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Development of a Ag/glass die attach paste for high power and high use temperature applications
Keywords: Ag / glass, SiC, 300C Continuous operating temperature
The power densities of certain semiconductor devices, such as SiC, GaN, and the like, are increasing with requirements for higher continuous use temperatures with high thermal properties. For example, one application is for a continuous use temperature of 300°C with SiC devices with a die attach process at about 370°C. This paper describes development of an Ag/glass die attach paste, which demonstrates high reliability, for high temperature continuous use. Organic adhesives, such as Ag epoxies, thermoplastics, etc. are not applicable because of high temperature degradation at 300°C. Existing Ag/glass adhesives do not meet the requirements of high continuous use temperature, with lower die attach temperature. For example, DM3030 Ag/glass employs a vitreous glass which was designed for low temperature processing but does not have the 300°C required adhesion properties because of the low Tg of the glass. DM3355 contains a crystallizing glass designed for low temperature remelt that results in a crystallized structure upon cooling with a more robust structure (with higher reliability and good temp-cycle resistance). However, the crystalline remelt temperature is too low to result in high 300°C adhesion; once the crystalline phase remelts, adhesion decreases significantly. Described in this paper is the development of a unique crystallizing glass that has a crystalline remelt temperature of greater than 300°C and less than about 370°C. During the die attach process, the crystallized glass melts at about 350°C which wets the surfaces of the die and substrate. During the cool down of the die attach process, the glass crystallizes creating a robust structure with a remelt temperature greater than 300°C. Therefore, die adhesion remains high for a 300°C continuous use temperature, which is a requirement for this application. Another key requirement of a die attach material for use with SiC devices is power dissipation; i.e. heat dissipation. Very low interfacial thermal resistance has been demonstrated in this paper. Also shown is the impact of key oxide additions which result in a significant reduction of thermal resistance consistently to as low as 0.01 C*cm2/watt, as measured by the laser flash method. An inherent advantage of glass as the adhesive agent is its ability to wet oxide or metal surfaces. Thus, this allows the option to use bare die and substrates in lieu of metalized surfaces in certain applications, with the potential for significant cost savings. This new technology replaces the higher cost solder alloys and provides a high reliability option that meets the requirements for SiC device packaging.
Maciej Patelka, Development Engineer
NAMICS North American R&D Center - Diemat, Inc.
Byfield, Massachusetts

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