Terminal is a kind of accessory product used to realize electrical connection, which belongs to connector in industry.

With the higher degree of industrial automation and the stricter and more accurate requirements of industrial control, the demand for terminal is gradually increasing.

With the rapid development of the electronics industry,terminals are used more and more widely.

Classification of Terminals

Terminals can be divided into: WUK terminals, European terminal series, plug-in/out terminal series, transformer terminal series, building wiring terminal series, fence terminal series, spring terminal series, rail terminal series, through-wall terminal series, photoelectric coupled terminal series, 110,205,250,187,ring terminals,flag series terminals and sheath series, all kinds of tubular terminals, copper strips etc.

At present, the most widely used are PCB board terminals, hardware terminals, nut terminals and spring terminals.

The plastic insulation material and conductive parts of the terminal are directly related to the quality of the terminal, which determine the insulation performance and electrical conductivity of the terminal respectively.

As an accessory product of precision operation, the quality of motion control terminals is always concerned by customers, how to effectively check and prevent failures has become a topic of concern.

Common Failures of Terminal

From the point of view of the use of the terminal, the function that should be achieved is: the contact part must be connected, and the contact is reliable.

Where the insulation should not be connected, the insulation must be reliable.

There are three common forms of fatal failure of terminals:

1.Poor Contact 

The metal conductor inside the terminal is the core part of the terminal, which transmits the voltage, current or signal from the external wire or cable to the contact part of corresponding connector.

Therefore, the contact parts must have excellent structure, stable and reliable contact holding force and good electrical conductivity.

Due to unreasonable structural design of contact parts, wrong material selection, unstable mold, poor processing size, rough surface, bad heat treatment and electroplating, improper assembly, bad storage environment and improper operation, all will cause bad contact at the contact and matching parts.

2.Poor Insulation

The function of the insulator is to keep the contact parts arranged in the correct position and to insulate each other between the contact parts and the contact parts and between the contact parts and the shell.

Therefore, insulators must have excellent electrical, mechanical and technological properties.

Especially with the wide use of high-density and miniaturized terminals, the effective wall thickness of insulators is getting thinner and thinner.

This puts forward more stringent requirements on insulating materials, injection mold accuracy and forming technology.

Due to the existence of metal excess on the surface or interior of insulators, surface dust, flux and other pollution, organic material precipitates and ionic conductive channel formed by harmful gas adsorption film fuse with surface water film, moisture absorption, mildew, aging of insulation materials and other reasons, will cause short circuit, leakage, breakdown, low insulation resistance and other bad insulation phenomena.

3.Poor Fixation

Insulators not only play the role of insulation, but also provide accurate alignment and protection for protruding contacts. At the same time, they also have the functions of installation positioning, locking and fixing on the equipment.

Poor fixation, may affect the reliability of contact resulting in instant power outage, or even cause the disintegration of the product.

Disintegration refers to the abnormal separation between plug and socket, pin and Jack caused by unreliable structure due to material, design, process and other reasons when the terminal is in the state of plug and joint. It will cause interruption of power transmission and signal control in the control system.

Poor fixation will be caused by unreliable design, wrong material selection, improper selection of forming process, poor quality of heat treatment, mold, assembly, welding and so on.

In addition, the appearance is poor due to coating peeling, corrosion, bruising, flying edge of plastic shell, rupture, rough processing of contact parts, deformation and so on; Poor interchange caused by poor positioning and locking fit size, poor machining quality consistency, excessive total separation force and so on.

They are all common and frequently occurring issues.

These faults can generally be found and eliminated in time in the process of inspection and use.

How to Detect the Short Circuit of Terminal

Faults in wires and terminals may also be caused by a short circuit between the harness and the main body (grounding wire) or inside the associated switch.

Before checking, first check whether the terminal on the car body is secure, and then you can follow the following steps to test.

1.Check that the wires are on or off

First remove the terminals that control the computer ECU and sensors, and then measure the resistance between the corresponding terminals.

If the resistance value is not more than 1 ohm, the wire is normal for further inspection. 

When measuring wire resistance, it is best to gently shake the wire in both vertical and horizontal directions to improve the accuracy of the measurement, and note that, for most wire terminals, multi-meter rod should be inserted from the back end of the connector, but for waterproof terminal, rods with waterproof jackets can not be inserted from the back end, because a little carelessness during insertion will deform the terminal.

2.Resistance check of short circuit

First remove the wire terminals on both sides of the control computer ECU and the sensor, and then measure the resistance between the terminal strips of the connectors on both sides and the car body. 

During the measurement, one end of the multi-meter rod is connected with the car body, and the other end is measured on the wire connectors on both sides. If the resistance value is greater than 1 ohm, there is no short circuit fault between the wire and the car body.

