Abstract:
A method and apparatus for electrically insulating heat sinks in electronic power devices that includes the formation of an insulating layer on the face of the electronic device having the heat sink. The insulating layer may be formed from an epoxy resin, commonly known as &#34;solder mask&#34; in the manufacture of printed circuits.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for electrically insulating heat sinks in electronic power devices. 
     2. Discussion of the Related Art 
     It is known that some electronic power devices that dissipate considerable heat are provided with heat sinks. With reference to FIG. 1, the device includes an integrated circuit chip 1 placed within a package 2 made of epoxy resin. The chip is connected by solder to a heat sink 3 which is conventionally made of a metal having high thermal conductivity, typically copper. One face of the heat sink 3 protrudes outside the electronic device so as to convey the flow of heat to the surrounding air. The heat sink must have particular characteristics, namely high thermal conductivity and a large dissipation surface. 
     In some applications, such as for example high fidelity audio devices, it is desirable to connect the heat sink to the external metal chassis of the audio device, so as to further increase the dissipation surface and accordingly improve heat dissipation. However, this arrangement has the drawback that the chassis of the audio device may have an electrostatic or other potential that can negatively affect the operation of the integrated chip. In some cases, the voltage between the chassis, the heat sink, and the integrated chip can irreversibly damage the chip of the power device. 
     Accordingly, it is desirable to electrically insulate the heat sink from the mechanical structures with which it is placed in contact. However, at the same time it is desirable to maintain the thermal conductivity of the device, i.e. its heat dissipation, at an optimum level. 
     A possible solution to this problem is to provide an insulating layer 4 (FIG. 2) on the heat sink 3 during the molding of the package 2 that contains the chip 1. This method for insulating the heat sink is not advisable, since it is difficult to ensure the thickness of the insulating layer. On one hand, if the insulating layer is too thick, the heat sink is unable to dissipate heat externally. On the other hand, if the thickness is insufficient, some regions of the heat sink may remain exposed. 
     Another possible solution uses a layer of metal, typically aluminum, which is deposited on the heat sink and then oxidized. This method has the drawback that it makes the production process more complicated and has a high cost. 
     Another possible solution is to deposit an insulating layer by virtue of chemical treatments. This method is not advisable because chemical treatments are poorly selective and might contaminate the chip of the power device or cover the contacts 5 (FIG. 1) of the device. 
     SUMMARY OF THE INVENTION 
     The foregoing problems of the prior art are overcome by one illustrative embodiment of the invention, in which a method and apparatus are provided for electrically insulating heat sinks in electronic power devices, the method and apparatus providing effective electrical insulation of the heat sink and preserving the desired thermal conductivity characteristics of the device. The method and apparatus comprise the formation of an insulating layer on the face of the electronic device that comprises the heat sink and the resulting electronic device, respectively. 
     In one embodiment of the present invention, the insulating layer is an epoxy resin, commonly known as &#34;solder mask&#34; in the manufacture of printed circuits. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The characteristics and advantages of the invention will become apparent from the description of a preferred but not exclusive embodiment, illustrated only by way of non-limitative example in the accompanying drawings, in which: 
     FIG. 1 is a cross-sectional view of an electronic power device without heat sink insulation; 
     FIG. 2 is a cross-sectional view of an electronic power device with heat sink insulation; 
     FIG. 3 is a perspective view of a group of electronic power devices seated in a production mold; 
     FIG. 4 is a cross-sectional view of power devices seated in a production mold and being treated according to the method of the present invention; 
     FIG. 5 is a cross-sectional view of the package of the power device and of the mold, treated according to the method of the present invention; 
     FIG. 6 is a cross-sectional view of the package of the power device and of the mold, treated according to the method of the present invention; 
     FIG. 7 is a cross-sectional view of the power devices and of the mold, treated according to an additional step of the method of the present invention; 
     FIG. 8 is a cross-sectional view of power devices obtained with the method of the present invention; and 
     FIG. 9 is a cross-sectional view of the power devices seated in the mold and treated according to another embodiment of the method of the present invention. 
    
    
     DETAILED DESCRIPTION 
     With reference to the above figures, a group of power devices is each in a mold 6 so that an upper part 8, which comprises the face of the heat sink, protrudes above the outer wall of the mold, as shown in FIG. 3. 
