U.S. patent application number 09/727717 was filed with the patent office on 2002-10-17 for methods and compositions for ionizing radiation shielding.
This patent application is currently assigned to MAXWELL ELECTRONIC COMPONENTS GROUP, INC.. Invention is credited to Featherby, Michael, Layton, Phillip J., Li, Edward, Strobel, David J..
Application Number | 20020148979 09/727717 |
Document ID | / |
Family ID | 27739432 |
Filed Date | 2002-10-17 |
United States Patent
Application |
20020148979 |
Kind Code |
A1 |
Featherby, Michael ; et
al. |
October 17, 2002 |
METHODS AND COMPOSITIONS FOR IONIZING RADIATION SHIELDING
Abstract
The radiation shielding composition and method of the present
invention relate to a conformal coating material composed of a
matrix of densely packed radiation shielding particles, which are
disbursed within a binder. The shielding composition is applied to
objects to be protected such as integrated circuits, or to packages
therefor, as well as for protecting animals including humans from
unwanted exposure to radiation in outer space or other
environments.
Inventors: |
Featherby, Michael; (San
Diego, CA) ; Strobel, David J.; (Poway, CA) ;
Layton, Phillip J.; (San Diego, CA) ; Li, Edward;
(San Diego, CA) |
Correspondence
Address: |
FITCH EVEN TABIN AND FLANNERY
120 SOUTH LA SALLE STREET
SUITE 1600
CHICAGO
IL
60603-3406
US
|
Assignee: |
MAXWELL ELECTRONIC COMPONENTS
GROUP, INC.
|
Family ID: |
27739432 |
Appl. No.: |
09/727717 |
Filed: |
November 30, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09727717 |
Nov 30, 2000 |
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09375881 |
Aug 17, 1999 |
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6261508 |
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09375881 |
Aug 17, 1999 |
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08791256 |
Jan 30, 1997 |
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08791256 |
Jan 30, 1997 |
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08372289 |
Jan 13, 1995 |
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5635754 |
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08372289 |
Jan 13, 1995 |
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08221506 |
Apr 1, 1994 |
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60021354 |
Jul 8, 1996 |
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Current U.S.
Class: |
250/515.1 ;
257/E23.114; 257/E23.121; 257/E23.126; 257/E23.136; 257/E23.189;
257/E23.19; 257/E25.012 |
Current CPC
Class: |
H01L 2224/48247
20130101; H01L 2924/00014 20130101; H01L 23/3135 20130101; H01L
2924/00012 20130101; H01L 2224/45099 20130101; H01L 2924/00014
20130101; H01L 2224/45015 20130101; H01L 2224/48247 20130101; H01L
2924/00014 20130101; H01L 2924/00 20130101; H01L 2924/207 20130101;
H01L 2924/181 20130101; H01L 2924/00014 20130101; H01L 2924/15165
20130101; H01L 23/18 20130101; H01L 2924/1616 20130101; H01L
2924/1617 20130101; H01L 23/295 20130101; H01L 2224/48091 20130101;
H01L 25/0655 20130101; H01L 2224/05599 20130101; H01L 2224/45099
20130101; H01L 23/057 20130101; H01L 2224/85399 20130101; H01L
2924/3025 20130101; Y10T 428/256 20150115; H01L 2924/00014
20130101; H05K 9/0083 20130101; H01L 2224/49171 20130101; Y10T
428/25 20150115; H01L 2924/15153 20130101; H05K 9/0088 20130101;
H01L 2224/85399 20130101; H01L 2224/05599 20130101; H01L 24/49
20130101; H01L 2224/48091 20130101; H01L 2924/16195 20130101; H01L
2924/01078 20130101; H01L 24/48 20130101; H01L 2224/49171 20130101;
H01L 2924/00014 20130101; H01L 2924/01079 20130101; H01L 23/552
20130101; H01L 2924/181 20130101; H01L 23/055 20130101; H01L
2924/14 20130101; H01L 2924/01077 20130101 |
Class at
Publication: |
250/515.1 |
International
Class: |
G21F 001/00 |
Claims
What is claimed is:
1. A radiation shielding composition comprising: a binder; a
conformal coating material composed of a matrix of densely packed
particles dispersed within the binder to shield ionizing or other
radiation; and wherein said conformal coating material is composed
of a layer of high Z shielding particles interposed between a pair
of layers of low Z shielding particles, wherein each one of the
said shielding particles are encapsulated within said binder.
2. A radiation shielding composition according to claim 43 wherein
one of the pair of layers of low Z shielding particles is disposed
as an electric insulator for an electronic device and the other one
of the pair of layers of low Z shielding particles is disposed as a
protective outer layer for the electronic device.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/021354, filed Jul. 8, 1996, a copy of which
accompanies this application as Appendix "A," and which forms a
part of this application and is incorporated herein by
reference.
[0002] This application is a continuation-in-part patent
application entitled "Radiation Shielding of Integrated Circuits
and Multi-Chip Modules in Ceramic and Metal Packages," Ser. No.
08/221,506, filed Apr. 1, 1994, and is incorporated herein by
reference.
TECHNICAL FIELD
[0003] The present invention relates in general to a radiation
shielding coating composition and a method of making and using it.
