JPH1163932A - Wire height inspection in wire bonding - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inspect the height of wire efficiently after wire bonding. SOLUTION: A wire 6 is shot with the focus of a camera 8 fixed at a reference height to obtain the brightness distribution of the wire 6 in a cross direction from the obtained wire image. The degree of fuzziness is numericallyexpressed by the brightness distribution, and the relation between the numerically-expressed result and dislocation amount from the reference height in a height direction is obtained previously as a height conversion table. For wire height inspection, the dislocation amount is obtained from the height conversion table based on the fuzziness degree of the wire image, therefore, the height of the wire 6 can be obtained without any adjustment of the focus to the wire 6 at every time by moving the camera 8 vertically, thus it is possible to conduct wire height inspection after wire bonding promptly.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for inspecting a wire height in wire bonding for connecting a chip and a substrate.

[0002]

2. Description of the Related Art Wire bonding for connecting a chip to a substrate, such as a lead frame or a printed circuit board, on which the chip is mounted by wires is performed as follows. First,
An electrical spark is generated between the lower end of the wire drawn down from the lower end of the capillary tool and the torch, and a ball is formed at the lower end of the wire. Bonding is performed on the upper surface of the mounted chip. Next, after once moving the capillary tool upward, the lower end of the capillary tool is lowered toward the pad of the substrate while drawing a predetermined trajectory, and the wire is bonded to the substrate. And after wire bonding, to protect chips and wires,
Resin sealing is performed.

[0003] Before resin sealing is performed, an inspection for confirming the state of wire bonding is performed, and whether or not normal wire bonding is performed is inspected. One item of this inspection is a wire height inspection. If the variation in the wire height is large, it may cause defects such as short-circuiting between wires. Therefore, in the inspection after wire bonding, there is a method that can inspect the wire height with high reliability and efficiency. Desired.

Conventionally, the above wire height inspection is performed by a method of detecting the position of the wire in the height direction using a laser displacement meter, or by observing the wire with a camera and moving the camera up and down until the wire is focused. A method of obtaining the height of the wire from the amount of vertical movement of the camera at this time has been used.

[0005]

However, a method using a laser displacement meter and a method for moving a camera up and down are not well known.
A considerable inspection time is required for one inspection point. In the inspection after wire bonding, it is necessary to inspect tens or hundreds of wires in one chip, and if the conventional inspection method is applied as it is, enormous inspection time will be required. Was virtually impossible.

Accordingly, an object of the present invention is to provide a wire height inspection method in wire bonding that can efficiently inspect the wire height after wire bonding.

[0007]

According to the present invention, there is provided a method for inspecting the height of a wire in wire bonding, wherein a camera connects an image of a wire connecting a pad of a chip mounted on a substrate to a pad of the substrate, and obtains an image. A wire height inspection method for inspecting the height of the wire based on the data, the step of adjusting the focus of the camera to a preset reference height,
A step of imaging the wire with a camera, a step of quantifying the degree of defocus of the image of the wire on the screen, and a step of calculating the amount of deviation in the height direction from the reference height of the wire based on the quantified result, Comparing the shift amount with a preset determination reference value.

According to the present invention having the above structure, the degree of defocus of a wire picked up by a camera is converted into a numerical value, and this numerical value is converted into a deviation amount from a reference height based on data obtained in advance. The height of the wire can be quickly inspected without moving the camera up and down to focus on the wire.

[0009]

Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing a configuration of a wire height inspection apparatus in wire bonding according to an embodiment of the present invention, FIG. 2A is a plan view of a substrate on which the wire bonding is performed, and FIG. FIG. 3 is a flowchart showing a method for inspecting the height of the wire, FIG. 4A is a partially enlarged image view of the wire, and FIG. 4B is the luminance of the image of the wire. 5A is a partially enlarged image diagram of the wire, FIG. 5B is a graph showing the luminance distribution of the wire image, and FIG. 6 is a height diagram showing the amount of displacement of the wire in the height direction. 6 is a graph of a conversion table.

First, the configuration of a wire height inspection apparatus in wire bonding will be described with reference to FIG. FIG.
, A substrate 2 is placed on a movable table 1. A chip 4 is mounted on the substrate 2. The pads 3 on the substrate 2 and the pads 5 on the chip 4 are connected by wires 6. A Z table 9 is disposed above the movable table 1, and a camera 8 is mounted on the Z table 9. By driving the movable table 1, the substrate 2 moves in the horizontal direction, and the wire 6 connecting the chip 4 and the substrate 2 is positioned below the camera 8. The camera 8 is moved up and down by the Z table 9 to capture an image of the wire 6 positioned below the camera 8.

