Patent Publication Number: US-7709937-B2

Title: Method of manufacturing semiconductor device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a division of application Ser. No. 11/003,361 filed Dec. 6, 2004 now U.S. Pat. No. 7,285,444. 
     CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese patent application No. 2003-431915 filed on Dec. 26, 2003, the content of which is hereby incorporated by reference into this application. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a semiconductor device and a method of manufacturing the same, and more particularly to technology which is effective for a semiconductor device with exposed leads on the back surface of a sealing body. 
     A conventional semiconductor device consists of a semiconductor chip with multiple input/output pads, a chip mounting plate bonded to the semiconductor chip using an adhesive agent, inner leads with upward burrs at the most remote end from the chip mounting plate, and conductive wires for electrically connecting the input/output pads of the semiconductor chip with the inner leads. Here, the bottoms of the chip mounting plate and inner leads are exposed to the outside. (See Patent Literature 1.) 
     [Patent Literature 1] Japanese Unexamined Patent Publication No. 2001-77278 (FIG. 1) 
     SUMMARY OF THE INVENTION 
     In QFN (Quad Flat Non-leaded Package) semiconductor devices, leads are partially exposed in the peripheral area of the back surface of a sealing body and used as external terminals. In response to demand for a more compact QFN semiconductor device model, a package structure which uses a common GND pin and/or a common power pin to decrease the number of input/output pins has been developed. 
     In other words, a tab (chip mounting plate) which bears the semiconductor chip is exposed on the back surface of the sealing body and used as a common ground terminal. A protruding part of the tab from the semiconductor chip is connected with plural pads for grounding the semiconductor chip by wires and used as a common ground terminal (this kind of wire bonding is hereinafter called down-bonding). When the QFN semiconductor device is mounted on a board, the tab and the ground terminal of the board are directly connected by soldering. 
     In the above QFN structure, since sealing resin easily absorbs moisture, a high temperature reflow process for loading solder balls or mounting the QFN on a board, can cause peeling between the tab and resin or between leads and resin. Further, if a temperature cycle test should be conducted in this condition, stress would extend to wire connections of the tab, resulting in increase in the stress at the wire connections. Repeated stress at the wire connections might lead to a wire disconnection. 
     Consequently, the QFN quality might deteriorate. 
     An object of the present invention is to provide a semiconductor device which improves quality and a method of manufacturing the same. 
     Another object of the present invention is to provide a semiconductor device which permits cost reduction and a method of manufacturing the same. 
     A further object of the present invention is to provide a semiconductor device which improves reflow resistance and a method of manufacturing the same. 
     The above and further objects and novel features of the invention will more fully appear from the following detailed description and accompanying drawings. 
     Typical aspects of the invention will be briefly outlined below. 
     According to one aspect of the invention, a semiconductor device includes: a semiconductor chip which has a main surface, a back surface and side surfaces, and a semiconductor element and a plurality of electrodes which are formed over the main surface; a chip mounting plate which has a main surface, a back surface and side surfaces and is connected with the semiconductor chip; a common lead which is located outside of the semiconductor chip and connected with the chip mounting plate; a sealing body which resin-seals the semiconductor chip; a plurality of leads which are arranged in a line around the semiconductor chip as partially exposed on the back surface of the sealing body; a plurality of first wires which connect the plural electrodes of the semiconductor chip and the corresponding plural leads respectively; and a second wire which connects the electrode of the semiconductor chip and the common lead. The sealing body has a continuous portion which continues from the side surface of the semiconductor chip to its back surface to the side surface of the chip mounting plate. 
     According to another aspect of the invention, a semiconductor device includes: a semiconductor chip which has a main surface, a back surface and side surfaces, and a semiconductor element and a plurality of electrodes which are formed over the main surface; a chip mounting plate which is connected with the back surface of the semiconductor chip; a connecting portion for connection with the chip mounting plate; a common lead which is located outside of the semiconductor chip and connected with the chip mounting plate through the connecting portion; a sealing body which resin-seals the semiconductor chip; a plurality of leads which are arranged in a line around the semiconductor chip as partially exposed on the back surface of the sealing body; a plurality of first wires which connect the plural electrodes of the semiconductor chip and the corresponding plural leads respectively; and a second wire which connects the electrode of the semiconductor chip and the common lead. The second wire is connected to an area of the common lead other than its intersection with the connecting portion. 
