Note: Descriptions are shown in the official language in which they were submitted.
iZC~ 317
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A SELF-POSITIONING HE~T SPREADER
Technical Field
This invention relates generally to heat spreaders
and, more particularly, to a heat spreader adapted for
insertion into a mold cavity prior to plastic
encapsulation with a semiconductor device.
Background Art
Metal heat spreaders o various shapes ancl designs
have been commonly used to spread or conduct heat away
from a ~netal lead fra~e which is electrically connected to
an integrated circuit die. One application of such heat
spreaders has been to physically attach the heat spreader
to the metal lead frame wherein both the heat spreader and
lead frame are encapsulated ln plastic. In such an
application, a heat spreader is used to spread heat in a
relatively uniform manner throughout plastic and is not
used for contact to an external heat sink. When heat
spreaders have an exposed surface, poor adhesion between
plastic and metal may allow moisture to enter into the
package. Exposed metal surfaces may also result in a
fragile structure. A disadvantage with heat spreaders
that are directly connected to a metal lead frame is that
achieving the physical connection requires a separate
manufacturing step. A further disadvantage with such heat
spreaders is the fact that capacitance exists between the
heat spreader which acts as one long conducting surEace or
plate and the lead frame which acts as another conducting
surface or plate. Since the lead frame has different
voltages at various points during operation and the
distance separating the two conducting surfaces is small,
a large capacitance is created between the two surfaces.
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For some applications the capacitance does not present a
significant problem. ~lowever, in many hiqh speed
circuits, such as a microprocessor, the capacitance can be
a severe problem. Another pro~lem is t:he variation in
capacitance from part to part which results from variation
in the position oE the heat spreader with respect to the
lead frame. As a result of the variation oE capacitance
from part to part, a heat spreader configuration may ~e
acceptable for some parts but ~e unacceptable for other
parts having different lead frame configurations.
Brief Su~mary of the Invention
Accorclingly, an object of the present invention is to
provide an improved heat spreader which is selE-
positionin~ in a mold cavity prior to plastic
encapsulation.
~nother object of the present invention is to provide
an improved heat spreader having a configuration in which
capacitance between the heat spreader and a lead frame is
minimized.
Another object of the present invention is to provide
an improved heat spreader having substantially the same
amount of capacitance between itself and a lead frame from
part to part.
~ further object of the present invention is to
provide an improved heat spreader which is anodized and
self-positioning in a mold cavity prior to plastic
encapsulation.
Yet another object of the present invention is to
provide an improved heat spreader for use in a
semiconductor package havinq a low thermal impedance.
In carrying out the above and other objects oE the
present invention, there is provided, in one form, a self-
positionin~ heat spreader adapted Eor insertion into a
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mold cavity prior to encapsulation with an encapsulant
such as plastic. ~he heat spreader is placed into the
mold cavity and is parallel to a first surface of a metal
lead frame which has a semiconductor die connected to a
second surface. The heat spreader has a frame with
notches at opposite ends to reduce capacitance and which
define bifurcated limbs which are sized to extend to
within a predetermined tolerance of the end walls of the
mold cavity. In order to ma}se capacitance substantially
uniform from unit to unit, the inner edges of each limb
complement the portion of the lead frame which overlays
the limb. A pair of spaced-apart lateral standoffs extend
from each side of the heat spreader to within a
predetermined tolerance of the side walls of the mold
cavity. At least a pair of feet extend A predetermined
distance below the lead frame, and a central portion
extends to within a predetermined tolerance of the bottom
surface of the lead Erame. ~t leas-t one surface of the
heat spreader is covered with an insulation such as
anodization to prevent electrical contact between the heat
spreader and the lead frame.
Brief Description of the Drawings
FIG. 1 illustrates a top plan view of a self-
positioning heat spreader constructed in accordance with
the preferred embodiment of the present invention;
FIG. 2 illustrates a side elevational view of the
heat spreader of FIG. 1; and
FIG. 3 illustrates a cross sectional view of a heat
spreader accordin~ to this invention taken substantially
on line 3-3 of FIG. 1.
Detailed Description of the Preferred Embodiment
Shown in FIG. 1 is a heat spreader 10 constructed in
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accordance with -the preferred embodiment. Heat spreader
10 may be inserted into either a lower or an upper half of
a mold cavity (not shown). Although a lower thermal
impedance may be obtained when heat spreader 10 is
utilized in an upper half of a mold cavity, heat spreader
10 is typically utilized in a lower half of a mold cavity.
