Improved Manufacturing Process of Cu/Mo70-Cu/Cu Composite Heat Sinks for Electronic Packaging Applications

Abstract

Looking back at the development of various thermal management materials, one can see the limit imposed by nature or by physical laws. Single-element materials, like Al and Cu, have their advantages and disadvantages. Composite materials like CuW, CuMo, AlSiC, and Al-graphite are a tradeoff between the thermal conductivity (TC) and coefficient of thermal expansion (CTE). Using CuW as an example, a higher Cu content will increase the TC and lower the weight (both are desirable). On the flip side, a higher Cu content will also lead to a higher CTE (undesirable). CuW has many compositions like 50/50, 60/40, 70/30, 80/20, 85/15, 87/13, 89/11, and 90/10. The various compositions reflect the compromise between the conflicting needs for CTE and TC. It was almost impossible to improve both CTE and TC until a Cu/Mo70Cu/Cu laminate material was developed. Cu/Mo70Cu/Cu is a laminate material with Mo70Cu sandwiched by two thin layers of Cu. During the rolling lamination process, Mo particles inside the Mo70Cu composite is elongated in the X-direction (the rolling direction). The resultant Cu/Mo70Cu/Cu is anisotropic. The X-direction has a lower CTE (like 7.0 ppm/°C for a 1:4:1 ratio), and the Y-direction has a higher CTE (~9.0 ppm/°C for a 1:4:1 ratio). Material physics still works here, coupled with relatively high TC (up to 300 W/mK for through thickness), but this unique material feature makes it very attractive for long and thin dies like LDMOS. This chapter provides a detailed discussion of the background and fabrication of this magical material.