U.S. patent application number 10/620916 was filed with the patent office on 2004-03-04 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 | 20040040852 10/620916 |
Document ID | / |
Family ID | 31981467 |
Filed Date | 2004-03-04 |
United States Patent
Application |
20040040852 |
Kind Code |
A1 |
Rzeznik, Maria Anna ; et
al. |
March 4, 2004 |
Plating method
Abstract
The present invention provides compositions and methods for
immersion depositing highly adherent silver layers. The
compositions contain silver ions, water and a carboxylic
acid-substituted nitrogen-containing heterocyclic compound. 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: |
S. Matthew Calms
c/o EDWARDS & ANGELL, LLP
P. O. Box 9169
Boston
MA
02209
US
|
Assignee: |
Shipley Company, L.L.C.
Marlborough
MA
|
Family ID: |
31981467 |
Appl. No.: |
10/620916 |
Filed: |
July 16, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60407107 |
Aug 30, 2002 |
|
|
|
Current U.S.
Class: |
205/86 ;
205/263 |
Current CPC
Class: |
H05K 3/244 20130101;
C23C 18/54 20130101; C23C 18/42 20130101 |
Class at
Publication: |
205/086 ;
205/263 |
International
Class: |
C25D 005/54 |
Claims
What is claimed is:
1. A method of depositing a layer of silver on a substrate
comprising the step of contacting a substrate having a layer of a
metal that is less electropositive than silver with an immersion
silver plating bath comprising one or more sources of silver ions,
water and one or more carboxylic acid-substituted
nitrogen-containing heterocyclic compounds, wherein the bath has a
pH of less than or equal to 4.
2. The method of claim 1 wherein the substrate is a printed wiring
board substrate.
3. The method of claim 1 wherein the metal that is less
electropositive than silver is chosen from zinc, iron, tin, nickel,
lead, copper or alloys of zinc, iron, tin, nickel, lead and
copper.
4. The method of claim 1 wherein the carboxylic acid-substituted
nitrogen-containing heterocyclic compounds is chosen from picolinic
acid, quinolinic acid, nicotinic acid, isonicotinic acid, fusaric
acid, isonipecotic acid, nipecotic acid, 2,6-pyridine dicarboxylic
acid, piperazine-2-carboxylic acid, pyrrole-2-carboxylic acid and
piperolinic acid.
5. The method of claim 1 wherein the carboxylic acid-substituted
nitrogen-containing heterocyclic compound comprises a
nitrogen-containing heterocyclic moiety chosen from pyridine,
piperidine, piperazine, pyrrole, morpholine, pyrrolidine, triazole,
and imidazole.
6. A method of manufacturing a printed wiring board comprising the
step of contacting a printed wiring board substrate having a layer
of a metal that is less electropositive than silver with an
immersion silver plating bath comprising one or more sources of
silver ions, water and one or more carboxylic acid-substituted
nitrogen-containing heterocyclic compounds, wherein the silver
plating bath has a pH of less than or equal to 4.
7. An immersion silver plating bath comprising one or more sources
of silver ions, water and one or more carboxylic acid-substituted
nitrogen-containing heterocyclic compounds, wherein the silver
plating bath has a pH of less than or equal to 4.
8. The immersion silver plating bath of claim 7 wherein the bath is
free of cyanide ions, ammonia and ammonium ions.
9. The immersion silver plating bath of claim 7 wherein the
carboxylic acid-substituted nitrogen-containing heterocyclic
compound is chosen from picolinic acid, quinolinic acid, nicotinic
acid, isonicotinic acid, fusaric acid, isonipecotic acid, nipecotic
acid, 2,6-pyridine dicarboxylic acid, piperazine-2-carboxylic acid,
pyrrole-2-carboxylic acid and piperolinic acid.
10. The immersion silver plating bath of claim 7 wherein the
carboxylic acid-substituted nitrogen-containing heterocyclic
compound comprises a nitrogen-containing heterocyclic moiety
selected chosen from pyridine, piperidine, piperazine, pyrrole,
morpholine, pyrrolidine, triazole, and imidazole
11. The immersion silver plating bath of claim 7 further comprising
one or more thickness control agents.
