U.S. patent application number 17/130009 was filed with the patent office on 2021-06-24 for printed circuit surface finish, method of use, and assemblies made therefrom.
This patent application is currently assigned to LILOTREE, L.L.C.. The applicant listed for this patent is LILOTREE, L.L.C.. Invention is credited to Kunal Shah, Purvi Shah.
Application Number | 20210193346 17/130009 |
Document ID | / |
Family ID | 1000005432943 |
Filed Date | 2021-06-24 |
United States Patent
Application |
20210193346 |
Kind Code |
A1 |
Shah; Kunal ; et
al. |
June 24, 2021 |
PRINTED CIRCUIT SURFACE FINISH, METHOD OF USE, AND ASSEMBLIES MADE
THEREFROM
Abstract
A surface finish for a printed circuit board (PCB) and
semiconductor wafer includes a nickel disposed over an aluminum or
copper conductive metal surface. A barrier layer including all or
fractions of a nitrogen-containing molecule is deposited on the
surface of the nickel layer to make a barrier layer/electroless
nickel (BLEN) surface finish. The barrier layer allows solder to be
reflowed over the surface finish. Optionally, gold (e.g., immersion
gold) may be coated over the barrier layer to create a
nickel/barrier layer/gold (NBG) surface treatment. Presence of the
barrier layer causes the surface treatment to be smoother than a
conventional electroless nickel/immersion gold (ENIG) surface
finish. Presence of the barrier layer causes a subsequently applied
solder joint to be stronger and less subject to brittle failure
than conventional ENIG.
Inventors: |
Shah; Kunal; (Bothell,
WA) ; Shah; Purvi; (Bothell, WA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
LILOTREE, L.L.C. |
Redmond |
WA |
US |
|
|
Assignee: |
LILOTREE, L.L.C.
Redmond
WA
|
Family ID: |
1000005432943 |
Appl. No.: |
17/130009 |
Filed: |
December 22, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
16688995 |
Nov 19, 2019 |
10902967 |
|
|
17130009 |
|
|
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|
16068247 |
Jul 5, 2018 |
10566103 |
|
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PCT/US2017/012777 |
Jan 9, 2017 |
|
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|
16688995 |
|
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62276485 |
Jan 8, 2016 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01B 1/023 20130101;
H01L 23/49866 20130101; H01L 2224/0346 20130101; H01B 1/02
20130101; H05K 2203/072 20130101; H05K 2203/122 20130101; H01L
2224/05644 20130101; H05K 3/341 20130101; H05K 3/282 20130101; H01L
24/05 20130101; H05K 2201/0347 20130101; H01L 24/03 20130101; H05K
1/09 20130101; H05K 3/184 20130101; H05K 1/181 20130101; H05K 3/24
20130101; H01L 2224/05155 20130101; H05K 2201/0344 20130101; H05K
2203/0793 20130101; H01L 23/498 20130101; H05K 1/111 20130101; H05K
3/244 20130101; H01B 1/026 20130101 |
International
Class: |
H01B 1/02 20060101
H01B001/02; H01L 23/498 20060101 H01L023/498; H05K 3/24 20060101
H05K003/24; H05K 3/28 20060101 H05K003/28; H05K 1/09 20060101
H05K001/09 |
Claims
1. A method for making an electrical circuit assembly, comprising:
receiving a substrate, at least a portion of the substrate
including a conductive material layer that defines one or more
electrical conductors; applying nickel onto at least a portion of
one of the one or more electrical conductors; and applying a
barrier layer onto the nickel, the barrier layer comprising at
least a portion of a nitrogen-containing molecule.
2. The method of claim 1 further comprising applying immersion gold
subsequent to the applying the barrier layer.
3. The method of claim 1 wherein the barrier layer is applied using
a solution comprising an amine.
4. The method of claim 1 wherein the nitrogen-containing molecule
includes from one to six carbon atoms.
5. The method of claim 1 wherein the barrier layer is applied using
a solution comprising a diamine or a triamine.
6. The method of claim 1 wherein the barrier layer is applied using
a solution comprising diethylene triamine.
7. The method of claim 1 wherein the barrier layer impedes
oxidation of the nickel.
8. A method of plating one or more metallic regions of a substrate,
the method comprising: forming a nickel layer on at least a portion
of the one or more metallic regions; and performing a surface
treatment on the nickel layer by exposing the nickel layer to a
solution comprising nitrogen-containing molecules.
9. The method of claim 8 further comprising forming a gold layer on
at least a portion of the one or more metallic regions after the
performing the surface treatment.
