Tinned Round Copper Wire Testing

Tinned round copper wire inspection mainly conducts scientific testing and analysis on the appearance and dimension inspection, tin coating thickness and adhesion testing, mechanical property testing, electrical property testing, solderability testing and chemical composition analysis, 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 inspection: Mainly checks the surface quality, wire diameter uniformity and dimension deviations of the tinned round copper wire. Specifically including observing whether the tin coating is continuous, uniform and smooth, and whether there are the defects such as the exposed copper, oxidation, black spots and burrs, and using a precision micrometer or laser diameter measuring instrument to measure whether its diameter and non-circularity meet the tolerances specified by the standards, such as within ±0.005 mm.

Tin coating thickness and adhesion testing: This testing item aims to evaluate the quality and reliability of the tin coating. The tin coating thickness is precisely measured by the metallographic microscope method, electrolytic thickness measurement method or X-ray fluorescence spectrometry, usually in the range of 1-5 μm. The adhesion testing checks whether the tin layer cracks or falls off through the winding, cupping or jerking methods, ensuring that it remains intact in the subsequent processing.

Mechanical property testing: Mainly evaluates the tensile strength and elongation at break of the tinned round copper wire, which are the key indicators for measuring its toughness and processing performance. An electronic universal material testing machine is used to stretch at the rate specified by the standards, recording the maximum breaking force and elongation length, and calculating the tensile strength (such as ≥210 MPa) and elongation (such as ≥15%), to ensure that it can meet the process requirements such as the winding and braiding.

Electrical property testing: The core is the determination of the DC resistance of the conductor. At the standard temperature (such as 20 ℃), a double bridge or low resistance tester is used to measure the resistance value of the tinned round copper wire per unit length (usually 1 metre or 1 kilometre). The resistivity is the key parameter for measuring the conductive ability of the conductor, and must meet the requirements for the tinned soft round copper wire in the national standards (such as GB/T 4910), to ensure the low-loss current transmission.

Solderability testing: Evaluates the soldering performance of the tinned round copper wire in the electronic assembly. The commonly used methods include the solder bath method, solder ball method or wetting balance method. During the testing, the specimen is immersed in the solder bath at the specified temperature, and the spreading and wetting of the solder on the coating surface are observed. It is required to form the bright, continuous and fully covered solder joints within the specified time (such as 2-3 seconds), without the solder retraction or non-wetting phenomena.

Chemical composition analysis: Although the base material is the high-purity copper, the compositions of the copper material and tin coating still need to be verified. A spectrometer (such as the ICP-OES) is used to detect whether the contents of the impurity elements (such as the oxygen, sulphur and phosphorus) in the copper material are within the specified limits. At the same time, the tin purity of the coating and whether it contains the hazardous substances such as the lead and cadmium are analysed, to meet the requirements of the environmental protection regulations (such as the RoHS).

Testing Scope

Products of the different wire diameter specifications: The testing scope covers the full series of the tinned round copper wires from the ultra-fine wires (such as the diameter 0.05 mm) to the thicker wires (such as the diameter above 2.0 mm). The application scenarios of the products of the different wire diameters vary, and the testing emphasis is also different. The fine wires focus more on the dimensional precision and coilability, while the thicker wires pay more attention to the mechanical strength and resistance uniformity.

Products of the different hardness states: According to the different annealing processes, the tinned round copper wires are divided into the soft state, half-hard state and hard state. The soft-state wires (commonly used for the winding wires) need to emphatically test the elongation and repeated bending performance; the hard-state wires (commonly used for the jumper wires and lead wires) focus on the tensile strength and rigidity, and the testing standards and methods need to be adjusted accordingly.

Products of the different coating types: In addition to the routine pure tin coatings, it also includes the coatings containing the small amounts of the alloying elements (such as the copper, silver and cerium) and the lead-free tin coatings. The testing scope needs to cover these special coatings, verifying whether their thickness, composition, solderability and corrosion resistance meet the specific technical agreements or environmental protection requirements.

Raw materials and finished wires: The testing is not only for the final finished products, but also extends to the upstream of the production chain. Including the incoming inspection of the resistivity and chemical composition of the incoming copper rods, as well as the surface cleanliness and dimension inspection of the intermediate products (such as the bright copper wires) in the tinning process, realizing the whole process quality control.

Specialized wires for the specific application fields: For the tinned round copper wires used in the special fields such as the coaxial cable braided layers for the high-frequency signal transmission, precision instrument coils, automotive wiring harnesses and motor windings, the testing scope needs to add the specific items, such as the high-frequency resistance (skin effect), thermal shock resistance and chemical solvent resistance application performance testing.

Production process and process monitoring: The testing scope also includes the online monitoring and process parameter verification. For example, the online diameter measuring instruments monitor the wire diameter fluctuations in real time, the electrolytic thickness measuring instruments quickly sample the coating thickness, and the drawing solution, annealing furnace atmosphere and plating solution compositions are regularly analysed, ensuring the process stability.

