U.S. patent application number 09/860872 was filed with the patent office on 2002-11-21 for equipotential fault tolerant integrated circuit heater.
Invention is credited to Lau, James C..
Application Number | 20020170901 09/860872 |
Document ID | / |
Family ID | 25334241 |
Filed Date | 2002-11-21 |
United States Patent
Application |
20020170901 |
Kind Code |
A1 |
Lau, James C. |
November 21, 2002 |
EQUIPOTENTIAL FAULT TOLERANT INTEGRATED CIRCUIT HEATER
Abstract
Fault tolerance is incorporated within the integral electric
heaters of a reworkable electronic semiconductor component, such as
a reworkable multi-chip module, to increase production yield and
longevity of the rework feature. Components of the foregoing kind
contain a multilayer substrate to bond to a printed wiring board,
and, for rework, the component includes a plurality of electric
heaters arranged side by side on a bottom layer of the substrate.
When energized with current, the heaters generate sufficient heat
to weaken the adhesive or solder bond to the printed wiring board
without delaminating the layers of the substrate, allowing the
electronic semiconductor component to be pulled away from the
printed wiring board for rework. Additional circuitry is included
to automatically route heater current around, that is bypass, any
current-interrupting break(s) as may form in any of the electric
heaters giving the heaters a fault tolerance.
Inventors: |
Lau, James C.; (Torrance,
CA) |
Correspondence
Address: |
Robert W. Keller
TRW Inc.
Law Dept.
One Space Park Bldg. E2/6051
Redondo Beach
CA
90278
US
|
Family ID: |
25334241 |
Appl. No.: |
09/860872 |
Filed: |
May 18, 2001 |
Current U.S.
Class: |
219/209 ;
219/476; 257/E23.081 |
Current CPC
Class: |
H05K 3/3494 20130101;
H01L 2224/48091 20130101; H01L 2224/48227 20130101; H05K 3/368
20130101; H05K 3/4611 20130101; H01L 23/345 20130101; H05K 1/0306
20130101; H05K 2201/09263 20130101; H05K 1/0212 20130101; H05K
3/4629 20130101; H05K 2201/09681 20130101; H05K 1/167 20130101;
H05K 2203/176 20130101; H01L 2224/48091 20130101; H05K 7/20
20130101; H05K 1/141 20130101; H01L 2924/15311 20130101; H05K
2203/1115 20130101; H01L 2924/00014 20130101 |
Class at
Publication: |
219/209 ;
219/476 |
International
Class: |
H05B 003/00 |
Claims
What is claimed is:
1. In a reworkable semiconductor device designed for bonded
attachment to a printed wiring board, said semiconductor device
including at least one semiconductor chip, a multi-layer substrate
and a plurality of electric heaters; said multi-layer substrate
including a bottom layer for bonded attachment to said printed
wiring board and a top layer for bonded attachment to said
semiconductor chip; said bottom layer having a top surface for
attachment of said plurality of electric heaters and said bottom
layer being heat transmissive in characteristic wherein heat
produced by application of heater current through said electric
heaters is conducted throughout said bottom layer to said bottom
surface of said bottom layer for weakening any bonded attachment of
said multi-layer substrate to said printed wiring board, whereby
said semiconductor device may be removed from said printed wiring
board for rework, the improvement therein wherein each of said
plurality of electric heaters is of a predetermined structure and
conducts heater current equally and further comprising: fault
tolerant means for rendering said plurality of heaters tolerant to
current interrupting breaks, whereby said plurality of heaters may
continue to produce heat when heater current is applied,
notwithstanding the presence of a current interrupting break in a
heater.
2. The reworkable semiconductor device defined in claim 1, wherein
said fault tolerant means comprises: conductor means responsive to
the presence of current interrupting break in any one of said
plurality of electric heaters for routing heater current for said
one of said plurality of electric heaters through an adjacent one
of said plurality of electric heaters and around said current
interrupting break to maintain heater current through a remainder
of said one of said plurality of electric heaters.
3. The reworkable semiconductor device defined in claim 2, wherein
said conductor means further comprises: a plurality of straight
metal traces, said straight metal traces being spaced apart and
oriented in parallel; said straight metal traces oriented at right
angles to and intersecting said plurality of electric heaters and
being in electrical contact with said electric heaters at each
location at which said straight lines of said second plurality of
straight metal traces intersect said electric heaters of said
plurality of electric heaters.
4. The reworkable semiconductor device defined in claim 2, wherein
said plurality of electric heaters comprises an even number.
5. The reworkable semiconductor device defined in claim 2, wherein
said plurality of electric heaters are positioned side by side to
form a plurality of columns of electric heaters, and wherein each
of said electric heaters includes a first plurality of spaced
locations at predefined positions along said heater, each of said
first plurality of spaced locations in an electric heater acquiring
a voltage level distinct from the voltage level acquired by the
other of said first plurality of spaced locations in the respective
electric heater when a heater current flows entirely through said
electric heater, and in which said first plurality of spaced
locations in any one of said electric heaters corresponds to a like
plurality of spaced locations in another of said electric heaters
that is positioned adjacent thereto: and wherein said conductor
means further comprises: a plurality of groups of conductors, each
group comprising multiple conductor buses, said plurality of groups
numbering one less than said number of said plurality of electric
heaters, and each said group in said plurality being associated
with a pair of said plurality of electric heaters located in
adjacent columns; each conductor in a group being connected between
a respective one of said plurality of spaced locations in a
respective one of said electric heaters and the corresponding
spaced location in the adjacent one of said electric heaters of a
corresponding pair of electric heaters.
6. The reworkable semiconductor device defined in claim 5, wherein
each electric heater in said plurality of electric heaters,
exclusive of a first and a last one in said side-by-side
relationship of said plurality of electric heaters, further
includes a second plurality of spaced locations at predefined
positions along said heater, each of said second plurality of
spaced locations in an electric heater acquiring a voltage level
distinct from the voltage level acquired by the other of said
second plurality of spaced locations in the respective electric
heater when a heater current flows entirely through said electric
heater, and in which said second plurality of spaced locations in
any one of said electric heaters corresponds to a like plurality of
spaced locations in another of said electric heaters that is
positioned adjacent thereto.
7. The reworkable semiconductor device defined in claim 6, wherein
each of said plurality of electric heaters comprises a straight
conductor and extend in a first direction across said bottom
substrate layer; and wherein each conductor in said plurality of
groups of conductors extends comprises a straight conductor and
extends in a second direction transverse to said first
direction.
8. The reworkable semiconductor device defined in claim 7, wherein
each conductor in a group of conductors in said plurality of groups
of conductors is in alignment with a conductor located in every
other group of conductors to thereby define another straight
conductor extending across said bottom layer of said multi-layer
substrate.
