A Case Study of First-Level Die Attach with Nano-Ag Paste

Abstract: Because of the expectation that the current exemption from the EU RoHS Directive for solders with more than 85% Pb will not be extended beyond 2016, the identification of a practicable lead-free alternative to high-lead high-melting-point solders has been one of the most urgent challenges for the electronic component industry. So far all attempts to formulate a Pb-free solder alloy that could replace the high-lead solders have involved compromises in the ease of processing and/or service reliability. Since nano-Ag can be sintered at temperatures similar to those used in the reflow of high-lead solders, it has been increasingly considered as one of the most promising replacement candidates. Because once sintered the melting point returns to the 961.8°C of bulk Ag, joints made with nano-Ag have physical and mechanical properties comparable with those of Ag. That means that joints sintered with nano-Ag have higher reliability at elevated temperature and in thermal cycling, higher electrical and thermal conductivity compared with the conventional high-Pb solders. In a previous paper, the authors reported the stabilization of reactive nano-Ag particles by alcohol derivatives and compared the performance of joints between Cu and Si in thermal cycling with those made with Pb-5Sn. The cost of making joints with nano-Ag can be reduced by the addition of micro sized particles of Cu and in this paper the authors will present data obtained in a successful commercial application of this combination in semiconductor first-level die attach. Basic physical properties and performance in thermal cycling test will be reported and compared with the properties and performance of joints made with pure nano-Ag and Pb-5Sn.