U.S. patent application number 13/283784 was filed with the patent office on 2013-05-02 for multilayered ceramic capacitor with improved lead frame attachment.
This patent application is currently assigned to KEMET ELECTRONICS CORPORATION. The applicant listed for this patent is Keith Brown, James B. Byrd, R. Allen Hill, Philip M. Lessner, Reggie Phillips. Invention is credited to Keith Brown, James B. Byrd, R. Allen Hill, Philip M. Lessner, Reggie Phillips.
Application Number | 20130107419 13/283784 |
Document ID | / |
Family ID | 47137599 |
Filed Date | 2013-05-02 |
United States Patent
Application |
20130107419 |
Kind Code |
A1 |
Hill; R. Allen ; et
al. |
May 2, 2013 |
MULTILAYERED CERAMIC CAPACITOR WITH IMPROVED LEAD FRAME
ATTACHMENT
Abstract
A capacitor with improved lead frame attachment is described
wherein the improved lead frame attachment mitigates defects. The
capacitor comprises parallel conductive internal electrodes of
alternating polarity with a dielectric between the conductive
internal electrodes. A first copper undercoat is in electrical
contact with the conductive internal electrodes of a first polarity
and a second copper undercoat is in electrical contact with
conductive internal electrodes of a second polarity. A first lead
is in electrical contact with the first copper undercoat with a
first solder between the first lead and the first copper undercoat.
A second lead is in electrical contact with the second copper
undercoat with a second solder between the second lead and the
second copper undercoat.
Inventors: |
Hill; R. Allen;
(Simpsonville, SC) ; Lessner; Philip M.;
(Newberry, SC) ; Phillips; Reggie; (Fountain Inn,
SC) ; Brown; Keith; (Simpsonville, SC) ; Byrd;
James B.; (Gray Court, SC) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Hill; R. Allen
Lessner; Philip M.
Phillips; Reggie
Brown; Keith
Byrd; James B. |
Simpsonville
Newberry
Fountain Inn
Simpsonville
Gray Court |
SC
SC
SC
SC
SC |
US
US
US
US
US |
|
|
Assignee: |
KEMET ELECTRONICS
CORPORATION
GREENVILLE
SC
|
Family ID: |
47137599 |
Appl. No.: |
13/283784 |
Filed: |
October 28, 2011 |
Current U.S.
Class: |
361/305 ;
29/25.41 |
Current CPC
Class: |
Y10T 29/43 20150115;
H01G 4/008 20130101; H01G 2/06 20130101; H01G 4/2325 20130101; H01G
4/30 20130101; H01G 4/232 20130101 |
Class at
Publication: |
361/305 ;
29/25.41 |
International
Class: |
H01G 4/008 20060101
H01G004/008; H01G 7/00 20060101 H01G007/00 |
Claims
1. A capacitor comprising: parallel conductive internal electrodes
of alternating polarity with a dielectric between said conductive
internal electrodes; a first copper undercoat in electrical contact
with conductive internal electrodes of a first polarity; a second
copper undercoat in electrical contact with conductive internal
electrodes of a second polarity; a first lead in electrical contact
with said first copper undercoat with a first solder between said
first lead and said first copper undercoat; a second lead in
electrical contact with said second copper undercoat with a second
solder between said second lead and said second copper undercoat; a
solderable silver layer between said first copper undercoat and
said first lead; and a flexible layer between said solderable
silver layer and said first copper undercoat
2. The capacitor of claim 1 wherein said first copper undercoat has
a surface oxide content of less than 1 percent.
3. The capacitor of claim 1 wherein said first copper undercoat has
a glass frit content of less than 20 wt %.
4. The capacitor of claim 3 wherein said first copper undercoat has
a glass frit content of at least about 5 wt % to no more than about
8 wt %.
5. The capacitor of claim 1 wherein said first copper undercoat has
a film thickness of at least about 1 micron to no more than about
100 microns.
6. The capacitor of claim 5 wherein said first copper undercoat has
a film thickness of at least about 10 micron to no more than about
100 microns.
7. The capacitor of claim 1 wherein said internal electrodes
comprise a base metal.
8. The capacitor of claim 7 wherein said base metal is selected
from nickel, copper, titanium, tungsten and molybdenum.
9. The capacitor of claim 8 wherein said internal electrodes
comprises an alloy of nickel.
10. The capacitor of claim 9 wherein said alloy comprises at least
one material selected from the group consisting of Cu, Si, Ba, Ti,
Mn, Cr, Co, and Al.
11. The capacitor of claim 10 wherein said alloy comprises at least
95 wt % nickel.
12. The capacitor of claim 1 which does not have a plating layer
comprising nickel or tin between said first copper undercoat and
said first lead.
13. A capacitor comprising: parallel conductive internal electrodes
of alternating polarity with a dielectric between said conductive
internal electrodes; a first copper undercoat in electrical contact
with conductive internal electrodes of a first polarity; a second
copper undercoat in electrical contact with conductive internal
electrodes of a second polarity; a first lead in electrical contact
with said first copper undercoat with a first solder between said
first lead and said first copper undercoat; and a second lead in
electrical contact with said second copper undercoat with a second
solder between said second lead and said second copper
undercoat.
14. The capacitor of claim 13 wherein said first copper undercoat
is in direct electrical contact with said conductive internal
electrodes.
15. The capacitor of claim 14 wherein said first lead is soldered
directly to said first copper undercoat.