9. In combination with an electronic component containing at least
one semiconductor device mounted to a multi-layer substrate, said
substrate including a bottom layer for bonding said electronic
component to a printed wiring board, a fault tolerant heater
supported on an upper surface of said bottom layer of said
substrate, said fault tolerant heater comprising: first and second
elongate conductor traces formed on said upper surface positioned
in spaced relationship extending across said bottom layer of said
substrate and defining a surface region there between; a plurality
of columns of conductor traces formed on said upper surface of said
multi-layer substrate, said columns being positioned side-by-side,
one end of each column being in contact with a first of said
elongate conductor traces and an opposite end of each column being
in contact with a second one of said elongate conductor traces,
said columns including a first column, a last column and
intermediate columns between said first and last columns; each
column in said plurality of columns further comprising: a set of
closed conductor loops, said set of closed conductor loops
including an initial closed conductor loop, a final closed
conductor loop, and a number of intermediate loops; said initial
closed conductor loop including a side formed by said first
elongate conductor trace, and an opposed side shared in common with
an intermediate closed conductor loop next in position in said set;
said final closed conductor loop including a side formed by said
second elongate conductor trace, and an opposed side shared in
common with an intermediate closed conductor loop immediately
preceding in position in said set; and each intermediate closed
conductor loops sharing one side with at least one of said
intermediate closed conductor loops or said initial closed
conductor loop and sharing an opposed side with at least another
one of said intermediate closed conductor loops; and wherein each
closed conductor loop of an intermediate column sharing a side or
portion of a side with at least one closed conductor loop of
another column positioned to one side and sharing another side or
portion of a another side with at least one closed conductor loop
of still another column positioned on an opposite side of said
intermediate column.
10. The combination as defined in claim 9, wherein each said closed
conductor loop comprises a right polygon in geometry.
11. The combination as defined in claim 10, wherein said right
polygon comprises a square.
12. The combination as defined in claim 11, wherein said squares
are of identical size, wherein said columns define straight
sides.
13. The combination as defined in claim 9, wherein said closed
conductor loops in odd numbered ones of said plurality of columns
comprises a letter "T" shape in geometry, exclusive of an end one
of said closed conductor loops which comprises a rectangle in
geometry; and wherein said closed conductor loops in even numbered
ones of said plurality of columns comprises a letter "T" shape in
geometry, exclusive of an end one which comprises a cross-shape in
geometry.
14. The combination as defined in claim 13, wherein all closed
conductor loops of a "T" shape in said columns are of the same
size.
15. The combination as defined in claim 9, wherein at least said
odd numbered columns are identical in geometry.
16. In a electronic semiconductor component, said semiconductor
component including a semiconductor chip, said semiconductor chip
including a plurality of electrical interface leads, and a
multi-layer substrate having a predetermined area for supporting
said semiconductor chip on a printed circuit board, said
multi-layer substrate having dimensions defining said predetermined
area, said multi-layer substrate comprising a laminate of layers of
a predetermined material, said multi-layer substrate including
first electrical connections for electrical connection to said
electrical interface leads of said semiconductor chip and second
electrical connections for electrical connection to pre-defined
locations upon said printed circuit board, said lower most layer of
said multi-layer substrate comprising a plurality of electric
heaters connected electrically in parallel for producing heat
uniformly over said predetermined area, said heat being sufficient
to weaken said second electrical connections to said printed
circuit board and being insufficient to delaminate layers of said
multi-layer substrate from one another or to weaken said first
electrical connections, each of said electric heaters being of the
same predetermined length and extending between first and second
power supply bus bars extending along opposed edges of said lower
most layer of said multi-layer substrate, the improvement therein
wherein said lower most layer of said multi-layer substrate further
comprises: a plurality of electrical buses, said buses being
capable of generating heat responsive to current flow there
through; said plurality of buses being connected between a
respective one of said electric heaters and another one of said
electric heaters located adjacent to said one of said electric
heaters at distinctive spaced locations along said electric heaters
and oriented parallel to said first and second power supply bus
bars, wherein said buses are symmetrically distributed about said
multi-layer substrate; whereby during operation of said plurality
of heaters a pair of adjacent ones of said electrical buses routes
current from said one of said heaters through a portion of an
adjacent heater on the occurrence of a break in said one electric
heater and back into said one of said heaters, when said break in
said one heater is located between said pair of electrical buses.
Description
FIELD OF THE INVENTION
[0001] This invention relates to reworkable electronic
semiconductor components, including multi-chip modules ("MCMs"),
that incorporate electrical heaters integrally within the component
structure to produce the heat necessary to soften or weaken the
bond of the component to the printed wiring board to which the
component is attach, allowing removal of the component from a
printed wiring board for rework. More particularly, the invention
relates to a new heater structure for the electronic semiconductor
component that is fault tolerant to current-interrupting breaks as
may be formed or produced in any of the heaters. The invention is
applicable to substrate-to-printed wiring board attachments that
employ adhesive bonds, such as found in the thermoset adhesive lead
type components, or that employ reflow solder bonds, such as found
in ball grid array lead-less type components.
BACKGROUND
[0002] The present invention improves upon the invention of Berkely
et al presented in U.S. Pat. No. 6,031,729, granted Feb. 29, 2000
entitled "Integral Heater for Reworking MCMS and Other
Semiconductor Components" (hereafter the "Berkely et al '729
patent") assigned to TRW Inc., the assignee of the present
invention. In a broader aspect, the invention improves upon
electrical heater systems as may be applied in other ways than
presented in the foregoing patent by incorporating circuits that
provide fault tolerance to current-interrupting breaks in the
electric heaters of an electric heater system for an electronic
component that avoids disruption of heating.
[0003] A principal application of the present invention is with
reworkable Multi-Chip Modules, such as described in the cited
Berkley et al '729 patent. Multi-Chip Modules ("MCMs") perform a
variety of electronic functions, and are finding increasing use in
sophisticated electronic applications, particularly airborne and
space-borne application. By definition, an MCM contains two or more
semiconductor die or chips, as variously termed, and ancillary
electrical components, assembled in a single enclosed package, that
together comprise an electronic circuit function. The semiconductor
chips contain the micro-miniature integrated circuits, such as
processors, amplifiers, memory, and the like.
[0004] In one type of MCM structure, the semiconductor chips and
components are supported on a common base, consisting of an
integral multi-layer printed wiring structure, referred to as the
substrate. Often that substrate is formed of ceramic, an electrical
insulator that is rigid, allows for plated-on conductors of the
finest widths and spacing with the greatest accuracy and is able to
maintain a hermetic seal. Metallic conductors printed on various
layers of the substrate, and metallic vias through the layers,
serve to electrically connect the semiconductor chips to each other
and to the external interfaces of the MCMs.
