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Image Evaluation Method for Automatically Inspecting Soldered Joints by Means of X-Ray Films

IP.com Disclosure Number: IPCOM000039685D
Original Publication Date: 1987-Jul-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 3 page(s) / 67K

Publishing Venue

IBM

Related People

Baier, H: AUTHOR [+2]

Abstract

During the production of multilayer circuit boards for large IBM computers, fine pins, known as HARCONs, are soldered into the boards. The quality of the soldered joints is inspected by X-rays. For that purpose, the circuit boards along with the soldered pins, rotated by about 20o, are introduced into an X-ray system. During exposure, the soldered joints are represented as light areas on the film positioned behind the circuit board (Fig. 1). The light areas are caused by the X-rays being absorbed by the soldering tin and the pins but not by the circuit board material. As the films are negatives, the exposed areas appear as dark spaces. For inspection, the films are placed between two glass plates, mounted on a large X/Y table, and transilluminated.

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Image Evaluation Method for Automatically Inspecting Soldered Joints by Means of X-Ray Films

During the production of multilayer circuit boards for large

IBM computers, fine pins, known as HARCONs, are soldered into the boards. The quality of the soldered joints is inspected by X-rays.

For that purpose, the circuit boards along with the soldered pins, rotated by about 20o, are introduced into an X-ray system. During exposure, the soldered joints are represented as light areas on the film positioned behind the circuit board (Fig. 1). The light areas are caused by the X-rays being absorbed by the soldering tin and the pins but not by the circuit board material. As the films are negatives, the exposed areas appear as dark spaces. For inspection, the films are placed between two glass plates, mounted on a large X/Y table, and transilluminated. To permit using an automatic set-up for inspecting the circuit boards during their production, the following requirements must be met. - The evaluation accuracy and reliability must be very high, as erroneously detected

defects may lead to expensive repair work. At 30,000

soldered joints per circuit board, an error rate of

noticeably less than 1/10th promille is necessary. -The inspection time for a circuit board must be about 30 minutes, which at the required resolution means a speed of about

1 second for each TV image. For edge determination, a "capture" range around the desired position of a soldered joint is inspected. The position, the expected angle and orientation of the edges as well as the width and length are known from the reference data. Each line of the "capture" range is

(Image Omitted)

horizontally scanned (in the X-direction), and the points or picture elements (pels) are read from the pel storage. The points PL(nl/m) for the left edge and PR(nr/m) for the right edge are those pels at which the following values yield a maximum in the inspected line m: left edge: (P(n-2)+P(n-1))/2-((P(n+1)+P(n+2))/2 maximum right edge: (P(n+2)+P(n+1))/2-((P(n-1)+P(n-2))/2 maximum where P(n,m) is the grey value of the pel having its X-coordinate n in line m. The image noise is smoothened by averaging two pels in each case. The circuit (Fig. 2) shows a hardware implementation of the described edge detection algorithm. Instead of generating a new image containing the gradients along a line, the position of the left and the right edge point in the line is determined without changing the original image. The circuit is initialized by setting the edge position registers LKPR and RKPR and the increment (step) counter to
0. The two difference registers for the left and the right edge are set to 0 or to a minimum difference. Using a minimum difference prevents detecting the highest noise position as the edge position in edgeless lines. Then the start address of the section to be inspected is set in the address generator of the image evaluator. A microprogram control reads the pels along the line, transferri...