Note: Descriptions are shown in the official language in which they were submitted.
DESCRIPTION
ADHESION PROMOTER FOR PRINTED CIRCUITS
Technical Field
The present invention relates to printed circuits,
and particularly to improving the adhesion between cop-
per foil and a polymeric substrate.
Printed circuit boards are typically constructed
with a polymeric substrate such as a phenolic, epoxy,
poIyimide, polyester, or like oth~r resinl upon which is
bonded a copper ~oil which is etched to provide a conduc-
tor in the desired configuration. Copperl like other
pure metals, generally exhibits poor adhesion character-
istics or bonding to polymers, and intermediate conver-
sion coatings~are frequently helpful. ~amili~r examples
of this practice are the various~pre-paint treatments,
such as phosphate on steel, chromate on;zinc or alumi-
num, and anodic oxide on aluminum.
:
For printed circuits, it is typical to employ cop-
per oxide as the adhesion improving coating; howev2r,
the known methods for achieving the copper oxide ~oat-
ings involve the~use of highly corrosive chemicals which
become even more~dangerous at the elevated temperatures
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typically employed for processing. Moreover, the im-
provement in adhesion achieve~ through the use of known
technology has been erratic. In fact, adhesion after
treatment is sometimes no better than for clean copper.
In view of these difficulties, the question arises
as to why use copper oxide at all when clean etched
copper alone will give peel strengths of two to three
pounds per inch. However, it is known that copper is a
reactive metal and can react with the components or
decomposition products of the polymer substrate. For
instance, copper can compete for the amine curing
agents in epoxy resins. These chemical and physical
processes, aggravated by thermal cycling, can continue
for the life of the assembly and lead to ultimate fail-
ure in service. An oxide coating serves as a diffusionbarrier to prevent these reactions.
Accordingly, there is a present need for improve-
ments in achieving increased bonding strength through
the use of copper oxide coatings.
Back~round Art
In the preparation of printed circuits, a copper
foil is bonded to the polymeric substrate which may be
phenolic, epoxy, polyimide, polyester, or the like.
Prior to bonding, the foil is normally treated electro-
lytically to provide a specific surface structure asdisclosed in U.S. Patents 3,293,109; 3,318,758;
3,518,168; 4,049,481; 4,131,517; 4,176,035; and others.
It is believed that the copper foil on most single and
two-sided printed circuit boards is so treated.
~ultilayer printed circuit boards are assemblies of
several t~o-sided boards further bonded to each other
through layers of semicured polymeric material which are
subsequently cured at elevated temperatures and pressures
to form the complete assembly. Prior to -his assembly,
the copper foil of the two-sided boards is imaged and
etched to ~orm the inner layer circuits. The opposite
side of the conductor patterns mus, then be treated for
adhesion to the layer of polymer bonding one board to
another. Because the etched cond~ctors have no conti-
nuity for electrolysis, it is necessary to treat them
chemically.
Copper oxide is the most useful chemical conversion
coating for copper adhesion and has so been used since
the early days of printed circuit technology. The basic
patent in this field appears to be U.S. 2,364,993, which
discloses the use of sodium chlorite and sodium hydroxide
at high concentrations and temperatures near boiling.
Similar disclosures appear in other patents, such as
U.S. 2,460,896, U.S. 2,460,898, and U.S. 2,481,854,
which have been granted to the same assignee. These
four related patents teach the use of a treatment solu-
tion containing caustic in an amount which equals or
exceeds the chlorite. The concentration range disclosed
is from five grams per liter of chlorite and ten grams
per liter of caustic on the low side, to solutions satu-
rated with chlorite and containing one thousand grams
per liter of caustic. It is indicated that the lower
range will blacken copper in thirty minutes at 216-F,
while the higher range will do the same in one minute at
250rF. An intermediate composition containing 150 grams
per liter of chlorite and 150 grams per liter of caustic
is disclosed in U.S. 2,3~4,993 to blacken the copper
surface in five minutes at 200F.
While these patents appear to be directed toward
decorative applications, as well as non-refle~tive coat-
ings for the interior surfaces of optical instruments~
the same principles have been applied to the treatment of
copper foils for use in preparing printed circuits. Some
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examplary patents applying this technology to printed
circuit adhesion are: U.S. 2,955,974; U.S. 3,177,103;
U.S. 3,198,672; U.S. 3,240,66~; U.S. 3,37~,129; and U.S.
