U.S. patent application number 10/139085 was filed with the patent office on 2003-01-02 for plating method.
This patent application is currently assigned to Shipley Company, L.L.C.. Invention is credited to Jacques, David L., Rzeznik, Maria Anna.
Application Number | 20030000846 10/139085 |
Document ID | / |
Family ID | 23130004 |
Filed Date | 2003-01-02 |
United States Patent
Application |
20030000846 |
Kind Code |
A1 |
Rzeznik, Maria Anna ; et
al. |
January 2, 2003 |
Plating method
Abstract
The present invention discloses compositions for increasing the
adhesion of a layer of silver deposited from an immersion plating
bath as well as methods of increasing the adhesion of silver layers
obtained using these compositions. Such compositions and methods
are particularly useful in the manufacture of electronic
devices.
Inventors: |
Rzeznik, Maria Anna;
(Framingham, MA) ; Jacques, David L.;
(Northbridge, MA) |
Correspondence
Address: |
EDWARDS & ANGELL, LLP
Dike, Bronstein, Roberts & Cushman, IP Group
P.O. Box 9169
Boston
MA
02209
US
|
Assignee: |
Shipley Company, L.L.C.
Marlborough
MA
|
Family ID: |
23130004 |
Appl. No.: |
10/139085 |
Filed: |
May 4, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60293662 |
May 25, 2001 |
|
|
|
Current U.S.
Class: |
205/263 ;
205/261 |
Current CPC
Class: |
C23C 18/42 20130101;
C23C 18/1834 20130101; H05K 3/244 20130101; C23C 18/54 20130101;
C23C 18/1637 20130101 |
Class at
Publication: |
205/263 ;
205/261 |
International
Class: |
C25D 003/00; C25D
003/46 |
Claims
What is claimed is:
1. A method of improving the adhesion of a layer of silver
deposited from an immersion plating bath comprising the step of:
contacting a metal that is less electropositive than silver with an
azole compound prior to contacting the metal with an immersion
silver plating bath.
2. The method of claim 2 wherein the azole compound is selected
from the group consisting of triazoles, benzotriazoles, tetrazoles,
imidazoles, benzimidazoles, indazoles and mixtures thereof.
3. The method of claim 2 wherein the azole compound is
benzotriazole, substituted benzotriazole, imidazole or substituted
imidazole.
4. The method of claim 3 wherein the substituted imidazole is
selected from the group consisting of
(C.sub.1-C.sub.16)alkylimidazole and arylimidazole.
5. The method of claim 1 wherein the azole compound is selected
from the group consisting of methylimidazole, ethylimidazole,
propylimidazole, hexylimidazole, decylimidazole, undecylimidazole,
1-phenylimidazole, 4-phenylimidazole, hydroxybenzotriazole,
aminobenzotriazole, 2-imidazolecarboxaldehyde,
benzotriazolecarboxylic acid, 2-guanidinobenzimidazole,
2-aminoindazole, chlorobenzotriazole, hydroxyethylbenzotriazole,
hydroxyethylimidazole, hydroxybenzimidazole and 1,2,4-triazole.
6. The method of claim 1 wherein the azole compound is present in a
pretreatment composition in an amount from 0.005 to 50 g/L.
7. The method of claim 6 wherein the pretreatment composition is a
microetching composition.
8. The method of claim 1 wherein the immersion silver plating bath
comprises a source of silver ions, water and a complexing
agent.
9. The method of claim 8 wherein the complexing agent is a
multidentate ligand.
10. The method of claim 1 wherein the metal that is less
electropositive than silver is selected from the group consisting
of zinc, iron, tin, nickel, lead, copper and alloys thereof.
11. A method of depositing a layer of silver on a substrate
comprising the steps of: contacting a metal that is less
electropositive than silver with an azole compound; and then
contacting the metal with an immersion silver plating bath.
12. A bath for promoting the adhesion of a layer of silver
deposited from an immersion plating bath comprising an azole
compound, water and an acid.
13. The bath of claim 12 further comprising a source of copper
ions.
14. A method of manufacturing a printed wiring board comprising the
steps of: contacting a metal that is less electropositive than
silver with an azole compound; and then contacting the metal with
an immersion silver plating bath.
15. A method of manufacturing a printed wiring board comprising the
steps of: contacting a metal that is less electropositive than
silver with an etchant composition; contacting the metal with an
azole compound; and then contacting the metal with an immersion
silver plating bath to provide a layer of silver.
16. The method of claim 15 wherein the azole compound is selected
from the group consisting of triazoles, benzotriazoles, tetrazoles,
imidazoles, benzimidazoles, indazoles and mixtures thereof.
17. The method of claim 15 wherein the metal that is less
electropositive than silver is selected from the group consisting
of zinc, iron, tin, nickel, lead, copper and alloys thereof.
18. A method of improving the adhesion of a layer of silver
deposited from an immersion bath comprising the step of: contacting
a metal that is less electropositive than silver with a
microetching composition including water, an azole compound, and a
microetchant selected from sulfuric acid/hydrogen peroxide or an
alkali metal persulfate, prior to contacting the metal with an
immersion silver plating bath to provide the layer of silver.
19. A method of reducing tarnish of a layer of silver deposited
from an immersion bath comprising the step of: contacting a metal
that is less electropositive than silver with an imidazole compound
prior to contacting the metal with an immersion silver plating bath
to provide the layer of silver.
20. A method for manufacturing a printed wiring board comprising
the steps of: a) contacting a printed wiring board substrate having
pads, through holes, soldermask and a layer of a metal that is less
electropositive than tin with an immersion plating bath including a
source of tin ions, water and a complexing agent, to form a tin
deposit on the metal; b) then contacting the tin plated printed
wiring board substrate with an azole compound; c) then contacting
the tin with an immersion silver plating bath including a source of
silver ions, water and a complexing agent, to form an immersion
silver deposit on the tin deposit; and d) heating the silver-tin
deposit to form a tin-silver alloy.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates generally to the field of
metal plating. In particular, the present invention relates to the
field of immersion silver metal plating.
