Analysis of the factors affecting the current-carrying capacity of metal terminals of waterproof connectors: based on the experimental data of the current-carrying capacity curve and temperature rise of Tyco MCP630 terminals

1. How do we determine the performance of metal terminals through rigorous experiments?

We characterize the performance of metal terminals in the experiment mainly through the following two figures:

1.1 Practical performance: current-temperature rise curve

This curve is like an “electronic component medical report” that provides benchmark data on thermal stability during the design phase. It reflects the performance of metal terminals in a generalized, relatively normal environment (mainly characterized by thermal response). It is like measuring the stability, fuel efficiency and failure rate of a car under normal driving conditions. In low-voltage power distribution systems, engineers can use this to calculate the safety margin of connectors during continuous operation, to avoid insulation aging caused by uncontrolled temperature rise.

1.2 Performance limits: ambient temperature-current carrying capacity curve

This reflects the terminal's capacity under the boundary conditions of application performance. In other words, this graph reflects the maximum performance of the metal terminal under high standards and demanding usage environments. This is like measuring the extreme performance of a car under extreme track conditions, just like the maximum torque and maximum speed that Xiaomi SU7 Ultra will indicate. (Incidentally, Xiaomi SU7 Ultra is just like our LLT, it wants it all!)

2. How do the base material, plating and crimping method affect the current-carrying performance of the terminal?

2.1 Effect of contact material on current-carrying capacity

Combining the data analysis of diagram 11 and diagram 13, we can have a systematic understanding of the effect of changing the contact material on the current-carrying capacity.

When the conductor cross-sectional area is kept constant, changes in the material's electrical conductivity are directly reflected in performance. As can be seen from the current-temperature rise graph, after the contact material is changed from a high-performance copper alloy to an ordinary copper alloy (i.e., when the electrical conductivity is reduced from 64% to 22%), the maximum allowable current-carrying capacity measured under the USCAR standard temperature rise requirement of 55°C is only reduced by about 11%.

Moreover, the advantage of the contact material with better conductivity in terms of thermal stability is even more obvious in operating conditions with higher currents. However, judging from the temperature rise curve, if the temperature of the scene is relatively high, the difference between the two is not so obvious.

2.2 Effect of plating on current-carrying capacity

A comparison of the data in Diagram 11 and Diagram 12 shows that the plating does not appear to affect the current-carrying capacity of the metal terminals:

Whether it is tin (Sn) or gold (Au) plating, the current-temperature rise curves at low and normal temperatures almost overlap, indicating that the impedance properties of the plating have little effect on the current-carrying capacity of the terminals.

This phenomenon shows that when the resistance properties of the plating material (silver/tin plating) are similar, the effect of the plating itself on the current-carrying capacity of the terminals is not significant.

However, things are not so simple. We cannot just look at the experimental data without considering the interpretability of other characteristics of the data. We can clearly see that when the ambient temperature increases above 130°C, the current-carrying capacity of the tin-plated material decreases significantly compared to the silver-plated material.

If you haven't forgotten everything you learned in university about materials science, you know that silver plating is more stable at high temperatures and can withstand higher ambient temperatures, while tin plating has a lower temperature tolerance and its conductivity decreases at high temperatures.

2.3 The effect of connection method on current-carrying capacity

Comparing the data in Diagram 14 (cold crimping) and Diagram 17 (soldering), we can see that the structure, base material and plating are exactly the same, but the different connection methods have a direct and significant impact on the temperature rise properties of the terminals. The experimental data shows that the temperature rise of the soldered terminals is significantly lower than that of the cold crimped terminals.

Why is this?

In the field of electrical connector manufacturing, the essential differences between cold crimping and soldering determine the physical effects of the two processes. When the metal interface undergoes the high-pressure plastic deformation of cold crimping and the melting and crystallization of soldering, it forms two very different microscopic contact forms: the surface of the cold crimping part retains a micro-convex structure formed by the original machining texture, while the soldered surface shows a smooth interface of metallurgical bonding. This difference in surface topography, combined with the process-specific normal load, ultimately gives rise to two different conductive networks at the contact interface – the former forms discrete, micro-scale conductive spots via the tips of the micro-bumps, while the latter establishes a continuous and uniform conductive channel.

Experience shows that the density and size of the conductive spots directly constitute the “microscopic code” of the contact resistance. The discrete groups of contacts formed by the random contact of the micro-bumps on the cold-pressing interface often have an equivalent conductive area that is smaller than the continuous contact surface formed by welding. This difference in electrical conductivity, which is determined by the process genes, is precisely the key consideration for engineers when choosing a process route for designing high-reliability connectors – both to tame the micron-level undulations on the material surface and to precisely control the interface load in order to construct an ideal current path in the microscopic world.

In other words, the difference in processing conditions between cold crimping and soldering results in different surface roughness and loading forces of the two contact cross-sections. The size and number of contacts on the contact interface, however, depend precisely on the surface roughness and loading force of the material. Anyone with even a basic knowledge of electrical contacts in connectors understands that the size and number of contacts directly determine the contact resistance.

Therefore, the cold-pressing method may result in insufficient pressure or poor contact, leading to high contact resistance and thus a large temperature rise. The soldering method, on the other hand, melts the contact surfaces together at high temperatures, making the contact surface more even and firm, with lower resistance and a smaller temperature rise, thus improving the current-carrying capacity of the terminals.

Based on the above theoretical analysis, we can draw the following conclusions:

• Material selection strategy: If the design of the contact material is optimized, the practical performance and performance limits of the connector can be improved to a certain extent, provided that the conductivity of the material is significantly improved.
• Plating optimization strategy: If the design of the plating is optimized, the practical performance cannot be improved much, but the performance limits can be significantly improved.
• Wiring process strategy: If a better wiring process is selected, both practical performance and application performance will be significantly improved.