Failure analysis of RF coaxial connectors and cable components

作者： 沃锌科技 时间：2023-11-28 15:58:14 阅读：16

1、 Introduction

With the rapid development of science and technology, the application scope of electronic devices is becoming increasingly widespread, almost penetrating into various sectors of the national economy, including public security, communication, medical and other fields. Therefore, the reliability of electronic devices is increasingly concerned and valued by people. Electrical contact components such as connectors and relays are one of the most commonly used components in electronic devices. According to incomplete statistics, an electronic computer, radar, or aircraft has tens of thousands of connection points, and the reliability of electronic equipment is closely related to the quantity and quality of the components used. Especially in electronic devices with serial structures, the failure of any component, device, or node may lead to local or individual system failures. This article focuses on analyzing the failure modes and mechanisms of RF coaxial connectors and cable components, and provides a detailed discussion on how to improve their reliability.

2、 Failure modes and mechanisms of RF coaxial connectors and cable components

Although there are many types of RF coaxial connectors currently used domestically and internationally, there are mainly three types of connections:

(1) Screw connection type: such as APC-7, N, TNC, SMA, SMC, L27, L16, L12, L8, L6 and other RF coaxial connectors. This type of connector has the characteristics of high reliability and good shielding effect, so it is also the most widely used.

(2) Bayonet connection type: such as BNC, C, Q9, Q6 and other RF coaxial connectors. This type of connector has the characteristics of convenient and fast connection, and is also the earliest form of RF connector connection applied internationally.

(3) Push in connectors, such as SMB, SSMB, MCX, etc., have the characteristics of simple structure, compactness, small size, and easy miniaturization.

Although there are many types of connectors, from a reliability perspective, many issues are the same. This article focuses on analyzing the failure modes and mechanisms of RF coaxial connectors with the most widely used and diverse threaded connection types. Based on our more than ten years of practice, the common main failure modes are as follows.

2.1 Connection Failure

(1) Connection nut detached

In daily life, some users have reported occasional detachment of connecting nuts, which affects normal operation, especially for small connectors such as SMA, SMC, and L6. After our analysis, there are generally the following reasons:

a. Improper material selection by designers, in order to reduce costs, resulted in the misuse of non elastic brass seat ring materials, making nuts prone to detachment.

b. During processing, the groove depth of the nut installation snap ring is not deep enough, so the nut will fall off with a slight torque during connection.

c. Although the material selection was correct, the process was unstable, and the beryllium bronze elastic treatment did not reach the hardness value specified in the drawing. The snap ring had no elasticity, resulting in the nut falling off.

d. During testing, the user did not use a torque wrench and instead used a regular wrench to tighten the nut, causing the tightening torque to significantly exceed the military standard value, resulting in the nut (snap ring) being damaged and falling off.

(2) Pairing error

Since the reform and opening up, the number of imported instruments in China has been increasing, and users often report that they mistakenly believe the Q9 cable head purchased in the market is the internationally common BNC cable head. Due to its exact shape and BNC cable head, with only slight differences in size, it is not compatible with imported BNC instruments, and these phenomena are also common.

(3) Loose or detached inner conductor

Some designers divide the inner conductor into two parts at the support of the inner conductor medium and then connect them together with threads. However, for small RF coaxial connectors, the size of the inner conductor itself is Φ 1-2mm, thread the inner conductor. If conductive adhesive is not applied at the threaded connection, the strength of the inner conductor connection will be very poor. Therefore, when the connector is connected multiple times, under the long-term action of torque and tension, the threads of the inner conductor become loose and fall off, causing the connection to fail.

2.2 Reflection failure

(1) Increased reflection

Any type of connector has a certain service life. Taking SMA connectors as an example, the US and Chinese military standards stipulate a lifespan of 500 times. This is because when the connector is used for a long time and repeatedly inserted and removed more than 500 times, the pins and sockets have caused varying degrees of wear, and contact is no longer optimal. Therefore, during use, reflection may increase sharply. Many users still use connectors that should be scrapped from an economic perspective. This approach is not advisable. Because the performance of connectors that have exceeded their service life has significantly declined, it is difficult to ensure testing accuracy and data reliability if new connectors are not replaced in a timely manner.

(2) Open circuit

In previous work, it was found that some users mistakenly connected the N-type 50 Ω plug to the N-type 75 Ω socket. Due to the fact that the diameter of the 50 Ω pin is much larger than the size of the 75 Ω socket, the socket size exceeds the elastic limit and cannot be restored to its original size. Others found an open circuit when using it again, and the socket of the original 75 Ω socket was damaged.

(3) Short circuit

In the past, when we tested sealed connectors, we found that a few connectors had a high voltage standing wave ratio, and a few had total reflection. After repeated careful inspection, it was found that during the welding of the inner conductor, the solder flowed onto the surface of the glass insulator, causing a complete or partial short circuit and resulting in unsatisfactory performance.

2.3 Electrical contact failure

(1) Pin and socket not in contact

Usually, the material of the socket parts should be made of beryllium bronze or tin phosphorus bronze, but a very small number of manufacturers use 59-1 brass as the socket to reduce costs. Therefore, after plugging and unplugging once or twice, the socket is in an expanded state, and when reconnecting the pin or socket, it does not come into contact at all. Of course, in our work, even if beryllium bronze is used as the elastic component, due to lax inspection during work, individual parts that have not been closed after milling are mixed into qualified parts for elastic treatment and gold plating. However, it was not found during assembly, resulting in no contact during connection. It was not until testing that it was discovered that the standing wave was very large, even total reflection. After careful inspection, it was discovered that the unqualified socket was installed in the product.