[0002] Immersion or displacement plating is an electroless plating
process, but is given a separate classification in the art. In
immersion plating, deposition is by displacement of an elemental
metal from a substrate by metal ions in a plating solution. In
electroless plating deposition takes place primarily by
autocatalytic reduction of metal ions from solution. Such
electroless plating requires the presence of a reducing agent.
[0003] Immersion plating does not employ an external electric
current but rather is an electrochemical displacement reaction
which is driven by the position of the substrate metal in the
electromotive series relative to the metal to be deposited from
solution. Plating occurs when the dissolved metal ions in a plating
bath are displaced by a more active (less noble) metal that is
contacted with the plating bath.
[0004] In the manufacture of printed wiring boards, solderable
finishes are typically applied to printed wiring board substrates
having pads and/or through holes exposed through a mask, such as a
soldermask. Such solderable finishes are often applied by immersion
plating as electroless plating can also deposit metal on the
surface of the mask, which is undesirable. As an immersion plating
reaction is driven by the difference in electrochemical potentials,
plating will only occur at areas of exposed metal. For example,
U.S. Pat. No. 5,143,544 (lantosca) discloses a solution for
immersion plating a tin-lead alloy suitable as a solderable finish
on a printed wiring board substrate. However, there is a growing
demand for more environmentally acceptable alternatives to lead for
use in printed wiring board manufacture. Thus, the use of lead and
lead alloys in electronic components faces an uncertain future.
See, for example, U.S. Pat. No. 5,536,908 (Etchells et al.).
[0005] Silver is a more environmentally acceptable alternative to
lead and has been suggested for use as a solderable finish. As
discussed above, the preferred method of depositing such a
solderable finish is by immersion plating. For example, U.S. Pat.
No. 5,955,141 (Souter et al.) discloses certain immersion silver
plating baths suitable for depositing a layer of silver on a
printed wiring board.
[0006] One drawback of immersion silver plating is that the
resulting silver deposit is typically poorly adherent. In
conventional electrolytic and/or electroless silver plating
processes, the substrate to be silver plated is typically
pretreated with an agent to prevent immersion or displacement
plating of silver during subsequent electrolytic or electroless
silver plating. A wide variety of agents to reduce or prevent
immersion deposition of silver are known.
[0007] U.S. Pat. No. 4,452,673 (Takano) discloses a number of
compounds, including benzotriazole and benzimidazole, as a
pretreatment for preventing immersion deposition of silver from a
silver electroplating bath. The object of this patent is to
markedly suppress immersion silver plating on copper, nickel, iron
or their alloys. This patent does not disclose how to improve the
adhesion of a silver deposit obtained from an immersion plating
bath.
[0008] U.S. Pat. No. 5,194,139 (Kinase et al.) discloses an acidic
pretreatment solution for silver plating containing an acid and an
agent to prevent silver deposition by displacement (i.e. immersion)
from a high speed silver plating bath (i.e. an electrolytic plating
bath). A long list of suitable agents for preventing immersion
plating of silver is provided which includes nitrogen-containing
heterocyclic compounds and their analogs. Specific
nitrogen-containing heterocyclic compounds disclosed are
2,2'-dipyridyl, benzotriazole, 1-hydroxybenzotriazole, and
5,6-dimethylbenzotriazole, among others. The goal of this patent is
to inhibit immersion plating of silver as such immersion silver
deposits show poor adherence which cannot be improved by subsequent
electroplating.
[0009] Further, limitations on the use of immersion plating exist
in printed wiring board manufacture. Such limitations include
relatively slow plating rates and limited deposit thicknesses,
which are due to the self-limiting nature of immersion plating,
i.e. as the metal deposit builds, it tends to mask the underlying
base metal, thereby preventing further displacement. These problems
have conventionally been addressed using a broad range of additives
in the immersion plating bath, such as rate enhancers. However,
such additives may adversely affect other important characteristics
of the deposit, such as adhesion and deposit uniformity.
[0010] Therefore, there is a need for a method of improving the
adhesion of immersion plated silver deposits. There is a further
need for methods that do not adversely affect other important
characteristics of the silver deposit obtained from immersion
plating baths.
SUMMARY OF THE INVENTION
[0011] It has been surprisingly found that pretreating a metal that
is less electropositive than silver with an azole compound provides
a subsequently immersion deposited layer of silver having improved
adhesion as compared to silver deposits obtained from conventional
immersion deposition methods without such a pretreatment step.
[0012] In one aspect, the present invention provides a method of
improving the adhesion of a layer of silver deposited from an
immersion plating bath including the step of contacting a metal
that is less electropositive than silver with an azole compound
prior to contacting the metal with an immersion silver plating bath
to provide the layer of silver.
[0013] In another aspect, the present invention provides a method
of depositing a layer of silver on a substrate including the steps
of: contacting a metal that is less electropositive than silver
with an azole compound; and then contacting the metal with an
immersion silver plating bath to provide the layer of silver.
[0014] In a further aspect, the present invention provides a bath
for promoting the adhesion of a layer of silver deposited from an
immersion plating bath including an azole compound, water and a
chelating agent.
[0015] In still a further aspect, the present invention provides a
bath for promoting the adhesion of a layer of silver deposited from
an immersion plating bath including an azole compound, water, a
chelating agent, and a source of copper ions.
[0016] In yet another aspect, the present invention provides a
method of manufacturing a printed wiring board including the steps
of contacting a metal that is less electropositive than silver with
an azole compound; and then contacting the metal with an immersion
silver plating bath to provide a layer of silver.
[0017] In still another aspect, the present invention provides a
method of improving the adhesion of a layer of silver deposited
from an immersion plating bath including the step of contacting a
metal that is less electropositive than silver with a microetching
composition including water, an azole compound, and a microetchant
selected from sulfuric acid/hydrogen peroxide or an alkali metal
persulfate prior to contacting the metal with an immersion silver
plating bath to provide the layer of silver.
