Note: Descriptions are shown in the official language in which they were submitted.
7~
Case ~o. U-11,042
~IPRaVED E~T.FXS C~PPER ~
Back~round of the Invention
The present invention broadly relates to an improved
ccmposition and process for prcducing electr~less or catalytic
copper deposits on substrates, and par~icularly, nonconductive
substrates such as various plastics which have ~een subjected
to various pretreatments to render the nonconductive substrate
receptive to the copper deposit. Electroless ccpper plating
baths of the types heretofore k~own conventionally comprise an
agueous alkaline solution containing oopper ions, a complexing
agent for the copper ions to prevent precipitation thereof, a
reducing agent for reducing the oopper ions to the m~tallic
state, a pH regulator, a stabilizing agent, a rate co~troller
and optionally, but preferably, wetting agents to imprcve
coverage and distribution of the oopper deposit.
The increased use of decorative plated trLm
components on automobiles has provided impetus for further
research and development efforts to improve prior art
electroless copper plating solutions tu reduce their cost and
simplify their maintenan oe and oontr~l, to increase ~heir
stability and t~ provide uniform, ~aherent copper deposits at
commercially acceptable deposition rates. The present
inNention achieves the foregoing ~nefits and cbjectives by
employing a controlled oo~binatiGn of ccmplexing ag~nts
pro~id~ng a s~nergistic effect whereby a substantial reduction
, ~
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in the quantity of oomplexing agent required is achieved
providing for significant savings in the cost of make-up and
replenishment of such electroless copper baths. Additionally,
the present invention enables uperation of the bath under
commercial conditions at a lower pH level thereby reducing the
quantity of the alkaline pH control agent necessary which in
turn substantially reduces the degradation reaction of the
alkaline agent such as caustic and the reducing agent such as
formaldehyde providin~ for a significant reduction in the
consumption of reducing agent. Still a further advantage of
the bath of the present invention resides in the use of higher
than normal rocm temperatures enabling the bath to be operated
at a temperature up to about 150F to achieve t~e desired bath
activity for aut~catalytically dep~siting copper. Such higher
te~perature enables the bath to be ccoled to akout r
temperature during periods of nonuse such as during shutdcwn
over weekends at which temperature the b~th is of comp~ratively
lcw activity enhancing its stability and inhibiting
autocatalytic deocmposition during periods of nsnuse. At such
higher operating temperatures, evaporation of the bath normally
also occurs providing volumetric spaoe in the plating tank for
the additian of replenishing chemicals as is ne oe ssary during
normal operation due to oonsumption and drag-out of the
oonstituents. In oontrast, electroless copper plating baths of
the types heretofore known adap~ed tD operate at rocm
temperature have frequently req~LL~ed the removal of some of t~e
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~2~7ll~
operating solution to provide volumetric space for
replenishment which not only wastes valuable solution but has
also presented a waste disposal problem. The Lmproved
ccmposition of the present invention has been found to also
enable a substantial reduction, and in some instances a
co~plete elimination of the need of rate controllers such as
cyanide and iodide ccnpounds conventionally employed in prior
art electroless copper baths thereby further simplifying
control and replenishment of the bath and a cost reduction in
the materials required.
Summary of the Invention
The benefits and advantages of the present invention
are achieved in accordance with the conposition aspects
thereof, ~y an electroless copper plating bath oo~prising an
aqueous alkalLne solution containing c~pper ions in an amcunt
sufficient to autocatalytically deposit ocpper which usually
ranges in a concentration of about O.5 to about 30 grams per
liter (g/l), a reducing agent present in an ~cunt sufficient
to effect a reduction of the copper ions to the metallic state
of which ~ormaldehyde is preferred and can generally be pr~sent
in an amount of abo~t 0.1 up to about 40 g/l~ a complex~ng
agent presen~ in an amount to oomplex the copper io~s present
in the bath and usually present in a mol ratio of oorplexing
a~ent to copper ions of from about 1:1 up to about 5:1. The
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~87C~
co~plexlng agent oomprises a nixture of an amine ccmpound
selected from the group oonsisting of ethylenediamune-tetra-
a oe tic acid tEDTA) and N,N,N',N'-tetrakis (2-hydroxypropyl~
ethylenediamine (THPEDA3 and a hydroxy acid selected from the
group consisting of glucanic acid and gluooheptonoic acid as
well as the bath soluble and cGmpatible salts and mixtures
thereof wherein the hydroxy acid is present in a mol ratio
relative to the mols of amune compound of about 0.1 to about
3:1. The bath can further contain a stabilizing agent,
preferably a heterocyclic organic sulfide compound present in
an a~ount up to about 2.5 g/l, hydroxyl ions to provide a pH of
from about 9 up to about 14, and optionally, a rate controller
such as cyanide, iodide and derivatives thereof present in an i
amount up to about 1 g/l and a ~ath soluble and compatible
wetting agent which may be present in an amount up to about 10
g/l.
