这不是关于接触时间的长短，而是关于触点受力时间的长短。这篇讨论将解释设计力是怎样影响末期触点压力的。

1. 设计力
2. 尺寸公差
3. 末期触点压力
4. 永久变形
5. 应力松弛
6. 疲劳

《技术报导》的下一期将包括一篇关于触点压力重要性的内容详实的讨论。

可靠性和末期触点压力

电气连接器用于将电信号从一个元件传递到另一个元件。这些元件可以在一个装置内，也可以在不同的装置内。在任何情况下，对于需要传递而只允许最小限度变异的信号而言，连接器内的触点必须接触良好并能在接触期间抵抗腐蚀。连接器必须能满足元件使用期末期的性能要求。由于性能会随着时间而退化，这就意味着设计者必须认真考虑元件的期望使用时间以及期望的接触循环数。

所有的电气触点都可以在接触界面提供一定量的法向力。该设计力通常是在一个假设条件下进行计算的，即元件的所有尺寸都正好处于尺寸公差的中点。而事实上，由于在规定的尺寸公差内的产品的差异，触点实际承受的力与设计力是不同的。幸好，这些差异可以用于提供大于设计值的触点压力。然而，很可能实际的触点压力同样要小于设计者的期望值。

对于更加复杂的情况，由于众多因素的影响，触点产生的法向力会随着时间的变化而变化。末期触点压力几乎一定小于第一个循环产生的力。因此，必须增加设计力以确保末期触点压力足以维持良好的电气接触。图-1显示了末期触点压力是怎样降低至要求的力之下的，即便设计力大于要求的力。

永久变形是触点压力随着时间变化而降低的一种方式。如果一个触点在初始变形时即屈服，负载去除后，它将不能回复到原来的形状。这意味着该触点任何后续的变形都将会很小，且相应的触点压力也会降低。

应力松弛是导致触点压力损失的另一现象。当一个触点处于挠曲中，金属中会产生一定量的应力。在一个稳定的挠曲中，该应力会随着时间变化而松弛（降低）。因为应力产生触点压力，所以触点压力也会降低。此外，应力的降低同时还意味着，当接触断开后，触点将不会回复到原先的形状。因此，应力松弛可以被看作是一种延迟的永久变形。

应力松弛由于时间、温度、初始应力水平和材料的不同而不同。触点挠曲的时间越长，松弛的应力越多。另外，松弛速率随着温度的上升而增加。松弛速率还取决于初始应力水平，当应力向屈服强度接近时，松弛速率增加。同时，某些材料比其他材料更容易松弛。

疲劳也影响末期触点压力，但以间接的方式对其产生影响。一个触点期望承受的挠曲而不损坏的循环数取决于初始应力水平。当应力增加时，期望的循环数随之减少。因此，如果需要更多的挠曲循环，应力必须降低到相应的水平。但是，如果应力水平降低，触点压力也随之降低。

可以通过材料的细选来改进末期性能。具有较高屈服强度的材料允许产生较大的触点压力，因为它们可以允许较大的应力。具有较大应力松弛强度的材料可以随着时间的变化而保持较大的设计力。具有较大疲劳强度的材料在相同的循环数下可以产生更大的力，或者在相同的作用力下可以持续更长的时间。图-2显示了和图-1相同的接触，但此次采用的材料可以保持足够的设计力以产生良好的末期效果。为了维持足够的末期触点压力，接触必须可以产生大于电信号整体性所需要的最小力的初始力。选择材料时，必须认真考虑服务时间的长度、循环数和承受的温度极限。如果缩小元件的尺寸，增加接触尺寸以增加触点压力的空间就会更小。实际上，小型化通常会导致低触点压力和高应力。所以，适合该项目的最好的材料应是在接触末期可以最有效地保持最多的初始触点压力的材料。

参考：John Ratka：鲍辛格效应对连接器材料性能的影响（The Influence of the Bauschinger Effect in the Performance of Connector Materials）

It’s not how old the contact is; it’s how old the contact feels!! This discussion will explain how design forces can affect end-of-life contact force.

