Note: Descriptions are shown in the official language in which they were submitted.
RD-13,683
ELECTRICAL ASSEMBLY INCLUDING A CONDUCTOR
PATTERN BONDED TO A NON-METALLIC SUBSTRATE
AN3 METHOD OF FABP~ICATING SUCH ASSEMBLY
_ ~ _ . . ... . . _._ _ . .
Background of the Invention
My invention relates to an electrical assembly includin~
conductor pattern bonded to a non-metallic substrate in some
areasbut not bonded to the non-metallic substrate in other areas,
S and to a method of fabricating such assembly, and, more
particularly, to such assembly and method wherein a conductor
pattern is formed from a metallic sheet which is bonded to a
non-me~allic substrate by means of a eutectic bonding technique.
Various eutectic bonding techniques exist for bonding
a metallic sheet to a non-metallic substrate. T~ese techni~ues
all share in common the formation of a molten eutectic alloy,
typically derived in part from the metallic sheet,
between the sheet and the substrate at an elevated
temperature. Such elevated temperature is near, but somewhat
below, the melting point of the metallic sheet. The molten
eutectic alloy wets the non-metallic substrate and thus
assures a strong bond between sheet and substrate.
It is known that a metallic sheet, when heated to an
elevated temperature as it is being bonded to a substrate with
a eutectie bonding technique, is likely to plastically conform
to the shape of the substrate. This is desirable to attain a
uniorm bond between the metallic sheet and the substrate.
However, there are instances in which it ~ould be advantageous
to ensure that selected areas of a metallic sheet did not
plastically confor~ to the shape of the substrate and co~sequently
become bonded to the sheet. For example, where the metallic
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sheet is patterned to implement a conductor pattern upon which
heat-producing electrical devices, such as power transistors,
are usually mounted and which includes an electrical interconnect
member, it would be desirable to have ~he interconnect member
S bridge over part of the upper surface of the non-metallic
substrate. This would enable the conductor pattern to withstand
higher voltage, due to an increased distance of separation along
the surface of the substrate between the interconnect member and
a metallic heat sink at electrical ground, on whlch the
substrate is typically disposed.
Summary of the Invention
Accordingly, it is an object of my invention to provide
an electrical assembly including a conductor pattern bonded to
a non-metallic substrate by means of a eu~ectic bonding
technique, wherein the conductor pattern includes non-bonded
members facing the substrate but separated therefrom.
It is a further object of my invention to provide a
method of fabricating an electrical assembly including a
conductor pattern bonded to a non-metallic substrate by means of
a eutectic bonding technique, with the conductor pattern
including members which face the substrate but are not bonded
thereto.
It is a still further object of my invention to provide
a method of fabricating an electrical assembly including a
conductor pattern bonded to a non-metallic substrate by means of
a eutectic bonding technique and which includes non-bonded
members facing the substrate but separated therefrom,the method
not requiring additional process steps to implement the non-
bonded members.
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Briefly sta~ed, in carrying out a particular form of
my invention~ I expose select.ed areas o one side of a metallic
s~eet to a wet chemical etchant to partially etch through the
sheet. Thereafter, I bond the metallic s~eet to a non-metallic
substrate by forming at an elevated temperaturP a molten eutectic
alloy between the sheet and the substrate, which wets the sub-
strate and provides a strong,stable bond upon cooling. At least
in a useful range of cases, as discussed below,I have found that
the unetched metal of the metallic sheet in each selected area
does not plastically conform and bond to the substrate, even
J~es
~; though the remaining area of the metallic sheet ~ bond to
the substrate.
