Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVED M~O~ AND APPARATUS FOR A
SURFACE-MOUNTED FUSE DEVICE
DE'SCRIPTION
Technical Field
The invention relates generally to a
surface-mountable fuse for placement into and
protection of the electrical circuit of a
printed circuit board.
Related APplication
The present application is a
continuation-in-part application of U.S.
Serial No. 08/247,584, filed May 27, 1994.
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Bac~ouh~d Of The Invention
Printed circuit (PC) boards have
found increasing application in electrical and
electronic equipment of all kinds. The
electrical circuits formed on these PC boards,
like larger scale, conventional electrical
circuits, need protection against electrical
overloads. This protection is typically
provided by subminiature fuses that are
physically secured to the PC board.
One example ~f such a subminiature,
surface-mounted fuse is disclosed in U.S.
Patent No. 5,166,656 ('656 patent). The
fusible link of this surface-mounted ~use is
disclosed as being covered with a three layer
composite which includes a passivation layer,
an insulating cover, and an epoxy layer to
bond the passivation layer to the insulating
cover. See '656 patent, column 6, lines 4-7.
Typically, the passivation layer is either
chemically vapor-deposited silica or a thick
layer of printed glass. See '656 patent,
column 3, lines 39-41. The insulating cover
may be a glass cover. See '656 patent, column
4, lines 43-46. The fuse ~rom the '656 patent
has three layers protecting its fusible link.
In addition, the fuse from the '656 patent has
relatively thick glass covering. There are
several other features in the '656 patent fuse
which are unnecessary in the present
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invention. Thus, the present invention is
designed to solve these and other problems.
Summary Of The Invention
The invention is a thin film,
surface-mounted fuse which comprises two
material subassemblies. The first subassembly
comprises a fusible link, its supporting
substrate and terminal pads. The second
subassembly comprises a protective layer which
overlies the fusible link so as to provide
protection from impacts and oxidation.
The protective layer is preferably
made of a polymeric material. The most
preferred polymeric material is a polyurethane
gel or paste when the stencil printing step is
used to apply the cover coat. However,
polycarbonates will also work well when an
injection molding step is used to apply the
cover coat. In addition, the most preferred
supporting substrate is an FR-4 epoxy or a
polyimide.
A second aspect of the invention is
a thin film, surface-mounted fuse. This fuse
comprises a fusible link made of a conductive
metal. The first conductive metal is
preferably, but not exclusively, selected from
the group including copper, silver, nickel,
titanium, aluminum or alloys of these
conductive metals. A second conductive metal,
different from the first conductive metal, is
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deposited on the surface of this fusible link.
One preferred metal for the surface-mounted
fuse of this invention is copper. One
preferred second conductive metal is tin-lead.
Another preferred second conductive metal is
tin.
The second conductive metal may be
deposited onto the fusible link in the form of
a rectangle, circle or in the form of any of
several other configurations, depending on the
configuration of the fuse link. The second
conductive metal is preferably deposited along
the central portion of the fusible link.
Photolithographic, mechanical and
laser processing techniques may be employed to
create very small, intricate and complex
fusible link geometries. This capability,
when combined with the extremely thin film
coatings applied through electrochemical and
physical vapor deposition (PVD) techniques,
enables these subminiature fuses to control
the fusible area of the element and protect
circuits passing microampere- and ampere-range
currents. This is unique, in that prior fuses
providing protection at these high currents
were made with filament wires. The
manufacture of such filament wire fuses
created certain difficulties in handling.
The location of the fusible link at
the top of the substrate of the present fuse
enables one to use laser processing methods as
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a high precision secondary operation, in that
way trimming the final resistance value of the
fuse element.
5 Brief DescriPtion Of The Drawinqs
FIG. 1 is a perspective view of a
copper-plated, FR-4 epoxy sheet used to make a
subminiature surface-mounted fuse in
accordance with the invention.
FIG. 2 is a view of a portion of the
sheet of FIG. 1, and taken along lines 2-2 of
FIG. 1.
FIG. 3 is a perspective view of the
FR-4 epoxy sheet of FIG. 1, but stripped of
its copper plating, and with a plurality of
bores (partially shown), each having a
diameter D, spaced apart by a length L and a
width W, and routed into separate quadrants of
that sheet.
FIG. 4 is an enlarged, perspective
view of a cut-away portion of the bored sheet
of FIG. 3, but with a copper plating layer
having been reapplied.
