Method and apparatus for connecting wires between a semiconductor chip and corresponding pins

Abstract: A method for connecting wire leads (7) between a semiconductor circuit chip (1) and corresponding terminal connectors comprises the steps of, providing a bonding tool (9) having a working end (12) formed with at least a pair of grooves (13,14) of different length, holding one end of the wire (7) on the pin in one (13) of said grooves and bonding it, and holding the other end of said wire on the chip (1) in the other (14) of said grooves and bonding it. The grooves (13,14) have different lengths to allow of different wire spans across the bonded connection areas, on the chip and the pins.

Description: Field of the Invention This invention relates to a method for connecting wire leads between a semiconductor circuit chip and corresponding terminal connectors. The invention also concerns an improved tool head for implementing the method on stitch bonding equipment. Background Art As is well known, in the specific field of this invention, there exists a need to establish electrical connections between a semiconductor circuit chip and corresponding pins or rheophores on a supporting metal frame. The connecting pins are formed integrally with the frame and allow the semiconductor circuit to be mounted on and connected to a pre-printed electronic board. Provided on the semiconductor chip periphery are predetermined contact pads whereto the terminations of the circuit structure are led; thin wires are connected between said pads and the inward ends of the corresponding pins. This process step of providing the electrical connections between the semiconductor chip and the corresponding pins is obviously critical. In fact, at this stage of the semiconductor circuit manufacturing process, a chip has already passed an electric test checking the proper operation of its circuitry. Therefore, should the wire connections be performed in anything less than a workmanlike fashion, there would be a material risk that the circuit is unable to operate correctly and that it is rejected. An even more undesirable and dangerous event is that the device is initially found in proper working condition, but it fails to operate after some time of use. The importance of having the wire connection bonding carried out in a truly reliable manner is, therefore, self-evident. To this aim, some special equipment such as the stitch bonding machines have been developed. Each machine includes at its tooling end a bonding tool head driven automatically toward and away from a silicon chip mounted on a central portion of the frame. An apparatus for bonding wires to metal surfaces is described in the US Patent No. 3,218,702 of November 23, 1965. Such an apparatus relates to the pressure bonding or welding of metal elements, particularly to the bonding of fine wires formed of precious metals to small metallic areas on transistors, and comprises a bonding tool having a groove for receiving the wire during the bonding operation. The tool of this known bonding apparatus makes bonds of identical area. Furthermore, the GB Patent No. 2 177 639 of July 8, 1985 describes an improved ultrasonic wire bonder which can perform the simultaneous bonding of two connecting wires, still of identical bonding areas. Moreover, the JP Patent No.57-40948 of March 6, 1982 details a wire bonding method for semiconductor devices, providing planar bonding wedges, formed at double heads having different bonding areas. While achieving its basic objective, such conventional equipments aren't yet attaining a sufficient reliability level to ensure faultless bonding of the wires. Furthermore, the trend of today's technologies favors increased miniaturization on the chip, which still further reduces the area available for bonded wire connections. The underlying technical problem of this invention is to provide a method for improving the quality of the connections between a semiconductor circuit chip and the pins of its supporting frame, which method has such functional features as to overcome the limitations investing the prior art solutions. Summary of the Invention The solutive idea on which this invention is based start from the simple postulate that, to enhance the quality and effectiveness of a bonded connection, the bonding area must be enlarged. Based on this solutive idea, the technical problem is solved by a wire bonding method of the kind outlined above and as defined in the characterizing parts of Claims 1 to 3. This technical problem is also solved by a tool head for stitch bonding equipment, as defined in Claim 4 and following. The features and advantages of the method according to this invention will become apparent from the following description of an embodiment thereof, given by way of illustration and not of limitation with reference to the accompanying drawings. Brief Description of the Drawings In the drawings: Figure 1 shows, partly in cross-section, a schematic view of a semiconductor material chip mounted on a supporting frame and undergoing a wire bonding process according to the inventive method; Figure 2 is a schematic view of a tool head, according to the invention, as applied to stitch bonding equipment for implementing the process of Figure 1; Figure 3 is a vertical section view, taken along line III-III through the tool shown in Figure 2; Figure 4 is a further vertical section view of the tool shown in Figure 2, taken along line IV-IV; and Figure 5 is an enlarged scale detail view of Figure 3. Detailed Description With reference to the drawing views, generally and schematically shown at 1 is a chip of a semiconductor material, such as silicon, on which an electronic circuit, not shown because conventional, has been realized. The chip 1 is supported on and bonded to a central portion 3 of a frame 2 formed by a thin metal sheet. Understandably, the chip 1 and frame 2 would contact each other along the surface 4 of the semiconductor substrate, while the circuit would be in full view on the exposed surface of the chip. The frame 2 is formed integrally with plural connection terminals consisting substantially of contact pins 5 which, completing the manufacturing process, may be plugged on a pre-printed electronic board. A wire lead 7 is bonded between each of said pins 5 and a corresponding contact pad on the chip 1. Specifically, the wire is bonded to an inward end or post lead 8 of the pin 5 and to a pad 6 on the chip. Shown by way of example in Figure 1 is a wire 7 elongated between the post lead 8 of a pin 5 and a contact pad 6 located peripherally on the chip 1. The opposite ends of the wire 7 are, therefore, bonded to the end 8 of the pin 5 and to the connection pad 6, respectively. For this purpose, a stich bonding type of equipment, only partially shown, is provided. This equipment carries at its working end a bonding tool head 9 which can be driven automatically toward and away from the silicon chip 1 mounted on the frame 2, as well as to and from the pins 5. Specifically, the tool 9 comprises a bonding tip 11 adapted to abut both against one end of a wire 7 to be bonded to the pad 6 and to the other end of the wire 7 to be bonded to the pin 5. Advantageously, according to the invention, the tip 11 has a grooved bottom end 12 through which the connecting wire is run. More particularly, a pair of parallel grooves 13, 14 extend along that end 12. The grooves 13, 14 are substantially V-shaped in cross-section, with a flare angle of approximately sixty degrees. In addition, the grooves 13, 14 have different lengths. Preferably, and contingent on individual requirements, the first-mentioned groove 13 is about one third to one half longer than the second-mentioned groove 14. The first groove 13 has a substantially straight-line profile shape with curved opposite ends 17, as shown in Figure 4. The second groove 14 is instead closed at one end by a projecting pip 15. The profile shape 10 of this second groove 14 is clearly shown in Figure 3. The curved profile between the pip 15 and the line 10 the groove 14 is shown in Figure 5. The bonding method of this invention will now be described. While the wire is being bonded to the end 8 of the pin 5, it is held in the first groove 13, as shown on the right in Figure 1. Bonding is performed with the aid of ultrasound, or using another equivalent technique. Thereafter, the bonding equipment is operated automatically to bring the tip 11 to a pad 6 on the chip 1 whereto the other end of the wire 7 will be bonded. However, the wire end is presently held in the second groove 14 while being bonded, all this being shown on the left side of Figure 1. During this step of the bonding process, the pip 15 will indent and penetrate the wire 7 to weaken its cross-section, but this does not harm the bonded joint, even upon the wire being parted by a sharp pull as is customarily applied to thin wires. The method and apparatus according to the invention do solve the technical problem and afford several advantages, outstanding among which is surely the fact that the bonded wire connections are more effective and reliable than those to be obtained with prior art techniques. The differential groove tool, in fact, produces bonded connections to pads and pins which are appropriate for the space and dimensions available at the respective points of contact.