12. The immersion silver plating bath of claim 11 wherein the
thickness control agent is chosen from azoles, amino acids,
hydroxy-substituted aromatic compounds, sulfur-containing compounds
and hydantoins.
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 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. 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.
[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] 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.
[0007] 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.
[0008] Picolinic acid has been used in a tarnish remover
composition for silver and copper, see Indian Patent No 163677.
Zhuang et al., Huaxue Xuebao, 1985, vol. 43, no. 2, pp 120-125,
disclose the electrodeposition of silver from a plating bath
containing silver nitrate, ammonia and pyridine carboxylic acid.
This article does not disclose the use of picolinic acid in an
immersion silver plating bath.
SUMMARY OF THE INVENTION
[0009] It has been surprisingly found that an electroless,
particularly an immersion, silver plating bath containing as a
complexing agent one or more carboxylic acid-substituted
nitrogen-containing heterocyclic compounds 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 complexing agent. In addition,
bright silver deposits are obtained using these plating baths.
[0010] The present invention provides a method of depositing a
layer of silver on a substrate including the step of contacting a
substrate having a layer of a metal that is less electropositive
than silver with an immersion silver plating bath including one or
more sources of silver ions, water and one or more carboxylic
acid-substituted nitrogen-containing heterocyclic compounds,
wherein the silver plating bath has a pH of less than or equal to
4.
[0011] Also provided by the present invention is a method of
manufacturing a printed wiring board including the step of
contacting a printed wiring board substrate having a layer of a
metal that is less electropositive than silver with an immersion
silver plating bath including one or more sources of silver ions,
water and one or more carboxylic acid-substituted
nitrogen-containing heterocyclic compounds, wherein the silver
plating bath has a pH of less than or equal to 4.
[0012] The present invention further provides an immersion silver
plating bath including one or more sources of silver ions, water
and one or more carboxylic acid-substituted nitrogen-containing
heterocyclic compounds. Such compositions are typically free of
ammonia and free of cyanide. The present plating baths may further
contain one or more thickness control agents.
DETAILED DESCRIPTION OF THE INVENTION
[0013] As used throughout this specification, the following
abbreviations shall have the following meanings, unless the context
clearly indicates otherwise: .degree. C.=degrees centigrade;
ca.=circa=approximately; g=gram; L=liter; g/L=grams per liter;
mL=milliliters; wt %=percent by weight; DI=deionized;
cm=centimeters; .mu.in.=microinches; and .mu.m=microns=micrometers
(1 .mu.in.=0.0254 .mu.m).
[0014] 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. 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 clear that such numerical ranges are constrained to add up to
100%.
[0015] 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 bath containing one or more
sources of silver ions, water one or more carboxylic
acid-substituted nitrogen-containing heterocyclic compounds,
wherein the silver plating bath has a pH of less than or equal to
4. The present invention provides a layer of silver deposited from
an immersion plating bath having increased adhesion as compared to
a layer of silver deposited using a conventional immersion silver
plating bath. 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. In addition, the silver deposits obtained
from the present plating baths are bright.
[0016] Any water soluble silver salt may be used as the source of
silver ions in the present plating baths. Suitable silver salts
include, but are not limited to, silver nitrate, silver acetate,
silver sulfate, silver lactate, and silver formate. Typically, the
source of silver ions is silver nitrate. Mixtures of silver salts
may also be used. The one or more sources of silver ions are
typically present in an amount sufficient to provide silver ions in
solution in a concentration of from 0.06 to 32 g/L, more typically
from 0.1 to 25 g/L and still more typically from 0.5 to 15 g/L.
