There are two main causes of mechanical cracking. The first is an extrusion crack, which is generated during the operation of the component pick-up on the PCB board. The second is the deformation crack caused by the bending or twisting of the PCB board. Extrusion cracks are mainly caused by incorrect pick-and-place machine parameter settings, while bending cracks are mainly caused by excessive bending of the rear plate of the PCB board on which the components are welded.
How to distinguish between extrusion crack and bending crack?
A crush crack will show up on the surface of the component, usually a circular or half-moon crack with a color change, at or near the center of the capacitor. When additional stresses from subsequent processing are applied to the component, these small cracks spin into large cracks, including stresses caused by PCB buckling.
The bending crack is marked by a "Y"-shaped crack or a 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 the MLCC. Bending cracks are mainly located near the PCB solder joints.
How does the incorrect setting of the mounter parameters cause cracks?
The pick and place head of the placement machine uses a vacuum suction tube or center pliers to position the components. The adjustment of the parameters in the X, Y and especially in the Z direction is essential for collision avoidance elements. It is easily understood that excessive Z-axis lowering pressure will break the ceramic element. However, if the pick and place head of the mounter applies a large enough force at a location other than the central area of the porcelain body, the stress exerted on the capacitor may be large enough to damage the component.
Similarly, improper sizing of the patch pick and place head can easily cause cracks. The small diameter of the patch pick and place head will concentrate the placement force during the patch, which will cause the MLCC to crack because the smaller area is under greater pressure.
In addition, fragments scattered on the PCB can also cause cracks. When the capacitor is placed, the uneven surface of the PCB causes the downward pressure on the capacitor to be unevenly distributed, so that the capacitor can break.
How does PCB bending cause crack spinning?
When the ceramic chip capacitor MLCC is mounted on the PCB board, it becomes part of the circuit board. The FR-4 material is the most commonly used as PCB board, its stiffness is not big, easy to produce bending. The ceramic substrate of the chip capacitor will not bend with the bending of the plate, so it will be subjected to tensile stress.
Ceramic materials have high compression 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 recommend dosage is 50 ~ 75% of the welding belt height of the porcelain body. Too much solder will increase the tensile stress on the chip capacitor MLCC when the PCB is bent.
The inconsistent amount of solder will produce inconsistent stress distribution on the component, and stress concentration will occur at one end, resulting in cracks.
The pad size is equally important. In addition to accommodating placement variations, the correct pad size can balance the formation of the solder ribbons during the soldering process. Pad sizes not recommend in the manufacturer's detail specification are not recommended.
What are the factors that cause MLCC cracks?
The manufacturer's packaged product is unlikely to be cracked, and most SMD capacitor MLCC manufacturers take great care to ensure final appearance inspection quality and correct handling. In addition to the extrusion of the mounting process and the bending of the machining process, cracks are also caused by thermal shock, in-board testing and hydrogen absorption.
How do capacitor users detect cracks?
The first is to provide more resources to avoid the occurrence of cracks rather than to detect the presence of cracks. However, the crack can be detected on the board by using a resistance tester. Generally, there is a crack in the capacitor, and the resistance value will drop, or the resistance value will drop significantly after aging.
Note: Mark "WARNING" to avoid plate bending and direct component contact.
How to avoid cracks when using ceramic chip capacitor MLCC?
Correct pick and place position setting and minimal board bending are key. PCB splitting after surface mounting is a particularly delicate process, and any bending during splitting will introduce stress, as discussed above. In addition, the proximity and orientation of the MLCC to the PCB board split plane is extremely important. The split holes and slots on the PCB should be designed away from the MLCC. The mounting orientation of the MLCC should be parallel to the opening to ensure that the MLCC is subjected to minimal tensile stress when the PCB is bent. Placing the MLCC parallel to the cut line and away from the contact point is the best placement direction.