Inner lead bonding inspecting method and inspection apparatus therefor

Abstract: An inner lead bonding inspecting method comprises the steps of: irradiating an illumination light onto the planar surface of a bonding portion between an electrode bump provided on a semiconductor pellet and an inner lead, measuring a quantity of a reflected light from the surface, and judging whether the bonding state of the inner lead bonding is good or bad on the basis of the measured result. According to anotehr aspect, an inner lead bonding inspecting apparatus comprises irradiation device (12) for irradiating a light onto a planar surface of a bonding portion between an electrode bump provided on a semiconductor pellet and an inner lead; light quantity measurement device (13) for measuring a quantity of a reflected light from the inner lead surface, and judgement device (15) for comparing the measured quantity of reflected light with a reference light quantity to judge bonding state.

Description: BACKGROUND OF THE INVENTION This invention relates to a method of inspecting semi-manufactured articles after the ILB (Inner Lead Bonding) process which is the process for manufacturing TAB (Tape Automated Bonding) products, and an apparatus for inspecting the same, and more particularly to a method and an apparatus used in automated inspection of bonding state of inner lead bonding by observation of appearance. In accordance with the TAB system, inner leads of a carrier tape are aligned to a plurality of electrode bumps or projected connection electrodes formed on an IC pellet. Then, inner lead bonding process for simultaneous connection is carried out. In the conventional art, the judgment whether or not the bonding or connecting state between bumps and inner leads is good was conducted by a visual inspection by human being using a stereoscopic-microscope. In accordance with this method, a visual inspection operator drew an attention to the inner lead surface in the vicinity of the bonding portion of the inner lead to carry out a subjective judgment relying on sense peculiar to the operator such that "if the surface is shinning finely, the bonding state is judged to be acceptable, or if not so, the bonding state is judged to be unacceptable". Since the objective judgment as to whether or not bonding state is good cannot be conducted in accordance with the above-mentioned method, there may occur disagreement of the judgment due to difference between operators, or disagreement of the judgment due to lapse of time even in the case of the same operator. As a result, the quality becomes unstable. Further, since the human being makes a test by the eye, there is high possibility that oversight or misperception, etc. may occur. Accordingly, there are the problems that the criteria for judgment may change or artificial error may occur. On the other hand, although automation of the manufacturing process is always required, the conventional subjective testing method cannot cope with automation. SUMMARY OF THE INVENTION An object of this invention is to provide an inner lead bonding inspecting method and an inspection apparatus therefore wherein when the bonding state between electrode bumps on a pellet and inner leads is inspected, an approach is employed to express the degree indicating whether the bonding state is good or bad as a quantitative numeric value, thereby making it possible to conduct an objective quality judgment, and to provide numeric criteria for judgment in automated inspection. According to this invention, there is provided an inner lead bondig inspection method in accordance with claim 1, comprising the steps of: irradiating an illumination light onto the planar surface of a bonding portion between an electrode bump provided on a semiconductor pellet and an inner lead, measuring a quantity of a reflected light from the surface, and judging whether the bonding state of the inner lead bonding is good or bad on the basis of the measured result. According to this invention, there is provided an inner lead bonding inspection apparatus in accordance with claim 5, comprising: irradiation means for irradiating a light onto a planar surface of a bonding portion between an electrode bump provided on a semiconductor pellet and an inner lead; light quantity measurement means for measuring a quantity of a reflected light from the inner lead surface, and judgement means for comparing the measured quantity of reflected light with a reference light quantity to judge bonding state. In accordance with the inner lead bonding inspecting method according to this invention, an approach is employed for judging the degree indicating whether or not the bonding state between electrode bumps on a pellet and inner leads to measure a quantity of specularly reflected light from the inner lead surface at the bonding portion to express it as a quantitative numeric value. Accordingly, the objective judgment as to whether or not the bonding state is good can be made, thus contributing to improvement in the quality of products. In addition, numeric criteria for judging can be provided in automated test. