WIRE STACKED BONDING METHOD

Abstract: The invention relates to joining of fine wires, and more particularly to a wire bonding method suitable for connecting a number of wires within a narrow area. A first wire is bonded on a pad (1), and a second wire (3) is bonded on the position where the first wire (2) has been bonded. Bonding of a subsequent wire (4) is performed in that the wire is stacked and bonded in similar manner mentioned above. Thus a plurality of repair wires can be bonded within one pad even if the pad (1) area is very small.

Description: The present invention relates to the joining of fine wires, and more particularly to a wire bonding method suitable for bonding a number of wires within a narrow area. In a multi-layer wiring substrate, in general, a plurality of electronic parts are assembled at high density. On such a substrate, when a circuit change becomes necessary due to a design change or a design miss, wires are added later so as to deal with the circuit change. On the other hand, on account of the high-density assembling on the substrate or the miniaturization of the substrate, the bonding pad for bonding the repair wires becomes small in size and the repair wire becomes thin. Consequently, although most of such repair wires have been bonded by soldering conventionally, bonding by ultrasonic bonding such as described in "ELECTRONIC PACKAGING & PRODUCTION" FEBRUARY 1984, P.124 - P.127 has been performed in recent years. Ball bonding is another connecting method similar to ultrasonic bonding. Ultrasonic bonding or ball bonding as described in JP-A- 55-88348 or JP-A-55-12712 have been used as bonding methods of a lead wire for drawing an electrode of a semiconductor in the prior art. Fig. 9 shows a bonded state of a repair wire bonded with the ultrasonic bonding method, and Fig. 10 shows the bonded state according to the ball bonding method. In the bonding of a lead wire, only one wire is drawn per one bonding pad. However, on a multi-layer wiring substrate, a plurality of repair wires may be drawn from one connection pad. According to the prior art, the bonded state of a plurality of repair wires on a bonding pad is shown in FIGS. 11 and 12. FIG. 11 shows the case according to the ultrasonic bonding method, and FIG. 12 shows the case according to the ball bonding method. In the prior art, when a plurality of repair wires 2 are to be bonded on a bonding pad 1, since the plane bonding is performed as shown in the figure, the bonding pad 1 must have a prescribed size corresponding to the number of wires to be bonded. Consequently, if the connection pad 1 is very small, the wires of a prescribed number cannot be bonded. DE-Cl-3 519 418 discloses an apparatus for stacked soldering of ring shaped ends of wires on a soldering pad of a printed circuit board, necessitating a relatively large space as compared with ball bonding, and not suited, therefore, to bond a plurality of wires to a pad with a high two-dimensional bonding density and a high vertical stacking density. SUMMARY OF THE INVENTION: An object of the invention is to provide a wire stacked bonding method wherein wires of prescribed number can be bonded even if the bonding pad is miniaturized due to high-density assembling. Said object is achieved, according to the present invention, by a wire stacked bonding method wherein a plurality of wires are connected (Figs. 1, 7, 11, 12) on to a bonding pad, characterized by the following steps: A first series of steps including: A step (Fig. 2) of melting an end of one wire of a plurality of wires to form a ball at the end of the wire; a step (Fig. 3, 8) of positioning the ball on a planar surface of the bonding pad with the wire extending in parallel with the planar surface; and a step (Fig. 4) of heating and pressing the ball to form the ball into a first disc-like form and to bond the first disc-like form to said bonding pad; followed by a second series of steps including: A step (Fig. 2) of heating an end of another wire of said plurality of wires to form a ball at the end of the other wire; a step (Fig. 5) of positioning the ball at the end of the other wire on an exposed surface of the first disc-like form with the other wire extending in parallel with the surface of the bonding pad; a step of heating and pressing the ball to form a second disc-like form and to bond the second disc-like form onto the surface of the first disc-like form bonded to the bonding pad thereby forming a stack of the disc-like forms bonded onto the bonding pad. Thereby a plurality of wires can be bonded within one pad even if the pad area is very small. A particularly advantageous method according to the invention is claimed in claim 3. BRIEF DESCRIPTION OF THE DRAWINGS: FIG. 1 is a perspective view illustrating a bonding method according to an embodiment of the invention; FIGS. 2 through 5 are explanation diagrams illustrating the process to produce the bonded state by the bonding method of FIG. 1; FIG. 6 is a plane view of a multi-layer module substrate; FIG. 7 is an enlarged view of portion A in FIG. 6; FIG. 8 is an explanation diagram illustrating ball forming process for the second bonding; FIG. 9 is a perspective view illustrating the bonded state by the ultrasonic bonding method according to the prior art; FIG. 10 is a perspective view illustrating the bonded state by the ball bonding method according to the prior art; FIG. 11 is a perspective view illustrating the bonded state of plural wires bonded by the ultrasonic bonding method according to the prior art; and FIG. 12 is a perspective view illustrating the bonded state of plural wires bonded by a ball bonding method according to the prior art. DESCRIPTION OF THE PREFERRED EMBODIMENTS: An embodiment of the invention will be described referring to the accompanying drawings. FIG. 1 is a perspective view of an embodiment of the invention illustrating the state in which a plurality of repair wires are stacked and bonded on one pad, and FIGS. 2 through 5 are diagrams illustrating bonding process of stacked bonding according to the invention. FIG. 6 shows a high-density assembled