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24 October 1997 Optical Fresnel diffraction system for inspecting VLSI leads
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The leads on VLSI semiconductor flat pack chips are becoming more delicate and more closely spaced with time. Inspection is becoming more expensive because of the higher lead density and more necessary because of the increased risk of damage. A new optical system for lead inspection is proposed that uses self-interference of coherent light passing between the leads. According to the fractional Talbot effect, explained in the paper, there are planes of uniform intensity behind a perfect periodic structure such as an undamaged set of leads. Damage to leads takes a variety of forms. One or more leads may be skewed sideways with a potential for a subsequent short circuit to a neighboring lead after flow soldering. Alternatively, a lead may be bent up with a potential for an open connection on flow soldering. In our system, damage shows up as a deviation in intensity across the self-uniform plane. A computer simulation was written showing that small amounts of damage may be detected, measured and classified as skew or coplanarity or some combination. Laboratory experiments were performed to demonstrate the inspection capability and assess the difficulty in constructing a competitive system using this approach.
© (1997) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Alastair D. McAulay and Junqing Wang "Optical Fresnel diffraction system for inspecting VLSI leads", Proc. SPIE 3159, Algorithms, Devices, and Systems for Optical Information Processing, (24 October 1997);

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