Surface mount capacitors (especially multi-layer ceramic capacitors - MLCC) are prone to breakage or terminal detachment, mainly due to their own structure, material properties, and the combined effect of external stress. The following are the main reasons:
The brittleness of ceramic materials:
The core of MLCC is a ceramic dielectric layer (usually barium titanate, etc.). Ceramics themselves are very brittle and hard materials, with extremely poor resistance to bending and impact.
When the circuit board is subjected to any form of bending or twisting stress, this stress will be directly transmitted to the capacitors soldered on the board. Ceramics cannot absorb these stresses through plastic deformation, and once the stress exceeds its ultimate strength, it will cause cracks and ultimately fracture inside the ceramic body.
Structural design (sandwich structure):
MLCC is a typical "sandwich" structure: the internal ceramic dielectric layer and electrode layer are alternately stacked, and the metal terminals at both ends (usually tin plated or nickel/tin/silver plated) are connected to the PCB pads through soldering.
When the PCB is bent, the most stress concentrated areas are usually at the two ends of the capacitor (near the solder joints). Because this is the interface between a rigid ceramic body and relatively soft solder and PCB materials, stress cannot be effectively dispersed here.
Cracks usually initiate from a position near the end electrode of the capacitor body (inside the solder joint), and may extend upwards through the ceramic body, leading to complete fracture or causing the end electrode to separate from the ceramic body (terminal detachment).
Thermal stress (welding and temperature cycling):
Welding process (especially reflow soldering): Ceramics, metal end electrodes, solder, and PCB substrates have different coefficients of thermal expansion. During the heating and cooling process of welding, the degree of expansion and contraction of each part is different, which will generate thermal stress inside the capacitor and at the solder joints. If the temperature curve is set improperly (such as too fast heating/cooling rate), this stress will be greater, which may lead to the formation of internal microcracks.
Temperature cycle (during use): The device will experience temperature cycles when it is turned on or off or when the ambient temperature changes. The differences in expansion and contraction of different materials lead to periodic stress, and long-term effects can exacerbate existing microcracks or cause the formation of new cracks (thermal fatigue).
Bending/deformation of circuit board:
Assembly process: This is one of the most common reasons. During the process of PCB assembly, testing, transportation, and installation into the chassis, the circuit board may be bent or twisted due to improper operation (such as pressing or bending), uneven support, incorrect screw tightening sequence, interference with other components, etc.
Splitting operation: If the capacitor is located at the edge of the splicing board or near the V-cut/stamp hole, the mechanical impact and bending stress generated when separating the splicing board into single boards (manually bending or using a splitting machine) can easily cause damage to nearby capacitors.
Testing process: During ICT or FCT testing, excessive downward pressure or improper support of the probe may cause local plate bending.
Equipment use: If there is a vibration source during the operation of the equipment itself, or if the equipment casing is subjected to external impact (such as falling), it may also cause PCB deformation.
Design factors:
Capacitor size and position: Larger capacitors (especially those with larger aspect ratios) are more sensitive to bending stress. Capacitors located at the edges, corners, near connectors, near screw holes, or on the path of board splitting are at higher risk.
Capacitor direction: When the long axis direction of the capacitor is parallel to the expected bending direction of the board, high stress is more likely to occur at both ends than in the vertical direction.
Pad design: Unreasonable pad size and shape design (such as being too large or asymmetrical) may affect the shape of the solder joint, thereby changing the stress distribution.
PCB layout: Placing large capacitors or multiple capacitors in the easily bent area of the board will increase the risk. Capacitors near PCB support points or stress concentration points are also prone to damage.
Mechanical impact:
During the production, transportation, or use of equipment, unexpected drops, collisions, and other direct external impacts can instantly generate enormous stresses that exceed the load-bearing capacity of ceramics.
Solder joint stress:
Excessive or insufficient amount of solder, as well as poor soldering (such as virtual soldering, cold soldering), can cause the solder joint itself to become a stress concentration point or unable to effectively buffer stress.
In summary:
The fundamental reason for the fracture/terminal detachment of surface mount capacitors is that the brittle ceramic body cannot withstand the mechanical stress (bending, twisting, impact) or thermal stress (welding, temperature cycling) applied to it. These stresses are most commonly concentrated near the end electrodes of the capacitor, causing ceramic cracking or detachment of the end electrodes from the ceramic body in that area. Any bending deformation of circuit boards during production, assembly, and use is the main external factor that triggers such failures, especially for large-sized capacitors and capacitors located in stress sensitive areas.
How to avoid it?
Optimize PCB layout: Avoid placing large-sized MLCCs in high-risk areas such as board edges, corners, partition paths, near screw holes, and near connectors. Disperse and arrange large capacitors. Make the long axis of the capacitor perpendicular to the main bending direction (such as parallel to the board edge).
Improve the splitting process: Use more precise milling cutters to split the plates instead of V-cut or stamping, optimize the splitting parameters (speed, cutting tools), and avoid manual bending.
Standardized operating procedures: Strictly avoid applying unnecessary bending force to the PCB during assembly, testing, handling, and installation processes. Use appropriate fixtures to support the PCB.
Optimize welding process: Follow the recommended welding temperature curve, control the heating/cooling rate, and reduce thermal shock.
Choose the appropriate capacitor: In locations with high stress risk, consider using smaller capacitors, flexible end electrode capacitors, or choosing tantalum capacitors/polymer capacitors with stronger bending resistance (if electrical performance allows).
Optimize pad design: Refer to the component specification sheet for pad design.
Strengthen support: Add reinforcing ribs or support points in areas prone to bending.
By understanding the failure mechanism and taking targeted preventive measures, the risk of breakage or terminal detachment of surface mount capacitors can be significantly reduced.