# Patent

## Title
Method and contact point for establishing an electrical connection

## Abstract

A method for establishing an electrical connection between a first contact surface and a second contact surface, with a wire-bonding tool being used to provide a contact wire between the contact surfaces by bonding the contact wire to the first contact surface and subsequently leading it to the second contact surface, bonding it to the latter, and subsequently, separating it using the wire-bonding tool. After the contact wire has been separated from the second contact surface, the wire-bonding tool is used to provide the contact point with an additional contact securing element via the contact wire.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application is a division of prior application Ser. No. 09/701,224, filed Mar. 12, 2001, now U.S. Pat. No. 6,477,768.


FIELD OF THE INVENTION
The present invention relates to a method for establishing an electrical connection, features and a contact point.
BACKGROUND INFORMATION
A single-wire contacting method, called bonding, is known for establishing an electrical connection between at least two contact surfaces. In doing this, individual wires, in particular, gold or aluminum wires, are positioned between the contact surfaces to be bonded, using a wire-bonding tool. In bonding, the contact wire is bonded to the contact surfaces by applying ultrasonic pressure and heat. The free end of the bonding wire is first melted to form a ball, by applying thermal energy, and subsequently pressed onto the first contact surface, using a bonding capillary. The contact wire bonds to the contact surface as a result of atomic bonding forces (material fusion) arising at the boundary between the contact surface and the contact wire. During bonding to the first contact surface, the ball that was previously melted on is deformed into a nail head. The contact wire is then led to the second contact surface, using the wire-bonding tool. To prevent the contact wire from breaking away at the first contact point, the contact wire is formed into a loop. The contact wire is then pressed onto the second contact surface with the wire-bonding tool by again applying ultrasonic pressure and heat. This produces necking of the contact wire, causing the latter to form a rupture joint at which the contact wire breaks away from the second contact surface as the wire-bonding tool moves on. The contact wire is bonded to the second contact surface by a “stitch,” with atomic bonding forces again arising at the boundary between the contact wire and the second contact surface.
This known ball-wedge bonding method (ball bonding with the first contact surface, and stitch bonding with the second contact surface) produces a strong dependency between the materials of the contact wire and the contact surface, thus forming strong atomic bonding forces at the boundaries. Particularly when contacting the second contact surface, a relatively weak surface bonding forms between the stitch and the contact surface, resulting in contacting errors, particularly in the case of contact surfaces made of hard-to-bond materials.
SUMMARY OF THE INVENTION
The method according to the present invention offers an advantage over the related art in that it considerably improves the contact stability of the bond between the contact wire and the second contact surface. The fact that the wire-bonding tool provides the contact point with an additional contact securing element after bonding the second contact surface increases the contact stability of the second contact point (stitch or wedge) independently of the generation of atomic bonding force between the contact wire and the second contact surface.
In one preferred embodiment of the present invention, the additional contact securing element is provided by the ball shape, applied to the contact point and subsequently deformed by the bonding tool, at the end of the contact wire that remains free after contacting the second contact surface. This makes it possible, after forming the electrical connection between the contact wire and the second contact surface, to immediately form the ball on the end of the contact wire that is now free and to position it over the contact point as an additional contact securing element. A particularly preferred feature is to deform the ball with the wire-bonding tool so that the contact point overlaps, producing at least one, preferably two, additional bonding areas between the additional contact securing element and the contact surface. The atomic bonding forces generated cause the additional bonding areas to adhere to the contact surface, forming a sort of tensile strain relief for the contact wire bonded to the second contact surface. This very reliably prevents the contact wire from breaking away from the second contact surface. The possibility of the contact wire breaking away is now determined only by the rupture strength of the contact wire itself, and no longer by the adhesion between the contact wire and the second contact surface, i.e., the contact wire itself breaks before the contact point ruptures.
According to another preferred embodiment of the present invention, the production of the additional contact securing element can be precisely reproduced through wire-bonding tool settings, in particular, by programming a corresponding controller of the wire-bonding tool. This makes it possible to create identical contact securing elements among a large number of contacts, and these identical contact securing elements can be easily tested on the basis of a predictable, reproducible result. One particularly preferred feature is that a visual, preferably automatic visual, inspection of the contact point is carried out, in which the contact securing elements that are not precisely produced, i.e., according to the specified degree of reproducibility, are reliably detected. This makes it possible to achieve a sort of zero error rate in producing bonds that result in a higher production yield.
A contact point according to the present invention advantageously ensures a high contact stability between the contact wire and contact surface. Since the contact point includes an additional contact securing element which at least partially engages over the contact wire in the area of the contact point and forms at least one additional bonding surface with the contact surface, the available overall surface is advantageously increased for contacting the contact wire with the contact surface, enabling the contact point to withstand higher mechanical stresses. Particularly when used in safety-related components, this contact point can maintain highly redundant electrical connections.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a shows a first schematic view of the process steps in establishing an electrical connection.
FIG. 1b shows a second schematic view of the process steps in establishing an electrical connection.
FIG. 1c shows a third schematic view of the process steps in establishing an electrical connection.
FIG. 1d shows a fourth schematic view of the process steps in establishing an electrical connection.
FIG. 1e shows a firth schematic view of the process steps in establishing an electrical connection.
FIG. 1f shows a sixth schematic view of the process steps in establishing an electrical connection.
FIG. 1g shows a seventh schematic view of the process steps in establishing an electrical connection.
FIG. 1h shows a eighth schematic view of the process steps in establishing an electrical connection.
FIG. 2 shows a first embodiment of a contact point.
FIG. 3 shows a second embodiment of a contact point.
FIG. 4 shows a third embodiment of a contact point.
FIG. 5 shows a fourth embodiment of a contact point.
FIG. 6 shows a fifth embodiment of a contact point.



