[Xi'an Yuqi High-Voltage Sealed Electrical Connectors Co., Ltd.] Connector Series Tutorial (1), Connector Degradation Mechanism + Influencing Factors

[Xi'an Yuqi High-Voltage Sealed Electrical Connector Co., Ltd.] Connectors can be seen everywhere in life, and a good connector is crucial to product performance. So, what factors will affect the quality of the connector? In this article, we will discuss what factors can cause connector failure, and discuss the role of connector degradation mechanism on connectors. The main contents are as follows.

 

Connectors are used for connection between two separation systems. There are many reasons why separability is necessary, from ease of manufacturing to improvement in performance. However, when matched, the connector should not increase any unnecessary resistance between the systems. Increasing the resistance value may distort the signal or lose power and cause system failure. The mechanism of connector degradation is important because they are a potential source of increased resistance, thus leading to functional failure over time.

Let's briefly review the resistance of the connector first. Figure 1 shows a cross-section of a universal signal connector. The equations in Figure 1 represent the various resistance sources within the connector. Ro is the overall resistance of the connector, which is the resistance between the tail point of the conductor and the solder point of the PCB connector pin. The two connection resistors Rp.c are the resistances between the finger connection point and the corresponding pin. Similarly, the two body resistances (Rbulk) refer to the body resistance of the rear contact and the parallel body resistance between the two legs of the connector; there is also the contact resistance Rc at the interface or separation. The overall connector resistance is the sum of the individual constant connection resistances, the back contact and cavity connector resistances, and the separable contact resistances, because all of these resistances are connected in series.

 

Figure 1: Schematic diagram of connector resistance

For purposes of discussion, let us assume that the total resistance measured Ro is 15 milliohms. Taking this assumption into account, we speculated about the relative impact of connection resistance, body resistance, and separable contact resistance on the overall connector resistance.

In this example, these values are the resistance values of a typical soft-shell connector, and the body resistance will account for the majority of the total resistance, close to 14 milliohms. The connection resistance is several hundred microohms, and the rest is the contact resistance at the separable point.

Although the body resistance of the connector contacts is a contributor to the connector resistance, it is also the most stable. The bulk resistance of an individual contact is determined by the material from which the contact is made and its overall geometry. In this simple example, considering the resistance of the conductor length, it can be calculated by the following company: Rcond. = r l/a.

In this equation, r is the resistivity of the conductor (or the spring material in the connector),"l" is the length of the conductor, and "a" is the cross-sectional area of the conductor (or the geometry of the spring in the connector). For a given material, such as phosphor bronze and contact geometry, these parameters are constants, so the overall resistance of the connector is constant.

The connection resistance and the interface or separable connection resistance are variable. These resistors are susceptible to multiple degradation mechanisms, which will be discussed in a later article. It should be pointed out that connectors are affected by many factors, such as harsh environment, heat, life, vibration, etc. And the total connector resistance may vary from the original 15 milliohms to, for example, 100 milliohms, with the resistance changes occurring mainly in the separable and connection resistances. Separable interface resistance is the most likely to degrade because of forces and deformations generated at the separable point.

Simply put, the two main separable interfaces require certain forces and deformations. The biting force of the connector is the first and most obvious requirement. For high PIN number connectors, the bite force of a single PIN position must be controlled, and the contact normal force is one of the main parameters limited by this requirement. For example, the contact force for separable connections is tens to hundreds of grams, while the force for insulated crimp connections, or IDC, is on the order of a few kilograms, as is the force for the corresponding press-in connection. The high force in this connection provides greater mechanical stability and lower resistance values, which are much lower than for separable connections.

 

In the same case, higher connection forces allow greater deformation of the contact surface than separable connections. Crimp connections are the most obvious examples, such as significant deformation of crimp terminals and significant deformation of individual conductors. Both the force of the crimp connection and the corresponding PIN pin allow for greater deformation of the contact surface. As with higher forces, larger surface deformations of connections reduce their resistance compared to separable contact resistances.

Deformation of separable connection surfaces is also limited by another separable interface requirement: mating durability. High surface deformation usually results in high surface wear, which in turn can lead to loss of contact coatings, such as gold or tin on the contact surface. Loss of this coating will increase the corrosion sensitivity of the contact surface, which will be discussed in a future article.

The combination of the snap force and snap durability of the separable interface limits the deformation and mechanical stability of the separable interface compared to connections, and is also responsible for the lower electrical stability of the separable interface.

In general, the larger the contact area between two surfaces, the lower the resistance of the interface. In fact, for the resistance of the length of a conductor, the contact area between the two surfaces is similar to the equation Rcond. = r l/a。Since separable connections have a lower contact area than connections, they have a higher resistance.

In summary, compared to connections, the reduced force of separable connections leads to reduced mechanical stability, and the reduced contact area leads to higher resistance.

These problems, namely the reduction of contact force and the reduction of contact area, directly affect the degradation sensitivity of the separable contact interface. Figure 2 shows an enlarged schematic view of the separable contact interface. The figure shows that on the microscopic scale of this contact interface, all surfaces are rough. This means that the contact interface itself will consist of a distribution called a-point or uneven contact points, rather than a complete area of contact. This uneven structure is the reason for the increase in contact interface resistance. The reduced contact area, including the distribution of points a over a certain geometric area, depends on the geometry of the contact surface. One type of resistance, called a shrinking resistance, is created by the current being squeezed to flow through a single point a. Shrinkage resistance can be reduced by increasing contact area through various methods. Therefore, connectors always add some resistance to the electrical system. From this perspective, the primary goal of connector design is to control the size and stability of the resistor.

 

Figure 2: Microscopic view of the inherent surface roughness of the contact interface

As mentioned earlier, the magnitude of the interface resistance depends on the contact area created when the plug and socket contacts contact each other. There are two main factors affecting the stability of contact resistance: disturbance at the contact interface and corrosion at point a. How these factors affect the mechanism of connector degradation will be discussed later. In summary, these mechanisms include:

1. Corrosion occurs at and around the contact interface, thereby reducing the contact area. There are two corrosion mechanisms: surface corrosion, which directly affects the contact area; induction or fretting, which can increase the sensitivity of the contact interface to corrosion.

2. The integrity of contact plating is lost due to insufficient plating or plating wear, increasing the sensitivity to corrosion. Most connector contacts are a surface layer plated with a precious metal, such as gold, or a common plated surface, usually tin. One of the main purposes of these coatings is to protect the contact substrate (usually a copper alloy) from corrosion. The corrosion sensitivities of precious metals and non-precious metals are different and will be discussed separately later.

3. Loss of contact force leads to a decrease in mechanical stability and the contact interface is susceptible to fretting. The main mechanism leading to the reduction of connector contact force is excessive contact stress and stress relaxation. Due to the influence of time/temperature, stress relaxation refers to the loss of contact force over time.