Method and apparatus for testing an integrated circuit using controlled wirebonding and wirebonding removal

An apparatus for testing an integrated circuit includes a bond substrate having a location for an integrated circuit. The location on the bond substrate has a plurality of traces around each location. A first fixture holds the bond substrate in a fixed relation to first fixture and holds the integrated circuit in a fixed relation to the first fixture and the bond substrate. A wirebonder forms wirebonds between the traces and the bond pads. An electrical tester provides electrical signals from the traces to the bond pads to verifying the operation of the integrated circuit. A second fixture lifts the bond substrate while the integrated circuit remains held to the first fixture. A vibrator vibrates the first fixture in relation to the second fixture so that the wirebonds are broken at a predetermined location near the bond pad.

BACKGROUND OF THE INVENTION 
The present invention relates to the testing of bare integrated circuits 
through wirebonding and, more specifically, to breaking the wirebonds 
after testing. 
Integrated circuit technology has progressed so that it is possible to 
mount a bare integrated circuit directly to a circuit board without 
enclosing it in a package having solder pins for mounting. One method of 
mounting a bare integrated circuit is called flip chip, which is commonly 
known in the art. 
In using the flip chip technique, the integrated circuit is tested before 
installation on the circuit board. One method for testing flip chips is by 
wirebonding to the individual bond pads of the integrated circuits and 
exchanging signals between a test apparatus and the integrated circuit. 
One problem with testing a bare integrated circuit using wirebonding is 
that the wirebonds must be removed from the bond pads without damaging the 
integrated circuit. 
One known method for removing wirebonds uses a high pressure air pulse 
directed at the wirebonds to break the wirebonds and remove them from the 
integrated circuit. This technique results in wirebonds that are broken 
unevenly with varying amounts of wirebond connected to the bond pads of 
the integrated circuit. When the integrated circuits are attached to a 
circuit board, the uneven wirebonds may cause electrical continuity 
problems at the solder joint joining the bond pad to the circuit board 
traces. Continuity problems occur since the uneven leads may cause the 
integrated circuit not to lay flat on the circuit board traces. 
It would therefore be desirable to provide a method and apparatus of 
removing the wirebonds from an integrated circuit uniformly so that the 
integrated circuit can be reliably attached to a circuit board for its 
intended application. 
SUMMARY OF THE INVENTION 
One object of the invention is to advantageously provide a method for 
testing an integrated circuit using wirebonding while uniformly breaking 
the wirebonds so that the remaining portion of the wirebond can be 
reliably attached to the traces of a bond substrate. 
The apparatus of the present invention includes a bond substrate having a 
location for an integrated circuit. The location on the bond substrate has 
a plurality of traces around each location. A first fixture holds the bond 
substrate in a fixed relation to first fixture and holds the integrated 
circuit in a fixed relation to the first fixture and the bond substrate. A 
wirebonder forms wirebonds between the traces and the bond pads. An 
electrical tester provides electrical signals from the traces to the bond 
pads to verifying the operation of the integrated circuit. A second 
fixture lifts the bond substrate while the integrated circuit remains held 
to the first fixture. A vibrator vibrates the first fixture in relation to 
the second fixture so that the wirebonds are broken at a predetermined 
location near the bond pad.