10. The method of claim 8 wherein the surface treatment impedes
oxidation of the nickel layer.
11. The method of claim 8 wherein the solution comprises an
amine.
12. The method of claim 8 wherein the solution comprises a diamine
or a triamine.
13. The method of claim 8 wherein the solution comprises
siloxane.
14. The method of claim 8 wherein the substrate comprises a
fiberglass reinforced epoxy.
15. The method of claim 8 wherein the substrate comprises a
semiconductor material.
16. A method of plating a terminal of a substrate, the method
comprising: immersing the substrate in a first solution that forms
a nickel layer on at least a portion of the terminal; and immersing
the substrate in a second solution comprising nitrogen-containing
molecules that forms a conversion layer on at least a portion of
the nickel layer.
17. The method of claim 16 further comprising immersing the
substrate in a third solution after the immersing the substrate in
the second solution, wherein the third solution forms a gold layer
on at least a portion of an outermost surface of the terminal.
18. The method of claim 16 wherein the second solution comprises an
amine.
19. The method of claim 16 wherein the nitrogen-containing
molecules include from one to six carbon atoms.
20. The method of claim 16 wherein the second solution comprises a
diamine or a triamine.
Description
CROSS-RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 16/688,995, which was filed on Nov. 19, 2019,
entitled "PRINTED CIRCUIT SURFACE FINISH, METHOD OF USE, AND
ASSEMBLIES MADE THEREFROM", which is a continuation of Ser. No.
16/068,247, which was filed on Jul. 5, 2018, which issued as U.S.
Pat. No. 10,566,103, on Feb. 18, 2020, entitled "PRINTED CIRCUIT
SURFACE FINISH, METHOD OF USE, AND ASSEMBLIES MADE THEREFROM",
which is a 371 U.S. National Phase of PCT International Application
PCT/US/2017/012777, which was filed on Jan. 9, 2017, entitled
PRINTED CIRCUIT SURFACE FINISH, METHOD OF USE, AND ASSEMBLIES MADE
THEREFROM, which claims priority to U.S. Provisional Patent
Application No. 62/276,486, which was filed on Jan. 8, 2016. The
contents of each of the aforementioned applications are hereby
incorporated by reference for all purposes.
FIELD
[0002] This disclosure generally relates to a printed circuit
surface finish, method of use, and assemblies made therefrom.
BACKGROUND
[0003] Surface finishes are applied over copper conductor layers of
printed circuit boards (PCB). A patterned surface finish may act as
a mask to protect a selected pattern of copper conductor on a
substrate during etching of the copper. The surface finish can also
reduce or eliminate corrosion of the copper surface to ensure
suitable surface chemistry for the application of and reaction with
solder for making electrical and physical bonds with electrical
components.
[0004] Additionally, some surface finishes are especially good at
providing a smooth surface. A smooth surface is especially
important for mounting high-density components in high value
products such as cell phones, tablets, and laptop computers. One
currently available surface finish that is especially attractive
for high density component mounting is Electroless Nickel/Immersion
Gold, commonly referred to as ENIG.
[0005] Unfortunately, ENIG has a drawback in that it is prone to
forming brittle solder joints. In some cases, a condition referred
to as "black pads" has been found to correspond to brittle solder
joints. Brittle solder joints can fail (especially under vibration
and/or shock load) and lead to malfunction and failure of the
electronic assembly and the product in which the electronic
assembly operates.
[0006] FIG. 1 is a (not to scale) side sectional diagram of an ENIG
surface treatment, according to the prior art. As may be
appreciated, while one principal benefit of ENIG is intended to be
smoothness of the surface, deep intergranular boundaries 112 in the
electroless nickel 106 tend to cause openings in the gold layer 110
and a surface roughness that may be less than ideal.
SUMMARY
[0007] Embodiments are directed to a nickel-barrier layer-gold
(NBG) circuit surface finish observed to reduce corrosion-related
issues compared to a conventional electroless nickel-immersion gold
(ENIG) surface finish. The NBG surface finish was observed to
reduce "black pad" defects, known to affect ENIG. Experimental
results indicate the inventor has achieved solder joints exhibiting
improved robustness. Inventor contemplates NBG chemistry and
process will likely provide for better reliability of electronic
assemblies. Inventor contemplates the NBG surface chemistry enables
portable electronic devices having improved shock and vibration
capabilities.
