DE2032302C2 - Method for operating a bonding device - Google Patents

Description

The invention relates to a method for operating a bonding device according to the preamble of patent claim 1.

Such a method is known from US-PS 32 50 452. This method, used in the manufacture of semiconductor devices, for example transistors, is carried out semi-automatically, in particular the automatic provision of a measured length of a fine wire, the formation of a small ball at the end of the wire, the connection of the wire to a contact surface of the semiconductor component and the subsequent severing of the wire are carried out automatically. The method is carried out in such a way that the wire on the supply spool is always kept under a suitable tensile stress so that the wire coil does not become loose after the wire is severed. This tensile stress acts on the wire at all times, i.e. also inevitably during the attachment of the wire to the semiconductor component. In the manufacture of semiconductor components, it has proven problematic if the wires are attached to the contact surfaces under tensile stress. This tensile stress can result in the wire becoming detached from the contact surface again at a later point in time, which leads to the failure of the semiconductor component.

It is also known from US-PS 33 63 18 to constantly exert a tensile stress on the wire used to connect contact surfaces on semiconductor components so that the wire wound onto the supply roll always remains tightly wound and does not spring open.

The invention is based on the object of developing a method of the type described at the outset in such a way that the connections formed on the contact surfaces are as permanent as possible without there being a risk that the wire coil on the supply roll becomes loose.

According to the invention, this object is achieved with the features specified in the characterizing part of claim 1. When, in the method according to the invention, after the wire has been attached to the first contact surface, the contact tip is raised and moved over the second contact surface, the torque acting on the supply roll and unwinding the wire ensures that the wire emerges freely from the contact tip and does not have to be pulled out of the contact tip and from the supply roll, putting strain on the contact point formed. No tensile forces therefore act on the contact formed first or on the contact formed afterwards, which contributes to a considerable improvement in the reliability of the semiconductor component produced by the method according to the invention.

An advantageous development of the invention is characterized in patent claim 2.

The invention will now be explained in more detail by way of example with reference to the drawing. It shows

Fig. 1 is a plan view of a typical workpiece for a device for carrying out the method according to the invention;

Fig. 2a to 2d simplified side views of the workpiece according to Fig. 1 to explain the method according to the invention;

Fig. 3 is a side view of a contacting device for carrying out the method according to the invention, with individual parts broken away;

Fig. 4 is an enlarged side view of a portion of the contacting device according to Fig. 3 including a portion of the parts broken away in this figure;

Fig. 5 is a plan view of the parts of the contacting device shown in Fig. 4 and

Fig. 6 is a section taken substantially along line 8-8 in Fig. 5.

Fig. 1 shows a workpiece 10 with a transistor die 12 alloyed onto the flattened head of a metal pin or conductor 14. The conductor 14 and similar conductors 16 and 18 are secured in a glass plate 20. Lead wires 24 a and 24 b extend from the base and emitter contact surfaces to the contact surfaces of the conductors 16 and 18 , respectively. The lead wires are typically made of gold and have a diameter of 0.025 mm. The method to be described serves to attach lead wires 24 a and 24 b between the contact surfaces of the transistor 12 and the contact surfaces of the conductors 16 and 18 , respectively. The method can be used to contact any contact surfaces of any semiconductor with wires.

Fig. 2a to 2d show the procedure when attaching the lead wire 24 a to the transistor chip 12 and the conductor 18 . The transistor chip 12 is first brought into an operating position and then the contacting device is centered on the transistor chip with respect to a predetermined axis 26 , which is done by means of an electro-optical servo system. A contacting tip 28 lies well outside the field of view of the optical system during alignment of the contacting device. Its position is indicated by the dashed outline 28 a . The wire 24 leads from the contacting tip 28 designed as a hollow needle to a supply roll 182 which is not shown in Fig. 3. The wire 24 is kept under tension by a winding force acting on the supply roll 182. A ball 25 which was created when the wire end was passed through a flame prevents the wire from being pulled out of the needle.