Testing Methods

Dimension measurement method: The wire diameter and ovality are the basic testing items. Usually, a micrometer with the precision of 0.001 mm or a more precise laser diameter measuring instrument is used, and multiple points (usually at least 3 points, at a distance of more than 1 metre from each other) are selected on the sample for the measurement, taking the average value and the maximum deviation value. For the ultra-fine wires, the measurement needs to be carried out under a microscope with a micrometer eyepiece, ensuring the accurate results.

Coating thickness determination — metallographic microscope method: This is the arbitration method. The sample is vertically mounted to make a metallographic specimen, and after grinding, polishing and etching, the thickness of the cross-section of the tin coating is directly observed and measured under a metallographic microscope equipped with a micrometer scale. This method is intuitive and accurate, but is a destructive testing, and the sample preparation requirements are high and time-consuming.

Coating thickness determination — coulometric method (electrolytic thickness measurement method): A commonly used fast quantitative method. The sample is used as the anode and electrolyzed at a constant current in a specific electrolyte. When the tin coating is completely dissolved and the copper substrate is exposed, the electrolytic voltage has a sudden change. By recording the electrolytic time and current, the coating thickness is precisely calculated according to the Faraday's law. This method is fast, and the damage to the sample is small (only a tiny etching point remains).

Mechanical property testing — tensile testing method: Carried out in accordance with the standards such as GB/T 4909.3. The specimen of the specified gauge length (usually 250 mm or 500 mm) is clamped on a tensile testing machine and stretched at a constant rate (such as the standard rate for the copper wires) until the fracture. The testing machine automatically records the force-displacement curve, thereby calculating the key data such as the tensile strength, yield strength and elongation after fracture.

DC resistance determination — bridge method/resistance tester method: A Kelvin double bridge or high-precision micro-ohmmeter is adopted for the measurement. Before the testing, the sample needs to be placed in a constant temperature environment for a sufficient time to make the temperature uniform. During the measurement, the four-terminal method with the separated current terminals and potential terminals is used for the wiring, to eliminate the influence of the lead resistance and contact resistance, thereby obtaining the high-precision resistance value per unit length.

Solderability testing — solder bath method: The most commonly used qualitative evaluation method. The specimen treated with the flux is immersed in the molten solder bath at the set temperature (usually 235 ℃±5 ℃) at the specified angle and speed, maintained for the specified time (such as 2-3 seconds) and then withdrawn. After cooling, it is observed under a magnifying glass whether the solder coverage area reaches more than 95%, and the coating should be completely melted into the solder, forming the bright and smooth covering layer.

Testing Standards

Domestic Standards:

1. GB/T 4910, Tinned round copper wire

2. GB/T 4909, Test methods for bare wires (series)

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

International Standards:

1. IEC 60228, Conductors of insulated cables

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

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 mechanical outer diameter micrometers (resolution 0.001 mm), digital micrometers and laser online/offline diameter measuring instruments. The laser diameter measuring instrument uses the laser scanning shadow method to realize the non-contact, high-frequency (thousands of times per second) online real-time measurement, and automatically records the data, calculates the deviations and performs the statistical process control (SPC). It is the key equipment for ensuring the wire diameter consistency.

Coating analysis instruments: The core equipment includes the metallographic microscopes (with the image analysis system), coulometric coating thickness gauges and X-ray fluorescence spectrometers (XRF). The XRF can realize the non-destructive, fast multi-element coating thickness and composition analysis, and is particularly suitable for the rapid sampling inspection on the production lines. The scanning electron microscope (SEM) combined with the energy dispersive spectrometer (EDS) is used for the high-magnification observation of the coating morphology and micro-area composition analysis.

Mechanical property testing equipment: The electronic universal material testing machine is the core, with the high load and displacement measurement precision, and is equipped with the various fixtures to adapt to the clamping of the wire materials. An extensometer also needs to be equipped to precisely measure the strain. In addition, the wire torsion testing machines and repeated bending testing machines are used to evaluate the flexibility and fatigue resistance of the wire materials, and are also the important auxiliary equipment.

Electrical property testing equipment: Including the DC resistance testers (micro-ohmmeters), Kelvin double bridges and constant temperature liquid baths (for the sample constant temperature). The high-precision equipment requires the measurement uncertainty better than 0.5%. For the high-frequency applications, a network analyser may be needed to measure its AC resistance and characteristic impedance at the specific frequencies.

Solderability and surface analysis equipment: The solderability tester (wetting balance method) can quantitatively measure the wetting force and wetting time, which is more objective than the solder bath method. In addition, the scanning electron microscope is used to observe the crystal morphology of the coating, and the surface roughness tester is used to quantify the surface finish of the coating, all of which have the direct influence on the solderability.

Environmental and chemical analysis equipment: The salt spray test chamber is used to evaluate the corrosion resistance of the tin coating; the constant temperature and humidity chamber is used for the high temperature and high humidity aging testing. The inductively coupled plasma optical emission spectrometer (ICP-OES) or atomic absorption spectrometer (AAS) is used to precisely analyse the trace element contents in the copper substrate and plating solution, ensuring that the material purity meets the standards.