Keywords: surface mounted technology, speckle interferometry, thermal deformation.

One of the major concerns regarding electronic equipment is the significant discrepancy of material thermo-mechanical parameters: which is likely to cause component failure. The work presented in this paper aimes at obtaining information concerning the spatial distribution of thermal deformations in an application board-surface mounted module assembly. Also, an estimation of the thermal expansion coefficient is desired. For this purpose, experimental investigations are performed, using two optical measurement techniques: speckle interferometry (SI) and digital image correlation (DIC). Both of these techniques yield full field, high resolution displacement fields and require important amounts of data processing.

Speckle interferometry is based on the properties of coherent light in order to obtain the static or dynamic displacement information [1]. The primary results are interference images corresponding to image-plane holograms of the test object. The secondary results consist of an interferometric fringe pattern, usually reconstructed with the help of a temporal or spatial phase-stepping procedure. In order to obtain the two-dimensional displacement maps, a phase unwrapping algorithm must be applied.

Digital image correlation consists in capturing a series of images during the continuous deformation of a test specimen in order to evaluate the variations in the surface characteristics. A numerical analysis based on estimating by correlating the mutual displacement of different regions of the images is applied in order to obtain the displacement fields.

The displacement fields show important disparities between the lateral expansion of the printed circuit board (PCB) and of the module. The estimated coefficient of thermal expansion (CTE) for the PCB board is five to six times higher than the CTE of the components. The connections are subjected to supplementary stress induced by these disparities.

An estimation of localized strain concentrations induced by the thermal loading is then performed [2]. As the in-plane expansion is blocked by the supporting mechanism, the normal and shear strain may be considered mainly dependent on the out-of plane deformation. Therefore, the normal and shear strain fields are approximated by calculating the terms depending on the second derivative of the out-of-plane displacement. This is obtained by numerical differentiation of the experimental data fields.

Finally, the results offered by both experimental, computer-assisted techniques are compared with the results offered by a finite element model.

1

R. Jones, C. Wykes, "Holographic and Speckle Interferometry", Cambridge University Press, 1989.

2

D. Borza, I. Nistea, "Identification par voie optique des concentrations de contraintes à l'origine des défaillances d'une carte électronique embarquée", Neuvième colloque international francophone du club SFO/CMOI, Nantes, France, CD-ROM, 2009.

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