Question 1

Open Mode and the“1210 Rule” (1): What is TDK "Open Mode" capacitor?

Accepted Answer

A. In response to the market demand for a reduced risk MLCC with respect to board flexure cracks, TDK introduced a unique inner electrode design that compensates for the typical crack path induced by PCB bending. This "Y" crack has been well documented as a signature crack for board flexure [1] and has been a leading cause of MLCC failures in the market for years.

Excessive board bending during PCB manufacturing or final assembly is common, and the brittle nature of ceramic components is especially prone to damage after soldered to the PCB. TDK has responded to this problem with the Open Mode MLCC for applications where flex cracking is a high concern (see illustration in Figure 1).

Figure 1

>>Multilayer Ceramic Chip Capacitors Product site
>>Open Mode Capacitors List

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Question 2

Flex Crack Countermeasures (1): What is meant by the term "flex crack"?

Accepted Answer

A. A flex crack refers to a crack formed by excess bending of an MLCC once it has been soldered onto a PCB (Print Circuit Board).

These cracks differ in appearance to cracks created by other means, such as impact or electrical overstress.

>>Multilayer Ceramic Chip Capacitors Product site
>>Flex Crack Countermeasures in MLCCs Outline

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Question 3

Flex Crack Countermeasures (2): Why do flex cracks occur?

Accepted Answer

A. The dielectric material in an MLCC is a ceramic compound. Ceramics, by nature, are brittle materials that are strong under compression, but weak under tension.

The ceramic's brittle nature does not allow for plastic deformation. If the mechanical stress induced on an MLCC is too great, the part will crack as a method of reducing this stress.

MLCCs will rarely crack during a concave bend of the PCB, because the forces exerted on the part are compressive during these bends (Figure 1).
MLCC's are very susceptible to cracking during a convex bend of the PCB, because the part is undergoing tension (Figure 2).

Figure 1: An MLCC on a PCB undergoing compressive forces due to a concave bend

Figure 2: An MLCC on a PCB undergoing tensile forces due to a convex bend

>>Multilayer Ceramic Chip Capacitors Product site
>>Flex Crack Countermeasures in MLCCs Outline

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Question 4

Flex Crack Countermeasures (3): What does a flex crack look like?

Accepted Answer

A. The signature flex crack is sometimes referred to as a "45°crack". These cracks originate near the start of the termination (the origin of stress on the part) on the mounted side of the MLCC and continue upwards at a 45° angle (the direction of bending stress on the part) until it reaches the termination at the outer edge of the MLCC (Figure 1).

Figure 1: Stress origin and direction on an MLCC during a convex bend

The 45°crack is the most commonly seen flex crack, but other flex cracks are possible, particularly if the part underwent especially severe bending.

Sometimes a discoloration of the dielectric is visible on the surface, although external visual detection of a flex crack is rare. This area will appear lighter in color than the normal dielectric.

Usually, the crack is only found once the part has undergone a DPA (Destructive Physical Analysis), also called a cross-sectioning (Figure 4).

Sometimes, if the MLCC’s termination is removed, the flex crack can be observed, if the crack has propagated the surface (Figure 5).

These cracks can sometimes be visible on the bottom of the MLCC (Figure 6).
During cases of severe board stress, cracks could form on both ends of the MLCC.

Figure 2: A cross section of an MLCC that cracked due to excess board flexure

Figure 3: Bottom of an MLCC, showing the origin of a flex crack.

Figure 4: Bottom of an MLCC with the termination removed. Cracks are visible at both ends.

>>Multilayer Ceramic Chip Capacitors Product site
>>Flex Crack Countermeasures in MLCCs Outline

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Question 5

Flex Crack Countermeasures (4): What happens to the MLCC (Multilayer Ceramic Chip Capacitor) once it is cracked?

Accepted Answer

A. Once a crack forms, this can allow moisture and contaminants to penetrate inside the component. If the crack crosses the active stack (the overlapping area of the electrodes) this can lead to a low electrical resistance path or “leaky short” developing over time. Once this low resistance path forms, this can cause further breakdown of the dielectric due to overheating. This overheating can lead to a true short circuit developing (Figure 1).

Figure 1: A diagram showing a typical 45° crack. The red electrodes are where the crack has passed through the active stack. This represents a risk for a short circuit.