The invention more particularly relates to compositions and methods
for shielding microelectronic devices and other objects and
animals, with a radiation-hardened light weight coating for
withstanding the radiation hazards found in the space environment,
as well as other less hazardous environments.
BACKGROUND ART
[0004] Many of today's commercial integrated circuit (IC) devices
and multi-chip modules (MCM) cannot be utilized in deep space and
earth orbiting applications because of total dose radiation induced
damage. Commercial IC devices are developed and manufactured for
computer and mass market applications and are not designed to
withstand the effects of the natural space environment. The
radiation effects include solar flares, galactic cosmic radiation
and the Van Allen trapped electron and proton belts or man-made
radiation induced events (neutrons and gamma radiation).
[0005] Typical commercial silicon integrated circuits fail to
operate when exposed to total doses of two to fifteen kilorads(Si).
Common methods used to prevent radiation degradation in performance
are: 1) to design special radiation tolerant die, 2) to shield the
entire component and board assembly, or 3) shield the individual
component. There are weight, cost and time-to-market penalties
depending on the method. For example, specially designed radiation
tolerant die are time consuming and expensive to produce, since the
part must be redesigned to incorporate radiation hardening
techniques. Examples of such methods include U.S. Pat. Nos.
3,933,530; 4,014,772; 4,148,049; 4,313,7684; 4,402,002; 4,675,978;
4,825,278; 4,833,334; 4,903,108; 5,001,528; 5,006,479; 5,024,965;
5,140,390; 5,220,192; and 5,324,952, each of which patent is
incorporated herein by reference. Reference may also be made to
Japan patent 62-125651, Jun. 6, 1987, and articles entitled
"Effects of Material and/or Structure on Shielding of Electronic
Devices," R. Mangeret, T. Carriere, J. Beacour, T. M. Jordan, IEEE
1996; and "Novice, a Radiation Transport/Shielding Code", T. M.
Jordan, E.M.P. Consultants Report, January. 1960, the Japan patent
and such articles being incorporated herein by reference.
[0006] Such techniques delay the time to market the products. As a
result, these conventional radiation hardened devices are usually
two to three generations behind the current commercial
technological advances in both size and capabilities. There are
additional penalties in limited marketability and demand, and hence
low volume productions of the die result. Consequently, such
methods produce a more expensive product, which is technologically
behind the commercially available microelectronics, with slower
speed and less capability. Additionally, because of the limited
market for these products, they are frequently not available at
all.
[0007] Such radiation shielding methods involve using metal
shielding external to the package. Shielding by other mechanical or
electrical elements complicates the platform design, often
requiring complex three dimensional modeling of the design.
[0008] Another attempt at shielding includes disposing a small
shield on the surface of the package. Such a technique does not
provide effective three-dimensional shielding protection.
Additionally, the small external shield is generally thermally
mismatched to the package, and increases the size and weight of the
package.
[0009] Examples of system level shielding are disclosed in U.S.
Pat. Nos. 4,833,334 and 5,324,952, which are incorporated by
reference as if fully set forth herein. The U.S. Pat. No. 4,833,334
discloses the use of a protective box to house sensitive electronic
components. The box is partially composed of a high atomic weight
material to shield effectively against x-rays. However this
approach has the serious disadvantage of adding substantial bulk
and weight to electronic circuit assemblies protected in this
manner. Moreover, it would be expensive to provide this type of
protection to individual integrated circuits as manufacturing
custom boxes for each circuit configuration would be costly.
[0010] The method of shielding material on the outside of the
package is known as spot shielding. Such a technique is disclosed
in Japanese patent publication 62-125651, published Jun. 6, 1987,
which is incorporated by reference as if fully set forth herein.
This patent describes a spot shielded semiconductor device which
utilizes a double layered shield film to serve as a sealing cover
on an upper surface of a semiconductor package. Another double
layered shield film is attached to a lower surface of the package.
However, space qualified microelectronic parts must be capable of
withstanding the enormous forces exerted during acceleration
periods during space travel. The external shields are subject to
tearing or prying off from the sealing cover. The use of a double
layer shield film only slightly reduces the weight of the package,
but increases the size of the package unnecessarily. Also, thin
films are generally only effective at shielding electromagnetic
interference (EMI) radiation and are ineffective at shielding
ionizing radiation found in space. Examples of this type of EMI or
EMF shielding devices include devices disclosed in U.S. Pat. Nos.
4,266,239; 4,823,523; and 4,868,716, which are incorporated herein
by reference.
[0011] The significant disadvantage of the spot shielding method
includes an increase in weight and thickness of the device, and an
increase in exposure of the semiconductor to side angle radiation
due to the shielding being spaced apart from the semiconductor.
[0012] A far superior method of shielding involves using an
integrated shield, where the package itself is the shield. For
example, reference may be made to said U.S. patent application Ser.
No. 08/221,506, filed Apr. 1, 1994, entitled "RADIATION SHIELDING
OF INTEGRATED CIRCUITS AND MULTI-CHIP MODULES IN CERAMIC AND METAL
PACKAGES," now U.S. Pat. No. ______, which is incorporated herein
by reference. The material in the package and the package design is
optimized for the natural space radiation environment.
