Dr. Bob on Connector Degradation Mechanism
This is the first in a series of articles about connector degradation mechanisms. The purpose of this article is to provide a rationale as to why they are important to connector performance. Following articles will discuss degradation mechanisms in additional detail. At the end of this article, you will be able to provide feedback on this subject and also be able to participate in an online discussion with others in the industry. If you have specific questions regarding future topics, email them to firstname.lastname@example.org, and we will respond.
Connectors are used in order to provide a separable connection between two subsystems. There are many reasons separability is necessary, ranging from manufacturing convenience to performance upgrades. When mated, however, the connector should not introduce any unacceptable resistance between the systems. “Unacceptable” resistance means any resistance that could cause malfunction of the system either through signal distortion or power loss, depending on the application requirement. The reason connector degradation mechanisms are important is that they are the potential source of resistance increases, and, therefore, field failures over time.
Let’s begin with a brief review of connector resistance. Figure 1 shows a cross section of a generic signal connector. The equation in Figure 1 indicates the various sources of resistance within the connector. RO is the overall resistance of the connector, and is the resistance between a probe attached to the conductor behind the crimped connection, and a probe making contact to the appropriate pad on the printed circuit board containing the press-in compliant pin connection. Two permanent connection resistances, RP.C., are shown—the resistance of the crimped connection and the compliant pin connection. Similarly, there are two bulk resistances—RBulk—the bulk resistance of the post contact and that of the parallel resistance combination of the two beams of the receptacle contact. Only one interface, or separable contact resistance, RC, is shown, because it is difficult to separate the parallel resistances of the two contact points. The overall connector resistance is the sum of the individual permanent connection resistances, the post and receptacle contact bulk resistances, and the separable contact resistance, because all these resistances are electrically in series.
For the sake of discussion, let us assume that the measured overall resistance, RO, is 15 milliohms. Given that assumption, take a minute or two at this point and guesstimate the relative contributions of the permanent, bulk, and separable interface resistances to the overall connector resistances.
In this example, and these values are typical of a soft-shell style connector, the bulk resistances will make up the majority of the overall resistance, approaching 14 milliohms. The permanent connection resistances will be of the order of a few hundred microohms and the separable interface resistance of the order of a milliohm.
Though the bulk resistance of the connector contacts is by far the largest contributor to the connector resistance, it is also the most stable. The bulk resistance of the individual contacts is determined by the materials of manufacture of the contacts and their overall geometry. In a simple example, consider the resistance of a length of a conductor, which is given by
Rcond. = r l/a.
In this equation, r is the resistivity of the conductor, or in a connector, the contact spring material, “l” is the length of the conductor, and “a” is the cross-sectional area of the conductor, or in a connector, the geometry of the contact spring system. For a given material choice, say phosphor bronze, and contact geometry, these parameters are constants and, therefore, the bulk resistance of a connector is constant.
The permanent connection resistance and the interface, or separable connection, resistance, are variable. It is these resistance contributions that are susceptible to a variety of degradation mechanisms, as will be discussed in later articles. At this point, suffice it to say that if a connector is subjected to a test program to assess its performance, harsh environments, heat, age, vibration, etc., and the overall connector resistance changes from the original15 milliohms to, say, 100 milliohms, the change in resistance will occur in the separable and permanent connection resistances. The separable interface resistance is the most susceptible to degradation because the requirement for separability places limitations on the forces and deformations that can be applied to create the separable interface.
Simplistically, there are two major separable interface requirements that limit those forces and deformations. The connector mating force is the first and most obvious requirement. To realize high pin count connectors, the mating force of the individual contacts must be controlled, and the contact normal force is one of the major parameters that is limited by that requirement. For example, separable connection contact forces are of the order of tens to a few hundred grams, while insulation displacement connection, or IDC, forces are of the order of a few thousand grams, as are the forces in compliant press-in connections. The high forces typical of such permanent connections provide much greater mechanical stability and lower resistances than can be realized with the significantly lower forces of separable connections.
In a similar fashion, the higher forces of permanent connections allows for greater deformation of the contact surfaces, compared to that of separable connections. Crimped connections are the most obvious example where significant deformation of the crimp terminal, as well as of the individual conductor strands, are obvious. The forces of IDC and compliant pin interfaces also permit much larger deformations of the contact surfaces. As with the higher forces, the larger surface deformations of permanent connections reduce their resistance as compared to separable contact resistances.
The deformation of separable connection surfaces is also limited by another separable interface requirement: mating durability. High surface deformations generally result in high surface wear, which in turn may result in the loss of any contact coating, gold or tin for example, on the contact surface. The loss of such coatings will increase the corrosion susceptibility of the contact surface, as will be discussed in subsequent articles.
The combination of these two separable interface requirements, mating force and mating durability, limit the deformations and mechanical stability of separable interfaces, and is the reason for the lower electrical stability of separable interfaces, as compared to permanent connections.
The same limitations also explain why the electrical resistance of separable interfaces is lower than that of permanent connections. In general, the larger the area in contact between two surfaces, the lower the electrical resistance of the interface. In effect, the contact area between two surfaces is analogous to the cross-sectional area in the equation, for the resistance of a length of conductor, Rcond. = rl/a. Because separable connections have a lower contact area than permanent connections, they have a higher electrical resistance.
To summarize, the reduced force of separable connections results in lower mechanical stability and the reduced contact area results in higher electrical resistance as compared to permanent connections.
These same issues, reduced contact force and reduced contact area, directly affect the susceptibility of separable contact interfaces to degradation. Figure 2 shows a schematic illustration of a separable contact interface at high magnification. The main point of the figure is to illustrate that, on the microscale of such contact interfaces, all surfaces are rough. This dictates that the contact interface itself will consist of a distribution of contact spots, called a-spots or asperities, rather than a full area contact. This asperity structure is what gives rise to the contact interface resistance. The reduced contact area, consisting of a distribution of a-spots over some geometric area depending on the geometry of the surfaces in contact, introduces a resistance, called constriction resistance, that results from the current flow being constricted to flow through the individual a-spots. Constriction resistance can be reduced by increasing the contact area by various means, but never eliminated. Therefore, a connector must always introduce some resistance into an electrical system. From this perspective, the prime objective of connector design is to control both the magnitude and stability of the resistance that is introduced.
As mentioned, the magnitude of interface resistance is determined by the contact area that is created as the plug and receptacle contacts engage one another. There are two major factors that affect the stability of contact resistance: disturbance of the contact interface and corrosion of the a-spots. These effects define the connector degradation mechanisms that will be discussed in the following articles. In summary, these mechanisms are:
- Corrosion in and around the contact interface, so as to reduce the contact area. Two corrosion related mechanisms will be discussed: Surface corrosion, which directly impacts the contact area, and induced or fretting motions, which can enhance the susceptibility of the contact interface to corrosion.
- Loss of the integrity of the contact plating, by inadequate plating and/or plating wear, resulting in enhanced susceptibility to corrosion. Most connector contacts are plated with either a noble finish, such as gold, or a non-noble finish, generally tin. One of the main purposes of these platings is to protect the base metal of the contact spring, usually a copper alloy, from corrosion. The corrosion susceptibility of noble and non-noble finishes is different and each will be discussed separately.
- Loss in contact force, leading to reduced mechanical stability and increased susceptibility of the contact interface to fretting motions. The major mechanisms leading to reduction in contact forces in connectors are overstressing of contacts and stress relaxation. Stress relaxation is a time-dependent loss in contact force, due to time/temperature exposures.
Each of these degradation mechanisms will be discussed in more detail in following articles.
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