U.S. patent application number 11/479692 was filed with the patent office on 2008-01-03 for process for increasing the adhesion of a metal surface to a polymer.
Invention is credited to Raymond A. Letize.
Application Number | 20080000552 11/479692 |
Document ID | / |
Family ID | 38875348 |
Filed Date | 2008-01-03 |
United States Patent
Application |
20080000552 |
Kind Code |
A1 |
Letize; Raymond A. |
January 3, 2008 |
Process for increasing the adhesion of a metal surface to a
polymer
Abstract
A process for increasing the adhesion between a copper or copper
alloy surface and a polymeric material is disclosed. The process
comprises treating the copper or copper alloy surface with an
aqueous solution of cupric ions at a pH from 2.8 to 4.2 to form an
oxide conversion coating on the copper or copper alloy surface. The
so treated surface can then be bonded to the polymeric
material.
Inventors: |
Letize; Raymond A.; (West
Haven, CT) |
Correspondence
Address: |
John L. Cordani;Carmody & Torrance LLP
50 Leavenworth Street, P.O. Box 1110
Waterbury
CT
06721-1110
US
|
Family ID: |
38875348 |
Appl. No.: |
11/479692 |
Filed: |
June 30, 2006 |
Current U.S.
Class: |
148/261 ;
148/269 |
Current CPC
Class: |
H05K 3/385 20130101;
C23C 22/52 20130101; H05K 2203/0789 20130101 |
Class at
Publication: |
148/261 ;
148/269 |
International
Class: |
C23C 22/52 20060101
C23C022/52 |
Claims
1. A process for increasing the adhesion of a copper or copper
alloy surface to a material comprising polymers, said process
comprising: a) contacting the copper or copper alloy surface with a
composition comprising: 1. water; 2. a source of cupric ions; at a
pH from 2.8 to 4.2; such that a conversion coating is formed on the
copper or copper alloy surface.
2. A process according to claim 1, wherein the composition is
substantially free of azole compounds.
3. A process according to claim 1 wherein the composition also
comprises a buffer.
4. A process according to claim 4 wherein the buffer is selected
from the group consisting of potassium or sodium carbonate, sodium
citrate and sodium hydrogen phosphate, and potassium hydrogen
phthalate and sodium hydroxide.
5. A process according to claim 1 wherein the source of cupric ions
is selected from the group consisting of cupric sulfate, cupric
chloride, cupric acetate and mixtures of the foregoing.
6. A process according to claim 1 wherein the pH is from 3 to
4.
7. A process according to claim 3, wherein the composition is
substantially free of azole compounds.
8. A process according to claim 7 wherein the buffer is selected
from the group consisting of potassium or sodium carbonate, sodium
citrate and sodium hydrogen phosphate, and potassium hydrogen
phthalate and sodium hydroxide.
9. A process according to claim 8 wherein the source of cupric ion
source selected from the group consisting of cupric sulfate, cupric
chloride, cupric acetate, and mixtures of the foregoing.
10. A process according to claim 9 wherein the pH is from 3 to 4.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a composition and process
for improving the adhesion between a metal and polymer, especially
between copper foil and a polymeric substrate in the fabrication of
printed circuit boards. The composition and process are also useful
in increasing the adhesion between lead frames or chip carriers and
encapsulating resins.
[0002] Printed circuit boards are typically constructed with a
polymeric substrate such as a phenolic, epoxy, polyimide,
polyester, or other resin, upon which is bonded a copper foil which
is etched to provide a conductor in the desired configuration.
Copper, like other pure metals, generally exhibits poor adhesion
characteristics for bonding to polymers, and intermediate
conversion coatings are frequently helpful. Familiar examples of
this practice are the various pre-paint treatments, such as
phosphate on steel, chromate on zinc or aluminum, and anodic oxide
on aluminum.
[0003] For printed circuits, it is typical to employ copper oxide
as adhesion improving coating, however, the known methods for
achieving the copper oxide coatings involve the use of highly
corrosive chemicals which become even more dangerous at the
elevated temperatures typically employed for processing. Moreover,
the improvement in adhesion achieved through the use of known
technology has been erratic. In fact, adhesion after treatment is
sometimes no better than for clean copper.
[0004] 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 failure in service. An oxide coating serves as a
diffusion barrier to prevent these reactions. Accordingly, there is
a present need for improvements in achieving increased bonding
strength through the use of copper oxide coatings.
[0005] In 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 electrolytically to provide a specific surface structure as
disclosed in U.S. Pat. Nos. 3,292,109; 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.
[0006] Multilayer printed circuit boards are assemblies of several
two-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 this assembly, the copper foil of the two-sided boards is
imaged and etched to from the inner layer circuits. The opposite
side of the conductor patterns must then be treated for adhesion to
the layer of polymer bonding one board to another. Because the
etched conductors have no continuity for electrolysis, it is
necessary to treat them chemically.
[0007] Copper oxide is the most useful chemical conversion coating
for copper adhesion and has been used since the early days of
printed circuit technology. The basic patent in this field appears
to be U.S. Pat. No. 2,364,993, which disclosed 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. Pat. Nos. 2,460,896, 2,460,898, and U.S. Pat.
No. 2,481,854, which have been granted to the same assignee. These
four related patents teach the use of a treatment solution
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 saturated 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 degree F., while
the higher range will do the same in one minute at 250 degree F. An
intermediate composition containing 150 grams per liter of chlorite
and 150 grams per liter of caustic is disclosed in U.S. Pat. No.
2,364,993 to blacken the copper surface in five minutes at 200
degree F.
[0008] While these patents appear to be directed toward decorative
applications, as well as non-reflective coatings 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 exemplary patents applying this technology
to printed circuit adhesion are: U.S. Pat. Nos. 2,955,974;
3,177,103; 3,198,672; 3,240,662; 3,374,129; and U.S. Pat. No.
