The present invention relates to a wire bonding method and device for electrically interconnecting an electrode pad of a semiconductor pellet and an external terminal by means of a fine metal wire.
Recently, there have been used wire bonding devices called "digital wire bonder". These wire bonding devices use a solenoid as a wire clamp and a drive motor for vertically moving the wire clamp and a bonding tool.
FIG. 1 shows a wire bonding device of the type described above. A bonding heat unit 4 is mounted on an XY table 3 which can be displaced in the X and Y directions by means of an X-direction drive motor 1 and a Y-direction drive motor 2. The bonding head unit 4 has a bonding tool 6 through which a bonding wire 5 is passed and which is attached to a bonding arm 8 which in turn pivots about a pivotal point 7. There is also provided a swinging arm 9 pivotable about the pivotal point 7. When a shaft or eccentric pin 11 is rotated in the clockwise or counterclockwise direction by a drive motor 10, the swinging arm 9 is vertically (in the Z direction) moved. Upon vertical movement of the swinging arm 9, a clamp arm 12, a bonding arm lock pin 13 and the bonding arm 8 are caused to move vertically. The Z-direction drive motor 10 is provided with a rotary encoder (not shown) for detecting the angle of rotation and in response to the angle of rotation detected by the rotary encoder, the height of the bonding tool 6 can be detected. The pivotal point 7 is in the form of a double shaft so that both the bonding arm 8 and the swinging arm 9 can pivot about it. The motion of the swinging arm 9 is transmitted to the bonding arm 8 when an auxiliary bonding arm 15 is pressed against the bonding arm lock pin 13 by means of an arm lock solenoid 14. The auxiliary bonding arm 15 and the swinging arm 9 are also attracted with each other under the force of a linear motor 16. When the bonding arm 8 is vertically moved at a high speed, the bonding arm 8 is locked by means of the arm lock solenoid 14. When a wire is bonded, the bonding tool 6 is placed into contact with a bonding pad surface of a semiconductor pellet and the auxiliary bonding arm 15 is separated from the bonding arm lock pin 13. In this case, only a force required for bonding is applied to the auxiliary bonding arm 15 by the linear motor 16. A wire clamp 17 for clamping a bonding wire 5 and a clamp solenoid 18 are mounted on the clamp arm 12, and a wire clamp 25 and a clamp solenoid 26 are mounted on a stationary arm 27.
Referring further to FIGS. 2A and 2B, the mode of operation of the wire bonding device with the above-mentioned construction will be described. A ball is formed at the leading end of the wire 5 extended from the bonding tool 6 (time: a) and the bonding tool 6 together with the wire clamp 17 is lowered to a first bonding position A. In this case, the wire clamp 27 is closed so that friction is produced between the wire 5 and the clamp 25. As a result, the ball is clinched to the leading end of the bonding tool 6 (time: b). When the first bonding operation is accomplished, the bonding tool 6 and the wire clamp 17 are lifted by H and a desired length of wire is extracted from the bonding tool 6 (time: c). During this time, the XY table 3 is moved from the first bonding position to the second bonding position. Based upon the distance L between the first and second bonding positions, the lift H of the bonding arm 6 is determined by the following equation: EQU H=(1.5-2.0).times.L+.alpha.
where .alpha. is determined based on experiences. Thereafter the bonding tool 6 is gradually lowered (time: d) and the wire 5 is bonded at the second bonding position (time: e). In this case, the wire clamp 25 remains clamped. Upon completion of bonding, the bonding tool 6 is lifted (time: f). The bonding tool 6 is stopped temporarily and the clamp 17 is closed so as to clamp the wire 5. The bonding tool 6 and the clamp 17 are lifted again and then wire 5 is cut off (time: g). When the bonding tool 6 is lifted, a ball is formed at the leading end of the wire by means of an H.sub.2 O.sub.2 torch or an electric torch 21 (See FIG. 2B) (time: h). Thus, one bonding operation is completed.
With the above described wire bonding method and device, the length W.sub.1 of the wire 5 to be bonded cannot be determined by the vertical shift H of the bonding tool 6. The reason is that the length W.sub.1 of the wire 5 varies in response to various factors such as the velocity of the bonding tool 6, frictional resistance between the bonding tool 6 and the wire 5, frictional resistance between the wire clamps 17 and 27 and the wire 5, resistance encountered when the wire 5 is unrolled from a wire spool 19, the hardness of the wire 5, the buckling strength of the wire 5 and so on. As a result, the length W.sub.1 of the wire 5 sometimes becomes shorter or longer than the appropriate length of wire. If the wire 5 is too short, a wire 23 is placed into contact with the corner of a semiconductor pellet 20 thereby causing a short-circuit circuit (See FIG. 3A). On the other hand, when the wire 23 is too long, the wire 23 becomes in the form of a helicoid and makes contact with other wires or a lead frame 22 (See FIG. 3B). In some cases, as shown in FIG. 3c, the wire 23 is slacked and placed into contact with the lead frame 22. Furthermore, as shown in FIG. 3D, the wire 23 is bent laterally and placed in to contact with the lead frame 22.
In order to overcome the above problems, there has been proposed a method for rotating a wire spool 19 so as to control the length W.sub.1 of the wire 5. However, the adverse effects of the above described problems cannot be eliminated.
