Possible Reasons for the Rupture of PCBA MLCC Multilayer Ceramic Capacitors
In general, capacitors can develop micro cracks, which often lead to open circuit conditions and an increase in insulation resistance (IR). Interestingly, when a microcrack occurs, it is not uncommon for insulation resistance to decrease, resulting in a current leakage short circuit. This issue may stem from layer-to-layer short circuits caused by ruptures in the multilayer structure.
If you are unclear about the structure of MLCC, it is advisable to refer to previously published articles that detail the structure and processes of multilayer ceramic capacitors (MLCC).
Now, let’s explore the potential causes of micro-cracks in “multilayer ceramic capacitors.”
The causes of MLCC rupture can be broadly categorized into three main areas:
▪ Thermal Shock Failure
▪ Extrinsic Defect and Overstress Failure
▪ Intrinsic Defect
Thermal Shock Failure Principle:
Rapid fluctuations in temperature surrounding PCB components can induce thermal shock, often encountered during processes like wave soldering, reflow, touch-up, or repairs.
**The high temperature.** This occurs during the production of multilayer ceramic capacitors (MLCCs), which utilize a range of compatible materials. These materials exhibit different coefficients of thermal expansion and thermal conductivity due to their distinct properties. When these various materials coexist in the capacitor and the internal temperature changes rapidly, differing volume change rates result in mutual pushing and pulling, ultimately leading to PCB cracking.
Such cracking typically initiates from the weakest structural point or where stress is most concentrated. Common locations include the exposed end adjacent to the central ceramic interface or areas where maximum mechanical tension occurs (often at the four corners where the material is hardest). Thermal shock can result in the following types of phenomena:
1. **Nail-shaped or U-shaped cracks.**
The MLCC exhibits cracks resembling nails or U-shapes.
2. **Microcracks hidden within the capacitor.**
3. **Cracks originating from the exposed central part or the lower half of the junction between the central ceramic end and the exposed end.** These cracks may spread with distortion due to temperature changes or during subsequent assembly.
**MLCC thermal shock cracking**
The first type of cracking manifests as nail or U-shaped cracks, while the second involves microcracks that are concealed within the component. The main distinction is that microcracks experience less stress and result in relatively minor damage. The first type of cracks can usually be detected through metallography, whereas the second type is only identifiable once it reaches a certain severity.
(Note: “Metallographic” refers to the structural image of metal viewed under a high-power microscope.)
**The failure principle of PCB overstress:**
Distortion and fractures are typically due to external forces (extrinsic factors). This scenario often arises during SMT assembly or the overall product fabrication. Possible causes include:
1. If the pick & place machine improperly grips components, it can induce cracks. Misalignment, wear, or tilting of the centering jaw during part placement can lead to concentrated pressure, generating rupture points. Such cracks are typically visible on the surface or may appear as internal fractures between 2 to 3 electrodes; surface cracks usually follow the strongest pressure line and the direction of ceramic displacement. New SMT machines have moved away from this centering jaw design.
2. During capacitor mounting, if the suction nozzle either lifts or places parts excessively, bending and deformation may occur, resulting in cracks. This type of damage often manifests as round or crescent-shaped indentations on the component’s surface with uneven edges. The diameter of these half-moon or circular cracks corresponds to the nozzle size. Tension-related ruptures may also result from nozzle head damage, with cracks extending from one side of the component to the other, potentially leading to breakage at the capacitor’s base.
3. Non-uniform land-pattern layouts (for instance, one pad connected to a large area of copper foil while another is not) or asymmetric solder paste application can result in unequal thermal expansion forces during reflow oven passage. This discrepancy can lead to one side experiencing greater pulling or pushing forces, causing cracks.
4. Thermal shock during the soldering process and substrate bending or deformation post-soldering can also result in cracking.
4.1 During capacitor wave soldering, inadequate preheating temperature, insufficient time, or excessive soldering temperatures can easily lead to cracks.
