PCBA Tech - MLCC assembly quality in smt placement production

Multilayer ceramic capacitors (MLCC) are widely used in aerospace electronic equipment due to their small size, low cost, large capacitance per unit volume, high operating temperature, compact structure, good frequency characteristics and high reliability. MLCC can be applied to various circuits, such as oscillation circuit, timing or delay circuit, coupling circuit, decoupling circuit, filter circuit, high frequency noise suppression circuit, bypass, etc. In order to meet the further development of integrated circuit and surface mount technology.

MLCC is developing in the direction of high capacity and miniaturization, and strives to make multilayer ceramic capacitors have more layers and thin dielectric layers. However, as MLCCs become smaller (thin), MLCC assembly becomes more difficult and prone to failure problems. There are a large number of MLCC components in an important model product of a smt placementmanufacturer. According to the needs of product design, the assembled electronic equipment is subjected to temperature cycling and stress screening. After temperature cycling and stress screening, the phenomenon of failure of multilayer ceramic capacitors continues to occur,

which makes the reliability of the product and the production schedule slow The degree of weeks is affected. In response to the problems, the manufacturer established a research team to optimize the assembly quality of multilayer ceramic capacitors, hoping to find a solution as soon as possible to ensure product quality. Next, let's explain and analyze in detail.

1. Analysis of failure causes of multilayer ceramic capacitors

In response to the problems that appeared, the research team analyzed the causes of the problems. And investigated the MLCC component manufacturers, after a period of research and analysis, combined with the actual use, summarized the reasons that may cause the failure of the multilayer ceramic capacitor.

1. Failure of multilayer ceramic capacitors caused by external forces

1) Once the multilayer ceramic capacitor is soldered on the PCB, any external force will affect the multilayer ceramic capacitor on the PCB, such as smt patch proofing or excessive or sharp bending of the PCB during processing and assembly, causing the two welding ends to be opposite Directional mechanical stress, at the weakest position of the capacitor, generally produces cracks at the junction of the ceramic body and the metal electrode.

The crack may be very thin in the initial stage without penetrating the inner electrode. After temperature shock and stress screening, there will be cracks. Generally, such cracks cannot be detected with the naked eye. Normal tests are generally not found. Only when the cracks expand and moisture infiltrates at low temperatures, the failure will occur.

2) Since the multilayer ceramic capacitor body is brittle and has no leads, it is greatly affected by the force. Once the internal electrode is easily disconnected by the external force, the multilayer ceramic capacitor will fail. Any breakage or breakage of the multilayer ceramic capacitor terminal caused by external force will cause the multilayer ceramic capacitor to fail.

3) Due to the quality problem of the poor bonding force of the terminal (body and electrode) of the multilayer ceramic capacitor, the metal electrode is likely to fall off after the process of welding, warming, debugging and other external forces, that is, the body and the electrode are separated.

2. Failure caused by improper welding operation

1) Improper manual soldering operation or rework of electric soldering iron

The thermal shock caused by the soldering of the electric soldering iron to the multilayer ceramic capacitor is very common. Thermal shock occurs during soldering. If the operator touches the tip of the soldering iron directly to the capacitor electrode, the thermal shock will cause the multilayer ceramic capacitor body to microcrack, and the multilayer ceramic capacitor will fail after a period of time.

2) The tin on both ends of the capacitor is asymmetrical during welding

The tin on both ends of the capacitor is asymmetrical when soldering. When soldering, the tin on both ends of the capacitor is asymmetrical. When the capacitor is subjected to external force or a stress screening test, the soldering tin will seriously affect the ability of the multilayer ceramic capacitor to resist mechanical stress. The electrode cracked and failed.

3) Too much solder

The factors related to the degree of mechanical stress generated by the multilayer ceramic capacitor on the PCB include the materia l and thickness of the PCB, the amount of solder and the location of soldering. In particular, too much solder will seriously affect the ability of the chip capacitor to resist mechanical stress. The capacitor has failed.