Note: Descriptions are shown in the official language in which they were submitted.
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EN-04-84 PROCESS FOR CONDITIONING
THE SURFACE OF PLASTIC SU~STRATES PRIOR
TO METAL PLATING
5BACKGROUND OF THE INVENTION
This invention relates to the metal plating of plastics
and, in particular, to enhancing the adhesion of metal plating to
polyetherimide plastics, such as, the plastic substrate of elec-
tronic circuit boards, by conditioning the board prior to metal
plating of the hoard.
.
The metal plating of plastic parts is well-known to be
of considerable commercial importance because the desirable char-
acteristics of both the plastic and the metal are combined to
offer the technical and aesthetic advantages of each. Thus, a
part plated with a bright, metalllc flnish takes advantage of the
economies in cost and~welght afforded by substltuting molded
~;~ plastic parts for metal and,~ additionally, the plated finishes
are~not as susceptible to pitting~and corrosion~because there is
~ ~ 25no galvanic reaction between a plastic substrate and a plated
;~ ; metal. Such metal plating of plastics is; used for a wide range
of applications from decorative plating to radio frequency
shielding~
An lmportant process~ the preparation of electronlc
circuit boards which requires the electroless plating of a con-
ductive metal layer, usually copper, onto the plastic substrate
360f the board. At present, boards are predominately made of epoxy
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resins but there is a constant search for different materials
which will offer improved economies and other benefits. ~ne such
material is a polyetherimide resin ~7hich has a higher tempera-
5 ture stability than epoxy and which can be injection molded withthrough-holes, thus eli~inating the costly drilling step now re-
quired when preparing epoxy boards. Resins of this type are made
by General Electric Company under the trademark ULTEM. For con-
venience the following description will relate specifically to
the process for conditioning polyetherimide resins although it
will be understood that the process may be suitably employed for
lS other resin materials.
These boards vary in design and may have a copper layer
on each surface face of the polyetherimide resin (two-sided
boards) or they can be multi-layer boards which have a plurality
of inter~leaved parallel planar copper and resin layers. In both
type boards through-holes in the boards are metal plated to fa-
cilitate connection between the circuits on the copper~layers.
25 The problems in plating either;the through-holes or other plastic
parts of the board are well-known in the art and a number of me-
thods have been developed to improve the adhesion of the metal
i
~ plating to the plastic substrate.
30~
While different~uses require~differént adhesive
strengths, in general, it has been established that a minimum
peel strength of about 8 lbs. per linear inch as measured by the
35 Jacquet Peel Test is required to prevent a metal coating from
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blistering or peeling from a plastic surface during manufacture
and use. According to ASEP Guidelines, the test measures the
force needed to peel a one-inch wide plate strip 37.5 micrometers
+ 0.51 thick acting at 90 + 5 to the substrate surface. The
measurement is done with an Instron Tensometer programmed to peel
the plate strip from the substrate at the rate of 1.0 + 0.1 inch
0 per minute.
The present invention treats the plastic surface before
plating to enhance the adhesiveness of the metal plating using a
procedure generally known as a swell and etch technique and em-
p~oys solvents which condition or swell the plastic and oxidants
to etch the plastic. U.S. Patent No. 3,758,332 discloses the use
of chemicals such as methyl ethyl ketone, tetrahydrofuran, diox-
20 ane, pyridine, dimethylformamide, and an alcohol mixture compris- ;
~ ~ ~ing methyl ethyl ketone, ethanol and methanol as swellants for
; epoxy resin. The swelled plastlc is then exposed to an etchant
~; ~ for a sufficlent period of time to etch the swelled surface with-
25 out unduly weakening or otherwise adversely affecting the physi-
cal charactéristics of the plastic. Etchants are oxidizing
,
materials dnd are generally aqueous solutions containing materi-
als such as sulfuric acid, phosphoric acid, permanganate ions,
:
;~ 30 Cr+6 ions and~the like. U.S. Patent No. 4,086,128 shows pretreat-
ment of an~epox~ resin with an~organlC solvent comprising al-
;~ cohols, acids~, esters, ketones, nltriles, nitro compounds, and
polyhydric~compounds such as ethylene glycol, glycerine and lt
2-propylene glycol prior to etching with hydrogen peroxide and
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sulfuric acid. U.S. Patent No. 3,865,623 shows im~ersion of
epoxy resin in an organic solvent such as dimethylformamide to
render the epoxy receptive to an acid etch.
