Note: Descriptions are shown in the official language in which they were submitted.
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METAL ELECTRONIC PACKAGE
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The present invention relates to a process
for making a package to house an electronic
device and the package so produced. The package
is characterized by improved resistance to
thermal degradation and improved dissipation of
heat from the enclosed electronic device.
Several basic package designs are used by
the electronics industry to house integrated
circuits. The integrated circuits, usually
fashioned from a semiconductor material and
typically from silicon, must be protected from
the outside environment and must electrically
connect to external components. Balanced against
these requirements is a need to minimize cost of
manufacture and assembly of the packageO
The electronics industry has generally
followed one of two lines; maximimum protection
of the electronic device for example with the
ceramic dual-in-line package or minimum cost,
for example by the encapsulated plastic package.
There are distinct disadvantages to both
concepts.
The ceramic dual-in-line package (CERDIP)
is comprised of two pieces of alumina or other
ceramic glass bonded to a leadframe with a
suitable solder glass forming a hermetic
enclosure. A problem associated ~ith a CERD~P
is poor thermal dissipation. To maximize
3n thermal dissipation capabili~ies of a CERDIP,
beryllium oxide is sometimes substituted for
aluminum oxide~ While the thermal capabilities
of the package is improved, the cost increases
significantly. The CERDIP is generally used for
high value electronic requirements, for example,
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state of-the-art computers or military
applications.
Plastic packages usually comprise an
electronic device mounted on a lead frame
encapsulated with a plastic resin, generally an
epoxy, The plastic package is less expensive to
manufacture than the CERDIP and more adaptable
to automated procedures. The plastic package is
generally not hermetic and not suitable for high
value electronic requirements. Additionally,
the plastic package has poor thermal
dissipation.
Throughout this application, the term
"hermetic" is used in accordance with Military
Specification 883B and defined as a package
cavity leak rate of less than 5 x lO 8 cm3/sec
when measured using a helium tracer gas.
A package which seeks to overcome the
disadvantages of the CERDIP and the plastic
package is the metal package. The metal package
is comprised of a leadframe disposed between a
metaI or metal alloy cover and base component.
The leadframe is bonded to the base and cover
using an electrically insulating adhesive,
typically a solder glass or polymer adhesive.
Generally, a solder glass is used if a hermetic
package is desired. Examples of a metal package
using a solder glass may be found in U.S Patent
Nos. 4,524,238, 4,532,222, 4,S42,2~9, 4l607,276
and 4,656,499 all issued to Butt.
A problem associa~ed with the metal package
is a mismatch in the coeffiecient of thermal
expansion between the metal component and the
conventinal CERDIP solder glasses. The metal
components are typically copper or a copper
based alloy and have coefficients of thermal
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expansion ~CTE) of about 160 - 170X10 7 oc while
the low temperature solder glasses were originally
designed to match the CTE of aluminum oxide CERDIP
packages and have a CTE of about 49X10 7 oc. I~
the CTE mismatch exceeds 10% the glass may fracture
due to stresses introduced during thermal cycling
resulting in a loss of hermeticity.
Methods to get a close match in coefficients
of thermal expansion include the use of a graded seal
as disclosed in U.S. Patent No. 4,704,626 to
Mahulikar et al and the use of appropriately chosen
filler material such as calcium fluoride to change
the coefficient of thermal expansion of the glass as
disclosed in U.S. Patent Nos. 4,752,521 and 4,801,488.
Hermetic metal packages have the added
advantage of excellent dissipation of heat generated
by the electronic device during operation. As metal
packages contain a cavity, the surface of the
electronic device is not subjected to stresses during
thermal cycling as occurs with a molded plastic
package.
Metal packages have also been used to
replace components in plastic packages~ As with
glass sealed packages, a metal leadframe is disposed
between a metal or metal alloy base and cover
component. The package i8 sealed with a polymer
adhesive, typically an epo~y. Examples of a metal
package with a polymer adhesive sealant are in U.S.
Patent Nos 4,461,924, 4,480,262 and 4,594,770 all to
Butt and U.S. Patent No. 4,105,861 to Hascoe.
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Polymer adhesives are generally more
compliant than sealing glasses. There is no need to
match the coefficient of thermal expansion of the
package components to the adhesive. Thermally
induced stresses will be absorbed and damped by the
polymer adhesive. A difficulty with the metal
package utilizing a polymer adhesive is it is not
hermetic in accordance with Mil. Spec. 883B.
