1. Field of the Invention
The present invention relates to a bonding load correction method and a wire bonding apparatus which includes a bonding arm movable upward and downward by an elastic member.
2. Prior Art
A wire bonding apparatus in which the bonding arm is supported so that the bonding arm is moved upward and downward and driven by an elastic member is shown in FIG. 2.
The bonding arm 2 of this bonding apparatus has a bonding tool 1 at one end thereof and is fastened to one end of a supporting frame 3. The supporting frame 3 is attached to a moving table 5 via a plate spring 4 which is assembled in the form of a cross, so that the supporting frame 3 is swingable upward and downward as shown by an arrow V, and a moving table 5 is mounted on an XY table 6. The coil 8 of a linear motor 7 is fastened to another end of the supporting frame 3, and the magnet 9 of this linear motor 7 is fastened to the moving table 5. A linear scale 10 is mounted to the rear end (right-side end in FIG. 2) of the supporting frame 3, and position detecting section 13 that comprises a position sensor 11 and an encoder 12 is fastened to the moving table 5 so that the position sensor 11 faces this linear scale 10. The wire bonding apparatus further includes a heating block 15 which heats a workpiece (not shown), and this heating block 15 is raised and lowered by a raising-and-lowering mechanism 16.
Examples of a wire bonding apparatus of this type are described in Japanese Patent Application Pre-Examination Publication (Kokai) Nos. S58-184734 and H6-29343 and Japanese Examined Patent Application Publication (Kokoku) No. H6-80697.
With the structure described above, the supporting frame 3 and bonding arm 2 are caused to swing in the direction of arrow V about the plate spring 4 by the linear motor 7, and the bonding tool 1 is thus moved up and down. Furthermore, the moving table 5, supporting frame 3, bonding arm 2 and bonding tool 1 are moved in the horizontal (or X and Y) direction by the XY table 6. By way of a combination of the vertical movement and horizontal movement of the bonding tool 1, a wire (not shown) passing through the bonding tool 1 is connected between a first bonding point and a second bonding point on the workpiece (not shown). In other words, a ball formed at the tip end of the wire is bonded to the first bonding point, and the other portion of the wire is bonded to the second bonding point. During bonding of the wire to the first bonding point and second bonding point, a load or a bonding load is applied by the linear motor 7 so that the ball or wire is pressed against the bonding point on the workpiece by the bonding tool 1.
Next, the operation system for the above bonding apparatus and the control configuration of the linear motor 7 will be described. The operation system substantially comprises an external input-output means 20 and a computer 21.
The external input-output means 20 performs input-output of various types of information (required for the operation of the apparatus) with respect to the computer 21. This may be accomplished by manual operation or by operation based on on-line communication with external devices.
The computer 21 comprises a control circuit 22, an operating circuit 23, a reference coordinate register 24, a load limit value register 25 and a height position counter 26. The control circuit 22 controls the external input-output means 20, operating circuit 23, reference coordinate register 24, load limit value register 25 and height position counter 26.
In the reference coordinate register 24, the height position of the bonding arm 2 is stored. The value of the height position is inputted into a position control circuit 31 of the position control section 30 as one position command. When the value is inputted, the position control circuit 31 compares a previous position command and a new position command and generates an amount of movement of the bonding tool from the difference between the two position commands. This amount of movement is transmitted to a motor driver 33 of the position control section 30 as a driving signal 32.
In the load limit value register 25, a value which indicates the upper limit (value) of the bonding load is stored, and the load limit value register 25 transmits such a value to the motor driver 33 as limit information 27. The motor driver 33 generates electric power which is used to move the bonding tool 1 to the designated height position in accordance with the driving signal 32 and at the same time acts so as to limit the electric power in accordance with the limit information 27 so that the upper limit value of the bonding load is not exceeded. Generally, electric power is the product of voltage and current; therefore, actual control of the linear motor 7 can be accomplished by controlling either the voltage or current, or both. Accordingly, the following explanation describes the case where the driving current 34 (and not voltage) that flows through the linear motor 7 is controlled and the voltage value is fixed. The circuit described in Japanese Examined Patent Application Publication (Kokoku) No. H6-18222 may be cited as an example of the circuit that controls the driving current. When the driving current 34 generated by the motor driver 33 is applied to the coil 8 of the linear motor 7, a driving force is generated; and as a result of this driving force, the supporting frame 3, bonding arm 2 and bonding tool 1 are caused to swing about the plate spring 4.
Furthermore, the height position counter 26 of the computer 21 counts signals from an encoder 12 which converts signals from the position sensor 11 into a signal format which is suitable for being inputted into the computer 21 and generates an actual height position on the linear scale 10. The computer 21 is provided beforehand with the ratio of the amount of movement of the bonding tool 1 in the vertical direction to the amount of movement of the linear scale 10 in the vertical direction, and a quantization coefficient. Accordingly, the actual height position of the bonding tool 1 is determined by calculations performed (on the basis of the above-described value) by the operating circuit 23 on the value indicated by the height position counter 26. The term "height position of the bonding tool" refers to the height position at which the bonding tool 1 contacts the object to which a load is to be applied.
A bonding load calibration method within the bonding apparatus described above will be described.
First, a load cell 40 is placed on the heating block 15 so that the projecting part 41 (detection part) of the load cell 40 is positioned directly beneath the tip end portion of the bonding tool 1. This load cell 40 is connected to a load gauge 42, so that the load applied to the load cell 40 is constantly displayed by the load gauge 42. Next, the raising-and-lowering mechanism 16 is operated so as to raise and lower the heating block 15 and load cell 40, and the bonding arm 2 is adjusted to a horizontal position.
