Today’s demand for total product quality requires that customer specifications are met or—better yet—exceeded. To meet the standards for higher-density packaging associated with surface-mount technology (SMT), an optically based inspection system is essential.
Conventional off-the-shelf microscopes only provide a 2-D image of the specimen. When challenged with identifying hidden defects, a 3-D view is preferred.
By customizing an existing optical system to yield a 3-D view of a specimen, operator fatigue is reduced, inspection throughput is increased and hidden defects are more easily identified. For manufacturers, optical inspection systems help ensure the integrity of finished PCBs and hybrids while minimizing waste and achieving higher yields.
Efficient inspection of any SMD requires a system that incorporates zoom optics equipped with an oblique viewing mechanism. These systems rotate around the SMD specimen and provide a 360° view of the component. This allows thorough inspection of all surfaces without moving the component which, in turn, minimizes potential damage.
The key elements of an oblique viewing system include a high-quality zoom microscope with an oblique viewing accessory; an X-Y stage; either manual or motorized control of rotation, zoom and focus; a fiber-optic illuminator, and high-resolution video equipment. This system provides perpendicular and oblique views of surface-mount assemblies while rotating 360° around the specimen on the zoom microscope’s optical axis in a continuous clockwise or counter-clockwise direction.
Zoom Optics
Dependable, flexible zoom optics are necessary when selecting an appropriate inspection system. Zoom optics offer a range of zoom ratios and magnification ranges.
When coupled with various TV tubes, the zoom optics and an oblique viewing accessory yield a package with vertical and oblique magnification capabilities. Remember, with increased magnification, field of view decreases. Zoom optics with oblique viewing accessories should also have a working distance of at least 45 mm and be completely parfocal so the object stays in focus throughout the magnification range.
Oblique Viewing Accessory
An oblique viewing accessory gives you perpendicular and oblique views of surface-mount assemblies. The oblique viewing accessory with a zoom module offers a choice of magnification ranges. You can change between vertical or oblique viewing by turning a viewing-angle switch.
To provide enough clearance for a complete rotation, the oblique viewing accessory must be mounted on a boom-type stand to avoid physical damage to the specimen. The stage should be maneuvered under the optical axis.
With large complex PCBs, an X-Y axis travel as large as 22″ could be required. The stage should also include an antistatic surface to protect the PCB from static electricity.
Several rotation options are typically available on an oblique viewing accessory:
Manual rotation.
Motor drive operated by a fixed-speed foot switch.
Motor drive operated by a variable-speed foot switch.
Variable-speed single-axis hand joystick.
Some applications also demand motorized zoom and focus for the zoom optics. Many joysticks and stepper-motor drives are available for this function.
Illumination Systems
Uniform bright light is necessary for accurate inspections. A 40-point fiber- optic ring illuminator is excellent for this purpose. The 40 individual points of light result in more uniform and even illumination throughout the field. The height of the ring should be adjustable and provide an inside diameter of 8-1/4″ to allow the oblique viewing accessory to rotate inside the ring without interference.
A dual bifurcated fiber-optic illuminator is optional. This type of system consists of two dual fiber-optic bundles (each bundle consisting of a flexible dual gooseneck) that provide four light probes. The flexibility and versatility of the probes ensure accurate illumination positioning.
High-Resolution Video Equipment
Examining boards under a stereomicroscope for extended periods of time can be physically uncomfortable. You can also experience fatigue after spending hours looking through microscope eyepieces. These situations rapidly become counterproductive and allow defective SMDs to slip through inspection.
This problem can be eliminated by attaching high-resolution video equipment to the optics. Video also enables group viewing and provides the capability to capture specific images for later review.
Often when using video, there is a trade-off between image resolution and the field of view. In achieving high resolution, the field of view is often compromised. Image contrast, brightness and other factors are also affected. Simply increasing magnification does not necessarily produce the desired result. Zoom lenses, however, help resolve the conflict between resolution and the field of view. Typically, the video monitor’s field of view is matched with that seen through microscope eyepieces, otherwise cropping or vignetting will occur.
Optics/Video Mix
The combination of high-quality optics and high-resolution video for inspection will provide manufacturers with vital information to improve quality and increase throughput. As video technology improves and becomes an integral element of today’s high-quality optics, visual-inspection optical systems will remain a primary choice for quality inspection.
About the Author
Paul E. Nothnagle is Vice President/Director of Technology at Optem International. He has devoted his 36-year career to solving optical problems in semiconductor manufacturing. Mr. Nothnagle graduated from St. Bonaventure University with a B.S. degree in physics. Optem International, 78 Schuyler Baldwin Dr., Fairport, NY 14550, (716) 223-2372.
Copyright 1996 Nelson Publishing Inc.
October 1996