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Study on the Coefficient of Thermal Expansion of Molybdenum-Copper Alloy

The coefficient of thermal expansion of molybdenum-copper alloys can be optimized to match different packaging requirements by adjusting the ratio of molybdenum-copper. 

The manufacturing process has a significant impact on the uniformity and stability of the CTE, and precise control of the CTE can help improve the reliability of electronic packaging. With the development of advanced packaging technology, molybdenum copper will play a more important role in high heat load electronic devices.

The coefficient of thermal expansion (CTE) represents the rate of dimensional change of a material at temperature, usually measured in X 10⁻⁶/K. The CTE of molybdenum copper is between pure molybdenum (about 5.1 X 10⁻⁶/K) and pure copper (about 16.5 X 10⁻⁶/K), and the ratio of molybdenum to copper can be adjusted to achieve different thermal matching needs. 

The CTE of molybdenum copper depends mainly on the content of molybdenum and copper, and the typical CTE values for different composition ratios are as follows:

Mo70Cu30: about 6.5-7.5 X 10⁻⁶/K

Mo60Cu40: about 7.5-8.5 X 10⁻⁶/K

Mo50Cu50: about 9.0-10.0 X 10⁻⁶/K

Mo40Cu60: about 10.5-12.0 X 10⁻⁶/K

The higher the copper content, the greater the CTE, while the higher the molybdenum content, the lower the CTE.

Influencing factors

(1) Ingredient ratio

Due to the large difference in CTE between molybdenum and copper (5.1 X 10⁻⁶/K vs. 16.5 X 10⁻⁶/K), the thermal expansion coefficient of the alloy can be precisely controlled by adjusting the molybdenum-copper ratio to suit different packaging needs. 

(2) Manufacturing process

Powder metallurgy process: The uniformity of the CTE may be affected due to the possible presence of microscopic porosity.

Liquid-phase sintering process: improve the density of the material and make the coefficient of thermal expansion more stable.

HIP (Hot Isostatic Pressing) treatment: Reduces porosity and improves material uniformity to optimize CTE performance.

(3) Operating temperature range

CTE is not a fixed value and varies with temperature. In general, the CTE of molybdenum-copper is stable in the range of room temperature to 300°C, but at higher temperatures, the copper phase may cause the CTE to rise.

(4) Microstructure of materials

Fine, uniform copper phase distribution helps to reduce local uneven expansion.

Materials with high porosity are prone to uneven expansion during heating, which affects the stability of the package.

The importance of molybdenum-copper alloy thermal expansion matching

In electronic packaging, CTE matching of chips and packaging materials is critical to reduce thermal cycling stress and improve device reliability. For example:

The CTE of silicon (Si) chips is about 3.0 x 10⁻⁶/K, and Mo70Cu30 is recommended.

The CTE of gallium nitride (GaN) chips is about 5.6 x 10⁻⁶/K, and Mo70Cu30 or Mo60Cu40 can be selected.

The CTE of gallium arsenide (GaAs) chips is about 6.0 x 10⁻⁶/K, and Mo60Cu40 is suitable.

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