U.S. patent application number 15/423324 was filed with the patent office on 2017-05-25 for method for electroless plating of palladium phosphorus directly on copper, and a plated component therefrom.
The applicant listed for this patent is UYEMURA INTERNATIONAL CORPORATION. Invention is credited to JON E. BENGSTON.
Application Number | 20170145567 15/423324 |
Document ID | / |
Family ID | 57943551 |
Filed Date | 2017-05-25 |
United States Patent
Application |
20170145567 |
Kind Code |
A1 |
BENGSTON; JON E. |
May 25, 2017 |
METHOD FOR ELECTROLESS PLATING OF PALLADIUM PHOSPHORUS DIRECTLY ON
COPPER, AND A PLATED COMPONENT THEREFROM
Abstract
A solution comprising a palladium compound and a
polyaminocarboxylic compound has been found to be suitable as a
bath for electroless plating of palladium onto copper. Use of such
a solution produces a plated component comprising a copper surface
and a palladium plated coating having a thickness of between 0.01
micrometers (.mu.m) and 5 .mu.m. A method for electroless plating
of palladium onto a copper surface of a component includes
preparing a bath having a palladium compound and a
polyaminocarboxylic compound. The copper component is submerged in
the bath to plate a palladium layer on the copper surface of the
component. The component resulting from the plating method has a
palladium layer plated on the copper surface.
Inventors: |
BENGSTON; JON E.; (West
Hartford, CT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
UYEMURA INTERNATIONAL CORPORATION |
Ontario |
CA |
US |
|
|
Family ID: |
57943551 |
Appl. No.: |
15/423324 |
Filed: |
February 2, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14819150 |
Aug 5, 2015 |
9603258 |
|
|
15423324 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C23C 18/1637 20130101;
C23C 18/44 20130101; C23C 18/1651 20130101; H05K 2201/0338
20130101; H05K 1/09 20130101; H05K 1/118 20130101; C23C 18/54
20130101; H05K 3/187 20130101; H05K 3/244 20130101 |
International
Class: |
C23C 18/44 20060101
C23C018/44; H05K 1/09 20060101 H05K001/09; C23C 18/16 20060101
C23C018/16; H05K 3/18 20060101 H05K003/18 |
Claims
1. A method for electroless plating of palladium onto a copper
surface of a component, comprising: preparing a bath having a
palladium compound; a poly aminocarboxylic compound; hypophosphite
or a derivative thereof a pH adjuster; a complexing agent; a
reaction stabilizing mixture of phosphite at a concentration
between 0.1 grams per liter (g/l) and 5 g/l, nitrite at a
concentration between 0.1 g/l and 5 g/l, copper at a concentration
between 0.1 milligrams per liter (mg/l) and 10 mg/l, bismuth at a
concentration between 0.1 mg/l and 10 mg/l, and lead at a
concentration between 0.1 mg/l and 5 mg/l; and submerging the
component in the bath to plate palladium directly on the copper
surface of the component.
2. The method of claim 1, wherein the palladium compound is present
in the bath at a concentration between 0.2 grams per liter (g/l)
and 10 g/l and the polyaminocarboxylic compound includes at least
one of an ethylene diamine tetraacetic acid (EDTA) or a derivative
thereof at a concentration between 1 g/l and 20 g/l.
3. The method of claim 1, wherein preparing the bath includes:
dissolving the polyaminocarboxylic compound and the reaction
stabilizer in water to create a mixture; dissolving the palladium
compound in the mixture; adjusting pH of the mixture by dissolving
the pH adjuster into the mixture; and dissolving the hypophosphite
or a derivative thereof in the mixture.
4. The method of claim 3, wherein the pH of the mixture is adjusted
to be between 5.0 and 10.0.
5. The method of claim 1, further comprising cleaning the copper
surface of the component to be plated by palladium; and applying a
palladium activator to the copper surface of the component.
6. The method of claim 1, further comprising plating a gold layer
on the plated palladium layer using immersion or electroless
plating.
7. The method of claim 1, further comprising plating a silver layer
on the plated palladium layer using immersion or electroless
plating.