[0017] Carboxylic acid-substituted nitrogen-containing heterocyclic
compounds function as both complexing agents and adhesion
promoters. By "carboxylic acid-substituted nitrogen-containing
hetrocylic compound" is meant any nitrogen-containing heterocyclic
moiety having one or more of its hydrogens replaced by one or more
carboxylic acid groups (--CO.sub.2H). A wide variety of
nitrogen-containing heterocyclic moieties may be used, such as, but
not limited to, pyridine, piperidine, piperazine, morpholine,
pyrrole, pyrrolidine, triazole, and imidazole. Such heterocyclic
compounds may be fused to another ring such as benzotriazole,
benzimidazole, quinoline or isoquinoline, or may be further
substituted, or both. Suitable substitutent groups include without
limitation hydroxy, (C.sub.1-C.sub.10)alkyl,
(C.sub.1-C.sub.10)alkoxy, and halo. Exemplary carboxylic
acid-substituted nitrogen containing heterocyclic compounds
include, but are not limited to, pyridine carboxylic acids,
pyridine dicarboxylic acids, piperidine carboxylic acids,
piperazine carboxylic acids, and pyrrole carboxylic acids. The
pyridine carboxylic acids are particularly useful, such as
picolinic acid, quonolinic acid, nicotinic acid and fusaric acid.
Such compounds are generally commercially available, such as from
Sigma-Aldrich (Milwaukee, Wis.), or may be prepared by methods
known in the literature. The carboxylic acid-substituted
nitrogen-containing compound may be used in a variety of
concentrations but is typically 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. The molar ratio of the
complexing agent to silver ions is generally .gtoreq.1:1, and
typically .gtoreq.1.2:1, more typically .gtoreq.2.0:1, and still
more typically .gtoreq.3.0:1. In general, the total amount of the
one or more complexing agents is typically from 0.1 to 250 g/L, and
particularly from 15 to 220 g/L, more particularly from 25 to 200
g/L, and still more particularly from 40 to 150 g/L.
[0018] It will be appreciated by those skilled in the art that more
than one carboxylic acid-substituted nitrogen-containing
heterocyclic compound may be used. One or more other complexing
agents, i.e. any complexing agent that is not a carboxylic
acid-substituted nitrogen-containing heterocyclic compound, may
also be added to the present plating baths.
[0019] The pH of the present plating baths is typically less than
or equal to 4. Baths having higher pH values may be used, however,
the resulting silver deposit may be significantly thinner.
Preferably, the pH of the bath is <4. A particularly useful pH
range is from 3 to 4. Other useful plating baths have a pH<3,
such as a pH in the range of 2 to 3. A buffering agent may be added
to maintain the pH of the 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(s) from solution, when such
acids or bases are used in amounts sufficient to buffer the pH.
Exemplary buffering agents include, without limitation, citric
acid, tartaric acid, nitric acid, acetic acid and phosphoric
acid.
[0020] 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, thickness controllers, 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, more
typically from 0.1 to 25 g/L, and even more typically 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. When surfactants are
present, nonionic surfactants are preferred.
[0021] A wide variety of anti-tarnish agents for silver may be
used, such as triazoles, tetrazoles, and imidazoles. 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,
more typically from 0.005 to 25 g/L, and still more typically from
0.01 to 10 g/L.
[0022] Grain refiners are typically added to improve the appearance
of the silver deposit. Exemplary grain refiners include, without
limitation, (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 more typically from 0.05 to 100
g/L.
[0023] Deposit thickness may be controlled by the addition of one
or more thickness control agents. Any agent suitable for
controlling the thickness of an immersion silver deposit may be
added to the present plating baths. Such thickness control agents
include rate enhancers and rate inhibitors. Suitable rate enhancers
include, but are not limited to: amino acids such as glycine,
DL-lysine, .beta.-alanine, glutamic acid and DL-aspartic acid;
sulfur-containing compounds such as mercaptodiacetic acid;
hydroxy-substituted aromatic compounds such as 5-methoxyresorcinol;
and azoles such as imidazole, 2-imidazolidone and
2,4-imidazolidinedione. Suitable rate inhibitors include, without
limitation: azoles such as imidazole, 4-phenylimidazole,
1,2,4-triazolo[1,5-.alpha.]pyrimidine, 4-amino-1,2,4-triazole and
2-imidazolidone; and hydroxy-substituted aromatic compounds such as
5-methoxyresorcinol. Suitable thickness control agents are the
azoles, amino acids and the sulfur-containing compounds.
