Cold Pressing Head Testing

Cold press terminal inspection mainly conducts scientific testing and analysis on the appearance and dimension testing, material composition and mechanical property testing, crimping resistance testing, crimping cross-section analysis, tensile testing and environmental reliability testing, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Appearance and dimension testing: The appearances of the cold press terminals are checked visually or with the optical magnification equipment, confirming whether their surfaces have the defects such as the cracks, burrs, scratches, oxidation and deformations. At the same time, the precision measuring tools such as the callipers, micrometers and projectors need to be used to measure their key dimensions, such as the inner diameters, outer diameters, lengths and chamfer angles of the crimping areas, ensuring that they meet the design drawings and process specifications. This is the basis of guaranteeing the crimping qualities.

Material composition and mechanical property testing: Through the spectroscopic analysers or chemical analysis methods, the chemical compositions of the materials used for the cold press terminals are tested, verifying whether they are the specified copper, aluminum or copper-aluminum alloys. The mechanical property testing includes the hardness testing (commonly using the Rockwell or Vickers hardness testers), tensile strength testing and yield strength testing, to ensure that the materials have the sufficient strengths and toughness, and can withstand the huge pressures during the crimping processes and the long-term working loads.

Crimping resistance testing: The low resistance testers (micro-ohmmeters) are used to measure the joint resistances after the crimping is completed. This testing is crucial, aiming to evaluate the electrical connection qualities of the crimping. The resistance value changes before and after the crimping need to be compared. It is required that the joint resistances after the crimping are far lower than the resistances of the equal-length wires, and the resistance values are stable, to verify whether the crimping processes effectively eliminate the oxide films on the contact surfaces and form the low-resistance and reliable metallic connections.

Crimping cross-section analysis (metallographic testing): This is the most direct destructive testing method of evaluating the crimping qualities. The crimped joints are cut along the axes, and after the mounting, grinding, polishing and etching, the metallographic specimens are made and observed under the microscopes. The main checks are whether the filling between the wire strands and the inner walls of the cold press terminals is tight, whether there are the gaps, whether the wire strands are excessively cut and deformed, as well as the flow conditions of the metal grains, thereby intuitively judging the compactness and uniformity of the crimping.

Tensile (mechanical strength) testing: The material testing machines are used to carry out the axial tensile testing on the crimped joints until the joint failure (the wires are pulled off or pulled out of the crimping barrels). The maximum breaking forces are recorded. This value must be greater than the nominal breaking forces of the wires or the minimum pull-out force values specified by the relevant standards (such as GB/T 14315 and UL 486). This testing directly verifies the reliability of the crimped joints under the mechanical stresses.

Environmental reliability testing: Simulating the actual use environments, a series of accelerated aging testing is carried out on the crimped joints. The common items include the salt spray testing (evaluating the corrosion resistance), high and low temperature cycling testing (evaluating the stability under the thermal expansion and contraction) and heat and humidity testing (evaluating the electrical property retention capabilities in the humid environments), to predict their long-term service reliability and lives.

Testing Scope

Cold press terminals for the power industry: Covering the various cable lugs, wiring terminals and equipment clamps used from the low-voltage power distribution to the high-voltage power transmission and transformation fields. The testing emphases are their capabilities of carrying the large currents, long-term thermal stability, galvanic corrosion resistance and reliability under the high mechanical stresses, needing to follow the relevant power industry standards such as DL/T 5285.

Cold press terminals for the rail transit and vehicles: Applied to the wiring harness connections in the high-speed railways, subways, automobiles and other fields. The testing scope especially emphasizes their fatigue resistance in the vibration and impact environments, as well as the strict environmental adaptability such as the flame retardancy, oil resistance and high and low temperature resistance (-40℃ to +125℃ or even higher), needing to meet the standard requirements such as EN 50343 and ISO 6722.

