0054] As used herein, the term "firing" means heating the assembly
in an oxidizing atmosphere such as air to a temperature, and for a
time sufficient to volatilize (burn-out) all of the organic
material in the layers of the assemblage to sinter any glass, metal
or dielectric material in the layers and thus densify the entire
assembly.
[0055] It will be recognized by those skilled in the art that in
each of the laminating steps the layers must be accurate in
registration so that the vias are properly connected to the
appropriate conductive path of the adjacent functional layer.
[0056] The term "functional layer" refers to the printed green
tape, which has conductive, resistive or capacitive functionality.
Thus, as indicated above, a typical green tape layer may have
printed thereon one or more resistor circuits and/or capacitors as
well as conductive circuits.
[0057] It should also be recognized that in multilayer laminates
having greater than 10 layers typically require that the firing
cycle may exceed 20 hours to provide adequate time for organic
thermal decomposition.
[0058] The use of the composition(s) of the present invention may
be used in the formation of electronic articles including
multilayer circuits, in general, and to form microwave and other
high frequency circuit components including but not limited to:
high frequency sensors, multi-mode radar modules,
telecommunications components and modules, and antennas.
[0059] These multilayer circuits require that the circuit be
constructed of several layers of conductors separated by insulating
dielectric layers. The insulating dielectric layer may be made up
of one or more layers of the tape of the present invention. The
conductive layers are interconnected between levels by electrically
conductive pathways through a dielectric layer. Upon firing, the
multilayer structure, made-up of dielectric and conductive layers,
a composite is formed which allows for a functioning circuit (i.e.
an electrically functional composite structure is formed). The
composite as defined herein is a structural material composed of
distinct parts resulting from the firing of the multilayer
structure which results in an electrically functioning circuit.
EXAMPLES
[0060] Tape compositions used in the examples were prepared by ball
milling the fine inorganic powders and binders in a volatile
solvent or mixtures thereof. To optimize the lamination, the
ability to pattern circuits, the tape burnout properties and the
fired microstructure development, the following volume %
formulation of slip was found to provide advantages. The
formulation of typical slip compositions is also shown in weight
percentage, as a practical reference. The inorganic phase is
assumed to have a specific density of 3.5 g/cc for glass and 4.0
g/cc for alumina and the organic vehicle is assumed to have a
specific density of 1.1 g/cc. The weight % composition changes
accordingly when using glass and oxides other than alumina as the
specific density maybe different than those assumed in this
example.
TABLE-US-00004 Volume % Weight % Inorganic phase 41.9 73.8 Organic
phase 58.1 26.2
[0061] The above volume and weight % slip composition may vary
dependent on the desirable quantity of the organic solvent and/or
solvent blend to obtain an effective slip milling and coating
performance. More specifically, the composition for the slip must
include sufficient solvent to lower the viscosity to less than
10,000 centipoise; typical viscosity ranges are 1,000 to 4,000
centipoise. An example of a slip composition is provided in Table
3. Depending on the chosen slip viscosity, higher viscosity slip
prolongs the dispersion stability for a longer period of time
(normally several weeks). A stable dispersion of tape constituents
is usually preserved in the as-coated tape.
TABLE-US-00005 TABLE 3 Slip Composition Component Weight % Acrylate
and methacrylate polymers 18.3 Phthalate type plasticizers 1.8
Ethyl acetate/isopropyl alcohol mixed solvent 15.2 Glass powder
27.9 Alumina powder 36.8
[0062] If needed, a preferred inorganic pigment at weight % of 0.1
to 1.0 may be added to the above slip composition before the
milling process.
Glass Powder Preparation
[0063] The glasses for the Examples found herein were all melted in
Pt/Rh crucibles at 1350-1450.degree. C. for about 0.5-1 hours in an
electrically heated furnace. Glasses were quenched by pouring into
water, removed quickly and dried as a preliminary step and then
subjected to particle size reduction by milling in water. The
powders prepared for these tests were adjusted to a 1-3 micron mean
size by ball milling and screening. The mill slurry is dried in a
hot air oven and de-agglomerated by screening. The glass, alumina,
and tape medium are milled together to produce a slip suitable for
forming a tape by casting the slurry on a polymer based carrier
substrate. The tape is cast using tape casting equipment forming a
layer of uniform dimension and dried to form a flexible tape of
typically about 0.11 mm thickness.
Chemical Durability Test
[0064] Ten layers of tape were laminated to form samples having
post fire dimensions of 2.5.times.2.5 mm. The samples were fired in
a standard LTCC heating profile (190 min. door to door) and dipped
in two different mineral acids of 10% acid strength kept at
constant temperature of 40.degree. C., for a constant period of
time of 30 minutes each respectively. The weight loss (difference
in % of weight) due of leaching of tape components into acid were
taken as the measure of chemical durability. Two commercially
available LTCC tapes, 951-AT and 943-A5, (Dupont Company,
Wilmington Del.) were used as the control. Results are given
below:
TABLE-US-00006 % Weight loss Sample HCL H2SO4 LTCC (Present
Invention) 0.10 0.11 943-A5 (Low loss LTCC) 0.42 0.23 951-AT
(Standard LTCC) 0.01 0.01
The chemical resistance of the current invention is substantially
improved over the standard 943 tape.
