Cracking causes of chip MLCC and Test methods

2024-06-08

Multilayer Ceramic Capacitors (MLCCs) are essential components in many electronic devices due to their high reliability and performance. However, they also encounter several common issues. Like Cracking. Mechanical stress and PCB flexing can cause cracking on MLCC.
 

Mechanical stress, such as the mechanical/thermal stress when mounting on a substrate and deflection of the substrate after mounting.

The following conditions can cause mechanical stress during substrate mounting.

1.Excessive volume of solder

When an excessive volume of solder is used, the chip capacitor may crack due to the contraction of the solder.

2.Impact by mounting machine (impact from suction nozzle)

Due to trouble with the support under the suction nozzle, stress is applied to the center of the chip capacitor and cracking occurs.

3.Impact by Mounting Machine (impact of positioning chuck)

When the impact of the positioning chuck is excessive or the shape of the chuck is pointed due to friction,marks will remain on the external electrodes of the capacitors and internal cracks may occur.

The following conditions can lead to thermal stress during substrate mounting.

1.Insufficient preheating of soldering iron during corrections, or chip contacting the tip of soldering iron.

2.Insufficient preheating of flow soldering 

When a thermal stress exceeding the acceptable limits of the chip capacitor is applied due to insufficient

preheating before flow solder dipping,external and internal cracking will occur in the ceramic element.

3.Too short a cooling period after soldering.

The following conditions can produce mechanical stress after mounting.

Deflection of substrate -mounting of other inserted parts, substrate splitting, and substrate testing.

When the substrate is deflected by substrate breaks, etc. , cracks may occur as shown in the figure on the right.Cracks tend to occur when a larger volume of solder is used.

PCB flexing can cause cracks in the ceramic layers.

A flex crack occurs when an MLCC is excessively bent after being soldered onto a PCB, and it appears different from cracks caused by other factors like impact or electrical overstress.

The dielectric material in an MLCC is made of ceramic, which is inherently brittle. Ceramics are strong under compression but weak under tension and cannot undergo plastic deformation. When the mechanical stress on an MLCC becomes too intense, the ceramic material will crack to relieve the stress.

MLCCs rarely crack during a concave bend of the PCB because the forces applied during such bends are compressive. MLCC’s are very susceptible to cracking during a convex bend of the PCB, because the part is undergoing tension.

The characteristic flex crack, often called a “45° crack,” begins near the termination (where the stress originates) on the mounted side of the MLCC and extends upward at a 45° angle (following the direction of bending stress) until it reaches the termination at the outer edge of the MLCC. While the 45° crack is the most commonly observed flex crack, other variations can occur, especially if the component has been subjected to extreme bending.

Detecting a flex crack through external visual inspection is uncommon, though occasionally a discoloration of the dielectric may be seen on the surface, appearing lighter than the surrounding material. Typically, the crack is only discovered after the component undergoes destructive physical analysis (DPA), also known as cross-sectioning.

Sometimes, if the MLCC’s termination is removed, the flex crack can be observed, if the crack has propagated the surface. These cracks can sometimes be visible on the bottom of the MLCC.

Sometimes, if the MLCC’s termination is removed, the flex crack can be observed, if the crack has propagated the surface. These cracks can sometimes be visible on the bottom of the MLCC.

In situations where the board experiences significant stress, cracks may develop at both ends of the MLCC.

Once an MLCC is cracked, it becomes vulnerable to moisture and contaminants infiltrating its interior. If the crack extends across the active stack (the region where electrodes overlap), it can create a pathway with low electrical resistance, known as a "leaky short," which may worsen over time. The presence of this low-resistance pathway can cause overheating, leading to further deterioration of the dielectric material and potentially resulting in a genuine short circuit.

Any action that exposes the PCB to considerable bending post-soldering of MLCCs poses a risk of inducing cracks. Flexure in PCBs can occur in various scenarios, both during manufacturing and afterward. PCBs commonly experience significant stress during depaneling. Additionally, other instances where a board might bend during the manufacturing process include attaching connectors and wiring harnesses to connector sockets, sliding boards into housings without proper support, and securing boards onto screw mounts.

Cracks may lead to electrical failure or decreased capacitance, affecting device reliability.

How to test the Mechanical stress cracking and PCB flex cracking?

Mechanical Stress Cracking Test ways:

1.Visual Inspection: Examine the components for any visible cracks, especially around solder joints or along the body of the component.

2. Microscopic Examination: Use a microscope or magnifying glass to inspect the surface of the components for hairline cracks that may not be visible to the naked eye.

3.Mechanical Testing: Apply controlled mechanical stress to the components using equipment such as a bend tester or a stress test apparatus. This helps simulate real-world conditions and can reveal weaknesses in the components.

4.Thermal Cycling: Subject the components to repeated cycles of heating and cooling to simulate thermal stress. This can be done using environmental chambers that can control temperature variations.

PCB Flex Cracking Test ways:

1.Visual Inspection: Look for any visible signs of cracking on the surface of the PCB, particularly around areas of high stress such as near components or along the edges of the board.

2.X-ray Inspection: Use X-ray equipment to inspect the internal layers of the PCB for any hidden cracks or delamination.

3.Microsection Analysis: Cut and polish a cross-section of the PCB to examine it under a microscope. This can reveal any internal cracks or defects that may not be visible from the surface.

4.Bend Testing: Apply controlled bending or flexing to the PCB using specialized equipment to assess its flexibility and resistance to cracking.

5.Electrical Testing: Perform electrical continuity and insulation resistance tests to detect any issues caused by cracks in the PCB, such as short circuits or open circuits.

6.Environmental Testing: Subject the PCB to environmental stress tests, including temperature cycling, humidity exposure, and vibration testing, to simulate real-world conditions and assess its reliability under different scenarios.

By combining these testing methods, you can effectively evaluate the mechanical integrity and reliability of components and PCBs, identifying any potential issues before they cause failures in the field.

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