U.S. patent application number 10/763979 was filed with the patent office on 2004-08-05 for electroplating solution containing organic acid complexing agent.
Invention is credited to Hradil, George.
Application Number | 20040149587 10/763979 |
Document ID | / |
Family ID | 27760239 |
Filed Date | 2004-08-05 |
United States Patent
Application |
20040149587 |
Kind Code |
A1 |
Hradil, George |
August 5, 2004 |
Electroplating solution containing organic acid complexing
agent
Abstract
A solution for use in connection with the deposition of one or
more metals on electroplatable substrates. This solution includes
water; a metal ion; and a complexing agent. The complexing agent is
advantageously an organic compound having between 4 and 18 carbon
atoms which includes at least two hydroxyl groups and a five or six
membered ring that contains at least one oxygen atom. The compound
is present in an amount sufficient to complex the metal in the
solution and inhibit oxidation of the metal. In particular, the
complexing agent and metal ion are present in a concentration ratio
of between about 3:1 and 9:1 to reduce or minimize agglomeration of
the substrates during electroplating. If necessary, a suitable pH
adjusting agent can be included in the solution to maintain the pH
of the solution in the range of between about 3.5 to 5.5. At the
preferred pH range, the solution is particularly useful for
electroplating composite articles that have electroplatable
portions and non-electroplatable portions without deleteriously
affecting the non-electroplatable portions.
Inventors: |
Hradil, George; (No.
Scituate, RI) |
Correspondence
Address: |
WINSTON & STRAWN
PATENT DEPARTMENT
1400 L STREET, N.W.
WASHINGTON
DC
20005-3502
US
|
Family ID: |
27760239 |
Appl. No.: |
10/763979 |
Filed: |
January 22, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10763979 |
Jan 22, 2004 |
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PCT/US03/03688 |
Feb 7, 2003 |
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10763979 |
Jan 22, 2004 |
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10277234 |
Oct 22, 2002 |
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60357330 |
Feb 15, 2002 |
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Current U.S.
Class: |
205/252 ;
205/302 |
Current CPC
Class: |
C25D 3/60 20130101; C25D
3/32 20130101 |
Class at
Publication: |
205/252 ;
205/302 |
International
Class: |
C25D 003/32; C25D
003/60 |
Claims
What is claimed is:
1. A solution for use in connection with the deposition of one or
more metals on electroplatable substrates, which comprises: water;
a metal ion in an amount sufficient to provide a metal deposit on a
platable substrate; a complexing agent of an organic compound
having between 4 and 18 carbon atoms which compound includes at
least two hydroxyl groups and a five or six membered ring that
contains at least one oxygen atom, with the compound being present
in an amount sufficient to complex the metal to render it soluble
in the solution and to inhibit oxidation of the metal; and a pH of
the solution in the range of between 3.5 and 5.5, adjusted, if
necessary, by the addition of a suitable pH adjusting agent;
wherein the complexing agent and metal ion are present in a
concentration ratio of between about 2:1 and 9:1 to reduce or
minimize agglomeration of the substrates during electroplating.
2. The solution of claim 1 wherein the complexing agent has the
structure: 2wherein each R is the same or different and is hydrogen
or a lower alkyl group of 1 to 3 carbon atoms, T is R, OR, or
O.dbd.P(OR).sub.2--, Z is O.dbd. or RO--, n is 2-4 and Z can be the
same or different in each occurrence in the compound, and m is 1-3,
or the complexing agent is a soluble salt of such structure.
3. The solution of claim 2 wherein the complexing agent is ascorbic
acid, isoascorbic acid, dehydroascorbic acid, glucoascorbic acid,
galacturonic acid, glucoronic acid, or a salt thereof, or is
derived from a ketogluconate or heptagluconate and is present in an
amount of about 25 to 200 g/l.
4. The solution of claim 1 wherein the metal is tin and is added to
the solution as a stannous alkyl sulfonate salt, a stannous sulfate
salt, a stannous chloride salt, a stannous ascorbate salt, or
stannous oxide and is present in an amount of between about 5 and
100 g/l.
5. The solution of claim 4 further comprising a divalent lead salt
in an amount sufficient to deposit a tin-lead alloy from the
solution.
6. The solution of claim 1 which further comprises a conductivity
salt in an amount sufficient to increase the conductivity of the
solution.