An A / D converter 12 is connected to the camera 8. The AD converter 12 converts the image data captured by the camera 8 into image data. The image processing unit 13
The D-converted image data is taken in, and an image processing operation is performed. The determination unit 14 determines the displacement of the wire 6 in the height direction based on the result of the image processing. The driving unit 15 controls the driving of the Z table 9 to move up and down. The display unit 16 is a monitor that displays an inspection screen. The height conversion table storage unit 17 stores conversion data for calculating the amount of displacement in the height direction from the processing result of the wire image. The criterion storage unit 18 stores a criterion value for the amount of displacement in the height direction.

Next, a state after wire bonding will be described with reference to FIG. 2A and 2B, a chip 4 is mounted on a substrate 2. A large number of pads 5 are formed on the upper surface of the chip 4 along the edge. The pads 3 are formed on the substrate 2 at positions corresponding to the respective pads 5 of the chip 4. A wire 6 is bonded so as to connect the pad 5 of the chip 4 and the pad 3 of the substrate 2. As shown in FIG. 2B, the pads 5 of the chip 4 and the pads 3 of the substrate 2 are not on the same plane but have a height difference, and the wire 6 after bonding has a three-dimensional shape.

As shown in FIG. 2A, the wires 6 are arranged radially around the chip 4 in a plan view. Reference numeral 20 denotes a range in which the camera 8 captures images at one time, and the height of the wire 6 is inspected on the visual field 20. In FIG. 2, 16 wires are bonded around the chip 4, but in practice, several tens, and in some cases, hundreds of wires are bonded to one chip at a time. In addition, F shown in FIG.
This is a reference height of the wire 6 described later.

The apparatus for inspecting the height of a wire in this wire bonding is constituted as described above. Next, a method for inspecting the height of a wire will be described with reference to the drawings along the flow of FIG. First, in FIG. 1, the substrate 2 on which the chip 4 is mounted is placed on the movable table 1. Next, the movable table 1 is driven to horizontally move the substrate 2, and the wire 6 to be detected is positioned below the camera 8.

Next, the focus of the camera 8 is adjusted to the reference height of the wire 6 (ST1) (see the height F shown in FIG. 2B). In this state, the camera 6 captures an image of the wire 6 (ST2). The captured data is read into the image processing unit 13 via the AD conversion unit 12. Next, the degree of defocus of the image of the wire 6 is quantified (ST3).

Here, the digitization of the degree of defocus of the wire image will be described with reference to FIGS. FIG.
5A shows an image of the wire 6 captured in the visual field 20. FIG. FIG. 4A shows an image in a state where the camera 8 is in focus at the time of imaging, and FIG. 5A shows an image in a state where the camera 8 is out of focus, that is, a so-called out-of-focus state. 4 (b) and 5 (b) correspond to FIGS. 4 (a) and 5 (b), respectively.
3A shows the luminance distribution of the image of the wire 6 on the scanning lines S1 and S2 shown in FIG. In FIG. 4A, the focus of the camera 8 is in focus, and the boundary between the image of the wire 6 and the background image clearly appears on the screen. Therefore, the luminance distribution graph G1 shown in FIG. The luminance distribution has a steep luminance distribution in which the luminance suddenly increases.

On the other hand, in the image shown in FIG.
Since the camera 8 is not focused on the wire 6,
The boundary between the image of the wire 6 and the background image does not appear clearly, and the width of the image of the wire 6 appears thicker by the unclearness. Therefore, the graph G2 of the luminance distribution along the scanning line is shown in FIG.
Has a gentler shape than the brightness distribution graph G1 shown in FIG.

That is, the difference between the case where the camera 8 is in focus and the case where the camera 8 is out of focus when the wire 6 is picked up has a clear difference in the luminance distribution of the wire image obtained based on the obtained image data. Appear in shape. Therefore, by numerically expressing this difference in some form, the degree of out-of-focus is obtained numerically (hereinafter, the numerically-defined degree of out-of-focus is referred to as “defocus coefficient”), and this out-of-focus coefficient is referred to as the actual wire 6. In the height direction (difference from the reference height at which the camera 8 is focused), the height position of the wire 6 can be obtained while the camera 8 is fixed.