To be commercially useful, however, it is important
that the swell and etch process provide a consistent and repro-
ducible resin condi-tioning effect which results in the minimum
peel strength being achieved practically for greater than 99~ of
the parts being conditioned and plated. Without this reproduc-
ibility the costs to the printed circuit board industry would
be staggering and result in highly increased manufacturing
expenses.
SUMMARY OF THE INVENTION
It has now been discovered that the adhesiveness of
metal plating and, in particular, electroless metal plating, to
plastics such as polyetherimide can be enhanced by a swell and
etch process using specially formulated compositions under con-
trolled operating conditions. The method includes first expos-
1ng the plastic for a suitable time to a swellant composition
comprising a solution of a polar material, preferably having a
dielectric constant greater than about 15 and a dipole moment
greater than about 3 Debye Units~D) and an organic solvent such
as a ;carboxylic acid, ketone, hydrocarbon, ether, ester,
alcohol, polyhydric, e.g., glycol and~glycol ethers and esters.
The swelled plastic is then exposed to an etchant comprising Cr+6
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l ions, and, preferably, also including an acid such as H2S04, at
an elevated temperature, e.g., above about 160F., for a suf
ficient period to etch the swelled surface without unduly weak-
5 ening or otherwise adversely affecting the physical characteris-
tics of the plastic.
The resultant etched surface is now in a condition of
improved adhesion for any species such as printing inks, paints,
coatings, and, in particular, for metal coatlngs, which may be
deposited thereon. As will be appreciated by those skilled in
the art, rinsing oE the plastic may be performed at any staga of;
15 the process and removal of residual etchant or etchant-plastic
material may require further treatment if desired.
In general, the process of using the compositions of
20 the invention in manufacturing pr1nted circuit boards and, in
particular, boards containing through-holes, is a sequence of
steps commencing with a laminate or multi-laminate made from,
e.g., filled polyetherimide material. A predesigned series of
25 through-holes may be formed in the board by injection molding or
drilling. The board is then contacted with the swellant composi-
tion of the 1nvention aDd, aEter water rins1ng, the board is
etched at an elevated temperature with an oxidant such as a
30 so1ution of; H2504 and CrO3 and water rinsed.~ Treatment w1th
a reductant to lowar the oxidation state of the chrome resi-
; dues is then typically performed and the board rinsed provid-
ing a board ready for electroless metal plating using conven-
t1onal procedures.~ A preferred step is to pretreat~the board
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1 with a composition such as ENPLATE PC-4459 sold by Enthone,
Incorporated to remove hydrocarbon soils and promote the catalyst
and then to contact the board with a composition such as ENPLATE
5 PC-236 to eliminate drag-in. The board is now immersed in a
- catalyst, such as a tin-palladium solution, which conditions the
surfaces of the resin for electroless copper plating. ENPLATE
Activator 444 is exemplary of this type catalyst. The board i5
then immersed in a post activator such as ENPLATE PA-493 to
,
activate the catalyst by removing the excess tin and freeing the
metal palladium ions on the board, rinsed a-nd immersed in an '
15 electroless copper plating solution for a period of tlme suffi-
cient to plate copper to the desired thickness on the surfaces
and to plate the surfaces of the holes to form through-hole con-
nections between the laminate surfaces. ENPLATE CU-700 and other
:
20 similar plating compositions may be employed. The boards may
then be immersed in diIute sulfuric acid followed~by electro-
plating usl;ng conventional technlques if a thicker coating is
desired. Rinsing of the board between~steps may be convention-
ally employed as lS well~-k~nown ln~the art.
DETAILED DESCRIPTION OF THE INVENTION
It is preferred that~the polyetherimide resin contain
~ 30
other materials,~prefèrably ~ ller or reinforcing material,''~
e. g., glass flbers.~ Other~fil~lers~lnclude paper~, synthetic
;fibers, carbon black, alumina powders, silica powders, wax, etc.
