Article of manufacture

Abstract

An encapsulated electrical or electronic device having a first layer of electrically insulating material which is then covered by another layer filled with a thermally conductive material such as mesophase pitch based carbon fibers or graphite particles. Both layers are of the same base resin.

Description

This invention relates to encapsulated electrical and electronic devices and, more particularly, it relates to electrical and electronic devices encapsulated with both an insulating material and a thermally conductive material.

Specific techniques for encapsulating electrical and electronic devices are known as disclosed by Eickman et al. in U.S. Pat. No. 4,632,798. They also disclose that it is common practice to include, within the encapsulating resin, particulate filler material such as silica or alumina which serves to increase the thermal conductivity.

The market segments for electrical devices such as for windings of motors, transformers and solenoids and electronic devices such as microchips are increasingly moving to miniaturization of such devices. This in turn leads to a rise in internal equipment operating temperature resulting in not only a need for higher temperature ratings on insulation materials used for these applications, but also a need for materials with improved thermal conductivity properties to rapidly remove the heat generated.

SUMMARY OF THE INVENTION

On order to complement the move toward miniaturization of such electrical and electronic devices, an article of manufacture has been developed which comprises an electrical device encapsulated with a layer of an electrically insulating material that in turn is encapsulated with a layer of thermal conductive material forming an outer surface of the article. A single phase interface is found between the layers. The thermal conductive material comprises 10% to 70% by weight and preferably from about 15% to about 60% carbon fiber, the balance being made up of a matrix of resin or a combination of a resin and an alternate fiber or filler. The electrically insulating material is made of a matrix resin or a combination of a resin and a reinforcing fiber and/or a filler. It is important that the resins are the same to enable the formation of the single phase interface between the layers such that the layers appear as one at the interface.

Suitable resinous materials which may be used as the resin include, but are not limited to, polyethylene terephthalate, Teflon.RTM. PFA by E. I. du Pont de Nemours and Company (DuPont), amorphous copolyamides as described in U.S. Pat. No. 4,681,411, and Hytrel.RTM. 7246 by DuPont as well as thermosetting resins.

The carbon fibers are preferably centrifugally spun from mesophase pitch as disclosed in U.S. Pat. No. 4,861,653 application which is incorporated herein by reference.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic cross-sectional view of a transformer according to the invention.

FIG. 2 is a photograph enlarged 200.times. showing the interface between an electrically insulating material and a thermal conductive encapsulating material.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

Referring to FIG. 1, the embodiment chosen for purpose of illustration is an article of manufacture generally designated 10 that includes an electrical device consisting of a plurality of electrically active wire coils 12, 14 wrapped around a coil form 16 positioned on a metal core 18. The coils 12, 14 serve as the primary and secondary windings of a transformer with connections leading to external terminals (not shown). Surrounding the electrical device is an encapsulating first layer 20 of an electrically insulating material which in turn is encapsulated with a thermally conductive second layer 22 which also can serve as a case for the transformer.

The thermally conductive layer 22 exhibits a three dimensional arrangement of fibers within the resin matrix as estimated from shrinkage data in the x, y, and z coordinate axes directions from mold size to the final part. More particularly, essentially equal shrinkage of the final part in the x, y, and z directions indicates three dimensional isotropic fiber reinforcement while shrinkage of the final part that varies by several orders of magnitude between directions suggests highly oriented reinforcing fibers.

As best shown in FIG. 2, there is formed a single phase interface 21 between first and second layers 20 and 22 such that the layers appear as one at the interface. This single phase interface 21 is created by using the same base resin in each layer and molding techniques disclosed in the following Example. This, in turn, provides for a rapid heat transfer from layer 20 to layer 22 which could not be obtained with incompatible or dissimilar resins.

EXAMPLE

In Step I, a non-encapsulated transformer is introduced into a steel injection molding tool held in a standard injection molding machine. The cavity of the injection molding tool has (1) notches cut into them so as to allow the transformer leads to protrude from the cavity holding the transformer and (2) positioning pins to hold the transformer in place so that the injected polymer can flow completely around the transformer. A commercial grade DuPont Rynite.RTM. FR-530 NC-10 polymer which is a 30 wt. % chopped glass fiber reinforced flame retarded polyethylene terephthlate polymer is heated to a melt temperature of 280.degree. C. and injected into the closed steel mold containing the transformer. After two minutes, the transformer is removed form the original steel mold. step II, a second steel mold with similar notches and positioning pins having approximately 1/2" to 3/4" larger overall inner dimensions than the first mold replaces the first mold in the injection molding machine. A 50 wt. % pitch carbon mat fiber reinforced flame-retarded polyethylene terephthalate polymer made by compression molding alternate layers of flame retarded polyethylene terephthalate polymer film and carbon fiber bats made according to the disclosure in U.S. Pat. No. 4,861,653 which is subsequently chopped into 1/4" pellets and fed to the injection molding machine where it is heated to 280.degree. C. and injected into the tool cavity containing the previously encapsulated transformer of step I. This doubly encapsulated transformer is removed from the mold after 1 minute and 40 seconds. A two shot encapsulated transformer was tested for electrical insulation properties of the inner layer and thermal conductivity properties of the outer layer, according to the Underwriters Laboratories standard UL 1585, sections Rated Secondary Current Test, and 29, Rated Output Heating Test, and the temperature rise of 42.degree. C. was much less than the specified maximum temperature specified in the UL 1585 standard. The integrity of the electrical insulation was maintained.

In an alternative embodiment, the carbon fibers described above in connection with the second layer 22 are replaced with a sufficient loading (up to 60 weight percent) of graphite particles with a particle size range of from 0.5 microns to 420 microns and preferably less than 180 microns. These particles are present in such an amount and oriented within a plane of the resin by melting and flowing the resin that adjacent graphite particles touch each other forming thermal conductive paths throughout the resin matrix providing improved thermal conductivity as measured by Underwriters Laboratories Standard UL 1585.

Claims

1. An improved article of manufacture comprising: an electrical device having electrically active components, said components being encapsulated with a first layer of an electrically insulating resin, and a second layer of a thermally conductive material forming the outer surface of said article, said second layer comprising an electrically insulating resin of the same composition as the first electrically insulating resin filled with up to 60 weight percent graphite particles with a particle size range of from 0.5 microns to 420 microns, said graphite particles being oriented within a plane of the resin and present in such an amount that adjacent graphite particles touch each other forming thermal conductive paths throughout the resin matrix providing improved thermal conductivity, there being a single phase interface formed between said first and second layer such that the layers appear as one at the interface.

2. The improved article of manufacture of claim 1 wherein the particle size is less than 180 microns.