8. The method of claim 1, further comprising keeping the copper
surface of the component submerged in the bath until the palladium
layer has a thickness between 0.01 micrometers (.mu.m) and 2
.mu.m.
9. The method of claim 1, wherein the polyaminocarboxylic compound
includes ethylene diamine tetraacetic acid (EDTA) or a derivative
thereof at a concentration between 1 g/l and 20 g/l.
10. The method of claim 1, wherein the pH adjuster and the
complexing agent are the same and include ethylene diamine.
11. The method of claim 1, wherein the complexing agent is at a
concentration between 1 g/l and 20 g/l.
12. The method of claim 1, wherein the palladium compound includes
at least one of a palladium sulfate, a derivative of palladium
sulfate, a palladium chloride, a derivative of palladium chloride,
palladium acetate, a derivative of palladium acetate, palladium
tetramine sulfate or a derivative of palladium tetramine
sulfate.
13. A component having a copper surface with a palladium layer
plated directly thereon by a method comprising: preparing a bath
having a palladium compound; a polyaminocarboxylic compound;
hypophosphite or a derivative thereof a pH adjuster; a complexing
agent; a reaction stabilizing mixture of phosphite at a
concentration between 0.1 grams per liter (g/l) and 5 g/l, nitrite
at a concentration between 0.1 g/l and 5 g/l, copper at a
concentration between 0.1 milligrams per liter (mg/l) and 10 mg/l,
bismuth at a concentration between 0.1 mg/l and 10 mg/l, and lead
at a concentration between 0.1 mg/l and 5 mg/l; and submerging the
component in the bath to plate palladium directly on the copper
surface of the component.
14. The component of claim 13, wherein the palladium compound was
present in the bath at a concentration between 0.2 grams per liter
(g/l) and 10 g/l and the polyaminocarboxylic compound includes at
least one of an ethylene diamine tetraacetic acid (EDTA) or a
derivative thereof at a concentration between 1 g/l and 20 g/l.
15. The component of claim 13, wherein preparing the bath includes:
dissolving the polyaminocarboxylic compound and the reaction
stabilizer in water to create a mixture; dissolving the palladium
compound in the mixture; adjusting pH of the mixture by dissolving
the pH adjuster into the mixture; and dissolving the hypophosphite
or a derivative thereof in the mixture.
16. The component of claim 13, further comprising a layer of gold
or silver plated on the palladium layer by immersion or electroless
plating.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a division of application Ser. No. U.S.
14/819,150 filed Aug. 5, 2015 for COMPOSITION AND METHOD FOR
ELECTROLESS PLATING OF PALLADIUM PHOSPHORUS ON COPPER, AND A COATED
COMPONENT THEREFROM.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to plating palladium on copper
and, more particularly, to a method of electroless plating of
palladium phosphorus alloy directly onto a copper surface, and the
product resulting from the plating method.
[0004] 2. Description of Related Art
[0005] Metal plating is used in many industries for various
reasons. For example, metal plating is used in the electronics
industry to increase the ability to solder to a base metal, to
increase resistance to corrosion, to alter conductivity, and for
radiation shielding, among others. Various metals are used for
plating and each metal has its own attributes. For example, nickel
is relatively easy to plate onto a base metal and is relatively
inexpensive; however, it has a relatively low oxidation resistance
and malleability (meaning it may have a tendency to crack under
stress). On the other hand, gold is relatively oxidation resistant
yet is relatively expensive and has a tendency to tarnish from
copper diffusion into the gold forming an oxidized layer when
plated directly on copper. Palladium is highly corrosion resistant,
is relatively easy to solder to, and enables gold wire bonding when
used in conjunction with gold; however, it is difficult to plate
the phosphorus alloy directly on copper surfaces.
[0006] In some situations, multiple materials are plated on a
substrate in order to achieve a desired set of qualities. For
example, a copper surface may receive a nickel plating with a gold
plating on the nickel plating. This combination allows the final
coating to have the desirable properties of gold, and the nickel
plating blocks the copper diffusion and the resulting tarnish of
the gold. However, the combination of gold plating and nickel
plating is not typically suitable for gold wire bonding with the
exception of gold thicknesses well above 0.1 .mu.m, which can
undesirably increase the plating costs. Palladium can be plated
between the nickel and the gold layers, increasing the suitability
of the plating for wire bonding. This process can be relatively
expensive and time consuming as it utilizes three distinct
materials and plating processes. There are additional drawbacks.
