Reflow processes in micro-bumps studied by synchrotron X-ray projection nanotomography

In microelectronics, the trend to lower the device dimensions forces to adapt the characterization tools. Here, we report a 3D characterization study of SnAgCu alloys in micro-bumps (@m-bumps) used for flip chip packaging, with bumps diameters as small as 25@mm. Such a study of thick, bulk samples requires penetrating radiation and consequently hard X-rays provided by synchrotron radiation have been used in a specific tomographic scheme. The intermetallics compounds (IMCs) growing at the copper/SnAgCu alloy interface are revealed at different reflow conditions by using holography coupled to X-ray projection nanotomography. This characterization method for such a system as @m-bumps allows here to render and measure the volume of IMC depending of the heat treatment conditions. At the same time, the influence of the bump size on the @m-bumps microstructure was studied by using copper pillars of different diameter.