16. The capacitor of claim 13 wherein said first copper undercoat
has a surface oxide content of less than 1 percent.
17. The capacitor of claim 13 wherein said first copper undercoat
has a glass frit content of less than 20 wt %.
18. The capacitor of claim 17 wherein said first copper undercoat
has a glass frit content of at least about 5 wt % to no more than
about 8 wt %.
19. The capacitor of claim 13 wherein said first copper undercoat
has a film thickness of at least about 1 micron to no more than
about 100 microns.
20. The capacitor of claim 19 wherein said first copper undercoat
has a film thickness of at least about 10 micron to no more than
about 100 microns.
21. The capacitor of claim 13 wherein said internal electrodes
comprise a base metal.
22. The capacitor of claim 21 wherein said base metal is selected
from nickel, copper, titanium, tungsten and molybdenum.
23. The capacitor of claim 22 wherein said internal electrodes
comprises an alloy of nickel.
24. The capacitor of claim 23 wherein said alloy comprises at least
one material selected from the group consisting of Cu, Si, Ba, Ti,
Mn, Cr, Co, and Al.
25. The capacitor of claim 24 wherein said alloy comprises at least
95 wt % nickel.
26. The capacitor of claim 13 which does not have a plating layer
comprising nickel or tin between said first copper undercoat and
said first lead.
27. A capacitor comprising: parallel conductive internal electrodes
of alternating polarity with a dielectric between said conductive
internal electrodes wherein said conductive internal electrodes
comprise a precious metal; a first silver undercoat in electrical
contact with conductive internal electrodes of a first polarity; a
second silver undercoat in electrical contact with conductive
internal electrodes of a second polarity; a first solderable silver
layer in contact with said first silver undercoat; a second
solderable silver layer in contact with said second silver
undercoat layers; a first lead in electrical contact with said
first silver undercoat with said first solderable silver layer
between said first lead and said first silver undercoat; and a
second lead in electrical contact with said second silver undercoat
with said second solderable silver layer between said second lead
and said second silver undercoat.
28. The capacitor of claim 27 further comprising a flexible layer
between said solderable silver layer and said first silver
undercoat.
29. The capacitor of claim 27 wherein said first silver undercoat
is in direct electrical contact with said conductive internal
electrodes.
30. The capacitor of claim 27 wherein said conductive internal
electrodes comprise a material selected from silver, palladium,
gold, platinum and alloys thereof.
31. The capacitor of claim 30 wherein said conductive internal
electrodes comprises silver-palladium alloy or
silver-palladium-platinum alloy.
32. The capacitor of claim 27 which does not have a plating layer
comprising nickel or tin between said first silver undercoat and
said first lead.
33. A method for forming a capacitor comprising the steps of:
providing a monolith comprising parallel conductive internal
electrodes of alternating polarity with a dielectric between said
conductive internal electrodes; applying a first copper undercoat
in direct electrical contact with said conductive internal
electrodes having a first polarity; applying a second copper
undercoat in direct electrical contact with said conductive
internal electrodes having a second polarity; soldering a first
lead frame in electrical contact with said first copper undercoat;
and soldering a second lead frame in electrical contact with said
second copper undercoat.
34. The method for forming a capacitor of claim 33 further
comprising: forming a flexible termination prior to said soldering
a first lead frame.
35. The method of forming a capacitor of claim 34 wherein said
flexible termination comprises silver particles in a polymeric
matrix.
36. The method of forming a capacitor of claim 34 wherein said
flexible termination is formed by applying a paste and drying said
paste.
37. The method of forming a capacitor of claim 36 wherein said
paste comprises 77-79 wt % silver particles and 10-15 wt % organic
resin in an organic solvent.
38. The method of forming a capacitor of claim 36 wherein said
paste has a viscosity of at least 5 Pa-S to no more than about 50
Pa-S at 25.degree. C.
39. The method of forming a capacitor of claim 36 wherein said
paste has a viscosity of at least 7 Pa-S to no more than about 15
Pa-S at 25.degree. C.
40. The method for forming a capacitor of claim 34 further
comprising forming a solderable silver layer after said forming a
flexible termination and prior to said soldering a first lead
frame.
41. The method for forming a capacitor of claim 40 wherein said
forming a solderable silver layer comprises applying a solution
with a viscosity of 1-10 Pa-S at 25.degree. C.
42. The method for forming a capacitor of claim 40 wherein said
forming a solderable silver layer comprises applying a solution
with a solids content of 30-60 wt %.
43. The method for forming a capacitor of claim 33 wherein said
first lead is soldered directly to said copper undercoat.
44. The method for forming a capacitor of claim 33 wherein said
first copper undercoat or said second copper undercoat has an
surface oxide content of less than 1 percent.
45. The method for forming a capacitor of claim 33 wherein said
first copper undercoat or said second copper undercoat has a
surface glass content of about 5 to about 20 wt. percent.
46. The method for forming a capacitor of claim 45 wherein said
first copper undercoat or said second copper undercoat has a
surface glass content of about 5 to about 8 wt. percent.
47. The method for forming a capacitor of claim 33 wherein said
first copper undercoat or said second copper undercoat has a total
copper film thickness of at least 1 microns to no more than 100
microns.
48. The method for forming a capacitor of claim 47 wherein said
first copper undercoat or said second copper undercoat has a total
copper film thickness of at least 10 microns to no more than 100
microns.
49. The method for forming a capacitor of claim 33 further
comprising surface treating of said copper.
