Abstract:
A leadframe according to this invention is formed by bonding a single leadframe with a substrate using adhesive film or double-sided adhesive resin film, which is divided and attached to two or more predetermined points between them. This reduces the quantity of gas or contaminants generated from adhesives. Also, this results in the reduction of the stress generated during heat treatment of the leadframe and also in the elimination of warping of the lead frame due to thermal stress. Cracking does not occur on the resin because resin is removed easily and assuredly, and no air is left behind. This contributes to high reliability and increased productivity. The lead frame is further formed by bonding a plurality of metal substrates of different materials to single leadframe. This through more stable thermal behavior high reliability.

Description:
This application is a continuation of application Ser. No. 07/976,708 filed Nov. 16, 1992, now abandoned, which is a continuation of application Ser. No. 07/588,903, filed on Sep. 27, 1990, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a leadframe to be used for semiconductor assembly, and in particular to a leadframe of resin mold type to be used for high-speed and high heat-releasing type LSI, IC, etc. 
     As the method for chip packaging in semiconductor components such as LSI, IC, etc., a method to package the semiconductor chips on leadframe by resin mold or a method to package it by ceramics have been adopted in the past. 
     The resin mold package has high productivity of LSI and IC and it is produced at low cost and by mass production, while it is not suitable for highly heat-releasing type chips because heat-releasing property is low. Ceramic package can maintain the better characteristics of LSI and IC, while production cost is high and it is not suitable for mass production. 
     On the other hand, there are strong demands on small size and lightweight components in electronics industry. To cope with such demands, semiconductor package is now increasingly produced in smaller size and also with high pin count in the same size, and the manufacturing technique for such purpose has also been developed. To meet the demands for high pin count, it is necessary to shorten the distance between inner leads and to make inner lead finer and longer. However, when inner leads are made finer and longer, inner leads touch each other as they are bent during resin molding or when they are transported, and the reliability is deteriorated. For this reason, it has been customary in the past as shown in FIG. 8 to attach the tape 02 on many inner leads 01, and its strength is increased by attaching and fixing these inner leads 01. This requires, however, much labor for taping. 
     In this respect, a low priced resin mold package has been proposed in the Japanese Patent Publication No. 63-246851. This package is provided with the advantages of ceramic package and also maintains high-strength inner lead. This Publication discloses a multi-layer lead frame, in which the substrates consisting of polyimide film and metal plates are piled up and bonded on normal metal lead frame, and it is suitable for LSI with high heat-releasing property and high-speed type IC. 
     In the conventional type multi-layer leadframe, 4-10 leadframes are bonded together by continuous (serial form) side rail, and the substrates are produced independently from each other. Multi-layer leadframes are manufactured in such manner that, to each of the serial form lead frames, each substrate is bonded using adhesive or resin film having adhesives on both sides by aligning each substrate with the jig hole formed on the serial form side rail. 
     In the leadframe, as described in the above Patent Publication, leadframe is bonded on substrate or substrates are bonded with each other on the same plane, using double-sided adhesive tape, i.e. polyimide film coated with adhesive on both sides. This may result in the generation of gas from adhesive when it is hardened, or contaminants are increased and the reliability as package is deteriorated. 
     Also, resin tends to flow or withdraw poorly during molding resin sealing, and gaps are likely to occur. As the result, the air is left in the product, and productivity is deteriorated. 
     Further, because double-sided adhesive tape based on polyimide film is used, it is impossible to reduce the thickness of the package. 
     Because adhesive is coated on the entire surface of polyimide, stress is generated due to thermal expansion coefficient or thermal shrinkage of adhesive or polyimide film, and lead frame is warped or resin is cracked after packaging. The lead frame as described in the above Patent Publication or multi-layer leadframe now under development are produced by bonding a single metal material on leadframe. Because thermal behavior of adhesive, resin film having adhesive on both sides, leadframe material and metal substrate differ from each other, adhesive force is decreased during manufacturing process or leadframe may be warped. 
     Further, because each substrate is bonded with leadframe, it requires much labor, and also it is difficult to align lead frame with substrate. Thus, the productivity is not high. 
