Fig. 1: Element cracking (cross section)
Fig. 2: Major causes and process of short circuit failure
Flex cracking is due to excessive circuit board flexure. As for the causes of board flexure, there are various causes including problems during the manufacturing process, such as solder stress due to an inappropriate amount of solder, stress applied at the time of depaneling or screw fastening, or board flexure at the time of final assembly, in addition to drops, vibration, or thermal expansion during use.
Ceramics are strong in compression but weak in tension. Thus, when a soldered MLCC experiences excessive board flex, a crack is easily generated in the element.
A flex crack can cause an electrical conduction between opposing internal electrodes. It is also possible that a fail open can progress to a fail short with continue product usage.
If a crack on a capacitor element progresses to a short circuit failure, it may cause problems such as heat generation, smoking, or ignition; therefore, it is indispensable to take measures against them, particularly in equipment where reliability is essential.
A fine crack that occurs during the process from SMD mounting to set assembly might progress to cracking of the capacitor element when the product is sent to the market and used. Such a risk is higher in equipment exposed to vibration or shock, such as automotive electronics, railway equipment, or industrial equipment. In addition, the probability of the occurrence of a crack is higher in equipment that can experience frequent shock due to drops, such as keyless entry or smart entry equipment.
In equipment used in a humid environment, condensation is generated from water vapor and may enter into an element crack, causing ionization of the metal of the internal electrodes and ion migration. This will cause an open circuit failure to progress to a short circuit failure.
Fig. 3: Applications that require flex crack countermeasures (1/2)
Products exposed to vibration or shock
Equipment expected to experience shock due to drops
Equipment used in a humid environment
Manufacturing can cause cracks to occur in the capacitor element due to expansion and contracting of the board when an MLCC is directly attached to an aluminum circuit board, etc., having large thermal expansion, not to mention at areas near screws or depaneling. In addition, it is common that boards are bent excessively during board manufacturing or final assembly, and fragile ceramic components tend to get damaged when they are soldered to boards.
Fig. 4: Applications that require flex crack countermeasures (2/2)
MLCCs near screws or depaneling
Board having large thermal expansion
Design that requires the board to be bent excessively during board manufacturing or final assembly
Fig. 5: MEGACAP structure
MEGACAP is a type of MLCC in which metal caps are attached to the terminal electrodes, and is available in single and double stacked configuration (Fig. 5).
Fig. 6 is a comparison of the flex strengths of a regular terminal product and MEGACAP. Element cracks occurred in the regular product after it was flexed up to several millimeters. On the contrary, no cracking occurred when MEGACAP even after it was flexed 10 millimeters or more.
Fig. 6: Flex strength comparison with the 5750 size (Comparison between a regular terminal product and MEGACAP)
Fig. 7: Difference between a regular terminal product and
In the terminal electrode of a regular MLCC, the Cu under layer is plated with Ni and Sn. Soft termination is a type of MLCC in which a conductive resin layer is provided between the Cu and Ni plating layer (Fig. 7).
The resin layer absorbs stress accompanying expansion or shrinkage of the solder joints due to thermal shock or flex stress on the board and prevents cracking of the capacitor element.
Fig. 8 shows the data of a board flex resistance (critical bending) test. In a conventional product, cracks developed on the ceramics element even with a flex of about 4 mm. On the contrary, Soft termination can safely withstand twice as much flex.
Fig. 8: Comparison of flex strength in 3216 size products (comparison between a regular electrode product and Soft termination)
Fig. 9: When pressure is applied excessively, the terminal electrodes
peel off and prevent the occurrence of element cracks
When excessive stress was continuously applied, cracks developed on the ceramics element in a conventional product. On the contrary, in Soft termination, no cracks developed on the element, even though there was peeling of the nickel plating layer and the conductive resin layer. This shows that the conductive resin layer has an excellent effect to prevent element cracks.
However, it has been confirmed that no resin peeling occurs even at 6 mm, which exceeds the 5 mm of the flexure guarantee condition.
Fig. 10: Tumbling test result (comparison between a regular product and Soft termination)
The test condition was set to satisfy the requirements for mobile phone applications. No cracks developed after a drop test with 10,000 cycles, while passing an 85/85 humidity test.
This shows that the resin electrode parts absorbed shock.
Fig. 11: Two short circuit countermeasures of Serial design
Serial design (the CEU series) is a type of MLCC with the highest safety adopting dual safety designs for crack occurrence prevention and short circuit occurrence prevention.
Firstly, the conductive resin layers are inserted in the terminal electrodes, and the resin electrode layers absorb stress applied by flex or thermal expansion of the board, preventing crack occurrences.
Secondly, the internal electrodes adopt a special structure, which is equivalent to a serial connection of two capacitors. This structure will reduce the risk of short-circuiting if a crack should occur on the capacitor element.
Moreover, the CEU series is compliant with AEC-Q200 and can be used for automotive applications.
When regular products are replaced with Serial design (the CEU series) in power lines carrying a large current, safety can be easily enhanced due to their dual fail-safe function. Since just one Serial design (the CEU series) product can realize safety design which usually employs a serial connection of two regular products, mounting areas or mounting costs can also be reduced (Fig. 12).
Fig. 12: Image of replacement of regular products with Serial design (the CEU series)
Serial design also has thermal shock resistance.
For information on thermal shock resistance, please refer to ≫Solder Crack Countermeasures in MLCCs..
Fig. 13: Open mode structure
Open mode is a type of MLCC in which the gap between the terminal electrode and the internal electrode on the opposing terminal electrode side (called L-Gap) is longer than that of regular products.
By making the overlapping portion of the opposing internal electrodes shorter, it is ensured that the opposing electrodes do not overlap at places where cracks may occur.
Due to this, the risk of short-circuit can be reduced even if cracks should occur.
* The design concept of the Open mode is for the reduction of the risk of short-circuit mode breaking, and it does not mean that the products will always have an open-circuit when it is damaged.
Fig. 14: Failure simulations (comparison between a regular product and Open mode)
The features of each product are summarized in the table 15 below.
Table 15: Comparison of flex crack countermeasures for MLCCs
|image||Flex stress||Large capacity||Cost||Applications||Product information
|1) MEGACAP||★★★||★★★||★||Circuits requiring especially high reliability and large capacitance|
|2) Soft termination||★★||★★||★★||Circuits in which flex stress or thermal shock can become an issue|
|3) Serial design, the CEU series
(resin electrode s+ safety structure)
|★★||★||★★||Circuits in which flex stress or thermal shock can become an issue and a serial connection of capacitors is considered|
|4) Open mode||★★||★||★★★||Circuits that do not require very big capacitance but flex stress can be an issue|