Mounting method and mounting apparatus of electronic part

Abstract: The present invention relates to a mounting apparatus (30) for mounting an electronic part (3) utilizing ultrasonic wave and thermocompression bonding to form a flip-chip bonding system, comprising an ultrasonic vibration generating device (8), a horn (7) attached to the ultrasonic vibration generating device (8), a collet tool (6) for holding an electronic part (3) attached to the horn (7), a control unit (9) being connected to the ultrasonic vibration generating device (8), and a substrate conductor heating unit (4) connected to said control unit (9). To inspect the state of the formed joint, a vibrating measuring device (21) for monitoring a vibrating state of the electronic part (3) or the bump (5) is connected with the control unit (9). Thus, it is possible to measure a vibrating state of the electronic part (3) at the time of mounting is and to compare it with a reference wave form, to make a judgment as to whether or not an ultrasonic vibration is applied normally, and to judge the connection as defective or non-defective in accordance with the judgment on the ultrasonic vibration application.

Description: FIELD OF THE INVENTION The present invention relates to a method of and apparatus for mounting an electronic part to a substrate conductor, in particular, to a method and apparatus for mounting an electronic part to a substrate which enables inspection of the bonding junction (made by flip-chip bonding) between the electronic part and the substrate conductor. BACKGROUND OF THE INVENTION Conventionally, as a high-density mounting method of a semiconductor chip to a substrate conductor, a flip-chip bonding system has been utilized. Such a flip-chip bonding system works by forming a bump (either a soldered bump or metal bump) on an electrode of a semiconductor chip or an electrode of a substrate conductor, and joining the electrode of the semiconductor chip to the countered electrode of the substrate conductor via the bump, with the electrode of the semiconductor chip facing down. Since in the flip-chip bonding system the electrode can be taken out from an arbitrary position of the surface of the chip, the connection with the substrate conductor by the shortest distance is possible. In addition, the size of the chip does not increase even if the number of electrodes increases, and super-thin mounting can be performed. A soldering reflow method, an anisotropic conductive resin connecting method, a thermocompression bonding method, an ultrasonic wave, and the like, can be used to join the semiconductor chip or the bump provided on the electrode of the substrate conductor to the mated electrode in the flip-chip bonding method. According to the ultrasonic wave and thermocompression bonding method mentioned above, and as shown in the conventional apparatus of FIG. 12, in a mounting apparatus 10 of a semiconductor chip 3 as an electronic part comprising an ultrasonic vibration generating device 8, a horn 7, a collet tool 6, a substrate conductor heating unit 4 and a control unit 9, and executes the ultrasonic wave and thermocompression bonding connection utilizing the flip-chip bonding system. In this bonding process, a substrate conductor 1 is heated by the substrate conductor heating unit 4, and an ultrasonic vibration is applied to the collet tool 6 on the side of the semiconductor chip 3 by the piezoelectric element 8 or the like via the horn 7, so that bumps 5 formed on the semiconductor chip 3 diffusively join to electrodes 2 of the substrate conductor 1. In comparison with other methods, this ultrasonic wave and thermocompression bonding method has advantages such that a joint material is not required, the connection can be made in a short time, and the bonding can be performed at a comparatively low temperature. However, when using the above-mentioned conventional ultrasonic wave and thermocompression bonding connection methods utilizing the flip-chip bonding system, propagation of an ultrasonic vibration is unstable, causing a danger of formation of defective connections. Such problems may be also caused by slippage between a rear surface (upper surface in the drawing) of the semiconductor chip 3 and the collet tool 6, diffusion of a vibration from the horn 7, and deterioration of the piezoelectric element 8 due to aging, when using the conventional apparatus shown in FIG. 12. In addition, in the flip-chip bonding system described above, since the joined portion is hidden from view of the semiconductor chip 3, the joined portion cannot be visually inspected after the connection is made. Therefore, the occurrence of the defective joints can become a particularly serious problem. JP 9-51007 A discloses a mounting apparatus for a