[0018] In still a further aspect, the present invention provides a
method of reducing tarnish of a layer of silver deposited from an
immersion plating bath including the step of: contacting a metal
that is less electropositive than silver with an azole compound
prior to contacting the metal with an immersion silver plating bath
to provide the layer of silver.
DETAILED DESCRIPTION OF THE INVENTION
[0019] As used throughout this specification, the following
abbreviations shall have the following meanings, unless the context
clearly indicates otherwise: .degree.C=degrees centigrade; g=gram;
L=liter; g/L=grams per liter; mL=milliliters; wt %=percent by
weight; DI=deionized; cm=centimeters; v/v=volume per volume basis;
and .mu.in.=microinches.
[0020] The terms "printed circuit board" and "printed wiring board"
are used interchangeably throughout this specification. As used
throughout this specification, the term "complexing agent" includes
ligands and chelating agents. "Halo" refers to fluoro, chloro,
bromo and iodo. Unless otherwise noted, all amounts are percent by
weight and all ratios are by weight. All numerical ranges are
inclusive and combinable in any order, except where it is obvious
that such numerical ranges are constrained to add up to 100%.
[0021] The present invention provides a method of improving the
adhesion of a layer of silver deposited from an immersion plating
bath including the step of contacting a metal that is less
electropositive than silver with an azole compound prior to
contacting the metal with an immersion silver plating bath to
provide the layer of silver. The present invention provides a layer
of silver deposited from an immersion plating bath having increased
adhesion as compared to a layer of silver immersion deposited on a
less electropositive metal that has not been first contacted with
an azole compound. Surprisingly, the present invention also
provides an immersion deposited layer of silver having increased
adhesion as compared to an immersion silver bath containing an
azole compound. Thus, more adherent silver deposits are obtained,
which are less likely to flake or abrade off than conventional
immersion silver deposits. The increased adhesion of silver
deposits obtained according to the present invention also provides
better surface mount connections between a printed wiring board and
a surface mount device.
[0022] A wide variety of azole compounds may suitably be used in
the present invention. Suitable azoles include, but are not limited
to, triazoles, benzotriazoles, tetrazoles, imidazoles,
benzimidazoles, indazoles and mixtures thereof. Such azoles may
optionally be substituted. By "substituted azole" it is meant that
one or more hydrogens of the azole are replaced by one or more
other substituents. Suitable substituents include, but are not
limited to, hydroxy, (C.sub.1-C.sub.16)alkyl,
(C.sub.1-C.sub.6)alkoxy, (C.sub.1-C.sub.6)alkoxy-
(C.sub.1-C.sub.16)alkyl, hydroxy(C.sub.1-C.sub.16)alkyl, carboxylic
acid, carboxaldehyde, (C.sub.1-C.sub.10)alkoxycarbonyl, amino,
(C.sub.1-C.sub.6)alkylamino, (C.sub.1-C.sub.6)dialkylamino, aryl
such as phenyl and tolyl, halo, thiol, (C.sub.1-C.sub.6)alkylthio,
arylthio and the like. Such substituents may themselves by
substituted. For example, a (C.sub.1-C.sub.16)alkyl may have one or
more of its hydrogens replaced with another substituent group such
as hydroxyl, carboxylic acid, amino and the like.
[0023] It is preferred that the azole compound is benzotriazole,
substituted benzotriazole, imidazole or substituted imidazole, and
more preferably benzotriazole, imidazole,
(C.sub.1-C.sub.16)alkylimidazole, and arylimidazole.
Phenylimidazole is the preferred arylimidazole. Exemplary
(C.sub.1-C.sub.16)alkylimidazoles include methylimidazole,
ethylimidazole, propylimidazole, hexylimidazole, decylimidazole and
undecylimidazole. Suitable phenylimidazoles include
1-phenylimidazole and 4-phenylimidazole. Other suitable azoles
include, but are not limited to, hydroxybenzotriazole,
aminobenzotriazole, 2-imidazolecarboxaldehyde,
benzotriazolecarboxylic acid, 2-guanidinobenzimidazole,
2-aminoindazole, chlorobenzotriazole, hydroxyethylbenzotriazole,
hydroxyethylimidazole, hydroxybenzimidazole, 1,2,4-triazole,
histidine, 1,2,4-triazolo[1,5-.alph- a.]pyrimidine, and the like.
It will be appreciated by those skilled in the art that mixtures of
azole compounds may be advantageously used in the present
invention. Such azole compounds are generally commercially
available, such as from Aldrich Chemical Company (Milwaukee, Wis.)
and may be used without further purification.
[0024] The azole compounds may be used in the present invention in
a wide range of amounts. Typically, the azole compound is used in
an amount of from 0.005 to 50 g/L, preferably from 0.005 to 20 g/L,
and more preferably from 0.01 to 15 g/L. Particularly suitable
amounts of azole compound are from 0.1 to 10 g/L, more particularly
from 0.5 to 10 g/L, and even more particularly from 1 to 10 g/L.
The specific amount of azole compound depends upon the particular
azole chosen and its solubility in the pretreatment
composition.
[0025] The pretreatment compositions of the present invention
typically contain one or more azole compounds and water. Such
pretreatment compositions may be alkaline or acidic. The pH of the
pretreatment compositions may vary from 1 to 14. The pH of the
pretreatment compositions may be varied in order to increase the
solubility of the azole compound. For example, the solubility of
hydroxybenzotriazole can be increased by increasing the pH of the
pretreatment composition.
[0026] A wide variety of organic and inorganic acids can be used to
adjust the pH of the pretreatment compositions. Suitable inorganic
acids include, but are not limited to: hydrochloric acid,
hydrofluoric acid, fluoroboric acid, hydroiodic acid, periodic
acid, phosphoric acid, sulfuric acid, nitric acid, and the like.
Suitable organic acids include, but are not limited to:
alkylsulfonic acids such as methanesulfonic acid,
trifuloromethanesulfonic acid, ethanesulfonic acid and
propanesulfonic acid; arylsulfonic acids such as phenylsulfonic
acid, naphthalenesulfonic acid, toluenesulfonic acid; and the like.