Mechanical Strength
[0065] Ten layers of "Green tapes" were laminated and fired using a
standard LTCC heating profile. The modulus of rupture of several
duplicated parts was measured and the average values are given
below:
TABLE-US-00007 Sample Modulus of Rupture (psi) LTCC (present
invention) 28,900 943-A5 (low loss LTCC) 26,200 951-AT (standard
LTCC) 30.000
These results indicate that similar strength was attained in all
tape materials.
Processing Latitude
[0066] Ten layers of "green tapes" were laminated and fired using
three different heating profiles, keeping the maximum firing
temperature and time at maximum temperature constant at 850.degree.
C. for 10 minutes. The total time for three heating profiles were
75 minutes, 190 minutes and 380 minutes.
[0067] Two dielectric properties, dielectric constant and
dielectric loss, were measured at a constant frequency of 8-9 GHz
and taken as the measure of the processing latitude. Results are
given below:
TABLE-US-00008 Dielectric Constant Dielectric loss (K) (%) Profile
1 2 3 1 2 3 LTCC (Tape - glass #2) 6.94 6.41 6.13 0.14 0.08 0.11
LTCC (Tape - glass #6) 7.34 7.28 7.12 0.12 0.09 0.08 943-A5 (low
loss LTCC) 7.66 7.57 7.41 0.13 0.10 0.09 951-AT (standard LTCC)
7.53 7.51 7.54 0.74 0.68 0.43
The dielectric constant and loss are reduced for the LTCC tape
based upon glass #6. The rate of heating of the tape laminates
shows a progressive reduction in dielectric properties for most all
tape samples.
[0068] In additional studies of the effect of temperature, the 190
minute profile was used to fire tape laminates made with glass #2
at 825, 850, and 875 C. for a period of 10 minutes. The results are
given below:
TABLE-US-00009 Peak Dielectric Constant Dielectric loss Temperature
(K) (%) 825 C. 6.40 0.10 850 C. 6.41 0.08 875 C. 6.35 0.07
When the heating profile is fixed and the peak soak temperature is
changed as a simulation to process latitude, the properties are
observed to vary only slightly, thus indicating good process
latitude. The properties of tapes made from glass from the current
invention are observed to improve both the desirability of the
dielectric properties obtained and the stability with which the
properties can be reproduced.
[0069] The dielectric properties of the fired film of this
invention, which is a "devitrified glass-ceramic composite", the
properties of which depend on the quantity and quality of total
crystals and the remnant glass present in the composite. The LTCC
dielectric properties also depend on the conductor film which is a
"metal-devitrified glass-ceramic composite". By adjusting the
heating profile, a knowledgeable practitioner could alter the
ratios of components present in the fired film and thus accomplish
lower dielectric constant and/or better dielectric losses.
Tape Shrinkage and Refire Stability
[0070] The shrinkage values have been measured then calculated
using the "Hypotenuse" method, known to those skilled in the art.
All parts were fired at 850.degree. C. following a standard green
tape firing profile. The refires were conducted at 850.degree. C.
using a 30-minute above 800.degree. C. profile.
TABLE-US-00010 Dimensional Shrinkage in % Sample Initial 1X 2X 3X
LTCC current invention Part 1 11.35 11.32 11.27 11.24 Part 2 11.35
11.29 11.26 11.23 943-A5 Part 1 9.92 9.80 9.71 9.66 Part 2 9.52
9.42 9.36 9.34 Part 3 9.34 9.20 9.10 9.04 Part 4 9.27 9.17 9.11
9.09
[0071] The shrinkage data for the glass #1 based tape (943
commercial tape) shows more variation between parts and with
refires than the glass #6 based tape of the current invention. This
variation seen for the glass #1 tape makes manufacturing control of
tape shrinkage more difficult than the tape made using the current
invention. The variation of the improved tape is on the order of
0.11-0.12 between initial and 3.times. refired. The variation of
the standard tape ranges 0.3-0.18 for the same conditions. Since
the acceptable tolerance for the manufacturing specification for
shrinkage is +/-0.3%, it yields marginal process control. However,
the variation in shrinkage between parts is largest source of
variability. The initial firing of the 943 tape shows a difference
of 0.65. This exceeds the variation required in application. The
improved tape shows very little part to part variation and can be
made to meet current dimensional stability standards.
Conductor Compatibility/Re-fire Stability
[0072] "Daisy chain test", consisting of over 5000 squares of
conductor lines and over 300 via-fill conductors connected in
series was used to evaluate the conductor compatibility and re-fire
stability of the system. After each firing, the resistivity was
measured. Any conductor line break and/or conductor-via fill
conductor separation should show in the test by giving infinite
resistivity. No conductor opens were detected. The testing was
performed on 0.05, 0.11, and 0.25 mm thickness tape with the same
stability performance being shown (ie. no opens). The resistivity
of the Ag based conductor as a function of the number of refires
shows the high level of stability achieved in FIG. 7, using the
current invention tape based upon the #6 glass.
[0073] Fired tape samples made from glass #14, #20 and #6 when
compared to Glass #1 (Donohue reference) show no Ag interaction
damage in proximity to buried conductor lines having 0.127 mm lines
and spaces. The Glass #1 based tape in contrast shows localized
conductor erosion in proximity of the buried conductor lines within
the tape dielectric.
* * * * *