Cracking and failure of surface-mount capacitors are relatively common production problems. A crack can cause the component to short circuit, and may also result in abnormal heating or, in severe cases, ignition. The primary cause of capacitor cracking is usually bending stress applied to the PCB.
Mechanism of Capacitor Bending Cracks
When a PCB bends, stress is distributed along the board in the L direction, as shown below.
As the PCB bends, its surface stretches in the L direction. The pads on the surface move with the board, as illustrated below.
The crack generally begins on a plane perpendicular to the tensile stress and initiates near the external electrode end of the capacitor. This location is particularly vulnerable because the component is mechanically coupled to the PCB through its solder joints. When the pads move as the board bends, the resulting load is transferred into the capacitor body and its terminations.
The capacitor's mechanical strength and the stress generated by PCB bending determine whether the component will survive the deformation. As the bending displacement increases, the stress in the capacitor also increases. Once that stress exceeds the lower limit of the component's mechanical strength, a crack or other failure occurs.
The failure process can be understood as a progression rather than an instantaneous event. PCB deformation first produces tensile stress in the component. A small crack then forms near the external electrode and extends into the ceramic body. As the crack grows, it can approach or intersect the internal electrode structure. The electrical insulation margin is consequently reduced, and the capacitor may eventually develop leakage, intermittent conduction, or a direct short circuit.
Insulation resistance may change significantly after cracking. One point requiring particular attention is that insulation resistance may partially recover after the damaged capacitor is heated with a hot-air tool or soldering iron. This recovery does not prove that the component has been restored. Heating can temporarily alter the mechanical contact or moisture-related condition at the crack, while the physical damage remains. A capacitor that has experienced excessive bending should therefore be treated as suspect even if a subsequent electrical check appears normal.
Capacitor Bending-Strength Test Method
Using Murata capacitors as an example, the recommended reference pad design is shown below. The pad geometry is part of the mechanical test condition because it affects how board deformation is transferred through the solder joints into the component.
The test sequence is as follows:
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Mount the capacitor on the PCB.
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Increase the applied load until the PCB deforms.
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Measure the amount of PCB deformation when the capacitor is damaged or fails.
The deformation value at failure provides a mechanical indicator for the tested component and PCB configuration. It should be interpreted together with the pad design, soldering condition, component type, and test setup. A result from one configuration should not automatically be applied to every capacitor or PCB assembly.
Failure Criteria
Capacitor damage can be determined using either of the following methods:
- Capacitance method: determine failure by applying a limit to the capacitance change rate.
- DPA method: perform destructive physical analysis to detect cracks after testing. DPA is an abbreviation for Destructive Physical Analysis.
Capacitance testing provides an electrical screening method, while DPA provides direct physical evidence of cracking. These methods can therefore reveal different aspects of the same failure. A component may require physical analysis when the electrical change is small but the mechanical damage is suspected.
Measuring PCB Strain at the Capacitor Location
Strain describes the degree of deformation in an object. It represents the relative displacement or change in length between points within the object. Strain is a ratio and therefore has no unit; it is commonly represented by the symbol ε.
Because the displacement caused by mechanical loading is usually small, often at the micrometer scale, the resulting strain value is also small. For this reason, strain is commonly expressed using scientific notation. A strain of 10-6 is called one microstrain and is represented as με.
The basic relationship is:
ε = ΔL / L
where ε is strain, L is the original length before loading, and ΔL is the change in length caused by deformation.
For example, if a 1,000 mm-long PCB stretches to 1,001 mm, the change in length is 1 mm. The strain is therefore:
1 mm / 1,000 mm = 1 / 1,000 = 0.001 = 1,000 × 10-6 = 1,000 με
The equipment required for strain testing includes a computer, display, strain measurement instrument, and strain sensors.
The strain sensor is installed near the solder pads and at the location where the component is mounted.
The objective is to determine the stress applied at the component mounting location. In practice, the stress at the capacitor itself cannot be measured directly in a simple manner. Instead, the change in PCB deformation is measured, and the strain value is used to estimate the mechanical stress experienced by the component.
This approach connects the board-level mechanical event to the component-level failure mechanism. If a crack is found after a cutting, handling, or test operation, strain measurement can help identify whether excessive PCB deformation occurred at the capacitor location during that operation.
Examples of PCB Operations That Can Cause Capacitor Damage
Common failure cases include the following operations:
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PCB cutting.
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Test fixtures.
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PCB loading and unloading.
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PCB drops.
These operations can bend or locally deform the PCB, transferring tensile stress through the solder joints into the capacitor body. Preventing bending-related failures therefore requires attention not only to the capacitor and its pad design, but also to every operation that mechanically supports, cuts, probes, transports, or handles the assembled board.