[0013] Many conventional microcircuits are only available in
prepackaged form, or the die is already mounted onto the circuit
board. Therefore, it would be highly desirable to have technique
and shielding compositions for shielding parts already packaged or
mounted on a circuit board, or in bare IC die form. Such
compositions should be relatively inexpensive to manufacture and
use, and are compact in size. In this regard, such new and improved
techniques should be very convenient to employ in a highly
effective manner, and yet be relatively inexpensive to
manufacture.
SUMMARY OF THE INVENTION
[0014] The principal object of the present invention is to provide
a new and improved composition and method of radiation shielding in
outer space or other environments, whereby such shielding
compositions and methods are highly effective and relatively
inexpensive.
[0015] Another object of the present invention is to provide such a
new and improved method and composition, wherein the radiation
tolerance of the bare die to be shielded is greatly improved, and
the shielding is provided in all axial directions.
[0016] A further object of the present invention is to provide such
a new and improved method and composition, wherein satellite
designers can utilize current generation IC technological advances,
while improving delivery time.
[0017] A still further object of the present invention is to
provide such a new and improved method and composition, wherein IC
devices can be supplied relatively inexpensively due to the use of
commercially available dies at current market prices without undue
weight, excessive or bulky sizes or other undesirable or unwanted
design requirements.
[0018] Yet another object of the present invention is to provide
such a new and improved composition and method of radiation
shielding for protecting other objects or animals from unwanted
radiation.
[0019] Briefly, the above and further objects of the present
invention are realized by providing shielding compositions and
methods which are relatively inexpensive to use and highly
effective in outer space and other environments.
[0020] The radiation shielding composition and method of the
present invention relate to a conformal coating material composed
of a matrix of densely packed radiation shielding particles, which
are disbursed within a binder. The shielding composition is applied
to objects to be protected such as integrated circuits, or to
packages therefor, as well as for protecting animals including
humans from unwanted exposure to radiation in outer space or other
environments.
[0021] The inventive radiation shielding composition including the
densely filled conformal coating material is used for commercially
available integrated circuits or grouping of circuits, to protect
against natural and man-made radiation hazards of the spacecraft
environment, whether in earth orbit, geostationary, or deep space
probes. The inventive composition and methods are provided to
facilitate the design and manufacture of microelectronics, and to
coat externally the microelectronics with the inventive shielding
composition to improve radiation tolerance to natural space
radiation.
[0022] The inventive shielding composition, in one form of the
invention, includes a fabric and a flexible binder, used to shield
animals including humans in space or in other environments. As
humans prolong their stay in space, the risks from increased
exposure to ionizing radiation become more of a concern. The
conventional method of shielding using lead has two major
disadvantages. Lead is highly toxic, which is a disadvantage in
both manufacture and use. Lead is also relatively less dense. With
the inventive composition, the same equivalent shielding can be
obtained with a thinner high Z material such as tungsten. By using
a denser material, a thinner shield can be constructed, making
movement relatively easier. Since sources of radiation are not
limited to space, this same material has utility to shield humans
or other animals from radiation sources on earth.
[0023] The limiting factor is weight, and the energy and species of
radiation. Thin densely packed shields are not very effective on
high energy electromagnetic radiation such as gamma rays, and high
energy neutrons.
[0024] Additionally, the inventive conformal coating composition
and method are useful as a radiation shielding gasket between
enclosures. There are many radiation shielding utilities for the
inventive compositions and methods, depending on the choice of the
binder material.
[0025] The present inventive methods and compositions contemplate
using both plastic or ceramic packaged microelectronic devices, as
well as unpackaged die and encapsulating or coating the outer
surface of the device to provide shielding as required for the
anticipated radiation environment. Since fluences of species and
energy ranges of radiation vary in space, and since the optimal
shielding varies depending on the species of radiation, the coating
substance or material can be optimally tailored based on the
anticipated radiation that irradiates the part to be protected. In
all applications, the particles impregnated within the conformal
coating substance are designed to achieve the highest tap density
possible for the application.
[0026] The present inventive method preferably includes
calculating/modeling the anticipated radiation spectrum, the
required amount of shielding, as well as multiple layers of both
high Z and low Z shielding material. The inventive conformal
coating substance or material is then designed to meet that
requirement. For a standard Geosynchronous Orbit, the optimum
shielding entails a conformal coating having three layers; namely,
a high Z layer sandwiched between two low Z layers. For marking and
hermiticity, a layer of smooth unimpregnated coating material is
applied to the top layer.
[0027] For integrated circuit devices that have already been
packaged, the inventive conformal coating material can be applied
in various manners. These include, but are not restricted to, the
following inventive methods. One method relates to using a low
pressure (or high pressure depending on the package strength and
susceptibility) injection mold. The coating material is injected
into a mold containing the packaged part. Another method involves
"globbing" or putting a viscous conformal coating over a packaged
part. The part can be disposed within a mold, or elsewhere when the
shielding composition is applied. Another method involves spraying
or painting on the coating composition. The optimum method is to
coat all sides of the part uniformly with the shielding composition
to shield all sides equally from isotropic radiation, and
especially when the direction of the source of radiation is not
known.
