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This invention relates generally to packaging arrangements for energy
dissipating devices and, more specifically, to a packaging arrangement
having improved thermal characteristics, heightened resistance to the
penetration of moisture, and reduced thermo-mechanical stressing. The
arrangement of the present invention is especially well-suited for
packaging electrical, electronic and microelectronic components, and can
be used with single components, multiple components, or complete circuits
(e.g., printed circuit boards).
The use of plastic encapsulation as a packaging technique for electronic
devices is well known. Problems of corrosion and inadequate thermal
performance have been noted in plastic encapsulated devices. More
recently, with the advent of Very Large Scale Integrated (VLSI)
Technology, the additional problem of thermo-mechanical stressing has
arisen as a major concern to packaging designers.
Consideration of plastic encapsulation techniques currently in use raises
questions as to the basic soundness of this packaging method for all
applications. The Transfer Molding Process commonly used allows molten
plastic to flow around and past the device and the delicate bond wires,
before eventual cross-linking and cure. The close contact between the
somewhat delicate circuitry on the device and the encapsulant does not
appear to be as benign as was once presumed. Furthermore, the adhesion at
the plastic/leadframe interface has always been a source of concern, as
has the relatively poor thermal performance of the package which results
from the forced reliance on the low thermal conductivity plastic for
transporting the major portion of energy dissipated at the device.
Consideration of these problems reveals the need to identify more
effective techniques for protecting the subject device both during and
after completion of the packaging process, and for transferring heat from
the packaged device to its environment.
An object of the present invention is to provide a packaging arrangement
for energy dissipating devices which has improved thermal characteristics.
Another object of the present invention is to provide a packaging
arrangement for energy dissipating devices which offers a greater degree
of mechanical protection for the packaged device and associated
connections.
Yet another object of the present invention is to provide a packaging
arrangement for energy dissipating devices which is especially well-suited
for use with preformed thermoplastic packaging elements which are used to
form an outer shell or casing around the packaged device.
Still another object of the present invention is to provide a packaging
arrangement for energy dissipating devices which incorporates thermal
elements which are specially designed, or which can be easily modified or
adjusted, to regulate the transfer or flow of heat from the packaged
device.
Still another object of the present invention is to provide a packaging
arrangement for energy dissipating devices in which the thermal elements
used to regulate heat transfer from the device form a sealed inner
enclosure around the device to provide increased mechanical protection of
the device and to reduce the migration of moisture from the environment to
the area immediately surrounding the device.
Still another object of the present invention is to provide a packaging
arrangement for energy dissipating devices which includes one or more
heat-flow modifiers placed within the package to control the heat flow
rates in major flow paths so as to optimize the thermal performance of the
package for given sets of operating conditions.
Still another object of the present invention is to provide a packaging
arrangement for electronic devices which is especially well-suited for use
with high pin count devices.
These and other objects of the invention are attained in a packaging
arrangement which comprises first and second thermal elements extending in
generally planar directions adjacent respective first and second sides of
the energy dissipating device, a plurality of electrically conductive
leads extending from the device to the exterior of the packaging
arrangement, means for electrically isolating the leads from the thermal
elements and for effecting a seal between the leads and thermal elements,
an outer casing surrounding the first and second thermal elements, and
means for effecting a seal between the outer casing and the leads. At
least one of the first and second thermal elements is in thermal
communication with the packaged device. The plurality of leads extend
between the first and second thermal elements, and at least one of the
elements extends adjacent a portion of the leads and is in thermal
communication with these lead portions. The leads extend through the outer
casing to the device environment.
A preferred embodiment of the present invention further comprises a
heat-flow modifying element adjacent at least one of the first and second
thermal elements for regulating the flow of heat therefrom. In an
especially preferred embodiment of the invention, the thermal conductivity
of the heat-flow modifier may be adjusted or varied, depending upon
internal and external operating conditions. The modifier may comprise a
metallic element. One or more openings or channels may be provided in the
element to accommodate a heat flow regulating medium. Alternatively, the
heat-flow modifier may comprise a cavity of specified dimensions. The
cavity may be left empty so as to form an air gap, or may be filled with a
heat flow regulating substance (e.g., an expanded foam) to provide the
required thermal resistance necessary for optimal energy distribution in
the arrangement. At least a portion of an interior surface of the cavity
may be lined with a thin metal foil to provide resistance to moisture
penetration of the cavity.
In a preferred embodiment of the invention, a portion of the second thermal
element contains a material having a high energy of transformation from
solid to liquid, which portion is situated in close proximity to a surface
of the packaged device. The portion of the second thermal element close to
electrically active regions of the device will be coated with a thin
semiconductor grade dielectric material for assured electrical isolation.
