The disclosure of Japanese Patent Application No. 2013-033272 filed on Feb. 22, 2013 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a semiconductor device. The invention especially relates to a semiconductor device in which a semiconductor chip is soldered to a lead frame.
2. Description of Related Art
In a semiconductor device in which a semiconductor chip is soldered to a lead frame, a crack sometimes occurs in a solder layer due to a difference in a coefficient of thermal expansion between the lead frame and the semiconductor chip. Various technologies for restraining occurrence of cracks or restraining damages due to cracks are proposed. (Japanese Patent Application Publication No. 11-186331 A (JP 11-186331 A), Japanese Patent Application Publication No. 2009-106993 A (JP 2009-106993 A), and Japanese Patent Application Publication No. 2006-043733 (JP 2006-043733 A)).
The technology described in JP 11-186331 A is as follows. Projections are provided in a lead frame, and a semiconductor chip is mounted on the projections, and then soldered. Due to the projections, a thickness of a solder layer is equalized, and occurrence of cracks is restrained. The projections are made of a material that does not become wet with solder. Due to the projections that do not become wet with solder, a void occurs in a boundary between the projections and the solder. The boundary of the solder is gently curved along the void. This means that a shape of a fillet becomes gentle, thereby restraining occurrence of cracks.
The technology described in JP 2009-106993 A is as follows. Two metal wires are preliminary buried in a solder material. The solder material is placed on a lead frame, and a semiconductor chip is mounted on the solder material. When a solder is melted, the wires maintain a width between the lead frame and the semiconductor chip constant, thereby forming a solder layer having a constant thickness.
The technology described in JP 2006-043733 A is as follows. Metallic grains are mixed into a solder material. After soldering, even if a crack occurs in a solder layer, the crack stops progressing when the crack runs into the metallic grain. Therefore, damages due to cracks are reduced. In addition, since cracks are likely to occur in corner portions of a semiconductor chip that is rectangular-shaped in a planar view, metallic grains are mixed into the solder material metal to be concentrated around the corner portions of the semiconductor chip.
The present invention provides a semiconductor device that restrains damages when a crack occurs. The present invention restrains a damage due to a crack by arranging a metal bar having a specific shape at a specific position in a solder layer.
An aspect of the present invention includes a lead frame, a semiconductor chip soldered to the lead frame, and a metal bar. When viewed in a stacking direction of the lead frame and the semiconductor chip, the metal bar extends along one side of the semiconductor chip, inside a solder layer between the lead frame and the semiconductor chip. In addition, when viewed in the stacking direction of the lead frame and the semiconductor chip, the metal bar is arranged so that a part of the metal bar overlaps the semiconductor chip and the rest of the metal bar does not overlap the semiconductor chip. In a section of the metal bar in a plane that is perpendicular to a longitudinal direction of the metal bar, an outline of the metal bar on a side of a center of the semiconductor chip is curved so as to project on the side of the center of the semiconductor chip. The outline extends from an end portion of the metal bar on the semiconductor chip side to an end portion of the metal bar on the lead frame side.
Cracks are likely to occur in a fillet of the solder layer. The fillet means a region of a solder protruding from between solder joining surfaces of two objects. In the case of the semiconductor device, the fillet means a region of a solder protruding from between the lead frame and the semiconductor chip. In the aspect of the present invention, the metal bar is arranged in the fillet. In a section (a transverse section) of the metal bar, which traverses in a width direction (a direction perpendicular to a longitudinal direction of the metal bar), an outline of the metal bar on a side of a center of the semiconductor chip is curved so as to project to the center of the semiconductor chip. In the aspect of the invention, the outline of the metal bar extends from an end portion of the metal bar on the semiconductor chip side to an end portion of the metal bar on the lead frame side. Due to the metal bar that has the above-mentioned shape and is arranged as stated above, a crack that occurs on a surface of the fillet advances along a boundary between the solder and the metal bar. Because a surface of the metal bar is curved, the crack smoothly proceeds along the curve. The curve of the surface of the metal bar is a curved line that projects to the side of the center of the semiconductor chip. If a crack occurs between the metal bar and the semiconductor chip, the crack advances along the curve of the surface of the metal bar, and, in the end, stops at a contact point between the lead frame and the metal bar. Thus, the semiconductor device according to the aspect of the present invention prevents a crack from advancing to the side of the semiconductor chip. Therefore, a solder layer near the center of the semiconductor chip is not damaged.
An angle, which is made by a tangent of the outline of the metal bar at the contact point between the metal bar and the lead frame and a surface of the lead frame with which the metal bar is in contact, may be an acute angle on the side of the center of the semiconductor chip. Alternatively, the tangent of the outline of the metal bar at the contact point between the metal bar and the lead frame may be on a surface of the lead frame with which the metal bar is in contact. A crack that occurs between the semiconductor chip and the metal bar advances to the side of the center of the semiconductor chip once, along the curve of the surface of the metal bar. However, as the crack advances further along the curve, the crack advances in a direction away from the center of the semiconductor chip. In the end, the crack stops at the contact point between the metal bar and the lead frame. In this aspect, since a crack advances in the direction away from the semiconductor chip, it is further ensured that a crack is prevented from advancing to the center of the semiconductor chip.
Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:
First of all, some structures of a semiconductor device according to embodiments of the invention will be listed. Technical elements stated below are independent from each other, and, it can be considered that technical usefulness is achieved by one of or various combinations of the technical elements.
A metal bar may extend across both ends of one side of the semiconductor chip. In this type of form, it is possible to restrain damages due to cracks over the entire region of the one side of the semiconductor chip.
