Patent Application
20140308438
The present invention relates to improvements in materials usable for reducing adverse effects upon magnetostrictive materials, and more particularly to a coating that may be used to protect a microelectronic component that must be encapsulated or be rigidly supported, to thereby increase its efficiency.
The phenomenon of magnetostriction pertains to the change exhibited in a material's dimensions or shape when it is exposed to an external magnetic field. Another perspective on such behavior is that these magnetostrictive materials convert magnetic energy into mechanical energy, as exposure to the magnetic field induces deformation of the material, which is known as the Joule Effect, and which may be measured using a strain gauge to determine the relative displacement of particles in the object (i.e., the ratio of elongation with respect to the original dimension). Where a magnetostrictive material is induced by the Joule effect to elongate in a lengthwise direction, through rotation and reorientation of small magnetic domains therein—regions of uniform magnetic polarization—a corresponding decrease in the material dimension is experienced in the transverse direction, resulting in negligible changes to its volume. Increases to the strength of the magnetic field applied to the material further increase the induced strain (μL/L-units of micro-length per unit length), until its saturation value (λ) is reached, at which generally all the magnetic domains of the material are aligned with the applied magnetic field.
The reverse phenomenon also exists, where a mechanically induced change to the magnetostrictive material's dimensions creates a corresponding magnetic field, in what is referred to as the Villari Effect. This bi-directional coupling between magnetic and mechanical states is inherent to the particular material, and does not degrade with the passage of time, although a ferromagnetic material that is induced to undergo magnetostriction will not naturally be restored back to it initial magnetization state, after the magnetic field is removed, which reflects the magnetic memory of that material. Certain compositions of ferromagnetic materials will retain an imposed magnetization indefinitely and are termed “permanent magnets.” In general, energy must be supplied to drive back the magnetic domains in the material, by the imposition of a magnetic field in the opposite direction. This reluctance to retraceability is known as the hysteresis loop.
The magnetostrictive effect of certain materials (e.g., nickel) were productively used in early applications such as telephone receivers and torque meters, and the Villari effect is commonly used in contact-less sensors. While copper exhibits the greatest room temperature magnetostriction of any pure element and saturates at 60 microstrains, the 1970s discovery of “giant” magnetostrictive alloys (the alloying of elements whose magnetostrictive behavior results in strains greater than 1000 μL/L when exposed to small magnetic fields), is now successfully utilized as sound and vibration sources, for vibration controls, for motional controls, and for materials processing (see, “Handbook of Giant Magnetostrictive Materials,” by Goran Engdahl). Currently, the greatest room-temperature magnetostriction of an alloy is shown by Terfenol-D—a combination of terbium (Ter), iron (Fe), and the rare earth element dysprosium (D). Terfenol was developed by the Naval Ordnance Laboratory (NOL), and exhibits roughly 2000 microstrains in a magnetic field of 2 kOe (160 kA/m), so it is the most commonly used magnetostrictive material in engineering applications.
However, while the magnetostrictive effect may be harnessed for beneficial uses, there are also unproductive results and drawbacks. A common side effect is the annoying humming sound resulting from the 60 Hz applied magnetic field on an AC electrical transformer, where a maximum change in length occurs twice per cycle, producing a 120 Hz humming sound plus harmonics. A similar humming may be heard around high power electric lines carrying alternating current. In addition, a significant counterproductive result also occurs where magnetic devices, such as small high voltage power supplies, transformers, and inductive components, must be securely mounted or must be encapsulated, and because of this constraint on its dimensional changes, a loss in efficiency of the device results.
In applications where an external pressure is applied—either by restriction of deformation by a rigid encapsulant or contraction due to polymerization of a rigid encapsulant, or by the mere use of the magnetostrictive device in a high pressure application—it is desirable to surround the magnetic device with a suitable compressible media. Closed cell foams have been used in those applications where space and size permit. However, in small assemblies, particularly for microelectronics, use of the prior art foams and methods is not feasible. The present invention discloses a new coating that has been tested on microelectronics to successfully counter such losses in efficiency, and furthermore discloses specific application techniques/requirements.
It is an object of the invention to provide a means of reducing the magnetostrictive effect upon ferromagnetic materials.
It is another object of the invention to provide a method of improving the efficiency of ferromagnetic devices that are constrained by a rigid encapsulant.
It is a further object of the invention to provide a coating material that may be used as a buffer between a ferromagnetic device and a source of pressure applied to the device.
It is another object of the invention to provide a means of resisting pressure applied to a ferromagnetic device that does not result in significant loss in efficiency of the device.
It is also an object of the invention to provide a means of encapsulating a microelectronic device while maintaining its efficiency.
It is another object of the invention to encapsulate the surfaces of a microelectronic component with a protective coating having a thickness in proportion to an anticipated total micro-strain at saturation.
It is also an object of the invention to provide a coating on two sides of a microelectronic device to create a mechanical buffer against constraints thereat by the component's supporting means.
Further objects and advantages of the invention will become apparent from the following description and claims, and from the accompanying drawings.