Cold press terminals for the aerospace and military: This is the most demanding application field. In addition to the routine items, the testing scope must include the special property testing such as the material purity, ultra-high reliability verification, vacuum outgassing rates, extreme temperature resistance (ultra-low and high temperatures) and radiation resistance. All the processes need to comply with the strict process specifications and quality traceability systems.

Cold press terminals for the communications and data centers: Mainly used for the equipment grounding and power distribution units (PDU). The testing focuses on the stability and minimization of the crimping resistances, because the small resistance fluctuations or increases will cause the heat generation when carrying the large currents, affecting the system safety. At the same time, their space occupation and installation convenience in the dense wiring environments also need to be considered.

Cold press terminals for the new energy field: Used in large quantities in the photovoltaic power stations, wind power generation and energy storage systems. The testing needs to pay attention to their long-term outdoor weatherability (UV resistance and salt spray resistance), electrochemical corrosion protection of the DC systems, as well as the special process requirements when connecting the large cross-section aluminum wires or copper-aluminum transitions, ensuring the long-term low-resistance connections in the complex environments.

Cold press terminals for the general industry and household appliances: Covering the internal wiring of the factory automation equipment and household appliances. The testing scope is relatively basic, but the regular appearances, precise dimensions and firm crimping must be ensured, meeting the consistency requirements of the batch production, and complying with the relevant testing clauses of the safety certifications such as CCC, UL and VDE.

Testing Methods

Visual inspection and dimension measurement method: This is the most basic and fastest preliminary inspection method. The operators need to rely on the sufficient illumination, and when necessary, use the 2-10 times magnifiers or stereomicroscopes to check the appearance defects against the standard samples or drawings. For the dimension measurement, according to the standards such as GB/T 3177, the measuring tools of the appropriate precision grades (such as the digital callipers with the precision not lower than 0.01mm) are selected, and the multiple-point measurements are carried out at the specified measurement points and recorded.

Spectroscopic analysis method and chemical titration method: In the material composition testing, the spark direct-reading spectrometers (OES) or X-ray fluorescence spectrometers (XRF) can quickly and non-destructively analyse the contents of the main alloying elements. For the more precise composition requirements or arbitration analysis, the laboratory methods such as the chemical titration and atomic absorption spectrometry (AAS) need to be adopted, strictly operating according to the standard processes such as GB/T 5121.

Low resistance measurement method (four-terminal method): To eliminate the effects of the test leads and contact resistances, the Kelvin four-terminal measurement method must be adopted. The two current terminals of the constant current sources are connected to the two ends of the crimping areas, and the two voltage terminals of the high-impedance voltmeters measure the voltage drops on the inner sides of the current terminals, precisely calculating the resistance values through the Ohm's law. The testing currents are usually selected according to the wire cross-sectional areas, with the principle of not causing the obvious heat generation.

Metallographic sample preparation and microscopic observation method: According to the metal micro-structure inspection methods such as GB/T 13298. The samples are cold-mounted with the resins for the fixation, and the sandpapers of the different grits (from coarse to fine) and diamond polishing agents are used in sequence for the grinding and polishing. Then the appropriate etchants (such as the FeCl3 hydrochloric acid solution for the copper) are used to etch the surfaces. Finally, the micro-morphologies of the crimping interfaces are observed, photographed and analysed under the metallographic microscopes.

Axial tensile testing method: According to the wire testing method standards such as GB/T 4909.3. The two ends of the specimens are respectively clamped in the fixtures of the material testing machines, ensuring that the force axes coincide with the joint axes. The tensile forces are applied at the constant rates (such as 25mm/min or the standard-specified rates), and the force-displacement curves are recorded in real time until the specimen failure. The breaking force values are directly read from the curve peaks, and the failure modes (wire breaking, crimping port slipping, etc.) are recorded.

Environmental test chamber simulation method: The corresponding standard testing methods are selected according to the expected use environments of the products. For example, the salt spray testing is according to GB/T 10125 (neutral salt spray testing), placing the samples in the salt spray chambers at 35±2℃ with the 5% sodium chloride solutions for the continuous spraying of the specific time (such as 96 hours). The high and low temperature cycling testing is according to GB/T 2423.22, carrying out the multiple cycles between the high temperatures (such as +85℃) and low temperatures (such as -40℃). After the testing, the electrical and mechanical property changes are immediately measured.