     According to another aspect of the invention, a method of manufacturing a semiconductor device includes the steps of: preparing a lead frame having a chip mounting plate, a common lead connected with, and located outside of, the chip mounting plate, and a plurality of leads arranged around the chip mounting plate; mounting a semiconductor chip on the chip mounting plate; connecting electrodes of the semiconductor chip with the corresponding leads through first wires; connecting electrodes of the semiconductor chip with the common lead through second wires; forming a sealing body which resin-seals the semiconductor chip and the first wires and the second wires so that a continuous portion of sealing resin which continues from a side surface of the semiconductor chip to its back surface to a side surface of the chip mounting plate is formed and the plural leads are partially exposed on the back surface of the sealing body; and cutting the plural leads from the lead frame to make a separate package. 
     Main advantageous effects which are brought about by the present invention are as follows. 
     In the semiconductor device, since the sealing body has a continuous portion which continues from a side surface of the semiconductor chip to its back surface to a side surface of the chip mounting plate, the degree of adhesion among the semiconductor chip, the chip mounting plate and the resin is increased. This prevents peeling between the chip mounting plate and the resin during a high-temperature process. Hence, stress on the common lead in a temperature cycle test is reduced, thereby preventing disconnection in the wire connections of the common lead. The quality of the semiconductor device is thus improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view showing a semiconductor device according to a first embodiment of the present invention; 
         FIG. 2  is a back view showing the semiconductor device of  FIG. 1 ; 
         FIG. 3  is a fragmentary plan view showing the relation among a tab, a common lead and a semiconductor chip in the semiconductor device of  FIG. 1 ; 
         FIG. 4  is a sectional view showing the semiconductor device of  FIG. 1 ; 
         FIG. 5  is a fragmentary sectional view showing the package structure of the semiconductor device of  FIG. 1 ; 
         FIG. 6  is a fragmentary plan view showing the wiring structure of the semiconductor device of  FIG. 1  as seen through a sealing body; 
         FIG. 7  is a sectional view showing a semiconductor device according to a variation of the first embodiment of the present invention; 
         FIG. 8  is a fragmentary plan view showing a half-etched area of a common lead in a semiconductor device according to a variation of the first embodiment of the present invention; 
         FIG. 9  is a sectional view showing a semiconductor device according to a variation of the first embodiment of the present invention; 
         FIG. 10  is a back view showing the semiconductor device of  FIG. 9 ; 
         FIG. 11  is a fragmentary plan view showing suspension leads in a semiconductor device according to a variation of the first embodiment of the present invention; 
         FIG. 12  is a fragmentary sectional view taken along the line A-A of  FIG. 11 ; 
         FIG. 13  is a sectional view showing a clamped die in a resin molding process where the suspension leads as shown in  FIG. 12  are employed; 
         FIG. 14  is a flowchart showing a dampproof packing process in a method of manufacturing a semiconductor device according to the first embodiment of the present invention; 
         FIG. 15  is a flowchart showing a non-dampproof packing process in a method of manufacturing a semiconductor device according to the first embodiment of the present invention; 
         FIG. 16  is a fragmentary plan view showing the relation among a tab, a common lead and a semiconductor chip in s semiconductor device according to a second embodiment of the present invention; 
         FIG. 17  is a fragmentary plan view showing the wiring structure of the semiconductor device according to the second embodiment of the present invention as seen through a sealing body; and 
         FIG. 18  is a fragmentary plan view showing the relation among a tab, a common lead and a semiconductor chip in a semiconductor device according to a variation of the second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the explanation of preferred embodiments given below, basically descriptions of like elements will not be repeated except when necessary. 
     Although preferred embodiments will be described below in different sections or separately on an embodiment-by-embodiment basis, the descriptions are not irrelevant to each other unless otherwise specified. They are, in whole or in part, variations of each other and sometimes one description is a detailed or supplementary form of another. 
     In the preferred embodiments described below, even when a specific numerical figure (quantity, numerical value, amount, range, etc.) is indicated for an element, it is not limited to the indicated specific numerical figure unless otherwise specified or theoretically limited to the specific numerical figure; it should be understood that it may be larger or smaller than the specific numerical figure. 