7. The solution of claim 6, wherein the conductivity salt is an
alkali or alkaline metal sulfate, sulfonate, or acetate
compound.
8. The solution of claim 1 further comprising a surfactant in an
amount sufficient to enhance deposit quality and grain
structure.
9. The solution of claim 8 wherein the surfactant is an alkylene
oxide condensation compound and is present in an amount of about
0.01 to 20 g/l.
10. The solution of claim 1 further comprising an agent to promote
anode dissolution.
11. The solution of claim 10 wherein the agent to promote anode
dissolution is as potassium methane sulfonate, ammonium chloride or
a metal sulfide salt.
12. The solution of claim 1, wherein the substrates are composite
articles having electroplatable and non-electroplatable portions,
the pH adjusting agent is an acid or a base and the pH is adjusted
to the range of about 3.5 to 5.5 to enable electroplating of the
electroplatable portions of the articles without deleteriously
affecting the non-electroplatable portions.
13. A solution for use in connection with the deposition of tin or
tin-lead alloys on electroplatable substrates, which comprises:
water; a stannous ascorbate compound; a complexing agent of an
organic compound having between 4 and 18 carbon atoms which
compound includes at least two hydroxyl groups and a five or six
membered ring that contains at least one oxygen atom, with the
compound being present in an amount sufficient to complex tin ions
to render them soluble in the solution and to inhibit oxidation of
the tin ions; when desired, a divalent lead compound in an amount
sufficient to deposit a tin-lead alloy from the solution; and a pH
of the solution in the range of between 3.5 and 5.5, adjusted, if
necessary, by the addition of a suitable pH adjusting agent;
wherein the complexing agent and metal ion are present in a
concentration ratio of between about 2: land 9:1 to reduce or
minimize agglomeration of the substrates during electroplating.
14. The solution of claim 13, wherein the substrates are composite
articles having electroplatable and non-electroplatable portions,
the pH adjusting agent is an acid or a base and the pH is adjusted
to the range of about 3.5 to 5.5 to enable electroplating of the
electroplatable portions of the articles without deleteriously
affecting the non-electroplatable portions.
15. The solution of claim 13, further comprises at least one of a
conductivity salt in an amount sufficient to increase the
conductivity of the solution or a surfactant in an amount
sufficient to enhance deposit quality and grain structure.
16. The solution of claim 13, wherein the stannous ion is present
in an amount of between about 5 and 100 g/l and the complexing
agent is present in an amount of about 25 to 200 g/l and has the
structure: 3wherein each R is the same or different and is hydrogen
or a lower alkyl group of 1 to 3 carbon atoms, T is R, OR, or
O.dbd.P(OR).sub.2--, Z is O.dbd. or RO--, n is 2-4 and Z can be the
same or different in each occurrence in the structure, and m is
1-3, or the complexing agent is a soluble salt of such
structure.
17. The solution of claim 16 wherein the complexing agent is
ascorbic acid, isoascorbic acid, dehydoascorbic acid, glucoascorbic
acid, galacturonic acid, glucoronic acid, glucose-6-phosphate, or a
salt thereof, or is derived from a ketogluconate or heptagluconate
and is present in an amount of about 25 to 200 g/l.
18. The solution of claim 17, wherein the conductivity salt is an
alkali or alkaline metal sulfate, sulfonate, or acetate compound
and the surfactant is an alkylene oxide condensation compound and
is present in an amount of about 0.01 to 20 g/l.
19. A method for electroplating a metal deposit on a substrate
which comprises contacting a plurality of such substrates with the
solution of claim 1 and passing a current though the solution to
provide metal electrodeposits on the substrate without causing
significant agglomeration of such substrates during
electroplating.
20. A method for electroplating a tin or tin-lead deposit on a
composite article that includes electroplatable and
non-electroplatable portions which comprises contacting a plurality
of such articles with the solution of claim 13 and passing a
current though the solution to provide tin or tin-lead
electrodeposits on the electroplatable portions of the articles
without deleteriously affecting the non-electroplatable portions of
the articles.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of International
application PCT/US03/03688 Filed Feb. 7, 2003, a
continuation-in-part of U.S. non-provisional application Ser. No.