[0008] According to an embodiment, a barrier layer portion of a NBG
surface finish includes all or a portion of a reactive
nitrogen-containing molecule, deposited onto the surface of a
nickel layer prior to application of a thin gold layer. In various
embodiments, the nitrogen-containing molecule can include primary,
secondary, or tertiary nitrogen(s) covalently bound to carbon. The
nitrogen may be present as a substituted aliphatic hydrocarbon or a
substituted aromatic hydrocarbon. According to embodiments,
nitrogen can include a substituted siloxane with the nitrogen
covalently bound to silicon or with a nitrogen covalently bound to
a carbon group which is covalently bound to silicon.
[0009] According to an embodiment, a surface finish includes a
barrier layer including an electroless nickel layer over copper,
and a barrier layer disposed over the electroless nickel layer, the
barrier layer including an amine or amine fragment deposited on the
surface of the electroless nickel. According to an embodiment,
electroplated nickel may be substituted for electroless nickel.
[0010] According to an embodiment, a NBG surface finish is applied
to a conventional printed circuit board such as copper over
fiberglass-reinforced epoxy (e.g., FR4) or copper over
polyimide.
[0011] According to another embodiment, the NBG surface finish is
applied to an aluminum conductor of a semiconductor integrated
circuit.
[0012] According to an embodiment the barrier layer is deposited
onto the nickel layer by chemisorption from an aqueous solvent.
[0013] According to another embodiment, the barrier layer is
deposited onto the nickel layer by condensation of a
nitrogen-containing molecule from a vapor such as a butane
vapor.
[0014] According to an embodiment, an electronic device includes an
electrical circuit assembly including NBG surface finish. The
electronic device can exhibit improved shock and/or vibration
tolerance compared to use of other surface finishes.
[0015] According to an embodiment, a method for making an
electrical circuit includes applying a NBG surface finish.
[0016] According to an embodiment, a composition of matter includes
a barrier solution.
[0017] According to an embodiment, a composition of matter includes
a solder joint having a more tightly contained intermetallic layer
than a conventional ENIG-based solder joint.
[0018] According to embodiments, an electronic assembly, radio
assembly, optical assembly, or satellite or spacecraft subassembly
may include NBG conductors or solder joints made with NBG surface
chemistry.
[0019] According to an embodiment, a PCB surface finish includes a
barrier layer including a nitrogen-containing molecule over nickel.
The nickel is disposed on an underlying metal. The underlying metal
may include copper or aluminum, for example.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a (not-to-scale) side sectional depiction of an
electroless nickel-barrier layer-gold ENIG surface treatment,
according to the prior art.
[0021] FIG. 2 is a (not-to-scale) side sectional depiction of a
barrier layer-on nickel (BLON) surface treatment, according to an
embodiment.
[0022] FIG. 3 is a (not-to-scale) side sectional depiction of a
nickel-barrier layer-gold (NBG) surface treatment, according to an
embodiment.
[0023] FIG. 4 is a (not to scale) sectional diagram of a solder
joint over a copper pad made using an NBG surface finish, according
to an embodiment.
[0024] FIG. 5 is a flow chart showing a process for making a NBG
surface finish and/or an electronic assembly including solder
joints made from a NBG surface finish, according to an
embodiment.
[0025] FIG. 6 is a transmission electron microscope image 600 of a
sectioned NBG surface treatment, according to an embodiment.
[0026] FIG. 7 is a SEM image of a barrier layer surface over
electroless nickel, according to an embodiment.
[0027] FIG. 8 is a SEM image of the electroless nickel surface at
the same magnification as FIG. 7, according to an embodiment.
[0028] FIG. 9 is a surface profilometer output corresponding to the
surface of the barrier layer, according to an embodiment.
[0029] FIG. 10 is a surface profilometer output corresponding to
the surface of the electroless nickel layer, according to an
embodiment.
[0030] FIGS. 11-14 are glow discharge emission spectrograph (GDOES)
graphs, according to an embodiment.
[0031] FIG. 15 is a diagram illustrating an NBG surface finish
trace, according to an embodiment.
DETAILED DESCRIPTION
[0032] In the following detailed description, reference is made to
the accompanying drawings, which form a part hereof. In the
drawings, similar symbols typically identify similar components,
unless context dictates otherwise. The illustrative embodiments
described in the detailed description, drawings, and claims are not
meant to be limiting. Other embodiments may be utilized, and other
changes may be made, without departing from the spirit or scope of
the subject matter presented here.
[0033] Surface finishes provide a barrier between copper pads of
the PCB and solder used to make electrical and physical contact
with assembled surface mount components. The surface finish can
reduce diffusion of copper into the solder during reflow, thereby
reducing degradation in physical strength of the solder joint.
Various types of surface finishes ensure the reliability of
PCB-to-surface mount component physical and electrical continuity
at the solder joints, which is critical to an electronic device's
reliable operation over time.
