Before the question of optimizing contact force is addressed it is necessary to ask a more fundamental question:
“What does contact force do in a connector?”
The two most important functions of contact force have been mentioned earlier. Contact force creates and maintains the integrity of the metal-to-metal areas of the contact interface. The next question is:
“How much force does it take to accomplish each of these functions?”
The question about the force necessary to create an “acceptable” contact interface has a more accessible answer.
Figure 2.19 shows the relationship between contact force and the contact interface resistance, a reasonable measure of the acceptability of a contact interface. Note that contact resistance decreases continuously with increasing contact force, but at a decreasing rate. Between 10 and 100 grams the contact resistance changes from about 4 milliohms to about 1.2 milliohms, a decrease of about 70 percent, while between 100 and 200 grams the change is from 1.2 milliohms to about 0.7 milliohms, a decrease of about 40 percent. The most important data from this curve, however, is the fact that at a contact force of about 10 grams the contact resistance is about 4 milliohms, a value that, if it is stable, would be acceptable for most connector applications with the exception of high current requirements.
The phrase “if it is stable” is the key point of that sentence. It is highly unlikely that the friction forces created at a 10 gram contact force would be sufficient to ensure mechanical, and, therefore, electrical stability under most application conditions. The conclusion to be drawn then is that the most demanding contact force requirement is to maintain the integrity of the contact interface. This force will be higher, possibly significantly higher than 10 grams. With this information in hand, the question of the optimization of contact force can be addressed.
As noted earlier in this chapter, contact force impacts a number of performance characteristics. Consider some of the major effects.
Increasing contact force will increase:
• the wear rate of the contact finish
• the mating force
• the stresses on the contact spring
• the stresses on the connector housing
These are all negative effects.
On the other hand, increasing contact force will
• decrease the magnitude of the contact resistance
• increase the stability of the contact resistance
These are, of course, positive effects. The data in Figure 2.19 illustrate the first point. The second effect arises from the fact that increased contact force increases the friction forces at the contact interface which increases the mechanical stability of the contact interface, which, in turn increases the stability of the contact resistance.
Optimization of the contact force, then, reduces to determining the minimum contact force necessary to ensure mechanical stability of the contact interface in a given connector application. Unfortunately, the variability in mechanical and thermal driving forces for contact interface disturbance vary dramatically with the connector application so there is no single answer to that question; which is to say that there is no generally applicable minimum contact normal force requirement for a connector.