[0005] The foregoing elements are contained together in a single
enclosed four-sided package, often hermetically sealed, that serves
as a protective housing for the semiconductor chips and associated
components. The ceramic substrate, being hermetic, serves as the
bottom wall to the module. A metal wall, or seal ring, is brazed to
the substrate around the perimeter, encompassing the components and
a lid welded to the top surface of this seal ring hermetically
seals the components inside. A number of electrical contacts or
leads extend out the four sides of the MCM to provide external
electrical input-output connections to the MCM.
[0006] In practice MCMs are generally installed upon a printed
wiring board, much larger in area than an MCM, that contains the
electrical interconnections between the MCMs and other components
thereon. The larger wiring board is typically constructed of a
material such as glass-epoxy or glass-polyimide, a less expensive
and lower quality material than the ceramic of the substrate. For
airborne and space applications, MCMs are typically bonded to the
printed wiring boards. Bonding enhances thermal conductivity to the
MCM, and isolates mechanical loads from the input-output
connections of the MCM, which promotes longer product life. A
variety of adhesives, such as thermosetting epoxies or
thermoplastics, and solder are available to provide the
bonding.
[0007] To bond the MCM in place, as example, a layer of thermally
sensitive adhesive is applied to either the underside surface of
the MCM, or directly to the surface of the printed wiring board at
the location to which that component is to be placed. With the MCMs
and all other components for that circuit board properly
positioned, the board is then placed in an oven and the temperature
raised to cure or reflow the adhesive, attaching the MCMs and other
components in place. When removed from the oven and cooled down to
room temperature the MCMs are firmly attached to the printed wiring
board.
[0008] Solder is another known thermally sensitive adhesive
material used to fasten parts together. A second known technique
for fastening the MCM to the circuit board is the solder ball grid
array. Instead of incorporating electrical leads extending from the
side of the MCM package and using a separate adhesive for fastening
the MCM to the circuit board, as in the foregoing structure, the
electrical leads are instead formed by electrical vias extending
through the multiple layers of substrate to the underside surface
of the MCM package. At the underside the terminal end of those vias
typically appear by design arranged in regular rows and columns.
Minute solder balls or solder columns, different geometry's for the
dab of solder collectively referred to herein as solder balls, are
formed at the terminal ends of those vias on the underside of the
substrate.
[0009] In assembly, the MCM package is placed upon the printed
wiring board, the latter of which contains solder pads that mate
with the solder balls on the MCM package and the temperature is
raised above the solder eutectic at which the solder reflows. When
cooled, the solder solidifies and provides a firm mechanical
connection that fastens the MCM package to the printed wiring board
as well as completing the electrical connections to printed
circuitry on that wiring board. The foregoing connection apparatus
and technique is well known.
[0010] If failed components were detected during subsequent
electrical testing of the assembled board, the failed components
needed to be removed from the printed wiring board for repair or
replacement. The problem in reworking MCM's, whether fastened to
the circuit board by regular adhesives or with a solder ball grid
array, is recognized as endemic to other large size electronic
semiconductor components as well, even those that contain only a
single physically large semiconductor chip. As those skilled in the
art recognize, the more modern semiconductor chips are growing in
physical size as more and more circuit functions are expected to be
packed within a single die even in commercial devices, such as
cellular telephones. As a consequence large numbers of very fine
closely spaced wires are required to interface to the semiconductor
die. Because the wires must all extend into the die they are
necessarily physically small in width and must be packed closely
together, typically one mil in diameter separated by a two mil
space. However, conventional printed circuit board technology
typically provides semiconductor die interface connections with no
less than a four mil separation.
[0011] To resolve the apparent physical incompatibility in spacing
requirements, the approach taken has been to mount the
semiconductor chip onto an intermediate "interposer" substrate,
which is often formed of ceramic material. The printed wiring
formed on the substrate fans out from the microscopic spacing at
the location of the semiconductor die or chip to the wider spacing
and wider wiring required by the conventional printed circuit
board.
[0012] That electronic semiconductor assembly is then mounted onto
the printed circuit board. The electrical leads from the assembly
substrate are soldered to the mating solder pads on the printed
circuit board, or, should the substrate instead employ a solder
ball grid array, the solder balls are soldered to the mating solder
pads formed on the printed circuit board. As in the case of the
earlier described MCMs, in the foregoing arrangement, viewed in a
generic sense, one multi-layer printed circuit board is mounted
atop another printed circuit board. The dimension critical wire
bonding of the electrical leads to the chips, thus, is accomplished
on the ceramic substrate. Interconnect to the printed circuit board
is accomplished by soldering the electrical leads from the
substrate to mating pads on the conventional circuit board. With
such an interposer or intermediate substrate, in retrospect, one
recognizes the parallel between the foregoing structure and that of
the MCM, earlier described.
[0013] Heat was employed to assemble each of the foregoing
electronic components; and heat is the means that was typically
used to remove an MCM or other thermally bonded unit from the
printed wiring board. The difficulty and problems encountered in
removing MCM's from the printed wiring board for rework,
particularly in large size MCM's, those over 1.5 inch in a
dimension, using traditional techniques, such as application of a
heat gun, is described at some length in the Berkley '792 patent to
which the interested reader may make reference and need not be here
repeated.
[0014] The Berkely et al '729 patent describes a new structure by
means of which heat may be uniformly applied to the underside of
the substrate sufficient to permit detachment of the MCM from the
printed wiring board without damage. The reworkable MCM presented
in the Berkely et al '729 patent includes electric heater elements
formed in a metallized pattern typically printed on the bottom most
internal layer of the multi-layer substrate of the MCM; in effect
to form an integral heater assembly. In addition to the multiple
layers of the substrate that contains the printed-on metal
interconnections for the semiconductors and input-output
connections of the MCM, a dedicated bottom layer to the multi-layer
substrate contains a number of printed and fired-on resistive
conductors, suitably arranged in a pattern, such as a serpentine
pattern, each of which serves as a heater. When current is passed
through the heater, the resulting I.sup.2R losses in the conductor
of the heater is evoked as heat. Together, the multiple heaters
effectively covers the surface of the bottom layer with heat; and
the heat is conducted to the adhesive bond to the printed wiring
board. By design, the heat produced is sufficient to weaken the
bond between the substrate of the MCM and the printed wiring board,
but is insufficient to cause delamination of the multiple layers of
the substrate.
[0015] The Berkely et al '729 patent also discloses a preferred
embodiment in which the electrical conductors and supporting layer
that forms the heater (or heaters) are formed of the same conductor
and substrate materials used in the other layers of the multi-layer
substrate, such as aluminum oxide and tungsten, respectively,
permitting convenient manufacture of the heater as part of a
conventional substrate fabrication process.
[0016] By incorporating within the structure of the electronic
semiconductor components a heater that facilitates removal of that
component from its installed adhesive-bonded position on a printed
wiring board in the event of a semiconductor component failure,
individual electronic semiconductor components may be expeditiously
and efficiently removed and replaced. Any necessity for discarding
the entire printed wiring board, along with other good electronic
components, is avoided, eliminating the expensive procedure of
building the entire circuit board assembly anew.
