Wire Bonding Method and Apparatus

A method forming a bond wire connection includes providing a wire bonder including a bond wedge with a wire guide, and forming a wire bond loop by initially bonding a bond wire to a first bonding surface using the bond wedge, then moving the wire bonder in a loop pattern whereby the bond wire passes through the wire guide, and then bonding the bond wire to a second bonding surface using the bond wedge, wherein moving the wire bonder in the loop pattern comprises a retrograde movement whereby the wire bonder moves away from the second bonding surface, and wherein the wire guide is formed from a material with a higher material hardness than the bond wire.

BACKGROUND

Semiconductor devices are typically provided within a package that houses and protects the dies and associated electrical connections from the exterior environment. A discrete package may include one or more semiconductor dies mounted on a metal lead frame. A power module may include several dies mounted on a power electronics carrier, e.g., DBC (direct bonded copper) substrate, IMS (insulated metal substrate) or AMB (active metal brazed) substrate, and an enclosure over the power electronics carrier. In either type of package, electrical interconnections must be made between the various components, e.g., die to lead connections, die to die connections, bond pad to bond pad connections, etc. There are various ways to form these electrical interconnections. One such way is a wire bonding technique. It would be desirable to improve the quality, performance, and cost of wire bonding.

SUMMARY

A method of forming a bond wire connection is disclosed. According to an embodiment, the method comprises providing a wire bonder comprising a bond wedge with a wire guide, and forming a wire bond loop by initially bonding a bond wire to a first bonding surface using the bond wedge, then moving the bond wedge in a loop pattern whereby the bond wire passes through the wire guide, and then bonding the bond wire to a second bonding surface using the bond wedge, wherein moving the bond wedge in the loop pattern comprises a retrograde movement whereby the bond wedge moves away from the second bonding surface after bonding the first bond, and wherein the wire guide is formed from a material with a higher material hardness than the bond wire.

A semiconductor device is disclosed. According to an embodiment, the semiconductor device comprises a bond wire connection that forms an electrical connection of a semiconductor device, wherein the bond wire connection comprises a wire bond loop between a first bonding surface and a second bonding surface, wherein the wire bond loop is formed from a bond wire comprising copper with a diameter of between 400 μm and 500 μm, wherein a bond loop length of the wire bond loop is ≤5,500 μm, and wherein a bond loop height of the wire bond loop is ≤2,5000 μm.

DETAILED DESCRIPTION

Embodiments of a method of forming a bond wire connection and corresponding bond wire connection are disclosed herein. The bond wire connection comprises a wire bond loop between a first bonding surface and a second bonding surface. The physical attributes of the wire bond loop, specifically the relation between the bond loop height and the bond loop length, are advantageous. Conventionally when forming wire bond loops, there is a tradeoff between the minimum attainable bond loop height relative bond loop length. As the bond loop length decreases, the bond loop height must be increased to reliably effectuate the bond wire connection. The methods disclosed herein facilitate the formation of otherwise unattainably low bond loop height values relative to bond loop length. In particular, the methods comprise using a bond wedge with a wire guide that is harder than the bond wire form the wire bond loop. In addition, the methods comprise using a retrograde movement into the loop pattern wherein the bond wedge moves backwards and away from the intended subsequent bonding point. The combination of the stiffer wire guide bond wedge and retrograde movement cause a shaping of the bond wire during the loop formation that reliably creates low-profile wire bond loops.

Referring toFIG.1, a bond wire connection100is shown, according to an embodiment. The bond wire connection100forms an electrical connection of a semiconductor device. For example, the bond wire connection100may form an electrical connection between the structured metal pads of a circuit carrier, e.g., a PCB (printed circuit board), a DBC (direct bonded copper) substrate, IMS (insulated substrate) substrate or AMB (active metal brazed) substrate and/or may form an electrical connection between the bond pad from a semiconductor die and a circuit carrier, lead frame, or other semiconductor die. The bond wire connection100comprises a wire bond loop102between a first bonding surface104and a second bonding surface106. The first bonding surface104and the second bonding surface106are metal surfaces configured to mate with a bond wire. For example, the first bonding surface104and the second bonding surface106may be provided by one or two bond pads from a semiconductor die, circuit carrier, lead frame, etc. The first bonding surface104and the second bonding surface106may be provided from metal structures that comprise and/or are plated with Cu, Ni, Ag, Au, Pd, Pt, Ni, for example. As shown, the first bonding surface104and the second bonding surface106are on the same vertical level. In other cases, the first bonding surface104and the second bonding surface106may be vertically offset from one another, e.g., in the case of a wire bond loop102formed between a semiconductor die and a lead frame or circuit carrier. As shown, the bond wire connection100comprises a single wire bond loop102between two metal structures that are separated from one another. In other embodiments, a bond wire connection100may comprise multiple wire bond loops102, e.g., a bond wire may be affixed to a single bond pad, e.g., from a circuit carrier by multiple wire bond loops102to enhance mechanical attachment and/or lower electrical resistance. In that case, the first bonding surface104and the second bonding surface106can be from the same continuous metal structure.

