Things to note when using chip capacitors on lighting power supplies

MLCC (Multilayer Ceramic Capacitor) has now become one of the most commonly used components in electronic circuits. MLCC appears to be very simple, but in many cases, design engineers 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, and you must be careful when using it. ??The following discusses some problems and precautions in the application of MLCC.

With the continuous development of technology, chip capacitors MLCC can now have hundreds or even thousands of layers, and each layer is micron-level thick. So a slight deformation can easily cause cracks. In addition, for chip capacitors MLCC with the same material, size and withstand voltage, the higher the capacity, the more layers, the thinner each layer, and the easier it is to break. On the other hand, when the material, capacity and withstand voltage are the same, small-sized capacitors require thinner dielectric layers, which makes them easier to break. The hazard of cracks is leakage, and in severe cases, it causes safety problems such as internal interlayer misalignment and short circuit. Moreover, one of the troublesome problems of cracks is that they are sometimes hidden and may not be found during the factory inspection of electronic equipment, and they are only officially exposed when they reach the customer end. Therefore, it is of great significance to prevent chip capacitors MLCC from cracking.

When chip capacitors MLCC are subjected to temperature shock, cracks are likely to start from the welding end. In this regard, small-sized capacitors are relatively better than large-sized capacitors. The principle is that the heat conduction of large-sized capacitors does not reach the entire capacitor so quickly, so the temperature difference between different points of the capacitor body is large, so the expansion size is different, thus generating stress. This is the same as the fact that a thick glass is more likely to break than a thin glass when boiling water is poured into it. In addition, during the cooling process after the MLCC is soldered, the expansion coefficients of the MLCC and PCB are different, so stress is generated, leading to cracks. To avoid this problem, a good soldering temperature curve is required during reflow soldering. If wave soldering is used instead of reflow soldering, this failure will be greatly increased. MLCC should avoid the process of manual soldering with a soldering iron. However, things are not always so ideal. Manual soldering with a soldering iron is sometimes inevitable. For example, for electronic manufacturers that outsource PCB processing, some products are very small in quantity, and when the chip outsourcing manufacturer is unwilling to accept such an order, manual soldering can only be done; when samples are produced, manual soldering is generally also done; when rework or repair soldering is required in special circumstances, manual soldering must be done; when repairmen repair capacitors, manual soldering is also done. When manual soldering of MLCC is inevitable, it is necessary to pay great attention to the soldering process.

First of all, the process and production personnel must be informed of the problem of thermal failure of capacitors, so that they attach great importance to this problem in their minds. Secondly, the soldering must be done by specialized skilled workers. Strict requirements must also be met in the welding process, such as using a constant temperature soldering iron, the soldering iron should not exceed 315°C (to prevent production workers from increasing the welding temperature for speed), the welding time should not exceed 3 seconds, choose the appropriate solder flux and solder paste, clean the pad first, do not subject the MLCC to large external forces, pay attention to the welding quality, etc. The best manual welding is to tin the pad first, then melt the tin on the pad with the soldering iron, and then put the capacitor on it. The soldering iron only touches the pad and not the capacitor during the whole process (it can be moved closer), and then use a similar method (heat the tinned pad on the pad instead of directly heating the capacitor) to solder the other end.

Mechanical stress can also easily cause cracks in MLCC. Since the capacitor is rectangular (parallel to the PCB) and the short side is the solder end, it is natural that the long side is prone to problems when subjected to force. Therefore, the force direction should be considered when arranging the board. For example, the relationship between the deformation direction during board separation and the direction of the capacitor. During the production process, try not to place capacitors in places where the PCB may have large deformations. For example, PCB positioning riveting, mechanical contact of test points during single board testing, etc. will all cause deformation. In addition, semi-finished PCB boards cannot be stacked directly, etc.