[0017] The individual heaters in the MCM described in the Berkely
et al '729 patent are connected electrically in parallel between
elongate conductors along a pair of opposed sides of the substrate,
as example, along the front and rear sides of the substrate; and
each heater is formed of a fine line of metal. In one embodiment,
each heater forms a serpentine-like pattern in between the two
sides. A large number of such heaters cover the area of the
substrate, thirteen in one example of the '792 patent. As one
realizes, if the heater wire of an individual heater in the
foregoing structure is broken, that heater cannot conduct
electrical current. Since the heat produced by the heater wire is
produced by the I.sup.2R loss, being unable to conduct current, no
heat can be produced; and that produces a heating discontinuity in
the substrate that could wholly or partially negate the advantage
of the embodiment of the Berkely et al '729 patent.
[0018] A current-interrupting break could be produced during
fabrication processing of the substrate layers, as example, should
a piece of dust lodge on the substrate during plating. A second
possibility for creating a break is due to mishandling during
assembly of the MCM. As example, should an assembler inadvertently
scratch the substrate on another solid and scrape or cut through a
heater line. A third possibility occurs during rework of the MCM,
should the technician raise the voltage applied to the heaters to a
level that results in too high a current through a heater lead, one
or more of the heaters may overheat and, like a fuse, burn out,
producing a break in the line. Unless the break is large enough to
visually observe, it can only be found by testing. For consistency
it would be necessary to electrically test each substrate produced,
and that testing procedure takes time and effectively raises the
production costs. Irrespective of the underlying reason for a
current-interrupting break in a heater, the availability of some
means to automatically "patch up" the break or effectively minimize
the effect of a break in a heater as would give the MCM a fault
tolerant characteristic, and would be of benefit to and improve
upon the foregoing combination.
[0019] Accordingly, a principal object of the present invention is
to provide a reworkable MCM or other electronic semiconductor
component that employs an integral heating system for permitting
detachment of the component from a printed wiring board with a
heater system that is fault tolerant.
[0020] And another object of the invention is to eliminate the
necessity for testing of the integrity of heaters contained in a
reworkable electronic component so as to reduce production cost
without detracting from the effectiveness of the heaters, even
though one or more of the heaters contains a break.
SUMMARY OF THE INVENTION
[0021] In accordance with the foregoing objects, the invention
incorporates fault tolerance within the integral electric heaters
of a reworkable electronic semiconductor component, such as a
reworkable multi-chip module, to increase production yield and
longevity of the rework feature. Components of the foregoing kind
contain a multi-layer substrate to bond to a printed wiring board,
and, for rework, the component includes a plurality of electric
heaters arranged side by side on a bottom layer of the substrate.
When energized with current, the heaters generate sufficient heat
to weaken the adhesive or solder bond to the printed wiring board
without delaminating the layers of the substrate, allowing the
electronic semiconductor component to be pulled away from the
printed wiring board for rework. Additional circuitry, specifically
a series of buses, is included to automatically route heater
current around, that is, bypass any current-interrupting break (or
breaks) as may form in any of the electric heaters rendering the
heaters tolerant to that kind of fault.
[0022] The foregoing and additional objects and advantages of the
invention together with the structure characteristic thereof, which
was only briefly summarized in the foregoing passages, will become
more apparent to those skilled in the art upon reading the detailed
description of a preferred embodiment of the invention, which
follows in this specification, taken together with the
illustrations thereof presented in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] In the drawings:
[0024] FIG. 1 is a perspective view of an MCM that incorporates the
invention;
[0025] FIG. 2 is a top view of a circuit board containing a number
of the MCMs of FIG. 1;
[0026] FIG. 3 is a side view of the circuit board assembly of FIG.
2;
[0027] FIG. 4 is a partial side section of the MCM substrate drawn
in larger scale;
[0028] FIG. 5 is a top view of the top most layer of the MCM
substrate used in the MCM of FIG. 1, showing a typical layout of
signal and power conductors;
[0029] FIG. 6 is a top view of an intermediate layer of the MCM
substrate containing electrical vias which interconnect the surface
conductor layer of FIG. 5 to the conductors defining the heaters
heater pattern of FIG. 7;
[0030] FIG. 7 is a top view of the bottom most internal layer of
the MCM substrate used in the MCM of FIG. 1, showing the conductive
heater pattern;
[0031] FIG. 8 is a pictorial view of a portion of the heater and
FIG. 9 is the same pictorial view with a broken heater, both of
which are used in connection with the explanation of the operation
of the invention;
[0032] FIG. 10 is a bottom view of an alternative embodiment of the
invention used in a semiconductor component that employs a ball
grid array;
[0033] FIG. 11 is a side view of FIG. 10; and
[0034] FIG. 12 is a top view of FIG. 10.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0035] The invention is described in connection with a Multi-Chip
Module. Reference is made to FIG. 1, which illustrates one example
of a Multi-chip Module ("MCM") 1 in a top perspective view, with
the module lid 3 partially cut away. A plurality of semiconductor
dice or chips 7, only three of which are labeled, are mounted at
various locations upon a dielectric multi-layer substrate 9, and a
plurality of small electrical components 11, only two of which are
labeled, are also mounted to the substrate 9. The semiconductor
chips are not encased. The various junctions and metal traces
exposed on the top surface of the semiconductor chips are very
small in relative size and are not readily visible, nor illustrated
in the figure.
[0036] A wall or ring 13 of metal or ceramic material borders the
periphery of substrate 9, and is bonded in place on the substrate,
suitably by brazing. The wall serves to raise and support the lid 3
above the height of the confined semiconductor chips 7. A large
number of metal traces printed on and in the substrate 9, not
illustrated, define various power and signal paths, that
interconnect the various semiconductor chips 7 within the module
and/or provide electrical connections therefrom to external leads
14 to the module, extending in rows from the module's four
sides.
[0037] Even though no particular electronic circuit has been
illustrated in the foregoing figures, it should be understood that
the present invention is not directed to any particular electronic
circuit, or semiconductor package. Hence any illustration or
description of the details of any such electronic circuit or
package would only serve to introduce unnecessary complexity to the
present description and would not aid one to understand the
invention. Accordingly, other than to note the presence of such
elements in a practical module, such elements are neither
illustrated or described in detail.
[0038] The view of the foregoing MCM in FIG. 1 is the same in
appearance as the prior reworkable MCM, containing the same
electronic circuit function and features described in the Berkely
et al '729 patent, since the physical differences required by
incorporation of the invention are not visible from this view. The
MCM includes a fault tolerant embedded heater system that is not
visible in this figure. One approach for applying power to the
embedded heater is illustrated. Leads 14a and 14b on ends of the
row of leads on the right side of the figure, and leads 14c and 14d
on the ends of the rows on the left side, are provided exclusively
for supplying current to the heater circuit.