In such a conventional fabrication of an encapsulated
integrated circuit, an upper half of a mold cavity
containing a lead frame ~not shown) which is connected to
a semiconductor die is positioned within the lower half of
the mold cavity. The semiconductor die has electrical
wires extending therefrom to the lead frame. Both halves
of the mold cavity are Eilled with an encapsulant, such as
molten plastic, to form an encapsulated semiconductor
device.
In a preferred form, heat spreader 10 comprises an
elongated heat conductive frame 12 which is metal, such as
aluminum, and is complete]y anodized or insulated.
Although heat spreader 10 is preferably metal, heat
spreader 10 may be fabricated from any other material
which has a greater heat conductivity than the
encapsulant. While it will be understood that heat
conductive frame 12 is preferably elongated, a
substantially s~uare frame may also be used. At the two
ends of frame 12 is a notch 14 and and a notch 16,
respectively. A pair of bifurcated limbs 18 and 20
surround notch 14 and a pair of bifurcated limbs 22 and 24
surround notch 16. Limbs 18 and 20 terminate with an end
edge 26 and an end edge 28, respectively. Limbs 22 and 24
terminate with an end edge 30 and an end edge 32,
respectively. Limbs 18 and 20 have an inner ed~e 34 and
an inner edge 36, respectively, and limbs 22 and 24 have
an inner edqe 38 and an inner edqe 40, respectively.
Lateral standoffs 42, 44, 46 and 48 extend Erom the outer
sides of limbs 18, 20, 22 and 24, respectively. Extending
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a predetermined distance from the lower surface of heat
spreader 1~ are feet 50l 52, 54 and 56. A central portion
58 extends from the top surface of heat spreader 10 to
within a predetermined tolerance of the bottom 6urface of
S a lead frame (not shown).
Feet 50, 52, 54 and 56 help position and support heat
spreader 10 from the floor of the mold cavity. A primary
purpose of feet 50, 52, 54 and 56 is to elevate elongated
frarne 12 from the bottom surface of the mold cavity. As a
result, heat spreader 10 does not have an exposed surface
as a heat sink would and is essentially totally
encapsulated. Complete encapsulation is important since
plastic does not always strongly adhere to metal. When a
metal surface of heat spreader 10 is exposed/ moisture can
often enter the metal-plastic interface. Further, heat
spreader 10 may be inadvertently pulled out or just come
out of the plastic if exposed. Feet 50, 52, 54 and 56
therefore extend below heat spreader lO a predetermined
distance to insure that the lower surface of heat spreader
10 is totally encapsulated in plastic. Although four
` distinct feet have been shown in a preferred form, either
` as few as two or more than four feet may be used. When
- only two feet are used, both feet are positioned toward
the opposite end of heat spreader 10 from the end in which
the encapsulant enters. The flow of encapsulant allows
the end having no feet to be sligthly raised from the mold
cavity.
The purpose of central portion 58 i5 to put only a
portion of metal heat spreader 10 in close proximity to
the lead frame. The reason for putting only a portion of
heat spreader 10 in close proximity to the lead frame is
to minimize the capacitance which exists between the two
conducting surfaces. Such capacitance increases as the
the distance or thickness of the plastic dielectric
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between the surfaces decreases. However, the lead frame
and heat spreader 10 can not he separated by a large
distance without seriously reducinq the heat spreading
ability of heat s~reader 10. Central portion 58 may be
placed virt~lally ad~acent the lead frame because heat
spreader 10 is anodized to have an electrically insulatin~
skin. As a result, even if no plastic separates heat
spreader 10 and a lead frame, heat spreader 10 will not
electrically contact an electronic circuit which is
attached to the lead frame.
Lateral standoffs 42, 44t 46 and 48 are essential for
heat spreader 10 to be self-positioning into the mold
cavity. Lateral standoffs extend into within a first
predetermined tolerance of the side walls oE the cavity so
that when heat spreader 10 is dropped into the cavity,
heat spreader 10 i5 substantially parallel to the lead
frame. Furtherr when plastic is Elowing through the mold
cavity, heat spreader 10 will not be able to move
sideways. Ends 26, 28, 30 and 32 are also sized to extend
to within a second predetermined tolearance of the end
walls of the mold cavity. Similarly, when plastic flows
into the mold cavity, heat spreader 10 will not he able to
move lengthwise since limbs 18, 20, 22 and 24 are firmly
wedged against the end walls of the mold cavity.