Pigments, mold release~agents, and other conventional;additives
may also be employed in the~resin~eor speciflc purposes.
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1 It has been found that a swellant pretreatment is nec-
essary to enable the etching process to roughen, or to otherwise
condition, the surface and to provide the enhanced adhesion of
5 the invention~ The swellant composition comprises a solution of
a polar material and an organic solvent, which composition is
essential to provide a swelled plastic capable of being etched
and plated with the desired adhesiveness at a high degree of
reproducability as will be demonstrated in the examples.
The polar materials useful herein are those materials '
having dielectric constants greater than about 15, preferably
lS greater than about 20 r most preferably greater than about 30
and dipole moments greater than about 3 ~ and preferably greater
than about 3.5 D. Exemplary of these materials are dimethyl-
formamide, dimethylacetamider dimethyl sulfoxide, tetrahydrothio-
phene dioxide, N-methylpyrrolidone, hexamethylphosphoric tria-
mide, tetramethylurea, and acetonitrile. Other materials of
similar structures mdy also be utilized as the polar materials
25 hereof. Thus, other sulfoxides which are liquid at or near room
temperature may be so employed, e~g., those having the formula
O ~ ':
~ R-S-R
30 ~
whérein R and;~Rl may each comprise alkyl groups of from 1 to 4
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carbon atoms. ~Similarly, other sulfones may be so used, e.g.,
those of the formula
2-S2-R3
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1 wherein R2 and R3 may be lower alkyl (1 to 4 carbon atoms) or may
be linked to form a tetrahydrothiophene ring.
.
The second cornponent of the swellant composition may be
any suitable organic solvent selected from the carboxylic acids,
ketones, hydrocarbons, ethers, esters, alcohols, polyhydrics,
e.g., glycols and polyglycols, lncluding ethers and esters
thereof. In general, tbe organlc solvent will typlcally contaln
less than about 10 carbon atoms, with the proviso that the polar;
material be soluble in the organic solvent to produce the desired
composition. Exemplary solvents include acetic acid~n-pentane,
ethyl acetate, ethanol, methanol, and the like.
The preferred organic solvents because of their dem-
onstrated effectiveness are the polyhydrics such as the glycols,
and the ethers and esters thereof. These solvents may be repre-
sented by the formula RlO(AO)nR2 wherein Rl and R2 are inde-
pendently selected from the group consisting of hydrogen atoms,
aryl groups and alkyl and acyl groups of 1-4~carbon atoms, A is a
straight or branched chain C2 to C4 alkylene group, and n is an
integer of l to 4. Examples of these solvents include ethylene
glycol, ethylene ~lycol monomethyl ether, propylene glycol,
; propylene glycol monomethyl ether, ethylene glycol acetyl ester,
~;30 etc~
It will be appreciated by those skilled in the art
.
that the polar material and organic solvent swellant mixture
; 35 may comprise one or more compounds of each ingredient. The swel-
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l lant composition may also be used in the form-of an aqueous solu-
tion although it has been found that the water is desirably limi-
ted, in weight percent, to less than about 20%, and preferably
less than lO~ and more preferably less than 5~ and even 1%. In
general, water decreases the swellant effect of the composition
and the adhesiveness of the subsequent metal plating.
-lO Two particularly preferred polar materials are di-
methylsulfoxide (DMSO) and N-methylpyrrolidone and the preferred
.
solvents are the polyhydrics represented by the formula
RlO(AO)nR2. In general, compositions of the two components con-
15 tain, by volume, about 25% to 90% DMSO, preferably 50~ to 85~ and
most preEerably 70% to 80%, with the balance being the polyhydric
component. For compositions containing N-methylpyrrolidone a
ran~e of 15% to 70~, preferably 20~ to 50% and most preferably
20% to 30~ may be employed. A preferred solvent because of its
demonstrated effectiveness is propylene glycol monomethyl ether
wherein Rl is H,~A is C3, n~is l and R2 is methyl.
,
Suitable addltives can be employed in the swellant
composition for specific purposes such as wetting agents to
enhance the capability of spreading the c~mposition on the resin
surface.