FIG. 5 is a cut-away perspective
view of the flat, upward-facing surfaces of
the replated copper sheet, after the sheet was
masked with a multi-squared panel of an
ultraviolet (W) light-opaque substance.
FIG. 6 is a perspective view of the
reverse side of FIG. 5, rotated about one of
the fuse rows 27, but after the removal of a
.
.
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strip-like portion of copper plating from the
replated sheet of FIG. 5.
FIG. 7 is a perspective view of the
top-side of FIG. 6, rotated about one of the
fuse rows 27, and showing linear regions 40
defined by dotted lines.
FIG. 8 is a perspective view of a
single fuse row 27 from the sheet, cut away
from the other fuse rows, and cut away at one
edge of one of the fuses, after dipping the
sheet into a copper plating bath and then a
nickel plating bath, with the result that
copper and nickel layers are deposited onto
the base copper layer of the terminal pads,
including the grooves of the pads.
FIG. 9 is a perspective view of the
strip of FIG. 8, but prior to W light curing,
and showing a fuse-blowing portion 50 at the
center of fusible link 42 that is masked with
a W light-opaque substance.
FIG. 10 shows the strip of FIG. 9,
but after immersion into a tin-lead plating
bath to create another layer over the copper
and nickel layers, and after deposition of a
tin-lead alloy onto the central portion of the
fusible link.
FIG. 11 shows the strip of FIG. 10,
but with an added polymeric gel or paste layer
onto the top of the fuse row 27.
FIG. 12 shows the individual fuse in
accordance with the invention as it is finally
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made, and after a so-called dicing operation
in which a diamond saw is used to cut the
strips along parallel and perpindicular planes
to form these individual surface-mountable
~ 5 fuses.
Detailed Description Of
The Preferred Embodiment
While this invention is susceptible
l0 of embodiment in many different forms, there
is shown in the drawings and will herein be
described in detail a preferred embodiment of
the invention. It is to be understood that
the present disclosure is to be considered as
15 an exemplification of the principles of the
invention. This disclosure is not intended to
limit the broad aspect of the invention to the
illustrated embodiment or embodiments.
One preferred embodiment of the
20 present invention is shown in FIG. 12. The
thin film, surface-mounted fuse is a
subminiature fuse used in a surface mount
configuration on a PC board or on a thick film
hybrid circuit. One of these fuses is
25 typically known in the art as an "A" case
fuse. The "A" case fuse standard industry
size for these fuses is 125 mils. long by 60
mils. wide. The "A" case fuse is also
designated as a 1206 fuse. In addition, the
30 present invention includes even smaller sized
fuses which are compatible with standard sized
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surface mountable devices. In particular, the
present invention can be used within all other
standard sizes of such surface mountable
device sizes, such as 1210, 0805, 0603 and
0402 fuses, as well as non-standard sizes.
The invention generally comprises
two material subassemblies. As will be seen,
the first subassembly includes the fuse
element or fusible link 42, its supporting
substrate or core 13, and terminal pads 34 and
36 for connecting the fuse 58 to the PC board.
The second subassembly is a protective layer
56 which overlies the fusible link 42 and a
substantial portion of the top portion of the
fuse so as to, at least, provide protection
from impacts which may occur during automated
assembly, and protection from oxidation during
use.
The first subassembly contains and
supports two metal electrodes or pads 34, 36,
and the fusible element or link 42, both of
which are bonded to the substrate as a single
continuous film, as shown in FIGs. 5 and 6.
The pads 34, 36 are located on the top, the
bottom, and a the sides of the substrate or
core 13, while the fusible link 42 is located
at the top of the substrate 13. More
specifically, the pads 34, 36 extend into the
two grooves 16 (each groove 16 is one half of
each bore 14) in each fuse created by the
bores 14 and dicing operation during the
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process of manufacture, as will be further
described below.
As will be seen, in the preferred
embodiment, pads are made up of several
layers, including a base copper layer, a
supplemental copper layer, a nickel layer and
a tin-lead layer. The base copper layer of
the pads and the thin film fusible link are
simultaneously deposited by (1)
electrochemical processes, such as the plating
described in the preferred embodiment below;
or (2) by PVD. Such simultaneous deposition
ensures a good conductive path between the
fusible link 42 and the terminal pads 34, 36.
