With the continuous development of technology, the chip capacitor MLCC can now achieve hundreds or even thousands of layers, each layer is micron thickness. Therefore, it is easy to cause cracks when it is slightly deformed. In addition, the same material, size and chip capacitance MLCC under pressure, the higher the capacity, the more layers, the thinner each layer, and the easier it is to break. On the other hand, when the same material, capacity and withstand voltage, a small size capacitor requires that each layer of the medium be thinner, resulting in easier breakage. The hazard of cracks is leakage, and in serious cases, it causes safety problems such as short-circuiting between internal layers. And the crack has a very troublesome problem, sometimes it is more concealed, it may not be found in the factory inspection of electronic equipment, and it is officially exposed to the client. Therefore, it is of great significance to prevent the chip capacitor MLCC from cracking.
When the chip capacitor MLCC is subjected to temperature shock, cracks easily occur from the solder end. At this point, the small size capacitor is relatively better than the large size capacitor. The principle is that the large size capacitor does not reach the entire capacitor so quickly, so the temperature difference between the different points of the capacitor body is large, so the expansion is different. Stress is generated. This is the same as thick glass when it is poured into boiling water. In addition, during the cooling process after the chip capacitor MLCC is soldered, the expansion coefficients of the chip capacitors MLCC and the PCB are different, so that stress is generated and cracks are caused. To avoid this problem, a good soldering temperature profile is required for reflow soldering. If wave soldering is used without reflow soldering, this failure will increase significantly. MLCC is to avoid the process of soldering with a soldering iron. However, things are always less than ideal. Manual soldering of soldering irons is sometimes inevitable. For example, for electronic manufacturers of PCB out-of-process processing, some products are extremely small, and the manufacturers of patch-external manufacturers are not willing to pick up such singles, and can only be hand-welded; when the samples are produced, they are usually hand-welded; special cases are reworked or When repairing welding, it must be hand soldered; when the repairman repairs the capacitor, it is also hand soldered. When it is unavoidable to manually weld MLCC, it is necessary to attach great importance to the welding process.
First of all, it is necessary to inform the process and production personnel of the thermal failure of the capacitor, so that it is highly valued. Second, it must be welded by a skilled worker. It is also necessary to strictly meet the welding process. For example, it is necessary to use a constant temperature soldering iron. The soldering iron should not exceed 315 ° C (to prevent the production workers from increasing the soldering temperature), and the soldering time should not exceed 3 seconds to select the appropriate soldering flux and solder paste. First clean the pad, do not subject the MLCC to a large external force, pay attention to the quality of the welding and so on. The best hand soldering is to let the solder on the pad first, then the soldering iron melts the tin on the pad. At this time, the capacitor is placed. The soldering iron only contacts the pad and does not touch the capacitor (movable close) during the whole process. Weld the other end in a similar way (heating the tin pad on the pad instead of heating the capacitor directly).
Mechanical stress is also likely to cause cracks in the MLCC. Since the capacitor is rectangular (parallel to the PCB) and the short side is the soldered end, it is naturally prone to problems when the long side is subjected to force. Therefore, the direction of the force should be considered when arranging the plates. For example, the direction of deformation when the board is divided is the relationship of the direction of the capacitor. In the production process, wherever the PCB may cause large deformation, try not to put capacitors. For example, PCB positioning riveting, mechanical contact of test points during veneer testing, etc. will cause deformation. In addition, semi-finished PCB boards cannot be stacked directly, and so on.
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