To pract1ce the method of the lnvention the plastic -
substrate is contacted with the swellant composition at an ele-
vated temperature for a sufficient time to render the surface re-
35 ceptive to the etching process. Contacting procedures~may vary
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1 widely, e.g., 1 to 60 minutes at temperatures up to about 180F.,
and pre~erably 2 to 20 minutes at 140 to 160F. Satisfactory
results for glass filled polyetherimide resin are provided by
5 immersing the part in the composition for about 5 minutes at
150F. The time and temperature will r in general, vary inversely
as will be appreciated by those skilled in the art~ Other means
such as spraying, may be used for treating the plastic part~
The treated plastic part is then ready for oxidative
etching using a chrome containing solution and basically com-
prises contacting the treated plastic part with the etchant at an
elevated temperature for a sufflcient time to promote adhesion to
the surface. It is preferred to then rinse the etched plastic
part to remove excess solution and to remove any chrome residues
by neutralization with a ~aterial such as sodium bisulfite or
chemical reduction using reductants such as hydrazine and oxalic
:
acid.
The aqueous chrome containing etchant solutions are
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well-known in the art and preEerably comprise Cr+6 ions, e.g.,
Cr207= and~Cr03. An acid such as H2SO4 is preferably included
in the etchant solution and other acids like H3PO4 may also be
employed. A preferred etchant composition is an aqueous solution
of H25O4~and CrO3. Suitablé~add~ltlves may also be used such as
surfactants,~e.g., perfluorinated sulfonatesj to insure uniform
etching of the conditioned resin surface. The concentration of
the etchant solution may vary widely with the chromium component
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1 added as CrO3 being, by weight, about 100 grams/liter (g/l) to
saturation, preferably abou-t 200 to 600 g/l, and most preferably
about 300 to 500 g/l. The acid component is about 100 to 500
5 g/l, and more preferably about 200 to 400 g/l. A preferred
composition contains about 400-450 g/l CrO3, e.g., 420 g/l, and
?50-350 g/l H2SO4, e.g., 300 g/l.
An important feature of the invention is the tempera-
ture of the etching process. Conventional etching procedures
utilize a temperature which may vary over a broad spectrum from
as low as room temperature to the boiling point of the etching
15 solution. It has been found for the conditioned polyetherimide
resins however, that temperatures above about 160F. and prefer-
ably above about 170F. are necessary and that unexpected ad-
hesive properties to the metal coating are provided. While the
20etching time will vary depending on the concentration of the
etching solution and~temperature of the etching process as will
be appreciated by those skilled in the art, for the preferred
25 composition of 420 g/l CrO3 and 300 g/l H2SO4, an etcbing tiwe of
about 5-lS minutes at 170F. provldes excellent results. In
general, the etching time may be up to about 60 minutes-but is
typically less than 30 minutes.
~ ~
It is to be understood and stressed for both the swel-
~;lant and etchant procedures,~that the above concentrations,
;temperatures and time paraweters are all Interdependent and that
variations in temperature will produce variations in the other
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1 parameters ~hereby optimum results will be attained. In this
regard, the various parameters and their interdependency are well
known in the art and their interaction between one another is
5 also ~ell known or can be easily ascertained experimentally by
one skilled in the art.
An optional step of removing traces of deposits on the
10 etched resin may now be performed. Rinsing will remove deposits
but a preferred procedure is to contact the resin with a suitable
reducing agen~ for chromium ions such as NaHSO3, NaOH, and the
like. Immersion of the etched resin in a solution of 150 9/1
1 NaHSO3 for 5 minutes has produced satisfactory results. Typi-
cally, exposurc to the reducing agent ranges from 30 seconds to
10 minutes at a temperature ranging from room temperature to
160F.
If a smoother metal plated surface is desired, another
optional step is to treat the etched resin to dissolve part of
the filler material. Materials such as hydrogen ~luoride and
25 ammonium bifluoride may be suitably employed. ACTANE 70~sold
by Enthone,Incorporated has proven very satisfactory for this
purpose.
~The etcheù resin is now prepared for metal plating by
known means to~render the surface catalytic. Among them are
cleaniny, applying catalyst promoters, sensitl2ing using an aque-
,
ous tin chloride solution and then activating by means of pallad-
35 ium chloride. On the other hand, unltary baths may be employed
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for such purposes, such as the dispersions of colloidal
palladium and tin ions described in Shipley, U.S. Pat. No.