     Next, preferred embodiments of the present invention will be described in detail referring to the accompanying drawings. In all the drawings that illustrate the preferred embodiments, elements with like functions are designated by like reference numerals; and descriptions of these elements will not be repeated. 
     First Embodiment 
     The first embodiment of the invention will be described referring to  FIGS. 1 to 15 . 
       FIG. 1  is a plan view showing a semiconductor device according to a first embodiment of the present invention;  FIG. 2  is a back view showing the semiconductor device of  FIG. 1 ;  FIG. 3  is a fragmentary plan view showing the relation among a tab, a common lead and a semiconductor chip in the semiconductor device of  FIG. 1 ;  FIG. 4  is a sectional view showing the semiconductor device of  FIG. 1 ;  FIG. 5  is a fragmentary sectional view showing the package structure of the semiconductor device of  FIG. 1 ;  FIG. 6  is a fragmentary plan view showing the wiring structure of the semiconductor device of  FIG. 1  as seen through a sealing body;  FIG. 7  is a sectional view showing a semiconductor device according to a variation of the first embodiment of the present invention;  FIG. 8  is a fragmentary plan view showing a half-etched area of a common lead in a semiconductor device according to a variation of the first embodiment of the present invention;  FIG. 9  is a sectional view showing a semiconductor device according to a variation of the first embodiment of the present invention;  FIG. 10  is a back view showing the semiconductor device of  FIG. 9 ;  FIG. 11  is a fragmentary plan view showing suspension leads in a semiconductor device according to a variation of the first embodiment of the present invention;  FIG. 12  is a fragmentary sectional view taken along the line A-A of  FIG. 11 ;  FIG. 13  is a sectional view showing a clamped die in a resin molding process where the suspension leads as shown in  FIG. 12  are employed;  FIG. 14  is a flowchart showing a dampproof packing process in a method of manufacturing a semiconductor device according to the first embodiment of the present invention; and  FIG. 15  is a flowchart showing a non-dampproof packing process in a method of manufacturing a semiconductor device according to the first embodiment of the present invention. 
     The semiconductor device according to the first embodiment as shown in  FIGS. 1 to 4  is a compact leadless type one in which plural leads la are arranged in a line and partially exposed in the peripheral area of a back surface  3   a  of a sealing body  3 . According to the first embodiment, a QFN (semiconductor device)  5  is explained as an example of the above semiconductor device. 
     As illustrated in  FIG. 4 , the QFN  5  includes: a semiconductor chip  2  having a semiconductor element and plural pads (electrodes)  2   a  (see  FIG. 6 ) over its main surface  2   b ; a tab  1   b  as a chip mounting plate connected with the semiconductor chip  2 ; bus bars  1   i  as a common lead located outside of the semiconductor chip  2  and connected with the tab  1   b ; a sealing body  3  which resin-seals the semiconductor chip  2 ; plural leads  1   a  arranged in a line around the semiconductor chip  2  in a manner that their connected surfaces (partial)  1   g  are exposed in the peripheral area of the back surface  3   a  of the sealing body  3 ; suspension leads  1   e  which respectively lie in positions corresponding to the four corners of the sealing body  3  and are connected with the tab  1   b ; plural first wires  4   a  which connect plural pads  2   a  of the semiconductor chip  2  and the corresponding plural leads  1   a ; and plural second wires  4   b  which connect specific pads  2   a  of the semiconductor chip  2  with the bus bars  1   i . The sealing body  3  has a continuous portion  3   b  which continues from a side surface  2   d  of the semiconductor chip  2  to its back surface  2   c  and further to a side surface  1   h  of the tab  1   b.    
     In other words, in the QFN  5  according to the first embodiment, the bus bars  1   i  lie around the semiconductor chip  2  and plural specific pads  2   a  for grounding (GND) or power supply are connected with the bus bars  1   i  through the second wires  4   b  respectively by down-bonding so that the leads  1   a  are shared and the number of input/output pins is thus decreased for the purpose of more compactness. The bus bars  1   i  as a common lead are produced, for example, in an etching process where the leads  1   a  and the suspension leads  1   e  are also produced. 