semiconductor chip. This apparatus comprises a collet for holding the semiconductor chip, that is fixed to a moving part of a linear motor via a bonding head and an arm. The moving part of the linear motor is provided with a linear scale. The scale can be red with a detector for detecting the height of the collet. US 5,341,979 discloses a mounting apparatus comprising an ultrasonic vibration generating device and control unit, a horn including a heating coil and a tool/end-effector (collet tool) for picking up an electronic part. DD 217 088 A1 discloses in a wire bonding apparatus having an ultrasonic vibrating generating device, an ultrasonic horn and as a wire holder an ultrasonic bonding element. A vibration sensor is connected with the horn for controlling the excitation voltage supplied to the ultrasonic vibrating generating device via a preamplifier 16 and a power amplifier. Further, the output of the vibration sensor is input into an envelope detector the output of which is monitored to determine the first minimum of the amplitude of the vibration as a switch-off indication for the bonding process. JP 8-17876 A discloses a wire bonder having a tip for performing the bonding process, that is provided at the front end of an arm connected to an ultrasonic vibrator. To judge that the tip of the arm comes into contact with a bonding pad of a chip the electrical output of the ultrasonic vibrator is monitored by a monitoring means during a time when it generates no vibrational energy. The output of the monitoring means that corresponds to the output of ultrasonic vibrator is supplied to discriminating means for judging the moment when the tip is contacting the pad. SUMMARY OF THE INVENTION The object of the present invention is to provide a mounting apparatus and a mounting method for mounting an electronic part having a bump to a substrate conductor by means of an ultrasonic wave and thermocompression bonding for a flip-chip bonding system that make it possible to inspect the state of the formed joint. In a first embodiment of the present invention, a mounting device of an electronic part having a bump for forming an ultrasonic wave and thermocompression bonding connection utilizing a flip-chip bonding system is provided consisting of an ultrasonic vibration generating device, a horn connected to the ultrasonic vibration generating device, a collet tool in connection with the horn, a control unit, and a substrate conductor heating unit in connection with the control unit, wherein a vibration measuring device for monitoring a vibrating state of the electronic part or the bump is connected with the control unit. In a second embodiment of the present invention, a mounting method of an electronic part according to the first embodiment above, utilizing ultrasonic wave and thermocompression bonding methods in a flip-chip bonding system is provided for mounting the electronic part to a substrate conductor, wherein a vibrating state of the electronic part at the time of mounting is measured and compared with a reference waveform, a judgment made as to whether or not an ultrasonic vibration is applied normally and, using this information, the connection judged defective or non-defective. Specifically, while the defective or non-defective joint is compared with a reference, the mounting is performed by using means for monitoring the vibrational state of the electronic part or bump at the time of the mounting, i.e., the amplitude, vibrating speed, vibrating velocity or the like as described in the first and second embodiment described above. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a system conceptual diagram of the mounting apparatus of an electronic part of the present invention according to claims 1 and 2 of the present invention. Fig. 2 is a graph showing the change in the height of a tool head of the present invention versus change in time (hourly). Fig. 3(a) illustrates the position of the components of the present invention at the beginning of the mounting process of the semiconductor chip as described herein. Fig. 3(b) illustrates the second step of the mounting process described herein, wherein a predetermined weight is applied to the semiconductor chip. Fig. 3(c) illustrates the third step of the mounting process described herein, wherein both a predetermined weight and ultrasonic vibrations are applied to the semiconductor chip. Fig. 3(d) illustrates the last stage of the mounting process described herein, wherein the semiconductor chip has been connected to a substrate conductor, and the collet tool is raised and separated from the semiconductor chip. Fig. 4 is a graph showing the relationship of joint strength between a bump formed of Au and electrodes of a substrate conductor and height of the bump. Fig. 