U.S. patent application number 10/277234 was filed with the patent office on 2003-08-28 for electroplating solution containing organic acid complexing agent.
Invention is credited to Hradil, George.
Application Number | 20030159938 10/277234 |
Document ID | / |
Family ID | 27757599 |
Filed Date | 2003-08-28 |
United States Patent
Application |
20030159938 |
Kind Code |
A1 |
Hradil, George |
August 28, 2003 |
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. 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 2 and 10
and preferably to a pH of 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: |
27757599 |
Appl. No.: |
10/277234 |
Filed: |
October 22, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60357330 |
Feb 15, 2002 |
|
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Current U.S.
Class: |
205/118 ;
205/136 |
Current CPC
Class: |
C25D 3/60 20130101; C25D
3/32 20130101 |
Class at
Publication: |
205/118 ;
205/136 |
International
Class: |
C25D 005/02 |
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 if
necessary, a suitable pH adjusting agent to maintain the pH of the
solution in the range of between 2 and 10.
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 if
necessary, a suitable pH adjusting agent to maintain the pH of the
solution in the range of between about 2 and 10.
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 composite
articles that include electroplatable and non-electroplatable
portions which comprises contacting a plurality of such articles
with the solution of claim 1 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.
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 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 and 4,681,670. The formulations
described in these patents include complexing agents of components
such as citrates, gluconates, or pyrophosphates to complex the tin
and/or lead and render them soluble in the solutions at the
elevated pHs required.
[0005] The use of prior art solutions has 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
[0006] 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 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
[0007] 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 between 2 and 10.
At the most 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.
[0008] The complexing agent preferably has one of the following the
structures: 1
[0009] 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.
[0010] 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.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0011] 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. In particular, ascorbic acid
and related compounds are the most preferred for use as such
complexing agents.
[0012] 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.
[0013] 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 are
relatively low cost and are readily available.
[0014] 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, 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. 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 45 to 200 g/l.
[0015] 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.
[0016] The electroplating solutions can have any pH between 2-10,
but preferably is in the range of about 3 and 7.5, and more
preferably is about 4 to 5.5 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.
[0017] 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.
[0018] 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 weight to determine weight
loss that occurs due to attack of the articles by the solution
during immersion.
[0019] 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)
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] 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.
[0031] 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 fusing of such
parts.
[0032] 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.
[0033] 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.
[0034] It has also been discovered that the plating of the
non-conductive portions of composite articles and the fusing of the
composite articles can be minimized or largely eliminated by
fomulating the plating solutions of the present invention to have a
low throwing power. These solutions are specifically formulated to
not deposit metal at low current densities. This is contrary to
conventional practice where electrolytes are formulated to deposit
metal at as broad a current density range as possible. In fact most
conventional tin electroplating solutions go to great lengths to
extend the current density range of electrodeposition by adding
various additives. It has now been found that fusing of parts can
be minimized by limiting the current density range of
electrodeposition to higher current densities. It is believed that
fusing occurs due to metal deposition in the electrolyte film
between two parts in close contact or between the parts and the
current feeder. Because the deposition occurs in a thin film
between two conductive surfaces, it necessarily occurs at low
current densities. By formulating the electrolyte to not plate at
low current densities, fusing can be minimized.
[0035] It has been found that part fusing is closely dependent on
the plating bath composition and selection of the proper grain
refiner or surfactant is critical to minimizing fusing. In this
regard, simple electrolytes which contain only the metal salt and a
complexing agent have been found to electroplate surface mount
technology (SMT) components without fusing. The resulting tin
deposit is a dark gray matte and is not acceptable for commercial
use. When typical surfactants or grain refiners are added to the
electrolyte to improve the quality of the deposit, in almost all
instances very strong fusing is observed. It appears that the
cathode surface polarization resulting from surfactants and grain
refiners strongly influences part fusing. Moreover, it has been
found that electrolytes containing additives which impart limited
coverage at low current densities are less prone to fusing than
additives which impart high coverage at low current densities.
