Failure Analysis and Improvement of RF Coaxial Connector

Failure Analysis and Improvement of RF Coaxial Connector

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As an important part of passive components, RF coaxial connectors have good broadband transmission characteristics and a variety of convenient connection methods, so they are widely used in test instruments, weapon systems, communication equipment and other products. Since the application of RF coaxial connectors has penetrated almost all sectors of the national economy, its reliability has also attracted more and more attention. The failure modes of RF coaxial connectors are analyzed.

After the N-type connector pair is connected, the contact surface (electrical and mechanical reference plane) of the outer conductor of the connector pair is tightened against each other by the tension of the thread, so as to achieve a small contact resistance (<5m Ω). The pin part of the conductor in the pin is inserted into the hole of the conductor in the socket, and good electrical contact (contact resistance<3m Ω) is maintained between the two inner conductors at the mouth of the conductor in the socket through the elasticity of the socket wall. At this time, the step surface of the conductor in the pin and the end face of the conductor in the socket are not tightly pressed, but there is a gap of<0.1mm, which has an important impact on the electrical performance and reliability of the coaxial connector. The ideal connection state of the N-type connector pair can be summarized as follows: good contact of the outer conductor, good contact of the inner conductor, good support of the dielectric support to the inner conductor, and correct transmission of the thread tension. Once the above connection status changes, the connector will fail. Let’s start with these points and analyze the failure principle of the connector to find the correct way to improve the reliability of the connector.

1. Failure caused by poor contact of outer conductor

In order to ensure the continuity of electrical and mechanical structures, the forces between the contact surfaces of external conductors are generally large. Take N-type connector as an example, when the tightening torque Mt of the screw sleeve is standard 135N. cm, the formula Mt=KP0 × 10-3N. m (K is the tightening torque coefficient, and K=0.12 here), the axial pressure P0 of the outer conductor can be calculated to be 712N. If the strength of the outer conductor is poor, it may cause serious wear of the connecting end face of the outer conductor, even deformation and collapse. For example, the wall thickness of the connecting end face of the external conductor of the male end of the SMA connector is relatively thin, only 0.25mm, and the material used is mostly brass, with weak strength, and the connecting torque is slightly large, so the connecting end face may be deformed due to excessive extrusion, which may damage the inner conductor or dielectric support; In addition, the surface of the outer conductor of the connector is usually coated, and the coating of the connecting end face will be damaged by large contact force, resulting in an increase in the contact resistance between the outer conductors and a decline in the electrical performance of the connector. In addition, if the RF coaxial connector is used in a harsh environment, after a period of time, a layer of dust will be deposited on the connecting end face of the outer conductor. This layer of dust causes the contact resistance between the outer conductors to increase sharply, the insertion loss of the connector increases, and the electrical performance index decreases.

Improvement measures: to avoid bad contact of the outer conductor caused by deformation or excessive wear of the connecting end face, on the one hand, we can select materials with higher strength to process the outer conductor, such as bronze or stainless steel; On the other hand, the wall thickness of the connecting end face of the outer conductor can also be increased to increase the contact area, so that the pressure on the unit area of the connecting end face of the outer conductor will be reduced when the same connecting torque is applied. For example, an improved SMA coaxial connector (SuperSMA of SOUTHWEST Company in the United States), the outer diameter of its medium support is Φ 4.1mm reduced to Φ 3.9mm, the wall thickness of the connecting surface of the outer conductor is correspondingly increased to 0.35mm, and the mechanical strength is improved, thus enhancing the reliability of the connection. When storing and using the connector, keep the connecting end face of the outer conductor clean. If there is dust on it, wipe it with alcohol cotton ball. It should be noted that alcohol should not be soaked on the media support during scrubbing, and the connector should not be used until the alcohol is volatilized, otherwise the impedance of the connector will change due to the mixing of alcohol.

2. Failure caused by poor contact of inner conductor

Compared with the outer conductor, the inner conductor with small size and poor strength is more likely to cause poor contact and lead to connector failure. Elastic connection is often used between internal conductors, such as socket slotted elastic connection, spring claw elastic connection, bellows elastic connection, etc. Among them, the socket-slot elastic connection has simple structure, low processing cost, convenient assembly and the widest application range.

Improvement measures: We can use the insertion force and retention force of the standard gauge pin and the conductor in the socket to measure whether the matching between the socket and the pin is reasonable. For N-type connectors, diameter Φ 1.6760+0.005 The insertion force when the standard gauge pin is matched with the jack should be ≤ 9N, while the diameter Φ 1.6000-0.005 standard gauge pin and conductor in the socket shall have a retention force ≥ 0.56N. Therefore, we can take the insertion force and retention force as an inspection standard. By adjusting the size and tolerance of the socket and the pin, as well as the aging treatment process of the conductor in the socket, the insertion force and retention force between the pin and the socket are in a proper range.

3. Failure caused by failure of dielectric support to support inner conductor well

As an integral part of the coaxial connector, dielectric support plays an important role in supporting the inner conductor and ensuring the relative position relationship between the inner and outer conductors. The mechanical strength, thermal expansion coefficient, dielectric constant, loss factor, water absorption and other characteristics of the material have an important impact on the performance of the connector. Sufficient mechanical strength is the most basic requirement for the dielectric support. During the use of the connector, the dielectric support should bear the axial pressure from the inner conductor. If the mechanical strength of the dielectric support is too poor, it will cause deformation or even damage during the interconnection; If the thermal expansion coefficient of the material is too large, when the temperature changes greatly, the dielectric support may expand or shrink excessively, causing the inner conductor to loosen, fall off, or have different axis from the outer conductor, and also causing the size of the connector port to change. However, water absorption, dielectric constant and loss factor affect the electrical performance of connectors such as insertion loss and reflection coefficient.

Improvement measures: select appropriate materials to process the medium support according to the characteristics of the combination materials such as the use environment and working frequency range of the connector.

4. Failure caused by thread tension not transmitted to outer conductor

The most common form of this failure is the falling off of the screw sleeve, which is mainly caused by the unreasonable design or processing of the screw sleeve structure and the poor elasticity of the snap ring.

4.1 Unreasonable design or processing of screw sleeve structure

4.1.1 The structure design or processing of the screw sleeve snap ring groove is unreasonable

(1) The snap ring groove is too deep or too shallow;

(2) Unclear angle at the bottom of the groove;

(3) The chamfer is too large.

4.1.2 The axial or radial wall thickness of the screw sleeve snap ring groove is too thin

4.2 Poor elasticity of snap ring

4.2.1 The radial thickness design of snap ring is unreasonable

4.2.2 Unreasonable aging strengthening of snap ring

4.2.3 Improper material selection of snap ring

4.2.4 The outer circle chamfer of snap ring is too large. This failure form has been described in many articles

Taking N-type coaxial connector as an example, several failure modes of screw-connected RF coaxial connector which is widely used are analyzed. Different connection modes will also lead to different failure modes. Only by in-depth analysis of the corresponding mechanism of each failure mode, it is possible to find an improved method to improve its reliability, and then promote the development of RF coaxial connectors.


Post time: Feb-05-2023