5 is a system conceptual diagram showing the mounting apparatus of an electronic part having means for detecting the amount of compression (sinking amount) of the bumps by means of grids. Fig 6 is a system conceptual diagram showing the mounting apparatus of an electronic part having means for detecting a sinking amount by means of a change in capacitance. Fig. 7 is a system conceptual diagram showing the mounting apparatus according to claims 3 and 4 of the present invention. Fig. 8 is a graph comparing vibration waveform W and preset reference waveform W0 versus time. Fig. 9 is a graph showing that the vibration waveform W is decomposed according to frequency, and its respective spectrums are compared with corresponding reference wave spectrums of the reference waveform W0. Fig. 10 is a system conceptual diagram showing the mounting apparatus of an electronic part according to claims 5 and 6 of the present invention. Fig. 11 is a graph comparing a monitored waveform of an electric current i applied to a piezoelectric element and a reference electric current waveform i0 versus time. Fig. 12 is a system conceptual diagram showing a conventional mounting apparatus according to an ultrasonic wave and thermocompression banding connection utilizing a flip-chip bonding system. DETAILED DESCRIPTION OF THE INVENTION In Fig. 1, a mounting apparatus 20 of an electronic part according to the present invention comprises an ultrasonic vibration generating device 8, a horn 7, a collet tool 6, a substrate conductor heating unit (not shown), a control unit 9, and a laser displacement meter 11 attached to the control unit which acts as a distance measuring device for monitoring the distance of the semiconductor chip 3 attached to the collet tool 6 with respect to a substrate conductor (not shown), the substrate conductor also being connected to the control unit 9. The semiconductor chip 3 is mounted to the substrate conductor by the mounting apparatus 20 according to the following steps: First, as shown in Fig. 3(a), the semiconductor chip 3 formed on an electrode is attached to the head 12 of the collet tool, and bumps or projections 5 formed in the semiconductor chip 3 having a height H = 50 µm (for example Au bumps) are lowered so as to face electrodes 2 of the substrate conductor 1. Second, as shown in Fig. 3(b), just when the bumps 5 of the semiconductor chip 3 come into contact with the electrodes 2 of the substrate conductor 1, a predetermined weight R of about 0.1 to 200 grams/l bamp, which is suitably set according to the material, shape, size, number and the like of the bumps 5 (normally several hundred gw to 1kgw), is applied for generally about 10-10,000 msec. The application of weight R causes slight crushing and sinking of the bumps 5, into the electrodes 2. This weight is termed the first stage sinking amount H1. Third, as shown in Fig. 3(c), the predetermined weight R and an ultrasonic vibration US (lateral vibration) are applied for a predetermined time T2, generally about 50-10,000 msec, so that the electrode of the semiconductor chip 3 is joined to the electrodes 2 of the substrate conductor 1 by ultrasonic wave and thermocompression bonding of the bumps 5. Fourth, as shown in Fig. 3(d), the head 12 of the collet tool is separated from the semiconductor chip 3 so as to rise, and the sinking amount of the bumps 5, termed the second stage sinking amount H2, caused by the application of the predetermined weight R and ultrasonic vibration US, is monitored by the laser displacement meter 11 (distance measuring device) shown in Fig. 1, and a judgment is made as to whether or not the sinking amount H2 of the bumps 5 is in a desired range. Specifically, a judgment is made as to whether the joint obtained by the ultrasonic wave and thermocompression bonding connection utilizing the flip-chip bonding system is defective or non-defective. It was experimentally determined that the amount of compression, i.e., the sinking amount H2, of the bumps 5 at the second height stage as shown in Fig. 3(d) (in other words, the change in distance between the semiconductor chip 3 and the substrate conductor due to the crushing of the bumps 5 when the predetermined weight R and the ultrasonic vibration US are applied) and the joint strength between the electrode of the semiconductor chip 3 and the electrodes 2 of the substrate conductor 1 by means of the bumps 5 satisfy a graphical relationship, as shown in Fig. 4. In thermosonic banding, an almost uniform bonding is effected at a boundary between a semiconductor electrode and a bump. A bonding strength for each unit area is determined by the materials the semiconductor electrode and the bump are made of. It can be said that if strengths are equal, then bonding areas are