The present invention relates to welding electrodes, more particularly compliant electrodes used to gang weld the intermediate leads of an integrated circuit chip to the outer leads supported by a lead frame.
The conventional method for bonding the leads of a semiconductor chip to an outer lead frame has been the wire bonding process. Typically, the operator uses a microscope to position the bonding tool and work piece through hand operated controls and then bonds the leads together. This operation is repeated for each lead on the semiconductor chip thus requiring as many full operations as there are leads on a chip; e.g., 16 on today's conventional chip. In addition to its time and labor cost, such a one-bond-at-a-time processing often results in chip failures due to human error. Recently, gang bonding processes and machines have been developed and put into use. The latter processes use the film-carrier technique to effect gang bonding.
The foregoing processes effect thermocompression bonds. Although the prior art mentions welding as an alternative to thermocompression bonding, so far as is known to applicants, gang welding of leads for semiconductor devices has not been commercially successful. The present invention provides an electrode assembly for effecting gang welding of leads for semi-conductor devices.
The present invention provides a gang weld electrode assembly which may be used with automatic bonding apparatus such as that disclosed in copending U.S. patent application Ser. No. 511,835 filed Oct. 3, 1974 now U.S. Pat. No. 3,949,925, to simultaneous weld all 16 leads of a semiconductor chip in a single automatic operation. The apparatus shown and described in U.S. patent application Ser. No. 511,835 now U.S. Pat. No. 3,949,925 has previously been used for thermocompression bonding of leads.
Although welding has been used with solid state devices in the past, it has never been trouble-free. The main problems are critical alignment of the opposed electrodes and variations in part thickness of the work piece. The individual spot electrodes must be aligned such that the work surfaces are planar. Due to the size and configuration of the leads of semiconductor chips, a planar peripheral configuration must be provided. In addition, this configuration must be sustained for long periods of time in order that the electrode may be used on a production basis.
The second problem, variations in the thickness of the work piece (i.e., the leads to be welded together), is perhaps even more troublesome. It is imperative that a constant mechanical force be applied to the work piece before, during and after the time current flows through the electrodes in order to impose and maintain the proper conditions for heating and welding the leads together. If the electrodes fail to apply a steady pressure against the work piece, one or more lead pairs may not weld. Moreover, increased contact resistance may cause excessive heating, resulting in burning of the electrodes. Additionally, if there is a loss of contact between the electrode and the work piece, destructive arcing at the electrode faces may occur thereby shortening the life of the electrode.
The present invention overcomes both of these problems by providing a compliant electrode comprising a plurality of independent spot electrodes which can each follow the deformation of a single lead pair during the welding operation on a lead pair by lead pair basis. An approach toward the concept of using individual compliant electrodes is set forth in U.S. Pat. No. 2,266,424. In this patent, a plurality of independent electrodes are semi-independently biased against a work piece by a resilient rubber block. This welding electrode is, however, not concerned with the problems of welding semiconductor leads.
The compliant electrode described in the above-identified patent is inadequate for many reasons. Initially, rubber ages and is therefore neither durable nor dependable. In addition, rubber does not have a linear spring coefficient and cannot provide a controlled, known and stable spring constant. Furthermore, since each of the plurality of spot electrodes is attached to a single rubber block, the displacement of each of the spot electrodes is not truly independent of the others.
These and other deficiencies of the prior art are overcome and further improvements appropriate to welding semiconductor leads are provided by supporting each of the spot electrodes on a different one of a plurality of independent parallelogram support members in the manner disclosed below. Such a support provides a means for biasing the spot electrodes against the work piece with a desired very rapid spring rate thereby ensuring constant contact between the electrodes and the work piece during the entire welding operation.
Another feature of the present invention is control over current flow. A constant, predictable current flow is imperative to a consistent and satisfactory welding operation. Among the parameters which effect the current flow are the resistance of the work piece. As noted above, the thickness of the leads being bonded varies from lead to lead. Such variations in the thickness of the leads result in variations in work piece resistance, and therefor current flow. In order to compensate for these irregularities and to provide the proper current flow through each electrode, the electrode is designed such that the electrical resistance of the electrode is much greater than that of the work piece. When constructed in such a manner, the electrode approximates a current source and hence each individual weld is not significantly effected by variations in lead resistance. That is, quantum changes in work piece resistance become less significant since they represent a small percentage change in the overall resistance of the electrode/work piece electrical system.
For the purpose of illustrating the invention, there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.