[0039] The foregoing leads are wider than the other leads in the
respective rows, hence are capable of carrying greater levels of
current than the more narrow leads, and are required to conduct the
relatively large current required by the internal heater (or
heaters), not illustrated in the figure. Alternatively, one might
instead use a number of the more narrow leads, electrically
connected in parallel, to carry the heater current; or one may omit
the leads dedicated to the heater entirely, and make connection to
the heater circuit by soldering wires directly to the top surface
pads only when it becomes necessary to utilize the heater circuit
for rework.
[0040] For operation, the MCM is fastened to a larger printed
wiring board on which the MCM along with other MCMs and components
forms a larger electronic system. Such an assembly is pictorially
represented in FIG. 2, wherein eight such MCMs 1a through 1h are
secured to one side of a printed wiring board 8, As represented in
a side view in FIG. 3, printed wiring board 8 may contain like
numbers of MCMs on its opposite surface as well, such as
illustrated by MCMs 2a and 2b.
[0041] As is the conventional practice for MCMs, the MCMs bottom
surface is bonded to the printed wiring board 8. Bonding may be
accomplished with a thermoset or thermoplastic adhesive, as
represented to exaggerated scale at 15 in FIG. 3, or with solder in
the case of a Ball-Grid Array (BGA), later herein described in
connection with FIG. 9. A metal filled adhesive may be preferred
for thermal or electrical reasons.
[0042] An enlarged not-to-scale partial section of substrate 9 in
MCM 1 is presented in FIG. 4 to which reference is made. The
substrate is a laminate containing multiple layers formed of a
dielectric material, such as aluminum oxide, aluminum nitride or
beryllium oxide materials, including a bottom most layer 17, an
upper most layer 19, and one or more intermediate layers 21, 23 and
25. As later herein described, a plurality of electrical vias 27
extend through the multiple layers of the substrate to form a part
of the electrical path between contacts 29 on the upper surface and
conductors on lower layers 17 and 25. Additional metal vias may be
included there between as desired the sole function of which is to
conduct heat away from the semiconductor die. Conductor 31, located
on the bottom layer of the substrate, serves as one of the
terminals to the embedded heater, later herein described in
connection with FIG. 7.
[0043] FIG. 5 is a layout view of the surface of upper most layer
19 of substrate 9 drawn in larger scale, illustrating the conductor
layout on that substrate layer. The wide rectangular frame or loop
35 is recognized as the metallized pad onto which the seal-ring 13,
illustrated in FIG. 1, is brazed in typical practice. A large
number of very small sized metallized pads 36, 37, 38, and 39
evenly spaced in rows on the top, bottom, left and right sides in
the figure, are recognized as the pads onto which the leads 14,
illustrated in FIG. 1, are bonded or brazed. Four larger metallized
pads or conductors 26, 28, 29 and 30 are located at each of the
four corners, extending along the upper and lower edges of the
layer. The latter conductors serve as the contacts or terminals for
the electric heater illustrated in FIG. 7, later herein
described.
[0044] FIG. 6 partially illustrates a layout of an intermediate
layer 21 of substrate 9, intermediate to the upper most and bottom
most layers. The sets of small dots 41-44 at the four corners
represent electrical vias that extend through the layer. These
electrical vias connect each of the conductors 26, 28, 29 and 30 to
the heater metallization pattern upon bottom layer 17 of the
substrate illustrated in FIG. 7. The plurality of vias clustered at
each corner of the layer in FIG. 6 are necessary to carry the
required current level for the heater system, which is orders of
magnitude greater than that carried by a typical power or signal
via in normal circuit operation. A like set of conductive dots is
present in the additional intermediate substrate layers 23 and 25
(FIG. 4). Many more like vias, not illustrated, would also be
present across the layer, which serve to interconnect other pads on
the surface with conductor lines printed on the intermediate layer,
also not illustrated. These latter vias and printed-on lines
comprise the various power and signal paths for the normal MCM
circuit operation. Being unique to the particular circuit
application of the MCM and not necessary to an understanding of the
invention, those additional paths are not illustrated or described
in detail.
[0045] Should the substrate 9 contain more than the three layers
illustrated in the laminate, each additional intermediate layer
would contain a like structure of vias 41-44, to extend the
electrical connection between the surface pads and the heater
circuit on the bottom layer 17 through those additional layers.
[0046] Reference is made to FIG. 7, which is a layout of the bottom
most internal conductor pattern, printed on bottom layer 17 of
substrate 9 that defines the fault tolerant heater system
integrated within the MCM. A wide straight printed-on conductor 32
extends along the upper edge of the layer and a second wide
straight printed-on metal conductor 31 extends along the lower edge
of the layer, the pair of which serve as the electric terminals to
the heater. In operation, the conductors 31 and 32 are connected to
an external source of electric current, not illustrated, via
terminals 14a-14d to the MCM, earlier illustrated in FIG. 1, and
the vias, earlier described.
[0047] Apart from the two foregoing conductors the wiring pattern
is seen to be unusual in geometry and difficult to describe in
words. One approach to this description is to speak to the
complicated pattern in terms of two sets of conductors. The first
set is those conductors, which extend in a serpentine pattern from
the top to the bottom in the figure, here referred to as heater
conductors, such as numbered 45 through 50. The second set is the
short lengths that extend laterally in the figure, called buses,
such as buses numbered 51a-51I, 53a-53h and so on. The latter buses
connect locations on one serpentine shaped heater conductor to a
corresponding location on an adjacent serpentine shaped heater
conductor. It should be understood that all of the conductors on
this layer are formed at the same time during fabrication of the
layers of the substrate. Even so the conductors are treated and
discussed separately so that the functions of each section of
foregoing conductor and operation of the invention is better
understood.
[0048] In this layout, an even numbered plurality of printed-on
conductors, specifically six, 45 through 50, the heaters, each of
which extends between the upper and lower edges of the layer. Each
of those conductors is configured, in this example, in a serpentine
pattern of seventeen laterally extending loops over the distance
between conductors 31 and 32. In conducting current, each of those
serpentine shaped conductors serves as an electric heater. The
first of those conductors 45, as counted from the left side of the
figure, (and the third, fifth and all other odd numbered
conductors) extends up from conductor 31 on the bottom left side of
the figure, loops first in one direction, to the right in the
figure, and then reverses direction, the second direction, to the
left in the figure. The pattern of the conductor continues and
repeats with additional such extensions and loops until the end of
the conductor joins the laterally extending conductor 32.
[0049] The second of those heater conductors 46 (and the fourth,
sixth and all other even numbered conductors) extends from the
bottom of the figure up a short distance and first loops in the
second direction, to the left in the figure, and then reverses
direction, the first direction, to the right in the figure. the
conductor continues extending in those loops from the bottom of the
layer in the figure to the top at laterally extending conductor
32.
