Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Reexamination Certificate
1998-11-20
2001-10-16
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
Reexamination Certificate
active
06301971
ABSTRACT:
This invention concerns a device for testing the integrity of a bond between a semiconductor device and an electrical conductor thereof.
Semiconductor devices are small, typically from 0.2 mm square to 15 mm square. Around the edge of the semiconductor substrate numerous sites for the bonding of electrical conductors are provided, these sites are very small and typically about 0.05 mm wide and 0.1 to 0.7 mm apart. Very thin wires, usually about 0.025 mm in diameter, are bonded to respective sites, and connect these sites to associated electrical circuitry and components. It is necessary to test bond integrity at the sites in order to gain confidence that the bonding method is adequate and that the bond strength is sufficient. Difficulties arise because of the very small dimensions of the components, the precision with which the testing device must be positioned, and the very small forces and deflections which are to be measured.
A known test device has a tool, such as a miniature hook or tweezers, for engagement with the wire at a bond site. In practice the semiconductor substrate is restrained and the wire is pulled by the tool to test the bond strength in tension. A force transducer is incorporated in the device to measure the force necessary to break or non-destructively test the bond or wire, as the case may be.
The forces measured by the device are very small. Often these forces are of a similar order to friction and stiction forces within the test device itself, and accordingly great care is taken to reduce the effect of these forces as much as possible. Nevertheless, prior art testing devices inevitably suffer from small but significant variations in measured force due to the inconstant internal friction of bearing components, hysteresis and the like.
Another difficulty is that the movement of the tool must be carefully controlled to avoid a sideways component as the bond breaks, which might damage an adjacent bond or wire, or which might render calculation of the bond strength more difficult.
A further requirement is that sequential tests be undertaken with the minimum of delay. Accordingly the device should re-zero as quickly as possible so as to give maximum productivity.
A typical prior test device utilizes a tool at the tip of a cantilever arm, the arm having a strain gauge thereon to measure the applied load. The tip of such an arm inevitably moves in an arc, and this device is thus undesirable because a purely linear pull is not possible, and because the arcuate movement may result in collision with another wire or bond site. The danger of collision damage is particularly acute if the bond or wire should break suddenly when under load.
A parallelogram linkage has been proposed to ensure movement of the tool in an approximately straight line, but such a linkage retains a degree of sideways movement, and the pivot points of the linkage introduce extra sources of friction which, as noted above, are undesirable.
The present invention has been developed to take into account the aforementioned difficulties, and provide a frictionless test device which substantially eliminates the sideways component of tool movement.
According to the invention there is provided apparatus for testing the integrity of a bond between a semi conductor device and an electrical conductor thereof, the apparatus comprising a baseplate having two cantilever arms extending therefrom in substantially the same direction, a test head operatively fixed to the free ends of said arms, and measurement means to measure the force deflection characteristic of said arms on movement of said test head relative to said baseplate in the plane of said arms, wherein said arms are of unequal effective length, the test head having a test datum on one side of the arm with longer effective length, the arm with the shorter effective length being on the other side of the arm with the longer effective length, and the apparatus being proportioned such that a small deflection of said arms results in movement of said test datum substantially perpendicularly to the direction of extension of said arms.
Although configurations with convergent or divergent arms are possible, the geometry of movement is rather complex. Accordingly in the preferred embodiment the arms are substantially parallel and of unequal length, and the test head extends substantially orthogonally to the arms.
The unequal length cantilever arms ensure that the test datum can much more nearly follow a straight line over the test deflection, typically an order of magnitude better than prior art parallelogram linkage. In a preferred embodiment a typical deviation of 0.0005 mm is achievable over a deflection of 3.5 mm in a test head having the short arm about two thirds of the length of the long arm, and the test datum about the same distance away from the long arm. In contrast a typical equal length beam arrangement having a similar geometry and two long arms would have a deviation of 0.1 mm over the same deflection.
The cantilever arms may be machined from aluminium and be of appropriate dimensions to remain within their elastic range over the anticipated deflection of the test datum.
Preferably the apparatus includes a non-contact damping device, such as a pneumatic or magnetic damper, which can damp relative movement of the test head to permit rapid re-zero of the device. In a preferred embodiment the damper is of appropriate dimensions to ensure effective damping within 5 cycles.
The test head may include a rotatable shaft to permit the angular position of the test datum to be varied; such a modification is useful for orientation of a hook. A co-axial stepper motor is preferably provided to rotate the shaft.
According to another aspect the invention provides test apparatus having no moving parts and comprising a baseplate, two unequal length cantilever arms extending therefrom in substantially the same direction, and a test head operatively fixed to the free ends of said arms, the arms being substantially parallel and the test head being movable relative to said baseplate by resilient movement of said arms. Such apparatus eliminates the internal friction inherent in prior art linkage-type test devices.
REFERENCES:
patent: 4243360 (1981-01-01), Wright
patent: 4458865 (1984-07-01), Sandorff
patent: 4501398 (1985-02-01), Sandorff
patent: 5633455 (1997-05-01), Quate
patent: 5646338 (1997-07-01), Mercusot et al.
patent: 5807758 (1998-09-01), Lee et al.
patent: 5886268 (1999-03-01), Larsson
patent: 5892157 (1999-04-01), Syr'e
patent: 6078387 (2000-06-01), Sykes
Aw-Musse Abdullahi
Dage Precision Industries, Inc.
Fuller Benjamin R.
Kilpatrick & Stockton LLP
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