Alternatively, the second thermal element may comprise a metallic sheet
and a thermally conductive, electrically insulating elastomeric material
disposed between the sheet and the device. The elastomeric material
preferably includes an outer, relatively rigid elastomeric crust and an
inner, relatively soft elastomeric core.
In an especially preferred embodiment, the thermal elements substantially
surround the device to form an inner casing or enclosure around the device
to provide a degree of mechanical protection and to inhibit the transport
of moisture from the package environment to the area immediately
surrounding the device. The outer casing may be formed by an encapsulating
technique, but is preferably preformed of a thermoplastic material in at
least two sections, including a base and a cap. In this embodiment, one of
the thermal elements is mounted in, or formed integrally with, the base
section of the outer casing, while the other thermal element is similarly
mounted in or formed with the cap section of the casing. After the device
is mounted in the base section and appropriate connections are made, the
base and cap sections are sealed together to form the completed package.
The base and cap may be sealed by a fusion bonding technique, or by
injecting a molten thermoplastic material into a joint formed by adjacent
surfaces of the base and cap.
In yet another especially preferred embodiment of the invention, a seal
between the outer casing and the leads is formed by a seal assembly which
comprises a relatively soft, inner elastomeric core surrounding each
respective lead, and a relatively rigid, outer elastomeric crust
substantially surrounding the relatively soft inner core. The seal
assembly is compressed and heat cured in situ, after which the base and
cap are joined so as to establish a tight seal around each lead.
The means for electrically insulating the leads from the thermal elements
preferably includes a thermoplastic material or preformed film disposed
between the leads and the thermal elements. When the preformed encasement
technique is used, the thermoplastic film or material is preferably formed
from a thermoplastic which is the same as or compatible with the
thermoplastic used to form the base and cap sections of the outer casing
so that at least some bonding will occur between the film or material and
one or both of these sections when the package casing is formed and the
package is sealed.
Other objects, advantages and novel features of the present invention will
become apparent from the following detailed description of the invention
when considered in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view of an embodiment of the packaging arrangement of
the present invention, shown without an outer casing.
FIG. 2 is a sectional view of an embodiment of the present invention which
includes a micro-electronic device, associated thermal elements and
heat-flow modifier, and an outer thermoplastic casing.
FIG. 3 is a partial sectional view of another embodiment of the present
invention showing details associated with establishing a seal around the
micro-electronic device.
FIG. 4 is a partial sectional view of another embodiment of the present
invention showing encasement of a high pin count electronic device.
FIG. 5 is an enlarged sectional view of a portion of FIG. 4.
FIG. 6 is a sectional view of another embodiment of the packaging
arrangement of the present invention.
DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an embodiment of the present invention which includes an
electronic device 10 mounted on a metallic pad 12 with a bonding material
14 which, for example, may be a silver-filled epoxy. Bond wires 16 connect
respective bond pads on device 10 to a plurality of leads, represented by
the two leads 18 in FIG. 1, which provide means for connecting device 10
to the external environment. Immediately adjacent the device/metallic pad
combination is a thermal element designated by reference numeral 20.
Thermal element 20 is, in this embodiment of the invention, a thin
metallic sheet which is U-shaped in cross-section and which forms a recess
22 for receiving the device/metallic pad combination. Metallic pad 12 is
mounted to thermal element 20 so as to insure good thermal contact between
thermal element 20 and device 10. Although metallic pad 12 is incorporated
in the arrangements shown in FIGS. 1 and 2, its use may not be necessary
in every instance and its inclusion or omission has no effect on the
practicing of the present invention. The outer ends 24 of thermal element
20 extend for a distance immediately adjacent to respective end portions
26 of the representative leads 18. Outer ends 24 of thermal element 20 are
electrically isolated from the leads by, for example, a thermoplastic tape
or film 28 having a high electrical resistance. However, outer ends 24 are
in thermal contact with end portions 26 of leads 18.
A second thermal element 30 is positioned above device 10 and leads 18.
Thermal element 30 is also preferably formed of a thin metallic foil plate
which is electrically isolated from, but in thermal contact with, end
portions 26 of leads 18 in the same manner as was previously described in
connection with thermal element 20. The cross-sectional shape of thermal
element 20 may vary in accordance with design requirements. For example,
the center portion of thermal element 30 may be shaped as illustrated at
32 in FIG. 1 so as to establish good thermal contact with device 10. In
this case, a surface of portion 32 of thermal element 30 will be in very
close proximity to the top surface of device 10, but will be separated
from that surface by, for example, a thin layer of semiconductor grade
thermally conductive gel 33. This portion of thermal element 30 may also
be designed to contain a material having a high energy of transformation
from solid to liquid to modify the heat dissipating characteristics of the
arrangement.