[0050] Each of the foregoing heater conductors 45-50 is identical
in length, width and thickness. The geometry of each of the odd
numbered conductors is identical and that of the even numbered
conductors is identical. The geometry of the even numbered
conductors is seen to be the mirror image of the odd numbered
conductors. The upper end of each of the conductors connects to
conductor 32 by which those ends are placed electrically in common.
The opposite lower ends of those conductors are attached to
conductor 31 placing the opposite ends of the conductors
electrically in common. It should be recognized that the individual
printed-on serpentine conductors of the plurality, each of which
serves as an individual heater, collectively define one larger size
electric heater.
[0051] One side of alternate loops of one conductor is connected by
a conductor, here referred to as a bus bar, to the corresponding
side of the confronting loop in the next adjacent heater conductor.
Thus bus bar 51a connects the first side of the first loop in
conductor 45, viewed from the side nearest conductor 31 at the
lower edge, to the first side of the confronting (first) loop in
adjacent conductor 46. Bus bar 51b connects the same locations on
the third loop in conductors 45 and 46, 51c the fifth loop therein
and so on.
[0052] Reference is next made to the bus bar interconnections
between the second conductor 46 and the next adjacent conductor to
the right, the third conductor 47. In this bus bar 53a connects the
first side of the second loop in conductor 46 and the corresponding
side of the confronting (e.g. second) loop in conductor 47. Bus bar
53b connects the first side of the fourth loop in conductor 46 and
the corresponding side of the confronting (e.g. fourth) loop in
conductor 47; bus bar 53c connects the first sides of the sixth
loop of those conductors; and so on.
[0053] The foregoing pattern repeats with the bus bars connecting
loops of the third and fourth conductors 47 and 48. Bus bar 55a
connects the first side of the first loop in conductor 47 to the
first side of the confronting (first) loop in adjacent conductor
47. Bus bar 55b connects the same locations on the third loop in
conductors 47 and 48, 55c the fifth loop therein and so on. The bus
bar connections between the loops of the fourth and fifth
conductors, 48 and 49, follow that prescribed previously for the
bus bar connections between the second and third conductors 46 and
47. Thus the first side of the second loop in conductor 48 is
connected by bus bar 57a to the first side of the confronting
second loop in conductor 49; bus bar 57b to the first side of the
fourth loop in conductor 48 and the first side of the fourth loop
in conductor 49; bus bar 57c to the sixth loop in those conductors
and so on. The pattern of bus bar connections between the fifth and
sixth conductors 49 and 50 repeats those of the first and second
conductors and those for the third and fourth conductors. The buses
are all the same in thickness, width and length; and are of the
same width, thickness and material as the heater conductors
45-50.
[0054] In assembling the semiconductor die to the substrate during
manufacture of the MCM 1, the die or chip is attached to the
substrate using a thermally stable microelectronics adhesive. In
the inert-gas environment typical of a hermetic package, such
adhesives remain stable to temperatures in excess of 200.degree. C.
On the other hand, many commercially available thermoplastic and
thermosetting adhesives used for attaching components to circuit
boards have glass-transition temperatures (i.e., softening
temperatures) well below 200.degree. C. The latter temperatures are
readily attainable through use of the described MCM heater. Once
above its glass-transition temperature (Tg), the adhesive securing
the MCM or like component to the circuit board will yield under
mechanical load and the removal of the MCM from the circuit board
proceeds readily. No load is applied to the adhesive used to secure
the die or dice to the substrate, and while this adhesive may
soften at the removal temperature, it will harden upon removing the
heat source from the MCM.
[0055] Typical component removal temperatures, less than
200.degree. C., also have no damaging effect on the substrate
construction itself, as typical Multi-Chip and single-chip module
substrates are fabricated from ceramic materials which have been
laminated and sintered together at temperatures in excess of
1000.degree. C., forming a monolithic structure impervious to
moderately elevated rework temperatures.
[0056] For rework of the foregoing MCM, one polarity of the source
of current, not illustrated, is connected to the leads 14a and 14b
in the MCM illustrated in FIG. 1; and the opposite polarity source
is connected to leads 14c and 14d. Current flows via contacts 14a
and 14b into the MCM, into contacts 29 and 30, illustrated in FIG.
5, and from those contacts, flows down through the vertically
extending electrical vias, including vias 41 and 42, through the
multiple intermediate layers of substrate 9, to one end of the
heater metalization pattern on the bottom most substrate layer, and
across the pattern. From there the current flows through the
vertically extending vias, including vias 41 and 42 on the opposite
side of the intermediate layers, up to contacts 26 and 28, FIG. 5,
on the upper surface of the substrate. From the latter contacts the
current flows in parallel out leads 14c and 14d, and back to the
opposite polarity terminal of the current source. As an alternative
construction, the heater-dedicated leads 14a-14d may be omitted, in
which case an electrical circuit would be completed by soldering,
clipping, or conductive-adhesive attaching discrete wires from the
current source to the substrate heater contacts 26, 28, 29 and
30.
[0057] The refractory metal conductors ("traces"), being resistive
in character, produce an I.sup.2R loss, generating heat. That heat
passes through the bottom layer and into the adhesive material
bonding the substrate 9 to the circuit board 8. The circuit board
ultimately conducts the heat away from the adhesive to the
environment.
[0058] With a source of voltage connected across conductors 29 and
30, elsewhere herein described, and temporarily neglecting the bus
bars, current flows through each of the heater conductors 45-50,
generating heat, through the power losses generated in the
resistivity of those conductors, the I.sup.2R loss, essentially as
described in the prior Berkely et al '729 patent. Since the heater
conductors are identical in length, width, composition, material
and resistivity, and since the voltage applied across those
conductors is identical, then the current, I, through each of those
conductors should be identical. Likewise the I.sup.2R loss in each
conductor is identical, and the heat thereby developed is
identical. As those skilled in the art will appreciate, with the
heater circuit functioning as described, which occur during a
rework procedure, the buses 51, 53, 55, 57 and 59, even though
conductively connected to adjacent heater conductors, have no
effect on the foregoing operation and cannot carry current. Were
the buses removed, that removal would not affect the operation of
the heater.
[0059] The foregoing is better understood by making reference to
FIG. 8, which illustrates a small portion of the bottom layer of
the substrate, heater conductors 45 and 46 of two adjacent heaters
and bus bars 51a and 51b connected between a side of the
confronting loops formed in the heater conductors. With equal
voltage, +V, and identical heater conductors the current I drawn by
each of heater conductors 45 and 46 is identical. The voltage at
the left side of bus bar 51a is the voltage drop produced in
sections L1 and L2 of heater conductor 45 and is equal to
I.times..rho. (resistivity).times.(L1+L2). The voltage at the right
end of bus bar 51a is also equal to I.times..rho.
