A multi-layer ceramic capacitor (MLCC), commonly referred to as a "patch," is a critical component in electronic devices. One common issue that can occur is the formation of a "twisted crack." This article explores the causes of twisted cracks and provides effective methods for preventing them.
1. What is a Twisted Crack?
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 stress or distortion. These cracks are not visible from the outside of the MLCC, so to observe them, the component must be cut open. As shown in the cross-sectional view, a twisted crack typically starts at one end of the external electrode and extends diagonally across the dielectric layer.
2. The Cause of Twisted Cracks
Twisted cracks often occur when an MLCC is soldered onto a printed circuit board (PCB). Excessive mechanical force applied during handling, assembly, or operation can cause the PCB to bend or deform over time. When the board flexes, the copper pads on the board shift, which can lead to stress on the solder joints and the external electrodes of the MLCC.
As illustrated 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, and as a result, the solder joint deforms. This deformation then causes the external electrode of the MLCC to stretch, concentrating stress at one end. If this stress exceeds the dielectric strength of the MLCC, a crack forms.
3. Effects of Twisted Cracks
When a twisted crack develops, it can significantly impact the performance of the MLCC. If the crack extends from the lower to the upper electrode, it may reduce capacitance and cause an open circuit. Even if the crack is minor, it can allow moisture and flux residues to penetrate through the gap, leading to a drop in insulation resistance. In severe cases, excessive voltage or current flow could cause a short circuit.
Once a twisted crack occurs, it is nearly impossible to repair from the outside. Therefore, prevention is key. To avoid such issues, it's essential to minimize mechanical stress during production and installation.
4. How to Measure Distortion
To prevent twisted cracks, it's important to monitor and control the amount of mechanical distortion applied to the PCB. Distortion refers to the change in length of a material under stress. It is calculated using the formula:
ε = ΔL / L
Where:
- ε = strain (amount of distortion)
- ΔL = change in length
- L = original length
For example, if a 1000 mm rod stretches to 1001 mm, the strain is 1/1000 = 0.001 or 1000 με (microstrain).
5. Preventing Twisted Cracks
Preventing twisted cracks requires a combination of proper design and process control. Here are some key strategies:
- **Process Management:** Monitor and control the level of distortion throughout the manufacturing process. Standards vary depending on the application. For safety-critical products, the acceptable strain is usually around 500 με, while for general consumer products, it may go up to 1000 με.
- **Design Considerations:**
- Maintain sufficient distance between the board edge, screw holes, and connectors (e.g., at least 10 mm).
- Avoid placing components near corners or bent areas of the PCB.
- Use routing lines instead of punching to reduce stress concentration.
- Ensure pad size is smaller than the width of the MLCC to minimize stress.
- Design the PCB with copper foil patterns to prevent warping during reflow.
- Consider using MLCCs with resin-based external electrodes for applications prone to high distortion.
By implementing these measures, manufacturers can significantly reduce the risk of twisted cracks and improve the reliability of their electronic assemblies.
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