     In addition, the close fitness or sealing between metal substrate and mold resin are influenced by the stress or the shortage in resin quantity, and this results in the decline of reliability. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to offer a leadframe capable with higher reliability and a method to manufacture such leadframe. 
     It is another object of this invention to offer a leadframe, which can be produced with higher productivity and with less thickness and a method to manufacture such leadframe. 
     It is still another object of the invention to offer a leadframe hardly influenced by heat and a method to manufacture such leadframe. 
     To attain the above objects, the leadframe of this invention comprises a leadframe consisting of a plurality of leads, and single-layer or multi-layer substrates consisting of metal or resin are bonded on said leadframe, characterized in that said leadframe and said substrates are bonded together by adhesive film or resin film having adhesive on both sides furnished at a plurality of the predetermined points between said leadframe and said substrates. 
     The method for manufacturing the leadframe according to this invention is characterized in that adhesive film or resin film having adhesive on both sides are divided and attached to a plurality of the predetermined points of said leadframe, and that said substrate is bonded with said adhesive film or said adhesive of said resin film. 
     Further, the method for manufacturing the leadframe of this invention comprises lead frame consisting of a plurality of leads, and single-layer or multi-layer substrate consisting of metal or resin are bonded to said leadframe characterized in that adhesive film or resin film having adhesive on both sides are divided and attached to a plurality of predetermined points on said substrate, and that said lead frame is bonded with said adhesive film or said adhesive of said resin film. 
     Further, the method for manufacturing the leadframe according to the invention is a method to manufacture a leadframe, which comprises leadframe consisting of a plurality of leads and single-layer or multi-layer substrate consisting of metal or resin, characterized in that said leadframe and said substrate are bonded by adhesive film or resin film having adhesive on both sides and being divided and furnished on a plurality of predetermined points between said leadframe and said substrate, and that said adhesive film or resin film having adhesive on both sides are simultaneously bonded with said leadframe and said substrate. 
     The method for manufacturing the leadframe according to this invention is a method to manufacture leadframe, which comprises leadframe consisting of a plurality of leads and single-layer or multi-layer substrate consisting of metal or resin is bonded on said leadframe, characterized in that both said single leadframe and said single substrate are formed by a plurality of the same serial chains, and that the single leadframe and the single substrate are joined together in chain-like serial form. 
     Further, the method for manufacturing leadframe according to this invention comprises single leadframe formed by a plurality of leads and single layer or multi-layer metal or resin bonded to said single leadframe, characterized in that said single leadframe and said single substrate are formed by a plurality of the same serial chains and that the single leadframe and the single substrate are bonded together by the methods such as adhesive film, resin film having adhesive on both sides, welding or caulking in chain-like serial form. 
     The method for manufacturing leadframe according to the invention is characterized in that said single leadframe and said single substrate are aligned by jig holes formed on common points of the chain-like serial form single leadframe and the single substrate. 
     Further, the method for manufacturing leadframe according to the invention is characterized in that said serial form single leadframe and said serial form single substrate are independently separated in the gap of said lead before the assembling process of chips to be mounted, during the chip assembling process or after resin molding. 
     Further, the leadframe according to this invention is a leadframe formed by a plurality of metal substrates bonded to the single leadframe having a plurality of leads, characterized in that said plurality of metal substrates are formed by different materials. 
     Further, the leadframe according to the invention is a leadframe comprising a plurality of metal substrates bonded to single leadframe having a plurality of leads, characterized in that said plurality of metal substrates have different thickness. 
     Further, the leadframe according to the invention is a leadframe comprising a plurality of substrates bonded to single leadframe having a plurality of leads, characterized in that different surface treatment is provided on said plurality of metal substrates. 
     Further, the leadframe according to the invention is a leadframe comprising a plurality of metal substrates bonded to single leadframe having a plurality of leads, characterized in that leadframe is formed by adequately combining said plurality of metal substrates of different material, different thickness and different surface treatment. 