equal. Furthermore, if a bump is compressed by a load and ultrasonic vibration, the bonding area is increased. Accordingly, if shapes of bumps are equal to each other, the compression of the bumps, that is the sinking amounts thereof, and bonding areas are uniquely determined. And if bonding areas are equal to each other, then bonding strengths are also equal to each other. Hence, inspection of the joint formed between the semiconductor chip and the substrate conductor can be performed by measuring the amount of compression of the bump (the sinking amount). Therefore, for example, a standard of judgment is previously set so that when the joint connection between the bumps (Au bumps) 5 and the electrodes 2 is not less than 50 gw, the joint is non-defective, and when smaller than 50 gw, the joint is defective. When the sinking amount H2 at the second stage, as shown in Fig. 3(c), is not less than 25 µm, the joint is defective. A judgment is made by the control unit 9 as to whether the joint is defective or non-defective based on monitored data from the laser displacement meter 11, also referred to herein as the distance measuring device. According to the above method, since a product whose joint strength is not more than 25 gw is defective, it is treated as a defective product or, if the desired sinking amount H2 can be obtained by again applying a ultrasonic wave, a non-defective joint can be produced. Therefore, a defective joint formed on the first attempt does not necessarily influence production steps thereafter, and yield of non-defective products can be improved. As mentioned above, the mounting method is a method for monitoring only the sinking amount H2 at the second stage of the method of the present invention, as shown in Fig. 3(c), and may be enough to judge whether the joint is defective or non-defective. However, in order to judge the defectiveness or non-defectiveness of the joint more accurately, it is desired to also monitor the sinking amount H1 at the first stage of the process, as shown in Fig. 3(b), and use both the sinking amount H1 at the first stage and the sinking amount H2 at the second stage as data for judging the defectiveness or non-defectiveness of the joint. Therefore, in the above-described mounting method, the sinking amount H1 of the bumps at the first stage, where the predetermined weight R of about 0.1 to 200 grams/l bamp is applied for the predetermined time T1 (generally about 10-10,000 msec, but usually predetermined on the order of 100 msec) just when the bumps 5 come into contact with the mating electrodes 2, and the sinking amount H2 of the bumps at the second stage, where in addition to the application of predetermined weight R, ultrasonic vibration US is applied for the predetermined time T2 (generally about 50 to 10,000, but usually predetermined in the range of about 1000 to 2000 msec) are individually monitored by the laser displacement meter 11 acting as a distance measuring device. These measurements allow a judgment to be made by the control unit 9 as to whether or not the sinking amounts H1 and H2 of the bumps at the above stage, are respectively, in predetermined desired ranges, so that a determination may be made as to whether or not the joint between the semiconductor chip 3 and the substrate conductor 1 by the ultrasonic wave and thermocompression bonding connection utilizing the flip-chip bonding system is defective or non-defective. According to the above described mounting method, in the case where the sinking amount H1 of the bumps 5 at the stage of the process shown in Fig. 3(b), at the time when the predetermined weight R is applied for the predetermined time T1, deviates from a range of set values, the bumps are deemed abnormal, and slipping of the bumps may occur. For this reason, a judgment can be made at a stage before the joint is made as to whether the bumps are defective or non-defective. To make such a judgment, when the sinking amount H1 at the first stage and the sinking amount H2 at the second stage are separately monitored and evaluated, the defectiveness or non-defectiveness of the joint can be judged with a high degree of accuracy. Here, as the distance measuring device for successively monitoring the height of the head 12, i.e., monitoring the sinking amounts H1 and H2 in the above embodiment, devices other than the laser displacement meter 11 may be used. For example, a counter for counting a pulse number of a pulse motor may be used. In addition, as the distance measuring means, as shown in Fig. 5, grids G as degrees are formed on a side surface of the collet tool 6, which retains the electronic part and is joined to the substrate conductor in every 1 µm, and the number of grids passing a grid sensor 31 during application of the ultrasonic