Particularly useful thickness control agents are glycine,
DL-lysine, mercaptodiacetic acid and
1,2,4-triazolo[1,5-.alpha.]pyrimidin- e. It will be appreciated by
those skilled in the art that mixtures of thickness control agents
can be used. Such mixtures can contain two or more rate enhancers,
two or more rate inhibitors or a combination of one or more rate
inhibitors with one or more rate enhancers. Certain thickness
control agents can function as a rate enhancer at one pH and as a
rate inhibitor at another pH. Likewise, certain thickness control
agents can function as a rate enhancer when present in the plating
bath at one concentration and function as a rate inhibitor when
present at a different concentration. Such behavior is well known
to those skilled in the art or can be determined by simple
experimentation. Suitable thickness control agents are generally
commercially available such as from Sigma-Aldrich.
[0024] Typically, the thickness control agents may be present in
the immersion silver plating baths in an amount from 0.01 to 10
g/L, more typically from 0.1 to 5/L and still more typically from
0.5 to 3 g/L.
[0025] 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, ammonia
and ammonium ions (i.e. NH.sub.4.sup.+). More preferably, the
present plating baths contain only one or more carboxylic
acid-substituent nitrogen-containing heterocyclic compounds as
complexing agents, although one or more other complexing agents may
be used in combination with the nitrogen-containing heterocyclic
compound depending upon the desired application.
[0026] 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.
[0027] 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.
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.
[0028] Silver is deposited on any substrate having a metal layer
that is less electropositive than silver by contacting the metal
layer with the present plating baths. Such contact may be by a
variety of means, such as dipping, spraying, flood coating, and the
like. A particularly suitable method of contacting with the present
compositions is spraying in a flood mode.
[0029] 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.
Particularly useful metals are tin, copper or alloys of tin or
copper. 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.
[0030] Prior to silver plating, the metal to be plated is typically
cleaned. Cleaning removes oxides and organic contaminants from the
metal surface as well as resist residues that may remain from
incomplete development of photoresists as well as soldermask
residues from copper surfaces. Such cleaning may be by any suitable
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. The metal layer may be microetched
either before or after the cleaning step, and typically 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.
[0031] Optionally, the metal to be silver plated may be contacted
with a pretreatment composition after cleaning and before contact
with the silver plating bath. Any suitable pretreatment composition
may be used. Preferred pretreatment compositions include one or
more azole compounds, water and a chelating agent.
[0032] A wide variety of azole compounds may suitably be used in
the pretreatment compositions. Suitable azoles include, but are not
limited to, triazoles, benzotriazoles, tetrazoles, imidazoles,
benzimidazoles, indazoles and mixtures thereof. Such azoles may
optionally be substituted.
[0033] Particularly useful azole compounds are benzotriazole,
substituted benzotriazole, imidazole and substituted imidazole, and
more specifically 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. Such azole compounds are generally commercially
available, such as from Sigma-Aldrich (Milwaukee, Wis.) and may be
used without further purification. The azole compounds may be used
in the pretreatment compositions 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. The specific amount of azole compound depends upon
the particular azole chosen and its solubility in the pretreatment
composition.
[0034] Such pretreatment compositions may be alkaline or acidic and
have a pH of 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.
[0035] A wide variety of organic and inorganic acids or organic and
inorganic bases 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, and
nitric acid. Suitable organic acids include, but are not limited
to: alkylsulfonic acids, and arylsulfonic acids. Suitable inorganic
bases include, but are not limited to, alkali metal hydroxides,
alkali metal carbonates, and ammonium hydroxide. Suitable organic
bases include, but are not limited to, tetraalkylammonium
hydroxides, and amines. The acids and/or bases are typically
present in the pretreatment compositions in an amount sufficient to
provide the desired pH.
[0036] Preferably, the pretreatment compositions further include
one or more chelating agents. Such chelating agents may be
monodentate ligands, such as ammonia, cyanide, and pyridine, or
multidentate ligands. Preferably, the chelating agents are the same
as those used in the subsequent immersion silver plating bath.