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: FIG. 1 is a schematic view showing the arrangement used in an embodiment of this invention, FIGS. 2 and 3 are cross sectional views for explaining good and bad bonding states of the inner lead bonding, respectively, FIGS. 4 and 5 are views for explaining how the inner lead surface of good and bad bonding states in the embodiment of this invention is reflected or imaged, respectively, FIGS. 6 and 7 are circuit diagrams showing the configuration of a circuit for making a judgment of the bonding state, FIGS. 8 and 9 are cross sectional views for explaining reflection of a light on the inner lead surface of good and bad bonding states in the embodiment of this invention, respectively, and FIG. 10 is a flowchart showing the procedure of a test to which this invention is applied. DESCRIPTION OF THE PREFERRED EMBODIMENT Initially, the principle of measurement used in this invention will be described. In the good bonding state of the inner bonding (the state having negligible small junction resistance and a sufficient connecting strength), a film metal (e.g., Sn or Au of plating film) of an inner lead at the bonding portion of the inner lead and in the vicinity therewith is completely molten. The surface of the inner lead at the bonding portion (which may sometimes be referred simply as a bonding portion surface) is smooth. On the contrary, in the bad bonding state, the molten condition is imperfect, and many fine uneven portions are formed on the surface of the bonding portion, resulting in losing of smoothness. Accordingly, it is possible to detect whether or not the bonding state of the inner lead bonding is good on the basis of smoothness of the bonding portion surface, i.e., brilliance as appearance. FIGS. 2 and 3 are cross sectional views of the bonding portion for explaining the bonding states. In the good bonding state shown in FIG. 2, an eutectic region 5 by the eutectic reaction is produced between a surface metal of the electrode bump 2 provided on the semiconductor pellet 1 and a film metal 4 of an inner lead 3. For producing the eutectic reaction, it is required that sufficient heat is applied to the bonding portion, In the case of the inner lead bonding, heat is applied from a bonding tool 6 having a planar bottom surface heated to a predetermined temperature to the bonding portions via the upper side surface of the inner leads. Accordingly, in order to obtain a satisfactory bonding state, i.e., eutectic reaction, it is required that sufficient heat is applied also to the inner lead surface layer. To speak conversely, it can be said that if sufficient heat is applied to the inner lead surface layer, a satisfactory eutectic reaction is provided. On the contrary, in the bad bonding state shown in FIG. 3, the contact between the bonding tool and the upper side surface of the inner lead is bad, so sufficient heat is not applied to the inner lead surface layer. Accordingly, since the film metal 4 is not completely molten, the surface is not smooth and a satisfactory eutectic reaction cannot be obtained. From these facts, observation of the molten state of the film metal 4 makes it possible to make a judgment as to whether the eutectic reaction is good or bad, i.e., the bonding state is good or bad. Explanation has been given in connection with the case of bonding by the eutectic reaction (Au-Sn eutectic alloy) between the surface metal (e.g., Au) of the electrode bump and the film metal (e.g., Sn) of the inner lead. In addition, the case of bonding by thermal compression (Au-Au thermal compression) between the surface metal (e.g., Au) of the electrode bump and the inner lead film metal (e.g., Au) is substantially the same as above. Namely, in the bonding state by a good thermal compression, the inner lead surface at the bonding portion is smooth. Accordingly, it is possible to detect the degree of bonding state by brilliance of appearance on the bonding portion surface. In the conventional art, a method was employed to sensorially judge visually ."smoothness" of the bonding portion surface as "brilliance". In contrast, the method of this invention intends to measure brilliance of the surface, i.e., irradiate an illumination light to the surface to measure a quantity of specularly reflected light from the surface to numerically express "smoothness", thus to detect the degree of the bonding state. FIG. 1 is a block diagram showing a testing apparatus used in an embodiment of an inner lead bonding testing method according to this invention. A semiconductor pellet 10 which has undergone inner lead bonding of an article to be tested is mounted on an X·Y stage 11. This pellet 10 is illuminated from the upper side by an illumination device 12, and is imaged by an image pick-up device or imager 13 (e.g., ITV camera). In this instance, it is required for measuring brilliance of a bonding portion surface 