Ball bonding of semiconductor die

Abstract

A method for forming a ball bond for an integrated circuit formed on a semiconductor die includes forming (604) a ball at a first end of a wire inserted into a capillary tool, and lowering (606) the capillary tool toward a pad on the semiconductor die located on a support surface. The method further includes moving the support surface (608) relative to the capillary tool using a motor to bond the ball to the pad without using ultrasonic waves, and then raising (620) the capillary tool.

Description

Background Art

[0001] Wire bonding is used to electrically connect contacts within a semiconductor package. One end of a metal wire (e.g., gold, copper, etc.) is ball-bonded to a bonding pad on a semiconductor die, and the other end is stitched (or wedged) bonded to a lead on a lead frame. To form such a connection, the wire is fed through a capillary associated with a movable bonding head. For ball bonding, an electronic flameout (EFO) mechanism is used to form a ball on the exposed end of the wire. The ball is pulled against the end of the capillary and then pressed into position on a preheated bonding pad, where a combination of heat, pressure, and ultrasonic vibrations are used to adhere the ball to the surface of the bonding pad. With the ball end of the wire fixed to the bonding pad, the wire is released through the capillary when the bonding head is moved to a position at the appropriate lead on the lead frame. A stitch bond is formed on the lead, and the tail wire is released through the capillary, which is clamped and then cut off. A new ball is then formed, the wire end is prepared for the next ball bond, and the cycle is repeated. Summary of the invention

[0002] In one example, a method for forming a ball bond for an integrated circuit formed on a semiconductor die includes: forming a ball at a first end of a wire inserted into a capillary tool; and lowering the capillary tool toward a pad on a semiconductor die located on a support surface. The method further includes: moving the support surface relative to the capillary tool using a motor to bond the ball to the pad without using ultrasonic waves; and then raising the capillary tool. The present invention also discloses a wire bonding connection for an integrated circuit die pad made according to the disclosed process. BRIEF DESCRIPTION OF THE DRAWINGS

[0003] To describe various examples in detail, reference will now be made to the accompanying drawings, in which:

[0004] Figure 1 A bond head assembly for forming wire bonds on a semiconductor die is described according to an example.

[0005] Figures 2 to 5 A timing sequence of operations for forming ball bonds on a semiconductor die according to an example is described.

[0006] Figure 6 A method for attaching a semiconductor die to a leadframe according to an example is presented.

[0007] Figure 7 Examples of ball joints formed according to the disclosed examples are shown.

[0008] Figure 8 The relative uniformity of ball bonds formed according to disclosed examples is illustrated. DETAILED DESCRIPTION

[0009] As described above, ultrasonic waves are often used to bond balls formed at the ends of wires to pads on a semiconductor die. Ultrasonic waves provide high-frequency oscillations of a capillary containing the wire to be bonded to the die. The high-frequency oscillations of the ball caused by the ultrasonic energy result in the ball bonding to the pad on the die. Unfortunately, ultrasonic-based bonding of the ball to the die is not always stable and requires a relatively long time to make a sufficient electrical connection.

[0010] The disclosed examples use a scrubbing technique rather than ultrasonic waves to adhere the ball to the semiconductor die. The scrubbing technique involves lateral (back and forth) or circular motion of the ball relative to a pad on the semiconductor die. In some examples, the scrubbing motion is implemented by a motor that moves a bonding stage on which the semiconductor die resides relative to a capillary containing the wire during the ball bonding process. Depending on the diameter of the wire to be bonded to the die, an appropriate combination of friction force, friction amplitude, and number of scrubbing cycles is used. Examples of these parameters for different wire diameters are provided below.

[0011] Figure 1 An example of a bonding head assembly 100 is described. The bonding head assembly 100 includes a bonding head assembly housing 102 that can move in a plane defined by an X-axis and a Y-axis and is located near a bonding stage 104. The X-axis and the Y-axis define a plane parallel to the plane of the bonding stage 104. An integrated circuit (IC) 130 is shown as being located on the bonding stage 104. The IC 130 includes a semiconductor die 132 attached to a lead frame 134. The bonding head assembly 100 operates to bond one or more wires between a pad on the semiconductor die 132 and the lead frame 134, and the bonding is performed by a friction action rather than using ultrasonic waves. A motor 150 is attached to the bonding stage 104 to move the bonding stage 104 relative to the ball formed at the end of the wire, thereby moving the IC 130 and its semiconductor die 132, as described below.