3,481,777. Other known means for providing copper oxide coatings
include oxidation with alkaline permanganate as disclosed in U.S.
Pat. No. 3,544,389, hydrogen peroxide as disclosed in U.S. Pat. No.
3,434,889, and even air at elevated temperature as disclosed in
U.S. Pat. No. 3,677,828.
[0009] 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 the 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.
[0010] A thick coating of oxide tends to be fuzzy and velvety,
making it mechanically weak. Black powder can be wiped off with a
finger. During the multilayer processing operation at a high
temperature and pressure, the fibers can be crushed and partially
encapsulated by the flowing polymer. 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 manifestation of encapsulated oxide
fibers.
[0011] Further, a thick black oxide may not be homogeneous. Whereas
the outer surface will be cupric oxide, 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 during the high temperature pressing operation by combining
with residual oxygen, water or various decomposition 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.
[0012] In view of these difficulties, it would be desirable to
provide a thermally-stable oxide coating on copper to enhance its
adhesion characteristics to polymers. Additionally, 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 employed 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 coatings on copper which could be
operated at concentrations and temperatures below about 140 degree
F.
SUMMARY OF THE INVENTION
[0013] Thus a new process is proposed for creating an oxide coating
on the surface of copper or a copper alloy, said process
comprising:
[0014] (a) contacting the copper or copper alloy surface with a
composition comprising: [0015] 1. water; and [0016] 2. a source of
cupric ions; [0017] at a pH from about 2.8 to 4.2;
[0018] The copper or copper alloy surface can then be effectively
bonded to polymeric substrates or coatings with greater adhesion
and stability.
DETAILED DESCRIPTION OF THE INVENTION
[0019] The present invention relates to a unique composition and
process for the formation of an oxide conversion coating on a
copper or copper alloy surface which provides for improved adhesion
between metal and polymeric resins as in the manufacture of
multilayer printed circuit boards or lead frame components. The
proposed process comprises:
[0020] (a) contacting the surface of copper or copper alloy with a
composition comprising: [0021] 1. water; and [0022] 2. a source of
cupric ions; [0023] at a pH from 2.8 to 4.2; such that a conversion
coating is formed on the copper or copper alloy surface.
Preferably, the composition of this invention does not contain any
azole compounds or other corrosion inhibitors. The composition is
also preferably substantially free of hydrogen peroxide. The
conversion coating formed is resistant to scratching and to
discoloration.
[0024] The source of cupric ions can be any soluble source of
cupric ions such as cupric salts like cupric sulfate, cupric
chloride, cupric acetate and mixtures of the foregoing.
[0025] The pH of the composition is important. The pH should be
between 2.8 and 4.2, preferably between 3.0 and 4.0 and most
preferably between 3.2 and 3.8. Adjustment of the pH can be made
with any acid or base but is preferably made with an acid
corresponding to the anions of the cupric source, if acid is used,
or sodium hydroxide if base is used. It is especially surprising
and unexpected that the copper oxide conversion coating forms in a
stable way at an acidic pH.
[0026] Since pH is important, it is preferable for the composition
to also comprise a buffer system to control and maintain the pH
within the recommended range. Any buffer system that buffers the
solution within the prescribed pH range is acceptable. The inventor
has found the following buffer systems to be useful in this regard:
sodium or potassium carbonate, sodium citrate and sodium hydrogen
phosphate, and/or potassium hydrogen phthalate and sodium
hydroxide.
[0027] The copper or copper alloy surfaces can be contacted with
the composition of this invention by immersion, spray or flood. The
temperature of the composition should be between 120.degree. F. and
150.degree. F., preferably between 130.degree. F. and 140.degree.
F. The contact time varies with the method of contact and the
temperature, but can range from 15 seconds to 5 minutes.
[0028] Before the copper or copper alloy is treated with the
composition of this invention, the surface is preferably cleaned
and microetched. The cleaner can be any standard commercial acid or
alkaline cleaner which is designed to clean copper surfaces.
Suitable commercial cleaners include Core Clean AT and Omniclean CI
available from MacDermid, Incorporated. After the surface is
cleaned, it is also preferably microetched to roughen and activate
the surface. Suitable micro etches include peroxide/sulfuric or
cupric based microetches such as Microetch G-5, Microetch G-4 and
Multiprep 100 available from MacDermid, Incorporated. The important
point is for the surface to be clean and microroughened before
entering the composition of this invention.
BRIEF DESCRIPTION OF THE DRAWING
[0029] FIG. 1 is an electron microscope photograph showing the
surface of an oxide coating prepared by the process of this
invention.
[0030] The invention is further described in a non-limiting manner
in the following example:
EXAMPLE 1
[0031] Copper foil was processed as follows: [0032] 1. Clean in
Omniclean CI alkaline cleaner at 140.degree. F. for 5 minutes.
[0033] 2. Rinse. [0034] 3. Microetch in Multiprep 100 microetchant
at 100.degree. F. for 1.0 minutes. [0035] 4. Rinse. [0036] 5.
Immerse in the following composition of this invention at
140.degree. F. for 1.5 minutes: [0037] 68 g/l copper sulfate
pentahydrate [0038] 5 g/l potassium carbonate [0039] pH=3.85 [0040]
6. Rinse. [0041] 7. Dry.
[0042] The copper foil was then laminated to an epoxy prepreg using
heat and pressure and its adhesion properties were checked with the
following results: [0043] Initial Adhesion -7.0 lb/in. [0044]
Adhesion after 3-10 sec. solder shocks at 550.degree. F.-6.5 lb/in.
[0045] Adhesion after 6-10 sec. solder shocks at 550.degree. F.-4.5
lb/in.
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