In accor& n oe with the process asp~cts o~ the present
invention, a substrate t~ be oopper plated is immersed in the
electroless copper solution maintained at a temperature of
about rcom temperature (70F3 u~ to about 150F under agitation
for a period of ~ime sufficient tD eff~ct a unifonm, dense and
adherent ccpper deposit over the surfa oe s thereof to the
desired thickness. If desired, the substrate incorpDra~ing the
electroless ccpper deposit thereover can be subjected ~o
further oonventional electroplating operations to apply one or
37~
a plurality of overlying electrodeposits thereon to achieve the
d~esired physical characteristics and appearance.
Additional benefits and advantages of the present
invention will beccme apparont upon a reading of the
Desoription of the Preferred ~m~odi~ents taken in c~njunction
with the accompanying examples.
Description of the Preferred Embodiments
In accordance with the cc~position aspects of the
present invention, the aqueous alkaline electroless or
autocatalytic copper plating solution contains as its essential
constituents, copper ions in an amount sufficient to deposit
metallic copper on a substrate, a reducing agent for reducing
the copper ions to the metallic state, a oomplexing agent
present in an amDunt sufficient to maintain the copper ions in
solution in the alkaline ~edium, hydroxyl ions to provide an
alkaline pH and optionally, s~abiliæing agents, rate
cGntxollers and wetting agents present in amounts sufficient to
stabili~e the solution and provide appropriate platin~ actiYity
and good wetting and unifonmity of the copper deposit produced.
The oopper ions can be introdu~ed into the aqueous
alkaline solution in the form of any kath oluble and
compatible oopper salt in which t~e associated anion does not
have any deleterious effects on the plating characteristics of
the process. Typically, cupric chloride dihydrate and o~pper
sulfate pentahydrate can be used as t~e source of the copper
~ 87~S
ions. The conoe ntration of the copper ions can generally range
from as lcw as about 0.5 to as high as about 30 g/l with
concentrations of fram about 1 to about 5 g/l being preferred.
Higher con oe ntrations of oopper ions are required when the bath
is operated at or about room temperature to prGvide for a
satisfactory rate of copper deposition whereas lower
concentrations such as about 0.5 g/l can be employed when the
temperature of the bath is at an elevated te~perature such as
frcm about 140F to about 150F.
In additio~ to the copper ions, ~he aqueous alkaline
solution contains a reducing agent present in an a~cunt
sufficient to redu oe the cupric ions to the metallic state for
deposition on the surfa oe of the substrate being plated. For 1
this purpose, formaldehyde oomprises a preferred material li
although paraformaldehyde can also be satisfactorily employed.
In addition to the foregoin3, hypophosphite and hydrazine as
well as derivatives thereof have heretofore been us~d or
proposed for use as reducing agents but are generally not as
effective as fol~ ldehyde it æ lf. The oonoentration of the
reducing agent will vary in relationship to the copper ion
o~ncentration present in the bath and may range fram as low as
akout 0.1 up to about 40 g/l calculated on a weight equivalent '
basis as formaldehyde with amDunts of about 1 to about 5 g/l l;
being preferred.
In addition to the ccpper ions and xe~ucLng agent,
the aqueous aIkaline solution further oontaLns a- controlled
7~
mixture of an organic complexing agent to naintain the ccpper
ions in solution which otherwise would precipitate as copper
hydroxide in the alkaline medium. The total complexing agent
is controlled in concentration to provide at least about 1 mol
of complex m g agent per mDl of copper ions present with mol
ratios of ccmplexing agent to copper ions as high as about 5:1
being feasible. Preferably the ccmple,Ying ~gent is controlled
at a mol ratio of about 1.3:1 to a~out 3.5:1 with a mol ratio
of about 1.7:1 being typical.