[0012] Wire rod 114 and capillary rod 112 extend from housing 102. A mechanism including galvanometer arm 116 and pivot 118 is provided for raising and lowering capillary rod 112. Capillary 125 is shown in capillary rod 112. Additional components such as wire clamp 120, venturi 122, and wire spool (not shown) are provided to accommodate feed control of wire into capillary 125 and maintain control of the wire during the process of bonding wire to semiconductor die 132 and lead frame 134. Capillary 125 may include any of a variety of materials, sizes, and configurations based on the wire bonding application at hand.

[0013] The rotation mechanism 110 rotates the capillary rod 112 to align the capillary 125 with the target pad on the semiconductor die 132. The rotation mechanism 110 may include a stepper motor or other mechanical, electromechanical or pneumatic mechanism to rotate the capillary rod 112.

[0014] Figures 2 to 5 A process for bonding wires 202 to pads 204 on die 132 is illustrated. Figures 2 to 5 Shows the timing of the operation. Figure 2 1. A partial cross-sectional view of the capillary 125 is shown in FIG. 1. The wire 202 is guided into the central hole 203 within the capillary 125. For example, a wire spool may feed the wire through the venturi 122 and into the capillary 125. A ball forming mechanism, such as an electronic flameout (EFO) 210, is activated to generate an electric spark that melts the end of the wire 202, thereby forming a ball 220 at the end of the wire. The capillary 125 is then lowered toward the pad 204 at a predetermined speed in the direction indicated by arrow 230. In one example, the speed is 1.6 mm / sec, however the speed may be different than in other examples.

[0015] The ball 220 is in molten form when it contacts the pad 204. Figure 3 As shown, by contacting pad 204, the shape of ball 220 changes to a slightly flattened disc. The bottom edge 127 of capillary 125 is used to partially control the shape of ball 220 when ball 220 is pushed against pad 204. The diameter of ball 220 is Figure 2 The spherical shape in Figure 3 The parts in between are increased in flat shapes. Figure 3 The diameter of the middle flat ball 220 is shown as D1. Before the friction begins, the width of the pad 204 is designated as D2. In some examples, D1 is between approximately 70% of D2. The pad 204 is heated, or has been heated, to a temperature of approximately 250 degrees Celsius.

[0016] exist Figure 4 In the embodiment of the present invention, the motor 150 is activated to move the semiconductor die 132 and its pad 204 relative to the partially flat ball 220. Figure 4 The friction motion may be generally circular, as indicated by arrow 402 in FIG. 1 , or the friction motion may be generally linear. The friction motion and force exerted between the ball and the die causes the ball to deform and attach to the die pad 204. Due to the friction motion, the partially flattened ball further expands outward to provide a diameter D3, as shown in FIG. Figure 5 In some examples, after the friction motion, D3 is between approximately 90% and 95% of D2.

[0017] The frictional motion of the pad 204 relative to the ball 220 is characterized by various parameters, and the values ​​of the parameters depend on the diameter D5 of the wire ( Figure 5). Parameters include friction force, friction amplitude, and friction cycles. Friction force is the downward force of the ball 220 against the pad 204 during the friction motion. Friction amplitude is the distance traced by the ball 220 relative to the pad 204. For a circular motion friction motion, the friction amplitude is the radius of the circle traced by the ball 220 relative to the pad 204. For a linear motion friction motion, the friction amplitude is the lateral distance traced by the ball 220 relative to the pad 204. The friction cycle parameter is the number of complete cycles of the ball's motion relative to the pad 204. For example, if the friction motion is circular, then the number of friction cycles is the number of 360 degree cycles that the ball 220 advances through relative to the pad 204. If the friction motion is linear, then the number of friction cycles is the number of times the ball moves from one side of the pad 204 to the other side and then back.

[0018] The following table provides suitable values ​​of friction force, friction amplitude, and friction cycles for three illustrative wire diameters: 0.96 millimeters ("mils"), 1.3 mm, and 1.98 mm.

[0019] Wire diameter (mm) Friction (g) Friction amplitude Friction cycle 0.96 50 to 60 4 to 5.5 5 1.3 45 to 65 5 to 6 6 1.98 100 to 110 10 to 11 4

[0020] Figure 6 An example of a method of attaching a wire to a semiconductor die pad using friction is shown. At 602, a wire is inserted into a capillary tool (e.g., capillary 125). The wire may be fed from a wire spool via any of a variety of mechanical structures. At 604, Figure 6 The method includes forming a ball at one end of a wire that has been inserted into a capillary tool. In one example, the ball can be formed via electronic quenching as explained above.

[0021] At 606, the illustrative method includes lowering the capillary tool toward a pad on a semiconductor die located on a support surface (e.g., bonding station 104). The downward movement of the capillary tool toward the pad causes the ball to exert a force on the pad that depends on the diameter of the wire. Examples of such forces are provided in the table above.