multi-layer module substrate for wiring repair wires, and FIG. 7 is an enlarged view of portion A in FIG. 6. FIG. 8 shows a grip changing to perform ball formation for the second bonding. As shown in FIG. 6, a semiconductor element 16 is assembled on the multi-layer module substrate 15, and a bonding pad 1 for bonding the repair wires 2 is arranged on the periphery of the semiconductor element 16. In the multi-layer module substrate 15, if a circuit correction becomes necessary due to logical changes, design misses or the like, the circuit correction/change is performed by attaching the repair wires 2 to the bonding pad 1 later. Consequently, the number of the repair wires 2 to be bonded to the bonding pad 1 is not limited to one as in the case of a lead wire of semiconductor, but a plurality of repair wires 2 may be bonded. The repair wire 2 bonded to some connection pad 1 is wired to the connection pad 1 of another prescribed semiconductor element 16 and bonded to the prescribed bonding pad 1. Bonding of a plurality of repair wires 2 to one connection pad will now be described. When one repair wire 2 is to be bonded to one connection pad 1, the connection may be performed by means of ultrasonic bonding or ball bonding as shown in FIG. 9 or FIG. 10. Also when a plurality of repair wires are to be bonded to one bonding pad using the above-mentioned ultrasonic bonding or ball bonding, the bonding is inevitably performed in the plane arrangement as shown in FIG. 11 or FIG. 12. However, if the connection pad does not have sufficient size to arrange a plurality of repair wires in one plane for the bonding, the repair wires are stacked and bonded as shown in FIG. 1. That is, the first repair wire 2 is bonded on the connection pad 1, and the second repair wire 3 and the third repair wire 4 are stacked and bonded on the first repair wire 2. Next, the process of bonding by the stacking method will be described. As shown in FIG. 2, the terminal 7 of an electric torch device 6 approaches the top end of the repair wire 2 projecting from a capillary 5, and is discharged between the repair wire 2 and the terminal 7 so that the top end of the repair wire 2 is melted to form a ball 8. Next, as shown in FIG. 3, the repair wire 2 with the ball 8 formed on its top end is positioned on the bonding pad 1 using the capillary 5. Then the repair wire 2 is held in the horizontal direction with respect to the connection pad 1. At this state, pressure is applied by a heater piston 9 also known as heater chip, and the ball 8 is heated and pressed as shown in FIG. 4 thereby the ball 8 is deformed into a disc form 8' and bonded to the bonding pad 1 Next, the second repair wire 3 with a ball 10 formed on the top end similar to the above-mentioned method is positioned on the disc-shaped portion 8' of the first repair wire 2 already bonded as shown in FIG. 5, and then heated and pressed by the heater piston 9. In such manner, the second repair wire 3 can be bonded further on the first repair wire 2 which has been bonded on the bonding pad 1. Further regarding the third repair wire and so on, the repair wire is stacked and bonded in the above-mentioned way. Using the above-mentioned stacked bonding method, in case of wiring a repair wire 2 on a multi-layer module substrate 15 as shown in FIG. 6, circuit changes can be dealt with limited sized connection pads 1 as shown in FIG. 7, by stacking and bonding the prescribed plural number of repair wires 2. On the other hand, using the above-mentioned stacked bonding method, the repair wire 2 which is stacked and bonded on the bonding pad 1 as shown in FIG. 1 can be drawn out freely 360° in the horizontal direction with respect to the bonding pad 1. Also as shown in FIG. 7, when the bonding pads 1 are arranged closely and the repair wires 2 are passed between the bonding pads 1 and are connected to respective pads 1 so as to perform wiring, it is particularly effective that the repair wires 2 are aligned in a definite direction. In other words, if the repair wires 2 bonded to the bonding pads 1 are drawn freely in multiple directions, and if the bonding pads 1 are closely arranged and the coating of the repair wires 2 from the disc-shaped portions 8′ falls off much, the repair wires from the neighboring bonding pads 1 will be contacted with each other to cause the shortcircuit state. However, in the stacked bonding as shown in FIG. 7 where the repair wires 2 are drawn in a considerably definite direction even if the coating falls off from the disc-shaped portions 8′, the repair wires 2 only from the same bonding pad 1 are contacted with each other and therefore the electric shortcircuit state is not produced. In the first bonding of one repair wire, a ball can be formed using a capillary in similar manner to conventional ball bonding. However, when the repair wire 2 is wired using the capillary 5 as shown in FIG. 8, a ball for the second bonding cannot be formed in the wiring state as it is. Consequently, in order to form a ball for the second bonding, the repair wire 2 is held by a chuck 17 at the first bonding side of the second bonding ball forming position. The repair wire 2 is cut near the outlet of the capillary 5, and the ball for the second bonding is formed on the end portion of the repair wire 2 held by the chuck 17, and the ball is positioned on the prescribed bonding pad 1 by the chuck 17 so as to perform the second bonding. As another method without using a capillary, the repair wire is previously cut in a prescribed length, and balls are formed on both ends first to perform bonding in the second bonding as well as in the first bonding. According to the invention, a plurality of fine wires can be bonded to the bonding pad with a limited size. Since a plurality of fine wires are stacked and bonded each wire can be drawn freely 360° in the horizontal direction so as the wire can be wired freely. Regarding the bonding strength, since the wire need not to be crushed beyond the principle wire diameter, the cut off at the neck portion does not occur and the strength nearly equal to that of the principle wire can be obtained.