It has been discovered that a synergistic effect in
the complexing characteristics and in the activity and
stability of the bath are achieved when the complexing agent
comprises a contrDlled muxture of an amine oompound selected
from the group consisting of ethylenediaminetetraacetic acid
(EDTA) and N,N,N',N'-tetrakis (2-hydroxypropyl) ethylenediamine
(THPEDA) and an organic hydroxy acid selected from the group
consisting of gluconic acid and glucoheptonDic acid and
mixtures thereof in ad~ition to the bath sDluble and ccmpatible
salts of the foregDing. Ihe mDl ratio of the hydroxy acid to
EDrA may range from 0.1 to ab~ut 3:1. The particular mol ratio
will depend somewhat on the type of amune compound e~ployed in
combination with the hydroxy acid ccmpound. For ~xample, wh~n
IHPEDA co~prises the amine compound, the gluconi~ acid and/or
gluooheptonoic acid can be present in a mDl ratio relative to
the amone oompo~Dnd within a range of about 0.11:1 up to about
2.45:1, with mol ratios of abcut 0.2:1 to abYut 1~1 being
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124B70S
preferred and with a mol ratio of about 0.25:1 being optimum.
On the other hand, when the amLne compound comprises EDTA, the
gluconic acid and/or glucoheptonoic acid can be present m a
mol ratio of about 0.25:1 up to about 3:1, preferably at a mol
ratio of about 0.6:1 ~o about 2:1 with a mol ratio of about
1.3:1 being optim~. The controlled mixture of co~plexing
agents can conveniently be introduced in the bath in the form
of the alkali metal neutralized salts thereof which avoids any
significant acidification of the bath and a reduction in its pH
in comparison to that which occurs when the acid form of the
complexing agents are added. The sodium salts are particularly
satisfactory for this purpose.
By employing the specific controlled muxture of the
two complexing agents as hereinabove set forth in the
proportions indicated, a substantial reduction in the quantity
of complexing agent can be achieved and wherein the tw~ types
of complexing agents are individually present in an amcunt
insufficient ky itself to complex all of the copper ions
present.
The electroless copper solution is on the aIkaline
side and contains hydm xyl ions in an amount to provide a pH
ranging from about 9 up to about 14, with a pH range of about
10.5 ~o akout 12.5 being preferredO Typically, a pH of abDut
11.5 can be satisfactorily used which provides for a
commercially satisfacb~ry copper deposition rate while at the
same time requiring a lo~er hydroxyl ion oonoentration which
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redu oe s the tendency of reaction and degradation of the
formaldehyde reducin,g agent reguiiring lower replenis~ment
thereiof and further economy in the operation of the process.
m e pH of ~he copper electroless solution can be naintained
within the foregoing ranige by the addition of any alkali metal
hydroxide of which sodium hydroxide itself constitutes a
preferred material.
In addition to the foregoing constituents, it is also
oontemplated that the electroless copper bath can contain a
stabilizing agent of any of the various types known in the art
to inhibit the spontaneous oopper deposition on catalytic
particles formed in the bath during the plating operation which
rapidly depletes the solution of copper ions. A variety of
oompounds hhve heretofore been used or proposed for this
purpose of which 2-mercaptobenzothiiazole has been in widespread
use. Alternative stabilizing agents heretofore used or
proposed for use include 2.5-dimercapto-1,3,4-thiodizole,
8-nercaptopurine, o-phenanthroline~ 1-phenyl-5-mercapt~tetra-
zole, 2,2-dipyridyl, 2-(2-pyridyl)-benzimidazole,
benzothiazole-thioetherpolyethyleneglyool, thiazoles,
isothiazoles, thiozines, ben~otriazole, diazole, Imidazole,
guanidine, pyrimidine,2,2'-biquinoline, 2,9~dimethylphenanthro-
line and 4,7_diphenyl-1,10-phenanthrolLne. Such stabilizing
agents can optionally and preferably be employed in an ~maunt
up to abcut 2.5 g/l with ooncentrations of about O.0001 to
about 0.5 g/l being u~ually preferred.
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~ hile the novel ccmbination of complexing agents has
been found to obviate the necessity of incorporating rate
controllers in the bath of the types conventionally required,
it is oontemplated, under certain conditions, that rate
controlling agents such as cyanide, iodide, or derivative
compounds thereof can be incorporated in the electroless oopper
plating bath in 3m~unts usuall~ up to about 1 g/l. Such rate
controllers accomnodate the stabilizers by reta~ding the
plating rate of the bath. It has also been found that t~e use
of such rate controllers in relatively ~small prDportions
improves the lustre and ductility of the copper deposit. When
employed, such rate controllers can be u~ed in amounts up to
about 1 g/l with amounts of from about 1 pQm up to a~bout 200
ppm being m~re typical.
