Patent Publication Number: US-2016233150-A1

Title: Semiconductor device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2015-021443, filed on Feb. 5, 2015 and 2015-092059, filed on Apr. 28, 2015, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments of the present invention relate to a semiconductor device. 
     BACKGROUND 
     In recent years, a semiconductor package in which a metal heat sink of a connector is exposed from a sealing resin has been developed to reduce the thermal resistance of the semiconductor package. To further reduce the thermal resistance, a semiconductor package in which a metal heat sink larger than a semiconductor chip is mounted on the semiconductor chip has been also developed. 
     However, at the time of reflow soldering, the semiconductor chip can move in a space between a lead frame and the metal heat sink. When the metal heat sink of the connector is larger than the size of a top electrode of the semiconductor chip at that time, the metal heat sink cannot restrict or fix the position of the semiconductor chip. In this case, if a solder flows to spread on the metal heat sink in a wider range than the size of the top electrode of the semiconductor chip, the position of the semiconductor chip may be displaced in the space between the lead frame and the metal heat sink or the semiconductor chip may be inclined in the space between the lead frame and the metal heat sink. 
     When the metal heat sink of the connector is increased in size, thermal stress applied to the solder and the resin during a reflow increases. In this case, the level of a reliability test (such as a TCT (Thermal Cycle Test), a TFT (Thermal Fatigue Test), or a PCT (Pressure Cooker Test)) may be decreased. Furthermore, a shock may be applied to the semiconductor chip at the time of mounting or during handling of a product, which may lead to a defect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are a plan view and a cross-sectional view showing an example of a configuration of a semiconductor device  1  according to a first embodiment, respectively; 
         FIG. 2  is a cross-sectional view showing an example of a configuration of the first engraved part  71 ; 
         FIGS. 3A and 3B  are a plan view and a cross-sectional view showing an example of a configuration of the semiconductor device  1  according to a second embodiment, respectively; and 
         FIGS. 4A and 4B  show contact angles of the solder  51 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments will now be explained with reference to the accompanying drawings. The present invention is not limited to the embodiments. 
     A semiconductor device according to an embodiment includes a first metal part. A semiconductor chip is mounted on the first metal part and includes a first electrode on a top surface thereof. A solder is provided on the first electrode of the semiconductor chip. A connector is provided on the solder and includes a first portion provided around the solder on a first surface thereof. The first surface faces the first electrode. A contact angle with the solder in the first portion is larger than a contact angle with the solder in a region other than the first portion of the connector. A resin is provided around the semiconductor chip. 
     FIRST EMBODIMENT 
       FIGS. 1A and 1B  are a plan view and a cross-sectional view showing an example of a configuration of a semiconductor device  1  according to a first embodiment, respectively.  FIG. 1B  shows a cross-section along a line B-B in  FIG. 1A . 
     The semiconductor device  1  includes lead frames  10  to  12 , a semiconductor chip  20 , a source connector  31 , a gate connector  32 , a resin  40 , solders  50  to  52 , a plating  60 , and engraved parts  70  to  73 . 
     The semiconductor chip  20  is mounted above the lead frame (bed)  10  as a first metal part. While the lead frame  10  is covered with the resin  40 , parts ( 10 P) of the lead frame  10  protrude from the resin  40 . The protruding parts  10 P of the lead frame  10  protrude from the resin  40  and function as drain terminals. The lead frame  10  is electrically connected to, for example, a drain electrode provided on the rear surface of the semiconductor chip  20  and functions as a drain terminal. 
     The lead frame (post)  11  as a second metal part is separated from the lead frame  10  and is electrically isolated from the lead frame  10  by the resin  40 . The lead frame  11  is electrically connected to a source electrode (first electrode)  21  provided on the top surface of the semiconductor chip  20  via the source connector  31 . Protruding parts  11 P of the lead frame  11  protrude from the resin  40  and function as source terminals. 
     The lead frame  12  is separated from the lead frames  10  and  11  and is electrically isolated from the lead frames  10  and  11  by the resin  40 . The lead frame  12  is electrically connected to a gate electrode  22  of the semiconductor chip  20  via the gate connector  32 . A protruding part  12 P of the lead frame  12  protrudes from the resin  40  and function as a gate terminal. A low-resistance and high-thermal-conductivity metal such as copper, nickel-plated copper, silver-plated copper, gold-plated copper, copper alloy, or aluminum is used for the lead frames  10  to  12 . 