Additionally, the polymers are not thermally
stable and tend to degrade during post-cure heating
cycles. One method of evaluating the package sealant
is commonly called the pressure cooker test. The
test comprises immersion of a sealed package in a
pressure cooker at 121C, a relative humidity of 100%
and a pressure of 2100 gm/cm2 (30 psi). It has
been found that metal packages sealed with polymer
adhesives delaminate within a relatively short amount
of time during this test. This test is an
accelerated environmental test and indicates how well
the package will protect the electronic device from
air and moisture.
In accordance with the present invention, a
leadframe containing a centrally positioned die
attach pad is provided. An electronic device,
typically a silicon semiconductor chip, is bonded to
the die attach pad. The die at~ach pad is bonded to
the package base and the base and cover components
are sealed to the leadframe. The package goes
through at least one fewer sealing cycle than prior
art packages resulting in substantial improvements in
the functional life of the sealant.
In another embodiment of the present
invention, an aperture is provided in the base
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component. The die attach pad is sealed over the
aperture. The embodiment permits improved removal
of heat from the electronic device.
Accordingly, it is an object of the
s present invention to provide a metal package sealed
with a polymer which is more resistant to thermal
degradation.
It is a further object of the present
invention to provide a metal packaye with a polymer
10 sealant requiring fewer assembly steps than present
metal packages.
It is yet another object of the present
invention to provide a metal package with improved
thermal dissipation characteristics.
In accordance with a particular embodiment
of -the invention there is provided a package
containing an enclosure for housing an electronic
device, comprising:
a base component containing a first
20 aperture;
a cover component;
a leadframe with first and second opposing
~ sides clisposed between said base component and said
cover component, said leadframe containing a
25 plurality of leads disposed about a centrally
positioned die attach pad, said die attach pad
positioned to seal said first aperture and adapted
to support said electronic device, said electronic
device electrically connected to said leads;
a first bonding means to seal said base
component to said first side of said leadframe;
a second bonding means to seal said cover
component to said second side of said leadframe; and
a third bonding means to seal die attach
35 pad to said base component.
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In accordance with a further embodiment of
the invention there is provided a package containing
an enclosure for housing an electronic device,
comprising:
s a base component;
a cover component containing a second
aperture sealed with a plug;
a leadframe with first and second opposing
sides disposed between said base component and said
10 cover component, said leadframe containing a
plurality of leads disposed about a centrally
positioned die attach pad and adapted to support
said electronic device, said electronic device
electrically connected to said leads;
a first bonding means to seal said base
component to said first side of said leadframe;
a second bonding means to seal said cover
component to said second side of said leadframe; and
a third bonding means to seal said die
attach pad to said base component.
In accordance with a still further
embodiment of the invention there is provided a
package containi.ng an enclosure to house an
electronic device, comprising:
2s a base component containing a first
aperture;
a cover component containing a second
aperture, said second aperture sealed with a plug;
a leadframe with first and second opposing
sides clisposed between said base component and said
cover component, said leadframe containing a
plurality of leads disposed about a centrally
positioned die attach pad, said die attach pad
positioned to seal said first aperture and adapted
~s to support said electronlc device, said electronic
device electrically connected to said leads;
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a first bonding means to seal said base
component to said first side of said leadframe;
a second bonding means to seal said cover
component to said second side of said leadframe; and
a third bonding means positioned about the
edge of said first aperture and disposed between
said base component and said die attach component.
Also, in accordance with the invention,
there i5 provided a process for assembling a package
~o containing an enclosure to house an electronic
device, comprisiny the steps of:
providing an electrically conductive lead-
frame, said leadframe having first and second
surfaces and comprised of a plurality of lead
15 fingers disposed about a centrally positioned die
attach pad, said electronic device bonded to said
die attach pad and electrically connected to said
leads;
providing a first, second and third bond-
ing means;
providing a base component;
providing a cover componenti
bonding said base component to said first
surface of said leadframe with said first bonding
25 means and at essentially the same time bondlng sa:id
base component to said die attach pad with said
third bonding means; and
bonding said cover component to said
second surface of said leadframe with said second
30 bonding means.
These and further objects and advantages
will become more apparent from the following
description and drawings in which like reference
numerals depict like elements.
3s FIG. 1 is a cross-sectional representation
of a metal package as known from the pr~or art.
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FIG. 2 is a schematic representation of a
leadframe wi.th an electronic device afflxed thereto
in accordance with the present invention.
FIG. 3 is a cross-sectional
s representation of a metal package in accordance with
one embodiment of the present invention.
FIG. 4 is a cross-sectional representation
of a metal package in accordance with a second
embodiment of the present invention.
