Flexible Lighting Modules
05/01/2000
 Schematic diagram of the illumination module. View upward from the substrate showing the eight petals and inner ring, which are replaceable in plug-and-play manner. |
Four engineers focus on versatile lighting's serious impact on substrate imaging.
Machine vision, used for substrate recognition, component centering/inspection and machine setup procedures, is an essential component of today's advanced automated assembly equipment. Machine vision's primary task - distinguishing features of interest from the background - is easier if the input image has a high degree of contrast. Of all the parts that make up a machine's vision system, it's the lighting system that has the most influence on image contrast.
With automated assembly systems handling a variety of substrates, each with unique imaging properties, lighting systems must be versatile. Substrate types include FR4, flexible circuits and ceramics of various colors and shades. As this variety of substrates can challenge a relatively simple, monochromatic lighting system, manufacturers should address the challenge via flexible lighting modules.
Features
 The module. A polarized film is mounted over four of the LED petals. |
A flexible lighting module should address illumination wavelength; polarization; and illumination angle, intensity and symmetry. Each component is crucial for effective imaging.
Illumination Wavelength: Each substrate type has unique optical properties. Two of the key parameters that describe the optical properties of a substrate are reflectivity and transmission. The values of these parameters are not constant - they change as a function of wavelength. In general, a monochromatic lighting module is not well-suited for providing high-contrast images over a variety of substrate types. Wavelength flexibility allows the user to tune the properties of the lighting module to maximize image contrast.
Polarization: Polarized lighting is an important tool in imaging certain substrates. Light-colored ceramic substrates tend to challenge machine vision systems. Reasons for this are the similar reflectivity of metal features and the bright background. For this important substrate class, polarized illumination is an effective wedge to distinguish features from background.
 Cross-sectional side view shows the angle of the petals, chosen to optimize the focus of the light on the substrate. |
Angle, Intensity and Symmetry: Because the reflective properties of substrate features differ, it is important to have the flexibility to control the incident light's intensity. Excessive light can lead to camera blooming, while insufficient light results in poor image contrast. The incident light's angle is also important. The character of the surface features may require either low- or high-angle illumination. Symmetric illumination is important to eliminate spatial distortion of substrate features.
Toward a Flexible Design
 The module's lighting zones. An enclosed electrical bus in the rear of the housing provides voltage to all of the lighting elements. |
One such flexible lighting module consists of a support structure holding eight light-emitting diode (LED) petals and an inner LED ring. The petals are used for low-angle illumination, while the inner ring is for higher-angle illumination. Both the petals and the inner ring can be exchanged in a plug-and-play fashion. The module has three lighting zones: petal zone I, petal zone II and the inner ring (Figure 1). Each petal is a small printed circuit board containing 10 LEDs. The petals can have LEDs of various wavelengths ranging from blue to red: red (~ 660 nm), yellow (~ 595 nm), green (~ 525 nm), red/green, blue (~ 470 nm) and white. The illumination wavelengths of the module can be quickly and easily changed. To extend its illumination capabilities, the module can be equipped with a polarizer kit. The module is mounted to the housing that contains the substrate-imaging camera.
Imaging studies were conducted on two different substrate types: flexible circuits and ceramics. The key to effective substrate imaging is understanding the substrate's optical properties and altering lighting parameters to leverage these properties.
Flexible Circuits
 Figure 3. Plot of the transmission of polyimide. The plot shows the drastic change in transmission from blue light (~ 450 nm) to red light (~ 650 nm). |
To effectively image flexible circuits, which present a significant imaging challenge to many machine vision systems, it is crucial to understand the optical properties of the constituent material - polyimide. Flexible circuits typically consist of polyimide layers mounted on a metal stiffener. The layers contain copper or gold conductive traces that terminate at an IC pad site.
Figure 3 plots the transmission of polyimide for wavelengths throughout the visible spectrum. To generate this plot, light of various wavelengths was directed through a thin film of polyimide. The transmitted light intensity was measured and compared to the incident intensity.
This data shows that polyimide is nearly transparent (transmission > 80 percent) for wavelengths in the red portion of the spectrum (wavelength ~ 650 nm). There is also a dramatic reduction of the transmission in the 450 to 550 nm range. For wavelengths in the blue portion of the spectrum (wavelength <475 nm), polyimide is virtually opaque. This observation was used as a wedge to distinguish metal fiducials from the polyimide background of flexible circuits. In Figure 4, the two images of a flex circuit pad site demonstrate this effect.
 Figure 4. Images of polyimide-based flexible circuit. (a) Image taken with a conventional monochromatic lighting system equipped with red (660 nm) LEDs. |
The image of Figure 4a was obtained with a conventional monochromatic (~ 660 nm) lighting system. The metal traces of the pad site are virtually indistinguishable from the surrounding polyimide because polyimide is considerably transmissive to the red light of the illumination module. In the background areas surrounding the metal traces and fiducials, the red light is transmitted through the polyimide and reflects from the metal backing of the circuit. This results in a bright background. The copper features on the substrate also reflect the red light efficiently. The result is a bright feature on a bright background - a low-contrast image.