50. The method for forming a capacitor of claim 49 further wherein
said surface treating is acid cleaning or plasma cleaning.
51. The method for forming a capacitor of claim 33 wherein said
applying a first copper undercoat comprises applying a solution
with a viscosity of 20-50 Pa-S at 25.degree. C.
52. The method for forming a capacitor of claim 33 wherein said
applying a first copper undercoat comprises applying a paste
comprising about 75-90 wt % copper and about 5-20 wt % glass
frit.
53. The method for forming a capacitor of claim 52 wherein said
applying a first copper undercoat comprises applying a paste
comprises about 5-8 wt % glass frit.
54. The method for forming a capacitor of claim 52 wherein said
glass frit is cadmium and bismuth free.
55. The method for forming a capacitor of claim 33 wherein said
internal electrodes comprise a base metal.
56. The method for forming a capacitor of claim 55 wherein said
base metal is selected from nickel, copper, titanium, tungsten and
molybdenum.
57. The method of forming a capacitor of claim 56 wherein said
internal electrodes comprise an alloy of nickel.
58. The method of forming a capacitor of claim 57 wherein said
alloy comprises at least one material selected from the group
consisting of Cu, Si, Ba, Ti, Mn, Cr, Co, and Al.
59. The method of forming a capacitor of claim 58 wherein said
alloy comprises at least 95 wt % nickel.
60. The method of forming a capacitor of claim 33 wherein said lead
frame comprises a material selected from phosphor bronze alloy 510,
nickel iron alloy 42 copper iron alloy 194.
61. The method of forming a capacitor of claim 33 wherein said lead
frame has a thickness of 76-500 microns.
62. The method of forming a capacitor of claim 33 wherein said
soldering a first lead frame comprises soldering with a material
which is lead free.
63. The method of forming a capacitor of claim 62 wherein said
solder comprises 5-30 wt % antimony.
64. A method for forming a capacitor comprising the steps of:
providing a monolith comprising parallel conductive internal
electrodes of alternating polarity with a dielectric between said
conductive internal electrodes wherein said conductive internal
electrodes comprise a precious metal; applying a first silver
undercoat in direct electrical contact with said conductive
internal electrodes having a first polarity; applying a second
silver undercoat in direct electrical contact with said conductive
internal electrodes having a second polarity; applying a first
flexible layer on said first silver undercoat; applying a first
solderable silver layer on said first flexible layer; applying a
second flexible layer on said second silver undercoat; applying a
second solderable silver layer on said second flexible layer;
soldering a first lead frame in electrical contact with said first
silver solderable silver layer; and soldering a second lead frame
in electrical contact with said second silver solderable layer.
65. The method of forming a capacitor of claim 64 wherein said
first flexible layer comprises silver particles in a polymeric
matrix.
66. The method of forming a capacitor of claim 64 wherein said
first flexible layer is formed by applying a paste and drying said
paste.
67. The method of forming a capacitor of claim 66 wherein said
paste comprises 77-79 wt % silver particles and 10-15 wt % organic
resin in an organic solvent.
68. The method of forming a capacitor of claim 66 wherein said
paste has a viscosity of at least 5 Pa-S to no more than about 50
Pa-S at 25.degree. C.
69. The method of forming a capacitor of claim 68 wherein said
paste has a viscosity of at least 7 Pa-S to no more than about 15
Pa-S at 25.degree. C.
70. The method for forming a capacitor of claim 64 further
comprising forming a solderable silver layer after said forming a
flexible layer and prior to said soldering a first lead frame.
71. The method for forming a capacitor of claim 70 wherein said
forming a solderable silver layer comprises applying a solution
with a viscosity of 1-10 Pa-S at 25.degree. C.
72. The method for forming a capacitor of claim 70 wherein said
forming a solderable silver layer comprises applying a solution
with a solids content of 30-60 wt %.
73. The method for forming a capacitor of claim 64 wherein said
first silver undercoat or said second silver undercoat has a total
silver film thickness of at least 1 micron to no more than 100
microns.
74. The method for forming a capacitor of claim 73 wherein said
first silver undercoat or said second silver undercoat has a total
silver film thickness of at least 10 micron to no more than 100
microns.
75. The method for forming a capacitor of claim 64 wherein said
applying said first silver undercoat comprises applying a paste and
drying said paste.
76. The method for forming a capacitor of claim 75 wherein said
paste comprises at least about 75 wt % to no more than about 90 wt
% silver and at least about 5 wt % to no more than about 20 wt %
glass frit.
77. The method for forming a capacitor of claim 76 wherein said
paste comprises about 5-8 wt % glass frit.
78. The method for forming a capacitor of claim 76 wherein said
glass frit is cadmium and bismuth free.
79. The method for forming a capacitor of claim 64 wherein
conductive internal electrodes comprises a material selected from
silver, palladium, gold, platinum and alloys thereof.
80. The method for forming a capacitor of claim 79 wherein said
conductive internal electrodes comprise silver-palladium or
silver-palladium-platinum.
81. The method of forming a capacitor of claim 64 wherein said lead
frame comprises a material selected from phosphor bronze alloy 510,
nickel iron alloy 42 copper iron alloy 194.
82. The method of forming a capacitor of claim 64 wherein said lead
frame has a thickness of 76-500 microns.
83. The method of forming a capacitor of claim 64 wherein said
soldering a first lead frame comprises soldering with a material
which is lead free.
84. The method of forming a capacitor of claim 83 wherein said
solder comprises 5-30 wt % antimony.
