Wire Bonding Apparatus

This invention relates to a wire bonding apparatus for connecting between electrodes and external leads of semiconductor integrated circuit elements, such as ICs or LSI circuits, by bonding wires placed in position by appropriate ultrasonic waves from an ultrasonic horn.

In the assembly of semiconductor integrated circuit elements, a wire bonding step is included for connecting between the electrodes of a semiconductor pellet and the external leads of the circuit elements. Conventionally, a wire bonding apparatus has been used. Ultrasonic wave oscillation from an ultrasonic horn is provided to place a bonding wire between the electrodes of the semiconductor pellet located in the center of a lead frame supported on a heat block and the external leads.

However, the wire bonding step is very delicate and requires a wire bonding apparatus capable of performing very precise operations, for instance, contact bonding a 100 um diameter wire ball to a 100 um square aluminum pad. It is therefore necessary to minimize locational errors with regard to respective positions of the semiconductor pellet, the external leads, and the wire bonding apparatus.

For keeping the locational errors within a predetermined tolerance, certain design changes in the wire bonding apparatus of the prior art have been considered, such as making the operating pitch of an X-Y table closer or pattern recognition resolution finer. However, there are limitations in the above design changes, and, therefore, with the miniaturization of the electrode (the semiconductor pellet) in recent years, conventional wire bonding apparatus have had problems in that it is impossible to keep the locational error of the bonding location within a tolerance of about 5 .mu.m.

Accordingly, it is an object of the present invention to provide a wire bonding apparatus which can respond to the requirements according the above miniaturization of electrodes in recent years.

In accordance with the wire bonding apparatus of the present invention, an ultrasonic horn thereof is made of a metallic material having a thermal expansion coefficient of not more than about 5.times.10.sup.-6 /.degree.C.

The wire bonding apparatus of the present invention can keep the positioning error of the bonding location within about 5 .mu.m.

FIG. 1 is a perspective view of the whole wire bonding apparatus of the present invention.

FIG. 2 is a partial enlarged view of elements of the above wire bonding apparatus in the bonding step.

FIG. 3 is a side view of the ultrasonic horn 3, for explaining the limitation of the thermal expansion coefficient by example.

FIG. 1 is a perspective view of the whole wire bonding apparatus of the present invention. An X-Y table 1 moves in two (X and Y) directions within a plane to transfer a bonding head 2 disposed thereon. An ultrasonic horn 3 is fixed to the bonding head 2 and has a bonding capillary tip 4 at the end thereof. A bonding wire 5 is delivered from the bonding capillary tip 4. The ultrasonic horn 3 is formed of a metallic material having a very low thermal expansion coefficient, e.g., nickel steel (36% Ni:Invar) of which the thermal expansion coefficient is 0.9.times.10.sup.-6 /.degree.C. This nickel steel is usually called "Invar" and contains nickel in the proportion of 36%. The length of the ultrasonic horn 3 is generally from 80.times.10.sup.3 to 100.times.10.sup.3 .mu.m. An ITV camera 6 is fixed to a top of the bonding head 2. A lead frame on a frame feeder 7 moves in the direction of arrow 100 on a table 9 to feed a semiconductor integrated circuit element 8 under the bonding capillary tip 4 of the ultrasonic horn 3. FIG. 2 is a partial enlarged view of elements of the wire bonding apparatus in the bonding step. In FIG. 2, the semiconductor integrated circuit element 8 is held on a heat block 10 by a frame clamp 11 and comprises a lead frame 8a, a die pad 8b, and an IC pellet 8c. The heat block 10 includes a heater 12. The die pad 8b is located in the center of the lead frame 8a and separated from the external leads (not shown) which extend from the lead frame 8a. The IC pellet 8c is fixed (die-bonded) on the die pad 8b.

In such a wire bonding apparatus, since the ultrasonic horn 3 (having length of not more than 100.times.10.sup.3 .mu.m) is formed of a metallic material having a very low thermal expansion coefficient, e.g., Invar or an alloy thereof, stretch in the ultrasonic horn 3 due to thermal expansion can be kept within about 5 .mu.m, that is, locational error of the bonding location can be kept within 5 .mu.m.