3,481,777. Other known means for providing copper
oxide coatings include oxidation with alkaline perman-
ganate as disclosed in U.S. 3,544,389, hydrogen peroxide
as disclosed in U.S. 3,~34,889, and even air at elevated
temperature as disclosed in U.S. 3,677,828.
It is known that these oxide coatings are erratic
and too frequently provide surprisingly poor adhesion,
sometimes less than clean copper. A part of this problem
may be that the oxide coatings are often too thick--
presumably because it was thought that thicker coatings
were better because of increased specific surface area.
In actual fact, a thick oxide is inferior as an adhesion
promoter because it is mechanically weak and may not be
homogeneous.
A thick coating of oxide tends to be fuzzy and vel-
vety, making it mechanically weak. Black powder can be
wiped off with a finger. Figure 1 shows the open fibrous
structure af such a surface. During the multilayer pro-
cessing operation at a high temperature and pressure J the
fibers can be crushed and partially encapsulated by the
flowing polymerO This means that the bond can break
within the oxide layer, causing a cohesive, rather than
adhesive, failure. The so-called "oxide transfer",
seen as a dark stain occasionally appearing within the
epoxy surface after etching away the copper, is a mani-
festation of encapsulated oxide fibers.
Further, a thick black oxide may not be homoge-
neous. Whereas the outer surface will be cupric o~ide,
there will be a gradient through the thickness which
will be increasingly richer in cuprous oxide. This is an
unstable species which can be oxidized to cupric oxide
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during the high temperature pressing operation by com-
bining with residual oxygen, water or various decompo-
~ition products. The result of this reaction is a change
in volume, a breaking away of points of attachment, and
the creation of micro-voids which lead to poor adhesion.
In recent years, the industry has been moving in the
direction of thinner oxides by dilution of the conven-
tional chlorite-caustic baths and shortening the im-
mersion time as described in "Electronic Packaging and
Production", 18(3), ~9-78 (1978). This diluted solution
still requires a high temperature in the near-boiling
range to dehydrate any cupric hydroxide which may form.
The hydroxide, white to gray in color, is another un-
stable entity which can also undergo shrinkage in volu~e
lS and loss of adhesion during subsequent thermal opera-
tions. And, these solutions remain troublesome from a
corrosion standpoint in view of the high temperatures
required. As recently as October 1981, an all-stainless
steel apparatus was described using this chemistry at
2n 190 F. ("Printed Circuit Fabrication", October 1981, p.
46-50). However, concern was expressed at the corrosion
of stainless steel in the welded areas under these con-
ditions.
.
In view of these difficulties, it would be de-
sirable to provide a thermally stable oxide coating on
copper to enhance its adhesion characteristics to poly-
mers. ~dditionally, it would be desirable to provide a
solution capable of forming such an oxide coating which
was sprayable in conventional spray equipment, such as
that typically e~ployed for spray etching printed circuit
boards to enable increased productivity in continuous,
conveyorized applications. And, it would be desirable to
provide a composition and a process for forming oxide
coatin~s on copper which cGuld be operated at concentra-
tions and temperatures below about 140~F.
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Disclosure of the Invention
In accordance with the invention, I provide a comp-
osition and process for improving the adhesion of copper
foil to polymeric printed circuit substrates by oxidizing
the copper bonding surface. In its broad aspects, the
composition comprises an aqueous solution containing an
alkali metal chlorite or alkaline earth metal chlorite
at a concentration of from about 100 grams per liter to
saturation, and sodium hydroxide or potassium hydroxide
at a concentration of from about 5 to about 25 grams
per liter. The method in its broad aspect comprises:
cleaning the surface of a copper foil, and immersing
the foil in a solution as defined above at a temperature
within the range of from about 80 to about 200F for a
period of time effective to form an oxide layer on the
surface of the copper foil capable of increasing its
adhesion to a polymer substrate.
The composition and the process according to the
invention provide a significant advance over the prior
art procedure5 such as those taught in U.S. patent
2,364,993, in that it has been found that highly con-
sistent results in terms of improved adhesion can be
achieved at moderate temperaturesl at commercially-effi-
cient production rates, and at low concentrations of
caustic, where the concentration of caustic is maintained
at 25% or less of the concentration of the chlorite.
Thus, according to the present invention, the relative
amount of chlorite is increased while that of the caustic
is decreased.
~OS. Patent 2,364,993 and the ~hree other related
patents teach the use of a treatment solution containing
caustic in an amount which equals or exceeds the chlorite.