[0022] At 608, the method includes moving the support surface (e.g., bonding station 104) relative to the capillary tool using a motor to bond the ball to the pad without using ultrasound. The rubbing motion of operation 608 can be circular or linear as described above, or elliptical or other motion as desired. The rubbing amplitude and the number of rubbing cycles also depend on the wire diameter, examples of which are provided in the table above. Then, at 610, the capillary tool is raised by actively raising the capillary tool or actively lowering the support surface.

[0023] At 612, the illustrative method includes forming a stitch bond of the wire to a contact point on the lead frame 134. In one example, the stitch bond is formed by moving a capillary tool to form a loop in the wire, moving the capillary tool over a contact point on the lead frame, lowering the capillary tool to the lead frame, bonding the wire to the lead frame (e.g., using ultrasonic and / or friction), clamping and pulling the wire from the lead frame to cut the wire. The machine for attaching wire to the semiconductor die and lead frame is programmed to determine at 614 whether there is additional wire connecting the die to the lead frame, and if so, repeat the process at 604. However, if no additional wire is needed, the process stops.

[0024] In some examples, multiple semiconductor dies 132 are formed on a common wafer. The wafer is then singulated to obtain individual dies. Each such die has multiple pads to be wire bonded to corresponding connection points on a lead frame. In some examples, each of the wire bonds is performed according to the techniques described herein. Once all of the die pads are wire bonded to connection points on corresponding lead frames, the die and lead frames are then encapsulated in a molding compound (e.g., a polymer resin including filler particles made of silicon dioxide, aluminum oxide, or other suitable materials) as part of the packaging process.

[0025] The disclosed examples also relate to the resulting wire bonding connections for semiconductor die pads made according to the disclosed processes. Figure 7 An example of a wire 702 attached to a pad 706 on a semiconductor die is shown. A ball bond resulting from a friction motion without the use of ultrasonic waves is shown at 710. The resulting ball 220 has a more regular shape than would be the case with ultrasonic waves and without the use of friction motion. The disclosed friction action makes the resulting ball bond more uniform and regular than would be the case with ultrasonic waves or another non-friction bonding technique (as described below). The resulting ball bond connection is more stable and structurally stronger (e.g., less prone to cracking or otherwise experiencing structural failure), and is formed more quickly than when ultrasonic waves are used. Reference numeral 715 identifies the outline of the circumference of the wire 702 when the wire 702 is attached to the ball 220, while reference numeral 725 identifies the outline of the ball 220.

[0026] Figure 8A top view of a ball bond is shown, showing the semiconductor die pad 204, the outline 725 of the ball, and the outline 715 of the wire. The outline 715 of the wire is generally circular because the cross-sectional area of ​​the wire is circular. The outline 725 of the ball is not necessarily circular, but is more circular than when ultrasonic waves are used instead of the rubbing action described herein. Reference numeral 802 identifies the geometric center of the wire outline 715, and the distance between the geometric center 802 and the ball outline 725 is shown as the radius r. When a 360-degree arc is drawn around the outline 725 of the ball, the length of the radius r varies by less than about 4%. "CPK" refers to the degree to which the output of the process is centered between its lower and upper specification limits, and how the output varies. CPK can be expressed as the ratio of the distance of the mean of the output data to the closer specification limit to three times its standard deviation. In the exemplary rubbing process described herein, the distance between the center 802 and the wire outline 715 results in a CPK value of at least 2.0. In conventional ball bonding techniques using ultrasonic waves, the distance between center 802 and wire profile 715 is difficult to control and is typically characterized by a CPK ratio of less than 1.33 or 1.67.

[0027] In an exemplary embodiment, the term "substantially" means that a value or range of values ​​is the stated value or range of values, or is within ±10% of the stated value or range of values. In the present disclosure and claims, the terms "include" and "comprising" are used in an open-ended manner and should therefore be interpreted to mean "including but not limited to..." In addition, the term "coupled" is intended to mean an indirect or direct wired or wireless connection. Thus, if a first device is coupled to a second device, the connection may be through a direct connection or through an indirect connection via other devices and connections.

[0028] The above discussion is intended to illustrate the principles and various embodiments of the present invention. Once the above disclosure is fully understood, many changes and modifications will become apparent to those skilled in the art. It is intended that the appended claims be interpreted as covering all such changes and modifications.