[0034] One principal benefit of ENIG relates to the role of its
nickel layer, which acts as barrier layer to diffusion of
underlying copper into the solder joint. The ENIG process includes
application of an electroless nickel, followed by forming an
immersion gold layer over the electroless nickel. The immersion
gold layer provides a protective barrier for passivation of the
nickel surface. During the reflow soldering process, the gold layer
is dissolved into molten solder and the resultant intermetallic
structure forms a bond between electroless nickel atoms and the
solder.
[0035] According to embodiments, the conventional portions of an
ENIG surface finish are augmented by provision of a barrier layer
interposed between the electroless nickel and the gold. The barrier
layer includes one or more reactive nitrogen-containing
molecules.
[0036] As used herein, the term "electroless nickel" refers to
phosphorous-doped nickel, or alternatively, boron-doped nickel.
[0037] As used herein, the phrase reactive nitrogen-containing
molecule refers to a molecule that contains nitrogen and which has
the capability of reacting with a nickel surface to adhere at least
a nitrogen-containing portion of the molecule to the nickel
surface.
[0038] The inventor discovered a root cause of black pad defects in
an Electroless Nickel/Immersion Gold (ENIG) surface finish on a
printed circuit board (PCB). The inventor describes the phenomenon
as hyper corrosion activity during an immersion gold deposition
process on the nickel surface.
[0039] Embodiments of the barrier layer described herein were found
to reduce concentration of deposits of phosphorous-rich regions in
the reflowed solder joint. The reduction in phosphorous-rich
regions is associated with higher solder joint ductility and
reduced tendency toward brittle failure. The inventor contemplates
the NBG surface finish may reduce or eliminate corrosion-related
issues (e.g., black pad defects as well as micro-defects similar to
black pad but which may be invisible to the naked eye) and provide
for robust solder joints for better reliability, including improved
shock and vibration resistance, of electronic assemblies and the
products in which the assemblies function, compared to use of the
ENIG surface chemistry.
[0040] FIG. 2 is an idealized side sectional diagram of a barrier
layer-on nickel (BLON) surface treatment, according to an
embodiment. A metal (M) layer 102 is supported by a substrate 104.
In one example, the substrate is a fiberglass-reinforced epoxy such
as FR4 and the metal M is copper (Cu). The copper may be between
about 10 and 100 micrometers thick. In another example, the
substrate is a semiconductor such as single crystal silicon (Si),
glass, or semiconductor-on-glass and the metal M is aluminum
(Al).
[0041] A nickel (Ni) layer 106 is disposed on the surface of the
metal M. The nickel layer may typically be electroless nickel,
which includes a phosphorous (P) or boron (B) component. In a
conventional phosphorous-doped electroless nickel, the phosphorous
content may typically be 8-9 wt %. The nickel layer 106 may
typically be about 3-6 micrometers thick.
[0042] A barrier layer 108 includes a separately deposited
nitrogen-containing molecule or molecular fragment on the nickel
106 surface. The barrier layer 108 is understood to generally be
deposited to a depth of only one or a few atoms thick.
[0043] FIG. 3 is an idealized side sectional diagram of a
nickel-barrier layer-gold (NBG) surface treatment, according to an
embodiment. A metal (M) layer 102 is supported by a substrate 104.
In one example, the substrate is a fiberglass-reinforced epoxy such
as FR4 and the metal M is copper (Cu). The copper may be between
about 10 and 100 micrometers thick. In another example, the
substrate is a semiconductor such as single crystal silicon (Si),
glass, or semiconductor-on-glass and the metal M is aluminum
(Al).
[0044] A nickel (Ni) layer 106 is disposed on the surface of the
metal M. The nickel layer may typically be electroless nickel,
which includes a phosphorous (P) or boron (B) component. In a
conventional phosphorous-doped electroless nickel, the phosphorous
content may typically be 8-9 wt %. The nickel layer 106 may
typically be about 3-6 micrometers thick.
[0045] A barrier layer 108 includes a separately deposited
nitrogen-containing molecule or molecular fragment on the nickel
106 surface. The barrier layer 108 is understood to generally be
deposited to a depth of only one or a few atoms thick. A gold layer
110 may be deposited over the nickel layer 106 and barrier layer
108. According to embodiments, the gold layer is deposited by
immersion coating, which results in a depth of only one or a few
atoms.
[0046] A barrier layer 108 includes a separately deposited
nitrogen-containing molecule or molecular fragment on the nickel
106 surface. The barrier layer 108 is understood to generally be
deposited to a depth of only one or a few atoms thick.
