In a typical prior art integrated circuit package, an integrated circuit die is mounted on the die attach pad of a leadframe. Contact pads on the die are connected to leads of the leadframe by wire bonding. This assembly is encapsulated in a protective material such as mold compound that at least partially exposes the leadframe.
In conventional wire bonding a tool known as a capillary forms a ball bond on the bond pad of a die. A small diameter bond wire of the same metal as the ball bond has one end that is integral with the ball bond and a distal end. The distal end is attached to a corresponding lead of the leadframe with a stitch bond.
During ball bonding, the molten ball of metal is initially formed at the end of the capillary. The molten ball is then moved by the capillary to place it in contact with the metal die bond pad. Stitching is performed by pressing the capillary against the bond wire and a top surface of the corresponding lead. Heat and ultrasonic vibration are usually applied by the capillary at the same time to form the ball for a ball bond and to complete a stitch bond. The pressure, heat and vibration cause the ball or the distal end of the wire to partially melt and bond with the adjacent metal surface to which it is attached.
Ball bonding and ball bonding machines are described in detail in U.S. patent application Ser. No. 13/345,460 of Wade Chang, et al., filed Jan. 6, 2012, Pub. No. 2013/0175677, which is hereby incorporated by reference for all that it contains.
A wire bonding tool 70, as shown in
The die 20 is attached to the leadframe 40 by a layer of a low modulus attachment material, for example, a material having a modulus of elasticity less than or equal to about 100 MPa. Such low modulus materials are sometimes specified in certain applications, such as sensor products. However, such low modulus materials can create a problem with a ball bond 60 that is formed on the die bond pad 32 as a part of wire bonding operations. This problem is exacerbated with extremely small dies, e.g., dies having a top face 22 measuring less than about 1.0 mm×1.0 mm. The problem arises from the ultrasonic vibration emitted by the wire bonding tool 70 for heating the bond wire to form the ball bond 60. As first appreciated by the inventors, ultrasonic vibration of the die bonding tool 70 causes lateral vibration, indicated at 82, in the capillary 72. When a low modulus material is used to attach the die 20 to the DAT 42 this vibration is transmitted through the ball bond to the die, which is laterally displaced by the ultrasonic vibration, as indicated at 84. Such die displacement produces poor bonding between the die bond pad 32 and the ball bond 60. The inventors have discovered that the quality of the bond between a die bond pad and ball bond may be improved by the structure described below with reference to
In
Referring now to
The inventors believe that a structure such as described in
Other method embodiments may include the steps illustrated in Fig, 5 and other additional steps or variations of those steps. For example, the step of forming a stud bump may include forming a stud bump on a bond pad on a face of a die that has a face area of less than about 0.50 mm2.
Another example includes the additional step of attaching the die to a leadframe with a material having a modulus of elasticity of less than about 100 MPa.
Another example includes the additional step of attaching the die to a leadframe with a material having a modulus of elasticity of less than about 90 MPa.
In another example, the step of forming a ball bond comprises forming a ball bond using ultrasonic energy.
In another example, the step of forming a stud bump on a bond pad on a face of a die that has a face area of less than about 1.00 mm2 includes forming the stud bump with a capillary.
In another example, forming a stud bump on a bond pad on a face of a die that has a face area of less than about 1.00 mm2 includes forming the stud bump when the die is still an integral portion of a semiconductor wafer.
In another example, forming a stud bump includes forming a gold stud bump and forming a ball bond on the stud bump includes forming a gold ball bond on the stud bump.
While various embodiments of the invention have been specifically described herein, it will be obvious to those having skill in the art that the invention may be otherwise variously embodied. The appended claims are to be construed to cover all such alternative embodiments except to the extent limited by the prior art.
This application claims the priority of U.S. Provisional Patent Application Ser. No. 62/032212, filed Aug. 1, 2014, which is hereby incorporated by reference for all that it discloses.
Number | Date | Country | |
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62032212 | Aug 2014 | US |