This type of deposition also facilitates
manufacture, and permits very precise control
of the thickness of the fusible link 42.
After initial placement of the
fusible link 42 and the base copper onto the
substrate 13, additional layers of a
conductive metal are placed onto the terminal
pads 34, 36. These additional layers could be
defined and placed onto these pads by
photolithography and deposition techniques,
respectively.
This fuse may be made by the
following process. Shown in FIGS. 1 and 2 is
a solid sheet 10 of an FR-4 epoxy with copper
plating 12. The copper plating 12 and the FR-
~ epoxy core 13 of this solid sheet 10 maybest be seen in FIG. 2. This copper-plated
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FR-4 epoxy sheet 10 is available from Allied
Signal Laminate Systems, Hoosick Falls, New
York, as Part No. 0200BED130C1/ClGFN0200
C1/ClA2C. Although FR-4 epoxy is a preferred
5 material, other suitable materials include any
material that is compatible with, i.e., of a
chemically, physically and structurally
similar nature to, the materials from which PC
boards are made. Thus, another suitable
10 material for this solid sheet 10 is polyimide.
FR-4 epoxy and polyimide are among the class
of materials having physical properties that
are nearly identical with the standard
substrate material used in the PC board
15 industry. As a result, the fuse of the
invention and the PC board to which that fuse
is secured have extremely well-matched thermal
and mechanical properties. The substrate of
the fuse of the present invention also
20 provides desired arc-tracking characteristics,
and slmultaneously exhibits sufficient
mechanical flexibility to remain intact when
exposed to the rapid release of energy
associated with arcing.
In the next step of the process of
manufacturing the fuses of the present
invention, the copper plating 12 is etched
away from the solid sheet 10 by a conventional
etching process. In this conventional etching
process, the copper is etched away from the
substrate by a ~erric chloride solution.
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Although it will be understood that
after completion of this step, all of the
copper layer 12 of FIG. 2 is etched away from
FR-4 epoxy core 13 of this solid sheet 10, the
remaining epoxy core 13 of this FR-4 epoxy
sheet 10 is different from a "clean" sheet of
FR-4 epoxy that had not initially been treated
with a copper layer. In particular, a
chemically etched surface treatment remains on
the surface of the epoxy core 13 after the
copper layer 12 has been removed by etching.
This treated surface of the epoxy core 13 is
more receptive to subsequent operations that
are necessary in the manufacture of the
present surface-mounted subminiature fuse.
The FR-4 epoxy sheet 10 having this
treated, copper-free surface is then drilled
or punched to create holes or bores 14 along
four quadrants lOa, lOb, lOc, lOd of the sheet
10, as may be seen in FIG. 3. Broken lines
visually separate these four quadrants lOa,
lOb, lOc, lOd in FIG. 3. It should be further
noted that in FIG. 3, the bores 14 are lined
up into rows 27 and columns 29. Although only
four rows 27 of bores 14 are shown in FIG. 3
in one quadrant lOa for convenience, the rows
27 of holes 14 are actually disposed over
almost the entire sheet 10 in all four
quadrants lOa, lOb, lOc, lOd, as is designated
by the three dots 11 For the "603" standard
sizing of surface mounted devices mentioned
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above, the length L between the center of the
bores 14 is approximately 70 mils, and the
width W between the center of the bores 14 is
approximately 38 mils. For the "402" standard
5 sizing of surface mounted devices mentioned
above, the length L between the center of the
bores 14 is approximately 50 mils, and the
width W between the center of the bores 14 is
approximately 30 mils. Again, smaller and
10 larger standard and non-standard sizings are
possible for the present invention. The
diameter D (FIG. 4) for each bore 14 for the
"603" sizing is approximately 18 mils.
When the drilling or punching of the
15 bores 14 has been completed, the etched and
bored sheet 10 shown in FIG. 3 is again plated
with copper. This reapplication of copper
occurs through the immersion of the etched and
bored sheet of FIG. 3 into an electroless
20 copper plating bath. This method of copper
plating is well-known in the art.
This copper plating step results in
the placement of a copper layer having a
uniform thickness along each of the exposed
25 surfaces of the sheet 10. For example, as may
be seen in FIG. 4, the copper plating 18
resulting from this step covers both (1) the
flat, upper surfaces 22 of the sheet 10; and
(2) the vertical regions of the groves 16
30 and/or the vertical regions of the bores 14.