Optionally, and preferably, the electroless copper
plating bath further contains small controlled a~ou~ts of bath
soluble and compatible wetting agents to enhance uniformity of
ooverage of the copper plate on the substrate. Wetting agents
of the general types which can be satisfactorily used are those
oonventionally emplcyed in electroless ccpQer plat mg baths
amD~3 which are PlurDnic P85, available fro~ B~SF CDrporation,
cc~prisLng a ~Dnionic block copol~mer of ethylene oxide and
pro~ylene oxi~e; Gafac RE 610, available f~ GAF Corporation~
an anionic phosphate ester, or the like are typical~ ~kien
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emplo,yed, such ~etting agents can be incorporated in amDunts up
to about 10 g/l while amounts of abcut 0.1 to about 3 g/l are
re ~ypical and preferred.
In accordance with the proc~ss aspects of the present
invention, an aqueous alkaline electrolyte of the com~osition
as hereinbefore described is formed and is heated to an
operating temperature of from about room te~QeratUre l60F) up
to about 160F, and preferably from about 80 to a~aut 150F.
S~bstrates to be copper plated are subjected to appropriate
clean mg treatments, if ne oe ssary, to remove surfa oe
contaminants therefrom. In the case of nonoonductive
substrates such as plastics, for example, a preliminary
pre-treatment of the nonconductive substrate is p~rformed
including a tin-palladium oo~plex treating solution to forn
active sites on the surfaces thereof usually followed by an
accelerating treatment whereafter t~e pretreated plastic is
susceptible for autocatalytic copper deposition. As a ge~eral
rule, the plating rate of the solution can ~e adjusted ~o meet
any desired oommercial situation by varying the con oentration
of ccpper ions, reducing agent, tenperature, pH and co~plexing
agent ooncentration to attain the desired result. In~reasing
copper ion oonoentration, re~ucing agent conoentration,
increasing temperatule and pH and reducing oomplexiny agent
conoentration all contribute tcward an increased rate of
deposition of ospper. Generally speaking, a copper deposition
rate of about at least 20 microinches in 10 minutes is
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considered oommercially satisfactory. ~gitation of the
solution also increases plating rate and can be attained by air
agitation, cath~de rod agitation or other mechanical agitating
~ar s.
In order to further illustrate the present invention,
the following examples are provided. It will be understc3d
that the examples are provided for illustrative purposes and
are not intended to be limit m g of the sccpe of the present
invention as herein described and as set forth in the subjoined
claims.
EXA~LE 1
An aqueous alkaline electroless copper solution is
prepared by dissolving in water cupric chloride to ~rcvide a
copper ion ~oncentration of about 2 g/l (0.032 mol), EDI~ tetra
sodium salt in an amount of about 9 g/l ~0.023 mol), sodium
glucoheptonate dihydrate in an amount of about 9 g/l (0.032
mDl), formaldehyde as a reducing agent in an amount to pravide
a formaldehyde o~ncentration of abaut 3 g/l, sodium hydr3xide
to adjust the pH of the soluticn to about 11~6 and a sul~ur
conta~Ing stabiliz1ng agent such as 2-mer ~ okhiazole in
an amount of ab3ut 0~05 tD about 10 pFm. Ihe bath is a
oorresponding to a turncver of about five to ten times of the
copper ion 03ncentration ~ reple m shment of the original
copper oon oentration a~d the other bath CDnStitUentS. Th2 aged
bath oonta ms a mDl ratio of gluoo ~ onate to ECIA of abcut
~L2~B7~5
1.4:1 with a total complexor to oopper ion mDl ratio of about
1.7:1.
The bath at a temperature of about 60C (140F) in
the presence of air agitation is employed for depositing copper
on test panels producing a bright, smcoth, unifonm pink copper
deposit at a plating rate of about 45 microinc~es in 10
minutes. The bath is stable.
EXAMPLE 2
An aged aqueous alkaline electroless copper solution
is prepared identical to that as previously described in
Example 1 with the exception that no complexing agent is added.