While the low concentrations of 5 grams per liter of
chlorite and 10 grams per liter of caustic darkens the
copper surface to almost black in 30 minutes of reaction
at boiling (216F), this rate is far too slo~ in vie~ of
current demands for high productivity. The high concen-
tration of saturated chlorite and 1000 grams per liter
of caustic yields a thick, black, powdery coating in 1
minute at boiling (250-F), but the coating is mechani-
cally weak and unsatisfactory for adhesion as described
- above. Moreover, these high temperatures, above the
boiling point of water, are extremely hazardous in a
production scale operation which typically involves a
tank of several hundred gallons. For example, the addi-
tion of water to replace evaporation losses could cause a
strong eruption, spattering hot caustic solution to the
hazard of operating personnel.
According to the present invention, however, adhe-
sion-improving oxide layers can be obtained at moderate
temperatures of within the range of from about 80~ to
200-F in commercially practical periods o~ time--typically,
less than 10 minutes, and preferably from 1 to 7 minutes.
At a preferred concentration of sodium chlorite of about
160 grams per liter and sodium hydroxide of about 10
grams per liter, a brown to bronze-colored oxide is
formed at temperatures of about 35-F. The resulting
coating is about 8 microinches in thickness and does not
appear to change with varying time or temperature, al-
though the adhesion value increases with increasing time
and/or temperature. Coatings prepared according to the
preferred embodi~ents of the invention will be brown,
ranging from dark brown to lighter shades o~ brown,
having a bronze hue.
The reason for the relatively constant thickness
and texture is not understood. While not wishing to be
bound to any specific theory, it is possible that copper,
being slightly amphoteri~, dissolves as the cuprate ion
in a strongly alkaline soluticn as evidenced by the blue-
colored solution during the blackening operation. The
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cuprate ion is unstable and soon changes to cupric oxide
and precipitates. However, it is apparent that copper is
continuously being dissolved, leading to a somewhat amor-
phous texture including cuprous oxide under a crystalline
cupric oxide surface. On the other hand, the coating of
the invention is completely insoluble in the lower alka-
linity and emerges as a thin, completely homogeneous,
adherent layer.
The caustic can be sodium hydroxide, potassium
hydroxide, or a combination of the two. The chlorite
however, can be either an alkali metal chlorite such as
sodium or potassium chlorite, or an alkaline earth chlo-
rite such as calcium or magnesium chlorite. Mixtures of
suitable chlorites can also be employed. In addition to
these materials the solution can also include phosphates
and carbonates, for effects such as regulating alkalinity
of the solution.
The minimum concentration of sodium chlorite has
been found to be about 100 grams per liter for reliable
adhesion while the upper limit is sat~ration. The pre- -
ferred concentration will be about 160 grams per liter.
The concentration range for the hydroxide is about 10 to
grams per liter. Lower concentrations ~o not develop
enough adhesive strength within the constraints of a 1 to
7 minute dwell time, or more preferably a 3 to 5 minute
dwell time, in a conventional spray machine. For most
substrates, a level of hydroxide of about 16 grams per
liter is preferred. On the other hand, it is preferred
to maintain the caustic c~ncentration below about 15
grams per liter, or preferably near 10 grams per liter,
for polyimides which are somewhat alkali-sensitive.
Significantly, the ratio of hydroxide to chlorite
on a weight basis should be less than 1:4, and will
preferably be less than about 1:8. The most preferred
f' ~ f
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ratios will be within the range of from about 1:10 to
about 1:20, especially when the preferred concentrations
are employed.
The maximum temperature is limited only by the
temperature tolerance of the equipment. Most conven-
tional polyvinyl chloride spray etchers are limited to
about 140-E' and are equipped with safety devices to pre-
vent overheating. Since safety and energy conservation
are objects of the invention, it is preferred to operate
at or below this level~ although operation at 200~F has
been found satisfactory in steel tanks.
Brief Description of the Drawings
_.
The invention will be better understood and some
oE its advantages will become more apparent when the
following detailed description is read in light of the
attached drawings wherein:
Figure 1 is an electron-microscope-generated
photograph showing the surface of an oxide coating pre-
pared by a prior art method according to Example l; and
Figure 2 is a similar photograph but at a dif-
erent magnification of a coating prepared by the method
of this invention as set forth in Example 3.
Best Mode for Carrying Out the Invention
The following examples will aid in illustrating and
explaining the invention by describing the best mode
contemplated for performing it and comparing it to the
procedures of the prior art. The examples are for illus-
trative purposes and are not to be taken as limiting in
any regard. Unless otherwise indicated, all parts and
percen~ages are by weight.