These vertical portion of the grooves 16
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and/or bores 14 must be copper-plated because
they will ultimately form a portion of the
terminal pads 34, 36 of the final fuse as will
be further described below.
The uniform thickness of the copper
plating will depend upon the ultimate needs of
the user. Particularly, as may be seen in
FIG. 4, for a fuse intended to open at 1/16
ampere, the copper plating 18 has a thickness
of 2,500 Angstroms. For a fuse intended to
open at 5 amperes, the copper plating 18 has a
thickness of approximately 75,000 Angstroms
for a particular width of the fusable link.
After plating has been completed, to
arrive at the copper-plated structure of FIG.
4, the entire exposed surface of this
structure is covered with a so-called
photoresist polymer.
An otherwise clear mask is placed
over the replated copper sheet 20 from FIG. 4
after it has been covered with the
photoresist. Square panels are a part of, and
are evenly spaced across, this clear mask
according to the sizing of the fuse being
manufactured. These square panels are made of
an W light-opaque substance, and are
generally shown as the rectangle 30 shown in
FIG. 5. Essentially, by placing this mask
having these panels onto the replated copper
sheet 20, several portions of the flat,
upward-facing surfaces 22 of the replated
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copper sheet 20 from FIG 4. are effectively
shielded from the effects of W light.
It will be understood from the
following discussion that these square panels
5 will essentially define the shapes and sizes
of the so-called fusible link 42 and the upper
terminal areas 60 of the terminal pads 34, 36
- on the upper portion 22 of the fuse. The
fusible link 42 is in electrical communication
lO with the upper terminal areas 60. It will be
appreciated that the width, length and shape
of both the fusible link 42 and these upper
terminal areas 60 may be altered by changing
the size and shape of these W light-opaque
15 panels.
Additionally, the backside of the
sheet is covered with a photoresist material
and an otherwise clear mask is placed over the
replated copper sheet 20 after it has been
20 covered with the photoresist. A rectangular
panel is a part o~ this clear mask. The
rectangular panels are made of a W light-
opaque substance, and are of a size
corresponding to the size of the panel 28
25 shown in FIG. 6. Essentially, by placing this
mask having these panels onto the replated
copper sheet 20, several strips of the flat,
downward-facing surfaces 28 of the replated
copper sheet 20 are ef~ectively shielded from
30 the effects of the W light. The rectangular
panels will essentially define the shapes and
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sizes of the lower terminal areas 62 of the
terminal pads 34, 36, and the lower middle
portions 28 of sheet 20, as shown in FIG. 6.
The copper plating from a portion of
the underside of a sheet 20 is defined by a
photoresist mask. Particularly, the copper
plating from the lower, middle portions 28 of
- the underside of the sheet 20 is removed. The
lower, middle portions 28 of the underside of
the sheet 20 is that part of the strip along a
line immediately beneath the areas 30 of clear
epoxy, and the fuse links 42. A perspective
view of this section of this replated sheet 20
is shown in FIG. 6.
The entire replated, photoresist-
covered sheet 20, i. e., the top, bottom and
sides of that sheet, is then subjected to W
light. The replated sheet 20 is subjected to
the W light for a time sufficient to ensure
curing of all of the photoresist that is not
covered by the square panels and rectangular
strips of the masks. Thereafter, the masks
containing these square panels and rectangular
strips are removed from the replated sheet 20.
The photoresist that was formerly below these
square panels remains uncured. This uncured
photoresist may be washed from the replated
sheet 20 using a solvent.
The cured photoresist on the
remainder of the replated sheet 20 provides
protection against the next step in the
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17
process. Particularly, the cured photoresist
prevents the removal of copper beneath those
areas of cured photoresist. The regions
formerly below the square panels have no cured
5 photoresist and no such protection. Thus, the
copper from those regions can be removed by
etching. This etching is performed with a
ferric chloride solution through well known
etching concepts.
After the copper has been removed,
as may be seen in FIGS. 5 and 6, the regions
formerly below the square panels and the
rectangular strips of the mask are not covered
at all. Rather, those regions now comprise
15 areas 28 and 30 of clear epoxy.
The replated sheet 20 is then placed
in a chemical bath to remove all of the
rem~'n'ng cured photoresist from the
previously cured areas of that sheet 20.
