Novel methodology for real time thermal expansion characterization on ball grid array substrate stack-up materials

Abstract

When a plastic package is subjected to reflow, deformation and deviation from an ideal uniform planar flatness usually results. In the case of ball grid array (BGA) packages, warpage is caused by the thermal expansion mismatch between an organic substrate and other components constituting the package assembly. Substrate stack-up is mainly constituted of copper and core material. An accurate estimation of the coefficient of thermal expansion (CTE) of substrate materials becomes necessary for increasing package reliability during soldering reflow. This paper deals with in-situ characterization of in-plane deformation by adopting a methodology based on the digital image correlation (DIC) principle. First, the CTE measurement by DIC is validated by analysing copper laminates and comparing results with literature values. Then, the evaluation is performed for different components of the BGA substrate stack-up. The analysis allows to determine the CTE of these materials at various temperatures encountered during ball attach reflow process. Experimental results help to investigate the impact of the different components on the thermo-mechanical properties of organic substrates. In particular, it is found that copper has a major contribution during the heating-up stage of the soldering process, while core material behaviour becomes prominent during cooling-down.