The wire bond loop102is formed by a bond wire108. The bond wire108comprises a low electrical resistance metal that is suitable for semiconductor device interconnection. For example, the metal bond wire108may comprise copper, aluminum, silver, and alloys thereof. According to an embodiment, the metal bond wire108is a copper bond wire108, i.e., a bond wire108formed of substantially pure copper, e.g., 95% pure or 99% pure copper. A diameter of the bond wire108may be in the range of 25 μm and 750 μm, for example. According to an embodiment, the diameter of the bond wire108is in the range of 300 μm and 500 μm. In a more particular embodiment, the bond wire108is a copper bond wire108with a diameter of 400 μm, meaning that the nominal diameter value of the wire is 400 μm. The bond wire108may have a rounded cross-section. Other cross-sectional geometries such as rectangular geometries are possible as well.

Referring toFIG.2, a method of forming the bond wire connection100comprises providing a wire bonder200. The wire bonder200comprises a bond wedge202with a wire guide204. The bond wedge202may be connected to a shaft (not shown) and may be actuated by a programmable robotic mechanism. The wire guide204forms a channel on the bond wedge202that is dimensioned to retain and guide a bond wire108while it moves through the wire guide204. The wire guide204comprises a backing (not shown) that corresponds to a bottom surface of this channel. The wire bonder200is configured to feed a length of bond wire108through the wire guide204such that the bond wire108wraps around a tip206of the bond wedge202. The tip206can be used to apply mechanical pressure and ultrasonic energy and/or manipulate the bond wire108. The bond wedge202may additionally comprise a blade208that is between the wire guide204and the tip206of the bond wedge202. The blade208is slidable outward and is used to sever the bond wire108as needed.

According to an embodiment, the wire guide204is formed from a material with a higher material hardness than the bond wire108. Material hardness refers to the degree to which the material irreversibly deforms in response to an applied force, i.e., the plasticity of the material. Material hardness may be measured according to the measured according to the Rockwell, Vickers, Shore, or Brinell scale. Thus, in the case that the bond wire108comprises, copper, aluminum, or alloys thereof, the wire guide204may be formed from a metal with a higher material hardness than metal which forms the bond wire108. According to an embodiment, the wire guide204comprises any one or more of: Cu, Ni, Ti, Zn, Fe, and alloys thereof, wherein these metals or alloys are selected harder than the bond wire108. In one particular example, the wire guide comprises K88 copper, which is a metal alloy of copper that is harder than typical copper used in for bond wires. The wire guide204may alternatively be formed of or comprise other materials with a higher material hardness than the bond wire108, e.g., diamond, sapphire, etc. Additionally, the wire guide204is formed with a high stiffness. Stiffness refers to the propensity of the wire guide204to return to its original position after being subjected to a high force. In this case, the wire guide204may have a sufficiently high stiffness to perform the wire bonding process disclosed herein with a bond wire108, e.g., a copper bond wire at least 500 μm in diameter, and return to its original position with negligible bending or warpage. This high stiffness is obtained through a combination of using high material harness materials as disclosed above and through appropriate dimensioning and configuration of the bond wedge202.

Referring to the combination ofFIGS.1and2, the wire bonder200forms the wire bond loop102in the following way. Initially, the bond wire108is bonded to the first bonding surface104using the bond wedge202. This process may involve applying pressure to the bond wire108by the tip206of the bond wedge202in a vertical direction that is orthogonal to the first bonding surface104. Optionally, ultrasonic energy may be applied to the bond wire108and/or to the first bonding surface104while the pressure is applied, thereby deforming the bond wire108and creating a permanent substance-to-substance bond between the bond wire108and the first bonding surface104. Additionally, or alternatively, heat may be applied to the bond wire108and/or the first bonding surface104, thereby softening the bond wire108and/or the metal that forms the first bonding surface104before. The heat may be applied before, during, or after the application of pressure by the bond wedge202. After the bond between the bond wire108and the first bonding surface104is created, the bond wedge202is moved in a loop pattern whereby the bond wire108passes through the wire guide204. The loop pattern moves the bond wedge202in open-ended shape (e.g., as shown inFIG.4) between the first bonding surface104and the second bonding surface106. After moving the bond wire108in the loop pattern, the bond wire108is bonded to the second bonding surface106using the bond wedge202, e.g., in the same way as the process for bonding the bond wire108to the first bonding surface104.