Description
BACKGROUND
[0001] The present invention is related to an improved MLCC
capacitor. More specifically, the present invention is related to
an improved MLCC capacitor wherein the adhesion between the MLCC
and lead frame is improved.
[0002] Multi-layered ceramic capacitors (MLCC's) are widely used
throughout the electronics industry. In many applications the
MLCC's have leads of opposing polarity for attachment of the
capacitor to a circuit. At least one MLCC is between the leads.
Often multiple MLCC's are between the leads such as in a stacked
arrangement. A particular problem with leaded MLCC's is their
susceptibility to cracking due to a mismatch in the Coefficient of
Thermal Expansion (CTE) of various components. The thermal mismatch
causes stresses when subjected to temperature cycling or thermal
shock. Cracking due to thermal stress has been mitigated in the
prior art by methods such as the use of lead frame materials with a
CTE which is less than the CTE of the MLCC, as described in U.S.
Pat. No. 6,081,416, or by adding a plated flexible conductive
adhesive to the MLCC, as described in U.S. Pat. No. 6,388,864.
[0003] Regardless of the advances, the art is still lacking a
suitable solution to mitigate thermal stress induced cracking. In
particular, the cracking which occurs due to CTE mismatches in the
plating layers, which allow solder adhesion to the lead frame, are
now known to be a significant source of crack propagation.
SUMMARY OF THE INVENTION
[0004] It is an object of the invention to provide an improved
capacitor, particularly, a capacitor with a decreased propensity
for the crack propagation which occurs due to thermal cycling or
thermal shock.
[0005] Another object of the invention is to eliminate spalling
defects which are surface defects caused by a CTE mismatch between
the nickel plating layers and the MLCC ceramic body that occur
during reflow attachment of the MLCC to the lead frame.
[0006] A particular advantage is provided by eliminating the
plating layers of the MLCC. Additional advantages are realized by
application of a thin second coating of solderable silver over a
flexible silver conductive adhesive to enhance soldering without
plating. The flexible termination provides an extra level of
protection against thermal shock cracking in addition to the
protection provided by the lead frame such as by elevating the chip
from the board. An additional benefit of eliminating the plating
layers is the elimination of the spalling defect.
[0007] These and other advantages, as will be realized, are
provided in an improved capacitor. The capacitor comprises parallel
conductive internal electrodes of alternating polarity with a
dielectric between the conductive internal electrodes. A first
copper undercoat is in electrical contact with the conductive
internal electrodes of a first polarity and a second copper
undercoat is in electrical contact with conductive internal
electrodes of a second polarity. A first lead is in electrical
contact with the first copper undercoat with a first solder between
the first lead and the first copper undercoat. A second lead is in
electrical contact with the second copper undercoat with a second
solder between the second lead and the second copper undercoat.
[0008] Yet another embodiment is provided in an improved capacitor.
The capacitor has parallel conductive internal electrodes of
alternating polarity with a dielectric between the conductive
internal electrodes wherein the conductive internal electrodes
comprise a precious metal. A first silver undercoat is in
electrical contact with conductive internal electrodes of a first
polarity and a second silver undercoat in electrical contact with
conductive internal electrodes of a second polarity. A first
solderable silver layer is in contact with the first silver
undercoat and a second solderable silver layer is in contact with
the second silver undercoat layers. A first lead is in electrical
contact with the first silver undercoat with the first solderable
silver layer between the first lead and the first silver undercoat
and a second lead is in electrical contact with the second silver
undercoat with the second solderable silver layer between the
second lead and the second silver undercoat.
[0009] Yet another embodiment is provided in a method for forming a
capacitor. The method includes the steps of:
providing a monolith comprising parallel conductive internal
electrodes of alternating polarity with a dielectric between the
conductive internal electrodes; applying a first copper undercoat
in direct electrical contact with the conductive internal
electrodes having a first polarity; applying a second copper
undercoat in direct electrical contact with the conductive internal
electrodes having a second polarity; soldering a first lead frame
in electrical contact with the first copper undercoat; and
soldering a second lead frame in electrical contact with the second
copper undercoat.
[0010] Yet another embodiment is provided in a method for forming a
capacitor. The method includes the steps of:
providing a monolith comprising parallel conductive internal
electrodes of alternating polarity with a dielectric between the
conductive internal electrodes wherein the conductive internal
electrodes comprise a precious metal; applying a first silver
undercoat in direct electrical contact with the conductive internal
electrodes having a first polarity; applying a second silver
undercoat in direct electrical contact with the conductive internal
electrodes having a second polarity; applying a first flexible
layer on the first silver undercoat; applying a first solderable
silver layer on the first flexible layer; applying a second
flexible layer on the second silver undercoat; applying a second
solderable silver layer on the second flexible layer; soldering a
first lead frame in electrical contact with the first silver
solderable silver layer; and soldering a second lead frame in
electrical contact with the second silver solderable layer.
BRIEF DESCRIPTION OF FIGURES
[0011] FIG. 1 is a side schematic view of an MLCC attached to lead
frames.
[0012] FIG. 2 is a cross-sectional partial side schematic view of
an MLCC attached to lead frames.
[0013] FIG. 3 is a cross-sectional partial side schematic view of
an MLCC attached to lead frames.
[0014] FIG. 4 is a cross-sectional partial side schematic view of
an MLCC illustrating a crack.
[0015] FIG. 5 is a cross-sectional view of a spalling defect on the
surface of an MLCC near the end metallization.