The remaining component in the arrangement shown in FIG. 1 is a heat-flow
modifying element 34 which is shown positioned immediately adjacent a
portion of thermal element 20. Element 34 may take a variety of forms. For
example, element 34 may be a metallic element used to establish thermal
contact between thermal element 20 and external heat dissipating
structures. Alternatively, element 34 may take the form of a relatively
thick metallic body provided with channels to accommodate the flow of a
heat regulating fluid (see FIG. 2). As another alternative, element 34 may
comprise an air gap of specified dimensions (See FIG. 3). Numerous other
forms are possible and the use of more than one heat-flow modifier
adjacent either of thermal elements 20 or 30 is considered to be within
the scope of the present invention.
If the outer ends of thermal elements 20 and 30 are adequately sealed
around the end portions of the leads, the embodiment shown in FIG. 1
represents a workable packaging arrangement for device 10. Thermal
elements 20 and 30 distribute the energy dissipated by device 10 in a
relatively uniform manner over as large a plane as might be practically
allowed by overall size considerations. By virtue of their thermal contact
with the end portions of the leads, elements 20 and 30 provide for good
thermal communication between the leads and device 10, making possible the
establishment of high uniform lead temperatures. Maintaining the highest
possible lead temperatures is important to achieving the maximum thermal
performance of the metallic leadframe in many packaging arrangements, and
elements 20 and 30 contribute toward this end in the packaging arrangement
of the present invention.
Although the arrangement shown in FIG. 1 might conceivably be used as a
"complete" packaging arrangement, the further provision of an outer casing
of a material such as a thermoplastic is preferred. The outer casing will
preferably completely surround thermal elements 20 and 30, and a seal will
be formed between the outer casing and the leads where the leads extend
through the casing to provide an external connection point. If a plastic
encapsulation technique is used to form the outer casing, thermal elements
20 and 30 serve to form an inner enclosure around device 10, thus
protecting the device and delicate bond wires from contact with the
plastic encapsulant. Other advantages to such an arrangement include: the
establishment of a relatively constant planar temperature distribution
along the planes of the thermal elements and improved energy transport out
of the package; the establishment of high lead temperatures with the
potential for reducing moisture ingress along the plastic/lead interfaces;
and thermal elements 20 and 30 act as a barrier to moisture transport
directly through the plastic to the device. Such an arrangement provides a
unified approach to the problems of corrosion, thermal performance, and
thermo-mechanical stressing known to exist in devices packaged by
conventional encapsulation techniques.
Although the arrangement illustrated in FIG. 1 can be encapsulated to
provide an improved packaging arrangement, an outer casing formed by a
premolded encasement technique, incorporating the features discussed
above, is preferred. Such an arrangement is illustrated in FIG. 2. To the
extent possible, like reference numerals are used to identify components
in FIG. 2 which were previously discussed in connection with FIG. 1.
Depending portion 32 of thermal element 30 is not shown in the embodiment
illustrated in FIG. 2. Reference numeral 34 of FIG. 1 has been changed to
34' in FIG. 2 since a modified version of the heat-flow modifying element
is shown in FIG. 2. This modified version illustrates the provision of
channels 35 (represented by dashed lines) in the heat-flow modifying
element to allow for modification and/or enhancement of the thermal
characteristics of the modifying element by, for example, providing for
the flow of a heat regulating fluid or medium through the channels.
The outer casing illustrated in FIG. 2 comprises a two part injection
molded package which includes a base 36 and cap 38. These components are
separately molded by injection molding techniques which preferably allow
element 34' and thermal element 20 to become an integral part of base 36,
while thermal element 30 becomes an integral part of cap 38. Assembly of
the entire arrangement illustrated in FIG. 2 then involves mounting the
leadframe and pad 12 in base 36, molding the housing around the frame,
mounting device 10 on pad 12, forming the appropriate connections between
device 10 and leads 18, properly positioning cap 38 on base 36, and
sealing cap 38 to base 36. The advantages provided by thermal elements 20
and 30 and modifying element 34', as previously discussed in connection
with FIG. 1, are applicable in the arrangement illustrated in FIG. 2.
FIG. 3 shows a partial sectional view of another embodiment of the
invention in which additional details of the seals between the base and
the cap and between the leads and the thermal elements are shown. FIG. 3
includes base 40 and cap 42 which are preformed of a thermoplastic
material. Integrally molded with base 40 is a metallic thermal element 44.
In like fashion, thermal element 46 is integrally molded with cap 42.