vibration US are counted by a light, laser or the like based on the time when the semiconductor chip 3 on the head of the forward end of the collet tool comes in contact with the bumps, so that the sinking amount H2 of the chip at the second stage can be obtained. The grids G may be formed by a magnetic material and, if so, the counting may be executed by a magnetic sensor. The grids may also be provided on the forward end of the horn 7. Further, as another means for measuring the distance of the head with respect to the substrate conductor, measuring means utilizing measurement of capacitance may be used. For instance, as shown in Fig. 6, a flat type electrode B may be provided 1 mm below the bottom dead center of the horn 7, and capacitance C between the flat type electrode B and the horn 7, which is made of metal, is measured. Since the capacitance C is determined by the distance S between the horn 7 and the flat electrode B, the sinking amount of the semiconductor chip 3 is detected by measuring the capacitance C by a capacitance detecting meter. This measurement of capacitance allows judgement of the defectiveness or non-defectiveness of the joint. In addition, instead of the flat type electrode B, a coil horn can be directly mounted as the horn 7, and the distance between the coil horn and the substrate conductor can be obtained by an inductance component of the coil horn. Mounting apparatus 30, as shown in Fig. 7, includes, in addition to the components of the conventional mounting apparatus 10 shown in Fig. 12, a vibration measuring device 21 for monitoring a vibrating state of the semiconductor chip 3 or the bumps 5, which is connected with the control unit 9 of the electronic part. In the mounting apparatus 30, a vibration waveform W, for example, of the semiconductor chip 3 at the time of the mounting is measured and it is compared with a preset reference waveform W0, as shown in Fig. 8, and the joint is inspected to determine whether it is defective or non-defective by making a judgment as to whether or not the ultrasonic vibration US is applied normally. Since the vibration waveform generated by the ultrasonic vibration US of the semiconductor chip 3 and the bumps 5 changes constantly in the joint steps, a judgment as to whether or not the ultrasonic vibration US is normally applied to the joint portion can be made by a monitoring any deviation of the vibration waveform W from the reference vibration waveform W0. In other words, it is an object of one embodiment of the invention to measure the vibration waveform W when monitoring the vibrating state, and a laser Doppler vibration meter 22 is provided as the vibration measuring device 21. A laser beam of the laser Doppler vibration meter 22 is directed at the semiconductor chip 3, and the vibrating state of the semiconductor chip 3 at the time of the mounting (amplitude and vibrating speed) is monitored as a waveform. The obtained vibration waveform W is compared with the reference waveform W0 by the control unit 9, so that a judgment can be made as to whether or not the ultrasonic vibration US is normally applied to the bumps 5, and when the vibration waveform W deviates from the allowable range of the reference waveform W0, the joint is deemed defective. At the time of this judgment, as shown in Fig. 9, the vibration waveform W is decomposed according to frequency by FFT (fast Fourier transform), and spectrums W1 and W2 are respectively compared with corresponding reference wave spectrums W01 and W02 of the reference waveform W0 so that a comparison and judgment can be quantitatively made. Mounting apparatus 40, shown in Fig. 10, includes, in addition to the components of the conventional mounting apparatus 10 shown in Fig. 12, a measuring device for monitoring an electric current i applied to the ultrasonic vibration generating device (piezoelectric element) 8, which is connected to the control unit 9, so as to be positioned therein. A waveform of the measured electric current i is compared with a preset reference electric current waveform i0, as shown in Fig. 11, and the bump is inspected and judged to be defective or non-defective by making a judgment as to whether or not the ultrasonic vibration US is applied normally to the bumps 5. As can be seen in Fig. 11, since the electric current i to be applied to the piezoelectric element by the ultrasonic vibration generating device 8 changes constantly in the joint forming steps, a judgment can be made as to whether or not the ultrasonic vibration US is applied normally by monitoring any deviation of the corresponding waveform i from a reference electric current waveform io. When an electric current probe using a hole element is used as the measuring device for monitoring the electric current i flowing