Patent Publication Number: US-2019198203-A1

Title: Surface-mountable thin film resistor network

Description:
TECHNICAL FIELD 
     The invention relates to electronic parts, especially relating to surface-mountable thin film resistor networks. 
     BACKGROUND ART 
     Japanese laid open patent publication 2012-60105 discloses a semiconductor device, which is encapsulated in a molded resin body. A lead frame includes an, island, a hanging lead connecting the island to the outer frame, and a plural of lead terminals connected to the outer flame. A chip is fixed on the island, and the chip is connected to the lead terminals by wire bonding. These are encapsulated into a molded resin body, and unnecessary outer frame portion etc. are cut away to form the semiconductor device encapsulated in the molded resin body. 
     In these semiconductor devices, the hanging lead connecting the island to the outer frame is cut away at a package end surface after encapsulated in the molded resin body. 
     SUMMARY OF INVENTION 
     Technical Problem 
     A thin film resistor network integrated array consisting of metal film is formed on a chip in a semiconductor device, which is surface-mountable such as gull wing type. When a high voltage applying test, which applies a high voltage to the device, because of repeated micro discharges, a carbonized electrically conductive path is possible to be formed on surface of the molded resin package, and then tracking phenomenon, which cause insulation breakdown, is possible to happen. 
     The invention has been made basing on above-mentioned circumstances. Thus, an object of the invention is to provide a surface-mountable thin film resistor network, which can be prevented from tracking phenomenon occurring in the high voltage applying test. 
     Solution to Problem 
     The surface-mountable thin film resistor network of the invention includes a chip on which a thin-film resistor integrated array has been formed; an island on which the chip is fixed; a plurality of lead terminals extending outward around periphery of the island; wires connecting electrodes of resistors mounted on the chip and the lead terminals; and a molded resin package that encapsulate a portion, which includes the wires; 
     wherein a hanging lead extending from the island is cut at an end surface of the molded resin package, and an electrical insulation is applied to the cut section of the hanging lead. 
     In a high voltage applying test, a carbonized electrically conductive path is possible to be formed on a surface of the molded resin package, and then tracking phenomenon, which cause an insulation breakdown, is possible to happen. According to the present invention, because an electrical insulation is applied to the cut section of the hanging lead, which is located at opposite side, a creepage distance for preventing the tracking phenomenon can be secured on the surface of the molded resin package. Then in the high voltage applying test, generation of tracking phenomenon can be suppressed at wide voltage area. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a plan view of one section of a lead frame according to an embodiment of the present invention. 
         FIG. 1B  is a plan view at a step, in which connection portions of outer frame, tie bar etc. have been cut away from the lead frame of  FIG. 1A . 
         FIG. 2A  is a side view of the package, in which the cut section of the hanging lead is exposed, according to the embodiment of the invention. 
         FIG. 2B  is a side view of the package, in which the electrical insulation is applied to the cut section of the hanging lead, according to the embodiment of the invention. 
         FIG. 3A  is a plan view of the package according to the embodiment of the present invention. 
         FIG. 3B  is a front view of the package according to the embodiment of the present invention. 
         FIG. 4  is a graph, which illustrates an example of the result of the high voltage applying test. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention will be described below with referring to  FIG. 1A  through  FIG. 4 . Like or corresponding parts or elements will be denoted and explained by same reference characters throughout views. 
       FIG. 1A  illustrates a manufacturing step of the surface-mountable shin film resistor network. A chip  13  is fixed on an island  12  of a section  11  of a lead frame. A plural of lead terminals  14  is extending outward around periphery of the island  12 . An end of lead terminal  14  is connected to a corresponding electrode, that is, an end of a resistor (not shown) by a wire  15 . These are encapsulated in a molded resin package  20 . The lead frame consists of a thin plate of copper or copper alloy and a lot of sections  11  is continuously formed. 
     A thin-film (metal film) resistor network integrated array has been formed on the chip  13 . A semiconductor silicon chip is used for the chip  13 . In the embodiment, 12 pieces of resistor bodies are formed on the chip (not shown). Both ends of 12 resistor bodies, that is, total 24 ends of 12 resistor bodies are connected to total 24 lead terminals  14  by wires  15 . The chip  13  is not limited to a semiconductor chip, but also ceramics chip etc. can be used. 
     A tie bar  16  of one side intersects and connects total 12 lead terminals  14  of one side. Also, total 12 lead terminals of one side are connected to outer frame portion  17 . An island  12  on which chip  13  is fixed, is connected to outer frame portion  17  via hanging leads  18 . A portion  20  surrounded by dashed line in  FIG. 1A  and  FIG. 1B  is encapsulated in molded resin package. The portion  20  encapsulates full of island  12 , chip  13  and wires  15 , and a part of lead terminals  14  and hanging leads  18 . 
