PCBA Tech - Matters needing attention in SMT soldering chip capacitors

MLCC (Chip Multilayer Ceramic Capacitor) has now become one of the most commonly used components in SMT electronic circuits. On the surface, MLCC looks very simple, but in many cases, design engineers or production and process personnel have insufficient knowledge of MLCC. Some SMT 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 you must pay attention to it when applying it. -Let's talk about some problems and precautions in the application of MLCC.

MLCC (Chip Multilayer Ceramic Capacitor) has now become one of the most commonly used components in electronic circuits. On the surface, MLCC looks very simple, but in many cases, design engineers or production and process personnel have insufficient knowledge 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 you must pay attention to it when applying it. -Let's talk about some problems and precautions in the application of MLCC.

With the continuous development of technology, the chip capacitor MLCC can now achieve hundreds or even thousands of layers, each layer is micron-level thickness. Therefore, a slight deformation can easily cause cracks. In addition, the chip capacitor MLCC under the same material, size and withstand voltage, the higher the capacity, the more layers, and the thinner each layer, so the easier it is to break. Another aspect is that with the same material, capacity, and withstand voltage, a small-sized capacitor requires each layer of dielectric to be thinner, which makes it easier to break. The hazard of cracks is electric leakage, which can cause safety problems such as internal interlayer dislocation short circuit in severe cases. Moreover, a very troublesome problem of cracks is that they are sometimes hidden and may not be found during the factory inspection of electronic equipment. They are officially exposed at the client. Therefore, it is of great significance to prevent cracks in the chip capacitor MLCC.

When the chip capacitor MLCC is subjected to a temperature shock, it is easy to crack from the welding end. At this point, small-size capacitors are relatively better than large-size capacitors. The principle is that large-size capacitors do not conduct heat to the entire capacitor so quickly, so the temperature difference between different points of the capacitor body is large, so the expansion size is different, so Generate stress. The reason is the same as that when pouring boiling water, a thick glass is more likely to break than a thin glass. In addition, in the cooling process after the chip capacitor MLCC is soldered, the expansion coefficient of the chip capacitor MLCC and the PCB are different, so stress is generated, which leads to cracks. To avoid this problem, a good soldering temperature profile is required during reflow soldering. If wave soldering is used instead of reflow soldering, this type of failure will be greatly increased. For MLCC, it is necessary to avoid manual soldering with a soldering iron. However, things are always not so ideal. Hand soldering with a soldering iron is sometimes inevitable. For example, for electronics manufacturers that process PCBs, some products are very small. When the outsourcing manufacturers are not willing to accept this order, they can only solder by hand; when samples are produced, they are usually soldered by hand; in special circumstances, rework or rework or When repairing, welding must be done by hand; when repairing capacitors, the repairer is also doing manual welding. When MLCC is unavoidably welded by hand, it is necessary to attach great importance to the welding process.

First of all, the process and production personnel must be informed of the thermal failure of the capacitor, so that they can attach great importance to this problem in their minds. Secondly, it must be welded by specialized skilled workers. There are also strict requirements on the soldering process. For example, a constant temperature soldering iron must be used, 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. Choose a suitable solder flux and solder paste. Clean the pads first, so that the MLCC cannot be subjected to large external forces, pay attention to the quality of the soldering, etc. The best manual soldering is to put tin on the pad first, then the soldering iron melts the tin on the pad, and then put the capacitor on it. The soldering iron only touches the pad and does not touch the capacitor (which can be moved closer) during the whole process. Use a similar method (heat the tinned pad on the pad instead of directly heating the capacitor) to solder the other end.

Mechanical stress is also easy to cause cracks in MLCC. Since the capacitor is rectangular (the surface 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 direction of force must be considered when arranging the board. For example, the relationship between the direction of deformation when dividing the board and the direction of the capacitor. In the production process, try not to discharge capacitors wherever the PCB may have a large deformation. For example, PCB positioning and riveting, mechanical contact of test points during single board testing, etc. will cause deformation. In addition, semi-finished PCB boards cannot be stacked directly, and so on.