Notches 14 and 16 serve a two-fold purpose. Firstly,
by reducing the a~ount of metal associated with frame 12,
the amount of unwanted capacitance between frame 12 and
the lead frame is reduced. In some applications, the
inte~rated circuit which is used with the lead frame is
very capacitance sensitive. Therefore, a reduction in
conducting surface area is often essential to minimi~e
capacitance between heat spreader 10 and the lead frame.
Secondly, notches 1g and 16 are very helpful in allowing
plastic to freely flow into the mold cavity at either end.
If metal extended all the way across ends 26 and 28 and
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ends 30 and 32, plastic flow would be obstructed when
entering the mold cavity and flow problems could resul-t.
Another important feature of heat spreader 10 is the
design oE inner edges 34, 36, 38 and 40 which are made to
complement or conform to the portion of the lead frame
which overlays limbs 18, 20, 22 and 24, respectively. The
lead frame has a particular design and contour and oEten
has a radial design from a centrally located integrated
circuit die. By making heat spreader 10 follow the same
contour, the variation in heat spreader-lead frame
capacitance from unit to unit is substantially reduced.
Furthermore, inner edges 34, 36, 38 and 40 are centered
between two adjacent leads of a lead Erame design. As a
result, the heat spreader-lead frame capacitance variation
is further reduced. Thus, all semiconductor devices
utilizing a particular lead frame design will have
substantially the same heat spreader-lead frame
capacitance.
A further feature of heat spreader 10 is the
; 20 utilization of tapered edges 59, 60, 62 and 64 from end
edges 26, ~8 30 and 32, respectively. The purpose of
tapering end edges 26, 28, 30 and 32 is to allow more
plastic to exist around the corners of the integrated
circuit package and have a more durable part. Because
integrated circuits can receive physical abuse during
handling, this feature tends to increase the integrity of
the package.
An additional feature of heat spreader 10 is the
utilization of ribs 66 and 68 extendin~ from the upper
surface of heat spreader 10 to within a predetermined
tolerance of the bottom surface of the lead frame.
Although in the preferred form two ribs are shown, one o~
more ribs may be used. Further, the positionin~ of ribs
&6 and 68 is not critical and ribs 66 and 68 may be
located at any distance from notches 14 and lG and central
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portion 58. Ribs 66 and 68 function to keep the plastic
which is flowing in the mold cavity from twisting or
moving limbs 18, 20~ 22 and 24 up agAinSt the lead frame.
Therefore, the use of a rib would be more significant if
positioned on the end o the mold cavity where plastic is
entering. However, i-f the manufacture of central portion
58 is within the specified predetermined tolerance, rihs
66 and 68 are not essential to heat spreader 10.
Shown in FIG. 2 is a side elevational view of heat
spreader 10. As previously noted, the positioning of ribs
66 and 68 and feet 52 and 56 with respect to central
portion 58 is not critical. However, in the preferred
form, all ribs, feet, limbs and standoffs are
substantially symmetrical with central portion 58.
Shown in FIG. 3 is a cross sectional view of heat
spreader 10 taken substantially on line 3-3 of ~IG. 1. An
additional feature of heat spreader 10 now made apparent
is a channel 70 which exists under central portion 58 and
extends through notch 1~ and notch 16. Inner inclined
edges 72 and 74 elevate an inner section 76 of elongated
frame 12 to within a predetermined tolerance of the bottom
surface of the metal lead frame so that inner section 76
is higher than sides 78 and 80. Side 78 is the side which
includes limbs 20 and 24, and side 80 is the side which
` 25 includes limbs 18 and 22. The purpose for elevating inner
section 76 is to place a substantial amount of metal heat
spreader 10 as close as possible to a lead frame to
further reduce the thermal impedance. ~lso, by elevating
inner section 76, plastic may flow into the mold cavity
more freely. By now it should be apparent that the
features of heat spreader 10 have been optimized to
provide a low thermal impedance, free plastic flow and a
low and substantially consistent heat spreader-lead frame
capacitance. The features discussed also provide a heat
spreader which may be easily loaded into a mold cavity
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since the spreader is self-positioning.
While the invention has been described in the context
of a preferred embodiment, it will be apparent to thos2
skilled in the art that the present invention may be
S modiEied in numerous ways and may assume many emboditnents
other than that specifically set out and described above.
Accordin~ly, it is intended by the appended claims to
cover all modifications of the invention which fall within
the true spirit and scope oE the invention.
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