After completion of several of the
operations described in this specification,
this sheet 20 will ultimately be cut into a
plurality of pieces, and each of these pieces
becomes a fuse in accordance with the
invention, as will be further described below.
However, for the purpose of brevity, only a
cut-away portion of the overall sheet
including three rows 27 and four columns 29 is
shown in FIGS. 5 through 7. As may also be
seen from FIG. 5 through 7, the bores 14 and
grooves of the sheet 20 still include copper
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18
plating. These bores 14 and grooves 16 form
portions of the pads 34, 36. These pads 34,
36 will ultimately serve as the means for
securing the entire, finished fuse to the PC
5 board.
FIG. 7 is a perspective view of the
opposite side of the sheet 20 from FIG. 6.
~irectly opposite and coinciding with the
lower, middle portions 28 of the sheet 20 are
linear regions 40 on the top-side 38 of the
sheet 20. These linear regions 40 are de~ined
by the dotted lines of FIG. 7.
FIG. 7 is to be referred to in
connection with the next step in the
manufacture of the invention. In this next
step, a photoresist polymer is placed along
each of the linear regions 40 of the top side
38 of the sheet 20. Through the covering of
these linear regions 40, photoresist polymer
is also placed along the relatively thin
portions which will comprise the fusible links
42. These fusible links 42 are made of a
conductive metal, here copper. The
photoresist polymer is then treated with UV
light, resulting in a curing of the polymer
onto linear region 40 and its fusible links
42.
As a result of the curing of this
photoresist onto the linear region 40 and its
fusible links 42, metal will not adhere to
this linear region 40 when the sheet 20 is
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19
dipped into an electrolytic bath containing a
metal for plating purposes.
- In addition, as explained above, the
middle portion 28 of the underside o~ the
5 sheet 20 will also not be subject to plating
when the sheet 20 is dipped into the
electrolytic plating bath. Copper metal
previously covering this metal portion had
been removed, revealing the bare epoxy that
10 forms the base of the sheet 20. Metal will
not adhere to or plate onto this bare epoxy
using an electrolytic plating process.
The entire sheet 20 is dipped into
an electrolytic copper plating bath and then
15 an electrolytic nickel plating bath. As a
result, as may be seen in FIG. 8, a copper
layer 46 and a nickel layer 48 are deposited
on the base copper layer 44. After deposition
of these copper 46 and nickel layers 48, the
20 cured photoresist polymer on the linear region
40, including the photoresist polymer on the
fusible links 42, is removed from that region
40.
Photoresist polymer is then
25 immediately reapplied along the entire linear
region 40. As may be seen in FIG. 9, however,
a portion 50 at the center of the fusible link
42 is masked with a W light-opaque substance.
The entire linear region 40 is then subjected
30 to W light, with the result that curing of
the photoresist polymer occurs on all of that
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region, except for the masked central portion
50 of the fusible link 42. The mask is
removed from the central portion 50 of the
fusible link, and the sheet 20 iS rinsed. As
5 a result of this rinsing, the uncured
photoresist above the central portion 50 of
the fusible link 42 iS removed from the
fusible link 42. The cured photoresist along
the remainder of the linear region 40,
however, rl~m~l n~:.
Plating of metal will not occur on
the portion of the sheet 20 covered by the
cured photoresist. Because of the absence of
the photoresist from the central portion 50 o~
the fusible link 42, however, metal may be
plated onto this central portion 50.
When the strip shown in FIG. 9 is
dipped into an electrolytic tin-lead plating
bath, a tin-lead layer 52 (FIG. 10) is
overlain over the copper 46 and nickel layers
48. A tin-lead spot 54 iS also deposited onto
the surface of the fusible link 42, i.e.,
essentially placed by an electrolytic plating
process onto the central portion 50 of the
~usible link 42 This electrolytic plating
process is essentially a thin film deposition
process. It will be understood, however, that
this tin-lead may also be added to the surface
of the fusible link 42 by a photolithographic
process or by means of a physical vapor
deposition process, such as sputtering or
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evaporation in a high vacuum deposition
chamber.
This spot 54 is comprised of a
second conductive metal, i.e., tin-lead or
tin, that is dissimilar to the copper metal of
the fusible link 42. This second conductive
metal in the form of the tin-lead spot 54 is
deposited onto the fusible link 42 in the form
of a rectangle.