Heat-flow modifier 48 in this embodiment comprises an air gap of specific
dimensions located immediately adjacent the underside of thermal element
44 near the center of the package. The size of the gap determines its
effect on the heat transport performance of the surrounding package
elements. The size and location of thermal element 44 and heat-flow
modifier 48 may be altered, as desired, due to the flexibility of the
injection molding process used to form the base and cap. Also in the
center of the package is device 50 which is mounted by silver-filled epoxy
51 to metallic pad 52. Pad 52 is, in turn, mounted in good thermal contact
to the top side of thermal element 44. As with the other embodiments
illustrated in the Drawings, device 50 could be mounted directly to
thermal element 44, if desired, since element 44 provides a suitable
substrate for mounting of the device.
Bonding pads on device 50 are connected by bond wires 54 to leads 56 which
extend to the external environment. In FIG. 3, lead 56 is electrically
insulated from thermal element 44 by a thermoplastic film 58 which is
fusion-bonded to lead 56, and a compressed elastomeric seal assembly 60.
Seal assembly 60, which is mounted on the leadframe, comprises an outer
elastomeric crust 62, within which is cast a softer B-Stageable elastomer
64. When cap 42 is assembled to base 40, outer crust 62 of seal assembly
60 is compressed by thermal elements 44 and 46 to provide for maintenance
of a tight seal between these elements and around each lead. As previously
noted, the sealed thermal elements form an interior enclosure (generally
indicated by reference numeral 66 in FIG. 3) around device 50, bond wires
54, and other internal components of the package. Seal assembly 60
preferably extends around the entire periphery of the interior enclosure
formed by the thermal elements.
In the embodiment of FIG. 3, base 40 and cap 42 are sealed and bonded to
each other in the area generally designated 68 (i.e., external to seal
assembly 60) by an induction heating technique. This fusion bonded seal
extends around the entire perimeter of the package.
Rectangular portion 69 of base 40, which is located immediately adjacent
lead 56, is initially a trough formed during the injection molding of base
40. A complementary projecting tongue is likewise formed on cap 42 during
the injection molding process. When cap 42 and base 40 are sealed, the
tongue melts to fill the trough so as to enhance the characteristics of
the bond formed between base 40 and cap 42 during the induction heating
and subsequent fusion of materials forming these two elements.
An additional feature of the embodiment of the invention shown in FIG. 3 is
the use of a thermally conductive, electrically insulating elastomer 70
which is cast on the underside of thermal element 46 immediately over
device 50. Elastomer 70 is a relatively soft compound which is encircled
on its vertical edges by a relatively firm outer elastomeric crust 72. The
area of device 50 which is in actual contact with elastomer 70 is somewhat
restricted by bonding wire considerations, but nevertheless provides
adequate means for the effective transport of heat to thermal element 46.
As previously mentioned in connection with FIG. 1, this arrangement for
increasing thermal energy transport from the device to the upper thermal
element is only one of many possible configurations.
FIG. 4 shows an embodiment of the invention which is especially well-suited
for use with electronic devices having a large number of leads (e.g., in
excess of 84), maintained at center-to-center distances less than or equal
to 25 thousands of an inch. For such high pin count devices, the leadframe
material thickness decreases. For leadframe thicknesses on the order of 6
mils. (0.006 inches), leadframe design requirements force the size of a
200-pinout device to grow to approximately 0.460 inch per side for
successful wire bonding. The encapsulation of such large devices with
conventional techniques may result in problems related to device
thermo-mechanical stressing which may be deleterious to device reliability
and function, The embodiment shown in FIG. 4 employs the features of the
present invention to reduce, or eliminate, these problems. The particular
embodiment illustrated in FIG. 4 shows the present invention used in
combination with a Tape Automated Bonding (TAB) technique of connecting
the inner leads to the bonding pads of the device and the outer leads to
the leads of the leadframe. The additional advantage derived from this
arrangement relates to the relative ease and efficiency with which the
leadframes can be manufactured since the lead-fingers of the frames
terminate farther from the device where the spacing of individual
lead-fingers is not as close. A similar arrangement can be designed using
conventional wire bonding techniques.
The embodiment of FIG. 4 includes a preformed thermoplastic base 80, which
is formed by injection molding and which includes an integrally molded
metallic thermal element 82. Base 80 also includes a heat-flow modifying
element 84 which includes one or more cavities 85 immediately adjacent the
underside of thermal element 82. Cavity 85 may be filled with air,
expanded foam, or other substances to give the required thermal resistance
for optimal energy distribution throughout the package and to any
underlying structures. A thin metallic foil 86 may be provided along the
bottom and side walls of the cavity to provide resistance to moisture
penetration and possible condensation. In this embodiment, device 88 is
mounted (e.g., by silver filled epoxy) directly to the upper side of
thermal element 82.
