Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to a hermetically sealed
case-type package for an electronic component or electronic
circuit, which comprises an element for absorption or retention of
moisture.
Description of the Prior Art
It is known that the contaminant which plays a
leading but unfavorable role from the point of view of
reliability of a component, of an integrated circuit on a
silicon chip, or of a hybrid circuits the water content
of the surrounding atmosphere. In particular, moisture
produces corrosion of circuit connection terminals which
are usually of aluminum (corrosion resulting from hydra-
louses of aluminum). Moisture also permits solubilizationof ions, thus increasing the corrosion. This deleterious
effect is very marked in the case of integrated circuits,
all the more so as large-scale integration is greater and
terminals and connections are of correspondingly smaller
size.
There are a number of reasons for the presence
of water molecules within a case-type package containing
an electronic component or circuit. In the first place,
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if special precautions are not taken at the time of
sealing, the package contains ambient air with its usual
moisture content or relative humidity (usually in the
vicinity of 60 to 70 %). In order to reduce this effect,
one known expedient consists in removing the water
molecules as far as possible before bonding the cover to
the base of the package and in carrying Ott the swilling
operation in such a manner as to ensure maximum air-
tightness. By way of example, this result is achieved by
tin-gold brazing, by glass bonding in a dry atmosphere
such as nitrogen at less than 2 or 3 Pam of H20. However,
even under these conditions, water molecules remain within
-the package and, in addition, the package still exhibits
in-leakage. At the time of temperature variations to which
packages are usually subjected, a pumping effect occurs
through the leak locations. Thus in the cold state, the
reduced internal pressure causes outside air to be
admitted into the package at ambient humidity The water
molecules thus admitted are at least partially retained
chemically (absorbed) or physically (adsorbed) by the
elements placed within the package (the circuit, the
aluminum, the glass elements used for passivation, and so
on). At the time of a temperature rise, the gas discharged
to the exterior will therefore contain a smaller quantity
of water molecules than the gas initially admitted. It is
therefore apparent that the number of water molecules
A
tends to increase within the interior of the package.
There also exist internal sources which produce
an increase in humidity within the package and which are
of two different types :
- in one case, the water molecules which are adsorbed and
absorbed by the circuit and the package components will
be released during the lifetime of the component. In
order to overcome this disadvantage, it is customary
practice to carry out degas sing prior to encapsulation
at a temperature in the vicinity of 150C. This tempera-
lure, however, is too low. It is in fact known that OH
groups may still be present at a temperature of over
1000C ;
- in the other case, generation of water molecules takes
place within the package during the service life of the
package and is caused in particular by chemical recombine-
lion of OH groups and protons, by devitrification
reactions within the packages containing glass in the
case of passivation layers of printed circuits, and so on.
One known solution to this problem of humidity
consists in incorporating an electronic circuit for
detecting humidity and delivering an alarm signal when the
humidity within the package exceeds a predetermined limit.
When the alarm threshold is reached, the component is
considered as no longer serviceable and is replaced. It is
therefore apparent that this is a costly solution on two
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counts : firstly because it entails the need to incorporate
a detector circuit within the package and secondly because
it does not prevent the service Lowe of the component from
being limited by the problem of humidity.
Another neural solution consists in incorporating
in the package a material which is capable of retaining the
water molecules. The effectiveness of this solution is
related to the absorption capacity of the material. Thus, as
explained earlier, the package not only exhibits in-leakage
but there also exist internal sources of humidity. After a
certain period of operation, the material is consequently no
longer capable of retaining newly-arrived or newly formed
water molecules and the well-known disadvantages reappear.
SUMMARY OF THE INVENTION
The aim of this invention is to provide a
package which makes it possible to overcome the above-
mentioned drawbacks by incorporating in the package a
device which is capable of retaining water molecules as
they enter the package or as they form within the interior
of the package throughout the lifetime of the component
together with means for heating said device, for degas sing purposes. In
one embodiment this device comprises a material including a metal which
reacts with water and is thus capable of retaining the water molecules ;
said heating means are periodically used for diffusing the atoms of this
metal towards the surface which is in contact with the atmosphere of the
pac`:agG, thus renewing its moisture-retaining capacity.