     In the QFN  5 , the area of the main surface  1   c  of the tab  1   b  is smaller than the area of the main surface  2   b  of the semiconductor chip  2 ; in short, the QFN  5  has a small-tab structure. As a semiconductor device with a small-tab structure, the QFN  5  should at least have a continuous portion  3   b  which continues from the side surface  2   d  of the semiconductor chip  2  to its back surface  2   c  and further to the side surface  1   h  of the tab  1   b  as illustrated in  FIG. 4 . 
     As shown in  FIG. 3 , slits  1   m  are provided around the tab  1   b  and the bus bars  1   i  lie with the slits  1   m  interposed. Both ends of each bus bar  1   i  are connected with suspension leads  1   e . In order to permit wire-bonding of each bus bar  1   i  with the semiconductor chip  2 , the bus bar  1   i  must be located outside of the semiconductor chip  2 . The tab  1   b  is smaller than the semiconductor chip  2  (small-tab structure) Therefore, the periphery of the semiconductor chip  2  lies above the slits  1   m  and as a result of resin sealing, a continuous portion  3   b  which continues from the side surface  2   d  of the semiconductor chip  2  to its back surface  2   c  to the side surface  1   h  of the tab  1   b  will be formed as part of the sealing body  3 , as shown in  FIG. 4 . 
     In this small-tab structure, part of the sealing body  3  is in tight contact with part of the back surface  2   c  of the semiconductor chip  2 . It is desirable that the area in which a die bonding agent  6  can be put be smaller than the main surface  2   b  of the semiconductor chip  2 . This is because the die bonding agent  6  is highly hygroscopic and easy to come off during a high temperature reflow process. The bonding strength between the sealing body  3  and the lead frame  1  is lower than that between the sealing body  3  and the semiconductor chip  2 . If the area of the tab  1   b  should be larger than the main surface  2   b  of the semiconductor chip  2 , there would be no contact surface between the back surface  2   c  of the semiconductor chip  2  and the sealing body  3  and it would be difficult to prevent peeling of the semiconductor chip  2 . On the other hand, when the area of the tab  1   b  is smaller than the main surface  2   b  of the semiconductor chip  2 , there is a contact surface between the back surface  2   c  of the semiconductor chip  2  and the sealing body  3 . In this case, the bonding strength between the sealing body  3  and the semiconductor chip  2  is higher than that between the sealing body  3  and the lead frame  1 ; therefore, even if the die bonding agent  6  comes off, peeling of the semiconductor chip  2  can be prevented because of the existence of the contact surface between the back surface  2   c  of the semiconductor chip  2  and the sealing body  3 . 
     It is desirable that pads  2   a  of the semiconductor chip  2  which are connected with the bus bars  1   i  through the second wires  4   b  be ground electrodes or power electrodes which can serve as common pins. For example, when a bus bar  1   i  is connected with a ground pad  2   a , ground potential is applied to the bus bar  1   i.    
     As illustrated in  FIGS. 2 and 4 , in the QFN  5 , the back surface  1   d  of the tab  1   b  is exposed on the back surface  3   a  of the sealing body  3 . Hence, when the QFN  5  is mounted on a mounting board  7 , the leads  1   a  and the tab  1   b  can be connected with a land  7   a  as an electrode of a mounting board  7  by soldering, as shown in  FIG. 5 . 
     As illustrated in  FIG. 3 , both ends of each bus bar  1   i  are connected with suspension leads  1   e  but its other portion, which extends between the suspension leads, is not connected with any other member. In other words, the bus bars  1   i  are connected with the tab  1   b  through parts of the suspension leads  1   e . As illustrated in  FIGS. 2 and 4 , the back surface  1   d  of the tab  1   b , the back surfaces  1   j  of the bus bars  1   i , and the back surfaces  1   f  of parts of the suspension leads  1   e  are flush with each other and exposed on the same plane as the back surface  3   a  of the sealing body  3 . 
     As shown in  FIG. 4 , the semiconductor chip  2  is fixed to the main surface  1   c  of the tab  1   b  using a die bonding agent (for example, silver paste)  6  and the back surface  2   c  of the semiconductor chip  2  and the main surface  1   c  of the tab  1   b  are joined through the die bonding agent  6 . 