Mixtures of acids, such as mixtures of inorganic acids, mixtures of
organic acids or mixtures of inorganic and organic acids, may be
advantageously used. The acids are typically present in the
pretreatment compositions in an amount sufficient to provide the
desired pH.
[0027] Similarly, a wide variety of inorganic and organic bases may
be used to adjust the pH. Suitable inorganic bases include, but are
not limited to, alkali metal hydroxides such as sodium hydroxide
and potassium hydroxide, carbonates such as sodium carbonate and
potassium carbonate, ammonium hydroxide, and the like. Suitable
organic bases include, but are not limited to, tetraalkylammonium
hydroxides, amines, and the like.
[0028] Preferably, the pretreatment compositions further include
one or more chelating agents. Such chelating agents may be
monodentate ligands, such as ammonia, cyanide, pyridine and the
like, or multidentate ligands. Suitable multidentate ligands
include, but are not limited to: amino acids having from 2 to 10
carbon atoms; polycarboxylic acids such as oxalic acid, adipic
acid, succinic acid, malonic acid and maleic acid; amino acetic
acids such as nitrilotriacetic acid, pyridine carboxylic acid and
pyridine dicarboxylic acid; alkylene polyamine polyacetic acids
such as ethylenediamine tetraacetic acid ("EDTA"),
diethylenetriamine pentaacetic acid ("DTPA"),
N-(2-hydroxyethyl)ethylenediamine triacetic acid,
1,3-diamino-2-propanol-N,N,N',N'-tetraacetic acid,
bis-(hydroxyphenyl)-ethylenediamine diacetic acid,
diaminocyclohexane tetraacetic acid, or ethylene
glycol-bis-((.beta.-aminoethylether)-N,N'-t- etracetic acid);
polyamines such as or N,N,N',N'-tetrakis-(2-hydroxypropyl-
)ethylenediamine, ethylenediamine, 2,2',2"-triaminotriethylamine,
triethylenetetramine, diethylenetriamine and
tetrakis(aminoethyl)ethylene- diamine; citrates; tartrates;
N,N-di-(2-hydroxyethyl)glycine; gluconates; lactates; crown ethers;
cryptands; polyhydric compounds such as 2,2',2"-nitrilotriethanol;
heteroaromatic compounds such as 2,2'-bipyridine,
1,10-phenanthroline and 8-hydroxyquinoline; thio-containing ligands
such as thioglycolic acid and diethyldithiocarbamate; aminoalcohols
such as ethanolamine, diethanolamine, and triethanolamine; and the
like. It will be appreciated by those skilled in the art that a
combination of chelating agents may be employed in the present
invention. Preferably, the chelating agents are the same as those
used in the subsequent immersion silver plating bath.
[0029] In one embodiment, the present invention provides a bath for
promoting the adhesion of a layer of silver deposited from an
immersion plating bath including an azole compound, water and a
chelating agent.
[0030] Other additional components may optionally be used in the
pretreatment compositions. Such additional components include, but
are not limited to, surfactants, metal ions, and the like. The
surfactants may be anionic, cationic, nonionic or amphoteric.
Preferably, the surfactants are cationic or nonionic. When a
surfactant is used in the present pretreatment compositions, it is
typically present in an amount of at least 0.001 wt %, preferably
at least 0.005 wt %, and more preferably at least 0.01 wt %. In
general, the surfactant will be present in the compositions in an
amount no greater than 5 wt %, preferably no greater than 3 wt %,
and more preferably no greater than 2.5 wt %. Thus, the surfactant
is typically present in an amount in the range of 0.001 to 5 wt %,
preferably 0.005 to 3 wt %, and more preferably 0.01 to 2.5 wt %.
Mixtures of surfactants may suitably be used.
[0031] When metal ions are present in the pretreatment
compositions, it is preferred that they are the metal that is less
electropositive than silver on which silver is to be deposited. For
example, when the metal that is less electropositive than silver is
copper, it is preferred that copper ions are present in the
pretreatment composition. In another embodiment, the present
invention provides a bath for promoting the adhesion of silver
deposited from an immersion plating bath including an azole
compound, water, a chelating agent, and a source of copper ions.
The source of copper ions may be any that is sufficiently soluble
in the pretreatment composition to provide the desired amount of
copper ions. Suitable sources of copper ions include, but are not
limited to, copper nitrate, copper sulfate, copper chloride, copper
acetate, copper gluconate, copper citrate, copper alkanesulfonate,
copper arylsulfonate, and the like. Mixtures of copper ion sources
may also be used.
[0032] The amount of metal ions added to the pretreatment
composition depends upon the particular metal that is less
electropositive than silver, the particular azole compound used and
the pH of the pretreatment composition. For example, when copper
ions are present in the pretreatment composition, they are
typically present in an amount up to 1 g/L, and preferably up to
0.05 g/L.
[0033] The pretreatment compositions are typically prepared by
combining the azole compound and water, along with any suitable
acid or base to adjust the pH and any additional components. Such
components may be combined in any order.
[0034] A metal that is less electropositive than silver is first
contacted with the present pretreatment composition. Such contact
may be by a variety of means, such as dipping, spraying, flood
coating, puddle coating, and the like. A particularly suitable
method of contacting with the present pretreatment compositions is
spraying in a flood mode.
[0035] Suitable metals that are less electropositive than silver
include, but are not limited to, zinc, iron, tin, nickel, lead,
copper or alloys containing one or more of these metals. It is
preferred that the metal is tin, copper or alloys thereof. A
particularly suitable alloy is tin-copper. In an alternate
embodiment, such metal may itself be immersion deposited on a
suitable metal underlayer prior to depositing an immersion silver
layer according to the present invention. For example, the metal
may be tin, such tin deposit being first deposited, such as by
immersion, electroless or electrolytic deposition, on copper.
[0036] In general, the metal is contacted with the pretreatment
composition for a period of time sufficient to increase the
adhesion of a subsequently deposited layer of silver by immersion
plating. Such period of time depends upon the particular metal and
pretreatment composition used. Typically, a contact time of 1
second to 15 minutes is sufficient, preferably 5 seconds to 10
minutes, and more preferably 10 seconds to 5 minutes. A
particularly suitable contact time is from 30 seconds to 3
minutes.
