Effects of punch geometry and grain size in micron scale compression molding of copper

Abstract

Mechanical size effects at small characteristic dimensions are of intense current interest. In addition to the relevance for microfabrication, microscale compression molding provides a rich laboratory in which mechanical size effects manifest in interesting ways, especially as the characteristic molding dimension approaches the average grain size of the material being deformed. We investigate the effects of punch geometry and grain size on the mechanical response and gap filling behavior in micron scale compression molding. Micron scale long rectangular single-, double-, and triple- punch molding was conducted on polycrystalline Cu with varying grain sizes. Mechanical size effects were observed regarding the compression pressure and the material flow to fill the gaps between neighboring punches. The nominal compression pressure depended significantly on the punch width in single-punch molding and on the spacing between neighboring punches in double-punch molding. Analysis of double- and triple- punch molding data showed that both the punch geometry and the grain size of the molded Cu influence the Cu gap filling behavior in significant ways. Detailed microstructural examinations of molded Cu features were carried out to elucidate the underlying mechanisms. Our findings offer new data on micron scale plasticity and test cases for validating micron scale plasticity models.