FIG. l is a cross-sectional representation
of a prior art housing 10 for encapsulating an
electronic device 22. The typical method of
manufacture is to fasten the leadframe 16 to the
base component 12 using a first sealant 28. The
15 first sealant is generally a low melting solder
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glass or a polymer adhesive. A typical solder glass
for use in sealing electronic packages is a lead
borate glass matrix usually containing one or more
other glass components. One example of a solder
glass is the mixture 75% - 85% PbO, 0.5% - 16% ZnO
and 8% - 15% B203. Any solder glass with an
appropriate coefficient of thermal expansion may be
utilized. A typical polymer sealant is a heat
curable epoxy such as novalac.
A typical sealing profile for a solder glass
is about 410 - 430C for 10 minutes and for an epoxy
a typical curing cycle is about 150 to 170C for
about 30 to 120 minutes.
After the first sealant is cured and the
leadframe affixed in place, the electronic device 22,
often a silicon based semiconductor chip, is
attached. The electronic device is attached either
directly to the base component lZ by a die attach 32
or to a buffer (not shown) which is then fastened to
the base component. A buffer system is disclosed in
European Application No. 86 102 059.2 published
September 17, 1986.
The coef~icient of thermal expansion of the
base component or bufer determines the choice of die
attach material 32 used. If the CTE of the base
component or buffer is within about 10% the CTE of
the electronic device, typically 49X10 7 C, a hard
solder such as 98% Au/2~o Si may be used. If the CTE
of the base component or buffer does not approach
that of the electronic devicei a more forgiving die
attach is used. For example, a soft solder such as
95% Pb/5~ Sn or an electrically conductive polymer
adhesive such as a silver filled epoxy
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may be used.
A thermal treatment is required to activate
the die attach. If Au/Si solder is used this
thermal treatment generally is about 420C for
5 about 5 seconds. For Pb/Sn solder, a
temperature of about 200C for about 20 seconds
is used and for the filled polymer about 150C
for about 30 minutes. Regardless of the die
attach chosen, the first sealant 28 is subjected
to a second heating and cooling cycle.
The lead wires 20 are next bonded to the
electronic device 22 and inner lead portions 18
to electrically connect the electronic device to
the outside world through leadframe 16~ The
lead wires are typically~ gold or aluminium wires
or thin strips of copper foil if the process
known as tape automated bonding (TAB) is used.
Bonding may be by ultrasonic welding, thermal
bonding or thermal compression bonding. Some
2n heat of bonding may be transmitted through the
leads subjecting the first sealant 28 to another
thermal cycle. A typical bonding temperature is
about 240C for about 5 seconds.
Next, a second sealant 26 is applied to the
2S cover componet 14. The second sealant is
usually chosen to have the same composion as the
first sealant 28 to insure chemical
compatibility although this is not necessary as
disclosed in previously cited U.S. Patent No.
4,704,626. The cover component 14 is then
positioned adjacent to the leadframe 16 and
bonded to the side of the leadframe opposite the
base component. A sealing profile suitable to
cure the second sealant 26 is required. The
first sealant 28 is once more subjected to a
thermal cycle.
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The prior art package is now complete. If
first and second sealants 28,26 were solder glass
the package is hermetic, if an epoxy the package is
probably no-t hermetic. It has been found multiple
s thermal cycling of the first sealant 28 impacts
detrimentally on the integrity of the package. One
embodiment of the present invention is to provide
~or the manufacture of a housing for an electronic
device which does not subject the first sealant to
the multiple thermal cycles of the prior art.
FIGS. 2 and 3 illustrate the assembly of a
housing 10 for an electronic device 22 in accordance
with the present invention.
Referring to FIG. 2, a schematic of a
15 leadframe 16 as known in the art is shown. The
lead~rame 16 is comprised of an electrically
conductive material. Leadframes are typically
fashioned from Alloy 42 (an iron-nickel alloy
containing 58% Fe and 42% Ni), KOVAR ~ (an iron-
20 nickel-cobalt alloy containing 54% Fe, 29~ N1 and
17% Co), copper or a copper based alloy. The
leadframe may be pla-ted OI' clad with a second
material to enhance bonding to the sealant~ improve
corrosion resistance or improve appearance.
2s Typically the leadframe for a metal package is a
copper based alloy such as C638 described in U.S.