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. When
a surfactant is used in the present pretreatment compositions, it
is typically present in an amount of at least 0.001 wt %, more
typically at least 0.005 wt %, and still more typically at least
0.01 wt %. Mixtures of surfactants may suitably be used.
[0037] When metal ions are present in the pretreatment
compositions, it is preferred that they are the metal on which
silver is to be deposited. For example, when silver is to be
immersion deposited on copper, it is preferred that any metal ions
present in the pretreatment composition are copper ions.
[0038] 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 more typically up
to 0.05 g/L.
[0039] 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. Other suitable ranges of
contact times include, without limitation, 5 seconds to 10 minutes,
or 10 seconds to 5 minutes.
[0040] 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.
[0041] Silver is deposited according to the present invention by
contacting a metal that is less electropositive than silver with
the present immersion silver plating baths. Such contact may be by
dipping, spraying, flood coating, and the like. When used in
vertical plating equipment, the substrate is typically dipped in
the present silver plating bath. When used in horizontal plating
equipment, the substrate is typically contacted with the present
silver plating bath by spraying or flooding.
[0042] The contact time of the metal with the immersion silver
plating bath is that amount sufficient to deposit a desired
thickness of silver. Typically, the contact time is from 10 seconds
to 15 minutes, more typically from 20 seconds to 15 minutes, and
still more typically from 30 seconds to 12 minutes.
[0043] 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. Other suitable
temperature ranges are from 15 to 60.degree. C., and from 200 to
55.degree. C.
[0044] The silver deposit typically has a thickness of 35 .mu.in.
(0.9 .mu.m) or less, more typically 30 .mu.in. (0.76 .mu.m) or
less, and even more typically 25 .mu.in. (0.64 .mu.m) 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.
[0045] 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, metal foils such as copper
foil, 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 immersion silver plating bath containing one or more sources of
silver ions, water and one or more carboxylic acid-substituted
nitrogen-containing heterocyclic compounds to provide a layer of
silver, wherein the bath has a pH of .ltoreq.4. 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.
[0046] 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 immersion silver plating bath
including one or more sources of silver ions, water and one or more
carboxylic acid-substituted nitrogen-containing heterocyclic
compounds, to form an immersion silver deposit on the tin deposit;
and c) heating the silver-tin deposit to form a tin-silver
alloy.
[0047] 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, but not limited to, dipping, spraying, and flood coating. The
use of a tarnish inhibitor subsequent to plating is not required,
but may optionally be used. Other suitable post-treatments may also
be advantageously used.
[0048] 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, and lead-free solder bumps on wafers, such as tin-silver
and tin-copper-silver solders. The present invention is suitable
for use in vertical or horizontal plating equipment.
[0049] The following examples are expected 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
[0050] An immersion silver plating bath was prepared by combining
50 g/L picolinic acid and 1 g/L silver nitrate in water (final
volume 1L). The pH of the bath was adjusted to 3, The bath
temperature was adjusted to ca. 50.degree. C.
EXAMPLE 2
[0051] The procedure of Example 1 was repeated, except that another
additive was added to the bath. These additives are listed in Table
1. The pH of these baths was again adjusted to 3. The bath
temperature was adjusted to ca. 50.degree. C.
[0052] 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 silver plating bath of Example 1 or
the plating bath containing additional additives of this example
for 10 minutes.
[0053] The thickness of the resulting silver layer on the copper
panels for each silver formulation was determined by X-ray
fluorescence ("XRF") spectroscopy for a number of points on the
panel and the data are reported as thickness ranges in Table 1. The
picolinic acid and additive concentrations and the corresponding
effect on silver plate thickness, obtained from XRF measurements,
are illustrated below.
[0054] 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 were rated as
"failed." Silver deposits having good adhesion were not removed by
the tape were rated as "passed." The adhesion results are also
reported in Table 1.