     The leads  1   a  arranged in a line in the peripheral area of the back surface  3   a  of the sealing body  3  of the QFN  5  are partially exposed as connected surfaces  1   g  on the back surface  3   a  of the sealing body  3 . The connected surfaces  1   g  are solder-plated or palladium-plated. 
     The tab  1   b , suspension leads  1   e  and leads  1   a  are made of sheet metal such as copper alloy sheet metal. 
     The first wires  4   a  which connect pads  2   a  of the semiconductor chip  2  with the corresponding leads  1   a , and the second wires  4   b  which connect ground pads  2   a  of the semiconductor chip  2  with the bus bars  1   i  are, for example, gold wires. 
     The sealing body  3  is formed by resin molding. The sealing resin used in resin molding is, for example, thermosetting epoxy resin. 
     In the QFN  5  according to the first embodiment, since the sealing body  3  has a continuous portion  3   b  which continues from the side surface  2   d  of the semiconductor chip  2  to its back surface  2   c  to the side surface  1   h  of the tab  1 , the degree of adhesion among the semiconductor chip  2 , the tab  1   b , and the sealing body  3  is increased. 
     In other words, since the QFN  5 , provided with the bus bars  1   i , has a small-tab structure (the tab  1   b  bearing the semiconductor chip  2  is smaller than the semiconductor chip  2 ), the risk that the tab  1  peels off the sealing body  3  during a high-temperature reflow process is prevented. Hence, stress on the bus bars  1   i  in a temperature cycle test is reduced and disconnection of the second wires  4   b  in the wire connections of the bus bars  1   i  is prevented. 
     As a consequence, the reflow resistance of the QFN  5  is increased and its quality is improved. 
     Since the reflow resistance of the QFN  5  is improved, a reflow process can be done at high temperature (260° C.). This permits Pb-free packaging. 
     In other words, as shown in  FIG. 5 , the QFN  5  can be mounted on the mounting board  7  using Pb-free solder  10 . The Pb-free solder  10  is, for example, tin-silver-copper alloy solder. 
     However, it is also acceptable to use Pb solder for mounting the QFN  5  on the mounting board  7 . 
     In the QFN  5 , both ends of each bus bar  1   i  are connected with suspension leads  1   e  but its other portion, which extends between the suspension leads, is not connected with any other member. In other words, since the bus bars  1   i  are only connected with the suspension leads  1   e , even if peeling occurs between the tab  1   b  and the sealing body  3 , it is possible to stop spread of the peeling to the bus bars  1   i  and thereby prevent disconnection of the second wires  4   b  in the bus bars  1   i.    
     In the QFN  5 , it is desirable that the suspension leads  1   e  have the same thickness as the exposed portions (which include the connected surfaces  1   g ) of the leads  1   a  on the back surface  3   a  of the sealing body  3 . In short, preferably the suspension leads  1   e  should not be thinned by half-etching or a similar process. As a result, deterioration in the rigidity of the suspension leads  1   e  will be prevented, and for the structure in which the tab  1   b  is exposed on the back surface  3   a  of the sealing body  3 , in a resin sealing process, the tab  1   b  can be strongly pressed against a film sheet (sealing sheet)  8  placed on a die surface  9   d  of a lower mold  9   b  of a resin mold die  9  as shown in  FIG. 13 , so that resin sealing (transfer molding) is performed with tab  1   b  in tight contact with the film sheet  8  and consequently no resin burrs are generated on the back surface  1   d  of the tab  1   b.    
     In resin molding, it is also acceptable to perform resin sealing in a manner to cover plural device areas collectively. 
     Next, a variation of the first embodiment will be explained. 
       FIG. 7  shows a small-tab structure in which the tab  1   b  and the bus bars  1   i  are not exposed but embedded in the sealing body  3 . In this case, the wires can be arranged in areas corresponding to the tab  1   b  and bus bars  1   i , on the back surface  3   a  of the sealing body  3 , so that the packaging efficiency for the QFN  5  can be improved. 