(resistivity).times.(L1- +L2). Thus in accordance with ordinary DC
network analysis, the voltage across the bus bar 51a is the
difference of the foregoing two voltages, namely zero. With no
voltage appearing across the bus bar 51a, no current is able to
flow. The same situation is true for bus 51b. In each case the
voltage at each end of bus 51b is the voltage drop created by the
current, I, multiplied by the length of the portion of conductor 45
(and 46 respectively) from the point of connection to conductor 31
and the resistivity of the conductor. That voltage drop is only a
fraction of the source voltage +V. Accordingly, with the heaters
properly functioning the included bus bars have no effect. on the
functioning of the circuit and perform no function.
[0060] The operation changes should a break occur in a heater
conductor. Reference is made to FIG. 9 which illustrates a break 60
in a loop in heater conductor 46. That formed discontinuity
prevents current from flowing through the loop, a current
disrupting break. Absent buses 51a and 51b the voltage at the
juncture (and all along the upper portion of conductor 46 would
rise to the source voltage, +V, while the bottom portion of the
broken heater conductor would be at ground potential. And with
normal current flow in heater conductor 45, the voltage at the
location along conductor 45 at which bus 51b is positioned is the
sum of the IR drops in the portion of conductor 45 between bus 51b
and conductor 31.
[0061] With buses 51a and 51b connected in place, and break 60
present, the voltage at the right side of bus 51b would start to
rise. In so doing the voltage at the right side of bus 51b is
greater than that at the left side, creating a voltage difference.
Accordingly, current flows from conductor 46 through bus 51b and
into the adjacent heater conductor 45, contributing to an increased
current in a small section of that conductor.
[0062] Because of break 60, one end of bus 51a is at ground
potential (or as otherwise stated, at a lower potential than the
normal voltage drop across sections L1 and L2 of heater conductor
45. With the increased current through conductor 45, that voltage
drop would tend to increase from the static state. The foregoing
produces a higher voltage on the left side of bus 51a than on the
right, resulting in a potential difference. Accordingly current
will flow from the left to the right through bus 51a, back into the
lower section of the broken heater conductor 46, and thence to
ground. Current also continues to flow through the remainder of
heater conductor 45 and thence to ground also, a parallel path. The
relative portions of the current is inversely related to the
relative resistance of the two paths (L1+L2) and [L1+L2+bus 51a
length].
[0063] Effectively, a current path is formed around break 60 by
buses 51a and 51b. A like action occurs between any other pair of
buses when a break is present in a section of one of the heater
conductors of two adjacent heaters located between the two buses.
The foregoing description of operation does not attempt to define
the current in each branch except in a general way that is
sufficient to enable one to understand the operation.
[0064] Further, with increased current flow through the short
section of heater conductor 45, greater heat is generated in the
associated loop; and with current flow through buses 51a and 51b,
which, in normal operation absent break 60, could carry no current,
the buses now also generate some I.sup.2R loses and create heat.
Although the heat generated in the section of the break is not
quite the same as normal, the heat is almost uniform and serves the
desired function in the vicinity of the break. Effectively thus the
foregoing bus structure automatically corrects a break and renders
the semiconductor component heater system fault tolerant.
[0065] Each parallel heater, including the bus bars, is constructed
of a resistive material, preferably having a range of resistivity
of (and including) 0.01 ohms-per-square to 1.0 ohms-per-square. An
example of metals of the former resistivity is gold or copper of a
thickness of 20,000 Angstroms. An example of a metal of the latter
resistivity is that which is used in the preferred embodiment,
Tungsten of about 1,200 Angstroms thickness. The aspect ratio
conductor trace length to width ratio of each element is from 50 to
500 so that each heating element is able to generate one to five
watts of power with a five volt supply. The total power and target
temperature may be adjusted by increasing or decreasing the supply
potential.
[0066] A five ohm resistive parallel interconnected heater traces
that are one inch in length and 10 mils wide spaced 50 mils apart
should with a five volt supply should provide 20 watts of heating
power in a one square inch area.
[0067] In one conventional type of MCM substrate construction,
known as High Temperature Co-fired Ceramic (HTCC), refractory
metals such as tungsten are used as the main constituent of the ink
which is printed-on to form the printed conductors, the conductive
traces for the interconnections on each of the substrate layers and
the buses. Tungsten is compatible with the high firing temperatures
inherent in substrate fabrication. These refractory metal
conductors naturally lend themselves to the formation of heating
element structures and do not require any special materials or
process changes in the standard HTCC substrate fabrication
technique. Moreover, the resistivity of the conductor metalization
is such that practical resistances for heater elements can be
tailored through simple geometrical manipulation of the artwork
pattern used to form the heater element.
[0068] This same approach for incorporating the heater element is
applicable to any HTCC component substrate, for instance, certain
types of single-chip Quad-Flat Pack or Ball Grid Array packages.
Embedding equivalent heaters in non-HTCC type components is
possible, as later herein discussed, but, for practical reasons
those structures may require the incorporation of additional
materials or processes outside the normal fabrication
procedure.
[0069] The foregoing embodiment of the invention was described in
connection with MCMs and thermoset (eg. epoxy) or thermoplastic
adhesive for bonding to the printed wiring board. However, it
should be appreciated that nothing in the design precludes the use
of the invention with other types of adhesives and/or other
electronic components, all of which is understood to be within the
scope of the present invention. The foregoing emphasis on MCMs and
epoxy merely illustrates a preferred embodiment of the invention
and its application.
[0070] As earlier noted, the integral heater system of the Berkely
'729 patent and, hence, the fault tolerant heater system of the
present invention may be used with other thermally sensitive
fastening materials, such as solder. MCMs and other electronics
components may be connected to a printed wiring board using an
array of solder balls. Such Ball Grid Arrays ("BGAs"), well known
in the electronics art, employ small solder spheres or, sometimes
columns, as the mechanical and electrical connection between the
circuit board and the component, in lieu of extending electrical
leads 14 employed in the embodiment of FIG.
[0071] As example, the illustration of FIG. 10, not drawn to scale,
provides a bottom view of such an alternative embodiment of the MCM
of FIG. 1, constructed using a BGA for the electrical and
mechanical connections. For convenience the elements in this figure
are denominated by the same number primed as was used to identify
the corresponding element in the prior embodiment. In such an
alternative structure, the bottom side of the bottom layer 17' of
substrate 9' contains the solder balls 50, which are typically
arranged in rows and columns. Those solder balls are attached to
the ends of various electrical vias of conventional structure, not
illustrated, that extend through the bottom substrate layer and one
or more of the substrates multiple layers to extend electrical
paths to the appropriate electrical circuit and/or semiconductor
chips affixed to the substrates upper surface.