10/277,234 filed Oct. 22, 2002 and claims the benefit of U.S.
provisional application No. 60/357,330 filed Feb. 15, 2002.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to the deposition of metals
and more specifically to the deposition of tin or tin-lead alloys
on objects or articles composed of an electroplatable substrate,
such as metal, or a composite article having electroplatable and
non-electroplatable portions. The present invention also describes
a method for inhibiting the fusing of a plurality of such composite
articles during electrodeposition. This is relevant to the
electroplating of small electrical components that have large
surface areas per unit mass and are susceptible to fusing. Of
particular interest herein are electrical components such as
surface mounted capacitors and resistors that have metal portions
as well as ceramic, glass, or plastic portions.
[0003] The size of electronic components has been dramatically
reduced in recent years. This reduction in size has made these
components significantly more difficult to electroplate.
Additionally, many surface mount technology (SMT) components have
sensitive ceramic portions which can be damaged by highly acidic or
highly alkaline solutions. To avoid this problem, neutral or near
neutral pH electroplating solutions are desirable.
[0004] Neutral or near neutral pH tin and tin/lead alloy
electrolytes that are specifically formulated to be compatible with
sensitive ceramic SMTs are described in U.S. Pat. Nos. 4,163,700,
4,329,207, 4,640,746, 4,673,470, 4,681,670 and Japanese patent
application H02-301588. The formulations described in these patents
include complexing agents of components such as citrates,
gluconates, ascorbates or pyrophosphates to complex the tin and/or
lead and render them soluble in the solutions at the elevated pHs
required.
[0005] While Japanese patent application H02-301588 discloses that
the baths should be operable over a wide pH range of 2 to 9, the
examples illustrate near neutral baths (i.e., a pH of 6 to 7.5).
The baths of these examples were found to not be stable at pHs
below 6. Thus, an improvement in the stability of these baths is
desired and necessary.
[0006] The prior art solutions mentioned above have a persistent
problem of component coupling or agglomeration during
electrodeposition. It is quite common when tin or tin alloy plating
small components with flat surfaces that the components tend to
cluster together during plating. It is not uncommon when barrel
plating SMT components that up to 10% of the load may be coupled
(i.e., stuck together). Under some conditions, the entire load
fuses together in large lumps. The extent of this problem depends
on the plating solution composition as well as plating method and
geometry of the components. This problem is particularly pronounced
in tin-lead alloy electroplating.
[0007] The neutral or near neutral pH tin and tin/lead alloy
electrolytes that are specifically formulated to be compatible with
sensitive ceramic SMTs have some utility, but they do not address
the issue of part agglomeration or fusing. This is a particularly
significant problem when the parts are of relatively small size.
The present invention now provides a solution and process that
overcomes this problem.
SUMMARY OF THE INVENTION
[0008] The present invention relates to a solution for use in
connection with the deposition of one or more metals on
electroplatable substrates. This solution comprises water; a metal
ion in an amount sufficient to provide a metal deposit on a
platable substrate; and a complexing agent. The complexing agent is
advantageously an organic compound having between 4 and 18 carbon
atoms which includes at least two hydroxyl groups and a five or six
membered ring that contains at least one oxygen atom. The agent is
present in an amount sufficient to complex the metal and render it
soluble in the solution. In addition, the agent inhibits oxidation
of the metal ion in the solution. When the metal ion has the
ability to exist in the solution in at least two different valence
states, the complex agent prevents oxidation of the metal from a
lower valence state to a higher valence state. If necessary, a
suitable pH adjusting agent can be included in the solution to
maintain the pH of the solution in the range of above 3 but less
than 6, and preferably between 3.5 and 5.5. The complexing agent
and metal ion are present in a weight ratio sufficient to reduce or
minimize agglomeration of the substrates during electroplating. A
preferred ratio range is between about 3:1 and 9:1. At the most
preferred pH range, the solution is particularly useful for
electroplating substrates of composite articles that have
electroplatable portions and non-electroplatable portions without
deleteriously affecting the non-electroplatable portions.
[0009] The complexing agent preferably has one of the following the
structures: 1
[0010] wherein each R is the same or different and is hydrogen or a
lower alkyl group of 1 to 3 carbon atoms, T is R, OR, or
O.dbd.P(OR).sub.2--, Z is O.dbd. or RO--, n is 2-4 and Z can be the
same or different in each occurrence in the structure, and m is
1-3, or the complexing agent is a soluble salt of such structure.