32 of the inner lead on the pellet 10 to pick up a specularly reflected image of the surface by the image pick-up device 12. For this reason, in this embodiment, since a flat bonding head is used to implement bonding to a large number of inner leads at the same time, a half mirror 14 is provided so that an illumination light 30 is incident perpendicular to the principal surface of the pellet 10, and the illumination light and an image pick-up light 31 are caused to be coaxial. It is to be noted that an illumination light may be irradiated obliquely with respect to the principal surface of the pellet 10. In this case, it is required to allow a reflection direction in which an illumination light is specularly reflected on the pellet 10 and the optical axis of the image pick-up device 13 to be in correspondence with each other. An image signal from the image pick-up device 13 is inputted to a measurement device 15. At the measurement device, a quantity of specularly reflected light from the inner lead surface at the bonding portion is measured. An image picked up by the image pick-up device 13 is displayed on a monitor 16. On the basis of this image, inner leads to be measured are caused to be in correspondence with measurement positions on the screen in accordance with a manual operation by an operator, or an automatic operation by the pattern recognition. The portion in the vicinity of the inner lead bonding portion is imaged as images shown in Figs. 4 and 5. Fig. 4 indicates the good bonding state and Fig. 5 indicates the bad bonding state. The measurement device 15 measures brightness in a predetermined region 20 encompassed by single dotted lines on the inner lead surface at the bonding portion from the above-mentioned image. To realize this, in Fig. 4 or 5, a window 21 is opened in the predetermined region 20 to measure brightness only in this portion. The judgment as to whether the bonding state is good or bad on the basis of brightness may be preferably conducted, as shown in FIG. 6, by comparing a brightness measurement signal S B with a reference level L outputted from a reference level generator 41 at a comparator 51 to take out a judgment signal S therefrom. In the case of dividing the judged result into ones at several stages in dependency upon the reflection to take out the judged results thus divided, an approach may be employed, as shown in FIG. 7, to output different levels L 1 , L 2 and L 3 from a reference level generator 42 to compare these levels with a brightness measurement signal S B at comparators 52, 53 and 54 to take out outputs S1, S2 and S3 therefrom, respectively. The measurement device 15 comprises therein an image processing unit composed of the above-mentioned reference level generators and comparators, an image input/output section, an image memory, and a computation section, etc., and further comprises control signal generator means for controlling the operations of the image pick-up device 13 and the X·Y stage 11, etc. The procedure of the test according to this invention will now be described in more detail. In this embodiment, brightness in the predetermined region 20 of an image is expressed as a mean or average value of image signals of respective pixels constituting the region 20. Accordingly, brilliance of the bonding portion surface, i.e., a quantity of specularly reflected light from the surface is represented by the above-mentioned image signal value. Thus, the degree of the bonding state can be grasped or recognized as a quantitative numeric value. In the good bonding state, since the inner lead surface is smooth as shown in FIG. 8, most of rays of an illumination light 30 are specularly reflected, and are imaged. In contrast, in the bad bonding state, since fine uneven portions exist on the inner lead surface, many components are irregularly reflected as shown in FIG. 9, resulting in a small quantity of rays of light which contribute to image picking up. In FIG, 9, reference numeral 31 denotes a reflected light (broken lines). Namely, a quantity of specularly reflected light in the predetermined region 20 on the inner lead surface and brightness in the predetermined region 20 in the pictorial image shown in FIG. 4 or 5 have correspondence relationship with each other. In this embodiment, in order to judgment as to whether the bonding state is good or bad, good article limit sample (good articles of the lowest grade) or bad article limit sample (bad articles of the highest grade) used in the conventional visual inspection by the human being were utilized. Namely, an approach is employed to measure the image signal value indicating brilliance of the limit sample immediately before the test is started to store it at a reference level generator 41 or 42 in the measurement device 15 as a reference value of brilliance. The judgment as to whether the bonding state of articles to be tested is good or bad was carried out by comparing image signal values of the articles to be tested with the reference value. It is to be