Patent Application
20080116548
The present invention relates to wirebonding and more particularly to a wire bond and a method of forming the wire bond.
Wirebonding is a widely used technique for providing electrical connection between a semiconductor chip and a chip carrier. Wirebonding typically involves forming a first wire bond on a bond pad of the semiconductor chip and a second wire bond on a lead finger of a lead frame or a pad surface of a substrate. Referring now to
Wire bond interconnections formed between the semiconductor chip and the chip carrier may be evaluated visually and/or by mechanical testing methods. A peel strength test is one such mechanical testing method. The peel strength of a wire bond provides an indication of the likelihood of bond failure. The peel strength test is performed by placing a hook under the wire proximate to a wire bond and applying a lifting force to test the strength of the adhesion of the wire bond to the bonding site. Typically, when a second wire bond is identified as having low peel strength, one or more bonding parameters of the second wire bond such as, for example, bonding force, bonding time, and ultrasonic frequency and power, are varied to increase the peel strength of the second bond. However, this often leads to a reduction in the thickness of the bond area of the second bond, making the second bond more susceptible to heel cracking. Heel cracking significantly reduces the pull strength of the second bond and is also known to cause premature cycling failures. Thus, a need exists for a wire bond with increased peel strength and bond area thickness, and a method of forming such a wire bond.
The following detailed description of preferred embodiments of the invention will be better understood when read in conjunction with the appended drawings. The present invention is illustrated by way of example and is not limited by the accompanying figures, in which like references indicate similar elements. It is to be understood that the drawings are not to scale and have been simplified for ease of understanding the invention.
The detailed description set forth below in connection with the appended drawings is intended as a description of the presently preferred embodiments of the invention, and is not intended to represent the only form in which the present invention may be practiced. It is to be understood that the same or equivalent functions may be accomplished by different embodiments that are intended to be encompassed within the spirit and scope of the invention. In the drawings, like numerals are used to indicate like elements throughout.
The present invention provides, in a semiconductor package having a plurality of wire bond interconnections between a semiconductor chip and a chip carrier, a wire bond including a first bond formed at a first location on a bonding site, and a second bond formed at a second location on the bonding site such that the second bond at least partially overlaps the first bond.
The present invention also provides a semiconductor packaged device including a lead frame having a plurality of lead fingers, and a chip attached to the lead frame. The chip includes a plurality of bond pads. Ones of the bond pads are electrically connected with respective ones of the lead fingers by a plurality of wires such that a bond connecting one of the wires to a corresponding lead finger is a forward-folded type stitch bond.
The present invention further provides a method of forming a wire bond including the steps of forming a first bond by bonding a first portion of a wire at a first location on a bonding site with a bonding tool, and forming a second bond by bonding a second portion of the wire at a second location on the bonding site with the bonding tool such that the second bond at least partially overlaps the first bond.
Referring now to
Referring now to
In the embodiment shown, the first stitch bond 22 is formed after a first wire bond (not shown) is formed on a bond pad (not shown) of a semiconductor chip (not shown) with a first end of the wire 30. In the presently preferred embodiment, the first wire bond is a ball bond. Ball bonding is well known by those of skill in the art and therefore detailed descriptions thereof are not necessary for a full understanding of the invention. After forming the first wire bond on the bond pad, the bonding tool 32, which is still holding the wire 30, rises and moves towards the bonding site 26, creating a loop shape. The first portion 28 of the wire 30 is brought into intimate contact with the first location 24 of the bonding site 26 by the bonding tool 32 and the first stitch bond 22 is formed on the bonding site 26 by applying a combination of heat, pressure, and/or ultrasonic energy. Heat may be applied using either a heated bonding tool 32, a heated pedestal (not shown) on which the chip carrier is placed, or both. Such formation of the first stitch bond 22 is well known in the art. Example wire bonder parameters for the formation of the first stitch bond 22 are ultrasonic energy: 12 mA, time: 12 ms, force: 300 g, and bonding temperature: 150° C. The present invention is not limited to a particular set of bonding parameters, as the optimum bonding parameters are dependent on wire type, pad metallization and device configurations.
In one embodiment, the bonding site 26 comprises a lead frame finger. Nonetheless, those of skill in the art will understand that the present invention is not limited to lead frame packaging. In alternative embodiments, the bonding site 26 may be a pad surface of a substrate. Lead frames, substrates and their respective bonding sites are known to those of ordinary skill in the art and therefore, detailed descriptions thereof are not necessary for a full understanding of the invention.