DETAILED DESCRIPTION
FIG. 1a shows an electrical connection 10 between a first contact surface 12 and a second contact surface 14. Contact surface 12 is provided on a substrate 16 and contact surface 14 on a substrate 18. Electrical connection 10 is produced by bonding in the known manner (ball-wedge bonding). To do this, a wire-bonding tool (not illustrated) is used to first heat the free end of a contact wire 20, forming it into a ball 21. A capillary nozzle of the wire-bonding tool is then used to press this ball 21 onto first contact surface 12, thus producing atomic bonding forces at the boundary between what is then a plastically deformed ball 21 and contact surface 12. The wire-bonding tool is then moved toward second contact surface 14, thus forming a loop 22 in contact wire 20. Contact wire 20 is pressed with the capillary nozzle onto second contact surface 14, where it is plastically deformed, thus producing atomic bonding forces between contact wire 20 and second contact surface 14. The plastic deformation of contact wire 20 (stitch) by the capillary nozzle simultaneously creates a rupture point at which contact wire 20 breaks after the capillary nozzle is removed. The design of second contact point 24 (wedge) has a relatively small contact area between contact wire 20 and contact surface 14. As a result, contact point 24 allows contact wire 20 to pull away from contact surface 14. Enormous contact problems arise, especially if contact surface 14 is made of a hard-to-bond material.
Electrical connection 10 illustrated in FIG. 1a is produced by a known bonding method (ball-wedge method). Such electrical connections 10 are established, for example, when microhybrid components are connected to microchips.
FIGS. 1b to 1h illustrate the method according to the present invention for establishing electrical connection 10, with this method being based on an electrical connection previously established according to FIG. 1a. In the following figures, identical components are always identified by the same reference numbers as in FIG. 1a and are not explained again.
FIG. 1b shows a schematic representation of a capillary nozzle 26 of a wire-bonding tool 28. Capillary nozzle 26 has a passage 30 through which contact wire 20 is fed. Suitable feed devices enable contact wire 20 to move through capillary nozzle 26. After electrical connection 10 shown in FIG. 1a has been established, contact wire 20 moves toward second contact surface 14, and its free end 32 is heated to a temperature above its melting point, using a thermal energy source. A surface tension causes the molten mass of contact wire 20 to form a ball 34. Melting a ball 34 onto end 32 takes place directly after establishing the connection between contact wire 20 and contact surface 14, as shown in FIG. 1a. As a result, it is not necessary to reposition capillary nozzle 26 in relation to contact point 24.
According to the next process step illustrated in FIG. 1c, a force and ultrasound are applied to capillary nozzle 26. This compresses ball 34, which undergoes plastic deformation. The shape of capillary nozzle 26 can influence the plastic deformation of ball 34. In the illustrated embodiment, the end of capillary nozzle 25 facing contact surface 14 has a circumferential ring-shaped ridge 36 that engages with an inner cone 38.
The plastic deformation of ball 34 follows the shape of this inner cone 38. The application of contact force F, combined with ultrasound energy, produces atomic bonding forces between ball 34 and, extending from center 40 of bonding point 24 to the wedge, causing deformed ball 34 adhere to contact point 24 during a motion away from contact surface 14, as shown in FIG. 1d. 
In a subsequent process step, illustrated in FIG. 1e, capillary nozzle 26 is moved laterally away from contact point 24. This movement is indicated by an arrow 40. If necessary, lateral movement 40 can be superimposed on the lifting of capillary nozzle 26 away from contact point 24 (FIG. 1d). Movement 40 is oriented so that its direction vector is more or less contrary to a longitudinal extension of contact wire 20 laid in loop 22. Direction of movement 40 is maintained until a vertex 44 of ring-shaped ridge 26 of capillary nozzle 26 has passed an imaginary perpendicular running through deformed ball 34 (perpendicular that is parallel to the axis of capillary nozzle 26). As shown in FIG. 1f, capillary nozzle 26 is moved in the direction of contact surface 14 so that ring-shaped ridge 36 strikes plastically deformed ball 34. Depending on contact force F′ applied, and under the influence of ultrasound, ball 34 undergoes a further plastic deformation, due to the outer lateral surface of