TD separate portions of the oomplex-free aged sollltion,
selected amounts and combinations of the EDIA and
glucoheptonate complexors are added and the sta~ility of the
result~ng solution under typical operating conditions is
cbserved. Tb sample A, 0.032 mol EDTA is added and the bath is
observed to be unstable. Tb a separate sample Bp a combination
of 0.023 mol ED~A and 0.014 mDl glucoheptonate is added
providIng a mDl ratio of gluoDheptonate to EDrR of about 0.6~
The ocpper deposit is semi-lustrous and provi~es a plating rate
of akout 60 micrDinches in 10 minutes.
I3 sample C, a nixture of 0.021 mDl gluoDheptonate
an~ 0.016:1 mol ED~A is added providing a relative mDl ratio of
gluoDheptonate to EDIA of about 1.3:1. The resultant
elect~Dless o~pper solution provides a pink copper deposit at
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deposition rate of about 56 micxoinches in 10 munutes. me
bath is stable.
To sample D, a mixture of 0.028 m~l glucoheptDnate
and 0.008 m~l EDTA is added provid mg a mol ratio of
glucoheptonate to EDTA of about 3.5:1. Ihe bath is unstable
and produ oe s a copper d~position rate at about 34 ~icroinches
in 10 minutes.
~ D sample E, 0.035 m~l glucoheptonate is added
without any EDrA. The bath provides a copper deposition rate
of about 16 microinches in 10 minutes. The bath is unstable
and decomposes.
The foregoing tests indicate that the use of
substantially equal mDlar amounts of EDrA or glucoheptonate by
themselves d~es not provide a stable aqueous aL~aline
electroless oopper solution. When employing a ocmbination of
glucoheptonate and EDTA within the mDlar ratios as hereinabuve
specified, namely, from about 0.1:1 to about 3:1 and preferably
frcm about 0.6:1 to about 2:1 mDls gluosheptonate to EDI~,
satisfactory ocpper deposits are obtained with relative ba~h
stability.
EX~MPLE 3
An aged aqueous alkaline electr~less oop2er s~lution
oontains abGut 2 g/l ~0.032 mDl) ocpper ions, about 9 g/l
(0.023 mDl) EDr~ about 5.3 g/l (0.021 mDl) gluoonate providing
a mol ratio of gl~oonate to EDT~ of about 0.9:1, about 3 g/l
7~5
formaldehyde and sufficient scdium hydroxide to provide a p~ oE
a~out 11.6. The bath further contains a sulfur stabilizing
compound such as 2-mexcapkobenzothiazole in an am~unt up to
about 0.25 ppm.
m e solution is at a temperature of abcut 60C
(140F) and is subject to air agitation. A bright pink copper
deposit is obtained on test panel surfa oe s at a plating rate of
about 37 microinches in 10 minutes.
E~2~PLE 4
An aged aqueous alkaline electroless c~pper solution
oontains 2 g/1 (0.032 mol~ copper ions, about 8 g/l ~0.027 m~l)
THPEDA, about 2 g/l (0.007 mol) sodium glucoheptonate dihydrate
to provide a glucoheptonate to ~.IPEDA ~Dl ratio of about
0.26:1, about 3 g/l formaldehyde and sufficient sodium
hydroxide to pr~vide a pH of about 12.2.
The solution is at a temperature of abGut 60C
(140F) and is subject to air agitation. A bright, smcokh,
pink oopper deposit is obtained on test panel surfaces at a
platiLng rate of about 140 microinches in 10 minutes.
E~E ~
An aged aqueous alkaline electroless copper solution
oontaLns about 5 g/l (0~08 ~ol) copper ions, about 15 g/l (0.05
mol3 T~PEDA, abcut 7.5 g/l ~n .02 mol) ED~, about 3.75 g/l
(0.013 ~ol) ~odium glucoheptonate dihydrat~ providiny a mol
~L291~7~i
ratio of gluooheptonate to the two amlne ccmpounds present of
about 0.18:1, about 5 g/l formaldehyde and sodium hydroxide to
provide a pH of about 12.2.
The solution is at a temperature of about 60C
(140F) and is subject to air agitation. A bright, pink oopper
deposit is obtained on test panel ~urfaces at a plating rate of
about 123 microinches in 10 minutes.
~ hile it will be apparent that the preferred
emtodlments of the invention di~closed are well calculated to
fulfill the objects above stated, it will be appreciated that
the invention is sus oe ptib}e to modification, variation and
change without departing from the proper soope or fair meaning
of the subjoined claims.
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