1. Design Force
2. Dimensional Tolerances
3. End-of-Life Contact Force
4. Permanent Set
5. Stress Relaxation
6. Fatigue

The next issue of Technical Tidbits will include an informative discussion about the importance of contact force.

Reliability and End-of-Life
Contact Force

An electrical connector is designed to pass an electric signal from one component to another. These components may either be in the same device or different devices. In any case, for the signal to pass through with minimal alteration, the contacts within the connector must maintain good contact force and resist corrosion over the life of the contact. Connectors must be designed to meet all performance requirements at the end of the useful life of the component. Since performance will degrade over time, this means that the designer must carefully consider how long the component is expected to last, and how many cycles the contact is expected to see.

Every electrical contact is designed to provide a certain amount of normal force at the contact interface. This design force is usually calculated under the assumption that all part dimensions fall exactly at the midpoint of the allowed dimensional tolerances. In reality, the actual force experienced by the contact will typically be different from the design force, because of actual product variation within the specified dimensional tolerances. With luck, these variations may serve to provide a contact force greater than the designed value. However, it
is equally likely that the actual contact force may be lower than what the designer anticipated.

To further complicate matters, the normal force generated by the contact will change over time, due to several factors. The end-of-life contact force will almost certainly be lower than the force generated by the first contact cycle. Therefore, the design force must be increased to ensure that the end-of-life force is adequate to maintain good electrical contact. Figure 1 shows how the end of life force may decrease below the required force, even though the design force is well above the requirement.

Permanent set is one method by which the contact force decreases over time. If a contact has yielded during the initial deflection, it will not return to its original shape when the load is removed. This means that any subsequent deflection of the contact will be smaller, and the corresponding contact force will be reduced as well.

Stress relaxation is another phenomenon that results in loss of contact force. When a contact is under deflection, a certain amount of stress will be generated in the metal. Under a steady deflection, this stress will relax (decrease) over time. Since the stress is what generates the contact force, the force will decrease as well. Additionally, the reduction in stress will also mean that the contact will not return to its original configuration when disconnected.? Therefore, stress relaxation can also be thought of as a delayed permanent set.

Stress relaxation is dependent upon time, temperature, the initial stress level, and material. The longer a contact is deflected, the more the stress will relax. In addition, the relaxation rate increases with temperature. The rate is also dependent on the initial stress level- as the stress approaches the yield strength, the rate increases. Also, some materials will relax much more easily than others.

Fatigue also affects the end-of-life contact force, but in an indirect manner. The number of deflection cycles a contact can be expected to experience without breaking depends on the initial stress level. As the stress increases, the expected number of cycles decreases. Therefore, as the contact needs to see more and more deflection cycles, the stress level must be reduced to allow this to happen. However, if the stress level is reduced, the contact force is reduced as well.

It is possible to improve the end-of life performance through careful material selection. Materials with higher yield strengths will allow for greater contact forces to be generated, since they allow for greater stress. Materials with greater stress relaxation resistance will be able to retain a greater amount of the design force over time. Materials with greater fatigue strength can generate greater force over the same number of cycles, or can last longer at the same amount of force. Figure 2 shows the same contact as in figure 1, this time using a material that retains enough of the design force to be viable at the end of life. In order to maintain adequate end-of-life contact force, the contact must be designed to give an initial force greater than the minimum required for electrical signal integrity. The length of service time, number of cycles, and the temperature extremes experienced must all be carefully considered when choosing a material. In this age of decreasing part size, there is less room to increase the contact dimensions in an effort to increase the force. Indeed, miniaturization often results in lower forces and higher stresses. Therefore, the best material for the job will be the one that most effectively retains greatest percentage of the initial contact force over the life of the contact.

REFERENCE: The Influence of the Bauschinger Effect in the Performance of Connector Materials - John

Ratka