For engineers specializing in hardware and connector design, they often encounter such problems: Even though copper was used, the terminals still heat up, the contact resistance fluctuates, and the vibration test fails outright?
Many people instinctively think: The higher the conductivity, the more reliable the connector. But in actual engineering, even when the conductivity of pure copper is at its maximum, it is rarely directly used as a contact component. This is because there is an inherent trade-off between "conductivity and strength" of the material. Today, we will thoroughly explain the conductivity of contact materials and after reading this, you will be able to directly apply it to drawing reviews and failure analysis.
Why are connector contacts rarely made of pure copper?
Pure copper (brass) has an IACS conductivity of 100%, and its conductivity is at the ceiling level among common metals. Its tensile strength is only 200-250 MPa.
The drawback is equally fatal: it's too soft. When pure copper is made into terminals, they will undergo permanent deformation after a few insertions and extractions, resulting in a direct drop in contact pressure and a subsequent sharp increase in contact resistance.
Therefore, the industry generally opts for copper alloys: they sacrifice some conductivity in exchange for sufficient mechanical strength. Here is a vivid analogy:
Conductivity is like the capacity of a highway; alloy elements are like toll booths set up on the highway, reducing the speed of vehicle flow (electronic) while making the entire road (material) more sturdy and durable.
Conductivity and strength, a pair of inherent seesaws
The essence of alloying is to introduce foreign atoms into the copper lattice. These foreign atoms will scatter electrons, causing continuous "collisions" of electrons, and the macroscopic manifestation is an increase in resistivity; at the same time, it hinders lattice slip, making the material less prone to deformation, and enhancing the strength.
In copper alloys, it is difficult to achieve both high conductivity and high strength simultaneously; one can only find a balance point.
Two types of strengthening routes in the industry:
Solid solution strengthening
Alloying elements are dissolved into the copper lattice, resulting in a significant increase in strength, but a significant loss of conductivity; suitable for scenarios with medium to low conductivity. Representative materials: brass, phosphor bronze.
Precipitation enhancement
The strengthening is achieved by precipitating tiny particles through heat treatment, which causes little damage to conductivity; however, the process is complex and costly. Representative materials: beryllium copper, copper-chromium-zirconium alloy.
Real failure case
A consumer electronics manufacturer aimed to reduce material costs and replaced the power terminal from phosphor bronze to brass. The static tests passed all, but after the vibration aging test, the contact resistance increased by three times.
The root cause was that brass had insufficient elasticity, and under vibration conditions, the terminal would undergo minor permanent deformation, resulting in loss of contact pressure. The saved material costs could not cover the later repair claim costs at all.
Simple selection logic:
For terminals with few insertion and removal times and long-term static storage, brass is sufficient;
For those that need repeated insertion and removal and withstand vibration and impact, phosphor bronze or beryllium copper should be preferred.
The thickness of the coating is not always the thicker the better! Many engineers misunderstood this.
A very common misconception: the thicker the coating, the better the conductivity.
Core mission of the coating: anti-corrosion, stable contact interface, not to reduce the body resistance of the base material. The vast majority of current flows through the base metal, and the coating only affects the very thin contact interface area.
The gold coating has a conductivity lower than pure copper (about 70% of pure copper), but it hardly oxidizes and can maintain a low contact resistance for a long time;
The tin coating has a lower conductivity, but the tin oxide film can be crushed under the contact pressure, revealing fresh metal, and is the mainstream choice for consumer products.
There is also a boundary for coating thickness: if the coating is too thick, it will change the mechanical properties of the contact component, and under force, it is prone to micro cracks, and the cost of precious metal coatings will increase significantly.
Select based on the working conditions, not blindly stacking thickness: ✅ Damp environment, high reliability requirements → Gold/Palladium-Nickel ✅ Indoor dry ordinary scenarios → Tin only ✅ Frequent board-to-board connectors → Prefer Gold
Temperature: The overlooked contact resistance amplifier
All metals have an increased resistance rate with temperature rise. For copper alloys, the general rule is that for every 1°C increase in temperature, the resistance increases by approximately 0.4%.
Take an intuitive example: at 25°C, the contact resistance is 5 mΩ; at 85°C, the resistance will reach approximately 6.2 mΩ, without considering the additional degradation caused by high-temperature accelerated oxidation.
This leads to two engineering points:
The resistance data obtained from the temperature rise test should be converted to the standard temperature to have horizontal comparison significance; directly comparing the resistance measured in different environmental temperatures will lead to distorted conclusions.
Be alert to the positive feedback of thermal runaway:
Current heating → Temperature increase → Resistance becomes larger → Heating further intensifies. For large current connectors, the contact resistance must be reserved with sufficient design margin.
?? Read the temperature rise report first to confirm two points: test environment temperature, working current; room temperature-looking data may fail when placed in a high-temperature complete machine environment.
How to understand the supplier's material data sheet
The three core parameters in the manual: resistance rate (μΩ·cm), conductivity (%IACS), resistance temperature coefficient (/℃).
Two easily-mistaken details:
The conductivity label is mostly a typical value, not the minimum value, and the actual batch performance will drop by a few percentage points; directly using the typical value for calculation will result in insufficient design margin.
For the same alloy, the conductivity of cold processing hardening (H state) is slightly lower than that of annealing (O state), and cold processing brings a large number of lattice defects. When selecting the material, the annealed state cannot be ignored.
High-conductivity copper alloys of ≥80% IACS are used for large current charging guns and busbar connectors, mostly relying on precipitation/ deformation strengthening, with a cost higher than ordinary phosphor bronze, and for large current design, it is recommended to refer to the manual of such materials.
Contact resistance = contraction resistance + coating resistance
Contact resistance consists of two parts:
Contraction resistance: It is generated by the compression of current through tiny contact spots, determined by the base material's resistance rate, the lower the resistance rate, the smaller the contraction resistance.
Coating resistance: It comes from surface oxidation and contamination films, depending on the material's chemical corrosion resistance, mainly solved by the coating.
This also explains why the industry standard solution has always been: copper alloy substrate + functional coating. The substrate withstands mechanical strength and provides conductive properties; the coating guards the contact interface and resists environmental corrosion, and both are indispensable.