Claims: A method of connecting wire leads (7) between a semiconductor circuit chip (1) and corresponding terminal connectors (5) comprising the following steps: providing a bonding head (9) comprising a bonding tip (11) having a working end (12) including at least a pair of different length grooves (13,14), defining at least first and second bonding surfaces with different areas and receiving said wire leads (7); holding one end of the wire (7) on the connector (5) in one (13) of said grooves and bonding it; holding the other end of said wire on the chip (1) in the other (14) of said grooves and bonding it; and indenting said other end of the wire (7) by means of a projection (15) closing one end of the other (14) of said grooves to facilitate breaking of the wire after bonding. A method according to Claim 1, characterized in that said one groove (13) is longer than said other groove (14). A bonding head for equipment operated in a stitch bonding mode to bond wire leads (7) elongated between a semiconductor circuit chip (1) and corresponding terminal connectors (5), comprising a bonding tip (11) having a working end (12) including at least a first and a second bonding surfaces having different areas, characterized in that said bonding tip (11) comprises at least a first (13) and a second wire-holding groove (14) having different lengths and defining said first and second bonding surfaces, the second wire-holding groove (14) being closed at one end by a projection (15) to indent said wire and facilitate breaking of the wire after bonding. A tool head according to Claim 3, characterized in that said grooves (13,14) lie mutually parallel along said working end (12). A tool head according to Claim 3, characterized in that the first (13) of said grooves is approximately one third to one half longer than the second (14) of said grooves. A tool head according to Claim 3, characterized in that said grooves (13,14) are substantially V-shaped with a flare angle of about sixty degrees in a cross-section perpendicular to the length of the grooves. A tool head according to Claim 3, characterized in that the first groove (13) has a substantially straight-line profile shape (16) with curved opposite ends (17), said opposite ends (17) having a curvature pointed toward the center of the bonding tip (11) in a cross-section parallel to the length of the groove.

Inventors: GRASSO ANTONIO, PINTO ANTONIO, GRASSO, ANTONIO, PINTO, ANTONIO
Applicants: CONS RIC MICROELETTRONICA, SGS THOMSON MICROELECTRONICS, CO.RI.M.ME. CONSORZIO PER LA RICERCA SULLA MICROELETTRONICA NEL MEZZOGIORNO, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1993
Main classification: H01L21
IPC codes: H01L21, B23K20