[0037] After the metal has been contacted with the pretreatment
composition, it is optionally rinsed, such as with water, and then
optionally dried. Such a rinsing step is preferred.
[0038] A layer of silver is next immersion deposited on the
pretreated metal. Any immersion silver plating bath may be used in
the present invention. Typical immersion silver baths contain one
or more sources of silver ions, one or more complexing agents and
water. Any water soluble silver salt may be used as the source of
silver ions, such as silver nitrate, silver acetate, silver
sulfate, silver lactate, silver formate, and the like. Preferably,
the source of silver ions is silver nitrate. The source of silver
ions is present in an amount sufficient to provide silver ions in
solution in a concentration from 0.06 to 32 g/L, preferably from
0.1 to 25 g/L, and more preferably from 0.5 to 15 g/L.
[0039] The one or more complexing agents are any which do not form
insoluble precipitates under the plating bath conditions employed.
Suitable complexing agents include the chelating agents described
above for the pretreatment compositions. It will be appreciated by
those skilled in the art that a combination of complexing agents
may be employed in the immersion silver baths.
[0040] The complexing agent may be used in a variety of
concentrations but is preferably present in the immersion silver
plating baths either in stoichiometrically equivalent amounts
(based on the amount of silver ion) or in a stoichiometric excess
so that all the silver ions may be complexed. The term
"stoichiometric" as used herein refers to equimolar. Preferably,
the one or more complexing agents are present in a higher molar
concentration than the silver ions. Preferably, the molar ratio of
the complexing agent to silver ions is .gtoreq.1:1, more preferably
.gtoreq.1.2:1, even more preferably .gtoreq.2.0:1, and still more
preferably .gtoreq.3.0:1. In general, the total amount of the one
or more complexing agents is from 0.1 to 250 g/L, preferably from 2
to 220 g/L, more preferably from 10 to 200 g/L, and particularly 50
to 150 g/L.
[0041] Immersion silver plating baths useful in the present
invention typically have a pH of from 1 to 14. Preferred immersion
silver baths have a pH of 1 to 10, and more preferably 2 to 10. A
buffering agent may be added to maintain the pH of the immersion
silver plating bath at the desired value. Any compatible acids or
bases may be used as the buffering agents and may be organic or
inorganic. By "compatible" acids or bases it is meant that the
acids or bases do not result in precipitation of the silver ions
and/or complexing agents from solution, when such acids or bases
are used in amounts sufficient to buffer the pH. Suitable buffering
agents include, but are not limited to, alkali metal hydroxides
such as sodium hydroxide or potassium hydroxide, carbonate salts,
citric acid, tartaric acid, nitric acid, acetic acid, phosphoric
acid and the like.
[0042] The immersion silver plating baths may optionally contain
one or more additional components. Suitable additional components
include, but are not limited to, surfactants or wetting agents,
anti-tarnish agents for silver, oxidation inhibitors, levelers,
grain refiners, defoamers, dyes, and the like. When surfactants are
used in the present immersion baths, they are typically used in an
amount of from 0.02 to 100 g/L, preferably from 0.1 to 25 g/L, and
more preferably from 1 to 15 g/L. Such surfactants may be anionic,
cationic, nonionic or amphoteric. The choice of surfactant will
depend upon the particular immersion silver plating bath used.
Nonionic surfactants are preferred.
[0043] Grain refiners are typically added to improve the appearance
of the silver deposit. Suitable grain refiners include
(C.sub.1-C.sub.6)alcohols and polyalkylene glycols such as
polyethylene glycol. Such grain refiners are typically used in an
amount from 0.02 to 200 g/L, and preferably from 0.05 to 100
g/L.
[0044] A wide variety of anti-tarnish agents for silver may be
used, such as triazoles, tetrazoles, imidazoles, and the like. Such
anti-tarnish agents are well known to those skilled in the art. The
anti-tarnish agents may be used in an amount from 0.001 to 50 g/L,
preferably from 0.005 to 25 g/L, and more preferably from 0.01 to
10 g/L.
[0045] Preferably, the immersion silver baths are free of reducing
agents capable of reducing silver ions. It is further preferred
that the immersion silver baths are free of cyanide ions and
ammonium ions.
[0046] The immersion silver baths are typically prepared by
combining the above ingredients in any order. Preferably, the baths
are prepared by forming a solution of complexing agent in water and
adding the source of silver ions to this solution. The optional
ingredients may be combined with the solution in any order.
[0047] The immersion silver plating baths may be agitated. Any form
of agitation may be used. Suitable agitation includes, but is not
limited to, stirring, shaking, swirling, aeration, sonication and
the like. Stirring may be accomplished by any suitable means, such
as with an overhead stirrer, paddle stirrer or stirring bar system.
Shaking may be accomplished in a variety of ways, such as by moving
the substrate to be silver plated back and forth or side to side
within the plating bath. Aeration may be accomplished by bubbling
or sparging a gas into the plating bath, or by means of jet spray,
such as through whirlpool jets. Sparging may be accomplished by
bubbling gas into the bath through a fritted means, such as a tube
having a frit composed of glass, poly(tetrafluoroethylene) or other
inert material. Any gas may be used such as air, oxygen or an inert
gas, and preferably air. In a particular embodiment, aeration is
used to agitate the bath. Swirling may be accomplished by moving
the bath or the substrate to be plated in a substantially circular
motion. It will be appreciated by those skilled in the art that a
combination of agitation methods may be employed, such as stirring
with aeration.
[0048] Prior to silver plating, it is preferred that the metal is
cleaned. Cleaning removes oxides and organic contaminants from the
metal surface as well as resist residues that may remain from
incomplete development of dry film photoresists as well as
soldermask residues from copper surfaces. Such cleaning may be by
any conventional cleaning processes and/or products. For example,
when the metal layer is copper or a copper alloy, it is preferred
that the metal layer is cleaned with an acidic cleaning
composition. Such cleaning procedures are well within the ability
of one skilled in the art. After cleaning, the substrate is
typically rinsed, such as with water, and optionally dried.