     The semiconductor chip  20  includes an arbitrary semiconductor element on a semiconductor substrate. For example, the semiconductor chip  20  has a drain of the semiconductor element on the rear surface thereof and has the source electrode  21  and the gate electrode  22  of the semiconductor element on the front surface thereof. As shown in  FIG. 1B , the semiconductor chip  20  is mounted above the lead frame  10  and is fixed thereto by the solder  50 . The solder  50  is provided between the lead frame  10  and the semiconductor chip  20 . 
     The source connector  31  is provided above the source electrode (first electrode)  21  of the semiconductor chip  20  and is fixed thereto by the solder  51  as shown in  FIG. 1B . The solder  51  is provided between the semiconductor chip  20  and the source connector  31 . The source connector  31  is also connected to the lead frame  11  by the solder  52 . The solder  52  is provided between the source connector  31  and the lead frame  11 . Accordingly, the source connector  31  electrically connects the source electrode  21  of the semiconductor chip  20  and the lead frame  11  to each other. The source connector  31  thus includes a bed-side connector  31   a  connected to the source electrode  21  of the semiconductor chip  20  via the solder  51  and a post-side connector  31   b  connected to the lead frame  11  via the solder  52 . In the present embodiment, the area of a surface (first surface) F 1  of the bed-side connector  31   a  facing the source electrode  21  is larger than that of the source electrode  21 . The area of the top surface of the bed-side connector  31   a  exposed from the resin  40  and covered with the plating  60  is also larger than that of the source electrode  21 . Accordingly, the bed-side connector  31   a  has a high heat dissipation performance. The thickness of the bed-side connector  31   a  is relatively large as shown in  FIG. 1B  and the thickness of the post-side connector  31   b  is smaller than that of the bed-side connector  31   a.    
     The gate connector  32  is provided on the gate electrode  22  of the semiconductor chip  20  and is fixed thereto by a solder (not shown). The gate connector  32  is connected to the lead frame  12  by a solder (not shown). The gate connector  32  thus electrically connects the gate electrode  22  of the semiconductor chip  20  and the lead frame  12  to each other. A low-resistance and high-thermal-conductivity metal such as copper, nickel-plated copper, silver-plated copper, gold-plated copper, copper alloy, or aluminum is used for the source connector  31  and the gate connector  32 . 
     The resin  40  seals around the semiconductor chip  20  and around the solders  50  to  52  and partially covers the lead frames  10  to  12  and the connectors  31  and  32 . The resin  40  thereby protects the semiconductor chip  20  and the solders  50  to  52  and isolates the drain, the source, and the gate from each other. Parts of the lead frames  10  to  12  and parts of the connectors  31  and  32  are exposed from the resin  40  and are covered with the plating  60 . 
     The plating  60  covers the parts of the lead frames  10  to  12  and the connectors  31  and  32  exposed from the resin  40 . The plating  60  protects the lead frames  10  to  12  and the connectors  31  and  32  from corrosion and improves the appearance. The plating  60  extends beyond the surface of the resin  40  to enhance the heat dissipation performance. Alternatively, the plating  60  can be recessed inward from the surface of the resin  40  to prevent an external shock from being applied to the semiconductor chip  20 . In this case, a shock-absorbing material such as grease can be coated on the plating  60 . 
     The source connector  31  according to the present embodiment has the first engraved part  71  as a first portion. As shown in  FIG. 1B , the first engraved part  71  is provided on the first surface (rear surface) F 1  of the bed-side connector  31   a  facing the source electrode  21 . The first engraved part  71  is provided around the solder  51  on the first surface F 1  of the source connector  31 . The first engraved part  71  has a property of being more likely to repel the solder  51  than the source connector  31 . That is, the first engraved part  71  is less likely to be wet with the solder  51  and has a lower wettability with the solder  51  than a part of the source connector  31  to be in contact with the solder  51 . In other words, a contact angle of the first engraved part  71  with the solder  51  is larger than that of a region of the source connector  31  other than the first engraved part  71  with the solder  51 . 
