Measuring and testing – Dynamometers – Responsive to force
Reexamination Certificate
2002-09-30
2003-12-30
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Dynamometers
Responsive to force
C073S862381, C073S862070
Reexamination Certificate
active
06668667
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an arrangement for mechanical loading of a bonding wire.
BACKGROUND OF THE INVENTION
Bonding wires of thick aluminum wire are used for purposes of electrical contact-making of the power components of power modules for gate and emitter terminals or for wiring within the module. The bonding technique is ultrasonic bonding, the wires which are circular in cross section having a diameter of typically 200 to 300 microns and arcs with 10 to 20 mm width and up to 5 mm height being implemented. In order to be able to transmit high currents via a contact, several wires are made parallel.
In special mounting techniques (integrated converter) bonding wires are also used as connecting elements between power modules and circuit boards which contain control electronics. When using this converter technique in traction drives or in drives for machine tools under shock and vibration stress high mechanical accelerations occur with resulting forces which can cause a considerable shift of the bond base points. This causes deformation of the wire and formation of considerable stresses in the wire which when the elasticity limit is exceeded lead to plastic deformation and subsequently to material fatigue. With a sufficiently large number of these mechanical loading cycles, which typically lie in the range of 1 to 10 million cycles, ultimately operating failure occurs in the form of material fracture. This fracture which is known as a “heel crack” occurs preferably at the wire site damaged previously generally by bonding, specifically at the kink point at the bond base which defines one end of the wire.
For a long time there were no effective indicators which could predict premature fatigue of the bond connection and thus probable operating failure. To safeguard the reliability of bond connections in integrated mounting technique therefore very time-consuming and thus also expensive loading tests, for example shock and vibration tests, were used. Due to time consumption these tests can only be used to a limited extent for studies on objects with a long service life.
Wire fatigue under vibration loading was studied in the past within the framework of loading tests in so-called vibration machines with adjustable vibration frequency and amplitude. These machines however can be used conventionally only for certain frequency ranges, limited to certain sample sizes, and are expensive.
In addition to mechanical loading of the wire, during operation moreover strong thermal loading of the wire occurs. The wire is on the one hand heated via electrical losses in the wire, on the other hand the chip surface acts as a heat source (power cycle load case). Associated with this the wire undergoes a change of length due to thermal expansion; this leads to arching of the wire. This deformation causes similar stress conditions in the wires, such as a displacement of the base points. Maximum bending stress occurs again at the kink point at the bond base. Addition deformation is superimposed on this loading and results from the thermal mismatch between the material, for example aluminum, of the bonding wire, and the material, for example silicon, of the chip surface, or the material, for example copper, of another contact surface, and causes mechanical stresses at the contact site of the bond base. The latter mechanism after a relatively large number of temperature cycles leads to formation of cracks at the contact site of the wire with the contact surface and ultimately to lifting of the wire away from the contact surface. The danger of wire lifting can be reduced by coating the bond base with a protective layer. In the latter case the deformation of the wire during thermal expansion contributes mainly to fatigue and ultimately leads to “heel crack”.
In order to simulate the wire deformation which results during thermal expansion, K. V. Ravi. E. M. Philofsky in “Reliability improvement of wire bonds subjected to the fatigue stresses”, 10th Proc. Reliab. Physics, Las Vegas, p. 143-148, 1972, suggested a test set-up in which the wire arc is periodically raised in the middle of the arc using a needle and in this way the arc height is increased. In this process the wire stress can lead to premature damage of the wire by the needle, pulling the wire can cause raising, and wire deformation differs considerably from the actual deformation upon thermal expansion.
H. Tomimuro, H. Jyumonji: “Novel Reliability Test Method for Ribbon Interconnections between MIC Substrates”, Proc. 36th Electronic Components Conference, Seattle, p. 324-330, 1986 discloses an electromechanical tester for thermomechanical fatigue of bonded gold bands. In this test the bonded ends of the gold bands are shifted up to 100 microns to or from one another via a piezoelectric actuator. Thus the thermomechanical loading of the gold band which forms in MICs (=Microwave Integrated Circuits) is simulated by the different coefficients of thermal expansion of the mounting materials during a temperature change.
Specifically this known tester is an arrangement for producing a mechanical load on a gold band which has
a carrier body,
a first contact surface which is fixed relative to the carrier body for attachment by bonding one end of the gold band,
a piezoelectric actuator which is mounted on a carrier body and which expands and contracts relative to the carrier body and in the direction to the first contact surface with a frequency of less than 1/60 Hz, and
a second contact surface attached to the actuator for attachment by bonding the other end of the gold band which moves back and forth upon expansion and contraction of the actuator relative to the carrier body and in the direction to the first contact surface with a frequency of less than 1/60 Hz of this expansion and contraction.
The gold band used in this known tester has a thickness of 20 microns and a width of 350 microns and thus a cross sectional shape which deviates sharply from the circular cross section of a bonding wire.
The ageing of a bonding wire by alternating temperature loading was studied in the past by so-called power cycles in which the components are periodically turned on and off. Based on the high thermal time constants, periods of 1-5 seconds are conventional so that service life tests can last a few weeks (see for example F. Auerbach, A. Lenninger: “Power-Cycling-Stability of IGBT modules”, IEEE Industry Applications Society, New Orleans, p. 1248-1252, 1997).
The degree of fatigue of the bond connection is checked by various tests: the shear tensile strength in a pull test (destructive, non-destructive), the shear strength in a shearing test and the kinking behavior in an air jet test. Detailed information about crack formation and possible acceleration factors for ageing can be obtained via REM (scanning emission microscopy), ultrasonic tests and chemical analyses (for example, Auger spectroscopy).
It has already been suggested that wire fatigue during a service life test be described via reliability indicators. The electrical resistance, the nonlinearity of the electrical resistance and the resistance noise are examples of these indicators.
The object of the invention is to make available an arrangement for mechanical loading of a bonding wire which makes it possible to simulate the mechanical loading case of displacement of the base points of the bond connection under vibration or shock stress.
This object is achieved by the features of claim 1.
According to this approach an arrangement for mechanical loading of a bonding wire is made available which has:
a carrier body,
a first contact surface which is fixed relative to the carrier body for attachment by bonding one end of the bonding wire,
a piezoelectric actuator which is mounted on the carrier body and which can expand and contract relative to the carrier body and in the direction to the first contact surface with a frequency of at least 0.1 Hz,
a second contact surface attached to the actuator for attachment by bonding the other end of the bonding wire which moves back and
Ellington Alandra
Lefkowitz Edward
Siemens Aktiengesellschaft
LandOfFree
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