News & Technicals

Factors Affecting the Resistance Between Electrodes

The contact resistance exists for a short period of time, usually in the early stage of welding, which is formed by two reasons:
1) The oxide or dirty material layer with high resistivity on the surface of the workpiece and the electrode will greatly hinder the current. Too thick oxide and dirty material layers can even make the current unable to conduct.
2) Under the condition that the surface is very clean, due to the microscopic unevenness of the surface, the workpiece can only form contact points on the rough surface. At the contact point, the current line is gathered. Due to the shrinking of the current path, the resistance at the contact is increased. The resistance Rew between the electrode and the workpiece is compared with Rc and Rw. Because the resistivity and hardness of the copper alloy are generally lower than that of the workpiece, it is very small and has a smaller effect on the formation of the nugget. We less consider its influence.

2. Welding Current, the influence of current on heat generation is greater than both resistance and time. Therefore, it is a parameter that must be strictly controlled during the welding process. The main cause of the current change is the fluctuation of the grid voltage and the change of the impedance of the secondary circuit of the AC welding machine. The impedance changes are due to changes in the geometry of the loop or due to the introduction of different amounts of magnetic metal in the secondary loop. For DC welding machines, the impedance change of the secondary loop has no obvious effect on the current.
3. Welding Time
In order to ensure the size of the nugget and the strength of the solder joint, the welding time and the welding current can complement each other within a certain range. In order to obtain a certain strength of the solder joints, high current and short time (strong condition, also called hard specification) can be used, or small current and long time (weak condition, also called soft specification) can be used. Whether to choose a hard specification or a soft specification depends on the properties and thickness of the metal and the power of the welding machine used. There are upper and lower limits for the current and time required for metals with different properties and thicknesses, which shall prevail when used.
4. Electrode Pressure
The electrode pressure has a significant effect on the total resistance R between the two electrodes. As the electrode pressure increases, R decreases significantly, but the increase in welding current is not large, which cannot affect the reduction in heat generation caused by the decrease in R. Therefore, the strength of the solder joint always decreases as the welding pressure increases. The solution is to increase the welding current while increasing the welding pressure.
5. Electrode Shape and Material Properties
Since the contact area of the electrode determines the current density, the resistivity and thermal conductivity of the electrode material are related to the generation and loss of heat. Therefore, the shape and material of the electrode have a significant influence on the formation of the nugget. With the deformation and wear of the electrode tip, the contact area increases, and the strength of the solder joint will decrease.
6. Surface Condition of the Workpiece
Oxides, dirt, oil and other impurities on the surface of the workpiece increase the contact resistance. An oxide layer that is too thick may even make it impossible for current to pass. Partial conduction, due to excessive current density, will produce splashes and surface burns. The presence of the oxide layer will also affect the uneven heating of each solder joint, causing fluctuations in soldering quality. Therefore, a thorough cleaning of the surface of the workpiece is a necessary condition to ensure high-quality joints.