1TABLE 1 Immersion Amount of Thickness Silver Plating additive
Range Tape Bath Additive (g/L) (.mu.m) Appearance Test Example 1 --
-- 0.091-0.18 Bright Passed Example 2A L-Glutamic Acid 0.21
0.12-0.23 Bright Passed 0.3 0.13-0.20 Bright Passed 0.5 0.073-0.12
Bright Passed Example 2B Glycine 0.1 0.10-0.22 Bright Passed 0.4
0.17-0.27 Bright Passed 1.0 0.19-0.31 Bright Passed Example 2C
DL-Lysine 0.01 0.071-0.20 Bright Passed 0.05 0.12-0.24 Bright
Passed 0.1 0.12-0.28 Bright Passed 0.5 0.38-0.52 Bright Passed 1.5
0.40-0.57 Bright Passed Example 2D Mercaptodiacetic Acid 0.1
0.19-0.27 Bright Passed 0.25 0.14-0.22 Bright Passed 0.30 0.26-0.42
Bright Passed Example 2E Imidazole 0.25 0.053-0.12 Bright Passed
0.50 0.049-0.13 Bright Passed Example 2F 4-Phenylimidazole 0.15
0.30-0.45 Bright Passed 0.25 0.16-0.39 Bright Passed 0.30
0.058-0.10 Bright Passed Example 2G 2-Imidazolidone 0.3 0.10-0.23
Bright Passed 0.5 0.067-0.09 Bright Passed 1.0 0.076-0.11 Bright
Passed Example 2H 1,2,4-Triazolo[1,5- .01 0.078-0.15 Bright Passed
.alpha.]pyrimidine .05 0.061-0.11 Bright Passed 0.25 0.094-0.12
Bright Passed 0.1 0.082-0.13 Bright Passed Example 2I
4-Amino-1,2,4-Triazole 0.31 0.070-0.11 Bright Passed Example 2J
2-Imidazolidone 0.3 0.10-0.23 Bright Passed 0.5 0.067-0.089 Bright
Passed Example 2K DL-Aspartic Acid 0.05 0.090-0.26 Bright Passed
0.10 0.10-0.17 Bright Passed 0.25 0.068-0.10 Bright Passed Example
2L 5-Methoxyresorcinol 0.01 0.087-0.16 Bright Passed 0.02 0.13-0.27
Bright Passed 0.05 0.083-0.13 Bright Passed 0.1 0.088-0.12 Bright
Passed Example 2M .beta.-Alanine 0.3 0.12-0.26 Bright Passed 0.5
0.075-0.24 Bright Passed 1.0 0.11-0.21 Bright Passed Example 2N
Hydantoin 0.3 0.22-0.32 Bright Passed 0.5 0.13-0.24 Bright Passed
1.0 0.18-0.27 Bright Passed
[0055] From these data it can be seen that highly adherent, bright
silver deposits were obtained. EXAMPLE 3
[0056] The procedure of Example 1 was repeated, except that 0.01
and 0.05 g/L 1,2,4-triazolo[1,5-.alpha.]pyrimidine was also added
to the compositions to provide Examples 3A and 3B. The pH of the
baths was adjusted to 3. In each case, the bath temperature was
adjusted to ca. 50.degree. C.
EXAMPLE 4
[0057] The procedure of Example 3 was repeated, except that 0.5 g/L
glycine was added to each of the compositions to provide Examples
4A and 4B. These additives are listed in Table 2. The pH of these
baths was again adjusted to 3. The bath temperature was adjusted to
ca. 50.degree. C.
[0058] 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 silver plating baths of Example 3 or
the plating baths of this example for 10 minutes.
[0059] The thickness of the resulting silver layer on the copper
panels for each silver formulation was determined according to the
procedure of Example 2 and the data are reported in Table 2. The
picolinic acid and additive concentrations and the corresponding
effect on silver plate thickness, obtained from XRF measurements,
are illustrated below. The silver plated copper panels were also
evaluated to determine the adhesion of the silver layer using the
procedure of Example 2. The adhesion results are also reported in
Table 2.
2TABLE 2 Immersion 1,2,4-Triazolo Thickness Silver [1,5-a]pyri-
Glycine Range Appear- Tape Plating Bath midine (g/L) (g/L) (.mu.m)
ance Test Example 3A 0.01 0 0.078-0.15 Bright Passed Example 4A
0.01 0.5 0.056-0.13 Bright Passed Example 3B 0.05 0 0.061-0.11
Bright Passed Example 4B 0.05 0.5 0.043-0.092 Bright Passed
[0060] The above data clearly show that highly adherent, bright
silver deposits were obtained.