3.Terminal appearance and contact pressure inspection

First of all, each terminal should be removed one by one, check whether there is rust and dirt on the connector terminal, and clean up the rust and dirt.

Then check whether the terminal piece is loose or damaged, whether the terminal is fixed firmly, and the terminal should not be loose when pulled gently.

On the contrary, if the plug terminal in which seat hole is pulled out more easily than other seat holes, the seat hole may cause poor contact failure in use.

How to Screen and Check by Precautionary Measures

In order to ensure the quality and reliability of the terminal and prevent the occurrence of the above failure problems, it is suggested that the corresponding screening technical requirements should be studied and formulated according to the technical conditions of the product, and the following targeted reliability inspection to prevent failure should be carried out.

1.Prevention of poor contact

1). Conduction test

In general, there is no such item in the product acceptance test of terminal manufacturers, but users generally need to carry out conduction testing after installation.

Therefore, it is suggested that manufacturers should increase 100% point-by-point conduction testing for some key models of products.

2).Instantaneous break test 

Some terminals are used in a dynamic vibration environment.

The experimental results show that only checking whether the static contact resistance is qualified or not can not guarantee the reliability of contact in dynamic environment.

Because connectors with qualified contact resistance still have instantaneous power outage during vibration, shock and other simulated environmental tests, it is best to conduct dynamic vibration tests to check their contact reliability for some terminals with high reliability requirements.

3). Single hole separation force test

The single hole separation force refers to the separation force in which the contact parts in the plug-in state change from static to moving, which is used to indicate that the pin and Jack are in contact.

The experimental results show that the separation force of single hole is too small, which may cause signal instantaneous break under load of vibration and impact.

Checking the contact reliability by measuring the separation force of a single hole is more effective than measuring the contact resistance.

Inspection found that the single hole separation force is too poor, the measurement of contact resistance is often still qualified. 

For this reason, in addition to developing a new generation of flexible plug-in contact featuring stable and reliable contact parts, the factory should not use automatic insertion force testing machine for key models, but should carry out 100% point-by-point single-hole separation force inspection of the finished products, to prevent signal breakage caused by relaxation of individual jacks.

2.Prevention of poor insulation

1). Insulation material inspection

The quality of raw materials decides the insulation performance of insulators.

Therefore, the choice of raw material manufacturers is particularly important, cannot blindly reduce the cost and lose the material quality. 

Large factory materials with good reputation should be selected.

And for each batch of materials to carefully check the batch number, material certificate and other important information.

Do a good job in the retrospective data on the use of materials.

2). Insulation resistance test

Some production plants are required to test the electrical performance after being assembled into finished products. As a result, the insulation resistance of the insulator itself is not up to standard, so the whole batch of finished products can only be scrapped.

100% process screening in the state of insulator parts is to ensure that the electrical performance is qualified.

3.Prevention of poor fixation

1). Interchangeability inspection

Interchangeability inspection is a dynamic check.

It requires that the same series of matching plugs and sockets can be plugged and connected to each other, from which it is found whether it is impossible to insert, locate and lock due to the poor size of insulators, contact parts, lack of parts or assembly, or even disintegrate under the action of rotating force.  

Another function of interchangeability inspection is to find out in time whether there are metal surpluses that affect insulation performance through thread, bayonet and other plugging and pulling connections.

Therefore, 100% inspection of the project should be carried out on some important terminals to avoid such major fatal failure accidents.

2). Endurance moment test

Endurance moment test is a very effective method to assess the reliability of terminal structures.

Each batch of samples should be tested for endurance moment, and problems should be found in time.

3). General measurement of crimped wire

It is often found that the individual core crimping is not in place, or can not be locked after it is sent, and the contact is unreliable.

The reason for the analysis is that there are burrs or dirt stuck in the teeth of individual installation holes.

In addition, due to the improper matching of wire diameter and crimping aperture, or due to mis-operation of crimping process, it will also cause the accident of weak crimping end.

For this reason, the factory should test all the installation holes of the sample of the delivered plug (seat) before the finished product leaves the factory, that is, use loading and unloading tools to simulate the wire with pins or sockets in place to check whether it can be locked.

According to the technical conditions of the product, check the pull-off force of each crimped wire.

Remarks at the end

The terminal is the foundation, if there is no reliable terminals, there is no reliable system engineering.

Various failure modes and failure mechanisms can be found through terminal reliability screening, and a large number of experiences and lessons can be analyzed and hidden dangers can be eliminated.

It provides a scientific basis for improving design, process, inspection and use, sets an important basis for revising and formulating the technical conditions of terminals, and provides a technical guarantee for preventing unnecessary losses.