The most preferred compounds include ascorbic acid, isoascorbic
acid (also called erythorbic acid), dehydroascorbic acid,
glucoascorbic acid, galacturonic acid, glucoronic acid, and
glucose-6-phosphate, or a salt thereof. Typical salts include
alkali or alkaline earth metals. These agents are generally present
in an amount of about 25 to 200 g/l.
[0011] It has been found that the ratio of the complexing agent
concentration to the tin and lead concentration is the primary
factor which controls the agglomeration of the substrates or
components during electroplating. Specifically, it is advantageous
to provide only enough complexer to render the tin or lead soluble
and to assure anode dissolution. A substantial excess of complexer
has been found to cause agglomeration of components during plating.
Therefore, one embodiment of the invention relates to the use of an
amount of complexing agent in the solution in a specific ratio to
the tin or lead ions such that only a small excess of free
complexing agent is present in the plating solution. The exact
ratio will depend on the complexer used as well as the solution
pH.
[0012] The solution also may include a conductivity salt in an
amount sufficient to increase the conductivity of the solution.
Preferred conductivity salts are an alkali or alkaline metal
sulfate, sulfonate, or acetate compound. A surfactant may be
included in an amount sufficient to enhance deposit quality and
grain structure. Also, an agent to promote anode dissolution can be
used. This agent may be potassium methane sulfonate, ammonium
chloride or a metal sulfide salt.
[0013] The invention also relates to a method for electroplating a
metal deposit on composite articles that includes electroplatable
and non-electroplatable portions. This method comprises contacting
a plurality of such articles with one of the solutions described
herein and passing a current though the solution to provide metal
electrodeposits on the electroplatable portions of the articles
without deleteriously affecting the non-electroplatable portions of
the articles. Preferred metal electrodeposits are tin metal or
tin-lead alloys and the preferred articles are electronic
components.
BRIEF DESCRIPTION OF THE DRAWING
[0014] Further advantages of the invention are illustrated in the
drawing figure, which is a graph of the results of a washer test,
where the washer count is plotted against the ascorbic acid/tin
concentration ratio.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] It has now been discovered that the fusing of the composite
article electronic components can be largely eliminated by
providing an electrolyte which includes one or more of the
complexing agents disclosed herein at a specific ratio to the tin
or lead contained in the electrolyte. In particular, ascorbic acid
and related compounds are the most preferred for use as such
complexing agents.
[0016] The complexing agents are preferably utilized in solutions
for electroplating tin or tin-lead deposits although they can also
be used in solutions for electroplating other metals, particularly
those metals that have multiple valence states. These complexing
agents help maintain the metals in the solution at one of their
lower valence states, thus facilitating the electroplating step and
avoiding oxidation of the metals which can affect proper operation
of the solution. Stannic tin is also complexed in these
systems.
[0017] Any of the complexing agents of the formulae given above can
be used in this invention. Advantageous complexing agents are
organic acids, with preferred agents including ascorbic acid,
isoascorbic acid, dehydroascorbic acid, glucoascorbic acid,
galacturonic acid, and glucoronic acid. Salts of these acids can
also be used, with the preferred salts being the alkali or alkaline
metal salts. Ketogluconates can be used because these compounds
convert in the bath to ascorbic acid. Heptagluconates are also
suitable since they convert in the solution to similar acidic
species. Any of these agents can be used at a typical amount of
about 25 to 200 g/l. The most preferred complexing agent is
ascorbic acid or an ascorbate salt because these compounds have a
relatively low cost and are readily available.
[0018] Ascorbic acid is included in the solution as simple ascorbic
acid, an ascorbate salt such as sodium or potassium ascorbate,
and/or as an ascorbic acid-metal complex, such as tin ascorbate for
instance. The latter is preferred when it is desired to utilize
other acidic components, such as organic acids or organic acid
salts, to maintain the desired solution pH. The amount of ascorbic
acid present should at a minimum be sufficient to render the metals
present in the solution soluble at the given pH of the solution but
should not greatly exceed this amount. As such the amount of
ascorbic acid required is proportional to the metal concentration.
At a tin concentration of 15 g/l, the preferred ascorbic acid
concentration is about 80 to 120 g/l.