EXAMPLE 5
[0061] The procedure of Example 1 was repeated, except that 0.02
and 0.05 g/L 5-methoxyresorcinol was also added to the compositions
to provide Examples 5A and SB. The pH of the baths was adjusted to
3. In each case, the bath temperature was adjusted to ca.
50.degree. C.
EXAMPLE 6
[0062] The procedure of Example 5 was repeated, except that 0.5 g/L
glycine was added to each of the compositions to provide Examples
6A and 6B. These additives are listed in Table 3. The pH of these
baths was again adjusted to 3. The bath temperature was adjusted to
ca. 50.degree. C.
[0063] Copper panels (2.times.6 inches or 5.times.15 cm) were
cleaned and microetched according to the procedure of Example 2.
After contact with the microetching composition, the copper panels
were then submerged in either the silver plating baths of Example 5
or the plating baths of this example for 10 minutes.
[0064] The thickness of the resulting silver layer on the copper
panels for each silver formulation was determined by XRF
spectroscopy according to the procedure of Example 2 and the data
are reported in Table 3. The silver plated copper panels were also
evaluated to determine the adhesion of the silver layer according
to the procedure of Example 2. The adhesion results are also
reported in Table 3.
3TABLE 3 Immersion 5-Methoxy- Thickness Silver Glycine resorcinol
Range Appear- Tape Plating Bath (g/L) (g/L) (.mu.m) ance Test
Example 5A 0 0.02 0.13-0.27 Bright Passed Example 6A 0.5 0.02
0.091-0.25 Bright Passed Example 5B 0 0.05 0.057-0.11 Bright Passed
Example 6B 0.5 0.05 0.060-0.13 Bright Passed
[0065] These data show that bright, highly adherent deposits were
obtained.
EXAMPLE 7
[0066] The procedure of Example 1 was repeated, except that 0.10
g/L 1,2,4-triazolo[1,5-.alpha.]pyrimidine was also added to the
composition to provide Example 7. The pH of the bath was adjusted
to 3. The bath temperature was adjusted to ca. 50.degree. C.
EXAMPLE 8
[0067] The procedure of Example 7 was repeated, except that another
additive was added to the bath. These additives are listed in Table
4. The pH of these baths was again adjusted to 3. The bath
temperature was adjusted to ca. 50.degree. C.
[0068] Copper panels (2.times.6 inches or 5.times.15 cm) were
cleaned and microetched according to the procedure of Example 2.
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
silver plating bath of Example 7 or the plating bath containing
additional additives of this example for 10 minutes.
[0069] The thickness of the resulting silver layer on the copper
panels for each silver formulation was determined by XRF
spectroscopy according to the procedure of Example 2 and the data
are reported in Table 4. The silver plated copper panels were also
evaluated to determine the adhesion of the silver layer according
to the procedure of Example 2. The adhesion results are also
reported in Table 4.
4TABLE 4 Immersion Amount of Silver Additive Thickness Tape Plating
Bath Additive (g/L) Range (.mu.m) Appearance Test Example 7 -- --
0.082-0.13 Bright Passed Example 8A L-Glutamic Acid 0.11 0.083-0.12
Bright Passed Example 8B DL-Aspartic Acid 0.05 0.075-0.14 Bright
Passed 0.10 0.081-0.11 Bright Passed Example 8C Mercaptodiacetic
Acid 0.3 0.14-0.28 Bright Passed Example 8D 2-Imidazolidone 0.51
0.070-0.20 Bright Passed Example 8E DL-Lysine 0.01 0.10-0.14 Bright
Passed 0.05 0.11-0.18 Bright Passed 0.1 0.15-0.22 Bright Passed 1.0
0.17-0.36 Bright Passed Example 8F .beta.-Alanine 0.1 0.082-0.10
Bright Passed
[0070] These data clearly demonstrate that bright, highly adherent
silver deposits were obtained.
