SMTA International Conference Proceedings


Author: Jennifer Muncy et al.
Company: Georgia Institute of Tech.
Date Published: 9/21/2003   Conference: SMTA International

Abstract: Over the last several years, higher density flexible substrates have been introduced into high volume electronics assembly. As with any new technology, there are new challenges that arise in processing, reliability testing, material selection, and even fixturing. One of the main assembly issues with laminate structures is warpage due to the CTE mismatch between the laminated materials. The goal of this research was to utilize moiré techniques to investigate out of plane deformation of a 35mm TBGA component while in a reflow profile. The moiré analysis gave the out of plane deformation as a function of time and temperature for the test vehicles used throughout the experimentation.

There are several reasons for PWB or substrate warpage: the inherent CTE mismatches of the substrate building blocks, the transient temperature difference in the laminated substrate materials, and the rigidity of the substrate. Warpage from all of the above causes could affect flip chip attachment yield for first level interconnects. Warpage could cause misalignment between the die and the substrate and/or prevent the solder balls from making contact with the substrate flip chip pads during reflow, therefore the deflection of the substrate at reflow temperatures should be well understood for proper reflow process characterization.

This out of plane deformation experienced in the reflow soldering process was investigated experimentally and studied via moiré techniques. The output from the moiré analysis gave real time flatness measurements of the substrates and aided in the development of a robust reflow process that resulted in yields of 98% or greater for a wide range of fluxes.

Key words: thermal warpage, shadow moiré, phase stepping, flip chip, ball grid array (BGA), and flex substrate.

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