Capacitor breakage is a common problem in electronic production, especially in ceramic capacitors (MLCC), which can lead to early product failure or reduced reliability. Preventing and controlling capacitor breakage requires comprehensive management from multiple aspects such as design, materials, processes, equipment, operation, and environment. Here are some key prevention and control measures:
1、 Prevention during the design phase
Optimize layout:
Stay away from stress concentration areas: Avoid placing capacitors near PCB edges, connectors, screw holes, V-cuts, or large/high stress components such as transformers and heat sinks.
Stay away from bending areas: If the PCB may bend during use (such as when installed inside a chassis), ensure that the capacitor is not located on the expected bending axis.
Directionality: For rectangular capacitors, the long side parallel to the expected PCB bending direction or board splitting direction (if inevitably close to the board splitting line) can usually withstand greater stress. Avoid the long side of the capacitor being perpendicular to the bending/splitting direction.
Optimize pad design:
Symmetry: Ensure that the size of the solder pads at both ends is symmetrical (length, width, spacing) to avoid excessive stress on one side due to mismatched thermal expansion coefficients.
Size matching: The size of the solder pad should match well with the size of the capacitor terminal electrode. Excessively large solder pads may lead to excessive solder, resulting in greater shrinkage stress during cooling; A too small solder pad can affect the welding strength and heat dissipation. Follow the component specifications or IPC standard recommendations.
Solder mask opening: Ensure accurate opening of the solder mask to avoid solder mask climbing onto the solder pad, affecting welding or generating stress.
Hot pads/heat dissipation channels: For power capacitors that require heat dissipation, design hot pads and heat dissipation channels reasonably to avoid local high temperatures or large temperature differences.
Consider CTE matching: When possible (especially for large-sized or high reliability capacitors), choose a capacitor substrate material (ceramic) with a CTE as close as possible to the PCB substrate material to reduce stress during thermal cycling.
2、 Material and incoming material control
Choose high-quality capacitors:
Choose suppliers with good reputation and stable quality.
Prioritize capacitor models with higher mechanical strength (such as flexible end electrodes) or bending resistance (usually indicated by suppliers).
For large-sized capacitors (such as 1206 and above), special attention should be paid to their mechanical reliability specifications.
Strict incoming inspection:
Implement IQC inspection, including visual inspection (for damage and cracks) and electrical performance sampling.
For high reliability products, sampling X-ray inspection or slice analysis can be considered to investigate internal microcracks.
MSD control:
Ceramic capacitors are usually humidity sensitive components. Strictly follow the storage (humidity card, drying cabinet), baking (according to grade and exposure time), and usage specifications of humidity sensitive components to avoid the "popcorn" effect or internal cracks caused by internal moisture vaporization during reflow soldering.
3、 Production process control
Printing:
Ensure uniform solder paste printing, consistent thickness, and accurate positioning. Monitor using laser steel mesh and SPI.
The design of steel mesh openings should be reasonable to avoid excessive or insufficient solder paste. Excessive solder paste can easily form "tombstones" or increase cooling stress during reflow; If it is too small, the welding strength will be insufficient.
Patch:
The key! Controlling mounting pressure: This is one of the most direct causes of mechanical damage. Accurately set the Z-axis height and mounting force of the mounting head to ensure that the capacitor is stably placed without excessive compression. Regularly calibrate the surface mount machine.
Selection and maintenance of suction nozzle: Use a suction nozzle with appropriate size and good condition to avoid scratching or hitting the capacitor.
Feeder adjustment: Ensure smooth feeding of the feeder and avoid components being impacted or stuck during the picking process.
Reflow soldering:
Accurate temperature curve: This is the key to preventing thermal stress. Optimize the temperature curve for the specific solder paste and components used.
Preheating zone: The heating rate should not be too fast (usually recommended to be<3 ° C/s), so that the PCB and components can heat up evenly and smoothly, reducing temperature difference stress.
Insulation zone: Ensure sufficient time to activate and evaporate the flux in the solder paste, reduce splashing, and make the component temperature more uniform.
Reflow zone: The peak temperature and time should be sufficient to completely melt and wet the solder, but not too high or too long, to avoid component overheating and damage or PCB deformation.