[0049] The azole compound adhesion promoters may optionally be
added to the cleaning baths. Such cleaning baths then provide the
dual functions of cleaning the metal and acting as a pretreatment
composition for subsequent silver plating of the metal. In this
way, the need for a separate treatment step is avoided.
[0050] The metal layer may be microetched either before or after
the cleaning step, and preferably after the cleaning step. Such
microetching is accomplished by contacting the metal layer on the
substrate with a microetching composition, such as sulfuric
acid/hydrogen peroxide or an alkali metal persulfate such as sodium
or potassium persulfate. When such a microetching step is used, the
metal layer may then optionally be rinsed with water or an acid,
such as with sulfuric acid, to remove any residues from the
cleaning and/or microetching steps.
[0051] In another embodiment, the present azole compound adhesion
promoters may optionally be added to the microetching composition.
Such microetching compositions then provide the dual functions of
microetching the metal and acting as a pretreatment composition for
subsequent silver plating of the metal. In this way, the need for a
separate treatment step is avoided. Thus, the present invention
provides a microetching composition including water, an azole
compound, and a microetchant selected from sulfuric acid/hydrogen
peroxide or an alkali metal persulfate. The present invention
further provides a method of improving the adhesion of a layer of
silver deposited from an immersion plating bath including the step
of contacting a metal that is less electropositive than silver with
a microetching composition including water, an azole compound, and
a microetchant selected from sulfuric acid/hydrogen peroxide or an
alkali metal persulfate prior to contacting the metal with an
immersion silver plating bath to provide a layer of silver.
[0052] Silver is deposited according to the present invention by
contacting the metal that is less electropositive than silver with
the present immersion silver plating baths. Such contact may be by
dipping, spraying, flood coating, puddle coating, and the like.
When used in vertical plating equipment, it is preferred that the
substrate is dipped in the present silver plating bath. When used
in horizontal plating equipment, it is preferred that the substrate
is contacted with the present silver plating bath by spraying or
flooding.
[0053] The contact time of the metal with the immersion silver
plating bath is that amount sufficient to deposit the desired
thickness of silver. Typically, the contact time is from about 10
seconds to 15 minutes, preferably from 20 seconds to 15 minutes,
and more preferably from 30 seconds to 12 minutes.
[0054] The present immersion silver plating baths may be used at a
variety of temperatures. Suitable temperatures include those in the
range of from 10.degree. to 70.degree. C., preferably from
15.degree. to 60.degree. C., and more preferably from 20.degree. to
50.degree. C.
[0055] The silver deposit typically has a thickness of 35 .mu.in.
or less, more typically 30 .mu.in. or less, and even more typically
25 .mu.in. or less. Following deposition, the silver layer may be
rinsed such as with water. The silver layer may optionally be dried
prior to subsequent processing steps.
[0056] In general, the metal that is less electropositive than
silver is a metal layer on a substrate. A wide variety of
substrates having a layer of a metal that is less electropositive
than silver may be plated according to the present invention.
Suitable substrates include, but are not limited to, jewelry,
decorative objects, object d'art, semiconductor packaging, lead
frames, solder bumps, metal powder and printed wiring board
substrates. The present invention is particularly suited for
depositing a solderable silver finish on a printed wiring board.
Further, the present invention provides a method of manufacturing a
printed wiring board including the steps of contacting a metal that
is less electropositive than silver with an azole compound; and
then contacting the metal with an immersion silver plating bath to
provide a layer of silver. Such a silver layer provides a
solderable finish on the printed wiring board. Such solderable
finishes are typically applied to a printed wiring board substrate
having pads, through holes and a mask, such as a soldermask. In
such a printed wiring board substrate, the exposed pads and through
holes generally contain a layer of copper.
[0057] In yet another embodiment, the present invention is suitable
for providing a printed wiring board substrate having exposed pads
and/or through holes including a tin-silver alloy as the solderable
finish. Thus, the present invention further provides a method for
manufacturing a printed wiring board including the steps of: a)
contacting a printed wiring board substrate having pads, through
holes, soldermask and a layer of a metal that is less
electropositive than tin with an immersion plating bath including a
source of tin ions, water and a complexing agent, to form a tin
deposit on the metal; b) then contacting the tin plated printed
wiring board substrate with an azole compound; c) then contacting
the tin with an immersion silver plating bath including a source of
silver ions, water and a complexing agent, to form an immersion
silver deposit on the tin deposit; and d) heating the silver-tin
deposit to form a tin-silver alloy.
[0058] A wide variety of post-treatments may be used to treat the
silver layer deposited according to the present invention. For
example, it is well known that silver, such as in silver films,
tarnishes upon prolonged exposure to air. Thus, in certain
applications it is desirable to contact the freshly deposited
silver layer with a tarnish inhibitor or anti-tarnish agent. Such
silver tarnish inhibitors are well-known to those skilled in the
art and include those described above. The silver deposit may be
contacted with the tarnish inhibitor by any suitable means, such as
dipping, spraying, flood coating, puddle coating and the like. The
use of a tarnish inhibitor subsequent to plating is not required,
but may optionally be used. Other conventional post-treatments may
also be advantageously used.
[0059] The present invention is particularly suitable for use in
the manufacture of a wide variety of electronic devices in addition
to printed wiring boards, such as lead frames, semiconductor
packaging, lead-free solder bumps on wafers, such as tin-silver and
tin-copper-silver solders, and the like.
[0060] In a further embodiment, the present invention also reduces
the amount of or inhibits the formation of tarnish on subsequently
immersion deposited layers of silver. For the reduction of or
inhibition of tarnish, it is preferred that an imidazole is used in
the pretreatment composition. Suitable imidazoles are those
described above. Such imidazoles may be used as a separate
pretreatment composition or may be added to any microetching
composition used to prepare the surface of the metal that is less
electropositive than silver for subsequent silver deposition. Thus,
the present invention also provides a method of reducing tarnish of
a silver layer deposited from an immersion plating bath including
the steps of contacting a metal that is less electropositive than
silver with an azole compound prior to contacting the metal with an
immersion silver plating bath to provide the layer of silver. The
amount of azole compound suitable to reduce or inhibit tarnish of
subsequently immersion deposited silver layers is that amount
sufficient to provide increased adhesion, as described above.
