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.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a divisional of prior application Ser. No. 10/251,417 filed Sep. 20, 2002 which is a divisional of prior application Ser. No. 09/701,224 filed Mar. 12, 2001. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to a method for establishing an electrical connection, features and a contact point.  
       BACKGROUND INFORMATION  
       [0003]     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.  
         [0004]     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  
       [0005]     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.  
         [0006]     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.  
         [0007]     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.  
         [0008]     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  
       [0009]      FIG. 1   a  shows a first schematic view of the process steps in establishing an electrical connection.  
         [0010]      FIG. 1   b  shows a second schematic view of the process steps in establishing an electrical connection.  
         [0011]      FIG. 1   c  shows a third schematic view of the process steps in establishing an electrical connection.  
         [0012]      FIG. 1   d  shows a fourth schematic view of the process steps in establishing an electrical connection.  
         [0013]      FIG. 1   e  shows a firth schematic view of the process steps in establishing an electrical connection.  
         [0014]      FIG. 1   f  shows a sixth schematic view of the process steps in establishing an electrical connection.  
         [0015]      FIG. 1   g  shows a seventh schematic view of the process steps in establishing an electrical connection.  
         [0016]      FIG. 1   h  shows a eighth schematic view of the process steps in establishing an electrical connection.  
         [0017]      FIG. 2  shows a first embodiment of a contact point.  
         [0018]      FIG. 3  shows a second embodiment of a contact point.  
         [0019]      FIG. 4  shows a third embodiment of a contact point.  
         [0020]      FIG. 5  shows a fourth embodiment of a contact point.  
         [0021]      FIG. 6  shows a fifth embodiment of a contact point. 
     
    
     DETAILED DESCRIPTION  
       [0022]      FIG. 1   a  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.  
         [0023]     Electrical connection  10  illustrated in  FIG. 1   a  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.  
         [0024]      FIGS. 1   b  to  1   h  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. 1   a . In the following figures, identical components are always identified by the same reference numbers as in  FIG. 1   a  and are not explained again.  
         [0025]      FIG. 1   b  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. 1   a  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. 1   a . As a result, it is not necessary to reposition capillary nozzle  26  in relation to contact point  24 .  
         [0026]     According to the next process step illustrated in  FIG. 1   c , 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 .  
         [0027]     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. 1   d.    
         [0028]     In a subsequent process step, illustrated in  FIG. 1   e , 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. 1   d ). 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  10  an imaginary perpendicular running through deformed ball  34  (perpendicular that is parallel to the axis of capillary nozzle  26 ). As shown in  FIG. 1   f , 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. 1   g . 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 .  
         [0029]      FIG. 1   h  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. 1   b ), the size and type of bonding parameters ( FIG. 1   f ), 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.  
         [0030]     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.  
         [0031]     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 ′.  
         [0032]     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.