     The mold 6 and the power devices 11 are inserted into a laminating machine during a lamination step. The machine may be of the type commonly used to deposit an insulating layer of epoxy resin on printed circuit boards. This resin is commonly termed &#34;solder mask.&#34; The laminating machine usually comprises two rollers on which two layers of material are rolled. The first layer 9 (FIG. 5) is the layer of epoxy resin, and a second layer 10 (FIG. 5) is a backing layer made of polyester. 
     The mold 6, together with the devices 11, are made to advance inside the laminating machine so that the devices 11 are subjected exclusively to the action of the upper roller, i.e. the roller that faces the protruding parts 8 of the power devices. The two layers 9 and 10 are deposited in a heated atmosphere to facilitate contact between the epoxy resin 9, which is still in the gel state, the mold 6, and the power devices 11. 
     The entire system is then cooled so as to stop the movement of the gel that constitutes the layer 9 of epoxy resin. Once the gel 9 has cooled, the backing layer 10 is peeled off so as to leave the gel layer 9 exposed. Since the gel-layer is spread over the entire work area (the mold 6 and the devices 11), it is selectively removed from the parts that cover the mold 6. 
     To remove layer 9 from the parts that cover mold 6, a mask 12 (FIG. 7) is applied that is provided with openings that allow the passage of ultraviolet rays only on the regions of the resin 9 that cover the power devices 11. These regions are accordingly exposed to UV rays 13 (FIG. 7) so as to cure or polymerize them. As a result, these regions cannot be removed with chemical treatments. 
     To remove the resin 9 placed on the mold 6, a chemical treatment with a saline solution of sodium carbonate, Na 2  CO 3 , is performed, etching the regions that have not been exposed and cured by the UV rays 13. In this manner, the structure shown in FIG. 8 is obtained, wherein the insulating layer 9 is left only on the devices 11. 
     The insulating layer 9 (epoxy resin) is still not mechanically strong (i.e., it can still be scratched and damaged mechanically). To improve the mechanical strength of the insulating layer 9, a further curing or polymerization with UV rays 13 is performed. Finally, heat curing in an oven is performed for about 8 hours at approximately 170° C., further polymerizing both the insulating layer 9 and the package 2 of the power device 11. To achieve better results, the upper part 8 of the package 2 should protrude only by an amount that is equal to or smaller than the thickness of the layer of resin 9, as shown in FIG. 6. If instead the upper part 8 protrudes by more than that amount, as shown in FIG. 5, air pockets 14a and 14b form. This effect may be undesirable, since after removal of the unwanted regions of the insulating layer 9 it leaves exposed surfaces of the device 11 and the heat sink 3. 
     The rolls of epoxy resin 9, in the dry state, are supplied by the supplier in guaranteed thicknesses. In this way it is possible to have a uniform thickness of, for example, 25 or 50 μm. It is furthermore not necessary to produce special rolls or special machines, since they are already available for the production of printed circuit boards. 
     Another advantage is that the invention uses the existing step for the heat curing of the package 2 of the device to also cure the insulating layer 9. 
     A second embodiment of the present invention shown in FIG. 9 spreads, by means of a doctor 16, gelatinous (liquid) resin 9 through a screen-printing frame 15 having a thickness that meets the thickness requirements of the above-described insulating layer that forms the devices 11. This step is followed by UV-ray curing and by heat curing as already described. The thickness of the insulating layer is controlled by the thickness of the screen-printing frame and by the density of the resin in the liquid state. 
     Devices formed in accordance with the above-described methods have been found to perform well. A film of dry resin with a thickness of 50μm insulates the heat sink from a 1000-V voltage with a loss of 5 nA. The junction-package heat resistance, R th , has been found to be on the order of 1.3-1.4×10 11  Ω, which fully meets the desired characteristics. The insulated devices have furthermore been kept inside a pressure cooker for over two hours at a temperature of about 100° C. After this treatment, the insulation did not show peeling. Likewise, the devices were subjected to dry etching with a temperature of about 175° C. for approximately eight hours, and the devices were immersed from the terminal end in solder for about 10 seconds and at the temperature of about 240° C. so as to make contact with the package over about 2 mm. The insulation did not separate in these last two cases either. 
     It is possible to use other materials for the insulating layer, such as nylon, mylar, acrylic resins, or different materials that compose the solder mask, such as acrylic materials and polyimide. 
     The insulating layer can furthermore be applied with other methods, such as for example electrochemically, chemically, by vacuum, by gluing, and so forth. 
     Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. The materials employed, as well as their shapes and dimensions, may be any required. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.