[0061] The present invention is suitable for use in vertical or
horizontal plating equipment.
[0062] The following examples are presented to further illustrate
various aspects of the present invention, but are not intended to
limit the scope of the invention in any aspect.
EXAMPLE 1
Comparative
[0063] A pretreatment composition was prepared by combining 100 g/L
of nitrilotriacetic acid ("NTA") and 0.05 g/L copper nitrate, which
was diluted to volume (IL) with deionized water. The pH of the
pretreatment was adjusted to 9.
EXAMPLE 2
[0064] The procedure of Example 1 was repeated, except that 1 g/L
of imidazole was also added to the composition. The pH was again
adjusted to 9.
EXAMPLE 3
[0065] Copper panels (2.times.6 inches or 5.times.15 cm) were
submerged in a commercially available acid cleaner to remove oxides
and organic residues from the copper surface, followed by rinsing
with water. The copper panels were next contacted with a
commercially available sulfuric acid/hydrogen peroxide-based
microetching composition to produce optimum copper surface
uniformity and texture, followed by rinsing with water. After
contact with the microetching composition, the copper panels were
then submerged in either the pretreatment compositions of Example 1
or Example 2 for 1 minute at room temperature. After pretreatment,
the copper panels were rinsed with water for 1 minute.
[0066] An immersion silver plating bath was prepared by combining
100 g/L of NTA, water, 6 g/L of picolinic acid, 0.005 g/L of
DL-lysine, and 1 g/L of silver nitrate. The pH of the bath was
adjusted to 9. The temperature of the bath was approximately
50.degree. C.
[0067] The pretreated copper panels were dipped in the immersion
silver plating bath for 10 minutes. The thickness of the resulting
silver layer on the copper panels for each pretreatment composition
was determined by X-ray fluorescence ("XRF") spectroscopy for a
number of points on the panel and the data are reported as a range
of thicknesses in Table 1.
[0068] The silver plated copper panels were also evaluated to
determine the adhesion of the silver layer. A 3.times.1 inch
(7.6.times.2.5 cm) strip of transparent adhesive tape, Scotch 610
brand available from the 3M Company, Minneapolis, Minn., was
applied to the surface of each of the silver coated copper panels.
The tape was then removed from each panel. Silver deposits having
poor adhesion were readily removed by the tape and rated as
"failed." Silver deposits having good adhesion were not removed by
the tape and rated as "passed." The adhesion results are also
reported in Table 1
1 TABLE 1 Pretreatment Composition Thickness Range (.mu.in.) Tape
Test Example 1 4.71-6.31 Failed Example 2 4.22-4.88 Passed
[0069] From the above data, it can be seen that pretreatment
compositions containing an azole compound provided a much more
uniform and adherent silver deposit than that obtained without such
azole compound pretreatment.
EXAMPLE 4
[0070] The procedure of Example 3 was repeated, except that the
silver immersion plating bath was prepared by combining 100 g/L of
NTA, water, 6 g/L of picolinic acid, 0.001 g/L of
hydroxybenzotriazole, and 1 g/L of silver nitrate. The pH of the
bath was adjusted to 9. The temperature of the bath was
approximately 50.degree. C. The deposit uniformity and tape test
data are reported in Table 2.
2 TABLE 2 Pretreatment Composition Thickness Range (.mu.in.) Tape
Test Example 1 5.01-6.06 Failed Example 2 3.71-4.48 Passed
[0071] From the above data, it can be seen that pretreatment
compositions containing an azole compound provided a much more
uniform and adherent silver deposit than that obtained without such
azole compound pretreatment.
EXAMPLE 5
[0072] Four pretreatment compositions, Examples 5-1, 5-2, 5-3 and
5-C, were prepared by combining 100 g/L of NTA and 0.05 g of cupric
ion. The pretreatment compositions of Examples 5-1, 5-2 and 5-3
further contained 1, 5 and 10 g/L of imidazole, respectively. The
pretreatment composition of Example 5-C was comparative and
contained no imidazole. These solutions were diluted to a final
volume of 1 L with deionized water and the pH adjusted to 9.
[0073] Four copper panels (2.times.6 inches or 5.times.15 cm) were
submerged in a commercially available acid cleaner to remove oxides
and organic residues from the copper surface, followed by rinsing
with water. The copper panels were next contacted with a
commercially available sulfuric acid/hydrogen peroxide-based
microetching composition to produce optimum copper surface
uniformity and texture, followed by rinsing with water.
[0074] Copper panel #1 was submerged in the pretreatment
composition of Example 5-C. Copper panel #2 was submerged in the
pretreatment composition of Example 5-1, copper panel #3 was
submerged in the pretreatment composition of Example 5-2 and copper
panel #4 was submerged in the pretreatment composition of Example
5-3.
[0075] The four copper panels were next dipped in a silver plating
bath containing 1 g/L of silver nitrate, 100 g/L of NTA, 6 g/L of
picolinic acid, and 0.1 g/L 1-hydroxybenzotriazole. The pH of the
bath was adjusted to 9.0 and the bath temperature was 50.degree.
C.
[0076] Following silver deposition, the panels were placed in an
oven set at 95.degree. C. and containing a water reservoir for 8
hours for steam testing. At the end of 8 hours, the panels were
removed from the oven and visually inspected. Panels #2-4 were
found to have significantly less tarnish than panel #1. These data
clearly show that the use of imidazole in a pretreatment
composition greatly reduces the amount of tarnish on a subsequently
immersion deposited layer of silver, as compared to a silver
deposit without such pretreatment.