     In the variation as shown in  FIGS. 8 to 10 , the back surfaces  1   j  of the bus bars  1   i  (opposite to the chip mounting side) are half-etched. In other words, the reverse faces  1   j  of the bus bars  1   i  (hatched in  FIG. 8 ) are made thinner than, for example, the exposed portions of the leads  1   a  on the back surface  3   a  of the sealing body  3  (portions of the leads  1   a  including the connected surfaces  1   g ) by half-etching or a similar process. In this structure, sealing resin gets into the spaces beneath the back surfaces  1   j  of the bus bars  1   i  during a resin sealing process; consequently, the resin sealing body  3  is formed as shown in  FIG. 9  where some resin fills the spaces beneath the back surfaces  1   j  of the bus bars  1   i.    
     When the bus bars  1   i  are thin, their rigidity is low; therefore, as the sealing body  3  warps, the bus bars  1   i  can warp, following the sealing body  3 . Besides, some resin of the sealing body  3  fills the spaces beneath the back surfaces  1   j  of the bus bars  1   i , thereby increasing the degree of adhesion between the sealing body  3  and the bus bars  1   i  (resin lock effect). 
     This reduces stress generated in the bus bars  1   i  and prevents peeling between the bus bars  1   i  and the sealing body  3 . As a result, disconnection of the second wires  4  in the bus bars  1   i  is prevented and the quality of the QFN  5  is improved. 
     In the QFN  5  structure in which the back surfaces  1   j  of the bus bars  1   i  are half-etched, the bus bars  1   i  are embedded in the sealing body  3  (not exposed on its back surface  3   a ) as illustrated in  FIG. 10 . 
     In a variation as shown in  FIGS. 11 and 12 , the back surface  1   d  of the tab  1   b  is protruded by amount P ( FIG. 12 ) toward the back surface of the tab  1   b  from the plane in which the resin mold die  9  is clamped for resin-sealing the suspension leads  1   e . For example, in an area outside the intersection of a suspension lead  1   e  and a bus bar  1   i , a bend  1   n  toward the chip mounting side (front side) of the suspension lead  1   e  and a bend  1   o  toward the opposite direction (back side) are made. Here, the step produced by the bend  1   n  at the tab side is larger by amount P than the step produced at the die clamping side (step level difference) so that the back surface  1   d  of the tab  1   b  protrudes backward by amount P from the plane (back surface  1   f ) in which the resin mold die  9  is clamped. 
     Accordingly, as shown in  FIG. 13 , when the suspension leads  1   e  are clamped by the resin mold die  9  for resin sealing, a spring force is generated due to the step level difference of the bend  1   n  of each suspension lead  1   e , resulting in force F which presses the tab  1   b  backward. Therefore, resin sealing is performed while the back surface  1   d  of the tab  1   b  is in tight contact with the film sheet  8  placed on the die surface  9   d  of the resin mold die  9 . 
     Alternatively, the retention of the suspension leads  1   e  can be improved by increasing the number of bends  1   n  ( FIG. 12 ) rather than making a step in the bend  1   n.    
     When the amount of step level difference is just P, the suspension leads  1   e  may be exposed on the back surface  3   a  of the sealing body  3 . If the suspension leads  1   e  should be exposed in the vicinity of the tab  1   b , the leads  1   e  might be short-circuited with the leads  1   a  during soldering for mounting on the mounting board  7 . Therefore, the step of the bend in  1   n  the vicinity of the tab  1   b  should be large enough ( FIG. 12 ) so that the leads  1   e  are not exposed on the back surface  3   a  of the sealing body  3 . Short-circuiting with the leads  1   a  is thus prevented. 
     As a consequence, the risk that resin burrs are generated on the back surface  1   d  of the tab  1   b  is avoided. 
     The bus bars  1   i  provide a common grounding means. In addition, the tab  1   b  connected with the bus bars  1   i  is mounted on the mounting board  7  for the QFN  5  by directly soldering it to the land  7   a  of the mounting board  7  as a large grounding area, so the ground potential is strengthened or stabilized, which offers a further advantage for a high-frequency version of the QFN  5 . 
     Next, a method of manufacturing the QFN  5  according to the first embodiment will be explained. 
     First, a lead frame  1  as shown in  FIG. 6  is prepared. This lead frame  1  includes: a tab  1   b  which can bear a semiconductor chip  2 ; suspension leads  1   e  which support the tab  1   b  at its corners; bus bars  1   i  which are connected with the tab  1   b  through parts of the suspension leads  1   e  and located outside of the tab  1   b ; and plural leads  1   a  arranged around the tab  1   b . In the lead frame  1 , the leads  1   a , suspension leads  1   e , tab  1   b  and bus bars  1   i  are inside a square area formed by four frame edges  1   p  (one device area). 