[0016] FIG. 6 is a cross-sectional view of a spalling defect
showing a crack under the end metallization formed by the spalling
damage.
[0017] FIG. 7 is a schematic cross-sectional view showing PME
MLCC's terminated with solderable silver end metallization attached
to lead frames via a solder attachment method.
[0018] FIG. 8 is a schematic cross-sectional view showing fired
copper terminated BME MLCC's or fired silver terminated PME MLCC's
coated with a cured silver-filled epoxy flexible termination layer
wherein the flexible termination layer is plated with typically
nickel and tin or tin-lead.
[0019] FIG. 9 is a side schematic view of MLCC's attached to lead
frames.
[0020] FIG. 10 is a side cross-sectional partial view of an MLCC
attached to lead frames.
[0021] FIG. 11 is a side cross-sectional partial schematic view of
an MLCC attached to lead frames.
[0022] FIG. 12 is a flow chart representation of a method of
forming a capacitor.
DESCRIPTION OF EMBODIMENTS
[0023] The present invention is related to an improvement in the
attachment of a lead frame to an MLCC. More particularly, the
present invention is related to an end metallization for an MLCC
which allows for attachment to a lead frame with minimal crack
propagation when the finished capacitor is subjected to thermal
cycles or thermal shock. The improved metallization eliminates the
need for plating of the MLCC's before soldering to a lead. The
improved metallization utilizes the application of a low viscosity
conductive-adhesive with solderable surface characteristics. This
layer can then be soldered directly to the lead frame thus
eliminating the plating step.
[0024] The invention will be described with reference to the
various figures which are an integral, non-limiting, component of
the disclosure. Throughout the various figures similar elements
will be numbered accordingly.
[0025] In FIG. 1, an example of a lead frame 2, soldered to an
MLCC, 1, is shown as typically done to improve temperature cycling
resistance, flex robustness, or to reduce microphonics. A single
MLCC chip, 1, or a plurality of chips can be attached to lead
frames, 2, in vertical or horizontal orientations by attachment
layers, 4, between the metallization, 3, and lead frame, 2. FIG. 2
illustrates a partial cross-sectional view of the attachment. The
attachment layers, 4, comprise a plating barrier layer, 5, which is
attached directly to the lead frame, 2, by solder, 6. FIG. 3
illustrates the additional use of a conductive adhesive, 9, between
the plating layer, 5 and metallization, 3. FIG. 4 shows a crack, 7,
which may form under the metallization, 3. Thermal stresses occur
under thermal shock or temperature cycling due to the CTE mismatch
of the MLCC and the lead frame. The crack may result in a failure
if it propagates into the active area and disrupts electrical
continuity of the internal electrodes, 8 and 8', of the MLCC.
[0026] In FIG. 5, a surface defect called spalling is shown.
Spalling is a particular problem that can occur during lead attach
processing when the MLCC surface temperature exceeds a certain
temperature, approximately 320.degree. C., which results in surface
cracking due to a mismatch in the CTE between the nickel plating
layer and the MLCC.
[0027] FIG. 6 shows a cross-sectional view of the spalling defect.
Typically, this defect does not extend into the active area of the
capacitor. However, because the spalling defect is typically
located at the interface of the nickel plating layer and the MLCC
surface, a crack under the end metallization can occur as shown in
FIG. 6. This crack at the end metallization interface can reduce
hermeticity of the MLCC and increase the risk of humidity
failures.
[0028] FIG. 7 shows a PME capacitor, 41, which is terminated with a
solderable silver end metallization, 40. This un-plated MLCC is
soldered, 42, directly to the lead frame, 43 without plating
layers.
[0029] FIG. 8 shows a flexible termination layer, 44, made of
silver-filled conductive adhesive is cured on top of a fired
copper-terminated, 45, BME MLCC or a fired silver-terminated PME
MLCC, 46. The flexible termination layer is plated with typically
nickel and tin or tin-lead, 47. The plated MLCC are connected to a
lead frame, 48 by solder, 49, attachment method.
[0030] An embodiment of the invention is illustrated in schematic
side view in FIG. 9. In FIG. 9 a stack of BME MLCC's, 10, is
illustrated. For the purposes of illustration only two MLCC's are
illustrated for convenience to achieve twice the capacitance of a
single MLCC. The number of MLCC's between electrodes is not
particularly limited herein. The lowest MLCC is illustrated above
the feet, 11, of the leads, 12, to isolate the MLCC from the
mounting substrate. The solderable fired copper metallization, 13,
is attached to the lead frames by a cured solderable silver epoxy,
14, which is attached to the lead frame by solder, 15.
[0031] Another embodiment of the invention is illustrated in
cross-sectional side view in FIG. 10 and schematically in FIG. 11.
In FIGS. 10 and 11, the MLCC, 20, comprises conductive internal
electrodes, 22, with a common polarity separated by dielectric, 24.
Internal electrodes of opposite polarity are not visible in FIG.
10. A sintered copper or silver undercoat, 13, is in direct
electrical contact with the internal electrodes, 22, having a
common polarity thereby forming a termination of the internal
electrodes. The conductive internal electrodes can be a base metal
electrode (BME) or a precious metal electrode (PME) with the
proviso that when the sintered undercoat is silver the internal
electrode is a PME. A flexible layer, 26, on the sintered copper or
silver undercoat provides electrical conductivity to a solderable
silver layer, 28, which is attached to a lead frame by solder, 15.