     Further, the leadframe according to the invention is characterized in that said plurality of metal substrates bonded mutually or said single leadframe is bonded to said metal substrate by the insulating adhesive film or by resin film having double-sided adhesive. 
     In the leadframe of this invention produced by such arrangement, said adhesive film or said resin film is partially divided and provided between said single leadframe and said substrate, and this reduces the quantity of gas or contaminants generated from adhesive. Also, it is possible to reduce the stress, which is generated during heat treatment of leadframe and to eliminate the warping of leadframe due to thermal stress. Moreover, resin flows through the gap between the adjacent adhesive films or resin films during resin molding, and this eliminates the gaps because resin is withdrawn easily and assuredly. For this reason, the air is not left behind, and this eliminates the cracking of resin by the moisture remaining in such air. 
     Thus, the reliability as package is ameliorated, and the productivity is also extensively improved. 
     If adhesive film is used to bond single leadframe and inner lead together, the total thickness of multi-layer leadframe becomes thinner, and more compact package can be obtained. 
     Further, serial form single substrate is positioned and bonded to serial form single leadframe. This results in shorter time to bond leadframe with substrate and higher accuracy for positioning of leadframe and substrate. This further ensures high productivity. 
     Further, leadframe is formed by combining two or more metal substrates with different material, different thickness and different surface treatment according to the invention, more stable thermal behavior is provided. This also extensively enhances the reliability of leadframe. 
     In addition, a plurality of metal substrates are bonded by the insulating adhesive film or the resin film having adhesive on both sides according to this invention, and this contributes to the formation of high-speed and highly heat-releasing multi-layer leadframe. 
     Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification. 
     The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of an embodiment of a leadframe according to the present invention; 
     FIG. 2 is a sectional view along the line II--II in FIG. 1; 
     FIG. 3 is a drawing to explain the manufacture of the leadframe in this embodiment; 
     FIG. 4 and FIG. 5 are a plan view of the leadframe and substrate of another embodiment of this invention; 
     FIG. 6 is a plan view of still another embodiment of the invention; 
     FIG. 7 is a sectional view along the line VII--VII of FIG. 6; 
     FIG. 8 is a plan view of a conventional type leadframe. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, description is given on the embodiments of this invention in connection with the drawings. 
     As shown in FIG. 1 and FIG. 2, leadframe 1 is formed in 3-layer structure, comprising a single leadframe 2 and a substrate 4 made of metal or resin and being bonded to said single leadframe 2 by adhesive film or resin film 3 having double-sided adhesive. The single leadframe 2 comprises a number of leads 2a, 2a, . . . , and it is formed by etching or stamping similar to the conventional method. Also, similarly to the conventional type products, the single leadframe 2 is furnished with gold or silver plating at necessary points to facilitate wire bonding. 
     The adhesive film or the resin film having double-sided adhesive on both sides is divided and furnished at minimum number of points a, a, . . . , on bottom surface of the bonding area on the single leadframe 2 in order to fix the substrate 4. Therefore, adhesive film or resin film is furnished with adequate spacing to each other. Adhesive film is, for example, thermosetting resin formed in film-like shape. Resin film has a base of polyimide film with thermosetting resin on both sides in form of double-sided tape. 
     To obtain the leadframe 1 with the above arrangement, two or more adhesive films or resin films 3 are attached on said predetermined points of the single leadframe 2 by taping machine as shown in FIG. 3. On the bottom surface of the adhesive film or resin film 3 furnished on the bottom surface of single leadframe 2, substrate 4 is mounted by aligning the position with high accuracy, and this substrate 4 is bonded to single leadframe 2 by the method such as press-fitting or thermal press-fitting. 
     When the leadframe 1 thus obtained is sealed with mold resin, resin flows through the gap between the adjacent adhesive films or resin films, and this eliminates the gaps and facilitates easy and assured withdrawal of resin. As the result, the air is not left behind, and no cracking of resin occurs due to moisture remaining in such air. 
     Tables 1 to 4 show the results of the comparison between the leadframe 1 of this invention and the conventional type leadframe. In this case, Table 1 shows the results with the lead frame according to the invention, Table 2 the quantity of generated gas, Table 3 the number of voids generated after molding, and Table 4 the generated stress. 
     