Brief Description o the Drawi~gs
While the specification concludes with claims particularl~
pointing out and distinctly claiming the subject matter which I
regard as my invention, it is believed that the invention will
be be~ter understood with reference to the following description,
taken in conjunction with t~e drawings, in which:
FIGURE 1 is a view in perspec~ive of a fragmentary 20 portion of an electricaL assembly including a conductor pattern
bonded to a substrate in accordance with my invention;
FIGURES 2-4 are views similar to FIGURE L illustrating
various steps in fabricating the electrical assembly of FIGURE l;
FIGURE 5 is an enlarged detail view taken at arrow 5
in FIGURE L;
FIGURE 6 is a view similar to FIGURE 1 illustrating an
alternative electrical asse~bly fabrica~ed in accordance with
my invention; and
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FIGURE 7 is a view similar to FIGURE 1, but in cross-
section, illustrating a fur~her alternative electrical assembly
fabricated in accordance with my invention.
Detailed Description of a Preferred
Embodiment of the Invention
Turning to FIGURE 1, there is shown a portion of an
electrical ass~mbly 10 fabricated from a preferred method in
accordance with my invention. Assembly 10 includes a conductor
pattern 12,fashioned from a metallic sheet such as copper, and
a non-metallic substrate 14, such as a ceramic (e.g., alumina or
beryllia). Conductor pattern 12 has been bonded to substrate
14 by a eutectic bonding technique.
Conductor pattern 12 includes an electrical interconnect
member 16, which is not bonded to substrate 14. Additionally,
conductor pattern 12 further include3 a feature 18 such as is
typically included in a completed conductor pattern 12.
A preferred method of fabricating assembly 10 ls
now described with reference to FIGURES 2-4.
As shown in FIGURE 2, I form photoresist pat~erns 20 and
22 on the upper and lower surfaces, respectively, of a metallic
sheet 24, using known photolithographic techniques. The photo-
resist pa~terns 20 and 22 may comprise, by way of example,
Laminar(R) dry film polymer, sold by Dynachem Corp. of
Santa Anna, California. The photoresist patterns 20 and 22
protect portions of the metallic sheet 24 which they cover
from "attack" or etching by an etchant, used as discussed below.
In particular, I cover the upper side of area 26 of
metallic sheet 24, which is to be fashioned into interconnect
member 16 (FIGURE 1) in accordance with my invention, with
portion 21 of photoresist pattern ~0; I do not cover its lower
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RD-13,683
side, however, with photoresist pattern 22. As to the remaining
areas of metallic sheet 24 that are illustrated,I either co~er
both their top and bottom sides with photoresist pat~erns 20
and 22, respectively, or cover neither of their top and bottom
sides, as,for exa~ple, in area 28. The provision of portion 21
of photoresist pattern 20 for implementing the interconnect
member 16(FIGURE 1) can be accomplished as par~ of the same
process steps required to implement photoresist pattern 20.
I etch metallic sheet 24, protected by photoresist
patterns 20 and 22, by spraying with,or immersing in,a wet
chemical etchant (not shown), such as ferric chloride , at least
where the metallic sheet co~prises copper. The wet chem~cal
etchant attacks or etches the surfaces of the metallic sheet 24
which are not protected by photoresist pattexns 20 and 22. In
particular, the wet chemical etchant etches from both sides of
metallic sheet 24, and,for e~ample, in area 28, metallic sheet
24 .is etched (or penetrated) co~pletely through. When the sheet
24 has been completely penetrated in area 28, it is only partially
penetrated in area 26, because the etchant cannot contact its
upper surface, which is covered by portion 21 of photoresist
pattern 20. At this point in time, sheet 24 has been etched
approximately half-way through in area 26. I prefer to remove
metallic sheet 24 from the wet chemical etchant at this point,
and,consequently, no alteration in the etching step is required
to implement interconnect member 16(FIGURE l) in area 26 of
sheet 24. ~etallic sheet 24, which is now patterned like conduc~or
pattern 12(FIGURE l), appears as illustrated in ~IGURE 3.
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I then remove photoresist patterns 20 and 22 from
the metallic sheet 24, shown in FIGURE 3I with a photo-
resist stripper, such as that sold by the Inland Chemical
Corp. of Oneida, New York and designated CODE AP-612.