EXAMPLE 6
[0077] A number of microetch compositions were prepared by
combining 800 mL/L DI water, 50 mL/L concentrated sulfuric acid, 50
mL/L hydrogen peroxide and 100 mL/L of a concentrate containing
organic acids. The microetch compositions were heated at 40.degree.
to 50.degree. C. The pretreatment composition of Example 6-C was
comparative and contained no azole compound. The other pretreatment
compositions all contained an azole compound. The particular azole
compounds and amounts are reported in Table 3.
3TABLE 3 Pretreatment Composition Azole Compound Amount (g/L)
Example 6-C -- 0 Example 6-1 Imidazole 1 Example 6-2 " 5 Example
6-3 " 10 Example 6-4 4-Phenylimidazole 1 Example 6-5 " 5
EXAMPLE 7
[0078] A number of copper panels (2.times.6 inches or 5.times.15
cm) were submerged in a commercially available acid cleaner to
remove oxides and organic residues from the copper surface,
followed by rinsing with water. Each copper panel was next
contacted with a microetching composition of Example 6, as shown in
Table 4.
[0079] The four copper panels were each next dipped in a silver
plating bath containing 1 g/L of silver nitrate, 100 g/L of NTA, 7
g/L of picolinic acid, and 0.1 g/L 1-hydroxybenzotriazole. The pH
of the bath was adjusted to 9.0 and the bath temperature was
50.degree. C. Following silver deposition, the uniformity of the
silver deposit was evaluated by XRF spectroscopy and the adhesion
of some of the silver deposits was evaluated according to the tape
test of Example 3. These data are also reported in Table 4.
4 TABLE 4 Thickness Range of Pretreatment Composition Silver
Deposit (.mu.in.) Tape Test Example 6-C 3.82-5.99 Failed Example
6-1 2.35-3.37 Passed Example 6-2 2.09-3.72 Passed Example 6-3
2.24-3.48 Passed Example 6-4 2.03-2.91 Passed Example 6-5 1.41-2.02
Passed
[0080] The above data clearly show that pretreatment compositions
containing an azole compound provided a much more uniform and
adherent silver deposit than that obtained without such azole
compound pretreatment.
[0081] Following silver deposition, a number of the panels were
also placed in an oven set at 95.degree. C. and containing a water
reservoir for 8 hours for steam testing. At then end of 8 hours,
the panels were removed from the oven and visually inspected. The
panels that were pretreated with an azole compound, i.e. a
composition according to Examples 6-1, 6-2 or 6-3, had
significantly less tarnish than the silver layer on the copper
panel that was not pretreated with an azole compound. These data
clearly show that the use of an azole compound in a microetch
composition greatly reduces the amount of tarnish on a subsequently
immersion deposited silver layer, as compared to the same silver
deposit without such pretreatment.
EXAMPLE 8
Comparative
[0082] A number of copper panels (2.times.6 inches or 5.times.15
cm) were submerged in a commercially available acid cleaner to
remove oxides and organic residues from the copper surface,
followed by rinsing with water. The copper panels were next
contacted with a commercially available sulfuric acid/hydrogen
peroxide-based microetching composition to produce optimum copper
surface uniformity and texture, followed by rinsing with water.
[0083] Immersion silver plating baths were prepared by combining 1
g/L of silver nitrate with 95 g/L EDTA. An additive was added to
each bath, as shown in Table 5. Each plating bath was diluted to a
final volume of 1 L with deionized water. The pH of each bath was
adjusted to 7.4 and the temperature of each bath was approximately
50.degree. C.
[0084] Cleaned and microetched copper panels were dipped in each of
the immersion silver plating baths. The panels remained in the
plating bath for 10 minutes. Following silver plating, the panels
were rinsed, the appearance of the silver deposit was visually
evaluated, and the silver deposits evaluated by the tape test of
Example 3. The results are reported in Table 5.
5TABLE 5 Thickness Range of Silver Deposit Deposit Additive Amount
(g/L) (.mu.in.) Appearance Tape Test None -- 9.5-13.5 Dark Failed
1-(2-Hydroxyethyl)-2- 0.1 3.8-5.3 Dark Failed imidazolidinethione
6-Aminoindazole 0.015 5.9-8.0 Slightly Failed discolored
2-Guanidinobenzimidazole 0.1 2.7-3.4 Slightly dark Failed
2-Imidazolecarboxaldehyde 0.03 7.11-11.17 Mostly brown Failed
DL-Methionine 3.3 3.4-5.5 Dark Failed Succinimide 10 3.19-4.33 Dark
Failed 2,2'-Dipyridylamine 1.5 4.04-5.80 Brown Failed
2,2'-Dipyridyl 0.15 2.54-4.61 Brown Failed
[0085] The above data clearly show that azole compounds in the
immersion silver plating bath do not provide increased adhesion as
compared to azole compounds used in the pretreatment compositions
of the present invention.
EXAMPLE 9
[0086] A printed wiring board substrate having exposed copper pads
and/or copper plated through holes and a soldermask is contacted
with a pretreatment composition containing 95 g/L of EDTA, water, 1
g/L of imidazole and 0.05 of copper nitrate. The substrate is then
rinsed with water and is then contacted with a silver plating bath
containing 95 g/L of EDTA, water and 1 g/L of silver nitrate at a
temperature of approximately 50.degree. C. A layer of silver is
then immersion deposited on the exposed copper pads and/or copper
plated through holes.
EXAMPLE 10
[0087] The procedure of Example 9 is repeated, except that the
imidazole is present in a 1:1 v/v sulfuric acid/hydrogen peroxide
microetching composition.
EXAMPLE 11
[0088] A printed wiring board substrate having exposed copper pads
and/or copper plated through holes and a soldermask is contacted
with a pretreatment composition containing 100 g/L of NTA, 0.05 g/L
copper nitrate, and 1 g/L of imidazole, diluted to volume (1L) with
deionized water and having a pH of 9. The substrate is then rinsed
with water and is then contacted with a silver plating bath
containing 100 g/L of NTA, water and 1 g/L of silver nitrate at a
temperature of approximately 50.degree. C. A layer of silver is
then immersion deposited on the exposed copper pads and/or copper
plated through holes.
* * * * *