Referring toFIG.3, the method of forming the bond wire connection100is shown at selected points in the loop pattern. As shown inFIG.3A, the loop pattern comprises a first movement300immediately after bonding the bond wire108to the first bonding surface104. The first movement300moves the bond wedge202vertically away from the first bonding surface104. According to an embodiment, the first movement300moves the bond wedge202in a vertical direction VD1that is orthogonal to the first bonding surface104. Alternatively, the first movement300may comprise lateral movement as well, e.g., the first movement300may move the bond wedge202vertically away from the first bonding surface104while also moving the bond wedge202in a lateral direction LD1towards the second bonding surface106.

As shown inFIG.3B, the loop pattern comprises a second movement302that is performed after the first movement300. The second movement302is a retrograde movement whereby the bond wedge202moves away from the second bonding surface106. That is, the bond wedge202is moved in an opposite direction as the lateral direction LD1that it needs to move in order to reach the second bonding surface106. According to an embodiment, the retrograde movement moves the bond wedge202exclusively in a lateral direction that is substantially parallel to the first bonding surface104, i.e., substantially orthogonal to the vertical direction VD1and opposite from the lateral direction LD1shown in the figures. In this context, the term substantially parallel refers to a movement that is nominally parallel and or within +/−5 degrees of parallel. Alternatively, the retrograde movement may comprise a vertical movement component, i.e., the retrograde movement may be intentionally tilted at an angle such that the bond wedge202moves vertically towards or away from towards or away from the first bonding surface104in the vertical direction VD1as it moves laterally away from the second bonding surface106.

As shown inFIG.30, the loop pattern comprises a third movement304that is performed after the second movement302. The third movement304moves the bond wedge202away from the first bonding surface104. According to an embodiment, the third movement304moves the bond wedge202in the vertical direction VD1that is orthogonal to the first bonding surface104. Alternatively, the third movement304may comprise a lateral movement as well, e.g., the third movement304may move the bond wedge202vertically away from the first bonding surface104while also moving the bond wedge202in a lateral direction LD1towards the second bonding surface106.

As shown inFIG.3D, the loop pattern comprises a further movement305that moves the bond wedge202laterally towards the second bonding surface106immediately after the third movement304. This further movement may comprise moving the bond wedge202in the lateral direction LD1that is substantially parallel to the first bonding surface104. Alternatively, this further movement may comprise vertical movement as well, e.g., the further movement may move the bond wedge202vertically towards or away from the first bonding surface104while also moving the bond wedge202towards the second bonding surface106in the lateral direction LD1.

Referring toFIG.4, an exemplary loop pattern400that may be programmed into a wire bonding machine is shown, according to an embodiment. The loop pattern400corresponds to a programmed route that the bond wedge202follows in between bonding the bond wire108to the first bonding surface104and bonding the bond wire108to the second bonding surface106. The loop pattern400comprises a first movement300performed immediately after bonding the bond wire108to the first bonding surface104, a second movement302performed immediately after the first movement300, a third movement304performed immediately after the second movement302, a fourth movement306performed immediately after the third movement304, a fifth movement308performed immediately after the fourth movement306, and a sixth movement310performed immediately after the fifth movement308and immediately before bonding the bond wire108to the second bonding surface106. The first movement300, the second movement302, and the third movement304are performed as described above. In this case, the first movement300and the third movement304each move the wire bonder200exclusively in the vertical direction VD1that is orthogonal to the first surface, and the second movement302is a retrograde movement that moves the wire bonder200exclusively in a direction that is parallel to the first bonding surface104and opposite from the lateral direction LD1. The fourth movement306moves the wire bonder200in a tilted direction that moves vertically away from the first bonding surface104and laterally towards the second bonding surface106the lateral direction LD1. The fifth movement308moves the wire bonder200in a tilted direction that moves vertically towards the first bonding surface104and laterally towards the second bonding surface106the lateral direction LD1. Thus, the fourth and fifth movements306,308form an arched pattern with an apex point in between the first bonding surface104and the second bonding surface106. The sixth movement310moves the bond wedge202vertically towards the second surface, thereby bringing the bond wedge202vertically to a location wherein the bond wedge202bond wedge may effectuate the bond between the bond wire108and the second bonding surface106.

