The solderability of the coating refers to the property that the coating is easily wetted by molten solder under certain test conditions. Its characteristics include two meanings: one is the bonding force of welding, that is, "welding quality", which is measured by the size of the wetting force within a given time; the other is the time required for welding, which is specified to achieve a certain degree of wetting Time required to measure. Therefore, the test of solderability should include these two aspects.
The methods for measuring the solderability of coatings include slot welding method, ball welding method and wet weighing method. For the quantitative test of general round wires, the ball bonding method is more accurate than the slot bonding method. Neither of these two test methods can display the wetting force, an important indicator of solderability, while the wet weighing method can comprehensively evaluate the technical indicators of solderability.
1. Slot welding method
Dip the sample soaked in standard flux into an electrothermal solder bath (solder temperature is controlled at 235±2°C, take it out after 3s, observe the wetting condition of the sample surface to judge the solderability.
2. Ball welding method
Place the flux-coated lead horizontally on the molten ball of solder (see Figure 7-2-19a). The size of the solder ball depends on the diameter of the lead. The lead drops into the solder ball, which splits evenly in two. From the moment the lead wire touches the heating block (Fig. 7-2-19b) to when the solder ball completely wraps the lead wire (Fig. 7-2-19c), this time is the brazing time.
Note that the soldering time starts when the lead reaches the surface of the heating block, not when it first touches the solder.
The test results of this method are consistent with the actual situation, and the operation is convenient.
3. Wetting weighing method
When the sample is immersed in the molten solder, the sample will be subjected to the combined effects of its own gravity, the buoyancy of the liquid solder, and the interfacial tension between the sample, solder and flux. If the shape and immersion depth of the sample are constant during the measurement, it can be considered that the gravity and buoyancy are constant, while the interfacial tension will change with the temperature of the sample and the action of flux. This change in interfacial tension reflects the wetting performance between the solder and the sample. The wet weighing method is based on this point to evaluate the weldability of the sample.
During the test, the sample coated with flux is suspended from a sensitive scale, dipped into the molten solder tank, and one end of the sample is inserted to a certain depth. At this time, the resultant force of the buoyancy force acting on the sample and the vertical component force of the interfacial tension is measured by the sensor and converted into a voltage signal, which is displayed by the digital voltmeter. Then record it as a function of time with a printer or Recorder.
See Figure 7-2-20 for a typical wetting curve tested by wet weighing method.
(1) Sample preparation and test conditions
Specimens should be carefully protected from contamination of the surface, especially by fingers. The test conditions are:
①Solder is made of Sn 60%, Pb 40%.
② Flux with rosin 25%, isopropanol or ethanol 75%.
③Solder temperature is controlled at 235±2℃.
④ The speed at which the sample is dipped into the molten solder is (25±2) mm/s, and the dipping depth is 2~5mm.
(2) Evaluation of test results
① Time required to reach a given degree of wetting: such as zero-crossing time, that is, the time required from when the sample is immersed in molten solder to F=0 (point G in Figure 7-2-20). The smaller the zero crossing time, the better the solderability.
② The degree of wetting achieved within a given time: IEC 50C (Secretariat), document No. 40 recommends that it be the wetting force (F) at 3s. The more negative the F, the better the solderability. However, it is generally expressed as a percentage of F to the theoretical wetting force.
The theoretical wetting force at the time of complete wetting can be calculated as follows:
F principle=-0.4P+0.08V
Among them, F theory represents the theoretical wetting force (mN);
P is the perimeter of the sample (mm);
V is the volume of the sample entering the molten solder part (mm³)
At 235°C, the surface tension constant of the specified flux and solder is 0.4N/m; the density of molten solder is approximately 8000kg/m³.
Wetting weighing method Tester has ready-made equipment at home and abroad (such as British MULTI-CORED company and Japanese Tamura Manufacturing Co., Ltd.).