[0028] For integrated circuits already attached to a board, either
in a bare die form or with an existing coating, the coating is
applied with a mold, by "globbing" the composition on, by spraying
or painting. To shield the top and bottom sides of the die
uniformly, the bottom of the board preferably is also shielded with
the inventive conformal shielding composition.
[0029] For multi-chip modules (MCMs) where there are multiple
integrated circuits within a single package, the inventive
conformal coating composition is applied in a similar manner as in
the monolithic packaged integrated circuit. Similarly, when there
are multiple bare integrated circuits, the inventive conformal
coating composition is applied in a similar manner as with the
single bare integrated circuit, wherein the coating composition is
applied to the entire area covered by the devices to be
shielded.
[0030] For system or boxes containing board level products
requiring additional shielding, the inventive conformal coating
composition can also be applied to any box or device to be shielded
from ionizing radiation. In this manner, with the use of a flexible
binder material such as latex, a gasket can be made for sealing two
objects, wherein the inventive gasket material also provides a
radiation shielding function.
[0031] Because of the flexibility of the inventive shielding
composition, radiation shielding can be achieved easily and
relatively inexpensively for applications that were either
previously considered to be excessively expensive or difficult to
shield.
[0032] For human radiation protection, the inventive composition
conformal coating include a latex or similar flexible binder. To
enhance the mechanical strength properties, a fabric material is
added and combined with the binder. In this form of the invention,
a high Z material, which is dense and nontoxic, can be inserted
within the layers of clothing material to add extra protection for
the wearer from unwanted radiation. Because of weight
considerations, the optimal shielding can be obtained in the
weightless environment of space. Lighter, thinner material is used
for gravity constrained environments. Additionally, the
impregnating particles can be tailored for the type of radiation to
be encountered, enabling optimal use of space and weight of the
material.
BRIEF DESCRIPTION OF DRAWINGS
[0033] The above mentioned and other objects and features of this
invention and the manner of attaining them will become apparent,
and the invention itself will be best understood by reference to
the following description of the embodiment of the invention in
conjunction with the accompanying drawings, wherein:
[0034] FIG. 1 is a diagrammatic sectional side view of a prior art
spot shielded prepackaged integrated circuit;
[0035] FIG. 2A is a diagrammatic sectional side view of a
conventional unshielded commercial package assembly;
[0036] FIG. 2B is a diagrammatic top view of the package assembly
of FIG. 2A;
[0037] FIG. 3 is a diagrammatic sectional side view of a prior art
module with multiple integrated circuit devices shielded
therewithin;
[0038] FIG. 4 is a flow chart illustrating a radiation shielding
method according to the present invention;
[0039] FIG. 5 is a graph of a typical total dose versus depth
curve, useful in understanding the present invention;
[0040] FIG. 6 is a diagrammatic sectional side view of a shielding
composition applied to a conventional package in accordance with
the present invention;
[0041] FIG. 7 is a diagrammatic sectional side view of a shielding
composition applied to a conventional chip-on board in accordance
with the present invention;
[0042] FIG. 8 is a diagrammatic sectional side view of a
multilayered conformal coating of shielding composition applied to
a conventional integrated circuit package in accordance with the
present invention; and
[0043] FIG. 9 is a pictorial, partially diagrammatic fragmentary
view of a shielding composition used for animal radiation shielding
in accordance with the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0044] The following description is of the best mode presently
contemplated for practicing the invention. This description is not
to be taken in a limiting sense, but is made merely for the purpose
of describing the general principles of the invention. The scope of
the invention should be ascertained with reference to the issued
claims. In the description that follows, like numerals or reference
characters will be used to refer to like parts or elements
throughout.
[0045] Referring now to the drawings, and more particularly to
FIGS. 1, 2A and 2B, there is shown a commonly used conventional
microelectronic package 10, which is a plastic package. FIG. 1
illustrates the package 10 with a spot shield attached. The
packages are comprised of a die 20, which is composed of silicon or
other semiconductor base. The die is generally attached to a die
attach pad 18 for support. The die is then bonded with multiple
lead wires 22, 24 to a lead frame with multiple leads 15, 16. This
entire assembly is encased within a package 13 composed of suitable
plastic material or other material such as ceramic.
[0046] If thermal conductivity properties are important
considerations, other materials such as ceramics are used, as shown
in FIG. 3, these are more difficult to work with and can be
conducting, necessitating an insulating feed through 25 to cover
the leads 15, 16.
[0047] A conventional method for shielding these packages is shown
in FIG. 1, where a pair of shielding plates 30 and 31, usually made
of a high Z material such as tantalum, is attached to the top and
bottom portions of the package 13 respectively by a suitable
adhesive (not shown).
[0048] As shown in FIG. 3, another prior art technique relates to
the use of integrated shielding technology, where the package
itself, is part of the shielding. FIG. 3 shows the integrated
shielding package 310 that also incorporates multiple die 320 and
321. The multiple die 320 and 321 on a die attach pad 318 employ
multiple lead wires 322 and 324, together with a lead frame with
multiple leads 315 and 316 and an insulating feed through 325, for
a package 313. This type of package is called an MCM or Hybrid
package. With multiple die within the package, the density of
functions increases, while the overall weight required to
accomplish the task is reduced. This type of packaging requires
base members 340 and 341, which can be made of various shielding
materials. For ionizing radiation, high Z materials can be used,
enabling the package itself to become the radiation shielding.