[0036] It is a widely held belief that solutions formulated for
plating of discrete components in barrels or other suitable
equipment must have high throwing power so that current will
penetrate the load and deposit metal within the bulk of the load.
It is also clear that plating speeds are so negligible at low
current densities that no substantial amount of metal is deposited
under these conditions, rather the majority of the metal is
deposited at high current densities near the plating barrel
circumference. Therefore, it has now been found that it is
unnecessary to provide a solution with high throwing power for
plating discrete components in a barrel or other suitable apparatus
as long as the parts themselves have no low current density areas
such as hollows or blind holes.
[0037] Furthermore, a plating solution that does not deposit metals
at low current densities will minimize the deposition of metal on
the non-conductive portions of composite articles. The phenomenon
of metal deposits extending from the conductive termination of the
article onto the non-conductive portions is commonly referred to as
creep or bridging. The extent of this phenomenon is primarily
dependent on the composition of the non-conductive material. For
example, ceramic materials that have some electrical conductivity
are more prone to metal creep than ceramic material that are
perfect insulators. It is believed that creep is caused by
electrical currents leaking from the conductive portions of the
article into the "non-conductive" composite portions during
electrodeposition. By restricting metal deposition to high current
density conditions, the deposition of metal onto the non-conductive
portions can be minimized or eliminated.
[0038] The plating solutions of the present invention can be and
preferably are formulated to have the following attributes and
advantages:
[0039] 1. they will deposit a white matte to a semi-bright
deposit.
[0040] 2. they will not damage the component to be coated.
[0041] 3. they will not deposit metal at low current densities.
[0042] When the parts to be plated are composite articles
containing ceramic or leaded glass portions, acid or alkaline
solutions will damage the ceramic or glass portions during
electroplating. Components such as SMT resistors, inductors and
capacitors are of this type. The pH of the electroplating solutions
for SMT components must be between about 2.5 and 9 in order to
minimize damage to the ceramic or glass portions of the article. To
achieve this pH, the tin must be in a complexed form. Prior art
complexing agents typically include citrates, gluconates, and
pyrophosphates. In order to plate a semi-bright deposit, however,
one or more organic additives are typically used. Most common
additives greatly increase the low current density coverage of the
solution resulting in fusing of the parts to be plated and
overplating of the non-conductive portions of the parts.
[0043] The low current density coverage of an electrolyte may be
reduced by operating the electrolyte at high metal concentrations,
operating at elevated temperatures, selecting additives which do
not increase low current density coverage (LCDC) or which reduce
the LCDC and/or any combinations of the same. For example, a high
metal ion content of at least about 25 g/l is preferred when tin is
the metal to be electroplated. As it has been found that high
temperatures generally increase parts fusing, elevated bath
temperatures are the least desirable method of decreasing the
LCDC.
[0044] Selection of the organic additives in the bath is
particularly important in maintaining the throwing power at a low
level. The most desirable additives can be determined by routine
testing on the specific plating solution of interest. These
additives include conventional surfactants and grain refiners, such
as condensation compounds of organic compounds such as single or
multiple aromatic rings as well as other organic condensation or
reaction products that have dye-like properties but which are not
surfactants. These compounds are generally known in the art and are
simply tested to assure that they do not impart a high throwing
power to the plating solution.
[0045] Other additives may be used in combination with surfactants
and grain refiners to reduce the throwing power of the electrolyte.
Ammonium chloride, ascorbic acid, and M-nitro phenol have been
found to reduce throwing power when used in conjunction with
various surfactants and grain refiners. Clearly, many other
additives would function in this way and the use of these other
additives is a subject of the present invention. One of ordinary
skill in the art can conduct routine testing to determine the best
combination of additives to use or not to use for any particular
electroplating solution.