     Then, a die bonding step is taken. In the die bonding step, the semiconductor chip  2  is fixed to the main surface  1   c  of the tab  1   b  as a chip mounting plate in the lead frame  1  using a die bonding agent  6 . 
     Then, a wire bonding step is taken. As shown in  FIG. 6 , pads  2   a  of the semiconductor chip  2  are connected with the corresponding leads  1   a  through first wires  4   a  (gold wires) and pads  2   a  of the semiconductor chip  2  for grounding (or power supply) are connected with the bus bars  1   i  through second wires  4   b  (gold wires, etc). 
     Then, a resin sealing (resin molding) step is taken. As shown in  FIG. 4 , at this step, a continuous portion of sealing resin  3   b  which continues from the side surface  2   d  of the semiconductor chip  2  to its back surface  2   c  to the side surface  1   h  of the tab  1   b  is formed, and also as shown in  FIG. 2 , the semiconductor chip  2 , first wires  4   a  and second wires  4   b  are resin-sealed to form a resin sealing body  3  in such a way that the plural leads  1   a  are partially exposed in the peripheral area of the back surface  3   a  of the resin sealing body  3 . 
     In case that a bend  1   n  as shown in  FIG. 12  is made in each suspension lead  1   e , when the suspension leads  1   e  are clamped between the upper mold  9   a  and lower mold  9   b  of the resin mold die  9  as shown in  FIG. 13 , a spring force is generated due to the step level difference of the bend in of each suspension lead  1   e , resulting in force (F) which presses the tab  1   b  backward. Therefore, during resin sealing, the back surface  1   d  of the tab  1   b  and the connected surfaces  1   g  of the leads  1   a  are in tight contact with the film sheet  8  placed on the die surface  9   d  of the lower mold  9   b  of the resin mold die  9 . 
     Under this condition, sealing resin is supplied into a cavity  9   c  of the resin mold die  9  for resin molding. Because the connected surfaces  1   g  of the leads  1   a  and the back surface  1   d  of the tab  1   b  are in tight contact with the film sheet  8 , no resin burrs are generated on the connected surfaces  1   g  of the leads  1   a  and the back surface  1   d  of the tab  1   b.    
     After resin sealing is finished, the leads are cut. The leads  1   a  and suspension leads  1   e  are cut from the edges  1   p  of the lead frame  1  ( FIG. 6 ) to make a separate package. 
     Next, how the semiconductor device according to the first embodiment is packed for shipment will be explained. 
     According to the first embodiment, the QFN  5  features improved reflow resistance, so a high-temperature reflow process can be carried out and non-dampproof packing is acceptable. In other words, because of the improved reflow resistance, even if there is some moisture absorption, it is possible to prevent the tab  1   b  and leads  1   a  (including the bus bars  1   i ) from peeling off the sealing body  3 . Hence, a product packed in a non-dampproof manner may be shipped, though a dampproof-packed product may also be shipped. 
     First, a dampproof packing process is explained with reference to  FIG. 14 . 
     After assembly of QFN  5  is finished, sorting is done (step S 1 ) and the QFN  5  is housed in a prescribed tray  11 . 
     After the sorting step, baking is done (step S 2 ). At this step, moisture of the QFN  5  is removed by baking. 
     After the baking step, a visual inspection is made (step S 3 ). At this step, the appearance of the QFN  5  is checked. 
     After the visual inspection step, taping is done (step S 4 ). At this step, the QFN  5  is loaded onto carrier tape  12 , taped with cover tape  13  and wound around a reel  14 . 
     After the taping step, vacuuming and sealing are done (step S 5 ). At this step, carrier tape  12  carrying the QFN  5  is put in a dampproof bag  16  together with silica gel (desiccant)  15  and the dampproof bag  16  is vacuumed and thermally sealed. 
     After thermal sealing, boxing up is done (step S 6 ). Specifically, the dampproof bag  16  containing the QFN  5  is put in an inner/outer box  17 . 
