A comparison between moire interferometry and strain gages for effective CTE measurement in electronic packages

Abstract

High-sensitivity laser moire interferometry technique has proven to be invaluable in characterizing electronic packages. It is often contrasted to the classical strain gage technique as a more powerful, whole-field alternative for the characterization of electronic packages. In this paper, we present a systematic comparison of the accuracy of thermal strains measured using the moire technique to that obtained using strain gages. The effective thermal strains in approximately twenty printed circuit board specimens were measured using the two techniques. The results of the two techniques showed good overall correlation, but differed from each other by as much as 2.73 PPM. The possible sources of error in each technique are identified and discussed. Of practical significance, the measured effective coefficient of thermal expansion varied in wide range by as much as 5 PPM about the commonly used value of 17 PPM. This has been shown in a related study as causing a significant reliability impact for a plastic ball grid array package assembly.