Abstract:
A package for semiconductor devices is encapsulated in an insulating resin. Multiple conductive leads project from one side of the package. Alternating leads are provided with an insulating coating which projects along a portion of their length. Leads which are not insulated are bent so as to displace them from the plane of the coated leads and space them further away from the coated leads. The bent leads are displaced a sufficient distance to provide a separation in air consistent with spacing standards for high voltage devices.

Description:
This is a Continuation-in-part of application Ser. No. 07/872,777, filed Apr. 23, 1992 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device encapsulated in resin and completely insulated for high voltages. 
     2. Description of the Prior Art 
     Semiconductor devices encapsulated in resin, completely insulated, and capable of resisting voltages up to 1500 V AC are known. An example of such a device is illustrated in U.S. Pat. No. 4,888,307. FIG. 12 of such patent is reproduced herein as FIG.  1 . 
     Such devices are not suited for use in the range of high voltages, because, in such high ranges, they do not comply with international electrical insulation and safety standards. This is particularly true for very small electrically insulated packages containing semiconductor chips. An example of such a package is known internationally by the designation “TO 220”. Other packages of the same dimensions have the same problem. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a semiconductor device encapsulated in resin and completely insulated, capable of resisting high voltages of at least 2250 V AC. In one embodiment the invention proposes to provide devices with the same overall dimensions as the TO 220 device and capable of resisting voltages equal to or greater than 2250 V AC. This is done while fully complying with applicable international standards. 
     Another object is to provide a device which is economical, reliable, and safe. 
     Therefore, in accordance with the present invention, a package for semiconductor devices is encapsulated in an insulating resin. Multiple conductive leads project from one side of the package. Alternating leads are provided with an insulating coating which projects along a portion of their length. Leads which are not insulated are bent so as to displace them from the plane of the coated leads and space them further away from the coated leads. The bent leads are displaced a sufficient distance to provide a separation in air consistent with spacing standards for high voltage devices. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     To further clarify the innovative principles of the present invention and its advantages as compared with the known art, possible embodiments applying said principles are described below as non-limiting examples, with the aid of the annexed drawings. 
     In the drawings: 
     FIG. 1 shows a perspective view (not to scale) of an electrically insulated plastic power transistor in accordance with the known art; 
     FIG. 2 shows a perspective view (not to scale) of an electrically insulated plastic device for high voltages in accordance with the present invention and placed in contact on a dissipator; 
     FIG. 3 shows a plan view of the device shown in FIG. 2; 
     FIG. 4 shows a cross-section of the device shown in FIG. 2; 
     FIG. 5 shows a perspective view of another plastic device in accordance with the present invention; and 
     FIG. 6 shows a cross-section view of the device shown in FIG. 2 in accordance with a variation which calls for a heat-dissipating plate incorporated in the package. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In summary, the semiconductor device encapsulated in resin and completely insulated for high voltages in accordance with the present invention includes at least one chip of semiconductor material. The chip is encapsulated in a resin body acting as the covering, preferably with a first side designed to be place in contact with an external dissipator. A metallic heat dissipation plate may possibly be provided on this device. The package has several conductive leads emerging in succession from one side of the covering, with such leads having the following properties: 
     Alternately, in the order in which they appear along the side of the package, the conductive leads have their surface exposed or covered for a certain length by an insulating coating. Exposed leads have a bend at a selected distance from the resin body, while covered leads are encapsulated along their length by projections of the resin body for at least the selected distance. 
     Each conductive lead not provided with an insulating coating has a first bend in a direction opposite from the side designed to be placed in contact with the external dissipator. Each such lead also contains a second bend in the direction opposite to that of the first. The second bend occurs after a length along the conductor of a value such that the distance between the end of such conductive lead and the end of any nearby leads is not less than the limit which standards impose as the minimum distance in air between the conductive leads. Such minimum distance is made with reference to the maximum value of the insulation voltage the device is intended to assure. 
     In each conductive lead provided with an insulating coating, the section covered by the coating is selected to be a minimum length (L) greater than a distance from the body at which bends are formed in the other leads. L is selected so that the clearance and creepage distances between the lead and the nearby leads is not less than the values called for by standards for the maximum insulation voltage which the device is intended to assure. 
     The clearance and creepage distances between any two conductive leads not immediately adjacent are no less than the values called for by the standards. 
     FIGS. 2,  3 , and  4  show perspective, plan and cross-section views, respectively, (not in scale) of an electrically insulated device suitable for use with high voltages in accordance with the present invention. The primary characteristics which distinguish the device concern the shape of the conductive leads  28  and  29 , and the resin coating  21  of the central lead. 