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According to the invention, there is provided a package for
an electronic component, comprising a base for receiving said component
and a cover placed over said component and fixed on said base in an
air-tight manner, an element which has the function of retaining water
molecules, positioned within the interior of said package, and means
for heating said element.
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It is clear from the foregoing that the device
in accordance with the invention is of relatively simple
design and also makes it possible to extend the useful
life of the component. Since the water molecules are in
fact retained as they form within a material placed for
this purpose, they are in no way liable to produce the
damaging effects mentioned earlier.
BRIEF DESCRIPTION OF THE DRAWINGS
Other features of the invention will be more
apparent upon consideration of the following description
and accompanying drawings, wherein :
- Fig. 1 is a top view of one embodiment of the
package in accordance with the invention ;
- Figs. pa, 2b and 2c are partial variants of
the previous figure, shown respectively in a top view
(Fig. pa) and in cross-sectîonal views (Figs. 2b and 2c) ;
- Fig. 3 is a sectional view of another embody-
mint of the package in accordance with the invention.
In these different figures, the same references
serve to designate the same elements and the real scale
has not been observed for the sake of enhanced clarity of
the description.
DESCRIPTION OF THE PREFERRED Embodiments
In the overhead view of Fig. 1, there is there-
fore shown a package which may be of the so-called "chip
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carrier" type and is designated by the reference Cc. This
package is composed of a base E of ceramic material, for
example, and of a cover CA of metal or of ceramic material
for example. In the figure, the cover is shown only in
phantom outline on the base E.
An electronic component Of which, in this example,
is an integrated circuit fabricated on a semiconductor
chip is deposited on the base E. The component Of has
different connection terminals such as those designated by
the reference numerals 1 and 2, the terminals being located
at the periphery of the component. The base E is adapted
to carry a predetermined number of conductors in the form
of metal deposits such as those designated by the reference
numerals 21 or 22. These conductors extend to the port-
phony of the base E. In aeeordanee with well-known
practice, the conductive deposits in this type of package
extend within half-bores 20 formed in the edge lace of the
base and terminate on the underfaee of the base, on which
they constitute the output connections of the package.
Finally, the connection terminals (1, 2) of the component
Of are connected to the base conductors (21, 22) by means
of leads such as the lead 24. There is also shown by way
of example one of the conductors (21) of the base which is
connected to ground.
In aeeordanee with the invention, the base also
carries a component A comprising an element 13 which is
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capable of retaining water molecules.
In the embod~lent which is illustrated in Fig 1,
the component A is added to the base. In other words, the
element 13 is deposited on a substrate 14 on which the
entire component A is constructed, the substrate being
deposited on the base E. The element 13 has two
connection terminals 11 and 12 connected respectively to
two of the conductors such as those designated by the
reference 22 which are carried by the base, the terminals
11 and 12 being connected to an electrically conducting
element which is not shown in the figure and the function
of which is explained in detail below.
The retaining element 13 can consist of any
material which is capable of retaining water molecules,
this being preferably the case even when the concentration
of such molecules is very low. More specifically, the
retaining element can consist, for example, of absorbent
and porous glass-type material in the elite group, also
known as "molecular sieve" in vacuum pump technology. In
order to provide maximum capacity for retention of water
molecules by the element 13, it is preferable to carry out
a degas sing treatment with a view to ensuring that the
number of water molecules carried by the retaining
element at the moment of closure of the package is reduced
to a minimum. This result is achieved by supplying power
to the retaining element 13, for example during the usual
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operation which consists in degas sing the package before
bonding the cover to the base. Any known means may be
employed for this purpose. In particular, in the embody-
mint described in the foregoing, it would be possible to
make use of a heating resistor connected to the terminals
11 and 12. The moisture-retaining element 13 can also be
formed by a materiel which reacts heroically with water, such as
a binary or ternary metal alloy in which at
least one alloying element reacts with moisture. Examples
10 worthy of mention include silicon, titanium, zirconium,
tantalum, vanadium, aluminum, tin, and so on. A suitable
alloy can consist of gold and silicon. In this case, water
retention is therefore achieved by chemical reaction.