The flexible layer absorbs stress from thermal or mechanical board
flexure and allows compliance when the different components of the
leaded device are under movement so as to protect the brittle MLCC
from cracking. The sintered copper or silver undercoat,
particularly in combination with the flexible layer, reduces
exposure of MLCC to aggressive plating solutions that could
compromise dielectric robustness. The lead frame is not visible in
FIG. 10. The material layers are shown in FIG. 11 and described
below:
[0032] A termination paste for preparing the sintered copper
undercoat preferably comprises about (a) 75% to 90% by weight of
solids in the paste being finely divided copper particles, (b) 5 to
20% by weight of solids the paste being cadmium-free and
bismuth-free glass frit and more preferably the glass frit
represents about 5-8% by weight of the solids (c) a volatile and
decomposable organic liquid vehicle suspending the solids. The
paste viscosity is suitable for applying by dipping with a typical
viscosity is 20-50 Pa-s at 25.degree. C. The paste is dried at
around 140.degree. C. for 10 minutes and sintered for 7 to 15
minutes between 800.degree.-900.degree. C. to achieve proper curing
for copper. The cured film thickness is preferably between 1 and
100 microns and more preferably 10 and 100 microns. A particularly
preferred copper undercoat has a surface oxide content of less than
1 percent and a surface glass content of less than 8 percent and a
total copper film thickness greater than 1 micron.
[0033] A termination paste for preparing the sintered silver
undercoat comprises about (a) 75% to 90% by weight of solids in the
paste being finely divided silver particles, (b) 5 to 20% by weight
of solids the paste being cadmium-free and lead-free glass frit and
more preferably the glass frit represents about 5-8% by weight of
the solids (c) a volatile and decomposable organic liquid vehicle
suspending the solids. The paste viscosity is suitable for applying
by dipping with a typical viscosity is 20-50 Pa-s at 25.degree. C.
The paste is dried around 140.degree. C. for 10 minutes and
sintered for 7 to 15 minutes between 650.degree.-750.degree. C. to
achieve proper curing. The cured film thickness is preferably
between 1 and 100 microns and more preferably 10 and 100 microns. A
particularly preferred silver undercoat has a surface oxide content
of less than 1 percent and a surface glass content of less than 8
percent and a total silver film thickness greater than 1
micron.
[0034] A flexible layer is preferably formed by applying a paste
which preferably comprises about (a) 77-79 wt % solids in the paste
being finely divided silver particles, (b) 10-15 wt % organic
resins such as epoxy and (c) an organic solvent such as but not
limited to butyl carbitol to adjust viscosity. The paste is dried
at around 140.degree. C. for 5-30 minutes and cured at
150-200.degree. C. for 10-90 minutes. The electrical conductivity
should be no more than 0.3 mohms-cm. The flexible layer is
preferably a silver-filled conductive adhesive layer. The
silver-filled conductive adhesive may be used as a paint with a low
viscosity with a viscosity of at least about 5 Pa-S to no more than
about 50 Pa-S at 25.degree. C. suitable for demonstration of the
teachings herein. More particularly, the viscosity of the
silver-filled conductive adhesive paint is in the range of 7-15
Pa-s at 25.degree. C. as measured with a Brookfield Viscometer
Model RV type viscometer with a Spindle 14 and small volume
adapter.
[0035] The solderable silver layer is preferably a silver-filled
conductive adhesive layer. The silver-filled conductive adhesive
may be used as a paint with a low viscosity of at least 1000 to no
more 10,000 cps (1-10 Pa-S) as measured with a Brookfield RVT,
Spindle #3, 20 RPM, @ 25.degree. C. suitable for demonstration of
the teachings herein. The solids content is between 30-60% and
solvents such EB acetate and PM acetate are used to adjust
viscosity. The resistivity of the dried film is <0.3
mohms-cm.
[0036] When a flexible layer is used an additional thin conductive
adhesive layer, 28, is preferably applied on top of the flexible
layer. The conductive adhesive layer has a solderable cured surface
unlike the flexible termination conductive adhesive layer. The
benefit of the conductive adhesive layer is that it can be dipped
and cured onto the part thus eliminating the nickel and tin plating
layers as commonly employed in the art and attached by a solder
layer to a plated lead frame. By not having the nickel and tin
plated layers, stress from the CTE mis-match between the nickel
under-plate layer and the sintered copper end metallization is
eliminated thus significantly mitigating the initiation of surface
cracks in the ceramic, often referred to as "spalling", which often
occur upon lead attachment due to the heat fluctuations from the
soldering operation.
[0037] An embodiment of the invention will be described with
reference to FIG. 12 wherein the process for forming the capacitor
is illustrated in the form of a flow chart. An MLCC is prepared at
80. The method of forming the MLCC is not particularly limited
herein and any method employed in the art is suitable for
demonstration of the teachings herein. The MLCC has conductive
internal electrodes of alternating polarity terminating at
different locations with a dielectric between the conductive
internal electrodes.
[0038] A copper or silver undercoat is applied to the MLCC at 82 to
form external terminations with at least one external terminal of
each polarity. In a preferred embodiment the external terminations
are on opposing sides of the MLCC yet other embodiments are
suitable for demonstration of the invention such as a plurality of
external terminations on a single face with each external
termination being in electrical contact with internal conductive
electrodes of only one polarity. The copper or silver undercoat is
preferably applied as a paste and cured, or fired, prior to the
application of subsequent coatings.
