System and method for in-line inspection of stencil aperture blockage

• US 6,606,402 B2
• Filed: 08/24/2001
• Issued: 08/12/2003
• Est. Priority Date: 12/18/1998
• Status: Expired due to Term

• Current Assignee: Cognex Corporation
• Original Assignee: Cognex Corporation
• Inventors: Wagman, Adam H.
• Primary Examiner(s): Dastouri, Mehrdad
• Application Number:
  US09/939,425
• Publication Number:
  US 20020081025A1
• Time in Patent Office: 718 Days
• Field of Search: 382/141, 382/144, 382/145, 382/147, 382/148, 382/149, 382/150, 382/168, 382/169, 382/170, 382/172, 382/174, 382/218, 382/219, 382/220, 348/87, 356/237.1, 356/241.1
• US Class Current: 382/150
• CPC Class Codes: G06T 1/0007 Image acquisition
• CPC Class Codes: G06T 2207/30141 Printed circuit board [PCB]
• CPC Class Codes: G06T 2207/30152 Solder
• CPC Class Codes: G06T 2207/30164 Workpiece; Machine component
• CPC Class Codes: G06T 7/001 using an image reference approach
• CPC Class Codes: H05K 1/0269 for visual or optical inspection
• CPC Class Codes: H05K 3/1233 Methods or means for supplying the conductive material and for forcing it through the screen or stencil

Abstract

A system and method for ascertaining blockage of one or more stencil apertures is provided. An analysis of pixels in an image of the stencil region that includes the aperture is made that includes determining the relative grayscale intensity of pixels acquired from the clean stencil and also the variance among nearby pixels. The grayscale and variance values for each pixel are combined to form a series of histogram groupings or bins, based upon levels. These values are used to compute parameters that represent how the apertures appear under the lighting conditions used to acquire the image, thereby allowing the informing of a decision function that estimates the probability that a given pixel represents unblocked aperture rather than solder paste.

Claims

1. A method for determining blockage of an aperture in a surface by a material comprising:

(a) providing a predetermined level and orientation of illumination to the surface having the aperture;

(b) acquiring a pre image of the surface prior to application of material thereonto;

(c) extracting pre-application image characteristics from the pre image, image characteristics including, (i) grayscale intensity values for each of a plurality of pixels and (ii) intensity variance values for each of the pixels based upon respective nearby pixels as a measure of surface texture, to form a histogram of the pre image.

(d) applying material onto the surface;

(e) acquiring a post image of the surface, after the act of applying;

(f) extracting post-application image characteristics from the post image, image characteristics including, (i) grayscale intensity values for each of a plurality of pixels and (ii) intensity variance values for each of the pixels based upon respective nearby pixels as a measure of surface texture, both characteristics based upon a model established from the pre-application image characteristics; and

(g) ascertaining blockage of the aperture by differentiating the grayscale intensity values for pixels representing the aperture from gray scale intensity values for pixels representing the material, the step of ascertaining including the steps of

i) establishing an image intensity histogram including a first plurality of histogram bins each containing grayscale intensity values for each of pixels of the pre image and pixels of the post image, each of the pixels being placed within respective predetermined intensity value ranges and wherein the histogram includes a second plurality of histogram bins for each of the first plurality of histogram bins, each second bin containing pixel intensity variance values falling within a respective predetermined intensity variance ranges for each of the pixel intensity values,

(ii) shifting histogram bins to a different value in response to measured bin values of other apertures so as to place each of the histogram bins at a similar location to thereby compensate for image characteristic differences between apertures in an overall surface.

2. The method as set forth in claim 1 further comprising weighting the grayscale intensity values, based upon the act of differentiating, into a plurality of classes having respective weight values including zero, one and at least one value between zero and one.

3. The method as set forth in claim 2 wherein the ascertaining act (g) further comprises summing the classes and thereby deriving a first weight value for the pre image and a second weight value for the post image and calculating the difference between the first weight value and the second weight value.

4. The method as set forth in claim 1 wherein the ascertaining act (g) further comprises modelling the pixel data representative of the aperture as a single peak in the histogram having at least one of the histogram bins therein.