[0019] Tin metal is generally added to the solution as a stannous
alkyl sulfonate salt, a stannous sulfate salt, a stannous chloride
salt, a stannous ascorbate salt, or stannous oxide and is present
in an amount of between about 5 and 100 g/l. When lead metal is
added for the purpose of depositing a tin-lead alloy, it may be
added to the solution as a divalent lead alkyl sulfonate salt,
sulfate salt, chloride salt, or ascorbate salt and is present in an
amount of between about 0.5 and 10 g/l.
[0020] The complexing agent is present in the solution in a
specific concentration ratio to the tin or lead ions such that only
the amount of complexing agent is just sufficient to complex the
metals without providing significant excess amounts. While a small
excess of free complexing agent may be present in the plating
solution, large excesses must be avoided in order to prevent
agglomeration of the substrates during electroplating. The exact
ratio will depend on the complexer used as well as the solution pH.
Typically, the ratio is above 2:1 but is less than 10:1. Useful
ratios range from about 3:1 to no more than 9:1. Also, higher pH
values will require complexor to total metal concentration ratios
of about 5:1 to 8:1 to maintain the metals in solution. The ratio
in any specific case may be established by routine
experimentation.
[0021] Any electroplatable substrates can be plated using the
solutions of the present invention. Generally, these substrates are
made of a metal such as copper, nickel, steel or stainless steel.
In today's commercial products, many parts that require
electroplating are being made in smaller and smaller sizes. In
particular, electronic components are a typical example of such
parts. Furthermore, these parts are composite articles that have
electroplatable and non-electroplatable portions. While the metal
portions are metals or metallic, the non-electroplatable parts are
typically ceramic, glass or plastic. The present solutions are
particularly useful for electroplating such composite articles.
[0022] The electroplating solutions should have a pH above 3 but
less than 6, but preferably is in the range of about 3.5 and 5.5,
and more preferably is about 4 to 5 so that the solution is
compatible with the electronic components that are to be plated.
When the components have metallic and inorganic portions, the
preferred pH range enables metal to be deposited on the metallic
portions without adversely affecting the inorganic portions.
Generally, very high or very low pH solutions will damage the
ceramic portions of the composite articles to be plated.
[0023] These solutions preferably do not contain appreciable
amounts of free acid or free base, although essentially any acid or
base can be used for pH adjustment. Generally, since the solution
is acidic, a base or basic component is utilized to convert free
acid to its corresponding salt. Preferred bases for this purpose
include sodium or potassium hydroxide as well as many others.
[0024] The solution is formulated to be compatible with the
substrates to be plated, and preferably to have no adverse effect
on the substrates. When composite articles that have
electroplatable and non-electroplatable portions are to be plated,
the solution should be formulated to not attack or crack the
non-electroplatable portions of the substrates. A simple test can
be used to determine substrate/solution compatibility. The articles
to be plated can simply be immersed in the proposed solution for a
period of time that is equal to or longer than that which is to be
used for the plating process. The temperature of the solution can
be that which approximates the temperature of the solution during
the plating process, or an elevated temperature can be used for an
accelerated test. The parts are immersed in the solution for a
desired time and then are recovered and weighed to determine weight
loss that occurs due to attack of the articles by the solution
during immersion.
[0025] For example, composite articles used for capacitor
manufacture now are being made with a low-fired ceramic. These
ceramics contain a larger proportion of glass than conventional
ceramics, and are more prone to attack during the plating process.
A simple comparison test was made to determine the compatibility of
various commercially available solutions and with a solution
according to the present invention. The capacitors were placed into
beakers containing equal amounts of these solutions, and weight
loss of the parts after 5 hours immersion was measured. The results
are shown in the following table:
1 Weight loss after 5 hours Solution immersion in the solution (%)
Competitor A (gluconate based 1.0% bath at pH of 3.5) Competitor A
(gluconate based 0.5% bath at pH of 4) Competitor B (citrate based
bath 5.0% with pH of 4.2) Present Invention (ascorbic acid 0.0%
bath with pH of 5)
[0026] This table shows that the present invention has essentially
no effect on the capacitors and is a substantial improvement for
the plating of such components compared to conventional baths. It
is theorized, that limiting the amount of complexor to the amount
required to complex the metal ions in the electroplating bath
minimizes the attack of the metal oxide ceramic substrate by free
complexor in the solution. This is a further advantage of the
current invention.
[0027] A particularly useful device for electroplating such
electrical components is disclosed in U.S. Pat. No. 6,193,858, and
need not be described further herein. To the extent necessary, the
entire content of that patent is expressly disclosed herein by
reference thereto.