[0047] According to embodiments, amine groups chemisorb to nickel
atoms situated at intergranular boundaries (deep crevices) 112 to
make the microstructure smoother. The smoothed microstructure is
contemplated to reduce galvanic hyper-corrosion during subsequent
application of an immersion gold layer 110. The reduction in
hyper-corrosion is contemplated to reduce a tendency for copper
migration from the copper layer 102, reduce coating porosity, and
reduce a risk of degradation of solderability compared to a
conventional ENIG surface treatment.
[0048] The barrier layer 108 is understood to passivate the atoms
of the nickel layer 106 at the surface and at the intergranular
boundaries 112 and deep crevices of the nickel layer 106. The
barrier layer 108 may be deposited by chemisorption of amine with
resultant smoothening of the microstructure.
[0049] According to inventors' hypothesis, in the presence of the
nickel surface, amine molecules may dissociate into a NH-- anionic
species. Chemisorption of amine at the intergranular boundaries 112
and smoothening of the microstructure reduces or eliminates
galvanic hyper corrosion (aka "black pads") and achieves robust
solder joints. As a result, brittle failure of the solder joint
and/or component fall-offs in the electronic assemblies may be
reduced or eliminated, leading to improved reliability of
electronic devices made according to the disclosure herein.
[0050] Embodiments include an electroless nickel layer having a
phosphorous content of 8-9 wt %. The inventor(s) contemplate that
embodiments herein may allow a reduction in P content in the
electroless nickel layer. The inventor(s) further contemplate that
a low concentration of additional metals including zinc and/or tin
may be included in the electroless nickel layer, which metals may
interact synergistically with embodiments herein to yield one or
more improved properties including improved shock resistance,
yield, traceability, vibration, joint shear strength, and/or joint
ductility.
[0051] FIG. 4 is a side sectional diagram of an assembly including
a solder ball 402 over the nickel 106. An intermetallic layer 406
lies between the solder 402 and the nickel layer 106. The nickel
layer is disposed, a copper pad 20 and a dielectric substrate 18,
according to an embodiment. Referring to FIGS. 1 and 2, during a
solder reflow process, the gold layer 16 is dissolved into the
solder, and the intermetallic layer 22 is formed between the
electroless nickel layer 12 and the solder 24. The composition of
the intermetallics include solder (tin alloy), electroless nickel,
and carbon. The intermetallic layer causes the solder joint to
exhibit desired ductility and to resist brittle failures.
[0052] FIG. 5 is a flow chart showing a method 500 for making a
circuit assembly including an NBG surface finish, according to an
embodiment. Beginning at step 502, a substrate is received, at
least a portion of the surface of the substrate including a
conductive material layer. In an embodiment, a substrate can
include a dielectric layer, such as fiberglass reinforced epoxy
(FR4) or polyimide, having a conductor, such as copper (Cu), on at
least one surface. For purposes of description, it is assumed that
the received substrate has been cleaned, such as by mechanical
abrasion followed by a solvent wash, and dried. In another
embodiment, the substrate includes a semiconductor, such as a
silicon wafer, with a conductor, such as aluminum (Al), on the
surface.
[0053] Proceeding to step 504, a mask is applied to the surface of
the circuit substrate and particularly the conductor. The mask is
selected (e.g., designed or laid out) to define individual
electrical conductors. Following application of the mask, the
exposed surface of the conductor may be pre-treated. Pre-treatment
can include dipping, spin coating, spraying, or otherwise applying
a material selected to attract or receive at least nickel atoms
during a subsequent step 508.
[0054] For example, pre-treatment of a copper conductor can include
immersion coating palladium (Pd) from a solution into a thin layer
on the surface of the conductor. An example procedure includes the
following steps:
[0055] 1. Use compressed air to remove dust and metal particles
from component.
[0056] 2. Immerse component in acetone bath for 60 seconds.
[0057] 3. Upon removal immediately immerse in 70% isopropyl alcohol
solution for 15 seconds.
[0058] 4. Rinse off isopropyl alcohol under running water.
[0059] 5. Immerse component in deionized (DI) water bath and
sonicate for 180 seconds.
[0060] 6. Remove component from DI water bath and remove excess
water with compressed air until no water droplets remain.
[0061] 7. Immerse component in 17.5% nitric acid solution for 60
seconds.
[0062] 8. Immerse component in 5% sulfuric acid solution for 30
seconds.
[0063] 9. Remove component from 5% sulfuric acid solution and rinse
under running DI water.
[0064] 10. Immerse component in DI water bath and sonicate for 180
seconds.