Referring again toFIG.1, the wire bond loop102is characterized by two dimensional parameters, namely, a height H1of the wire bond loop102and a length L1of the wire bond loop102. The height H1of the wire bond loop102refers to a maximum displacement between an apex point of the wire bond loop102and the furthest one of the first bonding surface104and the second bonding surface106. The length L1of the wire bond loop102refers to a distance between the contacting point of the wire bond loop102and the first bonding surface104and the contacting point of the wire bond loop102and the second bonding surface106. The length L1of the wire bond loop102includes a part of the bond wire108that contacts the metal surface. In the case of multiple wire bond loops102formed in succession, the length L1of the wire bond loops102may be measured at the midpoint of the contacting portion of the bond wire108.

The ratio between the height H1of the wire bond loop102and the length L1of the wire bond loop102is an important parameter that impacts cost and performance of the bond wire connection100. By reducing the height H1of the wire bond loop102at a given length L1, the electrical resistance of the connection may be reduced and the material costs for forming the bond wire connection100may be reduced. When compounded across a device with tens, hundreds or even thousands of these wire bond loops102, the benefits of such a height reduction may be significant. Current wire bonding technologies are limited in their ability to lower the height H1of a wire bond loop102. Moreover, there is an inverse relationship between the length L1of the wire bond loop102and the lowest achievable height H1of the wire bond loop102. That is, shorter wire bond loops102may require a higher wire bond loop102height H1in order to ensure that the bond wire108deviates from the bonding plane and does not cause an electrical short. Using a copper bond wire108with a diameter of 400 μm as an example, the minimum achievable height H1of the wire bond which enables a stable loop shaping by a conventional wire bonding technique is 2000 μm for a wire bond length L1of 5,500 μm, is 2,200 μm for a wire bond length L1of 5,000 μm, is 2,400 μm for a wire bond length L1of 4,500 μm, is 2,800 μm for a wire bond length L1of 4,000 μm, is 3,000 μm for a wire bond length L1of 3,500 μm, and is 3,500 μm for a wire bond length L1of 3,000 μm. In this context, a conventional wire bonding technique refers to a technique wherein the bond wedge202does not move in a retrograde movement and/or does not use a wire guide204with a high stiffness that is formed from a material with a higher material hardness than the bond wire.

The combination of moving the bond wedge202in a loop pattern that comprises a retrograde movement and using a wire guide with a high stiffness that is formed from a material with a higher material hardness than the bond wire108allows for the formation of wire bond loops102with a significantly lower ratio between the height H1of the wire bond loop102and the length L1of the wire bond loop102than the conventional wire bonding technique. Table 1 below provides exemplary values for the height H1of the wire bond loop102and the length L1of the wire bond loop102that may be obtained in a copper bond wire108with a diameter of 400 μm according to the presently disclosed techniques.

The above provided values are illustrative of a beneficial ratio between the height H1of the wire bond loop102and the length L1of the wire bond loop102that may be obtained by the presently disclosed technique. An equivalent beneficial improvement to the ratio may be obtained with different types of bond wires108, e.g., bond wires108having different thickness, hardness, material composition, etc., while not necessarily having the same absolute values as provided above.

Referring again toFIG.3, the beneficial improvement to the ratio between the height H1of the wire bond loop102and the length L1of the wire bond loop102may result from a manipulation of the bond wire108that occurs during the movement of the bond wedge202in the loop pattern400. Specifically referring toFIGS.3C-3D, the retrograde movement of the bond wedge202creates a kink110in the bond wire108, i.e., an acute curving of the bond wire108away from the second bonding surface106. This kink110remains in the bond wire108as the wire bonder200continues forming the wire bond loop102by moving towards the second bonding surface106. This kink110facilitates the reliable formation of low-profile wire bond loops102by instilling the necessary vertical clearance between the bond wire108and the first bonding surface104into the wire bond loop102and providing a weaker point that allows for greater manipulation of the wire bond loop102. The subsequent movement of the bond wedge202towards the second bonding surface106therefore does not result in creating too shallow of a departure angle. The kink110results from the combination of moving the bond wedge202in the retrograde movement and using a wire guide204with a high stiffness that is formed from a material with a higher material hardness than the bond wire108. This allows the wire guide204to instill the kink110as a permanent shape without damage or excessive wear to the wire guide204. By way of comparison, a bond wedge202with a relatively softer material, e.g., plastic, for the wire guide204may not be able to effectively create the kink and/or may become damaged by an acute retrograde movement when used in combination with a harder bond wire108.

