Custom Case Study
Our customer was designing a pulsed application that required a high power 1000 W resistor at high frequencies. Frequent cycling of high power can cause the different materials in the resistor to expand and contract rapidly at different rates. As a result, this thermal mismatch in the coefficient of thermal expansion (CTE) of the various materials can have very destructive effects, including device / system failure.
Standard flanged resistors in these types of applications typically fail after 600 cycles to 1000 cycles, while the customer in this application required significantly higher reliability for the design to be successful.
Vishay Barry needed to ensure that all materials and manufacturing processes involved were optimized to create a viable resistor solution with a consistent CTE. To achieve this, the Vishay Barry design team recommended two major changes.
- Utilizing a copper tungsten (CuW) flange: the CTE of a CuW flange is far closer to that of the resistor chip than a standard Cu flange. This choice minimizes the mismatch when under expansion / contraction at high pulse power and eliminates susceptibility to mechanical stress
- Incorporating gold-germanium (Au-Ge), flux-free brazing in a vacuum chamber: this minimizes the CTE mismatch and increases thermal conductivity. An Au-Ge eutectic is used to bond the chip’s substrate to the flange. The Au-Ge layer is chemically compatible with the Au-plated adjoining layers, and no embrittling metallics will form. The Au matrix exhibits outstanding ductility, corrosion resistance, low diffusivity, and low toxicity, as opposed to other materials like lead
The combination of the CuW flange, Au-Ge braze, and gold plating significantly reduced thermal stresses in the final device. Through careful material selection and device design, Vishay Barry minimized the CTE difference, increased fatigue resistance, and maximized thermal conductivity.
The final result was a significant improvement in reliability from 600 thermal cycles to over 500k, far surpassing the customer’s minimum requirement. This CuW “e-type” solution is now available as the industry-standard solution for any high thermal cycle application at DC or RF.
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