     For example, after the source connector  31  is mounted on the solder  51 , the solders  50  to  52  are reflowed while the semiconductor chip  20  is pressured in a state interposed between the lead frame  10  and the source connector  31 . At that time, if the solder  51  flows outside of the source electrode  21  on the first surface Fl of the source connector  31 , the semiconductor chip  20  may be displaced along with the solder  51  or the semiconductor chip  20  may be inclined. 
     On the other hand, according to the present embodiment, the first engraved part  71  is provided on the first surface F 1  of the bed-side connector  31   a.  The solder  51  is thereby restricted within a region R 71  of the source connector  31  enclosed by the first engraved part  71  between the source electrode  21  and the source connector  31  and is hard to spread out of the region R 71 . As shown by a dashed line in  FIG. 1A , the first engraved part  71  has a shape substantially similar to the planar shape of the source electrode  21  of the semiconductor chip  20 . When viewed from above the surface of the semiconductor chip  20 , the geometric center or the center of gravity of the planar shape of the region R 71  substantially matches the geometric center or the center of gravity of the planar shape of the source electrode  21 . Therefore, a range in which the solder  51  spreads is restricted within the range of the planar shape of the source electrode  21  (within the range of the planar shape enclosed by the first engraved part  71 ). 
     The first engraved part  71  includes, for example, a trench TR 71  and an oxide film OX 71  that covers the surface of the trench TR 71  as shown in  FIG. 2 .  FIG. 2  is a cross-sectional view showing an example of a configuration of the first engraved part  71 . The first engraved part  71  is formed by machining the rear surface of the source connector  31  using a laser or the like. The laser forms the trench TR 71  by gouging the source connector  31  and forms the oxide film OX 71  of the source connector  31  by oxidizing an inner surface of the trench TR 71 . For example, when the material of the source connector  31  is copper, the oxide film OX 71  is a copper oxide. The copper oxide is lower in the wettability with the solder  51  than copper. Therefore, the first engraved part  71  can suppress the solder  51  from spreading to the rear surface of the source connector  31  other than the region R 71 . The width of the first engraved part  71  can be, for example, about 10 to 50 micrometers. The same holds for other materials of the first engraved part  71  (such as nickel-plated copper, silver-plated copper, gold-plated copper, copper alloy, and aluminum). The semiconductor device  1  according to the present embodiment thus includes the source connector  31  having the first engraved part  71 . As described above, the first engraved part  71  can restrict the range in which the solder  51  spreads within the range of the planar shape of the source electrode  21 . Therefore, even when the area of the first surface F 1  of the bed-side connector  31   a  is formed to be larger than that of the front surface of the source electrode  21 , the solder  51  does not spread out of the source electrode  21 . The area of the part of the source connector  31  exposed from the resin  40  (the area of the metal heat sink) thereby can be formed to be larger than that of the source electrode  21  or that of the semiconductor chip  20 . As a result, the semiconductor device  1  according to the present embodiment can dissipate heat from the semiconductor chip  20  at a high efficiency. That is, the semiconductor device  1  according to the present embodiment can have a lower thermal resistance than conventional ones. The area of the part of the source connector  31  exposed from the resin  40  can be smaller than that of the top surface of the bed-side connector  31   a  of the source connector  31 . This increases the contact area between the resin  40  and the source connector  31  and can enhance the reliability of the semiconductor device  1 . 
     By restricting the range in which the solder  51  spreads within the region R 71 , the solder  51  is prevented from easily flowing out of the region R 71  and thus placement of the source connector  31  is defined in a self-aligned manner also in the reflow. Therefore, the positional accuracy of the source connector  31  is improved. Furthermore, in the reflow, the semiconductor chip  20  can be kept substantially parallel (horizontal) to the source connector  31  and the lead frames  10  to  12 . Accordingly, an open failure between the source connector  31  and the lead frame  11  or a short-circuit failure between the source connector  31  and other members can be suppressed. Because the solder  51  stays in the region R 71 , the solder  51  can be formed to be relatively thick. When the solder  51  is thick, the stress resistance is improved and thus the reliability (such as the TCT, the TFT, and the PCT) of the semiconductor device  1  is further enhanced. 