Cooling zone: Control the cooling rate (usually recommended<4 ° C/s) to avoid excessive shrinkage stress caused by rapid cooling. The ideal cooling curve should be close to linear.
Furnace temperature uniformity: Regularly test the furnace temperature uniformity to ensure that components with different positions and heat capacities on the PCB can meet the required temperature curve.
Avoid repeated furnace passing: Try to minimize the number of repairs or secondary furnace passing.
Wave soldering (if applicable):
Strictly control the preheating temperature and time to ensure that the components reach sufficient temperature before coming into contact with the solder, reducing thermal shock.
Control the contact time and temperature of wave soldering.
For double-sided panels with capacitors on the back, special attention should be paid to their heat shock resistance or other welding methods should be considered.
Cleaning (if applicable):
Avoid using overly strong cleaning methods or solvents to prevent impact or corrosion on capacitors or solder joints.
Ensure thorough drying after cleaning.
4、 Post assembly and testing
Board division:
The biggest source of risk! This is the most common cause of capacitor breakage.
Preferred milling cutter plate splitting: using a milling cutter (Router) plate splitting machine, with low vibration and stress. Ensure board path optimization and avoid capacitors.
V-cut board:
Avoid designing V-cuts near capacitors.
Use high-quality and sharp slitting knives.
Accurately control the plate pressure and depth.
Consider using support boards or fixtures to reduce PCB bending.
Absolutely avoid manual bending!
Plug ins and crimping:
Avoid plug-in or crimping processes that require strong operation near capacitors.
Use appropriate tools and fixtures.
Screw lock attachment:
Avoid locking screws near capacitors. If unavoidable, strictly control the torque, use a screwdriver with torque control, and ensure that the structure around the screw hole is stable.
The order of locking screws should be even.
Connector insertion and removal:
Avoid designing connectors near capacitors that require frequent insertion and removal, or ensure that connector insertion and removal do not cause local bending of the PCB.
Testing:
ICT testing: Ensure good support of the testing needle bed, reasonable positioning of the testing points, and avoid excessive pressure on the PCB or capacitor caused by the testing probe.
Functional testing: Operators or testing fixtures should avoid pressing or bending the PCB, especially in areas with capacitors, when operating the PCBA. Use support fixtures.
Handling and Storage:
Operators should handle the PCBA with care to avoid falling, impact, or excessive bending.
Use anti-static, cushioned turnover boxes or pallets to store and transport PCBA.
Avoid stacking heavy objects on the PCBA.
5、 Process monitoring and failure analysis
AOI inspection: Set up AOI at key workstations (such as after reflow soldering and board splitting) to check for defects such as capacitor offset, standing monument, obvious cracks, and missing parts.
X-ray inspection: Conduct X-ray inspection for capacitors under BGA or suspected internal cracks.
Functional testing and aging testing: Early detection of potential open circuits or intermittent faults.
Strict Failure Analysis:
Once a capacitor failure is discovered, a thorough failure analysis (FA) must be conducted to determine the fracture mode (mechanical stress? Thermal stress?) and root cause.
Observe the morphology of the fracture surface using a microscope (optical, electron microscope).
Slice analysis to observe the origin and direction of cracks.
Analyzing the location of the fracture (near the end electrode? In the middle?) can help determine the source of stress.
Continuous improvement: Based on process monitoring data and failure analysis results, continuously optimize design, process, and operational specifications.
Summarize key points
Splitting is of utmost importance: try to use milling cutters for splitting and strictly optimize the splitting path and parameters. Avoid manual bending!
Control mechanical stress: Throughout the entire production process (patch pressure, handling, testing, assembly), avoid any form of impact, compression, or bending.
Manage thermal stress: Accurately control the reflow soldering temperature curve (especially the heating and cooling rate) and peak soldering preheating.
Source design optimization: Reasonable layout and pad design are the foundation.
Incoming materials and MSD control: Ensure the quality of the components themselves and prevent moisture issues.
Process monitoring and FA: Early detection of problems, identification of root causes, and continuous improvement.
By systematically implementing these measures, the risk of capacitor breakage during the production process can be significantly reduced, improving product yield and long-term reliability. Remember, prevention is often more cost-effective than post repair.