Thin Film Coating Packaging of Devices Having Fusible and Wettable Links - Patent application

Abstract

To provide a method for creating a dielectric thin film coating for devices having a fusible element.SOLUTION: The method comprises mixing insoluble and soluble polymers in solid forms, and exposing the mixture to heat to create a melt mixture. The melt mixture is then dissolved in a solvent to create slurry, and the slurry can then be deposited on a device as a thin film coating to create an interior insulation layer or an external surface.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates generally to the field of circuit protection devices, and more particularly to fuses, thermally protected metal oxide varistors (TMOVs), and thermal cutoff devices (TCOs). [Background technology]

[0002] Fuses and TMOVs are commonly known circuit protection devices that are typically installed between a power source and a component of an electrical circuit to be protected. Devices such as fuses and TMOVs have a fusible link that melts under an overcurrent or overvoltage condition. Such devices may be low profile or wafer-based devices and may be coated with a dielectric coating that accommodates dewetting. However, current packaging for such devices is difficult to deposit due to its relative thickness. As such, current dielectric coatings are expensive and difficult to deposit on a scale of less than 50 μm.

[0003] Ideally, a coating for a fusible link meets three criteria. First, the coating should have dielectric properties. In particular, the coating should act as an insulator. Second, the coating should be able to cover the fusible link without restricting the link from flowing when the fusible link reaches its melting temperature. Some current coatings can hold the link in place after the fusible link reaches its melting temperature, thereby restricting the circuit from opening. Third, the coating should be able to be deposited while in the solution phase to create coatings on the scale of 50 μm or less.

[0004] Current methods of coating these devices fail to meet all three criteria. For example, some polymers, silicone polyesters, and polypropylenes, when used as coatings meet the first criterion of having dielectric properties, but fail to meet the second criterion of covering the fusible link without restricting the flow of the link after melting. Current devices may use polymers from the polyamide family, which meet the first two criteria, but fail to meet the third criterion of being able to be deposited from a solution phase, thereby resulting in a thin coating layer.

[0005] It would therefore be desirable to have a coating for low profile and wafer-based devices that has a fusible link that meets all three criteria. Summary of the Invention

[0006] Coatings for devices having fusible links may be made by a method that includes melt mixing an insoluble polymer with a soluble polymer and exposing the molten mixture to a solvent to create a slurry, which can then be deposited using any known technique, for example, spin coating, deep coating, drop coating, etc. Coatings made in accordance with the present invention may be used as insulating layers on internal devices or as external coatings.

[0007] In this way, devices made on the wafer and / or very small devices can have a thin dielectric coating that eliminates expensive packaging. The coating can accommodate controlled dewetting that allows for quick release of the device and also helps protect any exposed metal surfaces of the device from corrosion. [Brief description of the drawings]

[0008] [Figure 1]FIG. 1 is a photograph showing a prior art example of a device having a fusible link.

[0009] [Diagram 2] 1 is a flow chart showing steps in a method of producing and applying a coating of the present invention.

[0010] [Diagram 3] FIG. 2 is a schematic diagram of a device having a fusible link, showing the coating of the present invention in place on the device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The solution-based coating of the present invention creates a thin dielectric layer suitable for use over a fusible link. The coating serves as packaging and creates a low profile for the fuse or TMOV. The dielectric film accommodates the wetting characteristics of the fuse, creating a packaging film on the scale of 50 microns.

[0012] Figure 1 is a photograph of a portion of a device having a fusible element. The device has a metal electrode 102, typically made from copper, bridged by a solder ball 104. The solder ball 104 is designed to melt and open a circuit when a certain temperature is reached. The device of Figure 1 is representative of the type of device that would benefit from the coating of the present invention.

[0013] 2 is a flow chart illustrating steps in a method 200 for making a coating of the present invention. Insoluble and soluble polymers are combined in step 202. Insoluble and soluble polymers are commercially available and provided in the physical form of pellets or chips.

[0014] In some embodiments, the insoluble polymer may be selected from the group comprising polyamides, and their copolymers and graft materials (e.g., hot glue). In other embodiments, low temperature high density polyethylene (HDPE) or low density polyethylene (LDPE), low Tg (glass transition temperature) PET, soluble or semi-soluble EBA, EMA polymer family (referred to by the general term of hot glue), and wood pitch, often referred to as fatty acid or rosin, may be used as the insoluble polymer. In a preferred embodiment, the insoluble polymer is a polyamide. The insoluble polymer is generally in the form of pellets with sizes of 1 / 8 inch to 1 / 4 inch (0.32 cm to 0.64 cm).

[0015] Many soluble polymers can be used in the present invention. In some embodiments, the soluble polymer may be a soluble polyurethane or wax that is soluble in DMF or aromatic solvents, such as xylene, toluene, or anisole, or any other soluble polymer. In other embodiments of the present invention, the soluble polymer may be a polyamic acid or PVDF that is soluble in DMF or NMP. In general, any soluble polymer may be used as long as it has a melting point below about 130° C. or below the melting point of the solder link, does not restrain the molten solder, and better suppresses arcing. In a preferred embodiment of the present invention, a low melting temperature polyurethane is used as the soluble polymer. The soluble polymer is generally in the form of chips with sizes of ½ mm to 5 mm.

[0016] The insoluble and soluble polymers may be mixed in an insoluble to soluble ratio of about 75:25, although in preferred embodiments the ratio will be between 40:60 and 60:40.

[0017] The insoluble and soluble polymer pellets are melted by exposure to heat to create a molten mixture, step 204. The mixture may be melted and mixed by an industrial machine that includes a heating element and a mixing blade, or preferably an extruder that can heat and mix the insoluble and soluble polymers to create a molten mixture and then extrude the molten mixture into a wire or fiber having a relatively small cross section.