in the ultrasonic vibration generating device (piezoelectric element) 8, a noise present at the time of measurement can be reduced, and the measuring accuracy may be improved. When the waveform of the electric current i is compared with the reference electric current waveform i0 as shown in Fig. 11 (analysis of spectrums by means of FFT), it can be determined whether or not the ultrasonic vibration US is normally applied to the bumps 5, and when the waveform of the electric current i deviates from the allowable range of the reference electric current waveform i0, the joint is deemed defective. In addition, a voltage V may be applied to a piezoelectric crystal element, as decribed above using an electric current i, and the change in the voltage measured to determine the waveform. Then, this waveform can be compared with a reference waveform, as described above with an electric current i. The aforementioned embodiments referred to the case where the bumps 5 are provided on the electrode side of the semiconductor chip 3, but the same conditions also apply to the case where the bumps 5 are provided on the electrode 2 side of the substrate conductor 1. In addition, the aforementioned embodiments referred mainly to the mounting of semiconductor chips onto substrates but, needless to say, it can be applied to all electronic parts which can be flip-chip bonded by the ultrasonic wave and thermocompression bonding method. As is clear from the above detailed embodiments, in the mounting method and mounting apparatus of the electronic part according to the present invention, since when the mounting height, vibrational state and electric current of the ultrasonic vibration generating device (piezoelectric element) are monitored and compared with the reference values and reference waveforms, a judgment can be made as to whether or not the joint state and ultrasonic vibrating state of the electronic part by means of the bumps are normal simultaneously with the mounting. If any abnormality is measured or detected, the following countermeasures may be taken: (a) the mounting apparatus is immediately stopped and the abnormality is identified; (b) the product is marked as defective; (c) an ultrasonic wave is again applied and the product is reproduced; and (d) an ultrasonic wave is continually applied until the desired state is obtained. As a result, the above mounting method and mounting apparatus have the desirable quality in that the joint can be judged as defective or non-defective, and the product can be reproduced. REFERENCE NUMERALS 1 substrate conductor 2 electrode 3 semiconductor chip 4 substrate conductor hearing unit 5 bump (gold bump) 6 collet tool 7 horn 8 ultrasonic vibration generating device 9 control unit 10, 20, 30, 40 mounting apparatus 11 laser displacement meter (distance measuring device) 12 head 21 vibration measuring device 22 laser Doppler vibration meter R predetermined weight US ultrasonic vibration H1 sinking amount of the bumps at the first stage of the mounting process H2 sinking amount of the bumps at the second stage of the mounting process G grid B flat type electrode C capacitance W vibration waveform W0 reference waveform W1, W2, vibration waveform spectrum W01, W02, reference wave spectrum i electric current i0 reference electric current waveform v voltage

Claims: A mounting apparatus (30) for mounting an electronic part (3) utilizing ultrasonic wave and thermocompression bonding to form a flip-chip bonding system, comprising an ultrasonic vibration generating device (8), a horn (7) attached to the ultrasonic vibration generating device (8), a collet tool (6) for holding an electronic part (3) attached to the horn (7), a control unit (9) being connected to the ultrasonic vibration generating device (8), and a substrate conductor heating unit (4) connected to said control unit (9), characterized in that a vibrating measuring device (21) for monitoring a vibrating state of the electronic part (3) or the bump (5) is connected with the control unit (9). A mounting method for mounting an electronic part (3) to a substrate conductor (1) comprising the use of ultrasonic wave and thermocompression bonding utilizing a flip-chip bonding system, according to claim 1, wherein a vibrating state of the electronic part (3) at the time of mounting is measured and compared with a reference wave form, a judgment is made as to whether or not an ultrasonic vibration is applied normally, and the connection is judged defective or non-defective in accordance with the judgment on the ultrasonic vibration application.

Inventors: FUJI TOMONORI, SUZUKI KAZUTAKA, FUJI, TOMONORI, SUZUKI, KAZUTAKA
Applicants: TAIYO YUDEN KK, TAIYO YUDEN CO., LTD.
Application year: 1999
Main classification: G01R31
IPC codes: B23K20, H01L21