The semiconductor chip is a rectangle when viewed from a stacking direction of a lead frame and the semiconductor chip, and the metal bar may extend along a short side of the semiconductor chip. In a case where the shape of the semiconductor chip is a rectangle in a plan view, stress caused by a difference in a coefficient of thermal expansion between the semiconductor chip and the lead frame is larger in a long-side direction than that in a short-side direction. Hence, cracks are more likely to occur on the short side than on the long side of a fillet. According to the above-mentioned structure, it is possible to restrain damages due to cracks over the entire region on the short side where clacks are likely to occur. As a matter of course, it is desired that metal bars are arranged on both long side and short side.
When a section of the metal bar is either circular or elliptical, and the metal bar is viewed in the stacking direction of the lead frame and the semiconductor chip, the center of the section of the metal bar may be located on an outer side of the semiconductor chip. In a direction that is parallel to a surface of the lead frame with which the metal bar is in contact and is perpendicular to one side of the semiconductor chip, a solder may cover from an edge of the one side of the semiconductor chip to the center of section of the metal bar. When viewed in a direction perpendicular to the lead frame (the stacking direction of the lead frame and the semiconductor chip), if the metal bar is arranged so that the center of the section of the metal bar is located on the outer side of the semiconductor chip, a solder material expands more easily from the edge of the one side of the semiconductor chip to an area directly above the center of the section of the metal bar. Thus, the fillet expands more easily from the semiconductor chip side to the lead frame side. As the fillet expands, cracks are less likely to occur.
A groove, to which the metal bar is fitted, may be provided in the lead frame. Alternatively, a surface of the metal bar, which faces the lead frame, may be horizontal. When either one of the above-mentioned shapes is employed, the metal bar does not roll, and positioning of the metal bar is done easily.
Solder wettability of a surface of the metal bar on the side away from the center of the semiconductor chip may be higher than solder wettability of a surface of the metal bar on the side closer to the center of the semiconductor chip. The side away from the center of the semiconductor chip means the fillet side. The above-mentioned wettability characteristics allow the solder material to flow easily to the fillet side, thereby making the shape of the fillet gentle.
A semiconductor device according to a first embodiment of the invention will be explained with reference to the drawings.
Metal bars 6 are arranged inside the solder layer 5.
The metal bars 6 are made of the same material as a bonding wire. The metal bars 6 are located inside a fillet 5a of the solder layer 5. To be more specific, the metal bars 6 are arranged so that, when viewed in the direction perpendicular to the lead frame 4, the metal bars 6 partially overlap the semiconductor chip 3 and the rests of the metal bars 6 do not overlap the semiconductor chip 3. As shown in
Since the section of the metal bar 6 is a circle, in a section that goes across the metal bar 6 in a width direction of the metal bar 6 (in a transverse section of the metal bar 6), an outline of the metal bar 6 on the chip center CP side, which extends from an end portion of the metal bar 6 on the semiconductor chip side to an end portion of the metal bar 6 on the lead frame side, is curved so as to project on the chip center CP side. In other words, the outline of the metal bar 6 has an arc shape, and projects to the chip center CP side. This means that an angle Tha, which is made by the lead frame 4 and a tangent PL of the section of the metal bar at a contact point AP between the metal bar 6 and the lead frame 4, is an acute angle. Thus, if a crack occurs in the fillet 5a between the semiconductor chip 3 and the metal bar 6, the crack advances along a boundary between the metal bar 6 and the solder layer 5, and stops at the contact point AP between the metal bar 6 and the lead frame 4 in the end. At this time, although the crack goes closer to the chip center CP once along the arc section of the metal bar 6, the crack then advances in a direction away from the chip center CP (see a thick arrow CK in
Also in the semiconductor device 102 according to the second embodiment, in a section that goes across the metal bar 106 in a width direction of the metal bar 106 (in the section in
As shown in
The semiconductor devices according to the embodiments of the invention have been explained so far. Any of the semiconductor devices according to the embodiments prevents a crack, which has occurred on the surface of the fillet, from advancing towards the chip center CP inside the solder layer, thereby reducing a damage caused by the crack.
Points to be noted with regard to the technologies explained in the embodiments of the invention will be stated. A metal wire (i.e., metal bar) in the embodiment has a circular section. The outline shape of the section of the metal wire is preferably either a circle or an ellipse, but may be a curved line that satisfies conditions stated in the embodiments. The outline shape of the section of the metal wire may be a curved line so that the outline shape on the side closer to the chip center projects to the chip center side, in order to allow a crack to advance easily along the outline. It is preferred that the curved line is continuous in a mathematical sense, in other words, a differentiable curved line. When the outline is bent, a crack might move away from the metal wire, starting from a bent point. It is more preferred that an angle, which is made by the surface of the lead frame, and the tangent at the contact point between the curved line of the outline shape of the section of the metal wire and the lead frame, is an acute angle on the chip center side.
Manufacturing costs are reduced when a material for the metal wire is the same as a material for a bonding wire that connects a terminal of the semiconductor chip with a terminal of the lead frame.
In any of the semiconductor devices according to the embodiment of the invention, a thickness of the solder layer is larger than a diameter of the metal wire, and the metal wire and the semiconductor chip are not in contact with each other. The metal wire and the semiconductor chip may be in contact with each other.
Although the specific embodiments of the present invention have been explained in detail, they are embodiments only, and do not limit the present invention. The present invention includes the specific embodiments listed above with various modifications and changes. Technical usefulness is achieved by one of or various combinations of the technical elements explained in this description and the drawings.
Number | Date | Country | Kind |
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2013-033272 | Feb 2013 | JP | national |