Ferromagnetic materials exhibit the property of magnetostriction—a linkage between the material's mechanical and magnetic states—where the magnetic domains within the material are induced to change direction by a magnetic field, resulting in small changes in its dimensions. Microelectronic devices utilizing such materials, and which are constrained by their means of support or by being encapsulated by a rigid material, exhibit a loss in efficiency due to the constraint of the magnetostrictive properties of the ferromagnetic material.
The present invention discloses a buffer material that provides exceptional protection for a microelectronic device, thereby improving its efficiency with respect to magnetostrictive materials therein. The composition includes a gas filled polymer shell microsphere carried in an elastomeric polymer binder. Dry expanded Expancel microspheres may be used, and are preferably about 20 microns in diameter. The microspheres form 80% of the composition by volume. The gas used to expand the microspheres may be isobutane. The polymer binder is preferably a low viscosity dimethyl silicone configured to exhibit a hardness of less than 25.
In general, a coating of the disclosed composition may be applied uniformly over each of the surfaces of the microelectronic component, prior to its encapsulation. Using a thickness of approximately 15 mils may suffice in most cases (note—a “mil” equals a thousandth of an inch).
However, for electronic assemblies where space is limited and/or where alloyed materials are used for the magnetic core, the thickness of the composition to be applied may be determined mathematically to provide a mechanical buffer based on the particular component and its usage, and based on the particular encapsulation material. Specifically, the thickness of the composition applied may be determined based upon the overall expected dimensional changes of the encapsulation material, due to its coefficient of thermal expansion and an expected operating temperature range of the component, plus the expected shrinkage of that encapsulation material during polymerization and the overall mass which shall be exerting a force upon the magnetic core, plus the dimensional changes of the component as a result of the flux density resulting in magnetostriction of the magnetic core.
Furthermore, where space considerations are critical, the thickness of the composition may be calculated for, and applied to, specific corresponding pairs of surfaces, to be in proportion to the total micro-strain at saturation, in that direction, to provide necessary buffering with minimal impact on component volume. Each pair of surfaces of a six-sided microelectronic component would thus have a respective coating thickness tailored for that direction, so that when paired together the coating on those planes would be capable of accommodating maximum positive magnetostriction in that surface-to-surface direction. The coating may alternatively be tailored according to the orientation of the magnetic domains of the ferromagnetic material of the microelectronic part, and the constant direction of a magnetic field, so that only the component's pair(s) of mounting surfaces that will experience positive magnetostriction need be coated.
In general, it may be advantageous to coat all of the planes of interest on the microelectronic components (i.e., applying the coating onto six sides of the block-shaped component). Tests on a typical transformer operated at moderate flux density with and without the use of the coating of the present invention have shown a reduction in losses of greater than 50%.
Ferrite magnetics, as well as other ferromagnetic materials, exhibit the property of magnetostriction, which is where the magnetic domains within the material change direction while in the presence of a magnetic field, resulting in small changes in the dimensions of the material. Magnetic devices which are constrained by their means of support or which must be encapsulated with a rigid material, exhibit a loss in efficiency due to the constraint of the magnetostrictive properties of the ferromagnetic material. Encapsulation of such electronic components is common in hostile environments (e.g., “down-hole” mining/drilling applications, deep-sea operations, etc.), and for military applications, the requirements for which may be found in MIL-T-27E.
Although previously disclosed foams and other compressible media have been satisfactorily employed where space is not critical, use of those inventions in smaller assemblies, particularly for microelectronic components, has not proved successful for meeting both the space requirements and the buffering requirement, to improve the efficiency of those devices. Tests conducted using the coating of the present invention have demonstrated markedly improved efficiency in the operation of such constrained microelectronic devices, while minimizing impact to the volume required for the component.
The problem with microelectronic devices is exacerbated by the high flux densities that are desirable in order to reduce the size and weight of magnetics, which is where the magnetostrictive effect is most pronounced. The buffering system of the present invention significantly reduces the compressive and expansive forces upon magnetic cores, thereby reducing losses typically experienced in devices which employ ferromagnetic materials having a high magnetostriction coefficient and which operate at moderate to high flux densities. The system of the present invention is a specially devised coating, which may be applied uniformly, or may alternatively be applied in accordance with one of the particular methods disclosed herein.
A diagram of the composition of the present invention is illustrated in
The composition also includes gas-filled polymer shell microspheres carried in the elastomeric polymer binder. The microspheres may form 80 percent of the composition by volume. The suitable microspheres are preferably 20 microns in diameter or smaller, and may be formed of acrylonitrile or vinylidene Chloride. The microspheres may be the dry, expanded, gas filled Expancel® microspheres that are available from AkzoNobel, in Duluth, Ga. The gas utilized may be isobutane. The microspheres are ideally dispersed uniformly throughout the binder, which is initially achieved by mixing during preparation.