For example, nickel plating is traditionally both thick and rigid,
making it unsuitable for flexible printed circuit boards (PCBs) as
well as when distances between components to be plated are
relatively small. Money, time, and real estate costs could be
reduced and usability increased by eliminating the nickel plating
and plating the palladium phosphorous directly on the copper.
However, a suitable method for plating palladium phosphorous
directly onto copper is not known in the art.
[0007] Electrolytic plating is one available plating method but it
is typically unsuitable for electronic components. Plating by
electrolytic process requires the base metal (i.e., the copper and
a second metal part) to each be submerged into a solution of water
soluble metal salts. A current is applied to the base metal and the
second metal part, making the base metal a cathode and the second
metal part an anode. The electrical current reduces the ionic metal
in the solution, forming a solid metal coating on the cathode (the
base metal). However, electrolytic plating has a significant
drawback--any surface to be coated must be made cathodic. Thus,
electrolytic plating is unsuitable for coating any component having
multiple insulated surfaces to be plated.
[0008] Immersion plating is another plating method used in the art,
yet is typically unsuitable for plating palladium directly on
copper. Immersion plating includes submerging a base metal into a
solution having electrolytes and metal salts. When the base metal
is submerged, the electrolytes corrode the surface of the base
metal, making the surface electrically cathodic relative to the
dissolved metal. The metal salts in the solution are then reduced,
forming a metal plate on the base metal. A first limitation with
immersion plating is that the plating is limited to a relatively
thin layer because as soon as the surface of the base metal is
coated, the reaction can no longer occur. Another limitation with
immersion plating, especially of palladium onto copper, is that the
resulting palladium coating is granular and porous, making it
unsuitable for soldering and wire-bonding.
[0009] Some attempts have been made for electroless plating
(autocatalytic plating) of palladium phosphorus onto copper;
however, no consistent or commercially suitable results have yet
been obtained. Electroless plating is performed by creating a bath
including a metal salt and a reducing agent. When the reducing
agent is exposed to a catalyst (typically the surface of the
substrate to be plated or a film thereon), the reducing agent
donates electrons, causing the metal salt to precipitate on to the
surface of the substrate. Accordingly, for electroless plating to
work, the surface of the substrate must be catalytic. Nickel,
palladium, and cobalt are examples of known catalysts for
electroless palladium phosphorus plating. Copper, however, is not a
catalyst for palladium plating, and in fact is a catalytic poison,
meaning that it effectively prevents the electroless plating
reaction.
[0010] There is an unfulfilled need for a component that has
palladium phosphorus plated directly on a copper surface, a method
for suitably plating palladium onto copper, and a bath composition
for performing the plating. The inventor has found a solution to
this unfulfilled need.
SUMMARY OF THE INVENTION
[0011] A solution comprising a palladium compound, hypophosphite
and a polyaminocarboxylic compound, such as at least one of an
ethylene diamine tetraacetic acid (EDTA) or a derivative thereof,
has been found to be suitable as a bath for electroless plating of
palladium phosphorus alloy onto surfaces of metals and, in
particular, for electroless plating of palladium phosphorus alloy
onto copper surfaces.
[0012] Use of such a solution produces a plated component
comprising a copper surface and a palladium phosphorus plated
coating having a thickness of between 0.01 micrometers (.mu.m) and
5 .mu.m.
[0013] A method for electroless plating of palladium phosphorus
onto a copper surface of a component includes preparing a bath
having a palladium compound and a polyaminocarboxylic compound,
such as at least one of an ethylene diamine tetraacetic acid (EDTA)
or a derivative thereof. The copper surface of the component is
submerged in the bath to plate a palladium phosphorus layer on the
copper surface of the component.
