**Multi-Layer Ceramic Capacitors and the Issue of Twisted Cracks**
A multi-layer ceramic capacitor, commonly referred to as a "patch," is a crucial component in modern electronic devices. However, one common issue that can occur during its use is the formation of a "twisted crack." This article explores the causes of twisted cracks and discusses effective methods to prevent them.
**What is a Twisted Crack?**
01
To better understand what a twisted crack looks like, refer to Figure 1. A twisted crack is a type of internal fracture caused by mechanical distortion. These cracks are not visible from the outside of the patch, so we often need to cut the component to examine it under a microscope.
From the cross-sectional view, you can see that a twisted crack typically starts at one end of the external electrode and extends diagonally through the ceramic body.
**Representative Example of a Crack**
**The Cause of Twisted Cracks**
02
Twisted cracks usually occur when a patch is soldered onto a printed circuit board (PCB). If excessive mechanical stress is applied—such as bending or aging of the board—the resulting deformation can cause the copper pads on the board to shift.
As shown in Figure 2, when the PCB is bent, the top side stretches while the bottom side compresses. This movement causes the copper pads to shift left or right, which in turn affects the solder joints.
When the solder deforms, the external electrodes of the patch move as well, leading to concentrated tensile stress on one end of the electrode. If this stress exceeds the dielectric strength of the ceramic material, a crack forms.
**The Impact of Twisted Cracks**
03
If a twisted crack propagates from the lower external electrode to the upper one, it can reduce the capacitance of the component, causing an open circuit in the system. Even minor cracks can be problematic if they reach the internal electrodes. In such cases, flux residues containing organic acids and moisture may penetrate through the crack, reducing insulation resistance.
Moreover, if the voltage load becomes too high and current flow increases, the worst-case scenario could lead to a short circuit. Once a twisted crack occurs, it's nearly impossible to repair from the outside. Therefore, prevention is key. The best approach is to avoid applying excessive mechanical force during the manufacturing and assembly processes.
**What is the Amount of Distortion?**
04
To prevent twisted cracks, it’s important to understand how much mechanical force is being applied. One way to measure this is by calculating the amount of distortion.
Distortion refers to the change in length per unit length when a force is applied. It is calculated using the formula:
**ε = ΔL / L**
Where:
- ε = amount of distortion
- ΔL = change in length
- L = original length
For example, if a 1000 mm rod stretches to 1001 mm, the distortion is **1 mm / 1000 mm = 0.001**, or **1000 μST** (microstrain).
**How to Prevent Twisted Cracks**
05
To prevent twisted cracks, both design and process management play critical roles. First, measure the amount of distortion in each step of the production process. Then, set appropriate standards for acceptable levels of distortion.
Typical values are:
- 500 μST for life-critical products
- 1000 μST for general consumer electronics
However, even with the same level of distortion, different board types and thicknesses can result in varying stress levels. Therefore, it's essential for customers to adjust these standards based on their experience and the specific application.
Some key preventive measures include:
1. **Maintaining proper distances** between the board edge, screw holes, and connectors (e.g., at least 10 mm).
2. **Optimizing layout** to avoid placing patches in areas where stress might concentrate, such as corners or bent parts of the board.
3. **Choosing the right dividing lines**—using etching instead of punching where possible.
4. **Designing pad sizes** to ensure the pad width is smaller than the patch’s width.
5. **Using copper foil mode** in PCB design to minimize warping during reflow.
6. **Selecting resin-coated external electrode capacitors** for applications prone to large distortions.
By implementing these strategies, manufacturers can significantly reduce the risk of twisted cracks and improve the reliability of their electronic systems.
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