Machine vision challenges: the key to success: flexibility with illumination and lens choice.

Problem:

Semiconductor packaging and component assembly companies face several vision-related problems. The companies must place parts that are getting both smaller [such as 0.25 mm sq. die or 0201 capacitors) and larger (such as 50 mm ceramic column grid arrays (CCGAs) or 22 mm semiconductor chips] on different substrates like ceramic, flexible circuits or printed circuit boards (PCBs). Many pick and place machines handle a variety of components and materials that stretch the limits of their vision capabilities.

Among the machine vision challenges associated with imaging new products and applications is selecting the proper illumination intensity, angle and wavelength for light sources. Another challenge is choosing a lens concept that will be forgiving for the focal plane.

Problem Solved:

The light-emitting diodes (LEDs) used for light sources vary in intensity, color and wavelength. Since results can vary drastically, using the correct angle is critical to illuminate different substrates and components, including: boards with several surface finishes; flexible circuits; ceramics; solder balls; columns; bare dies; and flip chips with various dielectric coatings. Incorporating higher-quality LEDs, multiple wavelengths and circular, side or through-the-lens (on-axis) lighting boards can lead to greater flexibility.

For example, the polyimide material of a flexible circuit is almost transparent in the standard red portion of the spectrum, where the wavelength is 660 nm (Figure 1). However, in the blue portion of the spectrum, with a wavelength of less than 470 nm, the material is virtually opaque (Figure 2). As evidenced by the line scans plotting pixel intensity, the result of using blue LEDs is a dramatic improvement in contrast and image quality.

[FIGURES 1-2 OMITTED]

When dealing with substrates of varying thicknesses, choose a lens that allows the depth of field to increase. The depth of field is the range of lens-to-object distance over which the image will be in sharp focus. A system with a large depth of field and high resolution can be mutually exclusive because high-resolution optics tend to reduce the depth of field. Instead, use a telecentric lens for greater flexibility. Telecentricity determines the magnification change as an object's distance changes. A telecentric system possesses the same magnification, regardless of its distance from the lens. As an object moves in and out of focus, the image size will remain constant, increasing the depth of field.

No magical combination of lenses, cameras, lighting or vision engines will capture all users' needs. However, new technologies, smart packaging and efficient integration can offer the flexibility needed to handle different problems.

Acknowledgment: The author wishes to acknowledge Robert Timms for his contributions to this column.

Richard Boulanger is vice president of the Advanced Semiconductor Assembly Division of Universal Instruments Corporation, Binghamton, NY; e-mail: [e-mail].