Note: Descriptions are shown in the official language in which they were submitted.
CA 02298684 2000-02-O1
WO 99/57949 PCT/US99/09551
INTERMEDIATE LAYER TO IMPROVE PEEL STRENGTH OF COPPER FOILS
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority from provisional patent
application Serial No. 60/046,598 filed May 15, 1997.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
In the design and implementation of electronic circuitry,
there is a strong incentive to make the circuit layouts as
dense as possible. In the interests of making circuit designs
more dense, it is desirable that the circuitry board has a low
dielectric constant (Dk) and a high glass transition
temperature (Tg). However, it has been observed that the peel
strength value for a copper-clad laminate can be significantly
reduced when a low Dk, high Tg resin system is employed,
relative to the peel strengths obtained when a standard resin
system is employed. Peel strength refers to the strength of
adhesion of coated copper layers to a circuit board substrate
and high peel strength is generally desirable. Peel strength
may be even more severely reduced when low or very low profile
copper foils are employed. However, low or very low profile
copper foils are critical to the success of circuit board
laminates for use in very dense circuit aesigns.
Typically, when attaching copper foils to fiberglass
impregnated epoxy resin laminates for circuit board
construction, no intermediate adhesive layer is required.
Usually, if the rougher side of the copper foil is placed in
contact with the uncured resin laminate, and heat is applied,
the mechanical interaction between the foil an the resin of the
laminate is sufficient to hold the foil to the laminate.
However, as the material of the board is altered, particularly
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as nonpolar materials are added to the board to increase its
dielectric properties, this mechanical bond can become
insufficient. This problem is made worse by the use of so-
called low or very low profile copper foils which are thinner
and which therefore also tend to have smaller surface
irregularities.
Studies conducted using both present commercial products
and products that are under development have shown that few
foil and low Dk boards are currently capable of meeting the
requirements of new military specifications (MIL-S-13949/4D)
for peel strength in dense circuitry after thermal stress
conditioning. One specification requires minimum peel strength
of 8.0 pounds per linear inch (pli).
One form of adhesive that is sometimes used to and in
bonding of resin to foil is to first coat the foil with a layer
of uncured liquid resin itself which is intended to assist an
integrating the foil to the laminate. Often such upper foils
are coated with silane, which is believed to aid adhesion.
U.S. Patents 5,525,433 and 5,629,433 describe some
multifunctional epoxy compounds which can be used to attach
foils to a laminate pre-preg.
There exists a need for a method of making a copper-clad
laminate having an increased peel strength relative to prior
art laminates.
BRIEF SUMMARY OF THE INVENTION
The present invention is a method of manufacturing a
copper-clad laminate having enhanced peel strength comprising
the step of applying an organic resin coating selected from the
group consisting of high molecular weight epoxy or phenoxy
resins or combinations thereof to a surface of a copper foil
and laminating the copper foil to a low Dk laminate.
The present invention is also a copper-clad laminate made
by applying an organic resin coating to a surface of a copper
foil and laminating the copper foil to a laminate, the organic
resin selected from the group consisting of epoxy or phenoxy
resins or combinations thereof.
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It is an object of the present invention to provide a
copper-clad laminate having a low dielectric constant, a high
glass transition temperature, and a peel strength that meets
industry standards.
Other objects, advantages, and features of the present
invention will become apparent after review of the
specification and drawings.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
Fig. 1 is a plot of peel values (pli) of copper-clad
laminate after thermal stress conditioning as a function of the
number average molecular weight (Mn) of the organic resin
coating applied to the copper foil prior to lamination.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is a method of manufacturing a low
Dk, high Tg copper-clad laminate having enhanced peel strength
comprising the step of applying a high molecular weight organic
resin coating to a surface of the copper foil prior to
lamination. The present invention is also directed to the
laminate materials and circuit boards made using this method.
The organic resin coating for use within the present
invention is preferably an uncured epoxy or phenoxy resin, or
combinations thereof, having an average molecular weight
greater than 4500. It has been found that high molecular
weight epoxy and phenoxy compounds provided greater peel
strength for low Dk and high Tg boards than other adhesives of
lower molecular weight.