[0049] As shown in FIG. 4, the inventive method includes, as
indicated in box 100, determining the inherent radiation tolerance
of the die to be shielded. This test can be accomplished by a
Cobalt-60 source or other penetrating irradiation source. Without
the knowledge of what the inherent radiation tolerance is for the
individual semiconductor device, the designer does not know how
much or whether shielding is necessary.
[0050] The next step as indicated at 102 involves determining the
radiation spectrum and dose depth curve of the particular mission
or radiation requirement of the application. For orbits around the
earth, this is calculated using conventional radiation transport
codes in conjunction with conventional radiation spectrum tables.
The dose depth curve is generally represented as a total radiation
dose versus thickness of equivalent aluminum shielding as shown in
FIG. 5. Although not preferred, steps indicated at 100 and 102 can
be omitted if the application is unknown and the designer desires
only to enhance whatever the radiation tolerance of the integrated
circuit to be protected.
[0051] Knowing the inherent radiation tolerance of the integrated
circuit device, as indicated at 100 and the dose depth curve as
indicated at 102, the amount of shielding required can be
determined to bring the integrated circuit device within tolerance
as indicated at 104.
[0052] Knowing the spectrum of radiation for the application, the
layering of the inventive shielding material is tailored as
hereinafter described in greater detail with reference to FIG. 8.
High Z material is more effective at stopping electrons and
Bremsstrahlung radiation, and less effective in stopping protons.
Low Z material conversely is more effective at stopping protons and
less effective at stopping electrons and Bremsstrahlung
radiation.
[0053] The next step, as indicated at 106, requires determining the
form of the integrated circuit. For a prepackaged part, the amount
of shielding is limited by the lead length on the bottom of the
device, unless extenders are used. The most appropriate method of
application of the inventive shielding composition is then
determined as indicated at 108. The part is coated in a mold (not
shown), using a dam (not shown), and the coating can be globbed,
sprayed, injected or painted on. For die that are already mounted
on the board (not shown), the methods mentioned above are
effective, but to insure uniform radiation shielding, the bottom of
the board underneath the part is also coated with the same
thickness of the inventive shielding composition. The coating
material is applied as indicated at 110 and then allowed to cure as
indicated at 111. Temporary extenders are preferably used to
provide thorough wetting throughout the binder. As an example, a
preferred extender for epoxy is a high boiling point ketone.
[0054] Additionally, by adjusting the properties of the binder, the
bulk electrical properties of the shield composition is adjusted to
be either insulating or conductive.
[0055] Upon completion of coating the parts, testing is then
performed electrically and mechanically, as indicated generally at
112. For space applications, the parts require space qualification
testing.
[0056] There are various different methods of application of the
inventive shielding composition as contemplated by the invention
and as indicated in FIGS. 5, 6, 7 and 8. However, the following
examples are intended to be representative and not all inclusive of
the possible application methods falling within the scope of the
present invention.
[0057] Referring now to FIG. 6, a coating method of the present
invention is illustrated for a die 600 attached to a substrate 604.
It should be understood that a multiple die device, such as the one
shown in FIG. 3, may also be protected as will become apparent to
those skilled in the art.
[0058] The die is wire bonded at 606 and at 607 to lead frame
devices 602 and 603, respectively, to complete electrical
connections between the die and systems (not shown) outside of the
package. A radiation shielding conformal coating composition is
applied to the outside of the package 610. The package can then be
applied to a board 615 or any other attachment system by any
suitable conventional technique.
[0059] The radiation shielding conformal coating composition 610 is
applied uniformly on the outer surface of the package to insure
uniform radiation protection in accordance with the present
invention. The coating can be applied by injection molding, mold
casting, spraying, globbing or brushing the material onto the part
to be protected.
[0060] Referring now to FIG. 7, another method of application
according to the invention includes applying the radiation
shielding conformal coating composition generally indicated at 709
to an integrated circuit device 700 previously attached to a board
730. The board 730 may have other devices such as a pair of devices
701 and 702 not requiring protection. The device 700 is attached to
the board via wire bonds 720 and 721. The radiation shielding
conformal coating composition 709 is then applied both on top of
the device 700 at 711 and directly underneath the device 77 at 710
on the board 730.
[0061] An area greater than the size of the device 700 is covered
with radiation shielding conformal coating composition 710 on the
bottom of the board 730. This is required to insure that the entire
integrated circuit device is protected from radiation.
[0062] The radiation shielding conformal coating composition 709 is
applied by the same method as described in connection with the
inventive method of FIG. 6.
[0063] Referring now to FIG. 8, to enhance radiation shielding
performance, multiple layers of the inventive radiation shielding
conformal coating composition are applied. Using conventional codes
such as NOVICE, different shielding layering are developed for each
type of orbit. An optimum shielding geometry for a Geosynchronous
Orbit is shown in FIG. 8.
