​Understand the main internal factors of TVS short circuit failure

Understand the main internal factors of TVS  short circuit failure

Simply understand the main internal quality factors that cause TVS  short circuit failure:

The failure modes of TVS s include short circuits, open circuits, and degradation of electrical characteristics. Short circuit failure is common and has a serious impact on the circuit. Once a short circuit occurs and a TVS  fails, the high energy released often damages the protected electronic equipment. The TVS device mainly consists of three parts: a chip, an electrode system, and a shell. The chip is usually formed by diffusion process on a single crystal silicon chip. If poorly controlled in TVS manufacturing process engineering, it may cause inherent defects in TVS devices, reduce TVS yield and reliability, and easily lead to screening or failure in use.

1. Chip bonding interface voids

The typical cause of short circuit in TVS s is poor sintering of the  core, inner lead assembly, and base copper sheet, resulting in large areas of voids at the sintering interface. Voids may be caused by uneven solder or contamination or oxidation of materials at the bonding interface, resulting in poor wetting of the solder, resulting in poor fusion welding between the solder and the chip or metal electrode during soldering. When the cavity area is large, the current converges near the burning point, making it difficult for the  core to dissipate heat, resulting in concentration of thermoelectric stress and local thermoelectric generation. In severe cases, it can cause thermal runaway and cause device burnout. A small void area can accelerate the thermal fatigue of the solder, causing fatigue cracking in the solder layer, which increases the thermal resistance of the device and ultimately leads to overheating and burning of the device.

2. Table defect

The failure caused by TVS pipe surface defects is often batch based. The main reasons for chip mesa damage during the TVS manufacturing process are that the hydrofluoric acid and nitric acid mixture used for chip etching and forming is too concentrated or too high, resulting in severe reactions. After soldering, when performing alkaline corrosion cleaning, the concentration of the corrosion solution is too high, and the temperature is too high, resulting in excessive alkaline corrosion cleaning.

3. Strong accumulation layer or strong inversion layer on the surface

Even if the TVS device chip surface is intact, TVS short circuit failure is prone to occur on the surface. This is because the periodicity of the crystal structure is interrupted on the surface, and there are often many lattice defects on the semiconductor surface caused by grinding, polishing, sandblasting, slicing, etc. The residual chemicals, gases, or other pollutants during adsorption and corrosion can make the semiconductor surface charged. The Surface charge is passivated by the protective glue, and the free carriers in the semiconductor are adsorbed or repelled. A surface space charge layer such as a surface accumulation layer Depletion region or an inversion layer is formed at the edge of the pn junction. Under the action of external voltage, the strong accumulation layer or strong inversion layer causes the electric field intensity at the edge of the pn junction to be greater than that at the edge of the internal pn junction. At a voltage lower than the rated breakdown voltage, the critical electric field is reached, leading to carrier multiplication effect, resulting in the concentration of current at the edge of the pn junction, excessive power density, and burning out due to high temperature.

4. Chip cracks

Chip cracks are an important internal quality factor that causes short circuit failure of TVS s. It may be caused by residual stresses such as grinding, polishing, sandblasting, slicing, and residual deformation after sintering, or it may be caused by mismatched stresses between the protective adhesive and electrode system on the chip during temperature changes.