>>Multilayer Ceramic Chip Capacitors Product site
>>Flex Crack Countermeasures in MLCCs Outline

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Question 6

Flex Crack Countermeasures (5): What are some common ways that flex cracking occurs?

Accepted Answer

A. Any activity that subjects the PCB to significant bending after the MLCCs have been soldered on the board risks cracking.

There are many ways in which a board could experience flexure, both during and after manufacturing.

PCBs frequently undergo a great amount of stress during depaneling. A few additional examples of ways in which a board could bend during the manufacturing process are: attaching connectors and wiring harnesses to connector sockets, sliding the boards into housings (without supporting the board), and screwing down the boards onto screw mounts.

>>Multilayer Ceramic Chip Capacitors Product site
>>Flex Crack Countermeasures in MLCCs Outline

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Question 7

Flex Crack Countermeasures (6): How can I detect a flex crack?

Accepted Answer

A. A flex crack is most frequently detected by electrical measurement as a decrease in insulation resistance. The MLCCs IR (Insulation Resistance) will decrease because the crack creates a weak point in the dielectric. Decreases in IR are usually detected by increases in leakage current.

A loss of capacitance or an increase in dissipation factor are also electrical indicators that a crack has occurred.

>>Multilayer Ceramic Chip Capacitors Product site
>>Flex Crack Countermeasures in MLCCs Outline

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Question 8

Flex Crack Countermeasures (7): How can I help prevent flex cracking?

Accepted Answer

A. The best way to prevent cracks is to ensure the board is not subjected to unnecessary external forces. Installing board stiffeners or supports can help prevent a board from bending. Board flexure during manufacturing can often be overlooked. Examining the manufacturing process can lead to finding ways reduce the stresses induced on the boards.

Additionally, part placement on the boards plays a large role in the amount of stress induced on the part during a bend. Placing parts farther away from depaneling planes and screw mounts can help prevent exposing MLCCs to unnecessary stress during manufacturing. Parts placed parallel to the depaneling planes will experience less stress (Figure 1).

Excess solder can lead to increased stress on MLCCs during bending. Care should be taken to use the soldering specifications provided by the manufacturer.

Figure 1: A diagram demonstrating that the stress on the capacitor during depaneling is greatest to least in the following order A > B = C > D > E.

>>Multilayer Ceramic Chip Capacitors Product site
>>Flex Crack Countermeasures in MLCCs Outline

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Question 9

Flex Crack Countermeasures (8): Does TDK offer any products providing solutions for flex cracking?

Accepted Answer

A. TDK has many flex cracking solutions including, Open Mode, Soft Termination, Serial Design, and MEGACAP.

Open Mode caps have an extended "L" gap (the distance between the termination and the electrode) as shown in Figures 1 and 2. This means that when the common 45° crack occurs, it will not travel through the active stack, and therefore will create an open circuit instead of a short circuit.

Figure 1: A diagram showing the "L" gap on a standard MLCC.

Figure 2: A diagram showing how the expanded "L" gap helps prevent a short circuit from flex cracking.

Figure 3: A diagram showing the location of the conductive resin layer (shown in green for emphasis).

TDK's Soft Termination MLCC's have a layer of conductive resin built into the termination (Figure 11). This layer of resin, located between the Cu and Ni layers, helps absorb mechanical stresses from bending and vibration to prevent a crack from occurring. When the part undergoes extreme bending, the termination is designed to separate from the resin before a crack develops.

Figure 4: A diagram showing the internal architecture of a serial design capacitor. A short has not occurred, even though the part has cracked.

TDK's Serial Design capacitors are effectively two capacitors in series, in a single ceramic body (Figure 12). These caps are designed to satisfy certain specifications for series caps on battery lines.

The two capacitors ensure that if one cracks, there still exists a second capacitor. This helps prevent a short circuit in the event of a crack. However, TDK's serial design is also equipped with Soft Termination. This helps prevent a crack in the first place.

Figure 5: A diagram showing the construction of a TDK MEGACAP.

TDK's MEGACAP is an MLCC with a metal lead frame available in single and stacked type (Figure 13). The metal leads absorb much of the stress from bending and vibrations.

>>Multilayer Ceramic Chip Capacitors Product site
>>Flex Crack Countermeasures in MLCCs Outline

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Question 10

Commentary video of flex crack countermeasures for MLCCs (Multilayer Ceramic Capacitors)

Accepted Answer

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