[0064] As shown in FIG. 8, in accordance with the present
invention, a die 800 having an integrated circuit package 804
containing lead frame devices 802 and 803 is encased within a
multiple layer radiation shielding composition generally indicated
at 820, prior to mounting the shielded die to a board or substrate
815. The multiple layer shielding composition 820 comprises a layer
of high Z particles 811 interposed between a pair of outer and
inner layers of low Z particles 810 and 812. The low Z layer 812 is
applied directly to the outer surface of the die 800 in accordance
with the method described in connection with FIG. 6. Thereafter,
the intermediate high Z layer 811 is then applied to the outer
surface of the inner low Z layer 812.
[0065] The outer low Z layer 810 is then applied to the outer
surface of the intermediate high Z layer 811 to complete the
shielding protection for the die 800. The shielded die 800 is then
connected electrically and mounted to the board 815 by conventional
techniques.
[0066] The high Z material is effective in stopping electrons and
Bremsstrahlung radiation, while the low Z material is more
effective in stopping protons. A Geosynchronous orbit is dominated
by trapped electrons, so it is preferable that the intermediate
high Z layer 811, is thicker than the other two low Z layers. It
will become apparent to those skilled in the art that the multiple
layer coating method of the present invention can be used in
connection with the protection of many different types and kinds of
integrated circuit devices and the like. Additionally, the coating
method can be applied by any method including, but not limited to,
those described in connection with the method of FIG. 6.
[0067] Referring to FIG. 9, there is shown a flexible shielding
material, which is composed according to the present invention. The
material 900 contains the inventive radiation shielding
composition, and is flexible and pliable to serve as clothing for
humans or gasket material for parts (not shown). The conformal
coating material 900 includes a flexible binder such as latex. The
material 900 is impregnated with a fabric such as a cloth woven
material 910 for strength. The cloth material can be composed of
conventional materials such as cotton or polyester. For extra
strength, nonwoven fabric such as Kevlar or Teflon material can be
used for the fabric.
[0068] Considering now the inventive radiation shielding
composition forming a part of the foregoing inventive methods and
materials, the following examples of shielding compositions are
given to aid in understanding the invention, but it is to be
understood that the particular procedures, conditions and materials
of these examples are not intended as limitations of the present
invention.
EXAMPLE I
[0069]
1 10.0 parts by weight high tap density tungsten powder 0.15 part
by weight premixed epoxy up to 0.50 part by weight ketone
[0070] The tungsten powder serves as a high Z material for
radiation shielding purposes. The epoxy serves as a binder to help
adhere the composition to a surface, and the ketone is added as an
extender.
[0071] To formulate the inventive composition, the ingredients of
Example I are mixed thoroughly, and then the mixture is applied to
a part. The applied mixture is in the form of a paste, and is
heated slowly at a suitable low temperature such as 40.degree. C.
for about one hour to remove a substantial portion of the ketone
extender without disrupting the integrity of the packed tungsten
powder. The mixture is then heated at about 60.degree. C. for about
16 hours to retain the stability of the composition. The
temperature is then increased to about 150.degree. C. for an
additional period of time of about 0.5 hours. The resulting mixture
has the desired consistency of a paste, and retains its stability
due to the foregoing multiple heating phases.
EXAMPLE II
[0072] In general, the ingredients of the present Example can be
adjusted to accommodate variations in the foregoing described
inventive methods and applications.
[0073] The shielding powder can be any suitable high Z radiation
shielding powder such as osmium, iridium, platinum, tantalum and
gold. In general, any high Z material may be employed having an
atomic number of 50 and above. More preferably, the range of atomic
numbers can be between 60 and 100, inclusive. The most preferred
range of atomic numbers is between 73 and 79, inclusive.
[0074] The shielding powder can also be a low Z material, such as
the one mentioned in connection with the description of the
inventive method of FIG. 8. The low Z shielding powder is
preferably selected from the group consisting of copper, nickel,
carbon, titanium, chromium, cobalt, boron, silicon, iron and
nitrogen. In general, any suitable low Z material may be employed
having an atomic number of 30 and below, but the most preferred
group of low Z materials is selected from the group consisting of
copper, nickel, carbon, iron, titanium, silicon and nitrogen.
[0075] In general, the shielding powder can be any suitable
material composed of a matrix of densely packed shielding
particles. The preferred material is tungsten (Example 1) having a
packing density of at least 150 grn per cubic inch.
[0076] There can be between about 0.10 and about 0.50 parts by
weight of a binder in the form of a suitable resin. The binder can
be a urethane. The exact quantity of the binder determines the
final density and strength of the shielding afforded by the
inventive composition. A more preferred range of the binder is
between about 0.13 and about 0.30.
[0077] Also, in general, the extender assures complete wetting of
the powders and adjusts the viscosity of the paste to suit the
application method.
EXAMPLE III
[0078]
2 10.0 parts by weight high tap density tungsten powder 0.15 part
by weight premixed epoxy up to 0.50 part by weight latex
[0079] This example of the inventive material may be used for the
method described in connection with FIG. 9, wherein a fabric may be
embedded therein for reinforcing purposes. Any suitable elastomer
may be employed for the latex.
[0080] As shown and described in the accompanying provisional
patent application in Appendix A, there is described and shown a
further and more detailed disclosure of the inventive methods and
compositions. In the Appendix A, the inventive radiation shielding
composition is identified by the trademark "RADCOAT."