The bonding arm 2 is adjusted horizontally because it is desirable that the undersurface of the bonding tool 1 which contacts the bonding point be parallel to the bonding surface during bonding. The bonding tool 1 is fastened to the bonding arm 2 at right angles to the direction of extension of the bonding arm 2, so that the bonding tool 1 faces directly downward. Furthermore, the bonding arm 2 and the bonding tool 1 swing about the plate spring 4. Accordingly, it is desirable that the bonding tool 1 be in a vertical position, i. e., that the bonding arm 2 be in a horizontal position, when the bonding tool I contacts the bonding point.
When the bonding arm 2 has thus been adjusted to a horizontal position, a command is sent to the computer 21 using the external input-output means 20 so that the bonding arm 2 is placed in a horizontal position. As a result of this command, the control circuit 22 sends control information for this purpose (horizontal positioning) to the position control circuit 31 via the reference coordinate register 24; and in addition, the control circuit 22 sends limit information 27 which is used to limit the driving current 34 to the motor driver 33. Furthermore, the position control circuit 31 sends a driving signal 32 which is used to generate a driving current 34 to the motor driver 33. On the basis of this driving signal 32, the motor driver 33 generates a driving current 34 of the specified polarity and magnitude and outputs this driving current 34 to the coil 8. However, in cases where the driving current 34 exceeds the limit value specified by the computer 21, the magnitude of the driving current 34 cannot exceed the limit value.
Afterward, instructions concerning the movement of the bonding arm 2 are sent out from the computer 21 in the manner described above.
Next, when the bonding load (for instance 20 g) is set via the external input-output means 20, this load is applied to the load cell 40 as a result of the above-described operation. In this case, since the actual bonding load value is displayed by the load gauge 42 which is connected to the load cell 40, it is necessary to adjust the driving current 34 so that this value is equal to the set bonding load. The correspondence between the limit information 27 specified by the computer 21 and the value of the actually limited bonding load is thus altered by manual operation of the external input-output means 20. When the set bonding load agrees with the actual value of the bonding load displayed by the load gauge 42, the bonding load is set at a different value (for instance, 200 g), and then the above-described operation is repeated. In this way, the error between the set bonding load and the actual bonding load is minimized.
In the system wherein the supporting frame 3 is supported by a plate spring 4 so as to swing upward and downward as described above, when no driving current 34 flows through the coil 8 or when no driving force is generated in the linear motor 7, the bonding arm 2 stops at the equilibrium position B as shown in FIG. 3(b) which is between the driving force of the plate spring 4 and the weight balance of the bonding tool 1, bonding arm 2, supporting frame 3, coil 8 and linear scale 10, etc. supported by the plate spring 4. The driving force of the plate spring 4 in this case acts in a direction which causes the bonding arm 2 to return to the equilibrium position B.
More specifically, when the bonding arm 2 is in a position A which is higher than the equilibrium position B as shown in FIG. 3(a), a driving force which pushes the bonding arm 2 downward toward the equilibrium position B is generated. On the other hand, when the bonding arm 2 is in a position C or D lower than the equilibrium position B, as shown in FIG. 3(c) or 3(d), then a driving force which pushes the bonding arm upward toward the equilibrium position B is generated. The equilibrium position B varies according to the mechanical factors; therefore, the bonding arm 2 in the equilibrium position B is not necessarily to a horizontal position as shown in FIG. 3(c). Here, the term "mechanical factors" refers to mechanical errors arising from the working precision, assembly, adjustment, etc. of the parts that makes up the bonding head part.
As the bonding arm 2 is displaced toward the higher position A from the balance position B, the force with which the bonding arm 2 is pulled downward by the plate spring 4 increases, so that the driving force of the plate spring 4 is added to the original bonding load. Accordingly, there is a corresponding increase in the actual bonding load. Furthermore, as the bonding arm 2 is displaced downward toward the lower position C or D, the force with which the bonding arm 2 is pulled upward by the plate spring 4 increases, so that the original bonding load is canceled. Accordingly, there is a corresponding decrease in the actual bonding load. As seen from the above, the prior art system has a problem. In other words, as the height position of the bonding arm 2 is apart from the equilibrium position B determined by the plate spring 4, error between the set bonding load and the actual bonding load increases.
Furthermore, in the conventional bonding load calibration method, the set bonding load is merely adjusted with the height position of the bonding point set at the horizontal position C of the bonding arm 2; and absolutely no consideration is given to the relationship between the driving force of the plate spring 4 and the weight balance of the bonding tool 1, bonding arm 2, supporting frame 3, coil 8 and linear scale 10, etc., supported by the plate spring 4. In the mean time, the height of the bonding points of the workpieces to be bonded varies according to the type of workpiece involved; and even in the same workpiece, in some cases, the first and second bonding points have different heights. Accordingly, when bonding is performed at a position other than the equilibrium position B, a driving force is generated in the plate spring 4, and this driving force affects the bonding load, so that the actual bonding load differs from the set bonding load.
Furthermore, in the prior art described above, the load gauge 42 must be read by an attendant, and the actual bonding load must be adjusted by this attendant so that the bonding load is kept equal to the set bonding load. Accordingly, the problem of correction error caused by human factors arises.