After the electro-optic system is aligned with the transistor die 12 , the contacting tip 28 is placed in a predetermined position with respect to the axis 26. The predetermined position is chosen so that the needle is over an appropriate area of the expanded contacts of the transistor die 12 and the contacting tip is then lowered to press the ball 25 against the contact pad as shown in solid lines in Fig. 2a. Both the ball 25 and the contact pad are heated to a contacting temperature so that the pressure exerted by the contacting tip 28 on the ball 25 bonds the wire 24 to the contact pad.

The contact tip 28 is then raised and performs a lateral movement such that its tip moves along the dashed line 32 ( Fig. 2b) and is then lowered into a second position fixed relative to the axis 26 , which is selected so that the edge of the wire 24 is pressed against the contact surface of the conductor 18. The direction of the winding force acting on the supply roll 182 is reversed for the duration of the movement of the contact tip along the line 32, so that the wire is unwound by means of an air stream through the contact tip 28 , thereby avoiding stress on the wire and the connection point. In this way it is possible to move the contact tip at high speed without destroying the connection or the wire. The pressure of the contact tip on the conductor 18 leads to a staple contact 33.

Next, the contact tip 28 is raised, allowing a piece of wire 24c to emerge, as shown in Fig. 2c. The wire 24 is then clamped opposite the contact tip 28. If the contact tip 28 now continues its upward movement, the wire breaks at the point where it is weakened by the tack contact 33. The contact tip 28 then moves along the line 34 so that the piece of wire 24c is guided through a flame 36 , forming a new ball 27 , as shown in Fig. 2d. After passing the flame 36 , the wire is released so that the winding force now acting on the coil again draws the ball 27 upwards against the tip of the contact tip 28. The contact tip then moves further into its starting position shown at 28a in Fig. 2a.

The method described above and illustrated in Fig. 2a to 2d has decisive advantages. As a result of reversing the direction of the force acting on the supply roll and the driven dispensing of the wire from the contacting tip by means of an air jet, the stress on the connection between the wire and the contact surface of the transistor chip 12 is significantly reduced or completely eliminated. This also reduces the risk of faulty contacts, which can be due to the lead wire becoming detached from the transistor chip, the ball 25 becoming detached from the contact surface and the wire becoming detached from the ball 25. Almost more importantly, if such a fault occurs, the wire 24 is not pulled out of the contacting tip 28 as a result of the winding force acting on the supply roll. Instead, the driven dispensing of the wire 24 ensures that a piece of wire emerges from the end of the contacting tip so that tack contacting can take place regardless of whether ball contacting has been achieved or not. The drive of the supply roll in the unwinding direction for dispensing the wire 24 is maintained until the wire is clamped in the process step according to Fig. 2c so that premature tearing of the wire 24 after tack contacting or an error in carrying out the tack contacting does not result in the wire 24 being pulled out of the contact tip 28. Once the wire 24 is clamped, this condition is maintained until the wire has been passed through the flame 36 and until a new ball 27 has formed at its end in preparation for the next work cycle. If the ball contact or the tack contact were not carried out successfully, the flame 36 cuts off the protruding wire end and the contacting device is thus prepared for the next work cycle, even if an error occurred in the ball contact or the tack contact. A contacting cycle can be carried out in about one second, so that about 60 contacts can be made per minute. The importance of maintaining the functionality of the contacting device despite faulty contacting steps is obvious, since the slipping of the wire 24 from the contacting tip 28 would make it necessary to shut down the contacting device until the wire was re-threaded into the contacting tip and provided with a new ball. Such repair times would result in a considerable loss of production.

The essential parts of the contacting device for carrying out the method explained with reference to Fig. 2a to 2d are shown in Fig. 3. A key component of the contacting device is a U-rail-shaped frame 52. Within the frame 52 , a first carrier element 54 ( Fig. 3) is mounted on tapered roller bearings with low friction and is movable in a direction which will be referred to below as the X coordinate. The carrier element 54 is driven in the direction of the X coordinate by means of a spindle (not shown), which in turn is driven by an X stepper motor. A second carrier element 62 is connected to the first carrier element 54 via low-friction tapered roller bearings for movement in a direction which will be referred to below as the Y coordinate. The second carrier element 62 is driven by means of a second spindle (not shown), which in turn is driven by a second or Y stepper motor which is attached to the first carrier element 54 .