The tin-lead spot 54 on the fusible
link 42 provides that link 42 with certain
advantages. First, the tin-lead spot 54 melts
upon current overload conditions, creating a
fusible link 42 that becomes a tin-lead-copper
alloy. This tin-lead-copper alloy results in
a fusible link 42 having a lower melting
temperature than the copper alone. The lower
melting temperature reduces the operating
temperature of the fuse device of the
invention, and this results in improved
performance of the device.
Although a tin-lead alloy is
deposited on the copper fusible link 42 in
this example, it will be understood by those
skilled in the art that other conductive
metals may be placed on the fusible link 42 to
lower its melting temperature, and that the
fusible link 42 itself may be made of
conductive metals other than copper. In
30 addition, the tin-lead alloy or other metal
deposited on the fusible link 42 need not be
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22
of a rectangular shape, but can take on any
number of additional configurations.
The second conductive metal may be
placed in a notched section of the link, or in
5 holes or voids in that link. Parallel fuse
links are also possible. As a result of this
flexibility, specific electrical
characteristics can be engineered into the
fuse to meet varying needs of the ultimate
10 user.
As indicated above, one o~ the
possible fusible link configurations is a
serpentine configuration. By using a
serpentine configuration, the e~fective length
15 of the fusible link may be increased, even
though the distance between the terminals at
the opposite ends of that link remain the
same. In this way, a serpentine con~iguration
provides for a longer fusible link without
20 increasing the dimensions of the fuse itself.
The next step in the manufacture o~
the device of the invention is the placement,
across a significant portion of the top o~ the
sheet 20 between the terminal pads 34, 36, of
25 a protective layer 56 (FIG. 11). This
protective layer 56 is the second subassenbly
of the present ~use, and forms a relatively
tight seal over the portion of the top of the
sheet where the fusible links 42 exist. In
30 this way, the protective layer 56 inhibits
corrosion of the fusible links 42 during their
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useful llves. The protectlve layer 56 also
provldes protectlon from oxidatlon and impacts
durlng attachment to the PC board. Thls
protectlve layer also serves as a means of
5 provldlng for a surface for pick and place
operatlons whlch use a vacuum plck-up tool.
Thls protective layer 56 helps to
control the melting, ionization and arcing
which occur in the fusible link 42 during
10 current overload conditions. The protective
layer 56 or cover coat materlal provldes
deslred arc-quenchlng characterlstlcs,
especlally lmportant upon lnterruptlon of the
fuslble llnk 42.
The protectlve layer 56 may be
comprlsed of a polymer, preferably a
polyurethane gel or paste when a stencll prlnt
operatlon ls used to apply the cover coat. A
preferred polyurethane ls made by Dymax
20 Corporation. Other similar gels, pastes, or
adhesives are suitable for the invention. In
addition to polymers, the protective layer 56
may also be comprised of plastlcs, conformal
coatings and epoxies.
This protective layer 56 is applied
to the strips 26 using a stencll prlntlng
process whlch includes the use of a common
stencll printlng machlne. In the past, an
lnjectlon of the material into a die mold was
performed while the sheet 20 was clamped
between two dies. However, stencil printing
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is a much faster process. Specifically, it
has been found that the use of a stencil
printing process while using a stencil
printing machine, at least, doubles production
5 output of the number of fuses from a previous
die mold operation. The stencil printing
machine is made by Affiliated Manufacturers,
Inc. of Northbranch, New ~ersey, Model No. CP-
885.
In the stencil printing process, the
material is applied to the sheet 20 in strips
simultaneously, instead of two strips at a
time in the die mold/injection filling
process. As will be further explained below,
15 the material is cured much faster than the
injection fill process because in the stencil
printing process, the cover coat material is
completely exposed to the W radiation from
the lamps as opposed to the injection filling
20 process where you have a filter that you have
to transmit the energy from the lamp to the
coating itself because the mold itself acts as
a filter. Furthermore, the stencil printing
process produces a more uniform cover coat
25 than the injection filling process, in terms
of the height, the width of the covet coat.
Because of that uniformity, the fuses can be
tested and packaged automatically. With the
injection filling process it was sometimes
30 difficult to precisely align the fuses in
testing and packaging equipment due to some
CA 02224070 1997-12-0~
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non-uniform heights and widths of the cover
coat.
