SMA Adapter Construction
One only must type “SMA Adapter” in a browser to find thousands of choices. The multitude of options can be daunting, especially if they are all visually very similar. Further study reveals that SMA Adapter construction varies within various manufacturers and distributors. We will focus on adapter contacts to determine which material meets the requirements of a specific application. It will be possible to identify the proper adapter for an application by understanding the pros and cons of Adapter contacts are known.
Contact Base Materials
A connector or adapter contact will have a greater impact on the interconnect’s durability and survivability more so than the body. A Female contact is the most critical between the mating contacts because it has a spring mechanism that is subject to wear. Let’s understand a Male and Female contact.
• Male Contact – There are two main metals that are used for a Male contact, brass and Beryllium Copper (BeCu). Brass is by far the most material for Male contacts, while Beryllium Copper is used for smaller contacts, such as an SMP interface. Brass is much easier to machine, lower cost, and it hardness is acceptable for many application. It is the preferred Male contact material for standard coax connections (BNC, DIN 7/16, Type N, TNC, etc.), miniature connectors (MCX, SMA, SMB, SSMA, etc.), and micro-miniature connectors (I-PEX MH4, MMCX, SSMB, SSMC, etc.). A Brass Male contact can easily handle 500 mates and demates, but BeCu may be used in situations where more durability is required, such as 5000 mates and demates, due to BeCu’s increase hardness. BeCu is also the material of choice for Male contacts for blind-mate connections (BMA, SMP, SMPM, SMPS, etc.) where contact hardness is required to prevent bending or breaking a contact in mis-aligned connections.
Figure 1 SMA Male Contact
• Female Contact – It is the Female contact where proper selection is needed. Brass and BeCu are both used for standard, miniature, and micro-miniature connectors but the Female contact’s elasticity and durability will be greatly affected by the base material. Brass, for example, has a very low elastic limit, thus it will only be able to handle a few mates and demates before the Female contact’s tines (springs) are deformed and will no longer return to their original state. In other words, Female contact tines will spread when mated and will not return to their original closed state when demated. The Female contact tines will eventually spread wide enough to cause a poor or no contact when mated to the Male contact.
Figure 2 SMA Female Contact
BeCu is the material of choice for Female contacts for its elastic resilience. BeCu will not deform even if cycled 500 times.
Contact Plating
Connector and adapter contacts can be plated with various material, with gold being the most common. We will cover the various types of plating materials and thicknesses for commercial and high reliability applications, such as defense and space.
• Hard Gold: Hard gold plating stands out as a widely preferred option for coaxial contacts, boasting exceptional attributes such as excellent corrosion resistance, low contact resistance, and high thermal conductivity. The robust gold surface ensures durability and wear resistance, rendering it well-suited for repeated mating and unmating cycles. However, it is crucial to note the various versions of plating available to ensure the selection of the most appropriate one. Among the diverse plating specifications, MIL-DTL-45204 emerges as the gold standard for durability, encompassing several levels of plating thicknesses tailored to specific applications. In the subsequent discussion, we will delve into these MIL-DTL-45204 levels and explore additional commercial plating options to provide a comprehensive understanding of the available choices.
o MIL-DTL-45204 Class 0, featuring a coating of 30 micro-inches of gold over nickel, finds widespread use in general defense and industrial applications. This class strikes an effective balance between durability and cost, making it a practical choice for applications requiring as many as 500 mating cycles.
o MIL-DTL-45204 Class 1, which is 50 micro-inches over nickel, is a more durable solution that well suited for high mate/demate applications, such as in a test lab and for military applications for applications that see over 500 mates.
o Gold Flash is seen in commercial applications. It is a thin plating of gold, usually 10 micro-inches or less and provides the least durability and greater chance for corrosion. It has the best cost performance and is used in applications where the connection will only experience a few mates. An example of an application would be a low-cost commercial antenna application that will only be tested then connected once.
• Gold-Flashed Palladium-Nickel (80/20): It offers the advantages of both gold and palladium-nickel, providing good electrical conductivity and moderate wear resistance. Frequently employed in data transmission applications through electronic connectors, it strikes a balance between efficient signal transfer and durability. The incorporation of a gold flash layer not only enhances surface properties but also contributes to improved conductivity. Meanwhile, the palladium-nickel base ensures stability, making this plating an optimal choice for reliable and enduring performance in electronic connectivity, particularly in scenarios demanding both signal integrity and robustness.
• Copper-Tin-Zinc Alloy: This ternary alloy, consisting of copper, tin, and zinc, serves as a plating material to nickel. It provides a well-balanced set of properties and is frequently employed in RF connectors. Specifically chosen for Low-PIM (Passive Intermodulation) applications, it eliminates nickel and its magnetic properties, mitigating non-linear effects on communication signals. Industry terms for this ternary alloy include white bronze, tri-metal, Copper-Tin-Zinc, and tri-alloy.
• Silver known for its exceptional electrical and thermal conductivity, outshines all other metals in these properties. However, it does tend to tarnish, resulting in the formation of a thin layer of silver sulfide on its surface. Surprisingly, this tarnish layer acts as an electrically resistant barrier, affecting radio frequency (RF) signals. In the past, silver was commonly used for RF connectors over 50 years ago. However, due to the electrical resistance caused by tarnishing, it has been gradually replaced with other plating. Despite this, silver still finds its place in larger and higher-power applications where its unique properties remain advantageous.
Conclusion
Choosing a connector or adapter involves numerous options. Without a clear understanding of the distinctions among metals and plating, decisions might rely solely on price and frequency considerations. Making selections based on these limited criteria can result in adapters or connectors with intermittent performance or potential electrical opens in the future. In various instances, thousands of devices have required retesting due to suboptimal connections, emphasizing the critical importance of informed decision-making in connector and adapter selection.
