Analysis of Fracture of Surface mount Capacitors
Fracture of surface mount capacitors (MLCC) is a common and serious failure mode in electronic assembly and use. The essence of its fracture is that the mechanical or thermal stress borne by the brittle ceramic body (dielectric layer) exceeds the material strength limit. The following is a systematic analysis of the reasons:
1、 Main fracture mechanism and stress source
Mechanical stress (most common cause):
PCB bending/twisting:
Improper splitting operation: Excessive mechanical stress or blunting of the splitting blade during V-Cut splitting; The stamp hole design is unreasonable; Manually bend the board.
Assembly process: Excessive tightening of screws causes local deformation of the PCB; Excessive force on connector insertion and removal; Improper pressing of the testing fixture.
Usage process: equipment falling, impact; The equipment is subjected to continuous vibration during transportation or use, especially in automotive electronics and portable devices.
Point stress impact: During operation, tools (such as tweezers) accidentally hit capacitors; Touching the nozzle of the hot air gun during maintenance; Other components or structural parts directly compress the capacitor.
Improper PCB design/layout:
Place capacitors in high stress areas: near board edges, screw holes, connectors, PCB splicing separation lines (V-Cut lines), board corners. These positions experience the greatest deformation during partitioning, assembly, or under stress.
Large capacitors parallel to the dividing line: When the long axis of the capacitor is parallel to the V-Cut line or the direction of force on the dividing board, it bears greater bending stress and is more prone to cracking.
Mismatch in solder pad design: The size of the solder pad is too large or too small, causing the capacitor body to be suspended or excessively tightened after soldering.
Thermal stress:
Temperature cycle/shock:
Improper reflow soldering/wave soldering temperature curve: The heating/cooling rate is too fast, resulting in significant thermal stress between the ceramic body and the PCB (FR4 material) due to the huge difference in coefficient of thermal expansion (CTE) (ceramic CTE ≈ 7-10 ppm/° C, FR4 CTE X/Y ≈ 14-17 ppm/° C, Z-axis up to 50-80 ppm/° C). During the cooling process, stress accumulates on the fragile ceramic body.
Severe temperature changes in the working environment: equipment frequently switches on and off or operates in extreme temperature difference environments (such as car engine compartments).
Localized overheating: During maintenance, the temperature of the hot air gun is too high or the heating time is too long, resulting in uneven heating of the capacitor itself or the surrounding area.
Material and manufacturing defects (relatively few, but not negligible):
Microcracks inside ceramic media: tiny defects generated during the capacitor manufacturing process (sintering, cutting) that become stress concentration points and the starting point for subsequent propagation.
Poor bonding between the end electrode and the ceramic body: The bonding strength between the end electrode and the ceramic body is insufficient, and it is easy to crack from the joint under stress.
2、 Fracture characteristics and effects
Typical location: It is most commonly found on the ceramic body near the end electrode at the bottom of the capacitor body, with a 45 ° oblique crack (caused by the tensile/shear stress of PCB bending). Cracks may also appear in the middle or top of the capacitor body.
Failure mode:
Electrical open circuit: The crack completely cuts off the internal electrode path.
Intermittent faults/parameter drift: Microcracks cause changes in the distance between electrodes or poor contact, resulting in decreased capacity, increased losses, and decreased insulation resistance.
Short circuit: Cracks can cause metal bridging or contamination intrusion between internal electrodes (less common, but more dangerous).
Consequence: Causing abnormal circuit function, performance degradation, or even complete failure, and in severe cases, may lead to even more serious system failures.
3、 Solutions and preventive measures
Optimize PCB design and layout:
Stay away from high stress areas: Avoid placing MLCC near board edges (at least>3mm), V-Cut lines (at least>5mm), screw holes/mounting posts, connector crimping areas, and board corners, especially for large sizes (such as 1206, 1210, and above) and high voltage/high capacitance capacitors.