To maximize the buffering capability of the composition, the microsphere fill is desirable maximized. Utilizing microspheres having a diameter of 20 microns or less optimizes the volume of microspheres, but also maintains a sufficiently high mean free path in the binder. The use of larger spheres resulted in minimizing the volume of spheres that might be mixed into the adhesive vehicle. In another embodiment, the use of smaller, random sized spheres—being smaller than 20 microns—is also desirable. Also, utilizing microspheres to form greater than 80% by volume of the composition, while furthering its ability to absorb dimensional changes of the microelectronic component and the rigid encapsulation coating, negatively impacted the application and workability of the composition. Improved flow of the composition for its application, while also accommodating higher loading ratios of the microspheres, was obtained through the use of a lower viscosity binder. The binder is preferably a low viscosity dimethyl silicone, which may result in a viscosity ranging between 10,000 and 50,000 cps. Two different viscosity levels are preferably utilized for different applications of the composition, as discussed hereinafter.
For its preparation, the low viscosity dimethyl silicone is mixed with a solvent, currently toluene, which may alternatively be a reactive diluent. The Expancel microspheres are blended into the mixture of solvent and dimethyl silicone, with mechanical stirring.
Application of the composition may be by a brush, a roller, a syringe, or other suitable application means, after which the coating requires a period of time to dry. The composition may be mixed at the time of use or it may be mixed and stored in sealed bottles, which require agitation to reduce material separation prior to application. A roller and vibrator may be utilized to maintain the dispersion after formation of the composition.
Microsphere loading of greater than 70% by volume is achieved with the composition as described, as the microspheres may be compressed to several atmospheres of pressure to thereby provide exceptional mechanical buffering for the microelectronic component.
The coating may be applied over each of the surfaces of the component (all of the planes of interest), which may be necessary in certain cases, such as for microelectronic components to be utilized in “down-hole” (mining/drilling) applications, deep-sea operations, etc. As seen in
However, for electronic assemblies where space is limited and/or where alloyed materials are used for the magnetic core, the thickness of the composition to be applied may be determined mathematically to provide a mechanical buffer based on the particular component and its usage, and based on the particular encapsulation material. Specifically, the thickness of the composition applied may be determined based upon the overall expected dimensional changes of the encapsulation material, due to its coefficient of thermal expansion and an expected operating temperature range of the component, plus the expected shrinkage of that encapsulation material during polymerization and the overall mass which shall be exerting a force upon the magnetic core, plus the dimensional changes of the component as a result of the flux density resulting in magnetostriction of the magnetic core. (Note, for a discussion of flux density, see, U.S. Pat. No. 6,084,499 to Faulk for “Planar Magnetics with Segregated Flux Paths,” the disclosures of which are incorporated herein by reference).
Polymer encapsulation materials for use on microelectronic components typically include epoxides and urethanes, but other materials may also be used, such as the rubbers (Polyisoprene, polybutadiene, neoprene, etc.), polysulfides, and pefloroelastomers. The linear coefficient of thermal expansion (TCE) for those families of materials range between 12 ppm/deg C. to greater than 200 ppm/deg C. In most magnetic applications epoxides are the primary choice with typical TCE's being in the range of 40-80 ppm/deg. C. However, the materials of choice for encapsulation of microelectronic components in space limited applications have a TCE ranging between 12 ppm and 20 ppm. The encapsulating material may expect to experience a temperature ranges of +130 degrees C. to −55 degrees C.
Where space considerations are critical, the thickness of the coating may be tailored—be correlated—to the dimensions of the component (i.e., TS≠TL), since the resultant total micro-strain experienced is dependent upon the overall dimensions (μL/L-units of micro-length per unit length). Therefore, in the “L” direction, which may undergo a greater total micro-strain, the coating thickness TL may be greater than the thickness TS, as the mechanical buffering to be provided in the “S” direction by the coating of thickness TS, generally speaking, need not be nearly as large.
With respect to the shrinkage of the encapsulating material, the linear shrinkage (S) of the material in a single plane is given by the relationship: S=L*TCE*Δt, where Δt is the change in temperature (degrees C.). Also, the inherent shrinkage during polymerization (curing) of the encapsulating material may generally range from 0.1 to slightly greater than 0.4%. (0.4% is on the lower side for most epoxides). Therefore, for a one by one inch rectangle cured at 125 degrees C., the total shrinkage “S” would be 0.004 inches due to the polymerization, and 2.16E-3 inches for the reduction in temperature from the 125 cure temperature to −55 degrees C., a total of 6.16E-3 inches.
In view of the required mechanical buffering to be provided for the microelectronic component, based on the maximum anticipated micro-strain at saturation, a proportionality factor may be used to determine the optimal coating thickness for each of its three pairs of sides.
Another consideration may further reduce the necessity of utilizing a uniform coating thickness, a practice which would unnecessarily sacrifice space about the microelectronic component—space which may not ideally or practically be available. Where the component is merely to be rigidly supported and not be completely encapsulated by a rigid material, a determination of the magnetic domains in the microelectronic component 101 of
In the exemplary use of the composition of the current invention on the ferrite transformers in the “PV” power supply board shown in
The examples and descriptions provided merely illustrate a preferred embodiment of the present invention. Those skilled in the art and having the benefit of the present disclosure will appreciate that further embodiments may be implemented with various changes within the scope of the present invention. Other modifications, substitutions, omissions and changes may be made without departing from the spirit of this invention.