3,011,920 or the soluble complexes of noble metals, stannous
ion and anions as described in Zeblisky, U.S. Patent No.
3,672,938. The surface may now be plated with a film of
rnetal by electroless plating. Activating and plating com-
positions and methods for copper metal electroless deposition
are described in U.S. Patent Nos. 2,874,072; 3,011,920;
3,075,855; 3,095,309; 3,672,938; and 3,736,156. Other methods
of deposition may also be used such as vacuum vapcr deposition,
electrolytic plating or a combination of electroless plating
and electrolytic plating.
The present invention will now be described in detail
by reference to the following examples.
~XAMPLE I
The following example illustrates the process of using
the swellant and etchant compositions to enhance the adhesiveness
of electroless plated copper to glass-filled polyetherimide
resin.
An injection molded plaque of ULTEM glass-filled
polyetherirnide resin;was metallized using the following proce-
dure:
(a) immerse the plaque for 5 minutes at 150F. with
mild agitation in a solution comprising, by volume, 75%
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DMSO and 25% propylene glycol monomethyl ether (PGMME~;
(b) rinse for.5 minutes in running water;
- (c) etch for 10 minutes at 170F. with mild agitation
in a solution comprising 420 g/l CrO3 and 300 g/l H2SO4;
(d) rinse for 5 minutes in runniny water;
(e) neutralize ~or 5 minutes at ro m temperature in a
solution comprising 150 g/l Na~SO3;
(f) rinse in running water for 5 mlnutes;
(g) immerse in conditioning cleaner ENPLATE
: PC-4459 for 5 minutes at 150F.;
.
(h) rinse in running water for 5 minutes;
ti) immerse in ENPLATE PC-236 for 2 minutes at room
temperature;
(j) immerse in one-step palladium catalyst ENPLATE
Activator 444 for 5 minute.s at 75F.;
(k) rinse with runnlng water; :
: (1) immerse in post activator solution ENPLATE
:: ,
PA-~93 for 5 minutes at room:tempe~rature;
(m) rinse ln runnin;g water for:5 minutes;
;(n) plate in an~electrolesS copper solution ENPLATE
CU-700 for:30 minutes~at 118F.; : ;
:(o)~rinse with-running water;
(p~ mmerse~in ~g~sulf~urlc~aci~d~for l~minute;~
: (q)~:e~leotroplate in an:ac~i~d~::copper~electrolyte:~to about
~: ~ : 0. 001 inch;~
5 ~ ( r l r ~ n~e w i th ~ ru ~ A; A g~ wa t er 'o r 2 r l nu t os ~a nd
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1 (s) air dry.
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The metal layer was tested for adhesion using the
Jac~uet Peel Test and an ultimate peel strength of greater than
s
11 lbs/in was obtained. The peel strength generally increases
with time to a maximum (ultimate) value and the ultimate peel
strength as used herein represents the adhesion value obtained
10 for the metal layer greater than 3 days after metallization.
EXAMPLE II -
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EXAMPLE I was repeated except that the etched board
15 was treated with ACTANE 70 prior to step (g) and a smooth ad-
hesive coating was obtained.
.
EXAMPLE III
The procedure of EXAMPLE I was repeated except that a
swellant composition containing, by volume, 25% N-methylpyrroli-
done and 75~ PGMME was used ln step (~a) instead~of the DMSO
composition and imme~rsed~for 10 minutes lnstead of ~5. An ultim-
ate peel~strength of greater than 10 lbs/in was obtained.
EXAMPLE IV
30 ~ Step~(a)~of EXAMPLE~ was repe~ated uslng IOO~ DUSO and
the plaque's surface~was attacked and unsuitable for further
process~l~ng.~ Slm~ilar results were obtained~whe~n 100% N-methyl-;~
pyrrolidone was used~ in Step (a). ~ ;
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1 tions of the several features described herein may be made
without departing from the spirit and scope of ~he invention. It
is therefore apparent that the foregoing description is by way of
illustration of the invention rather than limitation of the
inventon.
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