4.2 Direct contact between the soldering iron and capacitor body during PCB touch-up can create local overheating or excessive pressure, both of which may lead to cracking.
4.3 Post-welding, bending the substrate during board cutting or machine assembly can also induce cracks.
When the board bends under mechanical stress, the ceramic’s movement is constrained by the termination position and solder joints, causing cracks to form outside the ceramic’s termination interface, typically at a 45-degree angle.
**Distortion and fracture failure.** In cases where SMT stage ruptures are minor, they may not be detectable via metallography. However, ruptures and distortions that occur after SMT production are usually identifiable through metallographic examination.
**MLCC material failure and rupture**
MLCC material failures can generally be classified into three main categories of defects. These failures originate from internal capacitor issues and can significantly compromise product reliability, often due to improper MLCC processing or material selection.
1. **Electrode interface failure and bond line rupture (Delamination).**
Such defects typically result in larger cracks, primarily due to high void levels in the ceramic or voids between the dielectric layer and opposing electrode, leading to dielectric layer cracking and potential leakage risks.
2. **Voiding.**
Voids usually form between adjacent inner electrodes, sometimes spanning multiple electrodes. These defects can result in short circuits and leakage currents. When significant gaps form, capacitance may be adversely affected. The root cause typically lies in poor process control, such as contamination or inadequate sintering of ceramic capacitor powder.
3. **Firing cracks.**
Cracks tend to form perpendicular to the electrodes and often initiate from the edges or terminals. Such defects can lead to excessive current leakage, jeopardizing component reliability. The primary cause is often excessive cooling during the MLCC manufacturing process.
**In conclusion:**
Cracks induced by thermal shock typically propagate from the capacitor’s surface inward. Ruptures resulting from excessive mechanical tension can appear on the surface or within the component, spreading at approximately 45-degree angles. Material failures often lead to cracks either perpendicular or parallel to internal electrodes. Additionally, thermal shock-induced ruptures generally extend from one terminal to another. Ruptures caused by the pick and place machine may show multiple fracture points beneath the terminal, whereas distortions in the circuit board usually yield only a single rupture point.
In general, capacitors can develop micro cracks, which often lead to open circuit conditions and an increase in insulation resistance (IR). Interestingly, when a microcrack occurs, it is not uncommon for insulation resistance to decrease, resulting in a current leakage short circuit. This issue may stem from layer-to-layer short circuits caused by ruptures in the multilayer structure.
If you are unclear about the structure of MLCC, it is advisable to refer to previously published articles that detail the structure and processes of multilayer ceramic capacitors (MLCC).
Now, let’s explore the potential causes of micro-cracks in “multilayer ceramic capacitors.”
The causes of MLCC rupture can be broadly categorized into three main areas:
▪ Thermal Shock Failure
▪ Extrinsic Defect and Overstress Failure
▪ Intrinsic Defect
Thermal Shock Failure Principle:
Rapid fluctuations in temperature surrounding PCB components can induce thermal shock, often encountered during processes like wave soldering, reflow, touch-up, or repairs.
**The high temperature.** This occurs during the production of multilayer ceramic capacitors (MLCCs), which utilize a range of compatible materials. These materials exhibit different coefficients of thermal expansion and thermal conductivity due to their distinct properties. When these various materials coexist in the capacitor and the internal temperature changes rapidly, differing volume change rates result in mutual pushing and pulling, ultimately leading to PCB cracking.
Such cracking typically initiates from the weakest structural point or where stress is most concentrated. Common locations include the exposed end adjacent to the central ceramic interface or areas where maximum mechanical tension occurs (often at the four corners where the material is hardest). Thermal shock can result in the following types of phenomena:
1. **Nail-shaped or U-shaped cracks.**
The MLCC exhibits cracks resembling nails or U-shapes.
2. **Microcracks hidden within the capacitor.**
3. **Cracks originating from the exposed central part or the lower half of the junction between the central ceramic end and the exposed end.** These cracks may spread with distortion due to temperature changes or during subsequent assembly.
