Improved Manufacturing Process of Cu/Mo70-Cu/Cu Composite Heat Sinks for Electronic Packaging Applications
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/ˇăC for a 1:4:1 ratio), and the Y-direction has a higher CTE (~9.0 ppm/ˇăC 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.
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