                       TABLE 1______________________________________Leadframe     Attachment     Substrate______________________________________Embodi-  0.15 mmt;  20 μm adhesive film;                            Alloy 42 +ment 1 alloy 42   divided into 5 Ag platingEmbodi-  0.15 mmt;  20 μm adhesive film;                            Glass epoxyment 2 alloy 42   divided into 8 substrateEmbodi-  0.15 mmt;  20 μm adhesive on                            Polyimidement 3 copper     both sides     substrate;             25 μm polyimide film;                            Copper + Ag             divided into 5 platingEmbodi-  0.15 mmt;  10 μm adhesive onment 4 copper     both sides             50 μm polyimide film;             divided into 8______________________________________ 
    
     
                       TABLE 2______________________________________Quantity of the generated gas           Quantity generated______________________________________Prior art         100Specimen 1 of this invention             30Specimen 2 of this invention             25______________________________________ 
    
     
                       TABLE 3______________________________________The number of voids after molding         Number of voids generated______________________________________Prior art       6/9Specimen 1 of this invention           0/5Specimen 2 of this invention           0/5______________________________________ 
    
     
                       TABLE 4______________________________________Stress      Warping of              Ratio of cracking      leadframe              on package______________________________________Prior art    0.35 mm   100Specimen 1 of        0.12 mm   35this inventionSpecimen 2 of        0.13 mm   30this inventionSpecimen 3 of        0.21 mm   50this inventionSpecimen 4 of        0.26 mm   50this invention______________________________________ 
    
     As it is evident from Table 2-4, the quantity of generated gas, warping of the leadframe due to stress and the ratio of cracking are extensively reduced in the leadframe 1 of this invention. 
     FIG. 4 and FIG. 5 are to explain another embodiment of the method for manufacturing leadframe 1. The same number refers to the same component in the above embodiment, and the detailed description is not given here. 
     As shown in FIG. 4, four single leadframes 2, 2, . . . are formed in serial formation connected by side rails 5, which serve as outer frame. Similarly, the substrates 6, 6, . . . as many as the single leadframes 2 are formed by the serial side rails 7, 7, . . .. On these serial side rails, jig holes 8, 8, . . . and 9, 9, . . . of the same size are furnished at the predetermined positions. The positions of these jig holes 8 and 9 are determined in relation to the center of the serial leadframe 2 and the center of the serial single substrates 6. In this case, the relation between of the positions of jig holes 9 and single substrates 6 corresponds to the relation of the positions of jig holes 8 and single leadframes 2. By inserting jig guide pins (not shown) into the jig holes 8 and 9, the positions of single leadframes 2 and single substrates 6 are automatically and accurately aligned during die attach in the assembling process or in resin molding. 
     To produce the leadframe with such arrangement, the serial single leadframe 2 is aligned with the serial single substrate 6 by inserting guide pins into jig holes 8 and 9, and these are bonded together by press-fitting or thermal press-fitting in serial formation using adhesive film or resin film having adhesive on both sides. 
     Then, single leadframe 2 and single substrate 6 are individually separated by machine at the gap of the lead 2a before assembling process of the chips to be mounted, during chip assembling process or after resin molding. 
     Tables 5 to 6 summarize the results of the comparison between the lead frames produced by the manufacturing method of this embodiment and those produced by the conventional manufacturing method. In this case, Table 5 shows the bonding time of substrate and lead frame, and Table 6 the position accuracy between substrate and leadframe. 
     