[0081] While particular embodiments of the present invention have
been disclosed, it is to be understood that various different
modifications are possible and are contemplated within the true
spirit and scope of the appended claims. There is no intention,
therefore, of limitations to the exact abstract or disclosure
herein presented.
APPENDIX "A"
Outline of Ionizing Radiation Shielding of Microcircuits Using
Filled Conformal Coatings
[0082] This invention provides additional ionizing radiation
resistance to virtually any sensitive electronic device, with or
without a package. Specifically, resistance to total dose ionizing
radiation can be enhanced by adding shielding to any device without
modification to its form or function. Additional shielding,
particularly under the device could be accommodated if lead lengths
could be increased.
[0083] This invention utilizes a filled conformal coating in a
versatile system that can apply high density material for localized
shielding to many types of electronic components from exposed die
to finished components mounted on a printed circuit board. It is
suitable for low volume customized usage and does not require
expensive tooling or equipment to implement The material is
intended to provide easy application of a form of radiation
shielding to existing electronic or other sensitive devices. The
material developed here is essentially a (polymer or glassy) matrix
highly loaded with dense (high specific gravity) particles.
Tungsten (dispersed in epoxy) provides the most convenient,
efficient and readily available radiologically dense material.
[0084] Selection of the of the most appropriate combinations of
particle sizes and shapes can increase the loading of the composite
to provide the maximum density of the composite while retaining the
workability of the paste prior to curing. Selection of the polymer
can provide optimum adhesion of the particles to each other and to
the component being shielded and influences the compatibility,
workability and the mechanical and electrical properties of the
composite. Some polymers allow the composite to be electrically
insulating even at the high loading desired for maximum
density.
[0085] Spherical powders with a tap density of the order of 200
grams/cubic inch can produce composites with a specific gravity of
13 when mixed in the approximate ratio of 10 grams of powder to
0.15 grams of polymer. Fugitive solvents that are compatible with
the chosen matrix can be added to ensure wetting of all the
particles and satisfactory rheology of the paste. The rheology of
the paste can be adjusted to suit the application (e.g. casting,
molding, syringe or spray) and end use (e.g. bare die or mounted
package). The polymer binder chemistry can also be varied to suit
the application method.
[0086] Lower density pastes can be used if thicker coatings can be
tolerated. This widens the range of particle sizes and morphologies
that can be incorporated into the paste. The additional polymer can
be beneficial in terms of rheology, adhesion, electrical and
thermo-mechanical properties.
[0087] The insulating nature of the shielding paste can be enhanced
by precoating the powders or by precoating the component. Allowing
the insulating paste to completely cover the component-including
the leads can improve the level of shielding. Depending on the
rheology of the paste, the coating material can be built up by
spray or spatulation or if a lower viscosity paste is used, a dam
can be placed around the device until it has cured or solidified in
place.
[0088] Section 1 The RADCOAT.TM. Concept
[0089] The concept for RADCOAT.TM. is based on RAD-PAK.RTM. and
similar radiation hardening techniques which uses localized dense
shields around a sensitive (electronic) device. Maximizing the
density (specific gravity (S.G.)) of the shielding material,
optimizes the efficiency of the shielding and minimizes the
thickness and total mass of the shields. (Density and specific
gravity are used interchangeably,. Density values used here have
units of grams per cubic centimeter (gm/cc), specific gravity is
numerically the same but has no units.
[0090] The disadvantage to RAD-PAK.RTM. shielding is the package
has to be individually designed and prefabricated for specific
customized packages and applications and have to be prepared
differently based on whether they are permanently brazed to the
ceramic or soldered on as a lid. Long lead times and expensive
customized tooling are a consequence of these requirements. They
are not usually suited to attaching to finished devices. There is
therefore a need for a material that would overcome these
limitations. The approach taken here is to use a thick film paste
that would adhere to and conform to any device. Optimizing of the
paste includes maximizing the content (mass) of the high "Z"
material in the paste to maximize the density of the resultant
shield. (Where "Z" refers to the atomic number. In the "Art", high
Z refers to elements with atomic numbers greater than roughly 40.)
The vehicle supports the high "Z" powder and eventually bonds the
mass in place on the package or device. The vehicle has to have
other desirable properties which will be discussed later.
[0091] The usual processing of refractory powdered metals involves
high temperatures and pressures. Obviously this is not compatible
with electronic devices so a different approach has to be taken.
The most obvious way is to mix the powder in a liquid suspension
that later hardens after application. Epoxy is a suitable medium.
One problem is that it is usually difficult to add more than about
60 volume percent (v/o) of powder in the resin before it becomes
`too dry` or otherwise unmanageable. The effective density of such
a composition would only be less than 12 grams/cc. The most
promising shielding effectiveness lies with manipulating the
`packing density` of the "high Z" powder in the resin vehicle to
maximize the final density of the composite.