     As shown in  FIGS. 1A and 1B , the source connector  31  has coined parts (first trenches)  78  on the rear surface thereof. The coined parts  78  are provided in a region of the rear surface of the source connector  31  other than the region R 71  being in contact with the solder  51 . The depth of the coined parts  78  can be about  100  micrometers, for example. The resin  40  is filled in the coined parts  78 . This increases the contact area between the resin  40  and the source connector  31  and enhances an adhesion property between the resin  40  and the source connector  31  due to an anchor effect. As a result, levels of the reliability test, such as resistance to reflow, resistance to temperature cycling, and resistance to humidity can be improved. 
     The source connector  31  further includes the second engraved part  72  as a second portion. As shown in  FIG. 1B , the second engraved part  72  is provided on a surface (second surface) F 2  of the post-side connector  31   b  facing the lead frame  11  as the second metal part. The second engraved part  72  has a property of being more likely to repel the solder  52  than the source connector  31  similarly to the first engraved part  71 . That is, the second engraved part  72  is less likely to be wet with the solder  52  and has a lower wettability with the solder  52  than a part of the source connector  31  to be in contact with the solder  52 . In other words, a contact angle of the second engraved part  72  with the solder  52  is larger than that of a region of the source connector  31  other than the second engraved part  72  with the solder  52 . The solder  52  is thereby restricted within a region R 72  of the source connector  31  enclosed by the second engraved part  72  between the post-side connector  31   b  and the lead frame  11  and is hard to spread out of the region R 72 . That is, a range in which the solder  52  spreads is restricted within the region R 72  enclosed by the second engraved part  72  in the post-side connector  31   b . Because the solder  52  is hard to flow out of the region R 72  due to restriction of the range in which the solder  52  spreads within the region R 72 ; placement of the source connector  31  is defined in a self-aligned manner in the reflow and the positional accuracy of the source connector  31  is improved. Furthermore, in the solder reflow, the semiconductor chip  20  can be kept substantially parallel (horizontal) to the source connector  31  and the lead frames  10  to  12 . Accordingly, an open failure between the source connector  31  and the lead frame  11  or a short-circuit failure between the source connector  31  and other members can be suppressed. Because the solder  52  stays in the region R 72 , the solder  52  can be formed to be relatively thick. When the solder  52  is thick, the stress resistance is improved and thus the reliability of the semiconductor device  1  is further enhanced. 
     The second engraved part  72  has a shape in which the planar shape of the post-side connector  31   b  of the source connector  31  is partitioned into plural pieces as shown by a dashed line in  FIG. 1A . The solder  52  is thus partitioned into plural pieces between the post-side connector  31   b  and the lead frame  11 . Accordingly, even when a crack due to stress occurs at a part of the solder  52 , the crack is hard to propagate to other parts of the solder  52 . This enhances the reliability of the semiconductor device  1 . A configuration of the second engraved part  72  can be identical to that of the first engraved part  71  shown in  FIG. 2 . 
     The lead frame  10  as the first metal part further has the third engraved part  70  as a third portion as shown in  FIG. 1B . The third engraved part  70  is provided on a surface (third surface) F 3  facing the semiconductor chip  20 . The third engraved part  70  has a property of being more likely to repel the solder  50  than the lead frame  10 . That is, the third engraved part  70  is less likely to be wet with the solder  50  and has a lower wettability with the solder  50  than a part of the lead frame  10  to be in contact with the solder  50 . In other words, a contact angle of the third engraved part  70  with the solder  50  is larger than that of a region of the lead frame  10  other than the third engraved part  70  with the solder  50 . The solder  50  is thereby restricted within a region enclosed by the third engraved part  70  between the lead frame  10  and the semiconductor chip  20  and is hard to spread out of the region. Such restriction of a range in which the solder  50  spreads prevents the solder  50  from easily flowing out of the range. Placement of the semiconductor chip  20  is thus defined in a self-aligned manner in the reflow and the positional accuracy of the semiconductor chip  20  is improved. Furthermore, the semiconductor chip  20  can be kept substantially parallel (horizontal) to the source connector  31  and the lead frames  10  to  12  in the reflow. Accordingly, an open failure between the semiconductor chip  20  and the lead frame  10 , an open failure between the semiconductor chip  20  and the source connector  31 , or a short-circuit failure between the semiconductor chip  20  and other members can be suppressed. By restricting the range in which the solder  50  spreads, the solder  50  can be formed to be relatively thick. When the solder  50  is thick, the stress resistance is improved and thus the reliability of the semiconductor device  1  is further enhanced. A cross-sectional shape of the third engraved part  70  can be identical to that of the first engraved part  71  shown in  FIG. 2 . 