ring-shaped ridge 36. Ball 34 undergoes a segment-like deformation. Ball 34 continues to change shape until segment 34′ projects laterally over contact wire 20 already bonded to contact surface 14 and comes into physical contact with contact surface 14 in additional bonding areas 48. This type of deformation generates atomic bonding forces between segment 34′ formed and contact surface 14, causes the segment to permanently adhere to the surface. After capillary nozzle 26 moves away, segment 34 spans contact wire 20 in the area of contact point 24 and holds the latter in place like a strap, as shown in FIG. 1g. Segment 34 provides a kind of tensile strain relief function to secure contact point 24 for contact wire 20. The remaining wire that broke away when capillary nozzle 26 was removed is visible in center 40 of contact point 24 in the form of a pointed elevation 50. The latter forms an additional positive-lock joint with contact wire 20 in the area of contact point 24.
FIG. 1h shows an enlarged representation of contact point 24 after electrical connection 10 has been established with additional contact securing provided by segment 34′. It is clear that segment 34′ has certain shape characteristics that are derived from the size of ball 34 (FIG. 1b), the size and type of bonding parameters (FIG. 1f), and the shape of ring-shaped ridge 36 of capillary nozzle 26. Because the shape of capillary nozzle 26, the magnitude of bonding forces, and the size of ball 34 are known or can be set, segments 34′ can be achieved in reproducible shapes. After contact point 24 has been produced, segment 34′ can be measured by an optical monitoring unit (not illustrated). Comparing the measured shape of segment 34′ to a previously stored shape makes it possible to draw conclusions about the quality of contact point 24. If the shape of segment 34′ matches the expected shape, a perfect, i.e., contact-secure and additionally secured contact point 24 can be assumed, making it possible to produce contact point 24 with zero errors. Fault-free usage values can thus be predicted, particularly when using electrical connections on microchips in safety/security systems.
The present invention is, of course, not limited to the embodiment illustrated in FIG. 1. In particular, different shapes can be selected for segment 34′. FIGS. 2 through 6 show different embodiments of segment 34′. The shape of segment 34′ can be selected, for example, by choosing a different shape for capillary nozzle 26 and varying the placement of capillary nozzle 26 when shaping ball 34 into segment 34′. In addition, the design of segment 34, and the way it bonds to contact surface 14, can be influenced by setting general bonding parameters, such as force F or the frequency and intensity of the ultrasound energy.
According to further exemplary embodiments, it is possible, in particular, to provide a more shallow depression between tip 50 and segment 34′. This means that the transition between segment 34′ and point 50 occurs through a relatively shallow depression, thereby improving the positive-lock joint between contact wire 20 and segment 34′ or contact point 24 in the example.
By way of example, FIG. 2 shows a segment 34′ that is designed in the shape of a ridge. In contrast to this, segment 34′ in FIG. 3 has a flatter design and merges with the material of tip 50. According to the embodiment illustrated in FIG. 4, segment 34′ has an even flatter design, so that it is almost shaped like a disk and also merges with the material of tip 50. FIG. 5 shows a further embodiment, in which segment 34′ is shaped like a shallow basin, with segment 34′ again merging with the material of tip 50. Finally, FIG. 6 shows an embodiment of segment 34′ in which segment 34′ is designed as a largely flat disk that has a ridge-shaped bulge in the direction of contact wire 20. Tip 50, in this case, is formed from the material of segment 34′ by deforming ball 34 accordingly. This can be achieved by a suitable design of capillary nozzle 26 and placement of capillary nozzle 26 while forming the ball into segment 34′.
It is also possible to modulate segment 34′ as a largely rectangular object having a defined elongation in the x-direction, y-direction, and z-direction by setting the bonding parameters and/or placement parameters of capillary nozzle 26 while shaping segment 34′. Parameters that can be set while shaping ball 34 into “rectangular” segment 34′ make it possible to set precisely reproducible dimensions in the x-, y-, and z-directions. A subsequent visual, in particular automatic visual, inspection of contact point 24 can be used to easily and effectively check contact point 24 for freedom from errors.