     As regards the shape of the leads it will be apparent that they follow each other with their tips in a staggered position. The tip of a given lead lies on a horizontal plane different from that on which lie the tips of the immediately proceeding or immediately following leads. This occurs because the outside leads  28 ,  29  have two bends, almost at right angles, near the plastic covering. The leads have their tips on a plane parallel to the dissipator  26  and at a distance therefrom greater than the tip of lead  27  and the dissipator. 
     The distance between the two planes, i.e., the distance between the two right-angle bends of the side leads, is selected such that the distance between the end of the central lead and the ends of the side leads is not less than the limit which applicable standards impose on the clearance distance for the maximum insulation voltage which the device must withstand. The clearance distance is defined as the minimum distance in air between two conducting elements, i.e., between close leads and between leads and the external dissipator. 
     The coating  21  of the central lead is made of the same molding resin and during the mold transfer process as the remainder of the package. Its length L (FIG. 3) must meet primarily the following two dimensioning criteria. 
     In accordance with the first criteria, the length L must be such as to ensure that the minimum distance M in air between the central lead section nearest the coating  21  and the side lead sections between the two consecutive bends is not less than the clearance distance. In this manner the clearance distance imposed by standards is complied with for every part of two adjacent leads. The clearance distance is met not only between the ends of the leads, but also between the initial uncovered section of the central lead and the bend of the side leads. 
     In accordance with a second criteria, the length L must also be such as to ensure a creepage distance between the central lead and the adjacent leads not less than that imposed by the standard. The creepage distance is the shortest path between two leads, or between a lead and the external dissipator, measured along the surface of the package. In the case of FIG. 3, the creepage distance between the lead  27  and the lead  29  is N=e+f+g. 
     Since the minimum distances between the two side leads measured in air, and along the surface of the package, are also not less than the clearance and creepage distances respectively called for by the standards for the reference voltage, with the above dimensioning the clearance and creepage distances between any lead and the others is fully complied with. By way of example, in the case of a device with the same overall dimensions as a TO 220 device, the length L is 6 mm and allows clearance and creepage distances between the central lead adjacent leads of 5.3 mm and 7.4 mm respectively. 
     Therefore, the length L together with the particular conformation of the side leads allows clearance distances M and creepages distances N between the leads not less than those called for by the standards for voltage of 2250 V AC. The distances called for by the standards are 3.0 MM and 3.2 MM, respectively. 
     Referring to FIG. 4, the clearance and creepage distances of the leads in relation to the external dissipator  26  are illustrated as distances P and Q=a+b+c. Compliance with the standard values is achieved by appropriately selecting the values of P and b. Once P has been selected, the a+c portion of Q is no longer variable. 
     Again in the case of a device having overall dimensions the same as the TO 220 standard, the distance P is set at 3.1 mm and the distance B at 1.2 mm. This fully complies with the aforesaid values of 3.0 mm and 3.2 mm, respectively, prescribed by the standards for the clearance and creepage distances which the leads must meet with relation to the external dissipator as well as each other. 
     In the embodiment example described above, reference has been made to creepage and clearance which permit an electrical insulation of 2250 V AC. However, it will be apparent to those skilled in the art that the general criteria of dimensioning indicated above are applicable also when it is intended to ensure greater insulation voltages. 
     In the example of FIG. 2, the device rests on the external dissipator  26  and is electrically insulated therefrom. However, if installation of the device does not require the dissipator, because the device can be mounted in free air, the device can withstand greater insulation voltages than those indicated above. The clearance and creepage distances of 5.3 mm and 7.4 mm respectively, in the example described above, ensure an electrical insulation of at least 2800 V AC. The standard for such voltage calls for 4.2 mm and 5.0 mm clearance and creepage distances, respectively. 
     In conclusion, the above-described inventive characteristics taken together make it possible to provide an encapsulated semiconductor device completely insulated in resin and suitable for containing at least one bipolar transistor chip or other similar semiconductor device. The device thus packaged is capable of withstanding higher voltages, for given overall dimensions, than conventional devices. 
     Although a single preferred embodiment of the present invention has been illustrated and described, it will be apparent to those skilled in the art that numerous variations and modification can be made thereto without going beyond the scope of the present invention. 
     For example, the solution described above is not necessarily limited to a device having three conductive leads, but can be extended to a device having more than three leads. For example, a five lead package may be fabricated, and an example of such a package for containing a five terminal circuit is illustrated in FIG.  5 . The dimensioning criteria indicated for the three lead device is easily directly extended to devices having more than three leads. 
     As a further alternative, the encapsulated resin device, instead of having a cross-section as shown in FIG. 4, could have the cross-section of FIG.  6 . In this device, the copper plate  31  is welded by means of an insulating layer  32  to a copper plate  33 . The chip is directly welded to the copper plate  31  as known in the art. Insulating layer  32  preferably consists of material such as Al 2 O 3 , AlN, resin, and so forth. Copper plate  33  acts as a dissipator incorporated in the package, and may optionally be placed in contact with another external dissipating plate. 
     While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.