Choose the Right Lighting for Inspection

Figure 1. Three surface types predominate in electronic products, but most products' surfaces are some combination of two or all three.

Select Areas to Light
As you work with the part or assembly you want to inspect, you'll have to think about the size of the area that requires illumination. For example, on a large PCB you may need to inspect only a few components, so only a small part of the PCB needs light. At first you might have thought about using a powerful (and expensive) flood lamp to light the entire PCB. A small quartz-halogen or fiber-optic light source, however, may suffice to illuminate a small area. If you must inspect a large PCB, sometimes it can be difficult to evenly illuminate the entire board even when using high-intensity light sources.

After you've considered the different effects of surfaces and the size of the area you want to illuminate, you'll have to choose from different sources of light (see "Choose a Light Type," at the end of this article). And, you'll have to select the lighting technique or techniques that will illuminate your product properly. For the most part, inspection systems rely on front, back, dark-field, and light-field illumination techniques.

Front illumination involves placing a light source and a camera on the same side of a product to provide overall illumination. The position of the light source can vary depending on what you want to inspect (Fig. 2a). You can also position a light source to provide front illumination from only one direction. At a high angle, such directional lighting lets an inspection system measure shadows and calculate the heights of components. Placing a light source nearly perpendicular to a camera provides oblique illumination (Fig. 2b). Use this technique to find surface irregularities such as burrs.

Coaxial illumination, another form of front lighting, directs light down a camera's line of sight (Fig. 2c). Employ this lighting technique to shine light directly into a hole or a small subassembly. Because this technique provides uniform illumination across your field of view, it can reduce shadows and some glinting.

Light can also illuminate a product from behind. Back lighting is often ideal for making dimensional measurements or checking for the presence or absence of components (Fig. 2d). Back lighting a translucent PCB lets you locate components and find holes unoccupied by a lead or solder. A variation of back lighting uses collimated light to produce images with sharp edges, even for curved surfaces such as those on a cylinder or ball (Fig. 2e).

Figure 2. You can choose from six types of lighting techniques to best illuminate the product you want to inspect. Some inspection systems may use a combination of the techniques above.

Both dark-field and light-field illumination techniques provide high-contrast images that can highlight specific components or defects. Dark-field illumination directs most reflected light away from a camera so surface variations or features appear brightly on a dark background field (Fig. 3a).

Under light-field illumination, defects or components appear as dark features on an otherwise bright image (Fig. 3b). You can use either front lighting or back lighting to produce dark-field or light-field images for your camera. Typically, inspection systems use back lighting to highlight cracks, particles, and air bubbles in translucent or transparent parts.

Figure 3. Both dark-field and light-field lighting provide high-contrast images that let you quickly detect defects or areas of interest.

Put Lighting to Work
Now that you better understand the choices of lighting and lighting techniques available to you, three brief examples will show you how to put lighting techniques into practice.

Inspect Printed Information
Problem: Your manufacturing process prints white characters on black plastic cases that will enclose small circuit modules. You must inspect the printing to be sure it is properly positioned and undamaged.
Solution: The black plastic provides a dull opaque surface, so there is already a high contrast between the black plastic and the white ink. Two lighting techniques will properly illuminate the cases so you can inspect the printing. First, you can use a circular fluorescent ring light (Fig. 4a) that produces uniform diffuse front illumination. You could also use polarized lighting from a fiber light guide (Fig. 4b). The polarized light helps reduce any small bright glints on a case"s surface. In this example, the light simply maintains an already high-contrast image across the part's entire surface.

Figure 4. A print-inspection system can use either a fluorescent ring light to provide uniform illumination or a highly directional light that overcomes any problems with glint.

Read Etched Data
Problem: Your inspection system must accurately read characters that have been etched into the surface of a silicon wafer.
Solution: Under normal lighting conditions, the characters are almost indistinguishable from the wafer"s highly polished metallic surface. The wafer has a specular and opaque metallic surface.

The two best solutions require light-field or dark-field illumination. Both techniques take advantage of the disruptions caused by the characters on the specular flat surface of the wafer. The characters appear bright on a dark background under dark-field illumination or dark against a bright background under light-field illumination.

Collimating the light from a red LED produces light-field illumination (Fig. 5a). Or, you can use two rectangular banks of LEDs to provide the illumination for dark-field images (Fig. 5b).

Figure 5. Collimated light from an LED provides light-field illumination for reading characters off a semiconductor wafer. Directional LED sources provide illumination for dark-field images.

Monitor Solder Deposition
Problem: You must inspect through-hole PCBs for solder defects such as missing material, unfilled holes, and solder bridges.
Solution: Keep in mind that the PCB is a complex "optical component." And surfaces on the board include flat, prismatic, and mixed. Solder joints, on the other hand, range from smooth, domelike structures to sharp cones. The solder joints are bright gray, but the PCB is a dark green with white silk-screened legends printed on it. The PCB's surface is highly specular and opaque, and objects can appear bright or dark, depending upon the angle of incident light.

This application requires that the inspection system acquire two images, each of which has its own lighting requirements (Fig. 6). Because the solder joints have various shapes, diffuse illumination will best light them. When you illuminate the PCB with diffuse light, the inspection system acquires an image that lets it detect empty holes and missing solder.

Figure 6. A combination of diffuse light and directional polarized light produced in the same housing lets a camera obtain two images of a PCB. The inspection system combines images to extract useful information.

Next, you should illuminate the PCB with directional polarized light that enhances the silk-screened legends and blocks specular reflections from the solder. The inspection system can then acquire a second image and subtract it from the first. The image processing effectively "removes" or de-emphasizes silk-screened legends so the inspection system doesn't confuse them with solder bridges. In this example, the difference in reflectivity between the solder and the legends produces a high-contrast image. T&MW

Carl H. VanDommelen is the Asia/Pacific technical manager at Acuity Imaging. He has taught courses in lighting for machine vision, and he has presented papers on lighting at international conferences.