By "enhanced peel strength" it is meant that the peel
strength after thermal stress conditioning exceeds the peel
strength of a control laminate in which lamination occurs
without applying an organic resin coating to a surface of the
copper foil prior to lamination. Preferably, the peel strength
is enhanced at least about 100% greater than the peel strength
of a comparable laminate lacking the organic resin coating on
the copper film. Determining the peel strength of a laminate
by standard methods is well within the ability of one of
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ordinary skill in the art.
As demonstrated in the examples below, peel strength is
affected by numerous factors, including the dielectric constant
(Dk) and glass transition temperature (Tg) of the laminate, the
profile of the copper foil, the number average molecular weight
(Mn) of the organic resin coating on the copper foil, and the
thickness of the organic resin coating applied. For a
particular laminate and foil, there exists a direct linear
relationship between peel strength and Mn of the organic resin
employed (Fig 1). For example, when a laminate product having
a low Dk and a high Tg (e. g., AlliedSignal Laminate Systems
product FR408) and a very low profile copper foil (e. g.
AlliedSignal Oak-Mitsui 1 oz. MLS) are laminated together, an
organic resin having an Mn of at least about 4500 is required
in order to obtain a peel strength of about 8.0 pli. However,
it is well within the ability of one of ordinary skill in the
art to determine the minimum Mn required using the guidance
provided herein.
By a laminate having a low Dk and a high Tg, it is meant a
laminate having a Dk of less than about 4.5 and a Tg of greater
than about 170°C. The Dk value of the fiberglass and the
impregnating resin are different and thus the Dk depends on the
resin content of the laminate. Low Dk laminates generally have
Dk values between 2.5 and 4.5 and preferably between 3.0 and
4Ø These values may be measured using a Hewlett Packard
Materials/Impedance Analyzer, Model 4291A, operating at 100
MHZ .
The peel strength increases with the treatment thickness
of the organic resin applied to the copper foil prior to
lamination. A copper coated laminate having a peel strength of
about 8.0 pli when a FR408 laminate, or its equivalent, is
laminated to a copper foil equivalent to CircuitFoils NTTW-HTE-
1/2 oz using a treatment of a 0.1 mil coating of organic resin
as specified here. The minimum treatment thickness required to
achieve a particular peel strength may vary depending on the
laminate, the copper foil, and the organic resin. The
preferred range of thickness of this organic resin is between
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.05 mils and .50 mils, with the most preferred being .08 to .16
mils. However, it is well within the ability of one of
ordinary skill in the art to determine the minimum treatment
thickness required for a particular selection of starting
materials.
Although the copper-clad laminates described in the
examples were prepared by applying the organic resin to the
copper foil prior to lamination, it is anticipated that the
organic resin could be applied to the laminate prior to
lamination of the copper foil. It is also specifically
envisioned that copper foils can be pre-treated with the
organic resin and stored until needed for lamination. What is
important is simply the use of the organic resin to bind the
copper foil to the laminate material.
The examples below describe a method for manufacturing a
copper clad laminate having a high peel strength using
commercial epoxy or phenoxy products obtained from Phenoxy
Specialties or Ciba-Geigy as the organic resin. It is expected
that any epoxy or phenoxy resin could be employed in the
practice of the present invention, provided that the resin has
a sufficiently high number average molecular number. It is
reasonably anticipated that any polymer that has a sufficiently
high molecular weight and which is reactive toward epoxy resins
or epoxy curatives could be used in the practice of the present
invention.
The nonlimiting examples that follow are intended to be
purely illustrative.
cla laminate
To determine whether an organic resin coating applied to
the copper foil surface prior to lamination enhances copper
adhesion to the FR408 laminate product, the following
experiment was conducted using the FR408 laminate product. A
copper-clad laminate was made by laminating, 2 plies of 7628-
(41% resin content) (.014 inches) and AlliedSignal Oak-Mitsui 1
oz. MLS (reverse-treated, coated, very low profile) copper foil
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using an organic resin, PAPHEN Phenoxy solution PKHS-40
(Phenoxy Specialties, Inc.) was diluted to 8% solids with MEK
solvent and brush applied to the copper foil prior to
lamination to the FR408 laminate product. As a control, the
laminate and copper foil were laminated without the resin.