[0065] 11. Remove component from DI water bath and dry thoroughly
with compressed air.
[0066] 12. Immediately cover component completely with lint-free
cloth until ready to proceed with activation and plating steps.
[0067] 13. Using palladium-based solution (0.1 g/LPdCb, 1 ml/L HCl
(37% concentrate)) (Use in concentration as supplied.) (Use at room
temperature.), briefly immerse the component in a bath containing
Pd solution. The component should only be immersed very briefly,
for 2 seconds or less and removed immediately.
[0068] 14. Upon removal, immediately immerse component in a bath of
DI water, agitate component in bath for 10 seconds to remove bulk
of remaining Pd solution from surface of component.
[0069] 15. Remove component from bath and rinse thoroughly using a
jet of water under pressure, going over every part of component in
detail.
[0070] 16. Repeat this thorough rinsing. Thorough rinsing is
crucial. No residue should remain on the component surface.
[0071] In another example, pre-treatment of an aluminum conductor
can include immersion coating zinc (Zn) from a zincate solution
onto a thin layer on the surface of the conductor. Immersion
coating is an atomic substitution of a more noble metal atom for a
less noble metal atom, in which plating thickness is limited by
steric availability of the less noble metal atom at the surface.
The presence of Pd or Zn on the surface defines a conductor area
with a higher affinity for a subsequently plated metal compared to
an area without the presence of the material. As used herein, the
material (e.g. Pd or Zn) coated onto the conductor may be referred
to as an activator.
[0072] Proceeding to step 506, the substrate may be cleaned to
remove the mask. Step 506 is a typical step in that cleaning may
typically occur between steps shown in the method 300. In the
interest of clarity, description of specific cleaning processes
between specific steps is omitted. Cleaning may include one or more
of an acid wash, a basic wash, mechanical abrasion, sonication, a
solvent wash, UV exposure, heat exposure, etc., followed by rinsing
and drying.
[0073] Proceeding to step 508, nickel is plated onto the areas of
the conductor surface coated with the pre-treatment material.
Accordingly, the nickel is applied onto the electrical conductor
areas defined by the mask. The nickel (with dopant, if electroless)
may be applied to a thickness of about 3-6 micrometers, for
example.
[0074] According to an embodiment, applying the nickel includes
applying electroless nickel to the activator applied to the
conductive material. In another embodiment, nickel may be
electroplated onto the conductor. The nickel may be electroplated
through the mask, for example. In cases where electroplating is
used, step 504 may consist essentially of applying the mask, and
the pre-treatment portion of step 504 may be omitted.
[0075] Electroless nickel typically includes a dopant that
cooperates with the nickel to self-catalyze the coating process. In
some embodiments, the dopant includes boron (B). In some
embodiments, the dopant includes phosphorous (P). A
phosphorous-doped electroless nickel may be applied using NiSQ4
chemistry by holding the substrate in a water solution including
(NiSQ4*6H20) in a range of 20-40 g/L, sodium hypophosphite in a
range of 20-30 g/L, sodium citrate in a range of 15-25 g/L, and
thiourea in a range of 1-5 mg/L.
[0076] In step 509, a water solution carrying a reactive
nitrogen-containing molecule is obtained. Step 509 may include
purchasing the solution or may include making the solution.
[0077] In an embodiment, making the water solution of the reactive
nitrogen-containing molecule in step 509 may include making a
solution, in deionized water, of an aromatic amine. In another
embodiment, making the water solution of the reactive
nitrogen-containing molecule in step 509 may include making a water
solution of an aliphatic amine. Making a water solution of an
aliphatic amine may include making a solution of 0.1 to 1 molar 1,4
diamine butane or making a solution of 0.1 to 1 molar diethylene
tri-amine. Making a water solution of a reactive
nitrogen-containing molecule may include making a water solution of
ammonium hydroxide. In another embodiment, making a water solution
of a reactive nitrogen-containing molecule includes making a water
solution of an amine-substituted siloxane. Making the water
solution of the reactive nitrogen-containing molecule in step 509
may include adjusting the pH to about 12.
Ammonium Hydroxide May be Used to Adjust the pH.
[0078] For embodiments where a less soluble nitrogen-containing
molecule is used, making the water solution in step 509 may include
adding an emulsifier and/or a detergent to the water.