**MLCC thermal shock cracking**
The first type of cracking manifests as nail or U-shaped cracks, while the second involves microcracks that are concealed within the component. The main distinction is that microcracks experience less stress and result in relatively minor damage. The first type of cracks can usually be detected through metallography, whereas the second type is only identifiable once it reaches a certain severity.
(Note: “Metallographic” refers to the structural image of metal viewed under a high-power microscope.)
**The failure principle of PCB overstress:**
Distortion and fractures are typically due to external forces (extrinsic factors). This scenario often arises during SMT assembly or the overall product fabrication. Possible causes include:
1. If the pick & place machine improperly grips components, it can induce cracks. Misalignment, wear, or tilting of the centering jaw during part placement can lead to concentrated pressure, generating rupture points. Such cracks are typically visible on the surface or may appear as internal fractures between 2 to 3 electrodes; surface cracks usually follow the strongest pressure line and the direction of ceramic displacement. New SMT machines have moved away from this centering jaw design.
2. During capacitor mounting, if the suction nozzle either lifts or places parts excessively, bending and deformation may occur, resulting in cracks. This type of damage often manifests as round or crescent-shaped indentations on the component’s surface with uneven edges. The diameter of these half-moon or circular cracks corresponds to the nozzle size. Tension-related ruptures may also result from nozzle head damage, with cracks extending from one side of the component to the other, potentially leading to breakage at the capacitor’s base.
3. Non-uniform land-pattern layouts (for instance, one pad connected to a large area of copper foil while another is not) or asymmetric solder paste application can result in unequal thermal expansion forces during reflow oven passage. This discrepancy can lead to one side experiencing greater pulling or pushing forces, causing cracks.
4. Thermal shock during the soldering process and substrate bending or deformation post-soldering can also result in cracking.
4.1 During capacitor wave soldering, inadequate preheating temperature, insufficient time, or excessive soldering temperatures can easily lead to cracks.
4.2 Direct contact between the soldering iron and capacitor body during PCB touch-up can create local overheating or excessive pressure, both of which may lead to cracking.
4.3 Post-welding, bending the substrate during board cutting or machine assembly can also induce cracks.
When the board bends under mechanical stress, the ceramic’s movement is constrained by the termination position and solder joints, causing cracks to form outside the ceramic’s termination interface, typically at a 45-degree angle.
**Distortion and fracture failure.** In cases where SMT stage ruptures are minor, they may not be detectable via metallography. However, ruptures and distortions that occur after SMT production are usually identifiable through metallographic examination.
**MLCC material failure and rupture**
MLCC material failures can generally be classified into three main categories of defects. These failures originate from internal capacitor issues and can significantly compromise product reliability, often due to improper MLCC processing or material selection.
1. **Electrode interface failure and bond line rupture (Delamination).**
Such defects typically result in larger cracks, primarily due to high void levels in the ceramic or voids between the dielectric layer and opposing electrode, leading to dielectric layer cracking and potential leakage risks.
2. **Voiding.**
Voids usually form between adjacent inner electrodes, sometimes spanning multiple electrodes. These defects can result in short circuits and leakage currents. When significant gaps form, capacitance may be adversely affected. The root cause typically lies in poor process control, such as contamination or inadequate sintering of ceramic capacitor powder.
3. **Firing cracks.**
Cracks tend to form perpendicular to the electrodes and often initiate from the edges or terminals. Such defects can lead to excessive current leakage, jeopardizing component reliability. The primary cause is often excessive cooling during the MLCC manufacturing process.
**In conclusion:**
Cracks induced by thermal shock typically propagate from the capacitor’s surface inward. Ruptures resulting from excessive mechanical tension can appear on the surface or within the component, spreading at approximately 45-degree angles. Material failures often lead to cracks either perpendicular or parallel to internal electrodes. Additionally, thermal shock-induced ruptures generally extend from one terminal to another. Ruptures caused by the pick and place machine may show multiple fracture points beneath the terminal, whereas distortions in the circuit board usually yield only a single rupture point.
