Precautions for using chip capacitors in LED drive circuits

    The full name of chip capacitors is multilayer (laminated, stacked) chip ceramic capacitors, abbreviated as MLCC. When MLCC is subjected to temperature shock, cracks are likely to occur from the soldering 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 at different points of the capacitor body is large, so the expansion size is different, thus generating stress. This is the same as when boiling water is poured into a thick glass cup, it is easier to break than a thin glass cup. In addition, during the cooling process after MLCC welding, the expansion coefficients of 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 also 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 manufacturers are unwilling to accept such orders, they can only do manual welding; when samples are produced, they are generally also manually welded; when rework or repair welding is required under special circumstances, manual welding is required; when repairmen repair capacitors, they also do manual welding. When manual welding of MLCC is unavoidable, it is necessary to pay great attention to the welding process.

　　As we all know, IC chip packages are divided into SMD and DIP. It is generally believed that the difference between SMD and DIP is mainly the size and welding method, which has little effect on system performance. In fact, it is not. Every trace on the PCB has an antenna effect. Every component on the PCB also has an antenna effect. The larger the conductive part of the component, the stronger the antenna effect. Therefore, for the same model chip, the antenna effect of a smaller package size is weaker than that of a larger package size.

　　For the same device, it is easier to pass EMC test if using SMD components than DIP components. In addition, the antenna effect is also related to the working current loop of each chip. To weaken the antenna effect, in addition to reducing the package size, the working current loop size, working frequency and di/dt should be reduced as much as possible. If you pay attention to the pin layout of the latest IC chips (especially single chips), you will find that most of them have abandoned the traditional way - GND in the lower left corner and VCC in the upper right corner, and arrange VCC and GND in adjacent positions in order to reduce the size of the working current loop.

　　Not only IC chips, but also resistor and capacitor (BUZ60) packages are related to EMC. 0805 package has better EMC performance than 1206 package, and 0603 package has better EMC performance than 0805 package. 0603 package is currently popular internationally. Parts package refers to the appearance and location of solder joints indicated when the actual parts are soldered to the circuit board. It is a pure spatial concept. Therefore, different components can share the same parts package, and the same components can also have different parts packages. Like resistors, there are traditional pin-insertion types. This type of component is large in size, and the circuit board must be drilled to place the component. After drilling, the component is inserted, and then it is passed through a tin furnace or sprayed with tin (it can also be hand-soldered). The cost is high. Newer designs all use small surface mount components (SMD). This type of component does not need to be drilled. Use a steel film to pour semi-molten solder paste into the circuit board, and then put the SMD component on it, and it can be soldered on the circuit board.