Failure Modes and Mechanisms of RF Coaxial Connectors
RF coaxial connectors come in a variety of types, such as APC-7, N, TNC, BNC, A-PC3.5, SMA, SMB, and SMC. Various adapters between these connectors are also widely used. Despite there being hundreds of connector types, many reliability issues are common. This paper focuses on the failure modes and mechanisms of threaded RF connectors like N-type and SMA connectors.
Based on our years of experience, the common failure modes include the following:
1. Connection Failures
(1) Mating Errors
Occasionally, users mistakenly identify the internationally standard SMA connector as China's L6 connector due to their similar sizes and shapes. For instance, the nut size for China’s L6 connector is M6, while the internationally standardized SMA connector nut is 1/4" x 36 threads (equivalent to M6.35).
Users who mistakenly connect an SMA plug to an L6 jack often find it too loose to work properly. Conversely, attempting to connect an L6 plug to an SMA jack results in a failure to tighten due to a size mismatch. Similar issues occur between China's L16 connector and the internationally recognized N-type connector.
(2) Loose Connection Nuts
Loose connection nuts are common, particularly with small connectors such as SMA connector and TNC connector types. The possible causes are:
a. Material selection issues: Inadequate elasticity of retaining ring materials leads to nut detachment.
b. Machining errors: Grooves for installing retaining rings on nuts are not deep enough, causing slippage under slight torque.
c. Incorrect material status: un-temper beryllium bronze may be mistakenly used, resulting in poor elasticity.
d. Excessive tightening torque: Over-tightening beyond specified limits causes nut detachment. Correct torque wrenches should be used.
(3) Nuts Cannot Be Fully Tightened
Possible causes include:
a. Mismatching connectors: N-type plugs connected to L16 jacks often fail to tighten.
b. Thread errors: Excessive thread angle errors or excessive plating thickness after plating.
c. Poor machining: Lack of specialized tools and gauges for processing imperial threads leads to poor interchangeability.
(4) Loose or Detached Inner Conductors
Some designs use threaded connections for inner conductor dielectric supports, which may loosen or detach after repeated mating.
2. Reflection Failures
(1) Increased Reflection
When the interface between SMA connectors is filled with dielectric, large temperature changes may cause the dielectric to protrude or retract beyond the specified limit, leading to increased reflection.
(2) Open Circuit
Jack contact made from beryllium copper or phosphor bronze may develop open circuits due to excessive hardness during heat treatment or stress concentration at the slotted root, causing spring tabs to break.
(3) Short Circuit
a. Small cable connector assembly: The close spacing between inner and outer conductors may result in accidental contact when the shield touches the inner conductor.
b. Excessive solder during assembly: Solder may flow from the outer conductor to the inner conductor, causing a short circuit.
3. Electrical Contact Failures
(1) No Contact Between Pins and Sockets
Jack parts are typically made from beryllium copper or phosphor bronze, slotted, and heat-treated to ensure good electrical contact. Skipping the slotting step before heat treatment can result in non-contact during assembly.
(2) Poor Contact
This issue causes signal instability and fluctuating performance. Common causes include:
a. Loss of elasticity: Elastic components may become loose after repeated mating, reducing contact pressure.
b. Poor soldering: Cold solder joints in the inner conductor or loose crimping of the outer conductor can cause contact failure during bending or twisting.
(3) Corrosion
RF coaxial connectors are typically made of copper alloys plated with gold, silver, or nickel. Poor plating processes can lead to corrosion and reduced durability. For example, inadequate protection of silver plating can result in blackening and increased contact resistance.
Design flaws may also contribute to corrosion. Although gold is highly stable, poor design (such as using a silver underlayer for gold plating) may cause corrosion due to micro-pores allowing silver molecules to diffuse to the surface.
To prevent corrosion, military standards prohibit silver as an underlayer for gold-plated parts. Standard plating should include copper, followed by nickel, and then gold. Some manufacturers skip the copper layer to reduce costs, compromising bonding strength and corrosion resistance.
4. Contamination Failures
(1) Reduced Insulation Resistance
Aging or impurity contamination of the dielectric, cracking, or electrical breakdown can cause reduced insulation resistance.
(2) Reduced Dielectric Strength
Incomplete cleaning of parts before assembly, metal burrs, or embedded sharp edges in the insulator may lower dielectric strength. Excessive or deep riveting can also crack or puncture the insulator.
Conclusion
The above failure modes are not isolated; they are interconnected. For example, poor contact between pins and sockets can increase contact resistance and lead to reflection failures. Similarly, connection failures can cause reflection issues. Therefore, any failure may result in the complete failure of the connector.
In addition to the mentioned failure modes, issues such as skipped machining steps or dimensional errors may occur, leading to assembly errors. These occasional failure factors are not discussed in detail.
