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Why Should We Do a Protective Coating for Molybdenum-copper Alloy (MoCu alloy)?

Molybdenum-copper alloy (MoCu alloy) is a pseudo-alloy composed of two insoluble metals molybdenum and copper. As a result, it gains characteristics both of these two metals, such as high thermal conductivity, low thermal expansion coefficient, non-magnetic, low gas content, good machining performance and special high temperature performance.

What is the protective coating?

In order to improve the high temperature resistance and welding performance of finished products, there can be a layer of coating on the surface, usually nickel or gold. Displacement plating, chemical roughening, chemical nickel plating, or cyanide gold plating are the most common used methods. 

We use scanning electron microscope and EDS to analyze its morphology and composition of the coating, and test welding and high temperature resistance by gold-tin welding according to GJB1941-94. The results show that the displacement plating method achieves the uniformity of the chemical properties of element Mo and element Cu at the interface of the Mo-Cu alloy substrate, and avoids the problem of excessive internal stress in the deposited crystal due to different chemical activities; chemical roughening increases the specific surface area at the interface of the substrate, increases the number of "mechanical bites" at the interface of the coating and the substrate, and improves the bonding strength of the coating and the substrate.

The high temperature resistance of the gold plating layer obtained on the surface of the MoCu substrate is ≥350℃, and the void ratio after welding is ≤30%, which meets the requirements of welding technical indicators. 

Molybdenum-copper alloy has high thermal conductivity, up to about 200W/m·K, and has the advantages of small thermal expansion coefficient and easy machining. It is used in spacecraft phased array antenna T/R module bottom plate and high-power microwave shell and power device carriers and other fields that have good application prospects. However, its surface wettability is poor and cannot meet the soldering functional requirements of solders such as PbSnAg, SnAgCu and AuSn.

In order to improve the weldability, it is necessary to coat a gold plating layer with excellent welding performance on the surface. however, as the surface composition of the molybdenum-copper alloy is composed of two metal elements with relatively different chemical properties, and the content of chemically inert Mo element is as high as 85%, it is difficult to coat the gold coating on the surface.

In aerospace products, the solders used in the early days were mainly low melting point solders such as PbSnAg and InSn. The peak temperature during the soldering process was all below 230°C, so the high temperature resistance of the gold coating is relatively low. With the development of T/R module housing and chip packaging technologies, high melting point solders represented by AnSn have been widely used. The peak soldering temperature is as high as 300℃. 

During the high-temperature soldering process, the gold plating layer and the molybdenum copper alloy substrate will expand due to thermal expansion. The thermal stress caused by the coefficient mismatch increases, resulting in quality problems such as skinning and blistering of the gold coating. Therefore, in order to meet the welding requirements of spacecraft stand-alone load products, higher requirements are put forward for the preparation and high temperature resistance of the gold coating on the surface of the MoCu alloy.

There are many grades for moly copper alloy, such as Mo85Cu15, Mo80Cu20, Mo70Cu30, Mo60Cu40, Mo50Cu50, Mo40Cu60. Different grades match different densities, thermal conductivities and coefficients of thermal expansion. Other customized grades also can be customized.

Chinatungsten is a professional manufacturer of molybdenum for more than two decades, if you have got any purchasing plan for related products, please feel free to contact us, we will offer you our most favorable prices and qualitied products. 

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