Patent Number: 
Section: description

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 invention. The scope e 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. 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. 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. 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). 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. 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. 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. 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. 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. 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. Additionally, by adjusting the properties of the binder, the bulk electrical properties of the shield composition is adjusted to be either insulating or conductive. 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. 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. 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. 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. 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. 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. 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. The radiation shielding conformal coating composition 709 is applied by the same method as described in connection with the inventive method of FIG. 6. 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. 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. 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. 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. 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. 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. 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. 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 40xc2x0 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 60xc2x0 C. for about 16 hours to retain the stability of the composition. The temperature is then increased to about 150xc2x0 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. In general, the ingredients of the present Example can be adjusted to accommodate variations in the foregoing described inventive methods and applications. 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. 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. 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. 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. Also, in general, the extender assures complete wetting of the powders and adjusts the viscosity of the paste to suit the application method. 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. 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 xe2x80x9cRADCOAT.xe2x80x9d. 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. 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. Selection 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. 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. 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. 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. The concept for RADCOAT(trademark) is based on RAD-PAK(copyright) 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. The disadvantage to RAD-PAK(copyright) 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 xe2x80x9cZxe2x80x9d material in the paste to maximize the density of the resultant shield. (where xe2x80x9cZxe2x80x9d refers to the atomic number. In the xe2x80x9cArtxe2x80x9d, high Z refers to elements with atomic numbers greater than roughly 40.) The vehicle supports the high xe2x80x9cZxe2x80x9d 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. 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 xe2x80x98too dryxe2x80x99 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 xe2x80x98packing densityxe2x80x99 of the xe2x80x9chigh Zxe2x80x9d powder in the resin vehicle to maximize the final density of the composite. The technical paper entitled xe2x80x9cThe Advantage of Low Pressure Injection Moldingxe2x80x9d by Peter Shaffer in Materials Technologyxe2x80x94March/April 1993 is a guide in directing the development of high density pastes. Particularly relevant excerpts are reproduced below: The way to obtaining high particulate loadings is well known. 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 (to 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. For an ideal four component packing system, the diameter ratios have been determined to be 316:36:7:1. This is frequently simplified to the rule of 7, each size differing from that of the next larger by a actor of seven The relative volume ratios were determined to be approximately 61:23:10:6. These determinations r made on near perfect-spheres of very narrow size ranges. Relatively Zings 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. The relative viscosities of a range of packing configurations have been calculated as shown in FIG. 1xe2x80x941. Further it has been demonstrated that at solute 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. 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. Referring to FIG. 10, shown is a graph illustrating density variation with respect to percent by weight of Grade 50M spheroidal high. Z material. The best powder encountered was a fairly coarse spherical powder with a significant proportion of a range of finer powders. 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. 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. 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. 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 epoxyxe2x80x94and latexxe2x80x94based composites have high resistances The siliconexe2x80x94based composites were highly conductive. If the binder completely wets the xe2x80x9chigh Zxe2x80x9d particles then the lowest free energy state for each particle would be surrounded by a thin film of liquid. This insulating film isolates the xe2x80x9chigh 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 siliconexe2x80x94based composites. The xe2x80x9chigh Zxe2x80x9d powders can be individually coated to ensure that no particle to particle contacts could occur which would compromise the insulating properties of the composite. 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. 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. 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. RAD-COAT(trademark) can be applied to parts on a board, chip-on boards, and to the individual prepackaged or unpackaged components for ionizing radiation shielding. 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.