Furthermore, the conventional bonding methods and devices have the following problems: (1) that the inertia of a swinging part is large so that a high-power driving means is required; (2) that in order to form a loop of wire, the bonding tool must be vertically shifted by some distance or height so that it takes time to vertically move the bonding tool; and (3) that in order to cut off wire into a predetermined length, the bonding tool must be temporarily stopped while it is being lifted and then the wire clamp must be closed. As a result, the number of bonding operations per unit time is limited.
There is a following problem in the movement of the bonding tool 6 in the XY direction. That is, in order to move the bonding tool 6 in the XY direction the X-direction and Y-direction motors 1 and 2 must be energized. For instance, assume that, as shown in FIG. 4, the wire bonding tool 6 must be shifted from the origin O to a desired point P (a,b). Then the drive motors 1 and 2 are simultaneously energized and when the desired value b in the Y-direction is attained, the drive motor 2 is stopped. Thereafter when the desired value a in the X-direction is attained the drive motor 1 is stopped. As a result, the bonding tool 6 describes a locus 1.sub.1 as shown in FIG. 4. The locus 1.sub.1 is bent at a point Q (b,b). The locus 1.sub.1 causes the bonding tool 6 to change its course abruptly at the point Q (b,b). As a result, the bonding wire carried by the bonding tool 6 is bent to be in the form of an undesired loop form.
In order to solve the above described problem, when the movement distances in the X-direction and the Y-direction are different from each other, the velocity in the direction of a shorter distance is decreased so that the bonding tool 6 describes a locus 1.sub.2 as shown in FIG. 4. Therefore, the locus 1.sub.2 does not cause the bonding tool 6 to change its course abruptly. The time required for the bonding tool 6 to move from the origin O to the desired point P (a,b) is dependent upon the longer distance of a or b. It follows, therefore, that the time required to move the bonding tool 6 from the origin O to the desired point P (a,b) is equal to the time required to move the bonding tool 6 from the origin O to a point R (a,c). But the lengths of the loci 1.sub.1 and 1.sub.2 are different from each other so that the velocity of the bonding tool 6 at which the bonding tool 6 moves from the origin O to the desired point P (a,b) is different from the velocity of the bonding tool 6 at which the bonding tool 6 moves from the origin O to the point R (a,c). Meanwhile, in order to accomplish a reliable bonding operation, the velocity of the bonding tool is limited to a predetermined value. When the bonding tool 6 is controlled in the manner described above, in some cases the velocity of the bonding tool 6 exceeds the predetermined value, because the velocity of the bonding tool 6 is different according to the direction of shift of the bonding tool 6. When the velocity of the vertical movement of the bonding tool 6 remains constant, the loci of the bonding tool 6 in the three-dimensional space are different, as shown in FIG. 5, according to the velocity of the horizontal movement of the bonding tool 6. As a result, the shapes of loop of bonding wire are dependent upon the locus of the bonding tool 6. Therefore, the reliable wire bonding operation cannot be ensured.
FIG. 6 shows the conventional control of the drive motors 1 and 2. In this case, as shown in FIG. 6C, the uniformly accelerative control is employed. That is, in the case of acceleration, the motors 1 and 2 are accelerated at .alpha..sub.1 and in the case of deceleration, the motors are decelerated at -.alpha..sub.1. In the case of the uniformly accelerative control, the velocity v is linearly increased and decreased as shown in FIG. 6B. The relationship between the time t and the displacement S is shown in FIG. 6A. When the motors are controlled in the manner described above, the acceleration of the motors is abruptly changed so that strong impact is imparted to the bonding tool driven by the motors. Thus, the bonding tool is subjected to vibration or oscillation. The higher the velocity, the higher the impact becomes so that the stable and smooth wire bonding operation cannot be ensured.
In addition to the mechanism for causing the vertical movement of the bonding tool as shown in FIG. 1, there are various mechanisms. For instance, in the case of a wire bonding device as disclosed in Japanese Patent Application Laid-Open No. 55-74152, the relationship between the angle of rotation of a drive motor and the angle of rotation of a bonding arm carried by a bonding tool is expressed by a complex equation including a trigonometric function. That is, the relationship between the vertical shift of the bonding tool and the angle of rotation of the drive motor is complicated. Therefore there arises the problem that the control of the bonding tool by means of the angle of rotation of the drive motor is complicated.
In the case of a mechanism for vertically moving a bonding tool of the type as disclosed in Japanese Patent Application Laid-Open No. 54-154273, a bonding arm and a bonding arm supporrtng member are vertically moved in unison so that the weight of the driven parts is heavy and consequently a high power drive motor is needed. Furthermore, a screw is used to translate the rotation of the drive motor into the vertical movement of the bonding tool so that there arises the problems that the response of the vertical movement is low because the vertical shift in response to the rotation of the screw is small.
In the case of a wire bonding device of the type as disclosed in Japanese Patent Application Laid-Open No. 57-35330, a drive wire which is wound around the rotating axis of a drive motor is coupled to a bonding arm so that the rotation of the drive motor is translated into a rectilinear movement of the bonding arm. This wire bonding device has the problem that the vibration of the drive wire causes the bonding arm to vibrate. Furthermore, the drive wire must be always under tension so that the bonding arm can follow the rotation of a pulley. As a result, additional driving force is needed in order to impart tension to the drive wire so that a high power drive motor is required.