Method for bonding a member made of a niobium titanium alloy to another member using an active solder

The use of niobium titanium alloys in coaxial cables can cause problems in the solder joint between coaxial cables and connectors. This is because niobium titanium alloys have surface oxides that are difficult to wetting with solder alloys, even when an aggressive flux is applied to remove the oxides. As a result, the joint fails significantly more often than the solder joint formed between a coaxial cable and connector made of a non-niobium titanium alloy. Because the need to minimize insertion losses while minimizing heat transfer makes it impractical to use alternative materials for coaxial cables, an alternative means of connecting cables to connectors is needed.

One such alternative is to electroplate a coaxial cable made of niobium titanium alloy with nickel or gold layers so that the nickel/gold layers can then be welded to the connector. However, we found that this method was not appropriate because a failure of the joint between the nickel layer and the niobium titanium layer would result in a failure of the entire joint.

Two other methods have also been tested. The first is based on a mixing process in which a copper sleeve and a beryllium-copper tube are pressed onto a coaxial cable and then the beryllium-copper tube (as the outer layer on the coaxial cable) is welded to a gold-plated beryllium-copper connector. This process improves the joint strength compared to the previous method. However, the joint still shows low tensile strength and is therefore unreliable due to the risk of joint failure. Another second method is the standard press joint. The method involves placing the connector around the bare wire of a coaxial cable and compressing the connector so as to deform the connector and grip the bare wire, thereby generating electrical contact. The collet at the end of the connector is then placed on the coaxial cable and the electrical contact is established by tightening the nut on the collet. This technology works well at room temperature, but is not suitable for applications where connectors must operate over a wide temperature range, including temperatures as low as 4K.