[0079] The reactive, nitrogen-containing molecule(s) may include
one or more of: R1-NH2, NH2-R1-NH2, NH2-R1-NH2 NH2,
NH2-R1-NH2-R2-NH2, R1-N--R2H, R1-N--R2 R3, A1-NH2, NH2-A1-NH2,
NH2-A1-NH2NH2, A1-N-A2H, A1-N-A2A3, A1-N--R1H, A1-N-A2 R1, A1-N--R1
R2, a NH2-substituted siloxane, and an amine-substituted siloxane;
where R1 is a first substituted or unsubstituted aliphatic group of
C18 or less; where R2 is a second substituted or unsubstituted
aliphatic group of C18 or less where R3 is a third substituted or
unsubstituted aliphatic group of C18 or less; where A1 is a
substituted or unsubstituted aromatic group; and where A2 is a
second substituted or unsubstituted aromatic group of C18 or less;
where A3 is a third substituted or unsubstituted aromatic group of
C18 or less.
[0080] In an embodiment, the reactive nitrogen-containing molecule
includes 1 to 6 carbon atoms. In another embodiment, the
nitrogen-containing molecule includes 3 to 5 carbon atoms. In an
embodiment, the nitrogen-containing molecule includes 1,4 diamine
butane. In an embodiment, the nitrogen-containing molecule includes
diethylene triamine.
[0081] Proceeding to step 510, a barrier layer is applied onto the
nickel, the barrier layer including at least a portion of a
reactive nitrogen-containing molecule. In an embodiment, steps 509
and 510 may include making a water solution of a reactive
nitrogen-containing molecule and exposing the water solution of the
reactive nitrogen-containing molecule to the nickel. Applying the
barrier layer to the nickel may include maintaining the temperature
of the water solution at 30-80.degree. C. while exposing the water
solution of the reactive nitrogen-containing molecule to the nickel
for 5-40 minutes.
[0082] In an alternative embodiment, applying the barrier layer to
the nickel can include dissolving the reactive nitrogen-containing
molecule in a vapor and condensing the nitrogen-containing molecule
out of the vapor onto the nickel. For example, a molecule such as
1,4 diamine butane may be carried by a butane vapor and may be
condensed onto the nickel therefrom.
[0083] According to another embodiment, the barrier layer is
deposited onto the nickel layer from a high molecular weight matrix
by adhesion of a solid or viscous liquid layer such as a
polyester-supported polymethyl methacrylate host polymer carrying a
nitrogen-containing molecule guest, followed by peeling away of the
polyester (optionally, aided by application of heat).
[0084] In an alternative embodiment, applying the barrier layer to
the nickel may include embedding the reactive nitrogen-containing
molecule in a polymer matrix supported by a transfer film, placing
polymer matrix side of the transfer film in close proximity to the
nickel surface, and applying heat to the transfer film to cause the
nitrogen-containing molecule to diffuse out of the polymer matrix
and condense on the nickel.
[0085] Optionally, the method 500 may include step 512, wherein a
layer of gold is applied over the barrier layer, such that the
individual electrical conductor have a nickel-barrier layer-gold
(NBG) surface treatment over the conductive material. In one
embodiment, gold is immersion coated onto the nickel and the
barrier layer disposed thereon. In another embodiment, gold is
electroplated over the nickel and the barrier layer disposed
thereon.
[0086] Proceeding to step 514, areas of the exposed conductive
material not carrying the applied nickel and barrier layer are
etched to form individual electrical conductors. In one embodiment
where gold is applied to the barrier layer, etching is performed
prior to applying the layer of gold. In another embodiment, etching
is performed after applying the layer of gold.
[0087] Proceeding to optional step 516, a pattern of solder paste
may be applied over the electrical conductors (including the
conductive layer, nickel, barrier layer, and optionally gold), the
pattern of solder paste corresponding to component mounting pads,
and a plurality of surface mount components may be picked and
placed onto the component mounting pads. The solder paste may be
reflowed to make respective solder joints between the electrical
conductors and the surface mount components to form an electrical
circuit, and the electrical circuit cooled and cleaned.
[0088] The solder joints made according to the method 500 (NBG
chemistry) were characterized by a reduction in areas with high
phosphorous concentration compared to solder joints made without
the barrier layer (ENIG chemistry). The solder joints made
according to the method 500 (NBG chemistry) were characterized by
more confined regions of intermetallics compared to solder joints
made without the barrier layer (ENIG chemistry). It is contemplated
that the solder joints made according to the method 500 (NBG
chemistry) may be characterized by a reduced probability of brittle
failure compared to solder joints made without the barrier layer
(ENIG chemistry).
[0089] FIG. 6 is a transmission electro micrograph 600 of a
sectioned NBG surface treatment, according to an embodiment. Of
particular importance is the continuous gold layer 110. The
thickness of the gold layer is about 50 nanometers. (In comparison,
the gold layer in a similar image of a conventional ENIG surface
treatment shows the gold layer discontinuous with sections
displaced vertically from one another.) A thin intermetallic region
602 lies immediately below the gold layer 110. The electroless
nickel layer 106 extends to the bottom of the image.