[0014] The component resulting from the plating method has
palladium phosphorus plated on the copper surface.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The exact nature of this invention, as well as the objects
and advantages thereof, will become readily apparent upon
consideration of the following specification in conjunction with
the accompanying drawings in which like reference numerals
designate like parts throughout the figures thereof and
wherein:
[0016] FIG. 1 is a step diagram of a system for plating palladium
onto copper including a suitable bath and a printed circuit board
having copper components;
[0017] FIG. 2 is a diagrammatic illustration of forming the bath
used in the system of FIG. 1;
[0018] FIG. 3 is a diagrammatic illustration of a method of plating
palladium directly on copper, according to the present invention;
and
[0019] FIG. 4 illustrates a copper component with palladium plated
directly on the copper and gold plated directly on the palladium
layer.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] Electroless plating of palladium phosphorus includes
dissolving a palladium salt and a hypophosphite salt (PO2.sup.-3)
into water or another liquid and then inserting a base metal to be
coated. The hypophosphite salt acts as the reducing agent in the
mixture. When the hypophosphite salt is exposed to a catalyst
(typically the base metal or a film thereon), a reaction begins
between the catalyst and the hypophosphite salt whereby the
hypophosphite salt donates electrons, resulting in the dissolved
palladium precipitating, or plating, on the base metal. Suitable
catalysts for hypophosphite include nickel, palladium, and cobalt.
Copper, however, is a catalytic poison, preventing the reaction
(thus the plating), from occurring.
[0021] For this reason, it has been believed that electroless
plating is an unsuitable method for plating palladium phosphorus
onto copper. However, after extensive effort, it has been
discovered that the hypophosphite will react with the palladium
when the bath includes ethylene diamine tetraacetic acid (EDTA)
mixed with the palladium and hypophosphite. The use of EDTA in an
electroless plating bath of palladium and hypophosphite allows the
direct plating of palladium onto copper.
[0022] FIG. 1 illustrates a system 100 for electroless plating of
palladium directly onto copper. The system includes a printed
circuit board (PCB) 101 having a substrate 103 and copper
components 104, and a bath 102 within which the PCB 101 with copper
components 104 is immersed. Although discussion herein is directed
to plating of components on a circuit board, one skilled in the art
will realize that the bath 102 can be used to electrolessly plate
palladium onto any object having one or more copper surfaces, such
as an integrated circuit, a switch, an electrical contact, a lead
of an electrical component (such as a resistor, capacitor, etc.),
or the like. In various embodiments, a bath similar to the bath 102
may also be used to electrolessly plate palladium phosphorus onto
copper alloys, nickel, and/or other metals.
[0023] The PCB 101 includes the copper components 104 coupled to
the substrate 103, which insulates the copper components 104. The
copper components may include a pad 106, an electrical contact 108,
a through-hole 110, or any other electrical component comprising
copper. Each of the copper components 104 have an exposed copper
surface to which palladium is deposited. Some of the copper
components 104 may be electrically connected together, such as the
pad 106 and the electrical contact 108, and others may be insulated
from each other, such as the pad 106 and the through-hole 110.
[0024] The bath 102 preferably includes a mixture of palladium 112,
EDTA 114, hypophosphite 116, a pH adjuster 118, a complexing agent
120, a reaction stabilizer 122, and water 124. The palladium 112
may include a soluble palladium compound, such as a palladium salt
or a derivative thereof. For example, the palladium 112 may include
palladium sulfate, palladium chloride, palladium acetate or
palladium tetraamine sulfate. Palladium tetraamine sulfate is the
preferred palladium 112. A suitable concentration of the palladium
112 is between 0.2 grams per liter (g/l) and 2 g/l, with a
preferred concentration of about 1 g/l. When used in this context,
"about" refers to the referenced value .+-.30 percent (%) of the
referenced value or .+-.40% of the referenced value. The
electroless plating of palladium will also occur if the
concentration of palladium 112 is above 2 g/l; however, palladium
112 is a precious metal so costs increase as the concentration of
palladium 112 increases.
[0025] The hypophosphite 116 may include any soluble hypophosphite
or derivate thereof, such as a hypophosphite salt. For example, the
hypophosphite 116 may include sodium hypophosphite, potassium
hypophosphite, ammonium hypophosphite, lithium hypophosphite,
magnesium hypophosphite, calcium hypophosphite, the triprotinated
acid form of hypophosphite, and/or any derivate of the above.