The stencil printing machine
comprises a slidable plate 70, a base 72. a
5 squeegee arm 74, a squeegee 76, and an overlay
78. The overlay 78 is mounted on the base 72
and the squeegee 76 is movably mounted on the
squeegee arm 74 above the base 72 and overlay
78. The plate 70 is slidable underneath the
base 72 and overlay 78. The overlay 78 has
parallel openings 80 which correspond to the
width of the cover coat 56.
The stencil printing process begins
by attaching an adhesive tape under the fuse
sheet 20. The fuse sheet 20, with the
adhesive tape, is placed on the plate 70 with
the adhesive tape between the plate 70 and the
fuse sheet 20. The cover coat material is
then applied with a syringe at one end of the
overlay 78. The plate 70 then slides
underneath the overlay 78 and lodges the sheet
20 underneath the overlay 78 in correct
alignment with the parallel openings 80. The
squeegee 76 then lowers to contact the overlay
78 beyond the material on the top of the
overlay 78. The squeegee 76 then moves across
the overlay 78 where the openings 80 exist,
thereby forcing the cover coat material
through the openings 80 and onto the sheet.
Thus, the cover coat now covers the fuse link
area 40 (FIGS. 8 & 9). The squeegee 76 is
_
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26
then raised, the sheet 20 is unlodged from the
overlay 78, and the sheet 20 is placed in a W
light chamber so that the material can
solidify and form the protective layer 56
(FIGS. 11 & 12). The openings 80 in the
overlay 78 are wide enough so that the
protective layer partially overlaps the pads
- 34, 36, as shown in FIGS. 11 & 12. In
addition, the material used for the cover coat
should have a viscosity in the gel or paste
range so that after the material is spread
onto the sheet 20, it will flow in a manner
which creates a generally ~lat top sur~ace 49,
but not flow into the holes 14 or groves 16.
Although a colorless, clear cover
coat is aesthetically pleasing, alternative
types of cover coats may be used. For
example, colored, clear materials may be used.
These colored materials may be simply
manufactured by the addition of a dye to a
clear polyurethane gel or paste. Color coding
may be accomplished through the use of these
colored gels and pastes. In other words,
different colors of gels can correspond to
di~erent amperages, providing the user with a
ready means of determining the amperage of any
given fuse. The transparency of both o~ these
coatings permit the user to visually inspect
the fusible link 42 prior to installation, and
during use, in the electronic device in which
the ~use is used.
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The use of this protective layer 56
has significant advantages over the prior art,
including the prior art, so-called, "capping"
method. Due to the placement of the
S protective layer 56 over the entire top of a
fuse body, the location of the protective
layer relative to the location of the fusible
link 42 is not critical.
The sheet 20 is then ready for a so-
10 called dicing operation, which separates the
rows and columns 27, 29 from one another, and
into individual fuses. In this dicing
operation, a diamond saw or the like is used
to cut the sheet 20 along parallel planes 57
(FIG. 11), and again perpindicular to planes
57, through the center of the holes 14, into
individual thin film surface-mounted fuses 58
(FIG. 12). One of the directions of cuts
bisect the terminal areas through the center
of the holes 14, thereby exposing and creating
the grooves 16 of the terminal pads 34, 36.
These grooves 16 appear on either side of the
fusible link 42.
This cutting operation completes the
manufacture of the thin film surface-mounted
fuse 58 (FIG. 12) of the present invention.
Fuses in accordance with this
invention are rated at voltages and amperages
greater than the ratings of prior art devices.
Tests have indicated that fuses which fall
under the "603" standard sizing would have a
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28
fuse voltage rating of 32 volts AC, and a fuse
amperage rating of between 1/16 ampere and 2
amperes. Even though the fuses in accordance
with this invention can protect circuits over
5 a broad range of amperage ratings, the actual
physical size of these fuses remains constant.
In summary, the fuse of the present
invention exhibits improved control of fusing
characteristics by regulating voltage drops
10 across the fusible link 42. Consistent
clearing times are ensured by (1) the ability
to control, through deposition and
photolithography processes, the dimensions and
shapes of the fusible link 42 and terminal
15 pads 34, 36; and (2) proper selection of the
materials of the fusible link 42. Restriking
tendencies are minimized by selection of an
optimized material for the substrate 13 and
protective layer 56.
While the specific embodiments have
been illustrated and described, numerous
modifications come to mind without
significantly departing from the spirit of the
invention, and the scope of protection is only
limited by the scope of the accompanying
Claims.