Gold (Au) and aluminium (Al) wire are most commonly used in wirebonding. Both gold and aluminum are strong and ductile and have similar resistance in most environments. Gold wire is sometimes doped with a dopant, such as beryllium (Be) or calcium (Ca) in order to stabilize it. Small-diameter aluminium wire is often doped with silicon (Si) or sometimes magnesium (Mg) to improve its breaking load and elongation parameters. In addition to gold and aluminum, copper (Cu), palladium (Pd) alloy, platinum (Pt) and silver (Ag) bonding wire are also known.
In one embodiment, the wire 30 has a diameter Dw of between about 20.3 microns (μm) to about 50.8 μm, although wires of other diameters may also be used and the invention should not be limited to a particular wire diameter. As is known by those of skill in the art, various size wires are available for connecting the semiconductor chip to the chip carrier, with the wire size being selected based on, among other things, the spacing between the bonding sites.
The wire 30 extends though a hole 34 in the bonding tool 32. In the embodiment shown, the bonding tool 32 is a capillary having an outside radius R defined by a curved surface of between about 0.01 millimeters (mm) and about 0.076 mm, an angle θ of between about 6° to about 11° between a bottom face 36 of the capillary and the bonding site 26, which is typically horizontal, and a hole diameter Dh of between about 24 μm and about 65 μm. The capillary bonding tool 32 may be made from ceramic, tungsten or ruby materials, as are typically used. Such a bonding tool is well known in the art and therefore, further description of the capillary is not required for a complete understanding of the present invention.
Once the first stitch bond 22 is formed, the bonding tool 32 is raised in an upward movement, as indicated by the arrow in
The bonding tool 32 is then moved back over the first stitch bond 22 and the first location 24 in a reverse movement as indicated by the arrow in
Referring now to
The second stitch bond 40 is formed on the bonding site 26 by applying a combination of heat, pressure, and/or ultrasonic energy. In one embodiment, for a QFN (Quad Flat No lead) type package, the second stitch bond 40 is formed using: ultrasonic energy: 12 mA, Time: 12 ms, Force: 300 g, and bonding temperature: 150° C. However, as previously mentioned, those of skill in the art will understand that the present invention is not limited to a particular set of bonding parameters. Rather, the optimum bonding parameters are dependent on wire type, pad metallization and device configurations.
The formation of the second bond 40 over the first stitch bond 22 increases the contact area between the wire bond 20 and the bonding site 26, as well as the thickness of the wire bond 20. The increase in contact area and wire bond thickness enhances the adhesion of the wire bond 20 to the bonding site 26, which in turn improves wire bond performance and reliability.
Referring now to
Referring now to
Referring now to
The first crescent-shaped area 48 has a thickness T of about two or more times a thickness of the first stitch bond 22. In preferred embodiments, the first crescent-shaped area 48 has a thickness T of at least about 4 um (for 1.0 mil wire for example); a thicker wire would have a thicker crescent-shaped area.
A comparison of the peel strength of the forward-folded type stitch bond and a conventional stitch bond was made by performing a peel strength test on a hundred (100) forward-folded type stitch bonds and repeating the test on a hundred (100) conventional stitch bonds. The results of the comparison are shown in Table 1 below.
As can be seen from Table 1, when taking 1.0 mil wire for example, the mean peel force of the forward-folded type stitch bonds is about 1.1 grams (g) higher than that of the conventional stitch bonds. Thus, it can be said that the forward-folded type stitch bond has increased peel strength. In preferred embodiments, the forward-folded type stitch bond has a peel strength of at least about —4.9 gram (1.0 mil wire for example); thicker wire would have an even greater peel strength improvement.
As is evident from the foregoing discussion, the present invention provides a wire bond with increased peel strength and bond area thickness, and a method of forming such a wire bond. Advantageously, the formation of a second stitch bond over a first stitch bond increases: the contact area between the wire bond and the bonding site, as well as the thickness of the wire bond. The increase in contact area and wire bond thickness enhances the adhesion of the wire bond to the bonding site, which in turn improves wire bond performance and reliability. Apart from increasing peel strength, the increase in wire bond thickness also reduces the risk of heel cracking. Further advantageously, because the present invention can be implemented using currently available wire bonders, there is no need for additional capital investment.
The description of the preferred embodiments of the present invention have been presented for purposes of illustration and description, but are not intended to be exhaustive or to limit the invention to the forms disclosed. It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but covers modifications within the spirit and scope of the present invention as defined by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 200610149236.7 | Nov 2006 | CN | national |
