Terminal spring force and contact resistance

Force and resistance are one chain

A terminal joint looks like two surfaces touching. In reality it is a small number of metal bridges surrounded by air and oxide. The spring pushes the surfaces together, the asperities deform, and the real contact area grows. That real area, not the apparent area of the palm, carries the current.

The chain runs from deflection to force, from force to true contact area, from area to constriction resistance, and from resistance to heat. A change anywhere in the chain shows up at the end. A spring that relaxes by a few newtons can raise the joint temperature without any visible change to the terminal.

This is why force and resistance should be specified together. A contact resistance limit without a force requirement can be met by a joint that is already on its way to failure. A force requirement without a resistance check can be met by a joint with a contaminated interface.

What sets the normal force

The normal force is the force the spring or the screw clamp applies at the contact interface. In a spring terminal, it comes from the deflection of a leaf or a cage. In a screw clamp, it comes from the torque and the stiffness of the clamp. In a crimped joint, it comes from the residual compression in the barrel, which is a different mechanism but follows the same logic.

The force depends on the spring geometry, the material and the amount of deflection. A stiff spring gives more force for the same travel, but it also needs more insertion effort and puts more stress on the housing. The design target is a force window: enough to break the oxide and hold the surfaces, but not so much that the plating is damaged or the spring yields.

Temperature changes the force. A spring that is deflected at room temperature loses force as it relaxes at a higher temperature. The loss depends on the material, the stress level and the time. A terminal that passes a force check when new can fall below the minimum after a few hundred thermal cycles.

• Spring geometry and material set the force per unit of deflection
• Insertion depth and cage design set how much deflection is reached
• Temperature and time cause stress relaxation
• Screw torque and clamp stiffness set the force in a screw terminal

The parts of contact resistance

Contact resistance has three parts. Bulk resistance is the resistance of the metal in the terminal and the conductor. Constriction resistance comes from the current squeezing through the small contact bridges. Film resistance comes from oxide, sulphide, contamination or plating on the surfaces.

Constriction resistance falls as the normal force rises, because more bridges form and each bridge grows. Film resistance behaves differently. A thin, soft film can be broken by the force, but a hard oxide layer can survive even at a high force. That is why a terminal can have a good force and a poor resistance if the surface is contaminated.

The measurement method matters. A two-wire probe includes the lead and probe resistance, which can be larger than the joint. A four-wire method passes a current through the joint and measures the voltage drop with separate sense leads, so the lead resistance drops out. State the current, the test points and the stabilization time.

Fretting and the film that grows

Fretting is small relative movement between two surfaces that are supposed to be fixed. Vibration, thermal expansion and load changes cause it. The movement breaks the oxide, exposes fresh metal, and the fresh metal oxidises again. Over many cycles, the oxide accumulates and the contact resistance rises.

Tin plated surfaces are prone to fretting corrosion because tin oxide is hard and does not conduct. A contact aid or a lubricant can reduce the movement and slow the oxide growth, but it does not replace a stable force. The geometry should limit relative motion at the interface, especially where the terminal is on a vibrating device.

A static resistance check will not show fretting. The joint can measure well on the bench and fail after a vibration test or a thermal cycle. If the application has vibration, add a cyclic test and measure resistance before and after, not just at the start.

Heat at the joint

The heat at a terminal joint comes from the current squared multiplied by the joint resistance. A joint with a resistance of a few milliohms can dissipate several watts at a high current. The heat raises the terminal temperature, which accelerates relaxation and oxidation, which raises the resistance further.

That feedback is why a temperature rise test is a better system check than a single resistance reading. Run the joint at its rated current and measure the temperature rise above ambient, along with the resistance. A joint that is stable at the rated current has margin. A joint that keeps climbing has a problem that a bench resistance check did not show.

The test conditions belong with the result. Ambient temperature, current, cable size, mounting orientation and the distance from the measurement point all change the number. A temperature rise quoted without those conditions cannot be compared with another test.

Spring materials and plating pairs

Phosphor bronze is the default spring material for terminals. It has good fatigue resistance and a moderate conductivity. Beryllium copper gives higher strength and better relaxation resistance, at a higher cost. Brass is cheaper and easier to form, but it relaxes more and is usually chosen for lower-stress parts.

Plating changes the interface. Tin is soft, breaks easily and gives a stable contact, but it frets. Silver has lower resistance and resists oxidation better, but it tarnishes in sulphur-bearing air and costs more. Nickel is hard and corrosion resistant, but its resistance is higher. The plating pair should be chosen for the environment and the number of mating cycles.

A thin plating over a rough surface can wear through at the contact spots. A thick plating can flake at a bend. The specification should state the thickness and the adhesion requirement, and the inspection should check the contact area after mating, not just the part before assembly.

Measuring force and resistance

Measure force at the deflection the terminal sees in service. A force gauge on the spring, or a load cell on a test fixture that holds the terminal at its working position, gives a number that can be compared with the drawing. A free-height force is not the same as an installed force.

Measure resistance with a four-wire millivolt drop, at a stated current, with the probes placed at defined points on the terminal and the conductor. Let the reading stabilise. A reading taken immediately after assembly can be lower than the settled value, especially if the surfaces are still moving.

Add a temperature rise test at the rated current for a qualification sample. Add a vibration or thermal cycle for the applications that need it. Record the force, the resistance and the temperature together, because the three numbers explain each other.

What to specify

The drawing should state the spring material and temper, the normal force at the working deflection with a tolerance, the contact resistance limit and the test current, the plating, and the number of mating cycles. For a high-current joint, add the rated current and the permitted temperature rise.

The test method should state where the force is applied, where the resistance probes sit and how long the reading is allowed to settle. Without those details, two laboratories can test the same terminal and report different numbers, and neither is wrong.

If the joint is critical, ask for the force and resistance after a thermal cycle, not only when new. That single addition shows whether the spring has margin and whether the plating is stable. It is the closest bench check to the conditions the terminal will meet in service.