Sodium hypophosphite is preferred. A suitable concentration of the
hypophosphite 116 is between 1 g/l and 10 g/l, with a preferred
concentration of about 5 g/l. Concentrations of hypophosphite 116
less than 1 g/l and/or greater than 10 g/l may also render the bath
102 suitable for the plating.
[0026] The EDTA 114 can include any water soluble
polyaminocarboxylic compound, such as diethylene triamine
pentaacetic acid, N-(Hydroxyethyl)-ethylenediamine triactetic acid
and nitrilotriacteic acid; however. EDTA 114 is typically preferred
and may be disodium EDTA, which is the preferred EDTA 114. A
suitable range for the concentration of the EDTA 114 is between 1
g/l and 20 g/l, with a preferred concentration of about 2 g/l.
Concentrations of the EDTA 114 less than 1 g/l and/or greater than
20 g/l may also render the bath 102 suitable for the plating.
[0027] The pH adjuster 118 can be any compound usable to adjust the
pH of the bath 102. In some embodiments, the pH adjuster 118 may
also function as the complexing agent 120. Alternatively, a
separate additional complexing agent 120 may be added to the bath
102. Preferred pH adjusters include sulfuric acid as the acid and
ammonium hydroxide as the alkaline. A preferred complexing agent
120 includes ethylene diamine. The ethylene diamine is an alkali
and may be used as both the pH adjuster 118 and the complexing
agent 120. A suitable range of the pH adjuster 118/complexing agent
120 is between 1 g/l and 20 g/l, with a preferred concentration of
the pH adjuster 118/complexing agent 120 being about 2 g/l.
Concentrations of the pH adjuster 118/complexing agent 120 less
than 1 g/l and/or greater than 20 g/l may also render the bath 102
suitable for the plating.
[0028] It is desirable for the bath 102 to have a pH between 5 and
10. It has been discovered that a preferred pH is between 7.4 and
9. In order to set the pH of the bath 102, pH adjuster(s) 118 are
added to the bath 102.
[0029] The reaction stabilizer 122 may include any compound usable
to stabilize the bath 102. For example, the reaction stabilizer 122
can include one or more of lead, tin, indium, bismuth, cadmium,
selenium, antimony, arsenic, copper, nickel, tellurium, phosphite,
iodide, iodate, bromide, bromate, nitrate, or nitrite. The reaction
stabilizer 122 is used to inhibit the catalyst from promoting the
reaction and developing solid palladium prior to immersion of the
PCB 101.
[0030] A preferred reaction stabilizer includes phosphite, nitrate,
copper, lead, and bismuth. The phosphite may have a concentration
between 0.1 g/l and 5 g/l, with a preferred concentration of about
1 g/l. The nitrate may have a concentration between 0.1 g/l and 5
g/l, with a preferred concentration of about 1 g/l. The copper may
have a concentration between 0.1 milligram per liter (mg/l) and 10
mg/l, with a preferred concentration of about 4 mg/l. The bismuth
may have a concentration between 0.1 mg/l and 10 mg/l, with a
preferred concentration of about 4 mg/l. The lead may have a
concentration between about 0.1 mg/l and 5 mg/l, with a preferred
concentration of about 0.75 mg/l.
[0031] With reference to FIGS. 1 and 2, a preferred method 200 for
forming the bath 102 of FIG. 1 is shown. One skilled in the art
will realize that the method 200 is not limited to the order of the
steps shown in FIG. 2. In step 202, a mixture is created by
dissolving the EDTA 114 and the reaction stabilizer 122 into the
water 124. By including the reaction stabilizer 122 in the bath 102
prior to the palladium 112, the palladium 112 is prevented from
premature reactions.
[0032] In step 204, the palladium 112 is dissolved into the mixture
formed in step 202. In step 206, the pH of the mixture is adjusted
by adding one or more pH adjusters 118 to the mixture. In a
preferred embodiment, the pH adjuster 118 and the complexing agent
120 may be the same compound and can be dissolved in either step
202 or step 206. In step 208, the hypophosphite 116 is added to the
mixture, completing the bath 102.