[0046] When plating composite articles, the present solutions can
be used in the equipment disclosed in U.S. Pat. No. 6,193,858 and
published International Application WO02/053809. The plating
solutions of the present invention can also be used in the rotary
plating apparatus described in U.S. Pat. Nos. 5,487,824 and
5,565,079 with improved results, since the deposition of tin on the
current feeder ring is substantially reduced, resulting in
significant reductions in maintenance required to replace and strip
the current feeder.
[0047] Therefore, the use of electrolytes with reduced LCDC in
rotary plating apparatus is also a subject of the present
invention. The use of the current invention is also advantageous
when using plating barrels since less metal will be deposited on
the danglers, and metal creep and parts fusing are reduced.
Therefore, the use of the LCDC electrolyte in barrel plating is
also a subject of the current invention.
[0048] Although the present invention is particularly advantageous
when plating composite components without media, the use of the
present invention to plate discrete articles mixed with media has
the significant advantages of reducing plating on the current
feeder and reducing or eliminating metal deposition on the
non-conductive portions of the composite articles.
[0049] A useful method for testing an electrolyte for LCDC is to
use a standard 265 ml hull cell test. The standard procedure for
running a hull cell is used. Typical conditions would be 1 A for 5
minutes, 0.5 A for 5 minutes or 0.25 A for 5 minutes, each using
paddle agitation. Hull cell panels prepared at 1 A have LCDC if the
back of the hull cell panel is largely unplated except for portions
extending less than 1 cm from the panel edge. Additionally,
electrolytes which are most preferred will have an unplated portion
on the front side low current density edge. This unplated portion
may be from 1/8" to {fraction (3/4)}" of an inch wide. Electrolytes
which exhibit this type of hull cell panel results normally are
much less prone to fusing than electrolyte which produce
significant plating on the back side of the hull cell panel. The
limiting current density at which metal will not be deposited may
be measured by preparing a hull cell panel at 0.25 A and
determining the current density at the edge of the metal deposit
using the appropriate Hull cell panel scale.
[0050] Additionally, when electrolytes with LCDC are used in the
SBE apparatus to late SMTs without media, it is generally found
that the current feeder is largely unplated by tin at the end of
the plating cycle and that the parts to do not fuse to the current
feeder. In contrast, when commercially available neutral tin
plating electrolyte is used to plate SMTs in the SBE without media
the parts seize within three minutes of the start of plating and
the current feeder is found to be completely coated with tin.
Therefore, the use of a tin or tin alloy electrolyte with LCDC is
necessary for successful plating of SMTs without media in the SBE
apparatus.
EXAMPLES
Example 1
[0051] A pure tin electrodeposit is obtained from the following
solution and under the following electroplating conditions.
2 Ascorbic Acid 100 g/l Tin (as a methanesulfonic acid salt) 15 g/l
Surfactant 0.5 ml/l pH adjusted with KOH to: 4.05
[0052] The above solution will deposit semi-bright tin at current
densities of up to 20 ASF.
Example 2
[0053] 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 Ascorbic Acid 100 g/l Tin (as a methane sulfonic acid salt) 15
g/l Lead (as a methanesulfonic acid salt) 1.5 g/l Potassium
methanesulfonic acid 40 g/l Surfactant 0.5 ml/l pH adjusted with
KOH to: 4.05
[0054] This solution will also deposit semi-bright 90% tin at
current densities of up to 20 ASF.
Comparative Example
[0055] 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 5 A, 6.5V for 15 minutes. At the end of the
plating cycle, none of the flat washers were fused together.
[0056] The same plating cycle was conducted using an electrolyte of
the following formulation:
4 Citric Acid 40 g/l Tin (as a methanesulfonic acid salt) 10 g/l
Lead (as a methanesulfonic acid salt) 1.5 g/l Potassium
methanesulfonic acid 40 g/l Surfactant 2.5 ml/l pH adjusted with
KOH to: 4.2
[0057] The load was plated at 5 A 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.
* * * * *