All of the following samples were processed in the
following pre-treat and lamination cycle. Although some
of the steps may be omitted by some operators in actual
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production, 311 are preferred for maximum adhesion and
were performed for these test,s.
1. Soak clean a 6 x 6 inch, 1 ounce copper
foil in Anodex 61X (MacDermid
Incorporated, Waterbury, Connecticut),
8 o~./gal.l 3 minutes at 160 F.
2. Cold water rinse.
3. Etch in ammonium persulfate, 1.51bs./gal.,
3 minutes at 75 F.
4. Cold water rinse.
5. Dip in sulfuric acid, 10~ by volume, 1
minute at 75-F.
6. Cold water rinse.
7. Oxide treatment as described in the follow-
ing specific examples.
; 8. Cold water rinse~
9. Air dry.
10. Oven dry 30 minutes at 225-F.
11. Laminate to FR-4 epoxy by a
conventional multilayer procedure; i.e.,
400 psi at 350-F for 45 minutes.
Following preparation, the samples are subjected to the
following standard peel test: Measure the adhesion value
(in pounds) of a 1 inch wide strip when pulled at a 90
angle from the substrate at a rate of 1 inch/minute.
Exampl~ Prior art)
An oxidizing solution was prepared with 200 grams
per liter sodium hydroxide and 100 grams per liter sodium
chlorite. The cleaned and etched copper foil was im-
mersed for 5 minutes at 200-F, and then rinsed and dried.
The appearance was black and velvet-like. Black
powder could be wiped off with a finger. Figure 1 is an
electron microscope photograph of the surface. This
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photograph was prepared at a magnification of 7500X at
90. (perpendicular)
When laminated to FR-4 epoxy, the peel test value
was 2.0 to 2.5 pounds per inch.
Example 2 - (Prior art)
Vsing the same solution as in Example 1, but at
150 F, a copper foil was immersed for 10 minutes.
The appearance was dark gray with no velvet nap.
The peel test value was 2.0 to 2.25 pounds per
inch.
Example 3 - (Invention)
The oxidizing solution consisted of 160 grams per
liter sodium chlorite and 10 grams per liter sodium hy-
droxide. The prepared copper foil was immersed for 5
minutes at 130 F. Figure 2 is an electron microscope
photograph of the surface. This photograph was prepared
as that in Figure 1, but at a magnification of 4000X at
45~ angle to accentuate the surface texture.
The surface appeared golden-brown with no apparent
change in surface texture.
The adhesion value was 6.5 pounds per inch.
Example 4 - (Invention)
The solution of Example 3 was used in a conven-
tional printed circuit spray etcher (Chemcut Model 315)
in a conveyorized spray mode. The dwell time was 3.25
minutes at 130~F.
; The appearance of the oxide film was medium brown
~ and the peel test value was 6.0 pounds per inch.
,,
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.,
Exam~les 5 - 21 - (Invention)
. The following examples, all immersion tank appli-
cations, demonstrate a range of parameters through which
the invention is effectiveO
Ex. NaClO2 NaOH Time Temp Peel Test
No. (gm/l) (~m/l) (min) ( F) (pounds/inch)
160 10 3 80 5.0
6 160 10 1 130 3.5
7 160 10 3 130 6.25
108 160 1~ 5 130 6.25
9 160 10 7 130 6.25
160 10 1 150 7.25
11 160 1~ 3 150 7.25
12 160 10 5 1~0 7.75
1513 160 10 7 150 7.50
14 160 10 1 170 7.75
160 1 n 3 170 ~.00
16 160 10 S 170 7.25
17 1~0 10 7 170 7.50
2018 100 8 1 130 2.50
19 100 8 3 130 4.50
100 8 5 130 5.50
21 100 8 7 130 5.75
It is evident that concentration, time, and temp-
erature are interdependent variables. Therefore, an
operator requiring maximum production can select a short
dwell time with a proportionately higher temperature.
One with temperature limitations can operate with a
longer dwell time or a higher concentration~
The above description is for the purpose of teach-
ing a person of ordinary skill in the art how to practice
the invention~ This description is not intended to de
tail all of the obvious modifications and variations of
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the invention which will become apparent to the skilled
worker upon reading. However, it is intended that all
such modifications and variations be included within the
scope of the invention which is defined by the following
claims.