## Claims

1. A bonding system for producing an electrical connection between a first contact surface and a second contact surface, comprising:
a programmable control unit;
a bonding capillary including a capillary nozzle, the capillary nozzle having an annular protuberance;
a deformed first ball placed on a contact point on the first contact surface; and
a contact wire via which a bond connection is produced between the first contact surface and the second contact surface, using the deformed first ball on the first contact surface and a second contact connection on the second contact surface, a second ball being positioned on the second contact connection using the capillary nozzle and being deformed by compression using the capillary nozzle, wherein the programmable control unit is programmed to control the positioning of the second ball and the steps of:
after the compression of the second ball, the capillary nozzle is lifted off the deformed second ball and moved according to a lateral movement,
the lateral movement is in a direction away from a direction vector of a longitudinal extension of the contact wire,
after the lateral movement, the capillary nozzle is moved in a direction of the second contact surface, so that the annular protuberance strikes the deformed second ball, and
a further plastic deformation of the deformed second ball is brought about by an outer surface of the annular protuberance, under an influence of an applied contact force F′, and under an influence of an ultrasonic operation.

2. The bonding system as recited in claim 1, wherein:
the further plastic deformation of the second ball results in the second ball being deformed into a slice.

3. The bonding system as recited in claim 2, wherein:
the further plastic deformation of the second ball results in the second ball being deformed to an extent that the slice projects laterally over the contact wire that is already bonded to the second contact surface, and
the slice forms at least one additional connecting region that is in contact with the second contact surface.

4. The bonding system as recited in claim 1, wherein:
the further plastic deformation of the second ball results in the second ball being deformed such that the deformed ball overlaps the second contact connection and at least one additional connecting region is formed.

5. The bonding system as recited in claim 1, wherein:
in order to form the second ball, an end of the contact wire is heated above a melting point thereof.

6. The bonding system as recited in claim 1, wherein:
the second ball is deformed in a plastic manner on a center of the contact point, by compression applied by the capillary nozzle.

7. The bonding system as recited in claim 1, wherein:
a shape of the capillary nozzle influences deformation of the second ball.

8. The bonding system as recited in claim 1, further comprising:
after the further plastic deformation, the deformed second ball is optically examined automatically.

9. The bonding system as recited in claim 2, wherein:
the slice is formed into the shape of a trough.