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

• X-ray projection nanotomography was performed for the first time on micro-bumps of different sizes (25 μm and 80 μm).
• Based on intermetallic (IMC) volume quantification, this method gives good accuracy in average IMC thickness evaluation.
• Comparisons of growth kinetics of reaction product between samples undergoing different heat treatments were made.
• The analysis of the IMC volume and morphology for bumps of different sizes revealed the same growth kinetic behavior.

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 (μ-bumps) used for flip chip packaging, with bumps diameters as small as 25 μm. 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 μ-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 μ-bumps microstructure was studied by using copper pillars of different diameter.

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