5. A method for determining blockage of an aperture in a surface by a material comprising:

(a) providing a predetermined level and orientation of illumination to the surface having the aperture;

(b) acquiring a pre image of the surface prior to application of material thereonto;

(c) extracting pre-application image characteristics from the pre image, image characteristics including, (i) grayscale intensity values for each of a plurality of pixels and (ii) intensity variance values for each of the pixels based upon respective nearby pixels as a measure of surface texture.

(d) applying material onto the surface;

(e) acquiring a post image of the surface, after the act of applying;

(f) extracting post-application image characteristics from the post image, image characteristics including, (i) grayscale intensity values for each of a plurality of pixels and (ii) intensity variance values for each of the pixels based upon respective nearby pixels as a measure of surface texture, both characteristics based upon a model established from the pre-application image characteristics; and

(g) ascertaining blockage of the aperture by differentiating the grayscale intensity values for pixels representing the aperture from grayscale intensity values for pixels representing the material by grouping them into a plurality of classes having respective weight values including zero, one, and at least one value between zero and one, and summing the classes to derive a first weight value for the viewed image prior to the applying act (c) and a second weight value after the applying act (d) and calculating the difference between the first weight value and the second weight value.

6. The method as set forth in claim 5 wherein the ascertaining act (g) further comprises differentiating the grayscale intensity values for pixels representing the aperture from grayscale intensity values for pixels representing the material based upon the intensity variance values, including differentiating pixels with high intensity variance values from pixels with low intensity variance values.

7. The method as set forth in claim 5 wherein the ascertaining act (g) further comprises differentiating the grayscale intensity values for pixels representing the aperture from grayscale intensity values for pixels representing the material based upon intensity variance values including differentiating between intensity variance values based upon categorizations determined using the pre-application image characteristics.

8. The method as set forth in claim 5 wherein the ascertaining act (g) includes differentiating the grayscale intensity values for pixels representing the aperture from grayscale intensity values for pixels representing the material based upon both grayscale intensity values and intensity variance values in a simultaneous differentiation.

9. The method as set forth in claim 6 further comprising weighting the grayscale intensity values, based upon the act of differentiating, into a plurality of threshold classes having respective weight values including zero, one and at least one value between zero and one.

10. The method as set forth in claim 6 further wherein the class histogram includes a first plurality of histogram bins each containing grayscale intensity values for each of pixels of the pre image and pixels of the post application image, each of the pixels being placed within respective predetermined intensity value ranges and wherein the class histogram includes a second plurality of histogram bins for each of the first plurality of histogram bins, each second bin containing pixel intensity variance values falling within a respective predetermined intensity variance ranges for each of the pixel intensity values.

11. The method as set forth in claim 5 further comprising, registering the pre-application image characteristics and the post-application image characteristics with respect to a model image data pattern defining an approximate shape and location of the aperture on the surface when it is free of material applied thereonto.

12. The method as set forth in claim 5 wherein the ascertaining act (g) further comprises, establishing an image intensity histogram including a first plurality of histogram bins each containing grayscale intensity values for each of pixels of the pre image and pixels of the post image, each of the pixels being placed within respective predetermined intensity value ranges and wherein the histogram includes a second plurality of histogram bins for each of the first plurality of histogram bins, each second bin containing pixel intensity variance values falling within a respective predetermined intensity variance ranges for each of the pixel intensity values.

13. The method as set forth in claim 12 wherein the ascertaining act (g) further comprises, shifting histogram bins to a different value in response to measured bin values of other apertures so as to place each of the histogram bins at a similar location to thereby compensate for image characteristic differences between apertures in an overall surface.

14. The method as set forth in claim 12 wherein the ascertaining act (g) further comprises modelling the pixel data representative of the aperture as a single peak in the histogram, the peak having at least one of the histogram bins therein.

15. The method as set forth in claim 12 wherein the pixels representing the aperture and the pixels representing the material are grouped into the plurality of classes based upon the first and second plurality of histogram bins.