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In-process 3D measurement for semiconductor assembly using fringe projection technique

Warpage is the microscale 3D deformation of the surface of semiconductor packages generated during the thermal reflow process for assembling the packages on a printed circuit board (PCB). Because warpage affects the reliability of the packages, its accurate measurement and control are important tasks during the reflow process. Shadow moiré and laser moiré techniques are commonly used for measuring warpage. However, the shadow moiré technique cannot be used for simultaneously measuring the warpage of the assembled package and the PCB because it requires a glass grating located very close to the sample, and the laser moiré technique has a critical error source of laser speckle. These techniques also require spraying the sample surface with reflective paint in order to ensure uniform surface reflectance, which is not suitable for in-process measurements. To solve these problems, Dr. Kang performed a comparison study on various warpage measurement techniques and developed a novel dynamic digital fringe projection (DDFP) technique and an automatic optical system for measuring the warpage of unpainted PCB assemblies with resolution of 2.3 µm during the reflow assembly process. The DDFP technique is an extension of digital fringe projection technique that projects fringe pattern onto the sample and record and analyze the fringe pattern image that is phase modulated by the height distribution of the sample. To measure unpainted PCB assemblies that have various surface reflectance, the DDFP technique generates and projects a dynamic fringe pattern, in which proper fringe intensity distributions are dynamically determined based on the various surface reflectance. To generate the dynamic fringe, the DDFP segments the package and PCB regions in an unpainted PCB assembly image and compensate for the mismatches in coordinates and intensities between the projected and captured images.

 

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The Flowchart of DDFP Technique

 

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Warpage of plastic ball grid array packages obtained with the DDFP system 

 

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Warpage of PCBs obtained with the DFP system 

 

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