DETAILED DESCRIPTION OF THE PRESENT INVENTION 
Referring to FIG. 1, generally a fixture 10 secures integrated circuits 12 
and a bond substrate 14 for testing using a wirebonding process. Bond 
substrate 14 is preferably a circuit board, lead frame or ceramic packages 
on a board carrier or other similar structure for testing an integrated 
circuit. 
Fixture 10 has several integrated circuit cavities 16 for assisting in the 
positioning of integrated circuits 12. Cavity 16 is slightly larger than 
the length and width dimensions of integrated circuits by 20 to 40 mils. 
Cavity 16 is useful in assisting in locating integrated circuits 12 but 
are not essential to the invention if another scheme of accurately 
locating the chip on fixture 10 is employed. Fixture 10, as shown for 
convenience, has the capacity to hold six integrated circuits 12. However, 
the number of integrated circuits 12 may be varied for different 
requirements. Each cavity 16 has a vacuum hole 18 to fixedly secure 
integrated circuit to fixture 10 during wirebonding and wirebond removal. 
To assist in locating bond substrate 14 in the proper location with respect 
to fixture 10, alignment posts 20 on fixture 10 fit into alignment holes 
22 on bond substrate 14. 
Fixture 10 also has a series of vacuum slots 24 by which bond substrate 14 
is secured to base 10. The size and location of vacuum slots 24 provide 
enough force to securely hold bond substrate 14 in place during the 
testing process. It would be apparent to those skilled in the art, that 
other securing methods for securing integrated circuits 12 and bond 
substrate 14 such as mechanical clips may be used. Vacuum securing is the 
most desirable since a sufficient vacuum can be provided to hold the bond 
substrate securely in place. 
Bond substrate 14 has a hole 26 for each of the integrated circuits 12. 
Holes 26 are sized slightly larger than the dimensions of integrated 
circuits in the length and width dimension by about 20 to 40 mils. The 
depth of holes 26 is preferably about the thickness of integrated circuit 
12, although the depth can vary since the wirebonding process is somewhat 
adjustable. Holes 26 are primarily used for locating. Holes 26 have a 
plurality of traces 28 spaced around each hole 26. Each trace 28 is 
adjacent a bond pad 30 when integrated circuit 12 is placed within hole 
26. Traces 28 are preferably formed of a highly electrically conductive 
material such as gold. 
Referring now to both FIGS. 1 and 2, once bond substrate 14 is secured to 
fixture 10 and integrated circuits 12 are secured to fixture 10, wirebonds 
32 are formed between bond pads 30 and traces 28 in a conventional manner. 
Depending on the wirebond material, fixture 10 may be heated during the 
wirebond process. If aluminum wirebonds are used, no heating is required. 
However, if gold wirebonds are used, fixture 10 is preferably thermally 
conductive and is heated between 160.degree. C. and 240.degree. C. to 
ensure a good electrical connection between wirebonds 32 and traces 28. 
Traces 28 are connected to circuit test equipment 29. Electrical test 
signals are passed to traces 28 from circuit test equipment 29, through 
wirebonds 32, through bond pads 30 and into integrated circuit 12 to 
verify the operation of integrated circuit 12. Verification preferably 
takes place while bond substrate 14 is secured to fixture 10. As an 
alternative, bond substrate 14 can be removed from fixture 10 and placed 
in a separate apparatus for testing. 
Referring now to FIG. 3, once testing is completed on integrated circuits 
12, bond substrate 14 preferably is released from fixture 10 while 
integrated circuit 12 remains secured via vacuum hole 18. 
A second fixture 40 lifts bond substrate 14 from fixture 10 by using a 
vacuum 42 through vacuum holes 44. 
Second fixture 40 lifts bond substrate 14 until wirebonds 32 about 5 to 10 
mils. Wirebonds should not be broken during this process. 
A vibrator 46 vibrates either second fixture 40 or fixture 10. Preferably 
fixture 10 is vibrated until wirebond 32 fatigues sufficiently to break at 
its weakest point. Vibrator 46 provides either linear motion or rotational 
motion both of which have been found to give desirable results. Vibrator 
46 is preferably a motor in a cam follower configuration. Vibrator 46 may 
also be a transducer or other vibrating element. Typically, wirebond 32 
takes between 1 and 2 seconds to break. The vibration amplitude is in the 
range of 5 to 10 mils using a motor and cam follower configuration with 
the motor rotating at 10 to 20 rpm. 
Referring now to FIG. 4, a wirebond 32 is shown bonded to bond pad 30 in a 
conventional manner. The weakest point of wirebond 32 is at position 50. 
It is therefore desirable to break wirebond 32 at position 50. 
In FIG. 5, wirebond 32 is shown after undergoing the above process. 
Wirebonds 32 consistently break at position 50. The remaining portion of 
wirebond 32 is used for bonding integrated circuit to a bond substrate 
preferably in a flip chip application. The consistent wirebonds remaining 
on bond pad 30 greatly improve reliability of the electrical connection 
when attaching the integrated circuit to a circuit board. 
Various modifications will be apparent to those skilled in the art. For 
example, the materials, shapes and sizes of the associated components 
which affect the process are modifications which are all within the true 
spirit of the scope of the appended claims.