       FIG. 1B  illustrates a step that unnecessary part of the leadframe have been cut away. That is, unnecessary part includes connection portions of tie bar  17  with lead terminals, where tie bar  17  intersects and connects lead terminals, connection portions of outer frame portion  17  with lead terminals, and connection portions of hanging leads  18  with outer frame portion  17 . At the step, a lot of lead terminals  14  is projected from the molded resin package  20 . After then, lead terminals  14  are bended, and then, a surface-mountable thin film resistor network (electronic parts) shown in  FIGS. 2A-2B  and  FIGS. 3A-3B  has been formed. 
     As an example, the electronic parts has a size that a length of molded resin package  20  is about 9 mm, a width of the package  20  is about 4 mm, and a height of the package  20  is about 2 mm. The lead terminal  14  has a size that a pitch is 0.635 mm and a width is 0.25 mm. Other than 24 pin-type, there are 20 pin-type, 16 pin-type and so on. Also, these have similar structure and sizes. 
     The hanging lead  18  is cut at an end surface  20   a  of length direction of the molded resin package  20 . Accordingly, a cut section  18   a  of the hanging lead  18  is exposed at an end surface  20   a  of the molded resin package  20  (see  FIG. 2A ). And, an electrical insulation  21  consisting of an insulative resin film or an insulative inorganic film is applied to the cut section  18 A of the hanging lead  18  (see  FIG. 2B ). 
     As the material for the insulative inorganic film, a silicon nitride film, a silicon oxide film, or an alumina film etc. can be used. As the material for the insulative resin film, an epoxy system resin film, or a polyimide system resin film, etc. can be used. As the method for applying an electrical insulation, the insulative resin film can be formed by spreading with dispenser or by dipping in liquid-phase resin and drying. Also, the insulative inorganic film can be formed by thermal oxidization method, CVD method, or sputtering method. 
     That is, the cut section  18   a  is covered by insulative resin or inorganic film. The electrical insulation  21 , which covers the cut section  18   a  of the hanging lead  18  at an end surface or both end surfaces of the molded resin package, may be an insulating material. Accordingly, the creepage distance of discharging path can be made longer, and higher breakdown voltage can be obtained in high voltage applying test. 
     In the high voltage applying test, a high voltage is applied between lead terminals located at opposite side (for example, P 1  and P 24 ), then increasing the voltage, and measuring the break down voltage (see  FIG. 3A ).  FIG. 4  illustrates a graph, which shows an example of a result of a high voltage applying test.   mark (in  FIG. 4 ) shows no electrical insulation  21 , that is, the cut section  18   a  of hanging lead  18  is exposed (see  FIG. 2A ). □ mark (in  FIG. 4 ) shows the electrical insulation  21  covered, that is, the cut section  18   a  of hanging lead  18  is covered by the electrical insulation  21  (see  FIG. 2B ). 
       FIG. 4  shows that the discharge frequency when increasing applied voltage is compared by the cut section  18   a  being covered by the electrical insulation  21  or not. When increasing applied voltage, in the case of no electrical insulation  21  on the cut section  18   a  of the hanging lead  18 , the discharge starts at A kV (see  FIG. 4 ). However, when increasing applied voltage, in the case of electrical insulation  21  applied on the cut section  18   a  of the hanging lead  18 , the discharge starts at B kV (see  FIG. 4 ). 
     According to the result of high voltage applying test, by insulating material such as insulative resin film or insulative inorganic film etc. covering on the cut section  18   a  of hanging lead  18 , the creepage distance of discharge path can be made longer. And, while maintaining same package structure of same size and same function, it is improved in withstand high voltage resistance characteristics. 
     As shown (  in  FIG. 4 ), in case of the cut section  18   a  of hanging lead  18  exposed (see  FIG. 2A ), the discharge path is thought to be from lead terminal (P 1 )→hanging lead  18 →island  12 →hanging lead  18 →lead terminal (P 24 ). (see  FIG. 1B ) 
     As shown (□ in  FIG. 4 ), in case of the cut section  18   a  of hanging lead  18  covered by electric insulation  21  (see  FIG. 2B ), the discharge path is thought to be from lead terminal (P 1 )→diagonally on the package  20 →via any one of P 13 -P 24 →lead terminal (P 24 ). (see  FIG. 3A )
 
Accordingly the creepage distance of discharge path can be fixed to be longer, and higher voltage can be applied corresponding to the creepage distance fixed to be longer.
 
     Then the effect by covering on the cut surface  18   a  with the electrical insulation  21  enables not only to extend the creepage distance of the discharge path but also to suppress the invasion of moisture etc. into the package. That is, corrosion of wiring can be prevented, which is caused by cell reaction with the moisture or the impurity on the surface of the package and wiring metal. 
     Although embodiments of the invention have been explained, however the invention is not limited to above embodiments, and various changes and modifications may be made within the scope of the technical concept of the invention. 
     INDUSTRIAL APPLICABILITY 
     The invention can be applicable for the electrical parts such as thin resistor networks, which is encapsulated in molded resin package.