MLCC (chip multilayer ceramic capacitor) has become one of the most commonly used components in electronic circuits. On the surface, MLCC is very simple, but in many cases, the design engineer or production and process personnel have insufficient understanding of MLCC. Some companies also have some misunderstandings in the application of MLCC, thinking that MLCC is a very simple component, so the process requirements are not high. In fact, MLCC is a very fragile component, so we must pay attention to it in application. Here are some problems and precautions in MLCC application.
With the continuous development of technology, MLCC can now achieve hundreds or even thousands of layers, each layer is micron level thickness. So a little deformation is easy to crack. In addition, MLCC with the same material, size and withstand voltage, the higher the capacity, the more layers, the thinner each layer, and the easier to break. On the other hand, when the same material, capacity and withstand voltage are used, the capacitance with small size requires that each layer of medium is thinner, which leads to easier fracture. The damage of crack is leakage, which causes the safety problems such as dislocation and short circuit between layers. And the crack has a very troublesome problem, sometimes it is relatively hidden, which may not be found in the factory inspection of electronic equipment, and it will be officially exposed in the client. So it is very important to prevent MLCC from cracking.
When MLCC is impacted by temperature, it is easy to crack from the welding end. At this point, the small capacitance is better than the large capacitance. The principle is that the heat conduction of the large capacitance does not reach the whole capacitance so fast, so the temperature difference at different points of the capacitance body is large, so the expansion size is different, resulting in stress. This is the same as pouring boiling water into a thick glass is more likely to crack than a thin glass. In addition, during the cooling process after MLCC welding, the expansion coefficients of MLCC and PCB are different, which leads to stress and crack. To avoid this problem, a good welding temperature curve is needed during reflow welding. If wave soldering is used instead of reflow soldering, the failure will increase greatly. MLCC should avoid manual welding process with soldering iron. However, things are not so ideal. Soldering iron by hand is sometimes inevitable. For example, for the electronic manufacturers of PCB outsourcing processing, some products are very small, when the patch outsourcing manufacturers are not willing to accept such orders, they can only weld by hand; when the samples are produced, they are usually welded by hand; when rework or repair welding under special circumstances, they must weld by hand; when the repairmen repair the capacitance, they are also welded by hand. When manual welding of MLCC is unavoidable, great attention should be paid to the welding process.
First of all, it is necessary to inform the process and production personnel of the thermal failure of the capacitance, so that they attach great importance to this problem ideologically. Secondly, it must be welded by specialized skilled workers. In addition, the welding process shall be strictly required, for example, constant temperature soldering iron shall be used, the soldering iron shall not exceed 315 ° C (to prevent the production workers from drawing fast and increasing the welding temperature), the welding time shall not exceed 3 seconds, the appropriate welding flux and solder paste shall be selected, the pad shall be cleaned first, the MLCC shall not be subjected to large external force, and the welding quality shall be paid attention to, etc. The best manual welding is to make the pad tin first, then the soldering iron melts the tin on the pad, and then put the capacitance on it. The soldering iron only contacts the pad in the whole process without touching the capacitance (it can be moved near), and then uses the similar method (heating the tinning cushion on the pad instead of directly heating the capacitance) to weld the other end.
Mechanical stress is also easy to cause cracks in MLCC. Because the capacitance is rectangular (parallel to PCB) and the short side is the welding end, it is easy to cause problems when the long side is stressed. Therefore, the stress direction should be considered when arranging plates. For example, the relationship between the deformation direction of the split plate and the direction of the capacitance. In the production process, where PCB may have large deformation, try not to discharge capacitance. For example, PCB positioning and riveting, mechanical contact of test point during single board test, etc. will produce deformation. In addition, the semi-finished PCB cannot be stacked directly, etc. More questions can be inquired directly!