noted that an approach may be employed to express brilliance of the inner lead surface at the bonding portion with a ratio between a quantity of illumination light incident to the surface and a quantity of specularly reflected light on the surface to determine in advance a reference value of this relative brilliance to carry out judgment the bonding state is good or bad. Although the area of the region on a pellet imaged by the image pick-up device 13 varies in dependency upon the size of the pellet, the number of electrode bumps, or the test grade, etc., an approach is ordinarily employed to finely divide the pellet principal surface into a plurality of regions to make test every finely divided regions. The outline of an inner lead bondig testing method in this embodiment will now be described with reference to FIG. 10. Initially, a pellet 10 to be tested is mounted on the X·Y stage 11 to adjust the X·Y stage, etc. to place, at a predetermined position, a region to be imaged of the pellet principal surface (step S11). Then, an illumination light 30 is irradiated from the upward direction perpendicularly onto the region to be imaged to pick up an image of the region to be imaged by the image pick-up device (monochromatic ITV camera) 13. An image signal from the image pick-up device 13 is transmitted to the measurement device 15, and is then stored into a density image memory of 8 bits per each pixel via the image input section of the measurement device 15. At this time, an approach is employed to open a window 21 in the predetermined region 20 (step S12) to measure a light quantity in that region (step S13). Then, a mean value of brightness of entire pixels within the window, i.e., the degree of whether the bonding state is good or bad is determined. Then, the mean value of brightness and a reference value obtained in advance from the limit sample are compared with each other to make judgment as to whether the bonding state is good or bad (step S14). Thus, a desired processing result is outputted to the image output section. When such a judgment is completed in connection with the total numer of portions to be measured (step S15), the test is completed. In contrast, when that judgment is not completed, movement of the position (step S16) is carried out to repeatedly execute the processing at the step S11 to that at the step S15. It is to be noted that the testing method according to this invention is not limited to the above-described amebodiment. While a semiconductor pellet of an article to be tested is mounted on the X·Y stage and is then moved in the above-mentioned embodiment, an approach may be employed, for example, to mount an article to be tested on a fixed stage to directly connect the image pick-up device onto the X · Y stage to move it. Reference signs in the claims are intended for better understanding and shall not limit the scope.

Claims: An inner lead bonding inspection method characterized by irradiating an illumination light onto a surface of a region including a bonding portion between an inner lead and an electrode bump, said bonding portion having a planar upper surface and being provided on a semiconductor pellet, obtaining an image of said region, recognizing the bonding portion and detecting the position of the bonding portion, opening a window in said image of said planar upper surface of the bonding portion in said region, measuring the quantity of reflected light within said window, judging whether the bonding state of the inner lead bonding is good or bad by comparing the measured quantity of reflected light with a reference level. An inspection method as set forth in claim 1, wherein the degree of the bonding state is detected on the basis of a plurality of reference levels with respect to the quantities of reflected light. An inspection method as set forth in claim 1, wherein said illumination light is directed normally to the surface of the inner lead. An inspection method as set forth in claim 1, wherein said observing is performed using a video camera. An inner lead bonding inspection apparatus characterized by comprising: irradiation means (12) for irradiating an illumination light onto a surface of a region including a bonding portion between an inner lead and an electrode bump, said bonding portion having a planar upper surface and being provided on a semiconductor pellet, means (13) for obtaining an image of said region, means (15) for recognizing the position of the bonding portion, for opening a window in said image of said planar upper surface of the recognized bonding portion and for measuring the quantity of reflected light within said window, and means (51) for judging whether the bonding state of the inner lead bonding is good or bad by comparing the measured quantity of reflected light with a reference level.

Inventors: SHIBASAKA MITSUSADA, YAMANAKA KAZUYUKI, SHIBASAKA, MITSUSADA, YAMANAKA, KAZUYUKI
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1990
Main classification: H01L21
IPC codes: H01L21, B23K31