In the present embodiment, the average gradient of the brightness curves of the brightness distribution graphs G1 and G2 shown in FIGS. 4B and 5B is used as a method for obtaining the defocus coefficient. That is, when the object is in focus, the outline of the wire 6 appears sharpest, and the luminance curve shows the steepest rise. Therefore, the average slope of the luminance curve is maximum, and in this case, the defocus coefficient is maximum. On the other hand, the more the focus is out of focus, the wider the wire image width is, so the luminance curve becomes gentler, the average gradient of the luminance curve becomes smaller, and the defocus coefficient becomes smaller according to the degree of defocus.

FIG. 6 is a graph G showing the relationship between the defocus coefficient obtained in this way and the actual deviation d in the height direction of the wire 6 (difference from the reference height in a focused state).
3 is a height conversion table shown in FIG. As shown in FIG. 6, the shift amount is 0 when the focus of the camera 8 is accurately located on the wire 6, and the defocus coefficient is maximum in this state. The defocus coefficient decreases as the shift amount d increases.

Returning to the flow of FIG. 3, the deviation d of the wire 6 from the reference height is obtained from the graph G3 based on the defocus coefficient obtained in ST3 (ST4). Here, the obtained shift amount d is compared with a preset determination reference value, and the shift amount d is in the range of A shown in FIG. 6, that is, the range in which the shift amount d is small and considered to be safe in quality (safety range). Is determined (ST5). If it is within the safe range, a good judgment is made immediately. On the other hand, if the shift amount d is outside the safe range, it is determined whether or not the shift amount falls within the range (dangerous range) B in FIG. 6 (ST6). In the case of falling within the range of B, a more detailed height inspection is performed in a step to be described later. If the deviation d exceeds the danger range of B, that is, if the deviation d is in the range of C, the defect is immediately determined. .

Next, a description will be given of a method of additionally inspecting the height of the wire 6 when it is determined in ST6 that the area is within the dangerous range. This inspection performs more detailed inspection on measurement points belonging to a so-called gray zone. First, the Z table 9 is driven to move the camera 8 up and down so that the camera 8 is focused on the wire 6 (ST7). Next, the moving distance of the camera 8 during this vertical movement is read,
The height of the wire 6 is determined based on the moving distance. Then, it is determined whether or not the obtained height of the wire 6 is within an allowable range (ST9). If it is within the allowable range, a good judgment is made, and if it exceeds the allowable range, a bad judgment is made.

The present invention is not limited to the above embodiment. For example, in the above embodiment, the average gradient of the luminance curve is used as the definition of the defocus coefficient. Any definition may be used so long as the change that appears can be expressed numerically. For example, it may be the ratio of the height of the peak point on the luminance distribution to the base of the luminance distribution curve.

[0024]

According to the present invention, the degree of defocus of a wire picked up by a camera is digitized, and the relationship between the digitized result and the amount of deviation from the reference height in the height direction is determined in advance. However, when inspecting the height of the wire, the amount of deviation is determined based on the degree of defocus of the picked-up wire image, so each time the camera is moved up and down to determine the height of the wire without focusing on the wire. Can,
Therefore, the height inspection of the wire after the wire bonding can be performed quickly.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a wire height inspection apparatus in wire bonding according to an embodiment of the present invention.

FIG. 2A is a plan view of a substrate on which wire bonding according to one embodiment of the present invention is performed. FIG. 2B is a side view of a substrate on which wire bonding according to one embodiment of the present invention is performed.

FIG. 3 is a flowchart showing a wire height inspection method according to an embodiment of the present invention.

FIG. 4A is a partially enlarged image diagram of a wire according to an embodiment of the present invention; FIG. 4B is a graph showing a luminance distribution of a wire image according to an embodiment of the present invention;

5A is a partially enlarged image view of a wire according to an embodiment of the present invention. FIG. 5B is a graph showing a luminance distribution of a wire image according to an embodiment of the present invention.

FIG. 6 is a graph of a height conversion table showing a shift amount of a wire in a height direction according to the embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Movable table 2 Substrate 3 Pad 4 Chip 5 Pad 6 Wire 8 Camera 9 Z table 12 AD conversion unit 13 Image processing unit 14 Judgment unit 15 Drive unit 16 Display unit 17 Height conversion table storage unit 18 Judgment reference storage unit

Claims (1)

[Claims] A wire height inspection method for imaging a wire connecting a pad of a chip mounted on a substrate and a pad of the substrate with a camera and inspecting the height of the wire based on image data obtained. A step of adjusting the focus of the camera to a preset reference height; a step of imaging the wire with the camera; a step of quantifying the degree of defocus of the wire image on the screen; and a quantified result. Determining the amount of deviation of the wire from the reference height in the height direction based on the reference value, and comparing the amount of deviation with a preset determination reference value. Height inspection method.