In this embodiment, in order to
renew the capacity of the element 13 for retention of water
molecules, the resistor connected to the terminals 11 and
12 also has the function of heating the element 13 at
predetermined intervals of time after closure of the
package in order to ensure that the atoms of the reactive
20 metal can diffuse towards the surface which is in contact
with the atmosphere of the package and through the oxide
layer which has already been formed thus renewing the moisture-
retaining capacity of the element 13.
Fig. pa is a fragmentary top view of an alter-
native embodiment of the preceding figure. There is again
shown in this figure a portion of the base E, the chain-
dotted outline of the cover CA, and a few conductors such
as the conductor 22 including two conductors which are
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designated by the reference numerals 25 and 26.
In the alternative embodiment of Fig. 2b, which
is a sectional view taken along the axis X-X of Fig. pa,
the resistor is made of resistive material 15 which is
deposited directly on the base E and partially on the ends
of the conductors 25 and 26 by screen-process deposition,
for example After treatment by baking, this resistive
material is coated with the material which constitutes the
moisture-retaining element 13 as shown in Figs. 1, pa, 2b
and which can also be deposited by screen process.
Should the moisture-retaining element 13 be a
metal alloy, the resistive material 15 of the alloy is
electrically insulated, for example by means of a thin
layer of glass (which is not shown in the drawings.
In another embodiment shown in Fig. 2c, again in
cross-section taken along the axis X-X, the moisture-
retaining material is also conductive, thus dispensing
with the resistive layer 15. In this case, the retaining
material designated by the reference numeral 16 is also
deposited on the base E and on the ends of the conductors
25 and 26. The material 16 can be formed of resistive
material such as a silk-screen printing ink having
a base of ruthenium oxide, palladium-silver, platinum-
:, :
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12
silver and the like, the specific surface of which is
increased so that the porous material thus produced is
capable of retaining water molecules.
It is also possible to dispense with the
resistive layer 15 when the moisture-retaining element 13
is a metal alloy. It is only necessary in this case to
select the constituents of the alloy in such a manner as
to obtain a resistive compound.
In any of the alternative embodiments described
lo in the foregoing, the component A requires only one
connection 22 which is left free by the electronic
component Of whilst the second connection can be the
ground connection 21.
Fig. 3 is a longitudinal sectional view thus-
treating another embodiment of the package in accordance with the invention.
In this figure, there is again shown the base E
which carries the component Of. This component is
connected electrically by means of its terminals l and 2
and by means of leads 24 to the conductors 22 of the base
E which pass through the half-bores 20. The package
further comprises a cover CA which is intended to be
joined to the base E by means of a glass bond 27.
In this embodiment, the moisture-retaining
I material 13 is carried by the interior of the cover CA in
which it is deposited by screen process, for example.
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13
The advantage of this alternative embodiment
over the preceding embodiments is twofold. The first
aspect is that a larger quantity of material 13 can be
deposited on the cover than on the base itself. The
second aspect is that pudgier can be supplied to the
material 13 without difficulty prior to bonding of the
cover CA to the base E. It is in fact known that this
bonding operation calls for heating of the cover either
in a furnace or locally. When the heating operation is
performed locally, the cover is heated by a tool 30 to a
temperature of the order of 500C to 650C. This operation
therefore also has the effect of heating the layer 13 and
of producing the desired degas sing action.
Where is used a material which reacts chemically with
water, there is provided means for heating the fixing material
to make its atoms to diffuse towards the surface which is in contact
with the atmosphere of the package.
As will readily be apparent, the different
embodiments described in the foregoing can be employed
20 cumulatively.
Furthermore, the invention is not limited to
the embodiments described earlier. From this it accord-
tingly follows that, although consideration has been given
to a package of the flat chip-carrier type containing a
single silicon chip, the invention is in fact applicable
to any type of package (DIP, CERDIP, and so on) which may
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14
be either flat or multilevel, whether the package is
intended to be used for a discrete component, for an
integrated circuit or for a hybrid circuit. Similarly,
the term "component" is used in this patent Application
solely for the sake of simplification since it will be
understood that this term is applicable to any one of these
three elements. It should likewise be noted that, while
it has been assumed in the foregoing description that the
package is of ceramic or metal, it can also be either
partly or entirely made of plastic (base or cover).
Furthermore, in order to degas the material which serves
to retain water molecules, the necessary power can be
supplied by means other than heating. By way of example,
such means can consist of exposure to ultraviolet
radiation.