S As depicted in FIGURE 4, I next place the patterned metallic
sheet 24 on substrate 14. I use a eutectic bonding
technique to bond the patterned metallic sheet 24 to substrate
14. I prefer to use eutectic bonding techniques of the type
described (and claimed) in the following U.S. Patents:
3,766,634 - G.L. Babcock et al and 3,994,430 - D.A. Cusano
et al, assigned to the assignee of this invention. These
techniques each involve the formation at an elevated
temperature of a eutectic alloy consisting essentially
of metal from a metallic sheet, e.g., copper, to be bonded
to a substrate, and a heat reactive substance, e.g., oxygen,
at least where the metallic sheet comprises copper. A
further eutectic bonding technique is described, (and claimed),
for example, in U.S. Patent 2,857,663 - J.E. Beggs, assigned
to the assignee of this invention, and involves forming
at an elevated temperature a eutectic alloy between a
metallic sheet and a substrate by interposing a metallic
shim between the sheet and the substrate.
During bonding of the patterned metallic
sheet 24 to substrate 14, I have found it useful
to support area 26 of the patterned metallic sheet
24, which implements interconnect member 16 (FIGURE
1), by an auxiliary support 30 when a long inter-
connect mem~er 16 is desired. Auxiliary support 30
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ll~'~;Zl
comprises a material to which area 26 of the patterned
metallic sheet 24 does not bond. At least where
metallic sheet 24 comprises copper and the heat reactive
substance (of the eutectic alloy) comprises oxygen,
auxiliary support 30 suitably comprises one of the
following materials: molybdenum, tungsten, firebrick,
and a ceramic such as alumina or beryllia, coated with
graphite on its upper surface. After the patterned
metallic sheet 24 has been bonded to substrate 14/ it
appears as conductor pattern 12 of assembly 10 (FIGURE
1). The use of auxiliary support 30 entails only a
minimal amount of extra effort in fabricating assembly
10 (FIGURE 1) .
Turning to FIGURE 5, an enlarged portion of
assembly 10 (FIGURE 1) is shown to illustrate dimensions
of interconnect member 16 that I have been able to
attain. With conductor pattern 12 comprising copper, and
the elevated temperature at which a preferred molten
eutectic alloy of copper and copper oxide forms being
about 1065 C, or within approximately 50 C of the
melting point of copper conductor pattern 12, i.e.,
1083C, I have been able to repeatedly fabricate an
interconnect member 16 having an unbonded length L
of 70 mils (i.e., 0.70 inch, or 0.178 cm) where its
thickness T is 5 mils (0.013 cm) and the thickness of
conductor pattern 12 is 10 mils ~0.025 cm). These
results were obtained without using auxiliary support
30 (FIGURE 4) during bonding. The width W of inter-
connect member 16 does not appear to have a significant
bearing on how long the length L can be.
The foregoing results described only one possible
implementation of interconnect member 16. Based on these
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results, elastic beam theory predicts that an interconnect
member 16 having an unbonded length L of 150 mils (0.469 cm)
could be obtained by utilizing, during bonding, auxiliary
support 30 ~IGURE 4), at least with the same conditions
applying as ~he foregoing conditions of type and thickness of
metal for conductor pattern 12 and eutectic alloy temperature.
Where conductor pattern 12 comprises copper with a thickness
of 5 mils (0.013 cm) and interconnect member 16 is half this
thickness (i.e., 2.5 mils or 0.006 cm), elastic beam theory
predicts that interconnect member 16 could have an unbonded
length L of 42 mils (0.107 cm) if an auxiliary support 30
(FIGURE 4) is not used, and an unbonded length L of 109 mils
(O.277 cm) i an auxiliary support 30 is used. Further, for
an interconnect member 16 with a given thickness, the higher
above substrate 14 it is situated during bonding, the longer
its length L can be and still re~ain unbonded to substrate 14,
because it is then less likely to plastically conform to
substrate 14 and become bonded thereto.