Patent Nos. 3,341,369 and 3,475,227 to Caule et al
or C724 as described ln U.S. Patent No. 4,$94,221 to
Caron et al. Alloys C638, C724, C7025 and similar
30 copper based alloys form a refractory oxide layer
which facilitates bonding of the alloy to a solder
glass. Alternatively, dilute copper alloys,
essentially pure copper with trace amounts of
additives, are also used as leadframes due to
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the high electrical conductivity they impart as
compared to other copper alloys and added
strength they have compared to pure copper. A
typical dilute copper alloys for use in
leadframes is C194 (2.35% Fe, 0.03% P, 0.12~ Zn
and the balance Cu),
The leadframe is comprised of lead ingers 17
which are made up of inner lead portions 18 for
bonding to the electronic device 22 and outer
lead portions 19 for electrically connecting the
electronic device to an external device, for
example, by insertion into a printed circuit
board. The leadframe also contains tie bars 34
to support the lead fingers 17 during assembly
operations. The tie bars 34 are generally
severed once the leadframe is in place to
el~ectrically isolate the leads from each other.
Some leadframes also contain a centrally
positioned die attach pad 36 which is connected
2n to the tie bars by die attach pad supports 38.
Leadframes with die attach pads are usually used
in plastic encapsulated packages to provide a
site to mount the chip and provide electrical
contact to the back side o the chip. The use
of a die attach pad in a plastic encapsulated
package is disclosed in U.S. Patent No.
4,697,203 issued to Sakai el al. The use o a
die attach pad with a metal package is discosed
in U.S. Patent No. 4,656,499.
3n In accordance with the present invention,
the electronic device 22 which is typically a
semiconductor chip made of silicon although
other semiconductor materials such as germanium
or gallium arsenide may also be used is attached
to the die attach pad 36 by a die attach
material 32. Dependent upon the composition of
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the leadframe I6 and the corresponding coefficient of
thermal expansion of the leadframe, the die attach
may be a eutectic solder such as gold based Au/Si or
lead based such as 95% Pb/5% Sn, or a soft solder
such as lead based G~% Pb/40~o Sn or 92.5% Pb/5%
Sn/2.5% Ag or a polymer adhesive such as a heat
curable epoxy. The list of die attach materials is
given for the purpose of example and is not meant to
be all encompassing, any suitable die attach rnaterial
could be used within the scope of the present
invention. If an epoxy die attach is used, it may be
conducting or insulating dependent upon whether
backside electrical contact is desired. The epoxy
may be made electrically conductive by filling i.t
with a conductive metal (for example, silver) powder.
The electronic device 22 is ne~t connected
to the inner lead portions 18 through bonding wires
20. These wires are frequently made of thin, about
0.25 mm (0.001 inch) diameter strands of gold or
aluminum. In the alternative, thin strips of copper
foil are used for TAB bonding as disclosed in U.S.
Patent No. 4,330,790 issued to Burns. The wires 20
are bonded by conventional wire bonding techniques to
bonding sites 39 on the electrically active face of
the electronic device 22. rrhe opposlte ends of the
wires are bonded to the inner portions 18 of the
leadframe 16 thereby creating an electrical
connection between the electronic device 22 and the
outer lead portions 19.
The leadframe 16 containing the die attached
and wire bonded electronic device 22 is now ready for
further assembly in accordance with the present
invention. Reference is made
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to FIG. 3 which shows an assembled package in
accordance with the present invention.
In a first process embodiment, a base
component 12 containing first sealant 28 and
third sealant 40 is provided. The first sealant
28 is selected from the group consisting of
nonconductive solder glasses, ceramics,
thermosetting polymers a;nd themoplastic
polymers. The third sealant 40 is selected from
ln the group consisting of solder glasses,
ceramics, metal alloy solders, thermosetting
polymers and thermoplastic polymers. Dependent
on whether the back side of the chip is to be
electrically connected to the package base or
1~ electrically isolated, the third sealant 40 is
either a dielectric or an electrical conductor.
The third sealant is usually made electrically
conductive by filling it with a conductive metal
powder. Garbon filling may be utilized~ The
third sealant 40 is also characterized by high
thermal conductivity. Preferred third sealants
40 for the present invention are sllver filled
epoxy or soft solder such as Pb/Sn~ The
leadframe 16 is positioned adjacent to the
sealant containing face of the base component.
Frequently, the thickness of the first sealant
28 is greater than the thickness of the third
sealant 40 and the difference is compensated for
by downsetting the die attach pad support 38lo
Downsetting also helps maintain elec~rical
isolation between the inner lead portions and
the die attach pad supports. In the
alternative, after curing the first and third
sealants, the die attach pad supports 38 may be
severed.
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The leadf rame is bonded to the base
component by an appropriate curing cycle for the
first and third sealants. An advantage of the
present embodiment over an embodiment described
hereinbelow is the package is open during the
thermal cure so by-products of the cure, for
example, chloride ions from an epoxy may escape
into the atmosphere.