The embodiment of FIG. 4 further includes a preformed thermoplastic cap 90
having an integrally molded metallic thermal element 92 on its lower
(interior) surface. A thermally conductive elastomeric compound 94, with a
relatively rigid outer crust 96, is disposed between thermal element 92
and device 88. This embodiment of the packaging arrangement is sealed by
an injection molding technique which involves injection of molten
thermoplastic material between base 80 and cap 90. The area of the seal is
generally represented by reference number 98 in FIG. 4. Such a sealing
technique can also be used with the embodiment of the invention
illustrated in FIG. 3.
As previously noted, the connections from the various leads to the device
bonding pads in FIG. 4 are established by a Tape Automated Bonding
technique. FIG. 5 shows an enlarged cross-section of area A of Figure 4,
illustrating the interrelationship between this technique and the
packaging arrangement of the present invention. Moving from bottom to top,
the various layers depicted in FIG. 5 include thermoplastic base 80,
metallic thermal element 82, a first layer of thermoplastic film 100, a
conductor layer or pattern 102 which may be deposited on thermoplastic
film 100 by a metallizing technique, upper thermoplastic layer 104, and
thermoplastic material 99 of the package housing base. In a preferred
embodiment of this invention, thermoplastic film 104 and thermoplastic
material 99 of the base are compatible so that fusion bonding occurs
between these two elements when the base of the package housing is molded.
FIG. 6 shows a modified embodiment of the present invention in which the
seals established between the leadframe, the thermal elements, the base
and/or the cap differ, in structure and sequence of manufacture, from
those shown in the previously discussed embodiments. The embodiment of
FIG. 6 shows a device 110 mounted on a metallic pad 112 which, in turn, is
mounted to a lower thermal element 114. Immediately adjacent the top side
of device 110 is a thermally conductive, electrically insulating elastomer
116 which is attached to the underside of upper thermal element 118. In
this embodiment of the invention, upper thermal element 118 is
approximately ten thousandths of an inch thick, while lower thermal
element 114 is approximately two thousandths of an inch thick. As
discussed in connection with the embodiment shown in FIG. 3 above,
elastomer 116 is bounded by a relatively rigid outer crust 120.
The embodiment of FIG. 6 further includes a thermoplastic base 122 and cap
124 which together form the outer casing of the package. A plurality of
leads 126 extend from the package and are sealed between base 122 and cap
124 by molded material 128 and by an elastomeric seal 130. Leads 126 are
connected to the device by bond wires 132. In this embodiment, the
leadframe is attached to base 122 by material 128 which is overmolded onto
elastomeric sealing 130 and which encapsulates portions of the leads of
the frame and fuses with base 122.
The preferred manufacturing sequence for the embodiment of FIG. 6 includes
the following steps: mounting elastomeric seal 130 to the leadframe;
mounting the frame and seal on the base and molding material 128 as shown
to firmly attach the leadframe and seal to base 122; positioning thermal
element 118, properly positioning the seal mold and molding cap 124 to
seal the base/leadframe/seal assembly.
Elastomeric seal 130 is preferably made of two sections, as shown, which
are placed on either side of the leadframe and then fully heat cured,
under pressure, on the frame. Lower thermal element 114 is integrally
molded to, or snap-fitted into position within, base 122 prior to final
positioning of the leadframe and seal assembly. Molded material 128
preferably surrounds at least a portion of leads 126, extends into base
122, and extends over elastomeric seal 130 to bond all of these components
together. Upper thermal element 118 is preferably snap-fitted into
position atop the assembly prior to molding cap 124 and sealing cap 124 to
base 122 and/or material 128.
Neither a thermal resistance air gap nor other heat flow modifying elements
are shown in the embodiment of FIG. 6. These features could, of course, be
added to this embodiment for the same reasons, and in a similar manner, as
discussed in connection with FIGS. 1-4 above.
Although the invention has been described by reference to preferred
embodiments relating to the packaging of an electronic device, other types
of energy dissipating electrical devices can be packaged in accordance
with the principles of the invention. It should also be noted that,
although the invention has been described and illustrated in terms of a
packaging arrangement for a single device, similar packaging arrangements
for use with multiple devices and/or entire printed circuit boards can be
devised and are considered to fall within the scope of this invention.
Although the invention has been described and illustrated in detail, it is
to be clearly understood that the same is intended by way of illustration
and example only and is not to be taken by way of limitation. The spirit
and scope of the invention are to be limited only by the terms of the
appended claims.
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