     As shown in  FIG. 1B , the lead frame  11  as the second metal part further has the fourth engraved part  73  as a fourth portion. The fourth engraved part  73  is provided on a surface (fourth surface) F 4  facing the post-side connector  31   b  of the source connector  31 . The fourth engraved part  73  has a property of being more likely to repel the solder  52  than the lead frame  11 . That is, the fourth engraved part  73  is less likely to be wet with the solder  52  and has a lower wettability with the solder  52  than a part of the lead frame  11  to be in contact with the solder  52 . In other words, a contact angle of the fourth engraved part  73  with the solder  52  is larger than that of a region of the lead frame  11  other than the fourth engraved part  73  with the solder  52 . The solder  52  is thereby restricted within a region enclosed by the fourth engraved part  73  between the post-side connector  31   b  and the lead frame  11  and is hard to spread out of the region. By thus restricting the range in which the solder  52  spreads, the solder  52  is hard to flow out of the range. The fourth engraved part  73  can be provided to face the second engraved part  72 . That is, the fourth engraved part  73  can have a shape partitioned into plural pieces on the top surface of the lead frame  11  and being identical to that of the second engraved part  72 . Accordingly, the fourth engraved part  73  can have an identical effect to that of the second engraved part  72 . Provision of both the second engraved part  72  and the fourth engraved part  73  can further enhance the reliability of the semiconductor device  1 . 
     SECOND EMBODIMENT 
       FIGS. 3A and 3B  are a plan view and a cross-sectional view showing an example of a configuration of the semiconductor device  1  according to a second embodiment, respectively.  FIG. 3B  shows a cross-section along a line B-B in  FIG. 3A . The second embodiment is different from the first embodiment in that the post-side connector  31   b  of the source connector  31  has a substantially equal thickness to that of the bed-side connector  31   a.  That is, the source connector  31  has a disk shape as a whole and has substantially equal thicknesses between a portion above the lead frame  10  and a portion above the lead frame  11 . Other configurations of the semiconductor device  1  according to the second embodiment can be identical to corresponding ones of the semiconductor device  1  according to the first embodiment. 
     In this manner, by forming the thickness of the post-side connector  31   b  to be substantially equal to that of the bed-side connector  31   a,  the area of a part of the source connector  31  exposed from the resin  40  (the area of a metal heat sink) is further increased. Accordingly, the semiconductor deice  1  according to the second embodiment can dissipate heat from the semiconductor chip  20  at a higher efficiency. The second embodiment can further obtain effects of the first embodiment. 
     The contact angle of a solder is obtained by putting a drop of a melted liquid solder on the surface of a solid material (the first to fourth engraved parts  71 ,  72 ,  70 , and  73 , the source connector  31 , and the lead frames  10  to  12 , for example) and measuring an angle formed at a contact point between the solder and the solid material by a first tangent line touching the solder and the surface of the solid material (an angle on a side on which the first tangent line and the surface of the solid material sandwich the solder). For example,  FIGS. 4A and 4B  show contact angles of the solder  51 .  FIG. 4A  shows a contact angle θ 1  of the solder  51  dropped on the surface of the source connector  31  (or the lead frame  10  or  11 ).  FIG. 4B  shows a contact angle θ 2  of the solder  51  dropped on the surface of the first engraved part  71 . The contact angle θ 2  of the solder  51  dropped on the surface of the first engraved part  71  is thus larger than the contact angle θ 1  of the solder  51  dropped on the surface of the source connector  31 . Similarly, contact angles of solders dropped on the surfaces of the second to fourth engraved parts  72 ,  70 , and  73  are also larger than contact angles of solders dropped on the surfaces of the source connector  31 , and the lead frames  10  and  11 , respectively. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.