Take you to understand the bending cracks of chip multilayer ceramic capacitor MLCC
Latest update time：2020-02-01
Reads：
Since the work date has been postponed and the scope of activities is limited to the yard of my hometown, I have more time.
I have been sorting out and summarizing the basic knowledge about capacitors at home these days, and writing out several key points separately.
Today we will introduce the bending crack problem
of MLCC
.
The concept of MLCC
Chip multi-layer ceramic capacitors (MLCC:
Multi-layer Ceramic Capacitors) are made of ceramic dielectric diaphragms with printed electrodes (internal electrodes) stacked in a staggered manner. They are then sintered at a high temperature to form a ceramic chip, and then the chip is Seal both ends with metal terminals (external electrodes).
Structure of MLCC
The internal structure diagram of MLCC is as follows: Based on the basic structure of a parallel plate capacitor, two electrodes are overlapped, and a dielectric is added between them. In order to store more charges, MLCC overlaps this structure many times. ;
The terminal electrode is composed of an inner layer of copper, a nickel plating layer, and a tin plating layer (this is very similar to the plating structure of the connector PIN pin). The inner layer of copper
connects
the superimposed internal electrodes
.
From another perspective, the internal structure can be simplified as shown in the figure below, and you can compare it up and down.
Definition and causes of MLCC bending cracks
It refers to the cracks formed due to the large bending of the MLCC welded on the PCB;
furthermore, it is the mechanical stress caused by the bending of the PCBA substrate during the manufacturing process, which causes the cracks in the MLCC;
for example, the depanel cutting of the PCBA and the installation of screws. Scenes such as fixing and unplugging and plugging connector harnesses (pictures from TDK official website).
In addition,
excessive or uneven solder
may also cause cracks in the capacitor; there was a case in which a power supply short circuit occurred on a certain board after a temperature cycle test; after further locating, it was found that a certain problem at the power supply There was a short circuit in the MLCC, which was eventually determined to be caused by a crack. The reason was found to be that this MLCC was repaired
by manual welding later
. It may be that the amount of solder on both ends of the capacitor was too much or uneven,
causing excessive or uneven stress. During the temperature cycle test, due to the different expansion coefficients of the substrate and the capacitor, cracks eventually occurred
.
The mechanism of bending cracks in MLCC
The internal structure of the capacitor has been introduced above. The dielectric material of MLCC is ceramic compound. Although it is resistant to compression, it is not resistant to stretching and is brittle.
In the concave bending of the PCB, since the force acting on this part is compressive force,
cracks are almost impossible to occur
on the MLCC
.
However, in the convex bending of PCB, since this part is subjected to tensile force, MLCC is prone
to cracks
.
Just like sticking a potato chip to a piece of cardboard and then bending the cardboard, it creates cracks, as shown below.
The process of generating bending cracks is summarized as follows:
The appearance and consequences of bending cracks in MLCC
The common cracks produced
are called 45-degree cracks
. These cracks start near the boundary between the terminal electrode of the MLCC and the ceramic body, extend upward (in the direction of bending stress) at an angle of 45 degrees, and reach the outside of the terminal electrode of the MLCC. Bending cracks can rarely be found by visual inspection on the outside, requiring further internal inspection;
of course, other bending cracks can occur if the area is subjected to excessive bending.
After a crack occurs, it may directly lead to a short circuit or open circuit;
however, as time goes by, even in the open circuit mode, moisture and foreign matter will penetrate into the interior of the MLCC, which may cause the insulation resistance to decrease and develop into a short circuit.
(Picture from TDK official website)
Solutions to Prevent Bend Cracks
The most effective way to prevent cracks in MLCC is
to avoid applying unnecessary external force to the PCB
.
For example, the fixation design of the product shell to the PCBA and increasing the number of fixing holes on the PCB can effectively reduce the stress on the PCB.
When arranging devices on the PCB, you should also consider keeping the capacitor away from the PCB split surface and around the screw holes;
in the figure below, the capacitor at the split position of the imposition should first be as far away from the split as possible, and then the capacitor should be parallel to the split line to reduce stress ( The main idea is
to place the capacitor perpendicular to the direction of stress caused by PCB deformation
).
There are currently many measures to protect the capacitor itself;
for example, adding a conductive resin layer (soft terminal) to the terminal electrode, changing the structure of the internal electrode plate, etc.
(Picture from TDK official website)
Summarize:
The above related content refers to the internal information of several capacitor manufacturers (TDK\Murata\KEMET). Each device manufacturer's official website has a lot of basic knowledge introduction, you can make full use of it.
All of the above are for reference only.
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