[0092] The technical paper entitled "The Advantage of Low Pressure
Injection Molding` by Peter Shaffer in Materials
Technology--March/April 1993 is a guide in directing the
development of high density pastes. Particularly relevant excerpts
are reproduced below:
[0093] The way to obtaining high particulate loadings is well known
[1]. Into an array of closely packed large particles is introduced
a quantity of smaller ones of such size that they fit into the
interstices between the larger ones. A small amount of an even
finer fraction is introduced to fill these smaller voids, and so
on, and so on. With monosized spheres, the theoretical packing
density in a close packed array is 74 v/o (volume percent). Perfect
bimodal (two discrete sizes) packing yields a theoretical limit of
86 v/o, trimodal (three modes), 90 v/o. In practice the theoretical
74 v/o is never reached. Instead typical powder loadings, almost
without regard for their composition, rarely exceed about 60
v/o.
[0094] For an ideal four component packing System, the diameter
ratios have been determined to be 316:38:7:1 [1]. This is
frequently simplified to the rule of 7, each size differing from
that of the next larger by a factor of seven. The relative volume
ratios were determined to be approximately 61:23:10:6. These
determinations were made on near perfect spheres of very narrow
size ranges.
[0095] Relatively speaking, obtaining the high particle loadings is
the easy part; making the system sufficiently fluid to flow is not.
The particles must be fully dispersed and all agglomerates broken
into their individual crystallites. Their surfaces must be fully
wetted by the fluid medium. Finally, the suspension must be
stabilized to prevent reagglomeration.
[0096] The relative viscosities of a range of packing
configurations have been calculated as shown in FIG. 1-1 [2].
Further it has been demonstrated that at volume fractions over
about 70 v/o, the viscosity should be expected to increase
dramatically, even in systems having an infinite particle size
distribution. In practice, most systems show greater tendencies to
high viscosities and dilatancy than these calculations would
suggest.
[0097] Section 3 Ranking of Powder Size and Shapes
[0098] A wide range of powders shapes are available, including
spherical crystalline and irregular. Some have high degrees of
agglomeration, some have an inherently wide range of particle
sizes, some are fairly uniform in size. As expected, the coarser
powders are better than the finer ones
[0099] The best powder encountered was a fairly coarse spherical
powder with a significant proportion of a range of finer
powders.
[0100] Section 5 Binder Selection
[0101] Binders appear to be interchangeable insofar as the
resultant density of the composite (for a particular powder) is
concerned. The nature of the binder does influence some other
properties as will be discussed later. Epoxies, probably more
thoroughly plasticized for fracture toughness may prove to be the
best choice and are likely to be NASA approved for the proposed end
use. Thermoplastic and thermosetting formulations are often
available which adds to the versatility of the system and can be
changed to suit the method of application. Preliminary results show
that the density/radiation protection performance is not dependent
on the binder chemistry. Therefore, any superior resin system can
be substituted at any time
[0102] Latex appeared to have the best overall properties as far as
preparation, application and cured properties are concerned. While
latex may not be the material of choice since it contains ammonia
and water in the uncured state, a suitable synthetic material that
is similar in consistency and behavior could be used that would be
acceptable for contact with electronic devices.
[0103] Section 6 Composite Properties
[0104] The best results so far from simple mixing of selected
powder and resin (with the aid of a fugitive wetting agent) has
been a S.G of 13.1 with 10 grams of powder mixed with 0.15 grams of
epoxy resin. If the composite had been fully dense, there would
have been 79 v/o of metal in the composite and the S.G would have
been 15.5. Approximately 12 v/o of voids must therefore be present
to account for the measured density value. This powder loading of
about 67 v/o resulting from a rather crude manual mixing technique
is quite good since the literature cited earlier indicated that it
difficult to routinely exceed 60 v/o.
[0105] Another consideration has to be the electrical properties of
the material since it may be in intimate contact with a wire bonded
bare die or a package with exposed leads. The high loadings of
metallic powder is expected to result in a conductive material from
extensive particle to particle contacts, but the epoxy- and
latex-based composites have high resistances The silicone-based
composites were highly conductive.
[0106] If the binder completely wets the "high Z" particles then
the lowest free energy state for each particle would be surrounded
by a thin film of liquid. This insulating film isolates the "high Z
" particles and results in a non-conductive composite. (Externally
applied pressure could disrupt this insulating film and force
particle to particle contacts). Contact angles greater than zero
would result in agglomeration of the particles and conductive paths
and this may account for the conductivity of the silicone-based
composites.
[0107] Section 7 Powder Modifications
[0108] The "high Z" powders can be individually coated to ensure
that no particle to particle contacts could occur which would
compromise the insulating properties of the composite.
[0109] Larger particle sizes which make the denser composites can
be coated with thin layers of an insulator without seriously
degrading the density of the particles.
[0110] Section 8 Application Methods
[0111] Suitable rheology needs to be maintained while maximizing
the particle content of the uncured paste. The easiest application
is to pour the paste over the device. A better method would involve
putting a dam around the to contain the shape and maintain the
appropriate shielding thicknesses. However, not all PWAs requiring
shielding may allow the use of dams.
[0112] Syringe application is another method. A lower solids paste
with a fast evaporating solvent constituent might allow a syringe
to be used. Such a formulation may also allow spraying to be used
to build up to the required thickness and shape in the same manner
gunite is applied.
[0113] RAD-COAT.TM. can be applied to parts on a board, chip-on
boards, and to the individual prepackaged or unpackaged components
for ionizing radiation shielding.
* * * * *