Peel strength measurements after thermal conditioning were made
according to standard methods known to one of ordinary skill in
the art. The results of this experiment are shown in Table 1.
Table 1
Peel Strength Results
Sample Peel Strength (pli) Tg Dk
Untreated control 2.3 182 4.1
Treated Sample 1 7.0 182 4.1
Treated Sample 2 6.9 I'77 4.1
EffP~t of mo~,ecular number weight (Mn) of organic resin on eel
strength
Additional epoxy-phenoxy resin products were evaluated for
peel strength enhancement. The products listed below were
selected because of their varied Number Molecular Weight, Mn.
A copper foil (standard HTE-1 oz. foil) enhanced with a resin
coating of these products were evaluated for copper peel
adhesion to the FR408 laminate product. The results are shown
in Table 2.
Table 2
Peel Strength Results
Sample O~;ganic Resin Sunolier Mn P eel. Thermal
S tress,
Eli
control 6.0
A Araldite PZ 3901 Ciba-Geigy 1062 6.0
B Araldite PZ 3907 Ciba-Geigy 3948 7.1
C PKHW-34 Phenoxy Specialties5500 9.5
D Araldite GZ488 Ciba-Geigy 13378 11.6
N40US
E PICHW-35 Phenoxy Specialties13000 12.0
3 5 F PKHS-40 Phenoxy Specialties13'000 12.2
A plot of Mn versus peel strength for these resins shows a
positive linear relationship between the Mn of the organic
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resin and peel strength (Fig. 1). The plot indicates that when
employing the FR408 laminate product or an equivalent and a
standard HTE-1 oz. foil, the resin must have a Mn of at least
about 4500 to meet the MIL-S-13949 specification of 8.0 pli.
Effect of tr,~atment thi cakness on heel strencrth
PKHS-40 phenoxy resin solution was diluted to various
levels of solids content and brush applied to produce different
dry film thickness levels on the copper foil substrate
(CircuitFoils standard profile NTTW-HTE-1/2 oz.) to evaluate
the effect of treatment thickness on peel strength of copper-
clad FR408 laminates. The treatment thickness was determined
using a Veeco MP-900 B-Backscatter thickness measuring unit.
The results indicate a minimum thickness requirement at or
about 0.1 mils in order to obtain significant improvement in
peel strength (Table 3). The preferred range is 0.5-.5 mils,
thus minimizing any increase in thickness caused by the
adhesive while supplying adequate strength, with the most
preferred thickness being .08 to .16 mils.
Table 3
Impact of Treatment Thickness
Peel strength (x~li)
Level Treatment Thic ness " mils ~;eated control
1 0.05 4.8 4.2
2 0.12 8.2 3.9
3 0.14 8.2 4.1
4 0.19 8.4 3.8
5 0.20 8.4 3.7
The best practice to date for manufacturing the treated
foil product on a large scale consists of reverse roll coating
foils as supplied from the foil manufacturer. However, any
process method of coating may be employed as long as the
resultant coating thickness is in the desired range.
A experiment was conducted using a variety of low and very
low profile copper foils to determine peel strength
enhancements using the PKHS-40 phenoxy resin treatment. The
results of this experiment are shown in Table 4.
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Table 4
Peel Strength Results
Peel stren gth (,gli)
Foil Vendor FoilProduct Tooth ProfileTreated ~
~trol
Oak-Mitsui TOBIII-ML- 1/2 low 9.2 ,
oz. 4.6
Oak-Mitsui TOBIII-ML- 3/8 low 7.8 2.9
oz.
Oak-Mitsui MLS- 1/2 oz. very low 8.0 3.5
Oak-Mitsui MLS- 1 oz. very low 9.1 3.7
Gould JTCS- 1/2 oz. low B.6 3.0
It is understood that the present invention is not to be
limited to the particular embodiments and examples illustrated
above but encompasses all such modifications and variations
thereof as come within the scope of the following claims.
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