[0090] FIG. 6 represents a composition of matter including an
Electroless Nickel (Ni--P)-Barrier Layer--Gold (Au) surface
treatment lying superjacent a conductor layer comprising at least
one additional metal, according to an embodiment. The at least one
additional metal may include copper and/or aluminum.
[0091] The surface treatment is expressed as a conductive pad
structure having increased carbon concentration near its surface
relative to an Electroless Nickel Immersion Gold (ENIG) surface
finish and a more confined concentration of gold near its surface.
The composition of matter exhibits a substantially continuous Au
layer, as shown in SEM section. As shown in FIG. 6, the composition
of matter may further include a continuous intermetallic layer
between the Au and Ni--P.
[0092] According to an embodiment, the barrier layer is deposited
from a water solution carrying at least one reactive,
nitrogen-containing molecule. The water solution from which the
barrier layer is deposited may further include ammonium hydroxide
and may be pH-balanced to about 12. In some embodiments, the water
solution includes an emulsifier.
[0093] According to an embodiment, the barrier layer is deposited
from a water solution carrying at least one reactive,
nitrogen-containing molecule corresponding to the group consisting
of: R1-NH2, NH2-R1-NH2, NH2-R1-NH2, NH2-R1-NH2-R2-NH2, R1-N--R2H,
R1-N--R2R3, A1-NH2, NH2-A1-NH2, NH2-A1-NH2 NH2, A1-N-A2H,
A1-N-A2A3, A1-N--R1H, A1-N-A2R1, A1-N--R1 R2, a NH2-substituted
siloxane, and an amine-substituted siloxane; where R1 is a first
substituted or unsubstituted aliphatic group of C18 or less; where
R2 is a second substituted or unsubstituted aliphatic group of C18
or less; where R3 is a third substituted or unsubstituted aliphatic
group of C18 or less; where A1 is a substituted or unsubstituted
aromatic group; where A2 is a second substituted or unsubstituted
aromatic group; and where A3 is a third substituted or
unsubstituted aromatic group.
[0094] In some embodiments, the nitrogen-containing molecule
includes 1 to 6 carbon atoms, total. In some embodiments, the
nitrogen-containing molecule includes 3 to 5 carbon atoms.
[0095] The nitrogen-containing molecule includes 1,4 diamine
butane, according to an embodiment. In another embodiment, the
nitrogen-containing molecule includes diethylene triamine.
[0096] FIG. 7 is a SEM image of a barrier layer surface over
electroless nickel, made according to an embodiment. FIG. 8 is a
SEM image of the electroless nickel surface at the same
magnification as FIG. 7. By comparison of FIG. 7 to FIG. 8, one can
readily see an improvement in surface smoothness caused by the
barrier layer.
[0097] ENIG surface treatments exhibit a relatively high
electroless nickel grain boundary expression, corresponding to a
rough surface characterized by electroless nickel patches on the
surface of the ENIG electrode. Portions of the rough surface of the
electroless nickel lie adjacent or superjacent to an intervening
gold monolayer. For comparison, the gold layer in an NBG surface
treatment is continuous, as may be seen in FIG. 6.
[0098] FIG. 9 is a surface profilometer output corresponding to the
surface of the barrier layer shown in FIG. 7. FIG. 10 is a surface
profilometer output corresponding to the surface of the electroless
nickel layer shown in FIG. 8.
[0099] Surface profiles were made using the same parameters. FIGS.
9 and 10 make clear the improvement in surface smoothness provided
by the barrier layer.
[0100] FIGS. 11-14 are glow discharge emission spectrograph (GDOES)
graphs corresponding to the concentrations of carbon, nitrogen,
oxygen, and gold according to depth below respective surface
treatment surfaces. Each graph compares the concentration of a
species in an NBG surface treatment compared to a ENIG surface
treatment.
[0101] FIG. 15 is a diagram 1500 illustrating an NBG surface finish
trace 1502 on an electrical circuit 1504, which is included in an
electronic product 1506. The inventors contemplate that the BLON
and ENIG surface finishes described herein may significantly
improve drop test results for electronic products using them.
[0102] While various aspects and embodiments have been disclosed
herein, other aspects and embodiments are contemplated. The various
aspects and embodiments disclosed herein are for purposes of
illustration and are not intended to be limiting, with the true
scope and spirit being indicated by the following claims.
* * * * *