[0033] The water 124 is heated to a temperature between 100 degrees
Fahrenheit (100.degree. F.) and 150.degree. F., preferably about
120.degree. F., prior to insertion of any components or after one
or more components have been added. This provides benefits over
other plating methods which usually require more heat. For example,
electroless plating of nickel typically operates at about
190.degree. F. Over time, a greater energy cost is incurred at such
a high temperature.
[0034] With reference now to FIGS. 1 and 3, a preferred method 300
for plating the copper components 104 is shown. In step 302, the
surfaces of copper components 104 are cleaned. For example, the PCB
101 may be immersed into a cleaner for a period of time. The
cleaner will provide detergency for removing any contaminants from
the surfaces of the copper components 104. The cleaner may include
commercial cleaners and preferably includes an acidic cleaner.
[0035] In step 304, the surfaces of the copper components 104 are
plated with a palladium activator, such as a discrete film of
palladium. The palladium activator is preferably a non-halogen
activator. The palladium activator is the initial catalyst in the
electroless plating reaction. Stated differently, the palladium
activator allows the surfaces of the copper components 104 to react
with the hypophosphite 116 and become plated. Preferably, the
palladium activator is plated on the copper by immersion plating,
however, other methods may be used to plate the palladium activator
on the copper.
[0036] In step 306, palladium is plated on the surfaces of the
copper components 104. The plating is accomplished by submerging
the PCB 101 into the bath 102 for a predetermined amount of time.
Initially, the hypophosphite 116 reacts with the palladium
activator, allowing the palladium 112 to plate the surfaces of the
copper components 104. After a first layer of palladium is plated
on the copper surfaces, the palladium continues to react with the
hypophosphite 116, causing additional palladium 112 to plate on the
copper components 104.
[0037] The reaction will continue to occur until either the PCB 101
is removed from the bath 102 or no more palladium 112 exists in the
bath 102. The thickness of the palladium layer is controlled by
submerging the PCB 101 in the bath 102 until the palladium is at
the desired thickness. While a variety of thicknesses can be
achieved, the desired thickness is between 0.01 micrometers (.mu.m)
and 2 .mu.m.
[0038] In optional step 308, gold may be plated onto the palladium
layer using an immersion method. Alternatively, silver may be
plated, instead of, or in addition to, the gold. The gold may be
applied at a thickness of between 0.01 .mu.m and 0.1 .mu.m. Because
of the nature of immersion plating, it is difficult to achieve a
gold layer having a thickness much greater than 0.1 .mu.m. In some
embodiments the gold or silver may be plated using other methods
such as electroless plating, thus allowing the gold and/or sliver
layers to have a thickness beyond this range.
[0039] With reference to FIG. 4, the pad 106 of the PCB 101 has a
palladium layer 400 and a gold layer 402. The pad 106 includes
copper portions that may be printed on, or otherwise coupled to,
the insulating substrate 103. The pad 106 also includes a surface
401 that was exposed prior to the plating process. Palladium may be
plated directly on the pad 106. That is, the palladium is plated on
the surface 401 directly, without other metals therebetween, using
a method similar to the method 300 of FIG. 3. The palladium layer
400 may have a thickness 404 that is between 0.01 .mu.m and 2
.mu.m.
[0040] In other preferred embodiments, the palladium layer 400 may
include one or more transverse pores 408 (i.e., pores extending
through the palladium layer 400) and/or one or more masked or
bridged pores 410 (i.e., pores that do not extend through the
palladium layer 400). Accordingly, the palladium layer 400 may have
a porosity value that represents the percentage of the volume of
the pores 408, 410 compared to the total volume of the palladium
layer 400 (including the volume of the pores 408, 410). Porosity as
used herein may be calculated using: the void caused by the
transverse pores 408 only, the void caused by the transverse pores
408 and the masked or bridged pores 410, or a combination of these
two methods. Plating palladium phosphorus using the method 200 of
FIG. 2 with a polyaminocarboxylic compound will result in a layer
of palladium having a lower porosity value than a palladium layer
electrolessly plated using a method that does not include
polyaminocarboxylic compound using any method for calculating
porosity.
