The single-sided printed base plate has low cost and wide application, and has the disadvantages of poor mechanical properties and many restrictions on production. Moreover, the solder joint stability and life performance of the single-sided substrate are not as good as the double-sided through-hole printed circuit board. Therefore, life assessment is particularly important. Figure 1 is a welding model.
The deterioration of welding is divided into the following three types:
The tension load of the 1 wire causes the joint surface of the wire to be loose and the copper foil connection portion to peel off.
2 The welding and the looseness of the wire part caused by the dynamic load. The dynamic load is a load that is stable and oscillating.
3 Welding creep caused by static load. A static load is a fixed load. Creep is the long-term backlog of static loads that causes permanent deformation inside the weld.
Second, the dynamic load
1. The two types 1 and 2 are faults caused by the welding deformation being affected by the load and exceeding the elastic limit.
This process is accelerated by the effects of temperature cycling. Figure 2 illustrates an example of a dynamic load causing a defective product. As shown in the figure, the wire of the double joint part is welded to the joint area, and the cycle of 100 ° C (8 hours) and normal temperature 25 ° C (16 hours) is repeated, and after 5 cycles per cycle, the vibration test is performed at room temperature for 1 hour. , so that the welded part receives dynamic load. The defective rate is a ratio at which the welding of the single-sided joining zone and the wire portion are inferior in contact after repeating this cyclic processing for 30 times.
2. Static load
When metal materials and plastic materials are subjected to load, they are deformed. This deformation can cause permanent deformation even if it does not exceed the elastic limit. As shown in Fig. 1, when welding a printed substrate, deformation generally occurs in the middle of the hole and the wire.
Figure 3 is the result of a weld creep model test for a printed substrate. The sample used a single-sided joint zone with a joint diameter of 2.5 mm, a hole diameter of 0.8 mm, and a wire diameter of 0.5 mm. Together with the tensile load on the weld surface, the time required for the creep variable to reach 0.5 mm at various temperatures was measured.
It can be seen that when the printed board is soldered, if the weight of the component loaded on one of the wires exceeds 10 g, the durability and stability of the product may be adversely affected.
3. Creep allows stress
As shown in Fig. 1, the area of ​​the creep generating surface in the welding is Sc, and the static load is W, then the creep stress is:
F=W/Sc
In the single-sided printed substrate, the creep stress is preferably kept below 20 to 30 g/mm2. In the double-sided through-hole base plate, the same size joint area is used, and the stress value is only 1/4 to 1/5 of that of the single-sided printed base plate. Therefore, from the perspective of improving product durability, the advantages of using a double-sided substrate are numerous.
The above has explained the life of the power supply unit. However, even if a malfunction occurs, it is necessary to ensure that the type of the fault is controlled within a safe range. The so-called safety first, this must be considered in the design.
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