Considering again FIGURE 5, my invention enables
conductor pattern 12, upon which heat-producing electrical devices
(not shown) are usually mounted, to properly operate at high
voltages with respect to a metallic heat sink (not shown), upon
which substrate 14 is typically attached. This is because inter-
connect member 16, fabricated in accordance with my invention,
overhangs part of the upper surface of substrate 14. This
departure from the teachings of the prior art increases the
length of an electrical creepage path 32 on the surface of
substrate 14 between conductor pattern 12 and the metallic heat
sink under substrate 14. With the length L of
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interconnect member 16 comprising 79 miLs (0.178 cm) J
conductor pattern 12 can be operated at more than 600 volts
with respect to the metallic heat sink without degenerative
flashover occurring along electrical creepage path 32. Because
interconnect member 16 is not bonded to substrate 14, it can
advantageously be bent upward to position 16' to ensure
that, in use, interconnect member 16 does not accidentally
make contact with substrate 14.
FIGURE 6 iLlustrates an alternative electrical assembly
40 which can be fabricated in accordance with my invention.
Assembly 40 comprises a conductor pattern 42 bonded to a
substrate 44, the bond between conductor pattern 42 and
substrate 44 comprising a eutectic alloy. Prior to bonding,
conductor pattern 42 must be placed upon substrate 44.
To prevent the otherwise unconnected portions 46 and 48 of
conductor pattern 42 from becoming misaligned with each other,
a mechanical interconnect member S0, similar to the above-
described electrical interconnect member 16, can be implemented
in accordance with my inven~ion. Interconnect member 50 can
then be easily severed with a sharp instrument, for example,
after conductor pattern 42 has been bonded to substrate 44.
A suitable procedure for fabricating conductor pattern 42
including mechanical interconnect member 50 follows basically
the same steps as the above-described procedure for fabricating
conductor pattern 12 (FIGURE 1) which includes electrical
interconnect member 16, with the resulting length of member 50
s,~'/qr
being-s~ to the length of member 16 where auxiliary
support 30 ~FIGURE 4) is used.
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FIGURE 7 illustrates a further electrical assembly 60
which can be fabricated in accordance with my in~ention. Assembly
60 comprises conductor patterns 62 and 64, which are both bonded
to a substrate 66,the bond co~prising a eutectic alloy. Con-
ductor patter~ 62 has less than half the thickness of conductorpattern 64. The thicker conductor pattern 64 includes a "via"
or interconnect member 68 (shown partially removed), ~abricated
in accordance with my inv~ntion, which brid~es over the thinner
conductor pattern 62, without making contact to it. In
fabricatin~ assembly 60, conductor pattern 62 is first bonded
to substrate 66. In a subsequent bonding step, the via 68
is bonded to substrate 66. Inasmuch as via 68 is similar to
mechanical interconnect member 50 ~FI&URE 6), a suitable method
~or fabricating conductor pattern 64 with the via 68 follows
from the foregoing description.
From the foregoing, it will be appreciated that my
invention can produce various electrical assemblies, each of
which includes a conductor pattern bonded to a non-metallic
substrate and having selected areas which are not bonded to the
substrate. It can also be appreciated that such nan-bonded
areas in a conductor pattern can be implemented with minimal
extra effort.
While my invention has been described with respect
to specific embodiments, many modifications and substitutions
2~ will be apparent to those s~illed in the art. For example,
metals other ~han copper, specifically mentioned above, could
be used for implementing a conductor pattern, such as conductive
metals described in t'~e above-referenced patents. Furthermore,
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other types of non-metallic substrates could be used instead
of ceramic, such as for example, a wafer of silicon
semiconductor material, which is refractory and which may
contain integrated electronic devices. Moreover, metal
from a metallic sheet could be removed with means other than
wet chemical etching, such as machining with a milling process
or with a laser. It is, therefore, to be unders~ood that
the appended claims are intended to cover these and all such
modifications and substitutions as fall within the ~rue spirit
and scope of my invention.
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