[0028] Improvements to the previously patented system have been
disclosed in Published International Application WO02/053809, the
entire content of which is expressly incorporated herein by
reference thereto. The immersion of the plating chamber into the
electrolyte, as disclosed in this application, represents a
significant improvement in that external soluble electrodes can now
be used.
[0029] It has been found that electrolytes which contain the
complexing agents of the present invention are capable of
electrodepositing tin or tin-lead alloys while minimizing the
fusing or coupling of the electroplated parts, as well as without
deleteriously affecting the non-electroplatable portions of the
articles. In this regard, these electrolytes are superior to those
of the prior art, and in particular to baths that are citrate
based. The complexing agent serves to maintain the tin and/or lead
in solution at the pH of the electrolyte. Certain complexing
agents, in particular ascorbic acid, also serves as a stabilizer
for preventing the oxidation of stannous tin to stannic tin.
[0030] L-ascorbic acid (AA) readily converts to L-dehydroascorbic
acid (DAA). In addition, DAA can easily return to AA by the
conversion of two ketone groups to hydroxyl groups on adjacent
carbons with the single bond connecting those atoms being converted
to a double bond. The ease in which AA converts to DAA renders AA a
strong reducing agent. In the plating solutions of the present
invention, AA assists in complexing the tin ions both in their
divalent and tetravalent states. This prevents or at least
minimizes the formation of tin oxides that would precipitate to
form sludge which deleteriously affects the performance of the
solution.
[0031] A preferred solution according to the present invention
comprises water, a divalent tin salt, and ascorbic acid as a
complexing agent, and optionally contains a divalent lead salt, a
salt to increase electrical conductivity, a surface active agent,
or an agent to promote anode dissolution.
[0032] The stannous salts which may be used in this invention
include stannous sulfate, stannous chloride, stannous oxide,
stannous methane sulfonic acid, stannous ascorbate or any other
suitable source of stannous tin. The stannous tin concentration in
the solution maybe from 5 to 100 g/l and most preferably from 10 to
50 g/l. As noted above, the complexing agents of the invention also
complex stannic salts, so that it is possible to add stannic salts
to the solution instead of or along with stannous salts without
concern.
[0033] The lead salts that may be optionally included to provide
tin-lead deposits include any solution soluble divalent lead salt
including, for example, lead methanesulfonate, lead acetate or lead
ascorbate.
[0034] The conductivity of the solution maybe increased if
necessary by the additional of a salt. If a pure tin solution is
desired, a simple salt such as potassium sulfate may be used. If a
tin-lead alloy is desired, potassium methanesulfonate or potassium
acetate would be appropriate. Metal sulfide salts can also be used
if desired. Any of these salts may be used to promote anode
dissolution and assist in electrodeposition.
[0035] Surfactants which are typically utilized in tin or tin alloy
electrolytes may be included in the solution to improve deposit
crystalline structure and improve deposit quality at high current
densities. Preferred surfactants include solution soluble alkylene
oxide condensation compounds, solution soluble quaternary
ammonium-fatty acid compounds, solution soluble amine oxide
compounds, solution soluble tertiary amine compounds or mixtures
thereof. One preferred surfactant is an alkylene oxide condensation
compound and is present in an amount of about 0.01 to 20 g/l. Other
conventional surfactants can be used as there is no criticality to
this component with regard to deposit appearance, although some
additives may perform better than others with regard to coupling of
the articles to be plated. One of ordinary skill in the art can
perform routine testing to determine the most appropriate
surfactants for any particular plating solution.
[0036] When bright deposits are desired, an aromatic aldehyde can
be added in an amount sufficient to act as a brightener. Other
conventional brighteners can instead be used if desired.
[0037] The substrates to be electroplated are preferably those
composite articles that have conductive and non-conductive
portions. While the metal portions are metals or metallic, the
non-conductive parts are typically ceramic, glass or plastic. The
present solutions are particularly useful for electroplating such
composite articles without deleteriously affecting the non-metallic
portions of the articles and without causing agglomeration or
fusing of such parts.