[0018] The temperature at which the insoluble and soluble polymers melt will depend on the selection of the particular polymer, but generally the mixture may be heated at least to the temperature of the material with the higher melting point. In other embodiments, the mixture may be melted at a temperature between the melting points of the insoluble and soluble polymers, and the mixing blades of the mixer create shear forces in the mixture that tend to increase the temperature of the mixture. In some embodiments of the present invention, the molten mixture may be cut into small pellet-sized pieces as it exits the extruder to increase the surface area of ​​the molten mixture.

[0019] Process step 206 is an optional step in which the molten mixture is allowed to cool and solidify, and in optional step 208, the solidified molten mixture is cut into chip or pellet sized pieces to increase surface area. In other embodiments of the invention, steps 206 through 208 may be skipped and the molten mixture may be extruded or otherwise placed in molten form directly into a solvent.

[0020] At step 210 of the process, the molten mixture is exposed to a solvent and mixed to ultimately create a slurry. The increased surface area of ​​the molten mixture created by cutting the molten mixture into pellet-sized pieces makes the process of creating the slurry easier. The solvent may comprise, for example, xylene, toluene, anisole, DMF (dimethylformamide), or NMP (N-methylpyrrolidone), or any solvent capable of dissolving the soluble polymer. The choice of solvent is highly dependent on the choice of soluble polymer. The molten mixture and solvent are mixed in a ratio of approximately 3 parts molten mixture and about 7 parts solvent by weight, again highly dependent on the choice of polymer and solvent. The resulting slurry may have a viscosity of 1 cps to about 10,000 cps.

[0021] Once the slurry is prepared, it may be conformally deposited on the device, either as an inner insulating layer or as an outer coating, in step 212. The coating may be applied by any known means, such as spin coating, deep coating, drop coating, etc.

[0022] 3 shows a typical device having copper electrodes 304 joined by a fusible link 302, typically a solder ball. The conformal coating of the present invention is shown in the drawing as 306.

[0023] Coatings made according to the methods disclosed herein meet all three design criteria described in the Background section. The methods create dielectric coatings that have insulating properties. The coating, once conformally deposited on the fusible elements, allows the fusible elements to flow only when the fusible elements reach their melting point. Finally, the coatings can be applied as thin films in the range of 50 μm thick, making them suitable for use with low profile TMOV and other wafer-based devices.

Claims

1. mixing an insoluble polymer and a soluble polymer in solid form; forming a molten mixture of the insoluble polymer and the soluble polymer by heating and mixing the insoluble polymer and the soluble polymer; exposing the molten mixture to a solvent to form a slurry; depositing the slurry onto a device to form a coating; 1. A method of making a thin film coating comprising: The method, wherein the device comprises a meltable element between two electrodes, and further wherein the thin film coating at least covers the meltable element.

2. 10. The method of claim 1, wherein the insoluble polymer is selected from the group comprising polyamides, copolymers, low temperature high density polyethylene, low density polyethylene, PET, soluble or semi-soluble EBA, and EMA polymers.

3. The method of claim 1 or 2, wherein the insoluble polymer is a polyamide.

4. 4. The method of claim 1, wherein the soluble polymer is selected from the group comprising soluble polyurethanes or waxes, polyamic acids, and PVDF.

5. 5. The method of claim 1, wherein the soluble polymer is a low melting temperature polyurethane.

6. 6. The method of any one of claims 1 to 5, wherein the solvent is selected from the group comprising xylene, toluene, anisole, DMF, and NMP.

7. 7. The method of any one of claims 1 to 6, wherein the insoluble polymer and the soluble polymer are present in the molten mixture in a ratio of from 40:60 to 60:

40.

8. 8. The method of claim 1, wherein the molten mixture and the solvent are mixed in a ratio of 3 parts by weight of molten mixture to 7 parts by weight of solvent.

9. 9. The method of claim 1, wherein the insoluble polymer and the soluble polymer are heated to the melting temperature of the component having the highest melting temperature.

10. The method of claim 1 , wherein the slurry is deposited on the device using a method selected from the group comprising spin coating and drop coating.

11. 11. The method of any one of claims 1 to 10, wherein the thin film coating has dielectric properties.

12. 12. The method of claim 1, wherein the thin film coating allows the meltable element to flow away from the electrode after the meltable element reaches its melting point.

13. 13. The method of any one of claims 1 to 12, wherein the thin coating is less than 50 microns thick.

14. one or more electrodes; a fusible link electrically joining the one or more electrodes; a thin film coating covering the one or more electrodes and the fusible link; The thin film coating is formed by depositing a slurry comprising a molten mixture of an insoluble polymer, a soluble polymer, and a solvent onto the device. device.

15. The device of claim 14 , wherein the slurry is deposited on the device using a method selected from the group consisting of spin coating and drop coating.

16. 16. The device of claim 14 or 15, wherein the thin film coating enables the fusible link to flow away from the plurality of electrodes after the fusible link reaches its melting point.

17. 17. The device of any one of claims 14 to 16, wherein the insoluble polymer is selected from the group comprising polyamides, copolymers, low temperature high density polyethylene, low density polyethylene, PET, soluble or semi-soluble EBA, and EMA polymers.

18. 18. The device of any one of claims 14 to 17, wherein the soluble polymer is selected from the group comprising soluble polyurethanes or waxes, polyamic acids, and PVDF.

19. 19. The device of any one of claims 14 to 18, wherein the solvent is selected from the group comprising xylene, toluene, anisole, DMF, and NMP.

Images