[0041] This relatively low porosity value provides several
advantages. If the porosity of a plated layer is too great,
materials on either side of the layer may undesirably react with
one another. For example, if the porosity of the palladium layer
400 is above a predetermined value, the copper of the pad 106 may
react with the gold layer 402, allowing corrosion of the pad 106
and/or the gold layer 402. Plating of palladium onto copper, prior
to the present invention, resulted in a porosity that was too
great, and allowing the gold layer to corrode relatively
quickly.
[0042] The palladium layer 400 may have gold plated on its surface
403 using immersion plating. The gold plating 402 may have a
thickness 406 of between 0.01 .mu.m and 0.1 .mu.m. The gold plating
402 increases the suitability of the pad 106 for soldering and gold
wire bonding. Greater gold thicknesses are possible using an
electroless gold plating method.
[0043] The pad 106 with the palladium layer 400 and the gold layer
402 provides advantages over components having a plated nickel
layer, a palladium layer, and a gold layer. For instance, nickel
has magnetic properties which will interfere with high frequency
signals. Elimination of nickel plating from the pad 106 reduces
this problem of interference with high frequency signals passing
through the pad 106.
[0044] Components having nickel, palladium, and gold layers have a
greater thickness than pad 106 because of the additional layer and
the greater thickness of the nickel layer. Additional thickness is
generally undesirable, especially so for particular applications.
For example, a flexible PCB, such as a printed cable, is
occasionally flexed. Due to the thickness of traditional plating
(and the relatively low malleability of nickel), this flexing can
result in cracking of the nickel layer. However, the reduced
thickness of the combination of the palladium layer 400 and the
gold layer 402 (and the greater malleability of palladium and gold)
reduce the likelihood of cracking due to flexure. Additionally,
electrical components are constantly being redesigned to have a
smaller footprint. For example, some components previously sized at
about 50+ .mu.m and spaced apart by about 50+ .mu.m may now be
sized at about 15 .mu.m and spaced apart by about 15 .mu.m. The
lesser thickness of the plating layers of the pad 106 allows
plating of smaller components. This is not so for plated layers of
nickel, palladium, and gold.
[0045] Using the method 300 of FIG. 3, the inventors cleaned
exposed copper surfaces of a PCB by submersion in an acid cleaner.
A palladium activator was then plated on the copper parts of the
PCB using immersion plating. A bath including palladium tetraamine
sulfate at a concentration of 1 g/l, sodium hypophosphite at 5 g/l,
disodium EDTA at 2 g/l, ethylene diamine at 2 g/l, and reaction
stabilizers including phosphite at 1 g/l, nitrate at 1 g/l, copper
at 4 mg/l, bismuth at 4 mg/l, and lead at 0.75 g/l was prepared.
The bath had a pH of 9.0 and was heated to 120.degree. F. The PCB
with the cleaned copper surfaces and the palladium activator was
submerged in the bath. Measurements taken showed that the palladium
plated onto the copper surfaces at the rate of about 0.025 .mu.m
per minute. The PCB was submerged in the bath for a total of 40
minutes. When removed, the copper of the PCB had an evenly
distributed palladium layer at 1 .mu.m thick.
[0046] To determine the impact of EDTA in the plating process, the
inventor conducted the following experiment. The PCB having exposed
copper surfaces was cleaned using the same type of acid cleaner as
used in the above process. The copper was immersion plated with a
palladium activator. A bath having the same concentrations of all
ingredients of the above process was prepared. However, this bath
did not include any EDTA. The bath had a pH of 9.0 and was heated
to a temperature of 120.degree. F. The PCB was submerged in the
bath for 40 minutes. After the 40 minutes, the copper surfaces on
the PCB were still clearly visible. The palladium did not plate the
copper unfailingly because the bath did not have any EDTA, a
critical element of this invention.
[0047] Exemplary embodiments of the bath, methods, and plated
component have been disclosed in an illustrative style.
Accordingly, the terminology employed throughout should be read in
a non-limiting manner. Although minor modifications to the
teachings herein will occur to those well versed in the art, it
shall be understood that what is intended to be circumscribed
within the scope of the patent warranted hereon are all such
embodiments that reasonably fall within the scope of the
advancement to the art hereby contributed.
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