A cover component 14 containing a
ln nonconductive second sealant 26 selected from
the sa~e group as the first sealant 28 and
frequently, although not necessarily identical
to the first sealant is positioned adjacent to
the unbonded side of the leadframe. The cover
1~ component is sealed to the leadframe by an
appropriate thermal cycle thereby forming a
housing for the electonic device.
An improvement of the present invention
over the prior art is the first sealant is
subjected to only one additional thermal cycle,
thereby reducing the thermal degradation.
A second embodiment of the present
invention seals the first 28 and second 26
sealants at ~he same time. A leadframe 16 with
the electronic device 22 already die attached
and wire bonded is disposed between a base
component 12 and a cover componet 14, First
sealant 28 is disposed between the first side of
the lead~rame and the base component. Third
3~ sealant 40 is disposed between the die attach
pad and the base component. Second sealant 26 is
disposed between~the cover component and the
second side of the leadframe~ An appropriate
thermal cycle is chosen to cure all three
sealants simultaneously.
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The advantag~ of the second embodiment of
the invention over the prior art is the irst
sealant 28 is subjected to only a cure cycle and
not to excess thermal cycles. The structure of
an electronic package made according to the
present invention resembles the metal package
sealed with solder glass disclosed in U.S.
Patent No. 4,656,499 to Butt entitled
Hermetically Sealed Semiconductor Casing.
ln However the Butt method of assembly is
completely different and does not encompass the
improvements necessary to reduce thermal
degradation of the sealants.
Another embodiment of the present invention
is shown in FIG. 4. A housing for an electronic
device lO is assembled using either of the above
disclosed embodiments. An aperture 42 is
provided in the base component 12'. The
aperture allows the escape of reaction
by-products generated during the sealin~ cycle.
The first sealant 28, second sealant 26 and
third sealant 40 may all be cured at the same
time and the accumulation of residue within the
enclosure 30 of the housing is less than with
previous embodiments. The third seaIant 40 is a
ring type seal surrounding the aperture 42. The
seal 40 bonds the die attach pad to the base
12'.
A second improvement realized by the
3n aperture 42 within the base componet 12' is the
proximity of the chip to outside influences. As
the electronic device 22 is operated,
electricity passes through the device. Some of
the electr~icity is converted nto heat due to
the internal ;resistance of the semiconductor
circuitry. This heat reduces the operating life
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of the semiconductor chip and is preferably
removed. Common means of removing the generated
heat are metal heatsinks located below the chip
and forced air or forced fluid cooling. An over
view of these heatsink means may be found in the
article entitled ~Future Packages' Heat Transfer
Will Affect PCB Designs~ by Ernel R. Winkler
which appeared in the April 1985 edition of
ELECTRONIC PACKAGING & PRODUCTION.
ln The aperture 42 permits a forced air or
forced fluid cooling means to be positioned in
close proximity to the electronic device 22.
The use of a cooling material with high heat
capacity, for example helium gas, results in
improved cooling. This improved cooling becomes
more important as the density of electronic
circuits on the semiconductor device increases
resulting in greater heat generation during
operation of the chip.
Rather than a fluid, a solid material with
high heat capacity, for example copper or silver
may be inserted into the aperture 42 and used as
a conduit theat sink) to transfer heat from the
electronic device.
Yet another embodiment of the present
invention is the inclusion of a second aperture
44 within the cover component 14'. The aperture
44 is a vent for reaction by-products generated
during the cure cycle. After the cure cycle i~
3n completed, an inert thermally conductive gas
such as helium or a thermally conductive,
electrically non-conductive liquid or powder may
optional~ly be injected into the package cavity
30 to provide additional thermal dissipation
means for the electronic device 22. The aperture
is then sealed by applying a suitable patch, for
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example a drop of solder 48 to the outer sur~ace
46 of the cover component. Alternatively, the
aperture 44 could be sealed by any preferred
means such as plastic. The apertures 42 and 44
can be employed alone or in combination as
desired.
While the present invention has been
described in terms of a metal package, the
techniques described are suitable for ceramic
packages, such as CERDIP, or plastic packayes
It is apparent there has been provided in
accordance with this invention means of
assembling electronic packages and the products
of those assembly means which fully satisfy the
objects, means and advantages set forth
hereinbelow. While the invention has been
described in combination with specific
embodiments thereof, i~ is evident that many
aIternatives, modifications and variations will
be apparent to those skilled in the art in light
of the forgoing description. Accordingly, it is
intended to embrace all such alternatives,
modifications and ~ariations as fall within the
spirit and broad scope of the appended claims.