Claims: A wire stacked bonding method wherein a plurality of wires (2, 3) are connected (Figs. 1, 7, 11, 12) on to a bonding pad (1), characterized by the following steps: A first series of steps including: a step of melting an end of one wire (2) of a plurality of wires to form a ball (8) at the end of the wire (2); a step of positioning the ball (8) on a planar surface of the bonding pad (1) with the wire (2) extending in parallel with the planar surface; and a step of heating and pressing the ball (8) to form the ball into a first disc-like form (8') and to bond the first disc-like form (8') to said bonding pad (1); followed by a second series of steps including: A step of heating an end of another wire (3) of said plurality of wires to form a ball (10) at the end of the other wire; a step of positioning the ball (10) at the end of the other wire (3) on an exposed surface of the first disc-like form (8') with the other wire (3) extending in parallel with the surface of the bonding pad (1); a step of heating and pressing the ball (10) to form a second disc-like form (8') and to bond the second disc-like form (8') onto the surface of the first disc-like form (8') bonded to the bonding pad (1) thereby forming a stack of the disc-like forms (8') bonded onto the bonding pad (1). A wire stacked bonding method according to claim 1, wherein each of said wires (2, 3) are arranged in a longitudinal direction extending in the same direction from a circumference of a stack of said disc-like forms (8'). A wire stacked bonding method according to claim 1 or 2, characterized by the following steps: a step of positioning an end of one wire (2) projecting from a capillary (5) near a terminal (7) of an electric torch device (6); a step of effecting electrical discharge between said wire (2) and said terminal (7) to melt said end of the wire (2) to form a ball (8); a step of positioning said wire (2) in parallel with a planar surface of the bonding pad (1) and positioning said ball (8) on the bonding pad (1) with the capillary (5); a step of heating said ball (8) and pressing said ball (8) with a heater piston (9) to deform said ball into a disc-like form (8') and to bond said disc-like form (8') to said bonding pad (1); and repeating the preceding steps to effect bonding of another wire (3) onto the bonded disc-like form (8') to thereby form a plurality of stacked and bonded disc-like forms (8') on said bonding pad (1). A wire stacked bonding method according to claim 1, wherein said steps of positioning each of a plurality of wires (2, 3) in parallel with the surface of the bonding pad (1) are effected with a capillary (5). A wire stacked bonding method according to claim 1, further comprising arranging a portion of each of the plurality of bonding wires (2, 3) to extend in a longitudinal direction parallel with a surface of the bonding pad (1). A wire stacked bonding method according to claim 1, wherein the steps of heating and pressing each ball (8, 10) to deform the ball into a disc-like form (8') are effected with a heater piston (9). A wire stacked bonding method according to claim 1, further comprising repeating the second series of steps to bond at least another wire (4) to the stack of wires (2, 3) bonded onto the bonding pad (1). A wire stacked bonding method according to claim 3, wherein each of a plurality of wires (2, 3) bonded to form said stack are arranged to extend in longitudinal directions in parallel with a surface of the bonding pad (1).

Inventors: HASEGAWA, HIROSHI, KOJIMA, TOSAKU, NETSU, TOSITADA, SUGIMOTO, KOICHI, TANI, MITSUKIYO, YANO, TAKESHI
Applicants: HITACHI, LTD.
Application year: 1987
Main classification: B23K20
IPC codes: B23K20, H01R4, H01L23, H01L21