[0072] In this embodiment the individual heaters 62A-62K are
straight lines in geometry and extend between the laterally
extending between upper conductor 32' and the laterally extending
lower conductor 31'. Those lines are evenly spaced across the area
of the substrate. The bar busses are also evenly spaced. The
geometry of the interconnecting buses are aligned to also form
straight lines. Those bus lines are oriented perpendicular to the
lines of the heater wires with each bar bus intersecting and
contacting each of the heaters. Both the bar busses and the heaters
are formed of identical metal and are the same in width and
thickness so as to possess identical resistivity characteristics.
In appearance the array of conductors forms a large multi-cell grid
and resembles a wire mesh screen.
[0073] As illustrated in the side view of FIG. 11 and the top view
of FIG. 12, to which reference is made, electrical vias 27' extend
from pads 42' and 29' on the upper surface of top layer 19' of the
substrate, through all intermediate layers of the substrate, to the
respective termini 31' and 32' of the heater element on the upper
surface of bottom layer 17'. Like the embodiment of FIG. 1,
semiconductor chips 7' are attached to the upper surface of the top
layer 19' of substrate 9' and the electrical interfaces from those
chips are wire bonded to appropriate pads on the substrates upper
surface. Electrical paths between the various pads, and from the
pads to the solder balls on the bottom of the substrate, are
completed with metallic traces printed on the substrates various
internal layers, and with vias through the layers, as necessitated
by the particular circuit function. The details of such
conventional interconnect structure, not being necessary to an
understanding of the invention, are not further described.
[0074] As illustrated in the side view of FIG. 11, the
semiconductor chips or dies 7' are attached to the upper surface of
the top layer 19' of substrate 9' and the electrical leads from
those dies or chips are wire bonded to appropriate solder pads, not
illustrated, on the substrates upper surface, the same as with the
embodiment of FIG. 1. Through those solder pads various electrical
paths are completed through and about the substrate and to the
electrical vias that terminate at the various solder balls, as
necessitated by the particular circuit functions for the
semiconductor chips, the details of such conventional structure not
being necessary to the understanding of the invention and are not
be further described.
[0075] The embodiment of FIG. 7, to which reference is again made,
contains six heater wires 45-50 that extend between the upper power
supply bus 32 and the bottom power supply bus 31. However, the
number of rows of buses 51a, etc. differs by one from the number in
the adjacent row. From the left, the first row of buses that
interconnect heater wires 45 and 46 contains nine parallel buses
51a-51i. The second row of buses that interconnect heater wires 46
and 47 contains eight parallel buses 53a-53h. The foregoing numbers
of buses is repeated in the third, fourth and fifth rows as is
evident from inspection of the figure. The layout is symmetrical
about the center.
[0076] Broadly speaking, it may be stated that the quantity of
buses included in the foregoing arrangement is divided into groups
of buses. That parsing amongst the groups of buses is uneven to the
extent that the groups of buses in the odd-numbered rows, counting
from the left, contain one more bus than the quantity of buses in
the groups of buses located in the even numbered rows. In contrast,
the corresponding groups of buses in the embodiment of FIG. 9, to
which brief reference is again made, contain the same number of
buses.
[0077] Although the foregoing heater structure was explained in
terms of heater conductors and interconnecting buses, inspection of
FIGS. 7 and 10 shows that the conductor patterns may also be
described in alternative language. Referring first to FIG. 7, which
is the more complicated wiring pattern, it is seen that the
individual heaters are formed of a serial arrangement or column of
closed conductor loops and the sides of the column are in contact.
Each closed conductor loop contains at least one side (or portion
of at least one side) in common with the next closed conductor loop
in the column. And each column of loops extends from conductors 32
and 31, the latter of which serve as a side to the upper and lower
end loops, respectively.
[0078] As example, consider the closed conductor loop formed at the
upper end of conductors 45 and 46 resembling the letter "T". The
front end of conductor 32 serves as a side to the end loop. Another
closed loop forming an identical "T" shape is positioned
immediately below the foregoing loop. A portion of the upper side
of the foregoing intermediate closed loops is shared in common with
the foregoing loop at the upper end of the column. All of the
intermediate closed loops in this first column are identical in
size and are of the T-shape, and each shares a one side or a
portion thereof with the preceding closed loop in the column and
shares another side or a portion thereof with the succeeding closed
loop in the column. The closed loop at the lower end of the column
(containing bus 51a) is rectangular in shape, and one wall thereof
is formed by the lower conductor 31.
[0079] In the next adjacent column of closed loops, the first
closed loop forms a "+" or cross in shape. The remaining loops in
the column are of the same size "T" shape as those closed loops in
the first column. As in the first column each of the closed loops
at the upper and lower ends contains a side formed by the
respective adjacent transverse conductors 32 and 31, respectively.
All of the intermediate closed loops in this second column are
identical in size and are of the T-shape, and each shares a one
side or a portion thereof with the preceding closed loop in the
column and shares another side or a portion thereof with the
succeeding closed loop in the column.
[0080] The line of closed conductor loops in the third and fifth
columns (and any other odd numbered column) are identical in size
and shape to those closed conductor loops of the first column, and
the line of closed conductor loops in the fourth column (and any
other even numbered column) is identical to the line of closed
conductor loops in the second column.
[0081] In addition it is seen that the closed loops in the columns
share a side with a closed loop in an adjacent column. The closed
loops in the end columns, the first and the fifth in the foregoing
embodiment, each share a side or sides with one or two closed loops
in the adjacent column. The closed conductor loops in each of the
other columns, intermediate the first and the fifth column, share a
side or sides with one or two closed conductor loops of a column to
one side, and share another side or sides with one or two closed
conductor loops of a column to the other side of the column.
[0082] The heater structure of FIG. 10 is more regular in shape and
less complex. In this embodiment the closed conductor loops are all
square in shape and are arranged in seven columns. Each column of
closed loops is straight sided.
[0083] As in the prior embodiment, each closed conductor loop has a
side in common with another closed conductor loop in the column.
The intermediate ones of the closed conductor loops in the column
contain one side that is shared with a preceding closed conductor
loop in the column and another opposed side that is shared with a
succeeding closed conductor loop in the column. Each closed
conductor loop in each column contains still another side that is
shared with a closed conductor loop of an adjacent column. Each
closed conductor loop in one of the intermediate columns shares
another side with a closed conductor loop in the column to the left
and shares an opposite side with a closed conductor loop on the
right.
[0084] As one appreciates from an understanding of the invention,
the conductor pattern may be of complex or simple in design and may
be reproduced in various designs and be described in many different
ways all of which come within the scope of the invention.
[0085] It is believed that the foregoing description of the
preferred embodiments of the invention is sufficient in detail to
enable one skilled in the art to make and use the invention.
However, it is expressly understood that the detail of the elements
presented for the foregoing purpose is not intended to limit the
scope of the invention, in as much as equivalents to those elements
and other modifications thereof, all of which come within the scope
of the invention, will become apparent to those skilled in the art
upon reading this specification. Thus, the invention is to be
broadly construed within the full scope of the appended claims.
* * * * *