The loop pattern400described with reference toFIG.4was used in combination with a 400 μm thick copper bond wire108, with bond loop lengths L1in the range of 2,000 μm to 15,000 μm. The second movement302of the loop pattern400, i.e., the retrograde movement, moves the bond wedge202by up to 600 μm. This loop pattern400may represent an at least nearly optimized pattern for this particular wire type. The optimal geometry and extent of retrograde movement may differ in the case of different bond wire108s. In general, a beneficial impact on the length L1to width ratio of a wire bond loop102may be realized by any loop pattern400that comprises a retrograde movement and utilizes a wire guide204with a high stiffness that is formed from a material with a higher material hardness than the bond wire108.

Example 1. A method of forming a bond wire connection, the method comprising: providing a wire bonder comprising a bond wedge with a wire guide; and forming a wire bond loop by initially bonding a bond wire to a first bonding surface using the bond wedge, then moving the wire bonder in a loop pattern whereby the bond wire passes through the wire guide, and then bonding the bond wire to a second bonding surface using the bond wedge, wherein moving the wire bonder in the loop pattern comprises a retrograde movement whereby the wire bonder moves away from the second bonding surface, and wherein the wire guide is formed from a material with a higher material hardness than the bond wire.

Example 2. The method of example 1, wherein the loop pattern comprises a first movement immediately after bonding the bond wire to the first bonding surface and a second movement immediately after the first movement, wherein the first movement moves the bond wedge vertically away from the first bonding surface, and wherein the second movement is the retrograde movement.

Example 3. The method of example 2, wherein the retrograde movement moves the wire bonder in a lateral direction that is substantially parallel to the first bonding surface.

Example 4. The method of example 2, wherein the loop pattern comprises a third movement immediately after the second movement, and wherein the third movement moves the wire bonder vertically away from the first bonding surface.

Example 5. The method of example 4, wherein the loop pattern moves the wire bonder laterally towards the second bonding surface immediately after the third movement.

Example 6. The method of example 5, wherein the loop pattern comprises a fourth movement immediately after the third movement and a fifth movement immediately after the fourth movement, wherein the fourth movement moves the wire bonder in a tilted direction that moves vertically away from the first bonding surface and laterally towards the second bonding surface, and wherein the fifth movement moves the wire bonder in a tilted direction that moves vertically towards from the first bonding surface and laterally towards the second bonding surface.

Example 7. The method of example 1, wherein the bond wire is a copper or copper alloy wire, and wherein the wire guide is formed from a metal with a higher material hardness than the copper or copper alloy wire.

Example 8. The method of example 7, wherein the wire guide comprises any one or more of: Cu, Ni, TI, Zn, Fe, and alloys thereof.

Example 9. The method of example 1, wherein the bond wire is a copper or copper alloy wire with a diameter of between 300 μm and 500 μm.

Example 10. The method of example 9, wherein the diameter of the bond wire is 400 μm.

Example 11. The method of example 10, wherein a bond loop length of the wire bond loop is ≤5,500 μm, and wherein a bond loop height of the wire bond loop is ≤1,800 μm.

Example 12. The method of example 11, wherein the bond loop length of the wire bond loop is ≤4,000 μm.

Example 13. The method of example 11, wherein the bond loop height of the wire bond loop is ≤1,400 μm.

Example 14. A semiconductor device, comprising: a bond wire connection that forms an electrical connection of a semiconductor device, wherein the bond wire connection comprises a wire bond loop between a first bonding surface and a second bonding surface, wherein the bond wire is a copper or copper alloy wire with a diameter of between 300 μm and 500 μm, wherein a bond loop height of the wire bond loop is between 1,200 μm and 2,200.

Example 15. The semiconductor device of claim 14, wherein the bond loop height is less than or equal to 2,000 μm.

Example 16. The semiconductor device of example 15, wherein the bond loop height is less than or equal to 1,400 μm.

Example 17. The semiconductor device of example 14, wherein a bond loop length of the wire bond loop is between 3,500 μm and 6,000 μm.

Example 18. The semiconductor device of example 17, wherein the bond loop length is less than 5,000 μm.

Example 19. The semiconductor device of example 18, wherein the bond loop length is less than 4,000 μm.

Example 20. The semiconductor device of example 14, wherein the bond wire is a copper wire, and wherein the diameter is 400 μm.