Claims

1. A method for forming ball bonds for an integrated circuit formed on a semiconductor die, comprising: forming a ball at a first delivery end of a wire inserted into a capillary tool; lowering the capillary tool toward a pad on the semiconductor die on a support surface; moving the support surface relative to the capillary tool using a motor to bond the ball to the pad at a first bond strength using a friction action of the ball relative to the pad and without using ultrasonic waves, the first bond strength being greater than a second bond strength if the ultrasonic waves are used; and The capillary tool is raised. 2 . The method of claim 1 , wherein moving the support surface relative to the capillary tool comprises moving the support surface in a circular motion relative to the capillary tool. 3 . The method of claim 1 , wherein moving the support surface relative to the capillary tool comprises linearly moving the support surface relative to the capillary tool.

4. The method according to claim 1, wherein: The wire has a diameter of approximately 0.96 mm; Lowering the capillary tool comprises applying a force in a range of 50 to 60 grams between the ball and the pad; and Moving the support surface relative to the capillary tool includes moving the ball relative to the pad with a lateral displacement in the range of 4 to 5.5 microns, and moving the ball relative to the pad for 5 cycles.

5. The method according to claim 1, wherein: The wire has a diameter of approximately 1.3 mm; Lowering the capillary tool includes applying a force in a range of 45 to 65 grams between the ball and the pad; and Moving the support surface relative to the capillary tool includes moving the ball relative to the pad with a lateral displacement in the range of 5 to 6 microns, and moving the ball relative to the pad for 6 cycles.

6. The method according to claim 1, wherein: The wire has a diameter of approximately 1.98 mm; Lowering the capillary tool comprises applying a force in a range of 100 to 110 grams between the ball and the pad; and Moving the support surface relative to the capillary tool includes moving the ball relative to the pad with a lateral displacement in the range of 10 to 11 microns, and moving the ball relative to the pad for 4 cycles.

7. The method of claim 1 further comprising heating the pad to a temperature of approximately at least 250 degrees Celsius, and wherein moving the support surface relative to the capillary tool comprises moving the support surface while the pad is at a temperature of approximately at least 250 degrees Celsius.

8. The method of claim 1 further comprising raising the capillary to form a loop in the wire and a stitch bond on a lead frame.

9. The method of claim 8, further comprising singulating a wafer to obtain the semiconductor dies and applying a molding compound to the semiconductor dies and the leadframe.

10. A wire bonding connection for a semiconductor die pad formed according to a process comprising the steps of: forming a ball at a first delivery end of a wire inserted into a capillary tool; lowering the capillary tool toward a pad on the semiconductor die on a support surface; moving the support surface relative to the capillary tool using a motor to bond the ball to the pad at a first bond strength using a friction action of the ball relative to the pad and without using ultrasonic waves, the first bond strength being greater than a second bond strength if the ultrasonic waves are used; and The capillary tool is raised. 11 . The wire bonding connection of claim 10 , wherein moving the support surface relative to the capillary tool comprises moving the support surface in a circular motion relative to the capillary tool.

12. The wire bonding connection of claim 10, wherein moving the support surface relative to the capillary tool comprises moving the support surface linearly relative to the capillary tool.

13. The wire bonding connection of claim 10, wherein: The wire has a diameter of approximately 0.96 mm; Lowering the capillary tool comprises applying a force in a range of 50 to 60 grams between the ball and the pad; and Moving the support surface relative to the capillary tool includes moving the ball relative to the pad with a lateral displacement in the range of 4 to 5.5 microns, and moving the ball relative to the pad for 5 cycles.

14. The wire bonding connection of claim 10, wherein: The wire has a diameter of approximately 1.3 mm; Lowering the capillary tool comprises applying a force in a range of 45 to 65 grams between the ball and the pad; and Moving the support surface relative to the capillary tool includes moving the ball relative to the pad with a lateral displacement in the range of 5 to 6 microns, and moving the ball relative to the pad for 6 cycles.

15. The wire bonding connection of claim 10, wherein: The wire has a diameter of approximately 1.98 mm; Lowering the capillary tool comprises applying a force in a range of 100 to 110 grams between the ball and the pad; and Moving the support surface relative to the capillary tool includes moving the ball relative to the pad with a lateral displacement in the range of 10 to 11 microns, and moving the ball relative to the pad for 4 cycles.

16. The wire bonding connection of claim 10, further comprising heating the pad to a temperature of approximately at least 250 degrees Celsius, and wherein moving the support surface relative to the capillary tool comprises moving the support surface while the pad is at a temperature of approximately at least 250 degrees Celsius.

17. The wire bonding connection of claim 10, further comprising raising the capillary to form a loop in the wire and forming a stitch bond on a lead frame.

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