[0039] In one embodiment the copper or silver undercoat can be
treated to improve adhesion of subsequent layers. Acid plasma
cleaning is a particularly preferred method for treating copper and
silver undercoat layers.
[0040] In one embodiment a flexible layer is applied at 84. The
flexible layer is preferably applied by coating a silver-filled
conductive adhesive onto the copper or silver undercoat followed by
drying and curing. The silver-filled conductive adhesive layer is
preferably applied by dipping or spraying onto the surface at the
appropriate location. Dipping is preferred due to the manufacturing
simplicity offered thereby.
[0041] A solderable silver layer is applied to either the flexible
layer or directly to the copper or silver undercoat at 86. The
solderable silver layer is preferably applied as paste which is
subsequently cured. A particular advantage of the copper or silver
undercoat layer and solderable silver layer is the ability to form
a conductive robust bond while excluding nickel and tin plating
layers. The substitution of nickel and tin layers with a solderable
silver layer mitigates the stresses caused by CTE mismatch between
the MLCC and the plating layers thus mitigating the propensity for
spalling to occur.
[0042] The solder is preferably lead-free and preferably comprises
5-30 wt % antimony.
[0043] The MLCC with a copper undercoat as the external termination
is placed within a lead frame at 88 and secured to the lead frame
by soldering at 90. In this embodiment, the lead frame is soldered
directly to the copper undercoat.
[0044] The lead frame preferably comprises, but is not limited to,
materials selected from phosphor bronze alloy 510, nickel iron
alloy 42 or copper iron alloy 194 and has a thickness of 76-500
microns.
[0045] The capacitor is finished at 92. Finishing can include
encasing in a non-conducting resin, testing and packing.
[0046] The MLCC is soldered to a plated lead frame. The solder
could comprise lead or, more preferably, could be lead-free. To
improve adhesion, a silver-containing solder is preferred when
silver containing layers are used to reduce leaching of the Ag in
the end metallization during soldering. To achieve a Pb-free robust
solder joint, an epoxy-containing flux may be used in the solder
formulation thereby providing additional mechanical strength at
higher temperatures thus preventing secondary reflow.
[0047] The dielectric is not particularly limited herein. The
conductive plates are separated by a dielectric as well known in
the art and exemplified in U.S. Pat. Nos. 7,211,740; 7,172,985;
7,164,573; 7,054,137; 7,068,490 and 6,906,907 each of which is
incorporated herein by reference. Conductive plates separated by
dielectric forms a capacitor as known in the art. While not limited
thereto, a dielectric layer with a thickness of about 0.2 .mu.m up
to about 50 .mu.m is suitable for demonstration of the teachings
herein. The number of dielectric layers stacked is generally from 2
to about 500 without limit thereto.
[0048] The conductive material which forms the internal electrodes
is not critical, although a base metal electrode (BME) is
preferably used due to cost considerations particularly when the
dielectric material of the dielectric layers has anti-reducing
properties. Typical base metals are nickel, copper, titanium,
tungsten, molybdenum, alloys or cermets of base metals or base
metal alloys with nickel being preferred. Preferred nickel alloys
are alloys of nickel with at least one member selected from Cu, Si,
Ba, Ti, Mn, Cr, Co, and Al, with such nickel alloys containing at
least 95 wt % of nickel being more preferred. Precious metal
electrodes (PME) can be used with the proviso that a sintered
silver undercoat is used. Preferred precious metals include silver,
palladium, gold, platinum and alloys thereof such as
silver-palladium and silver-palladium-platinum. The thickness of
the internal electrodes is not particularly limited although about
0.2 .mu.m to about 5 .mu.m is suitable for demonstration of the
teachings herein.
[0049] The multilayer ceramic chip capacitor of the present
invention generally is fabricated by forming a green chip by
conventional printing and sheeting methods using pastes, firing the
chip, and printing or transferring external electrodes thereto
followed by baking.
[0050] Paste for forming the dielectric layers can be obtained by
mixing a raw dielectric material with an organic or aqueous
vehicle. The raw dielectric material may be a mixture of oxides and
composite oxides as previously mentioned. Also useful are various
compounds which convert to such oxides and composite oxides upon
firing. These include, for example; carbonates, oxalates, nitrates,
hydroxides, and organometallic compounds. The dielectric material
is obtained by selecting appropriate species from these oxides and
compounds and mixing them. The proportion of such compounds in the
raw dielectric material is determined such that after firing, the
specific dielectric layer composition may be met. Raw dielectric
material in a powder form having a mean particle size of about 0.1
to about 3 .mu.m is suitable for demonstration of the teachings
herein. Dielectrics are well known and not limited herein.
[0051] A green chip may be prepared from the dielectric
layer-forming paste and the internal electrode layer-forming paste.
In the case of deposition by printing methods, a green chip is
prepared by alternately printing the pastes onto a substrate of
polyethylene terephthalate (PET), for example, in laminar form,
cutting the laminar stack to a predetermined shape and separating
it from the substrate.
[0052] Also useful is a sheeting method wherein a green chip is
prepared by forming green sheets from the dielectric layer-forming
paste, printing the internal electrode layer-forming paste on the
respective green sheets, and stacking the printed green sheets.
[0053] The binder is then removed from the green chip and fired.
Binder removal may be carried out under conventional conditions
where the internal electrode layers are formed of a base metal
conductor such as nickel and nickel alloys.