Figure 4b was obtained with an illumination module equipped with blue LEDs (~ 470 nm). Because polyimide strongly absorbs in the blue portion of the spectrum, the previously bright background is now dark. The copper features on the substrate reflect the blue light efficiently. The result is an improvement in image quality.
To quantify the improvement in image contrast, line scans were taken through a metal fiducial on the pad site. The line scans, displayed in the lower right corner of each image, plot pixel intensity versus pixel location across the fiducial site. Figure 4a shows poor contrast and little edge structure. In short, the image is unworkable. The line scan in Figure 4b shows good contrast and a marked reduction in the background.
Ceramic Substrates
What makes ceramic substrate imaging so challenging? First, variability: The substrates are manufactured in a variety of shades and colors, coupled with different pad metallurgy. Making matters more complex, several component manufacturers using flip chip technology choose to image the pad site through a pre-applied film of flux. White or gray ceramics are particularly challenging, as the metal pads and ceramic background both reflect light efficiently. This makes it difficult for a vision system to distinguish the pads/fiducials from the background. A wedge is needed to distinguish the features. In the study of the polyimide of flexible circuits, the wedge was illumination wavelength. With light-colored ceramic substrates, the wedge is polarized light.
Polarized light is effective on this class of substrates because metals and dielectrics reflect polarized light differently. When linear polarized light strikes a metal surface, the light is reflected with no net rotation of the polarization. This happens because metals consist of free electrons. When polarized light interacts with a sea of free electrons in a metal, the electrons oscillate in the electric field of the light and re-radiate the light with no net change in the polarization orientation. This is not the case with the dielectric ceramic background that surrounds the feature of interest. Upon reflection, dielectrics tend to randomize the polarization of the light. This behavior can be capitalized on by installing a polarizing kit on the lighting module and using a technique called cross-polarized illumination (Figure 5).
This kit has two elements: a polarizing film covering four of the eight illumination petals and a metal slide that has a second polarizer, which is in the camera aperture. The axis of this polarizer, frequently referred to as an "analyzer," is orthogonal to that of the polarizer covering the LEDs. The combination of these polarizers on the module results in what is known as cross-polarized illumination.
 Figure 6. Polarization effects on metals and dielectrics. |
A polarizer covering the LEDs ensures that the substrate is illuminated with linear polarized light. Note that the metal features and ceramic background reflect polarized light differently. The light reflected from the metal has no net change in the polarization. Because the orientation of the polarizer located in the camera aperture is orthogonal to that of the LED polarizer, most of the light reflected from metal features is blocked. Therefore, metal features appear dark on the camera.
In contrast, the ceramic surrounding the metal features randomizes the polarization on reflection. Because the polarization is randomized, a portion of the reflected light can pass through the central polarizer and reach the camera. Thus, the ceramic background appears as a shade of gray (Figure 6).
 Figure 7. Images of a ceramic substrate for flip chip assembly. (a) Image obtained by unpolarized ilumination. Note the bright metal features on a light background. |
In Figure 7, which illustrates the effect of cross-polarization, the image on the left was taken with unpolarized illumination. The flip chip pad appears as a bright feature on a light background. A line scan taken across a portion of a row shows a high noise level. The image on the right was taken with cross-polarized illumination. The metal features appear dark on a relatively light background. Line scans taken across the equivalent portion of the image show an increase by a factor of two over unpolarized illumination.
Summary
Manufacturers requiring advanced substrate imaging should consider using flexible lighting modules. When evaluating a lighting module, seek one that offers flexibility in illumination wavelength, polarization and angle. It should be effective on flexible circuits and ceramics - two substrate technologies that traditionally have been challenging for machine vision systems; while this article only highlighted these two substrate types, the module should also be capable of imaging a variety of other substrates. In short, the module should provide the tools to allow a much more systematic approach to substrate imaging.
Acknowledgments
The authors gratefully acknowledge Mick Wunder (ADFlex) for providing flexible circuits for imaging studies, Mike Gorenflo (Universal) and Pat O'Donnell (formerly at Universal) for their valuable input, and DuPont Corp. for providing the transmission spectrum of polyimide.
Authors
JOHN HERMAN is an applications engineer and JOHN RADICE is a mechanical engineer in the Advanced Semiconductor Assembly Division of Universal Instruments Corp. JOHN DISCIULLO is a mechanical engineer at IBM, Bromont, Canada. PETER CHIPMAN is an advisory manufacturing engineer at Seagate Technology. For more information, contact John Herman, P.O. Box 825, Binghamton, NY 13902-0825; 607-779-7205; Fax: 607-771-8116; E-mail: [email protected].