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Improved Process for High Temperature Large-Area Sintered Nanosilver Device Attachment
Keywords: large-area die-attachment, high temperature joint, nanosilver sintering
Sintered nanoscale silver joint provides a high-temperature alternative to solder materials due to its inherently higher melting point. It has been successfully shown to provide bond strengths comparable, if not superior, to those of solder. For large area joining, due to the nature of the paste and the sintering process, the issue of proper and complete drying needs to be addressed as defects generated during drying will carry over into the final joint microstructure. These defects in turn will affect the joint physical, electrical and mechanical performance. To bond large devices, an external pressure is usually applied, of the order of a few megapascals. Having the added requirement of pressure complicates the joining process but may be necessary to obtain reliable and reproducible bonding. Eliminating the need for pressure during some critical stage of the process will therefore be advantageous. For example, if the parameters can be adjusted to eliminate pressure during the sintering stage, it would greatly simplify the process and lower the cost. This study is focused on the effect of key hot pressing parameters (drying and sintering pressure and time) on the final bond quality of sintered nanosilver die-attachments. Samples of large area (10 mm by 10 mm) die-attachments are made by nanosilver joint sintering under different combinations of these parameters and bonding qualities are evaluated by die-shear testing. The importance and the interaction between the parameters are analyzed by fractional factorial design experiments. Based on the analysis, an improved sintering process was developed in which pressure was applied only during drying. Drying for less than 10 minutes under 3 MPa pressure at 180°C with no pressure during sintering was found to be satisfactory with die shear strengths exceeding 30 MPa.
Kewei Xiao, Research Assistant
Virginia Tech
Blacksburg, Virginia

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