The second support element 62 can thus be brought into any position relative to the frame 52 in the XY plane and thus forms a device which is usually referred to as a cross table.

An electro-optical system 74 serves to position the contact tip 28 with respect to the contact surfaces.

The workpiece 10 is supported by a clamping device 40 which is mounted on a switching chain which brings the workpiece into machining position so that the semiconductor device lies within the field of view of the optical system 74. The second carrier element 62 carries a contacting unit which is designated by the reference numeral 80 .

A wire feed device, designated as a whole by the reference numeral 180 , is mounted on the L-shaped element 116 , as shown in Fig. 5. The device includes a replaceable supply roll 182 for the wire 24 , which is mounted on a carrier roll 184 , 184a . The carrier roll 184 , 184a is mounted on a tube 186 and secured there by an annular shoulder of a screwed-on cap 188. Compressed air is introduced into the interior of the tube 186 via a channel 192 within the arm 194 which carries the tube 186. The arm 194 is attached to the L-shaped element 116 so that it moves together with the contacting unit. As best seen in Fig. 6, a number of openings 200 lead from the inner bore 190 of the tube 186 to the annular space between the tube 186 and the carrier roll 184. Through these openings 200 a quantity of air is supplied which is sufficient to support the carrier roll 184 constantly on a layer of air, so that a very low friction bearing is achieved. In addition, it will be noted that the openings 200 are not exactly radial, but slightly offset so that the air circulates along the circumference of the space between the tube 86 and the carrier roll, thereby exerting a winding force on the carrier roll 184 , that is, a force which causes the carrier roll to rotate in such a direction that the wire is rewound onto the supply roll 182 . A nozzle 202 periodically receives blasts of compressed air from a flexible hose 204 and directs a stream of air against the surface 184a of the carrier roll. This stream of air is sufficiently large to overcome the winding force created by the air exiting the openings 200 and to produce a resultant unwinding force. The air to the nozzle 202 for reversing the direction of rotation of the carrier roll 184 , 184a is controlled by a solenoid valve (not shown).

Normally the supply roll rotates at a relatively low speed. However, if the wire breaks for any reason, the air escaping from the openings 200 causes the carrier roll 184 , 184a to rapidly increase in speed in the direction in which the wire would be wound. Alternating dark and light segments 205 and 206 on the face of the carrier roll allow this serious defect to be detected so that operation can be stopped. Separate fiber optic strands 208 and 210 are combined and aimed at the part of the face provided with light and dark areas 205 and 206 , respectively. The light is guided to the face via the fiber optic strand 208 and the reflected light is returned to a photodetector via the fiber optic strand 210. The photodetector electronically determines the pulse frequency and the intensity of the light reflected from the light and dark segments. As soon as the frequency exceeds a specified value, a signal is generated that indicates a wire break.

Claims (4)

1. A method for operating a bonding device for producing an electrically conductive connection between a first conductive contact surface and a second conductive contact surface, of which at least the first contact surface is provided on a semiconductor surface, the bonding device comprising: a) a wire wound on a supply reel which provides the electrically conductive connection, b) a contact tip from which the wire emerges downwards, The procedure is as follows: c) the contact tip is moved so that it is located exactly above the first contact surface, d) the contact tip is lowered to fix the wire on the first contact surface, e) the contact tip is raised and moved sideways to a position above the second contact surface, f) the contact tip is lowered to connect the wire to the second contact surface, g) the contact tip is lifted and the wire between the second contact surface and the contact tip is severed, characterized in that in process step e) a torque is exerted on the supply roll which unwinds the wire, so that in process step f) no tensile forces act on the finished first contact. 2. Method according to claim 1, characterized in that the supply roll is mounted on an air cushion and is driven by at least one air stream having a tangential component.