Measuring and testing – Vibration – Resonance – frequency – or amplitude study
Reexamination Certificate
2002-05-31
2004-06-08
Williams, Hezron (Department: 2856)
Measuring and testing
Vibration
Resonance, frequency, or amplitude study
C073S574000, C073S589000
Reexamination Certificate
active
06745629
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of European Application Serial Number 02 005 435.9 filed on Mar. 8, 2002.
FIELD OF THE INVENTION
The invention relates generally to wire bonding, and specifically to a method and apparatus for the production and quality control testing of an ultrasonically bonded wire connection.
DESCRIPTION OF THE RELATED ART
Bonding is an extremely widely used method of connecting wires in electronic devices of all kinds, in order to form contacts with electronic components and in particular integrated circuits (chips). The quality of these connections determines to a considerable extent the performance and reliability of the relevant electronic devices. Therefore the manufacturers of these devices pay great attention to monitoring the quality of the connections, and the manufacturers of wire-bonding machines are faced with a demand for ever more reliable testing and process-control systems.
One of the furthest developed and most widespread bonding methods is ultrasonic (“wedge”) bonding, which basically represents a micro-version of friction welding. Here—as described, for example, in the applicant's patent U.S. Pat. No. 4,619,397—an aluminium wire in contact with a substrate surface, to which it is to be connected in such a way that the two materials remain in contact, is put into rapid oscillation by an ultrasound transducer and simultaneously pressed against the surface. Under the influence of the compression (bonding) force and the oscillation (bonding) power, an oxide coating on the surface is broken up and, as severe deformation and local heating occur, the wire and surface materials intermingle to produce bonding at the interface between wire and surface.
A more detailed description of this method is unnecessary here, as it has long been well known to those skilled in the art.
For testing bonded connections produced in this way, many methods have been established, of which reference is made here only to that disclosed in the applicant's patent U.S. Pat. No. 4,984,730. This document describes a test method based on monitoring a deformation of the wire during the bonding process and comparison with a standard or reference curve. When the measured deformation curve is too far from the reference curve, this is regarded as indicating inadequate quality of the bonded connection, and when such unacceptable deviations are detected, the procedure should be interrupted and the relevant process parameters readjusted.
A method of testing connections produced by ultrasonic bonding that is similar, in a certain sense, is described in the patent DE 44 47 073 C1. Here the firmness of the connection is the decisive parameter for the bond quality. It is proposed that the relevant parameter to be monitored is the velocity or time course of deformation of the wire to be bonded and the time course of the amplitude of the bonding tool (wedge amplitude) during the bonding process; the results of this monitoring are to be compared with reference data. This method enables the firmness of each individual connection to be tested with no substantial extra expenditure of time.
In the applicant's patent U.S. Pat. No. 5,314,105 a system is proposed for controlling an ultrasonic wire-bonding process in real time, or quasi-real time, by monitoring the time dependence of deformation of the bonded wire. Specifically, it is proposed that the power input to the ultrasound transducer should be kept at a high level until a marked increase in deformation appears in the time-dependence curve, at which point the power is reduced to a predetermined lower level. Furthermore—in a preferred procedure—it is proposed to turn the ultrasound transducer off completely when the wire deformation has substantially reached a predetermined final value.
In the applicant's patent GB 2 271 305 B a procedure for controlling a soldering or bonding process of the reflow type is proposed, in which the power applied to drive prespecified sections of the process is adjusted to different levels. In this document the application of this principle for a wire bonder of the kind cited above is also mentioned.
The previously proposed methods and apparatus have proved not to provide optimal results in all of the constellations of materials and processes encountered in practice. It has been impossible to control or regulate the bonding process in the initial phase (the first five to ten ms), because during this period the wire to be bonded was not deformed sufficiently to permit evaluation.
SUMMARY OF THE INVENTION
Hence it is the objective of the invention to provide an improved method of this generic kind as well as an apparatus suitable for implementing the method, which enable a further improvement in ensuring the quality of bonded wire connections.
This objective is achieved with respect to the method by a method having the characteristics given in claims
1
and
2
, and with respect to the apparatus by an apparatus having the characteristics given in claim
10
.
The invention includes the fundamental idea of monitoring, by means of a suitable parameter, a reaction of the bonding process upon the tool—the ultrasound transducer—and thereby detecting a substantial change of state of the work piece, namely the wire-substrate unit during formation of the bonded connection. It further includes the idea of utilizing as this parameter the impedance measured at the transducer. Finally, the invention includes the idea of comparing the recorded time dependence of the representative parameter (impedance) with a suitable reference, which is obtained during the manufacture of qualitatively perfect bonded connections.
The invention enables an improved control or regulation of the bonding process, in particular in its initial phase, because a quantity which has an informative time-dependence during this phase is used as an input variable for the control or regulation process.
In a first, relatively independent implementation of the idea behind the invention, the proposed method is a manufacturing procedure insofar as it comprises, in addition to the above-mentioned aspects, the concept of controlling the power input to the ultrasound transducer (the bond power) or the force that the bonding tool exerts on the wire (bond weight), or both of these variables.
In a second implementation of the idea behind the invention, the proposed method represents a test procedure for quality control of the bonded connection that is produced, insofar as it allows the decision as to quality of the bonded connection to be derived from the recorded time dependence of the impedance in comparison to the reference curve.
The inventor has established, on the basis of extensive experimental investigations, that a particular characteristic shape of the impedance-time curve, distinguished by a relatively steep rise to a clear maximum and a subsequent decline, initially steeper and then becoming less steep, can be taken as an unambiguous indicator of a qualitatively high-value connection. It will be evident that for different bonding-process parameters, in particular for wires differing in composition and dimensions as well as substrate surfaces of various kinds, the specific time dependence of the impedance and in particular the position of the maximum and the rise and fall gradients are different. However, for each constellation of process parameters suitable reference curves can be constructed with a limited number of experiments, which can be used for comparison with a time-dependence curve obtained for the ongoing process.
Evaluation of the curve shape for controlling or regulating the bonding process is preferably carried out by comparison of the overall time dependence recorded while the bonded connection is being produced, or else only a characteristic section of the overall curve, with a reference curve or a set of reference curves. A significant assessment of the quality of the bonded connection is provided, as far as can be seen at present, by evaluation including three predetermined im
F&K Delvotec Bondtechnik GmbH
Knobbe Martens Olson & Bear LLP
Saint-Surin Jacques M.
Williams Hezron
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