Will the chip capacitor fail? Cause analysis of chip capacitor damage

Date：2022-07-25 16:08:44Views：1001

The shape of the chip capacitor is similar to that of the chip resistor. Usually, the chip capacitor is a white substrate, and most of the tantalum electrolytic capacitors are black substrates, but the positive side is also marked with white polarity. Chip capacitor is one of the common electronic components on PCB. Although the size of chip capacitor is very small, it seems to have no effect, but it is not. The existence of chip capacitor has its certain application and function. Then, will the chip capacitor fail? What are the main failure causes of chip capacitor damage? This article has collected and sorted out some materials, which are expected to be of great reference value to readers.

What is the main cause of mechanical cracks in ceramic chip capacitor MLCC?

There are two main reasons for mechanical cracks. The first is extrusion crack, which occurs in the operation process of picking up and placing components on PCB. The second is deformation cracks caused by bending or twisting of PCB. The extrusion crack is mainly caused by the incorrect parameter setting of the picking and placing machine, while the bending crack is mainly caused by the excessive bending of the PCB board after the component is welded.

How to distinguish between extrusion crack and bending crack?

Extrusion cracks will be exposed on the surface of the component, usually round or half moon shaped cracks with changed color, which are located in or near the center of the capacitor. When the additional stress generated by the subsequent processing process is applied to the component, these small cracks will become large cracks, including the stress caused by PCB bending.

The sign of bending crack is a "Y" shaped crack or 45o angle oblique crack, which can be observed under the DPA section. Such cracks may or may not be observed on the outer surface of MLCC. The bending crack is mainly located near the PCB solder joint.

How does the incorrect setting of the parameters of the mounter cause cracks?

The pick-up and placement head of the mounter uses a vacuum pipette or a center clamp to position the components. 10. Parameter adjustment in Y direction, especially in Z direction, is very important to avoid collision with components. It is easy to understand that excessive z-axis falling pressure will break ceramic components. However, if the picking and placing head of the mounter applies enough force at a certain position instead of the central area of the porcelain body, the stress applied to the capacitor may be enough to damage the components.

Similarly, the improper size of the tip pick-up and placement head will easily cause cracks. The placement force of the small-diameter patch pickup and placement head will be concentrated during the patch placement, which will cause MLCC cracks because the small area is under greater pressure.

In addition, the debris scattered on the PCB will also cause cracks. When placing the capacitor, the uneven surface of PCB causes uneven distribution of downward pressure on the capacitor, so the capacitor will be broken.

How does PCB bending cause split spinning?

When the ceramic chip capacitor MLCC is mounted on the PCB, it becomes a part of the circuit board. The FR-4 material is the most commonly used PCB board. Its stiffness is not large and it is easy to bend. The ceramic matrix of chip capacitor will not bend with the bending of the plate, so it will be subjected to tensile stress.

Ceramic materials have high compressive strength and low tensile strength. When the tensile stress is greater than the strength of the porcelain body, cracks occur. The main factor affecting the bending strength is the amount of solder. The recommended dosage is 50~75% of the height of the welding strip for the porcelain body. Too much solder will increase the tensile stress on the chip capacitor MLCC when the PCB is bent.

Inconsistent amount of solder will produce inconsistent stress distribution on the component, and stress concentration will occur at one end, resulting in cracks.

Pad size is also important. In addition to adapting to placement changes, the correct pad size can balance the formation of the welding strip during the welding process. Pad sizes not recommended by the manufacturer's detailed specifications are not recommended.

What other factors cause MLCC cracks?

The products packaged by the manufacturer are unlikely to have cracks. Most MLCC manufacturers of SMD capacitors are very careful to ensure the quality of final appearance inspection and correct handling operations. In addition to extrusion during mounting and bending during processing, cracks can also be caused by thermal shock, in-board testing and hydrogen absorption.

How do capacitor users detect cracks?

The most important thing is to provide more resources to avoid cracks rather than to detect whether cracks exist. However, cracks can be detected on the plate by using a resistance tester. Generally, there are cracks in the capacitor, and the resistance value will decrease, or the resistance value will decrease significantly after aging.

Note: mark "warning" to avoid bending of the plate and direct contact with the components.

How to avoid cracks when using ceramic chip capacitor MLCC?

Correct setting of picking and placing position and minimum plate bending are the key. PCB splitting after surface mounting is a particularly delicate process, and any bending during splitting will cause stress, as discussed above.

In addition, the proximity and direction between MLCC and the dividing surface of PCB are extremely important. The design of holes and slots on PCB should be far away from MLCC. The mounting orientation of the MLCC should be parallel to the opening to ensure that the MLCC receives the minimum tensile stress when the PCB is bent. MLCC layout parallel to the cutting line and away from the contact point is the best placement direction.

During plate splitting, the stress on element a is the largest, followed by elements C and D. Elements B and E are in the best position, but element E is far away from the dividing line, so the stress is minimal. It is better to place the element away from the dividing line, because the closer it is to the dividing line, the greater the stress.

When selecting the voltage and capacitance value of chip capacitor, attention should be paid to:

1. As the capacitance value (UF level) of VCC capacitor, try to choose foreign big brands such as TDK capacitor or increase the voltage value of domestic capacitor by one level.

2. Try not to choose chip capacitors with high voltage and large capacity, such as capacitors above 100v/100nf. Generally speaking, as long as we pay attention to design and production, we can avoid the failure of most chip capacitors.