I introduction
With the rapid development of science and technology, the application scope of electronic equipment is becoming more and more extensive, which has penetrated into almost all departments of the national economy, including public security, communication, medical and other fields. Therefore, the reliability of electronic equipment has attracted more and more attention. Electrical contact components such as connectors and relays are one of the most used components in electronic equipment. According to incomplete statistics, there are tens of thousands of connection points for an electronic computer, radar or aircraft, and the reliability of electronic equipment is closely related to the quantity and quality of components used. Especially in the electronic equipment with series structure, the failure of any component, device or node may lead to the failure of local or various systems. This paper focuses on the failure mode and mechanism of RF coaxial connector and cable assembly, and discusses in detail how to improve its reliability.
II Failure mode and mechanism of RF coaxial connector and cable assembly
At present, there are many kinds of RF coaxial connectors used at home and abroad, but there are mainly three types in terms of connection types:
(1) Threaded connection type: such as apc-7, N, TNC, SMA, SMC, L27, L16, L12, L8, L6 and other RF coaxial connectors. This kind 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 kind of connector has the characteristics of convenient and fast connection. It is also the earliest connection form of RF connector in the world.
(3) Push in connection type: such as SMB, SSMB, MCX, etc. the connector of this connection type has the characteristics of simple structure, compactness, small volume, easy miniaturization, etc.
Although there are many kinds of connectors, many problems are the same from the perspective of reliability. This paper focuses on the failure mode and mechanism of RF coaxial connector, which is widely used and has the most varieties at present. According to our practice for more than ten years, the common main failure modes are as follows.
2.1 connection failure
(1) The connecting nut falls off
In daily life, some users report that sometimes the connecting nut falls off, which affects the normal work, especially for small connectors, such as SMA, SMC and L6. After our analysis, it is generally caused by the following reasons:
a. In order to reduce the cost, the designer misused the inelastic brass seat snap ring material, which made the nut easy to fall off.
b. During processing, the groove depth of the nut mounting snap ring is not enough, so the nut will fall off with a little torque during connection.
c. Although the material was selected correctly, the process was unstable, the beryllium bronze elastic treatment did not reach the hardness value specified in the drawing, and the snap ring was inelastic, resulting in the nut falling off.
d. During the test, the user did not have a torque wrench, but used an ordinary wrench to tighten the nut, so that the tightening torque greatly exceeded the specified value of the military standard, so the nut (snap ring) was damaged and fell off.
(2) Pairing error
Since the reform and opening up, China has imported more and more instruments, and users often complain that they mistook the Q9 cable head bought in the market for the BNC cable head commonly used in the world. Because its shape is exactly the same as that of BNC cable head, but the size is slightly different, it is not compatible with BNC imported instrument, and these phenomena are common.
(3) The inner conductor is loose or falls off
Some designers divide the inner conductor into two parts at the dielectric support of the inner conductor, and then connect them with threads. However, for small RF coaxial connectors, the size of the inner conductor itself is Φ 1 ~ 2mm, thread is processed on the inner conductor. If conductive adhesive is not coated on the threaded connection, the connection strength of the inner conductor is very poor. Therefore, when the connector is connected for many times, under the long-term action of torque and tension, the internal conductor thread will loosen and fall off, resulting in connection failure.
2.2 reflection failure
(1) Reflection increase
Any kind of connector has a certain service life. Taking SMA connector as an example, the American military standard and Chinese military standard stipulate that its service life is 500 times. This is because after long-term use and repeated plugging for more than 500 times, the pins and sockets have caused varying degrees of wear, and the contact is not the best state. Therefore, the reflection may increase sharply during use. Many users still use the connectors that should be scrapped from an economic point of view. This approach is not desirable. Because the performance of connectors beyond their service life has decreased significantly, it is difficult to ensure the test accuracy and data reliability if they are not replaced in time.
(2) Open circuit
In previous work, it was found that individual users mistakenly connected the n-type 50 Ω plug to the n-type 75 Ω socket. Because the pin diameter of 50 Ω is much larger than the jack size of 75 Ω, the jack size exceeds the elastic limit and cannot be restored to the original size. Others found an open circuit when they used it again. The socket of the original 75 Ω socket has been damaged.
(3) Short circuit
In the past, when we tested sealed connectors, we found that a few connectors had large VSWR and individual total reflection. After repeated and careful inspection, when the seal is connected to the welding inner conductor, the solder flows to the surface of the glass insulator, resulting in total or local short circuit and unqualified performance.
2.3 electrical contact failure
(1) The pins and sockets are not in contact
Usually, the socket parts should be made of beryllium bronze or tin phosphorus bronze, but a few manufacturers use 59-1 brass as the socket in order to reduce the cost. Therefore, after plugging in and out for one or two times, the socket is in the expanded state, and there is no contact when connecting the pin and socket again. Of course, in our work, even if beryllium bronze is selected as the elastic part, due to the lax inspection in the work, the individual parts that are not closed after milling the groove are mixed into the qualified parts for elastic treatment and gold plating, which are not found during assembly, so there is no contact during connection. Until the test, it was found that the standing wave was very large, even total reflection. After careful inspection, it was found that the unqualified Jack was installed in the product.