[0038] The pH of the electrolyte is preferably retained in the
range of about 4 to 5.5 when plating on composite substrate
electronic components is desired. The pH can be raised by the
addition of caustic, for example potassium hydroxide, ammonium
hydroxide, sodium hydroxide or the like, or can be lowered with an
acid such as sulfuric or methanesulfonic. An alkane or alkanol
sulfonic acid, such as methanesulfonic acid, is preferred for
tin-lead alloy solutions, since sulfuric acid can generate lead
sulfate which is insoluble in the solution and which would tend to
precipitate. As noted above, a pH of about 4 to 5.5 results in the
strongest inhibition of agglomeration of such metals. Furthermore,
the amount of ascorbic acid should not be in great excess to that
needed to complex the tin in order to inhibit and minimize
agglomeration.
[0039] Typical antioxidants used in tin and tin-lead solutions may
be included in the solution of the present invention (e.g.,
catechol or hydroquinone as disclosed in U.S. Pat. No. 4,871,429),
however ascorbic acid has been found to be effective in preventing
the oxidation of stannous tin to stannic tin in neutral or near
neutral pH plating solutions. As such, ascorbic acid serves the
dual function of acting both as a complexing agent and as an
antioxidant in the present solutions.
EXAMPLES
Example 1
[0040] A pure tin electrodeposit is obtained from the following
solution and under the following electroplating conditions.
2 Tin (as a methanesulfonic acid salt) 15 g/l Ascorbic Acid 100 g/l
(Concentration ratio 6.67:1) Surfactant 0.5 ml/l
[0041] The pH was adjusted with KOH to 4.05.
[0042] The above solution will deposit semi-bright tin at current
densities of up to 20 ASF.
Example 2
[0043] A semi-bright tin-lead deposit is obtained by adding 1.5 g/l
of lead methane sulfonate to the solution of claim 1 and plating at
the same conditions.
3 Tin (as a methanesulfonic acid salt) 15 g/l Lead (as a
methanesulfonic acid salt) 1.5 g/l Ascorbic Acid 100 g/l
(Concentration ratio 6.0) Potassium methanesulfonic acid 40 g/l
Surfactant 0.5 ml/l
[0044] The pH was adjusted with KOH to 4.05.
[0045] This solution will also deposit semi-bright 90% tin at
current densities of up to 20 ASF.
Example 3
[0046] The formulation of Example 1 was used to plate tin on 250
pieces of 8 mm diameter flat washers in a 2.5" by 4"barrell, 140 ml
of 2.5 mm diameter conductive balls were used as the media. The
load was plated at 5A, 6.5V for 15 minutes. At the end of the
plating cycle, none of the flat washers were fused together.
Example 4
[0047] The same plating cycle as in Example 3 was conducted using
an electrolyte of the following formulation:
4 Tin (as a methanesulfonic acid salt) 10 g/l Lead (as a
methanesulfonic acid salt) 1.5 g/l Citric Acid 40 g/l
(Concentration ratio 3.5) Potassium methanesulfonic acid 40 g/l
Surfactant 2.5 ml/l
[0048] The pH was adjusted with KOH to 4.2.
[0049] The load was plated at 5A and 9V for 15 minutes at the end
of the plating cycle, and only 12 pieces were not coupled together.
The remaining pieces were agglomerated in groups of up to 10 pieces
and were difficult to separate. This example clearly demonstrates
the superiority of the solutions of the present invention.
Example 5
Effect of concentration ratio on part agglomeration
[0050] The drawing figure illustrates the effect of the ascorbic
acid to tin concentration ratio on part agglomeration during
electroplating. Ascorbic acid baths having a pH of 4.2, 4.5 and 5
and using different ratios of ascorbic acid to tin were used to
plate tin on 250 pieces of 8 mm diameter flat washers in a 2.5" by
4"barrell, 140 ml of 2.5 mm diameter conductive balls were used as
the media. The load was plated at 5A, 6.5V for 15 minutes. At the
end of the plating cycle, the number of single washer and
agglomerated washers were counted and summed. Therefore, a count of
250 represents that no washers were agglomerated. If all the washer
were agglomerated into groups of two the count would be 125. As can
be seen from the figure, lower ratios of ascorbic acid to tin
result in less agglomeration. This effect is particularly
pronounced at the lower pH values, as it is seen that higher
complexer levels are required to maintain the tin in solution at
higher pH values. Therefore, the higher pH value results are still
consistent with the concept of limiting the amount of complexer in
excess of that required to maintain the tin in solution.
* * * * *