A common technique for prearranging an external connection of an integrated circuit is a wire bonding technique. This wire bonding technique includes bonding a wire to each metal pad of the chip of the integrated circuit, and to a respective metal lead defined by die stamping a metal assembly frame of the integrated circuit. The bonding of the wire, which is typically a small diameter gold or aluminum wire, is done by friction welding the wire to the metal surface of the pad while providing vibrating energy to the tool.
Alternative encapsulating techniques have been developed to allow for less delicate and critical ways to make the necessary external connections. A recently developed technique is known as a "flip chip" technique. The flip chip technique is based on the formation of a metal bump or ball on each connecting pad. The bumps are pressure contactable, thus eliminating the need of bonding between the pads and the leads.
For low power devices containing signal processing systems, the metal bumps required on the connection metal pads of the chip may be formed by electrochemical deposition of a suitable metal. For high power devices, the only reliable method to form such bumps on the respective metal pads is to friction weld pre-formed balls of gold, or an equivalent alloy, with a technique similar to the one traditionally used to bond a gold wire.
Suitable tools have been developed to form bumps or balls of gold, and to friction weld respective metal pads to the chip. One such device is a tiny capillary tool with a dielectric tip commonly formed of aluminum, zircon or other refractory oxide. The axial conduit of the capillary tool opens to a flat end face with a tapered mouth at the tip of the tool. The machine is equipped with means to feed a thin wire of gold or alloy through the capillary, and has clamps for functionally blocking the wire intermittently advanced through the capillary tool. A bump is formed by advancing the wire so that it protrudes out a few millimeters from the end face of the tool tip, and by arcing between the wire and a counter-electrode suitably placed in proximity of the tool tip. The arcing is sufficient to melt the metal, which conforms to the concave shape of the mouth of the capillary.
The metal ball created on the tool tip is pressed to the metal pad by lowering the tool over it, and vibrating power is applied to the tool causing the bonding by friction welding the metal ball to the metal pad. Once the bonding is accomplished, the clamp blocks the movement of the wire relative to the capillary, and the tool is pulled away. This breaks off the wire from the bonded metal bump. Thereafter, the tool shifts over a new metal pad. The clamp is released and the wire is advanced a predetermined distance. The clamp closes again and a new electrical arc melts the protruding portion of wire into a new metal ball. This new metal ball is bonded to a corresponding new metal pad.
This process produces bonded metal bumps having a short stump or tiny tail of wire left during the breaking off from the wire. This is caused by retracting the tool while maintaining clamped the feed wire. Commonly, the same machine is equipped with a second tool by which, through a dedicated realigning software, each metal bump after having been bonded to the respective metal pad is stamped. This is done for flattening the short stump of wire left by the breaking-off of the wire, and to produce a plastic flattening of the top of the metal bump. Leveled areas are produced suitable for establishing good electrical contact for assembling the device in a flip chip package.
The bump forming and bonding process, as well as the successive stamping, is done with commercially available machines. One of these machines is described in the brochure for the KS1488 Turbo model, released Feb. 27, 1996, by Kulicke and Soffa Industries, Inc, located in Pennsylvania. Also described in the brochure is the realigning software for stamping the metal bumps after they have been bonded to the metal pads. The tool for forming the metal ball and for bonding it to a metal pad of a chip is a commercially available machine which has a stylus with an axial capillary passage. The tip is primarily made of a dielectric refractory ceramic material with a tapered cavity through which projects the metal wire fed through the capillary.
Commonly, the end face of the tip is a convex surface with a large radius of curvature, and has a concave junction of a relatively large radius connecting with the conical surface of the dielectric tip, as shown in the data sheet of Aprova Ltd. of Lyss, Switzerland, 1992. This is relative to Tip Style CSS, Thermocompression (T/C) or Thermosonic (T/S) Tailless Capillary Bonding tips.
The step of forming and bonding a metal bump to a metal pad using a special capillary tool, and thereafter stamping the metal bump using a different tool, is burdensome in terms of manufacturing times. A number of specific tools are required, including software for realigning a second stamping tool to the same points on which a first tool has been already aligned for forming and bonding the metal bump.
It is evident that there is a need to form and bond a metal bump onto a metal pad, break off the metal wire and stamp the top of a bonded metal bump in an uninterrupted sequence of operations. This is to be done without repeating the alignment of a specific tool to the same metal pad, and without substituting tools.