PCB Technical - Analysis of possible causes of PCBA MLCC rupture

Possible reasons for the rupture of PCBA MLCC multilayer ceramic capacitors

In general capacitors (micro cracks), most of them will produce open circuit phenomenon and cause the insulation resistance (IR, Insulation Resistance) to increase. When a microcrack occurs on the hand, it is indeed common that the insulation resistance becomes small, resulting in a short-circuit phenomenon of current leakage. The cause may be due to the layer-to-layer short-circuit phenomenon when the layered structure is ruptured. .

If you are not very clear about the structure of MLCC, it is recommended that you first refer to the article that has been published before introducing the structure and process of multilayer ceramic capacitors (MLCC).

Let's talk about the possible causes of micro-cracks in general "multilayer ceramic capacitors".

The causes of MLCC rupture can be roughly divided into the following three directions:

▪ Thermal shock failure (Thermal shock)

▪ Extrinsic Defect, Overstress Failure (Extrinsic Defect, Overstress Failure)

▪ Intrinsic Defect

The failure principle of Thermal Shock:

When the temperature around the PCB parts rises and falls too rapidly, thermal shock will be formed, such as wave soldering, reflow, touch-up, or repair.

The high temperature. This is because in the manufacture of multilayer ceramic capacitors, a variety of different compatible materials are used. These materials have different coefficients of thermal expansion and thermal conductivity due to their different characteristics. When these different materials exist in the capacitor at the same time When the internal temperature changes rapidly, different ratios of volume changes will be formed, pushing and pulling each other, and finally causing the phenomenon of PCB cracking.

This kind of cracking often occurs from the weakest part of the structure, or the place where the structural stress is the most concentrated. It usually occurs near the exposed end joining the central ceramic interface, or where the greatest mechanical tension can be generated (usually where the crystal is the hardest The four corners of ), and the phenomenon caused by thermal shock may have the following types:

1. Cracks in the shape of nails or U-shaped.

The MLCC is shaped like a nail or U-shaped crack.

2. A small crack hidden inside the capacitor.

3. The exposed central part or the lower half of the junction between the central ceramic end and the exposed end begins to crack, and then it spreads along with the distortion as the temperature changes or during subsequent assembly.

MLCC thermal shock cracking

The first type of cracking is like a nail or U-shaped crack and the second type of microcracks hidden inside. The difference between the two is that the latter is less stressed, and the resulting cracks are relatively slight. The first type of cracks Obviously, it can generally be detected in metallography, while the second type can only be detected when it has developed to a certain degree.

(Note: "Metallographic" refers to the structure image of metal under a high-power microscope)

The failure principle of PCB overstress:

Distortion and fracture are usually caused by external forces (extrinsic). This situation usually occurs during the assembly of SMT or the whole product. The possible reasons are as follows:

1. The pick & place machine (pick & place machine) improperly grabs the parts and causes cracks. When the SMT placement machine picks and places parts, its centering jaw is caused by wear, incorrect alignment, or tilt. The concentrated pressure of the center claw will cause a great pressure or cutting force, and then form a rupture point. Such cracks are generally visible surface cracks, or internal cracks between 2 to 3 electrodes; surface cracks generally follow the strongest pressure line and the direction of ceramic displacement. The current new SMT machines no longer use this centering jaw design mechanism.

2. During the mounting process of the capacitor, if the suction nozzle of the mounter picks up the parts or places the parts too much, the parts may be bent and deformed and cracks may occur. This kind of rupture generally forms a round or half-moon-shaped indentation on the surface of the part, and has a non-round edge. And the diameter of this half-moon or circular crack will be the same as the size of the nozzle. Another type of rupture caused by tension may also be caused by the damage of the nozzle head. The cracks will extend from one side of the center of the component to the other. These cracks may spread to the other side of the component, and the rough cracks may cause The bottom of the capacitor is broken.

3. The corresponding land-pattern layout is not uniform in size (including one pad is connected to a large area of copper foil, and the other pad is not), or the solder paste is asymmetric during printing, It is also easy to be subjected to different thermal expansion forces when passing through a reflow oven, so that one side is lifted by a greater pulling or pushing force, resulting in cracks.

4. The thermal shock of the welding process and the bending and deformation of the substrate after welding can also easily lead to cracks.

4.1 During the wave soldering of the capacitor, the preheating temperature, insufficient time or too high temperature during soldering can also easily lead to cracks.

4.2 During the PCB touch-up process, the soldering iron head directly contacts the capacitor body, causing local overheating, or applying excessive pressure, which may also easily lead to cracks.

4.3 After the welding is completed, it is easy to cause cracks when the substrate is bent when the board is cut or the whole machine is assembled.

When the plate is bent and deformed under the action of mechanical force, the movable range of the ceramic is limited by the end position and the solder joints, and the crack will form outside the termination interface of the ceramic. This crack will start from the formed position, from a 45 degree angle. The termination spread.

Distortion and fracture failure. In the rupture failure caused by the SMT stage, if the rupture is slight, it cannot be detected by metallography. The rupture and distortion caused by the production stage after SMT can definitely be detected by metallography.

MLCC material failure and rupture

MLCC material failures are generally divided into three major categories of defects. Such failures usually come from the internal failure of the capacitor, and they are sufficient to damage the reliability of the product. Such problems are usually caused by the MLCC process or improper selection of materials. cause.

1. Failure between electrodes and rupture of the bond line (Delamination).

Such defects usually form larger cracks. The main reason is that the high voids of the ceramic or the voids between the dielectric layer and the opposite electrode cause the dielectric layer between the electrodes to crack, causing a potential leakage crisis.

T8 MLCC fails between electrodes and the bond line is broken. Failure between MLCC electrodes and rupture of the bond line.

2. Voiding.

Holes generally occur between two adjacent inner electrodes, sometimes even as large as multiple electrodes. Such defects often cause short circuits between electrodes and leakage currents. When a large gap is generated, it may also affect and reduce its capacitance value.

The reason for this type of defect usually comes from the improper process control of MLCC, such as foreign matter contamination or poor sintering of ceramic capacitor powder.

MLCC holes generally occur between two adjacent internal electrodes, sometimes even as large as multiple electrodes. Such defects often cause short circuits between electrodes and leakage current. MLCC holes. The cause of such defects usually comes from the process control of MLCC, such as foreign matter contamination or poor sintering of ceramic capacitor powder.

3. Firing crack.

The cracking direction of the burning rupture will be perpendicular to the electrode (electrodes), and most of it will crack from the electrode edge or terminal.

Such defects usually cause excessive current leakage and damage the reliability of the components.

The reason for this type of rupture is mostly caused by the excessive cooling of the MLCC manufacturing process.

in conclusion:

Cracks caused by thermal shock will spread from the surface of the capacitor to the inside of the component. The rupture caused by excessive mechanical tension can be formed on the surface or inside of the component, and these ruptures will spread at an angle of almost 45 degrees. As for the failure of raw materials, it will cause cracks in a direction perpendicular or parallel to the internal electrodes.

In addition, thermal shock rupture generally spreads from one terminal to zero. The rupture caused by the pick and place machine will have multiple rupture points under the terminal; the damage caused by the distortion of the circuit board is usually only one. Rupture point.