Method and apparatus for three dimensional inspection of electronic components

A method for three dimensional inspection of a lead on a part mounted on a reticle, the method comprising the steps of:

• a) locating a first camera to receive an image of the lead;
• b) transmitting an image of the lead to a first frame grabber;
• c) providing fixed optical elements to obtain two side perspective views of the lead;
• d) locating a second camera to receive an image of the two side perspective views of the lead;
• e) transmitting the two side perspective views of the lead to a second frame grabber;
• f) operating a processor to send a command to the first frame grabber and second frame grabber to acquire images of pixel values from the first camera and the second camera;
• g) processing the pixel values with the processor to obtain three dimensional data about the lead, including a rotation, an X placement value and a Y placement value of the part relative to world X and Y coordinates, by finding points on four sides of the part;
• h) using a part definition file that contains measurement values for an ideal part;
• i) calculating an expected position for each lead of the part for a bottom view using the measurement values from the part definition file and the X placement value and Y placement value;
• j) converting the pixel values into world locations by using pixel values and parameters determined during calibration wherein the world locations represent physical locations of the lead with respect to world coordinates defined during calibration;
• and
• k) calculating a Z height of each lead in world coordinates in pixel values by combining a location of a center of a lead from a bottom view with a reference point of the same lead from a side perspective view.

A method to inspect a ball grid array device comprising the steps of:

• (a) locating a point on a world plane determined by a bottom view ray passing through a center of a ball on the ball grid array device;
• (b) locating a side perspective view point on the world plane determined by a side perspective view ray intersecting a ball reference point on the ball and intersecting the bottom view ray at a virtual point where the side perspective view ray intersects the world plane at an angle determined by a reflection of the side perspective view ray off of a back surface of a prism where a value of the angle was determined during a calibration procedure;
• (c) calculating a distance L.sub.1 as a difference between a first world point, defined by an intersection of the bottom view ray with a Z=0 world plane, and a second world point, defined by the intersection of the side perspective view ray and the Z=0 world plane, where a value Z is defined as a distance between a third world point and is related to Li as follows:
• ##EQU8## wherein Z is computed based on the angle;
• (d) computing an offset E as the difference between the virtual point defined by the intersection of the bottom view ray and the side perspective view ray and a crown of a ball at a crown point that is defined by the intersection of the bottom view ray with the crown of the ball, and can be calculated from a knowledge of the angle and ideal dimensions of the ball where a final value of Z for the ball is:
• Z.sub.Final =Z-E.

A method for finding a location and dimensions of a ball in a ball grid array from a bottom image, the method comprising the steps of:

• (a) defining a region of interest in the bottom image of an expected position of a ball where a width and a height of the region of interest are large enough to allow for positioning tolerances of the ball grid array for inspection;
• (b) imaging the ball, wherein the ball is illuminated to allow a spherical shape of the ball to present a donut shaped image, wherein the region of interest includes a perimeter of the ball wherein the bottom image comprises camera pixels of higher grayscale values and where a center of the bottom image comprises camera pixels of lower grayscale value and wherein a remainder of the region of interest comprises camera pixels of lower grayscale values;
• (c) finding an approximate center of the ball by finding an average position of pixels having pixel values that are greater than a predetermined threshold value;
• (d) converting the region of lower grayscale pixel values to higher grayscale values using coordinates of the approximate center of the ball;
• and
• (e) finding the center of the ball.

A method for finding a reference point on a ball in an image of a side perspective view of a ball grid array, the method comprising the steps of:

• a) defining a region of interest in the image from an expected position of a ball wherein a width and a height of the region of interest are large enough to allow for positioning tolerances of the ball grid array;
• b) imaging the ball, wherein the ball is illuminated to allow a spherical shape of the ball to present a crescent shaped image having camera pixels of higher grayscale values, and wherein a remainder of the region of interest comprises camera pixels of lower grayscale values;
• c) computing an approximate center of the crescent shaped image by finding an average position of pixels that are greater than a predetermined threshold value;
• d) using coordinates of the approximate center of the crescent to determine a camera pixel as a seed pixel representing a highest edge on a top of the crescent shaped image;
• and
• e) determining a subpixel location of the reference point based on the camera pixel coordinates of the seed pixel that define coordinates of a region of interest for the seed pixel.