[0054] The term "direct" with reference to electrical contact is
taken to define an electrical connection between two layers with no
layer there between. When two layers of different composition are
combined a blended layer wherein one component diffuses into the
other thereby forming an intermediate composition is considered a
direct electrical connection.
[0055] Paste for forming internal electrode layers is obtained by
mixing an electro-conductive material with an organic or aqueous
vehicle. The conductive material used herein includes conductors
such as conductive metals and alloys as mentioned above and various
compounds which convert into such conductors upon firing, for
example, oxides, organometallic compounds and resinates.
Example 1
[0056] A BME MLCC with nickel internal electrodes was dipped in
copper paste consisting of an acrylic resin dispersed in
dihyro-terpeniol solvent. The solids content was about 77% by
weight of finely divided copper and about 10 percent by weight of
cadmium-free and bismuth-free glass frit. The viscosity of this
paste was adjusted to about 13 Pa-s by further additions of
dihydro-terpeniol. After dipping, the MLCC was dried at 140.degree.
C. for 20 minutes in air atmosphere and then cured, or fired, at
900.degree. C. for 10 minutes in an inert atmosphere.
[0057] The BME MLCC with fired copper end metallization was
soldered directly to a phosphor bronze lead frame that was plated
with a nickel barrier plating layer and a tin overplate layer. 91.5
Sn 8.5 Sb lead free HMP solder was reflowed above 280.degree. C. to
attach the fired copper end met directly to the plated lead frame.
The reflow was conducted within 24 hours of copper firing so as to
minimize oxide layers from building up on the fired copper end
metallization rendering it unsolderable.
Example 2
[0058] A BME MLCC with nickel internal electrodes was dipped in
copper paste consisting of an acrylic resin dispersed in
dihyro-terpeniol solvent. The solids content was about 77% by
weight of finely divided copper and about 10 percent by weight of
cadmium-free and bismuth-free glass frit. The viscosity of this
paste was adjusted to about 13 Pa-s by further additions of
dihydro-terpeniol. After dipping, the MLCC was dried at 140.degree.
C. for 20 minutes in air atmosphere and then cured, or fired, at
900.degree. C. for 10 minutes in an inert atmosphere.
[0059] The fired copper termination layer was then dipped in a
conventional solderable silver consisting of a silver-bearing,
thermoplastic paint. This layer of silver paint was dried at
30.degree. C. for 45 minutes, pre-cured between 60-70.degree. C.
for 30 minutes and cured at 150.degree. C. for 60 minutes.
[0060] The lead frame was attached to the cured silver with a 8.5%
Sb 91.5% Sn solder alloy rosin mildly activated (RMA) flux system
available from Henkel as 92ADA100AP85V using a resistance-heated
hot bar contact reflow soldering method. The soldering temperature
was >280.degree. C. in the solder joint. The resulting product
had adhesion peel strength greater than 0.60 kg and electrical
properties such as control capacitance, dissipation factor and
internal resistance similar to the MLCC.
Example 3
[0061] A BME MLCC with nickel internal electrodes was dipped in
copper paste consisting of an acrylic resin dispersed in
dihyro-terpeniol solvent. The solids content was about 77% by
weight of finely divided copper and about 10 percent by weight of
cadmium-free and bismuth-free glass frit. The viscosity of this
paste was adjusted to about 13 Pa-s by further additions of
dihydro-terpeniol. After dipping, the MLCC was dried at 140.degree.
C. for 20 minutes in air atmosphere and then cured (fired) at
900.degree. C. for 10 minutes in an inert atmosphere.
[0062] A flexible layer paste was applied to the copper undercoat
using a dipping process. The flexible layer paste consisted about
of 78% by weight of solids in the paste being finely divided silver
particles and about 12% by weight of epoxy organic resin and the
remainder being a an organic solvent, butyl carbitol. Further
additions of butyl carbitol were used to adjust viscosity of the
final paste to about 15 Pa-s. Following dipping, the flexible paste
was dried at 140.degree. C. for 20 minutes and cured at 180.degree.
C. for 60 minutes. The electrical conductivity was <0.3
mohms-cm.
[0063] The flexible termination layer was then dipped in a
conventional solderable silver consisting of .a silver-bearing,
thermoplastic paint. This layer of silver paint was dried at
30.degree. C. for 45 minutes, pre-cured between 60-70.degree. C.
for 30 minutes and cured at 150.degree. C. for 60 minutes.
[0064] The lead frame was attached to the cured silver with a 8.5%
Sb 91.5% Sn solder alloy rosin mildly activated (RMA) flux system
available from Henkel as 92ADA100AP85V using a resistance-heated
hot bar contact reflow soldering method. The soldering temperature
was >280.degree. C. in the solder joint. The resulting product
had adhesion peel strength greater than 0.60 Kg and electrical
properties such as control capacitance, dissipation factor and,
internal resistance similar to the MLCC.
[0065] The addition of the flexible layer between the copper or
silver undercoat and the solderable silver paint mitigates thermal
or mechanical cracking by changing the failure mode when thermal or
mechanical stresses are applied so as to result in a tear in the
cured epoxy flexible termination layer rather than a crack in the
MLCC. This provides a preferred failure mode which prevents the
MLCC from shorting out under thermal or mechanical stress.
[0066] The invention has been described with reference to the
preferred embodiments without limit thereto. One of skill in the
art will realize additional embodiments and improvements which are
not specifically stated but which are within the metes and bounds
of the invention as more specifically set forth in the claims
appended hereto.
* * * * *