                       TABLE 5______________________________________Bonding time of substrate and leadframe         Bonding time of multi-layer         leadframe______________________________________Conventional    50 sec./1 series (4 pieces)methodSpecimen of     10 sec./1 series (4 pieces)this invention______________________________________ 
    
     
                       TABLE 6______________________________________Position accuracy of substrate and leadframe       Position accuracy______________________________________Conventional  ±0.120 μmmethodSpecimen of   ±0.055 μmthis invention______________________________________ 
    
     As it is evident from Table 5 and Table 6, it is possible by the manufacturing method of leadframe of this embodiment to extensively shorten the bonding time of substrate and leadframe and to align the position of substrates and leadframes at high accuracy. This results in extensive improvement of the productivity of leadframes. 
     FIG. 6 and FIG. 7 are to explain another embodiment of the leadframe of this invention. The same number refers to the same component in the above embodiment, and detailed description is not given here. 
     As shown in FIG. 6 and FIG. 7, the leadframe 1 of this embodiment is formed in multi-layer structure, comprising two or more metal substrates 4 and 10 bonded to single leadframe 2 by adhesive film or resin film 3 having adhesive on both sides. 
     The metal substrate 4 is made of alloy 42 in the form of a ring and is bonded to outer periphery of the leadframe 1. The metal substrate 10 is made of copper material having excellent thermal conductivity and is bonded near the die pad 2b. Through such arrangement, the metal substrate 4 plays a role to protect the leadframe 1, and the metal substrate 10 efficiently disperses the heat from chip (not shown), which is assembled on the die pad 2b. This reduces the deformation or warping of the leadframe 1. 
     In the leadframe 1 of another embodiment, metal substrate 4 and 10 are made of copper material. In this case, the metal substrate 10 bonded to bottom surface of the die pad 2b of single leadframe has thickness of 1 mm, and this facilitates the release of heat. Further, the metal substrate 4 bonded to the lead unit near outer periphery of the leadframe 1 has the thickness of 0.15 mm, and this metal substrate 4 is furnished for the application as electrode. Resin seal is performed by aligning the bottom surface of metal substrate 10 for heat dispersion with the bottom surface of mold resin, and this contributes to the manufacture of the package with excellent heat releasing property. 
     In still another embodiment of the invention, metal substrate 10 is bonded to the center of a leadframe without die pad and formed from alloy 42. This metal substrate 10 is made of alloy 42 with copper plating of about 5 μm, and it is used for electrode. Metal substrate 4 is bonded to the periphery of the leadframe 1, and this metal substrate 4 is made of alloy 42 processed by palladium plating, gold plating or tin-nickel alloy plating to provide high adhesive property to resin. This contribute to the formation of a package with high reliability and strong resistance to the intrusion of moisture. 
     Further, in any of the above embodiments, high-speed and highly heat-releasing IC or LSI can be produced by using insulating adhesive film or resin film having adhesive on both sides for the bonding between metal substrates 4 and 10 or for the bonding between metal substrate 4 and 10 and single leadframe 2. 
     The present invention is not limited to the above embodiments, and various design modifications are conceivable. 
     For example, leadframe 1 is formed in 3-layer structure in the above embodiments, whereas it may be formed in multi-layer structure by furnishing adhesive film or resin film on the lower surface of metal substrate 4 or 10 and by bonding another metal substrate. 
     In the above embodiments, adhesive film or resin film 3 having adhesive on both sides are attached first on the single leadframe 2, whereas the adhesive film or the resin film 3 having adhesive on both sides may be attached first on the substrate or single leadframe 2, and adhesive film or resin film 3 having adhesive on both sides and the substrate 4 can be bonded together at the same time. 
     Further, the serial single leadframe and serial single substrate can be bonded together in serial form by welding or caulking. 
     Further, in the above embodiments, adhesive film or double-sided adhesive resin film 3 is divided into two or more points, whereas adhesive film or double-sided adhesive resin film 3 may be continuously furnished on metal substrates 4 and 10 and on single leadframe 2. 
     Further, it is possible to form lead frame by adequately combining two or more metal substrates of different materials, different thickness and different surface treatment. 
     It is also possible to bond single leadframe and two or more single substrates by welding or caulking.