Testing Standards

Domestic Standards:

1. DL/T 5285, Technical specification for hydraulic compression technology of overhead conductors and ground wires in power transmission and transformation projects

2. GB/T 14315, Compression type copper and aluminium cable lugs and connectors for power cable conductors

3. GB/T 4909.3, Test methods for bare wires — Part 3: Tensile test

4. GB/T 5121, Methods for chemical analysis of copper and copper alloys

5. GB/T 13298, Inspection methods of microstructure for metals

6. GB/T 3177, Geometrical product specifications (GPS) — Inspection of plain workpiece sizes

7. GB/T 228.1-2021, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

8. GB/T 10125-2021, Corrosion tests in artificial atmospheres — Salt spray tests

9. GB/T 2423.22, Environmental testing — Part 2: Test methods — Test N: Change of temperature

International Standards:

1. EN 50343, Railway applications — Rolling stock — Rules for installation of cabling

2. ISO 6722, Road vehicles — 60 V and 600 V single-core cables

3. ISO 6892-1, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

4. UL 486, Standard for safety for wire connectors

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Precision dimension measuring equipment: Including the digital callipers/micrometers, height gauges, two-dimensional image measuring instruments and coordinate measuring machines (CMM). The image measuring instruments can non-contactingly and quickly measure the two-dimensional profile dimensions; the coordinate measuring machines are used to obtain the precise coordinate data of the complex three-dimensional morphologies, evaluating the shape and position tolerances. They are the key equipment of testing the high-precision cold press terminal molds and products.

Material analysis instruments: The spark direct-reading spectrometers (OES) are used for the fast composition analysis before the furnaces; the desktop X-ray fluorescence spectrometers (XRF) are used for the non-destructive testing of the finished products; the micro hardness testers (such as the Vickers hardness testers) are used to measure the local hardness of the crimping areas; the universal material testing machines are used to complete the mechanical property testing such as the tensile and yield strengths, and can be equipped with the high and low temperature environmental chambers for the temperature-controlled testing.

Electrical property testing equipment: The core is the DC low resistance testers (micro-ohmmeters), whose measurement resolutions need to reach the micro-ohm (μΩ) or even nano-ohm (nΩ) levels, and have the four-terminal testing capabilities. For the large current temperature rise testing, the adjustable large current sources (up to several thousand amperes) and infrared thermal imagers or thermocouples are needed, used to monitor the temperature rises of the crimped joints under the rated currents.

Metallographic preparation and observation system: A complete system includes the cutting machines, mounting machines, grinding and polishing machines (automatic or manual), metallographic microscopes (with the CCD cameras and image analysis software). The cutting needs to use the precision cutting blades to reduce the thermal effects; the automatic grinding and polishing machines can ensure the repeatability and consistency of the sample preparation processes; the digital metallographic microscopes are convenient for the storage, analysis and comparison of the crimping cross-section images.

Environmental reliability testing equipment: Mainly including the salt spray test chambers, high and low temperature alternating heat and humidity test chambers, thermal shock test chambers and vibration test tables. These equipment can simulate and accelerate the harsh environmental conditions. Their core control parameters (such as the temperature uniformity, humidity precision, salt spray deposition rates, vibration frequencies and accelerations) must meet the requirements of the relevant testing standards (such as the GB/T 2423 series).

Dedicated crimping tensile testing fixtures: To cooperate with the material testing machines for the pull-out force testing, the dedicated testing fixtures need to be designed. The fixtures need to be able to firmly clamp the cold press terminals without damaging their structures, while ensuring that the tensile forces are strictly applied along the axes. For the cold press terminals of the different shapes and sizes, a series of replaceable clamping blocks or sleeves usually need to be equipped, to adapt to the diversified testing requirements.