The reason why the thermal expansion coefficient of the material of the ultrasonic horn 3 is limited to not more than 5.times.10.sup.-6 /.degree.C. is as follows. FIG. 3 is a side view of the ultrasonic horn 3 for explaining the above limitation of the thermal expansion coefficient by example. In FIG. 3, points A,B,C indicate the heat point, the middle point and the root point of the ultrasonic horn 3, respectively, and l.sub.1 and l.sub.2 indicate the length from point A to point B and the length from point B to point C, respectively. In this example, the full length of the ultrasonic horn 3 (the length from point A to point C) is 84.times.10.sup.3 .mu.m, accordingly l.sub.1 and l.sub.2 are 42.times.10.sup.3 .mu.m, respectively. Since the head portion of the ultrasonic horn 3 is affected more by the heat from the heat block 10 than is the root portion, the stretch in the ultrasonic horn 3 will be considered by way of dividing the ultrasonic horn 3 into two portions. TABLE 1 below shows the temperature rise at the point A,B,C of the ultrasonic horn 3 during wire bonding.

When the ultrasonic horn 3 is formed of stainless steel (SUS 304) having a thermal expansion coefficient .alpha..sub.S (where .alpha..sub.S =1.6.times.10.sup.-5 /.degree.C.) which has been used in the prior art, the respective lengths of stretch (extension) l.sub.1, l.sub.2 of the head portion and the root portion of the ultrasonic horn 3 are as follows: ##EQU1## (where T.sub.1, T.sub.2 are respective mean temperature differences.)

Accordingly, the total stretch (l.sub.1 '+l.sub.2 ') is 26.7 .mu.m. However, when the ultrasonic horn 3 is formed of a material having a thermal expansion coefficient .alpha. where (.alpha.=5.times.10.sup.-6 /.degree.C.) as in the present invention, the total stretch (l.sub.1 '+l.sub.2 ') is only 8 .mu.m. When the length of total stretch must be kept within 5 .mu.m, a material having a thermal expansion coefficient .alpha. where .alpha.=3.times.10.sup.-6 /.degree.C. should be used. However, even when a material having a thermal expansion coefficient of .alpha.=5.times.10.sup.-6 /.degree.C. is used, equal effects can result by suitably selecting the length of the ultrasonic horn 3 and the size and/or the material of the other parts.

Accordingly, the wire bonding apparatus of the present invention can satisfy the demands of recent years on the locational error constraint resulting from miniaturization of the electrode. Further, since high accuracy wire bonding is achieved, improvement in the quality of IC products can also be accomplished.

The action of the wire bonding apparatus in the bonding step is as follows. Firstly, the lead frame 8a, to the center of which the IC pellet 8c is contact bonded, is held on the heat block 10 by the frame clamp 11. Next the IC element 8 is disposed at a predetermined position for bonding (positioning controlled) automatically by use of ITV camera 6, and after moving the bonding capillary tip 4 of the ultrasonic horn 3 over the electrode of the IC pellet 8c by way of shifting the X-Y table 1, the ultrasonic horn 3 is lowered, and the bonding wire 5 is pressed against the electrode of the IC pellet 8c by the ultrasonic horn 3 to apply the ultrasonic wave, thereby bonding the wire 5 to the electrode. Then, the ultrasonic horn 3 is raised and at the same time the X-Y table 1 is shifted, so that the other end of the bonding wire 5 can be bonded to the external lead terminal of the lead frame 8a. After a time, the tip portion of the bonding wire 5 takes on a globular shape, completing the process. In practice, since the IC pellet 8c has a lot of terminals, the above process is repeated many times. Wiring by use of ultrasonic waves and thermocompression bonding is embodied as above.

In the above embodiment, only the ultrasonic horn 3 is formed of a metallic material having a low thermal expansion coefficient. In the field of the precision assembly, it is most important to maintain high accuracy by preventing displacement caused by heat. Therefore, if other constructional elements of the wire bonding apparatus, e.g., the heat block 10 and the frame clamp 11, which are directly affected by the heat, are also formed of a metallic material having a very low thermal expansion coefficient, increased accuracy in positioning control can be achieved.