Optimization direction: Make the long axis of large-sized capacitors perpendicular to the main bending direction of the PCB (usually parallel to the board edge or V-Cut line). When designing the panel, the long axis of the capacitor should be perpendicular to the dividing line.
Reasonable design of solder pads: Follow IPC standards or device specification recommendations to ensure good solder joint shape and avoid stress concentration caused by "pillow effect" or "dog bone" effect. Consider using "tear drops" or "stress relief pads".
Add support: Add supporting structures (such as glue or support pads) below or around large MLCCs.
Optimize panel design: Adopt more flexible connection methods (such as stamp holes instead of V-Cut), or use more advanced panel splitting processes (such as milling cutter cutting, laser cutting).
Improve manufacturing and assembly processes:
Optimize the partitioning process:
Prioritize using routing or laser cutting instead of V-Cut cutting.
If V-Cut is necessary, ensure that the depth and angle of the V-groove are accurate, the cutting tool is sharp, the cutting speed is moderate, and avoid severe impact.
After splitting, polish the edges of the board appropriately to remove burrs.
Control reflow soldering curve: Strictly control the heating rate (especially for large-sized/high-capacity MLCC) and cooling rate (key!) to make the temperature change gentle and reduce thermal shock. Follow the recommendations in the device specification sheet.
Standardized operation:
Avoid applying unnecessary mechanical force (such as excessive pressing or twisting) to the PCB during production, testing, and maintenance processes.
Use appropriate tools (anti-static tweezers) to avoid direct impact on the capacitor.
Standardize the tightening torque and sequence of screws.
Optimize the design of testing fixtures to avoid probes or pressure plates directly pressing on capacitors or causing excessive deformation of the PCB.
Glue reinforcement: For critical large-sized MLCCs that are in extremely high vibration environments (such as automobiles, industrial equipment) or difficult to avoid high stress areas, reinforcement glue (such as silicone rubber, epoxy resin) can be applied at the bottom of their sides to increase mechanical support and disperse stress. Attention should be paid to the selection of glue and process control.
Device selection considerations:
Choose flexible terminal capacitors: MLCC with "flexible terminal electrode" design is preferred. This design adds a special material with buffering effect (such as resin, metal particle composite material) between the end electrode and the ceramic body, which can effectively absorb PCB bending or thermal stress, significantly reducing the risk of ceramic body cracking.
Size selection: On the premise of meeting electrical performance and space requirements, try to choose small-sized capacitors (such as 0603, 0402 instead of 0805, 1206), which have stronger bending resistance.
Voltage/capacitance margin: Leave an appropriate margin to avoid the device operating near its limit parameters.
Strengthen process control and inspection:
Incoming inspection: Sampling inspection of MLCC used in critical locations (such as X-ray inspection of internal structure).
Process monitoring: Strictly monitor key process parameters such as reflow soldering temperature curve, splitting parameters, and screw tightening torque.
Failure analysis: After a fracture failure occurs, a thorough failure analysis (visual inspection, X-ray, metallographic sectioning, SEM/EDS, etc.) is conducted to accurately locate the root cause for targeted improvement.
4、 Summary
The core of the fracture of surface mount capacitors is that the brittle ceramic body is subjected to excessive mechanical or thermal stress. PCB bending (especially improper board splitting and layout in high stress areas) is the main cause, and thermal stress (rapid temperature changes) is also an important factor. Solving this problem requires a systematic approach:
Design side avoidance: Optimize PCB layout, orientation, and pad design to stay away from stress concentration areas.
Process control: Adopt a more optimal plate splitting method, strictly control the welding curve, and standardize operations.
Device selection: Prioritize the use of flexible end capacitors and make reasonable selections.
Process side assurance: Strengthen key process monitoring and failure analysis.
By implementing targeted preventive measures in various aspects such as design, materials, processes, and operations, the risk of fracture and failure of surface mount capacitors can be significantly reduced, and the reliability and yield of electronic products can be improved.