     After that, shipment is made (step S 7 ). Namely, the boxed QFN  5  is shipped. 
     Next, a non-dampproof packing process will be explained with reference to  FIG. 15 . 
     The non-dampproof packing process is the same as the dampproof packing process of  FIG. 14  except that either the baking step (S 2 ) or the vacuuming/sealing step (S 5 ) or both are omitted; namely the other steps (sorting S 11 , visual inspection S 12 , taping S 13 , boxing up S 14  and shipping S 15 ) are the same as in the dampproof packing process. 
     As stated above, according to the first embodiment, the QFN  5  features improved reflow resistance and thus the packing process may be dampproof or non-dampproof. However, since the baking and vacuuming/sealing steps are omitted in the non-dampproof packing process ( FIG. 15 ), the non-dampproof packing process is more advantageous in terms of production cost reduction and labor saving. 
     In addition, the QFN  5  permits Pb-free packaging. After the QFN  5  packed in a non-dampproof manner is shipped and unpacked by a customer, it may be used for Pb-free packaging. Therefore, the customer can enjoy the benefits of cost reduction and handling ease. 
     Second Embodiment 
       FIGS. 16 to 18  concern a semiconductor device according to the second embodiment of the present invention, in which  FIG. 16  is a fragmentary plan view showing the positional relation among a tab, a common lead and a semiconductor chip in a semiconductor device;  FIG. 17  is a fragmentary plan view of the wiring structure of the semiconductor device as seen through a sealing body; and  FIG. 18  is a fragmentary plan view showing the positional relation among a tab, a common lead and a semiconductor chip in a variation of the second embodiment. 
     The semiconductor device according to the second embodiment are the same as the QFN  5  according to the first embodiment except that the pattern of connection of the bus bars as common leads and the tab  1   b  is different from that in the first embodiment. 
     In a structure as shown in  FIGS. 16 and 17 , each bus bar  1   i  is connected with suspension leads  1   e  at both ends and also its portion between the suspension leads  1   e  is directly connected with the tab  1   b  by one or more connecting portions  1   k.    
     In this structure, second wires  4   b  are connected with portions of the bus bars  1   i  other than their intersections with the connecting portions  1   k  and suspension leads  1   e  (hatched) as shown in  FIG. 17 . 
     In this structure, even if peeling occurs between the tab  1   b  and the sealing body  3  and this peeling spreads through a connecting portion  1   k  to a bus bar  1   i , disconnection of the second wires  2  in the bus bars  1   i  do not occur because the second wires  4   b  are not connected with the above bus bar intersections with the connecting portions  1   k  and suspension leads  1   e.    
     In a variation of the second embodiment as shown in  FIG. 18 , the bus bars  1   i  are connected with the tab  1   b  only through the connecting portions  1   k  and the tab  1   b  is not directly connected with the suspension leads  1   e  and supported only by the bus bars  1   i.    
     The second embodiment and its variation as shown in  FIGS. 16 to 18  bring about almost the same effects as the QFN  1  as shown in  FIG. 1 . Specifically, even if peeling spreads from the tab  1  through a connecting portion  1   k  to a bus bar  1   i , the absence of a connected wire in the intersection prevents wire disconnection. Hence, the second embodiment improves reflow resistance like the first embodiment (QFN  5 ). 
     Since the bus bars  1   i  are connected with the tab  1   b  through the connecting portions  1   k , the location of the bus bars  1   i  is stabilized in the assembly process, making semiconductor device assembly work easier. 
     So far, preferred embodiments of the invention made by the inventors have been concretely described. However, obviously the present invention is not limited to the above embodiments but may be embodied in other various forms without departing from the scope and spirit thereof. 
     Although the above first and second embodiments concern QFN type semiconductor devices, the present invention may be applied to other types of semiconductor devices as far as they have bus bars  1   i  and are of the leadless type where the bus bars  1   i  are down-bonded. 
     In the abovementioned variation of the first embodiment, the bus bars  1   i  are thinned by a half-etching technique. However, other techniques of thinning such as press work and coining may be employed. In the above first embodiment, the back surfaces  1   j  of the bus bars  1   i  of the QFN  5  are half-etched in the process of manufacturing the lead frame  1 . 
     The present invention is useful for electronic equipment and semiconductor device manufacturing technology.