EXAMPLE 9
[0071] The procedure of Example 1 was repeated, except that 0.01
g/L 1,2,4-triazolo[1,5-.alpha.]pyrimidine was also added to the
composition to provide Example 9. The pH of the bath was adjusted
to 3. The bath temperature was adjusted to ca. 50.degree. C.
EXAMPLE 10
[0072] The procedure of Example 9 was repeated, except that another
additive was added to the bath. These additives are listed in Table
5. The pH of these baths was again adjusted to 3. The bath
temperature was adjusted to ca. 50.degree. C.
[0073] Copper panels (2.times.6 inches or 5.times.15 cm) were
cleaned and microetched according to the procedure of Example 2.
After contact with the microetching composition, the copper panels
were then submerged in either the silver plating baths of this
example or the plating bath containing additional additives of
Example 10 for 10 minutes.
[0074] The thickness of the resulting silver layer on the copper
panels for each silver formulation was determined according to the
procedure of Example 2 and the data are reported in Table 5. The
silver plated copper panels were also evaluated to determine the
adhesion of the silver layer according to the procedure of Example
2. The adhesion results are also reported in Table 5.
5TABLE 5 Immersion Amount of Silver Additive Thickness Tape Plating
Bath Additive (g/L) Range (.mu.m) Appearance Test Example 9 -- --
0.12-0.18 Bright Passed Example 10A L-Glutamic Acid 0.11 0.081-0.15
Bright Passed Example 10B DL-Aspartic Acid 0.10 0.11-0.17 Bright
Passed Example 10C Mercaptodiacetic Acid 0.10 0.16-0.30 Bright
Passed
[0075] These data show that bright, highly adherent silver deposits
were obtained using the present immersion silver plating baths.
EXAMPLE 11
[0076] A number of silver plating baths were prepared by combining
1 g of silver nitrate and either 15 g, 25 g or 50 g of picolinic
acid with 1 L of DI water. The pH of each bath was then adjusted
using acid or base as appropriate to provide the desired pH. Each
bath was then heated to ca. 50.degree. C.
[0077] Copper panels (2.times.6 inches or 5.times.15 cm) were
cleaned and microetched according to the procedure of Example 2.
Each copper panel was then immersed in one of the plating baths.
The copper panel were contacted with the plating baths for a set
time, after which the panels were removed from the bath, rinsed
with water and dried. The thickness of the resulting silver deposit
on each panel was then determined according to the procedure of
Example 2. The results are reported in Table 6.
6 TABLE 6 15 g/L 25 g/L 50 g/L pH Picolonic Acid Picolonic Acid
Picolonic Acid 3 0.17-0.28 .mu.m 0.19-0.22 .mu.m 0.13-0.18 .mu.m 4
0.029-0.10 .mu.m 0.019-0.025 .mu.m 0.029-0.033 .mu.m 5 0.016-0.020
.mu.m 0.023-0.33 .mu.m 0.022-0.029 .mu.m 6 0.024-0.025 .mu.m
0.029-0.045 .mu.m 0.021-0.027 .mu.m 7 0.013-0.018 .mu.m 0.023-0.028
.mu.m 0.023-0.028 .mu.m
[0078] The above data clearly show that when picolinic acid is used
as the sole complexing agent, much less silver is deposited when
the pH of the bath is greater than 4.
EXAMPLE 12
[0079] A procedure of Example 1 except that the picolinic acid is
replaced with one of the compounds in the amount shown in Table
7.
7 TABLE 7 Compound Amount (g/L) Nicotinic acid 45 Isonicotinic acid
40 Quinolinic acid 55 Fusaric acid 35 Isonipecotic acid 60
Nipecotic acid 65 2,6-pyridine dicarboxylic acid 58
Piperazine-2-carboxylic acid 30 Pyrrole-2-carboxylic acid 35
Pipecolinic acid 50
EXAMPLE 13
[0080] Copper panels are cleaned and microetched according to the
procedure of Example 2, and are then to be contacted with the
plating baths of Example 12, to deposit a layer of silver.
* * * * *