Die Attachment on Bare Copper Surface by Non-Pressure Silver Sintering in Inert Atmosphere

Abstract

Silver sinter material has been considered as a promising alternative die attach material for high-lead solder as it possesses excellent properties such as high melting temperature, high thermal and electrical conductivity. This paper focuses on the development of a safe-to-use micro-silver sinter paste (mAgic DA250) for non-pressure sintering on Cu surface in inert atmosphere as a possible drop-in solution for high-lead solder material. To be compatible to the present manufacturing process, this newly developed sinter paste is designed to be able to continuously dispense over 8 hours. We observed no missing pattern, tailing and needle clogging over 8 hours continuous dispensing. Ag metallized Si die was attached on bare Cu TO 220 lead frame by non-pressure sintering process in a convection batch oven. X-ray measurement was performed on the sintered samples showing that no void was formed in the sintered layer. An average die shear strength of 18 N/mm2 was achieved with the failure mode of cohesive break in the sintered layer. Silver sintered layer can be found on both the die backside and Cu lead frame surface after Si die was sheared indicating that strong adhesion was generated on both Ag and Cu surfaces. A rather dense sintered layer is strongly bonded onto Ag metallized die and Cu lead frame which is evidenced by cross-sectional SEM images. Additionally, the EDX result illustrates an interdiffusion of Ag and Cu occurring at the interface between silver sintered joint and Cu lead frame. Pressure cooker test with the test conditions of 121°C, 100% RH was performed for the unmolded sintered samples and a slight decrease in die shear strength from 18 N/mm2 to 15 N/mm2 was observed after 96 hours test demonstrating that the sintered joint can withstand the extremely high humidity condition. To study the effect of the paste storage conditions on the die shear strength, mAgic DA250 were stored separately in a freezer (−40°C) and a refrigerator (2-10°C) and subsequently used for nonpressure sintering process. A relatively similar die shear strength was obtained showing that mAgic DA250 sinter paste can be flexibly stored in either a freezer or a refrigerator.