Understand the main internal factors of TVs tube short-circuit failure — SUNMATE

Understand the main internal factors of TVs tube short-circuit failure

Briefly understand the main internal quality factors causing TVs tube short-circuit failure:

The failure modes of TVs tube include short circuit, open circuit and degradation of electrical characteristics. Short circuit failure is common and has a serious impact on the circuit. Once the TVS tube fails in short circuit, the high energy released will often damage the protected electronic equipment. TVs device is mainly composed of chip, electrode system and shell. The chip is usually formed on single crystal silicon by diffusion process. Poor control in TVs manufacturing process engineering may cause inherent defects of TVs devices, reduce the yield and reliability of TVs, and easily lead to failure in screening or use.

1. Chip bonding interface cavity

The typical cause of TVs tube short circuit is poor sintering of tube core, inner lead assembly and base copper, and large-area voids appear at the sintering interface. The cavity may be caused by the uneven solder or the poor wetting of the solder due to the contamination and oxidation of each layer of materials at the bonding interface, resulting in the lack of good fusion welding between the solder and the chip or metal electrode during welding. When the cavity area is large, the current converges near the burn-in point, which makes it difficult for the tube core to dissipate heat, resulting in the concentration of thermoelectric stress and local thermoelectricity. In serious cases, it causes thermal run-off and burns the device. Small cavity area can accelerate the thermal fatigue of solder, which will lead to fatigue cracking of solder layer, increase the thermal resistance of devices, and eventually lead to overheating and burning of devices.

2. Table defect

The failure caused by TVS tube surface defects is often batch. The main causes of chip mesa damage in TVs manufacturing process are that the formula of hydrofluoric acid and nitric acid mixed solution is too thick or too high and the reaction is violent during acid etching molding. When alkali corrosion cleaning is carried out after welding, the concentration of corrosive solution is too high and the temperature is too high, resulting in excessive alkali corrosion cleaning.

3. Surface strong accumulation layer or strong inversion layer

Even if the TVS device chip table is intact, TVs short-circuit failure is easy to occur on the surface. This is due to the periodic interruption of the crystal structure on the surface and the lattice defects caused by many grinding discs, polishing, sandblasting and slicing on the semiconductor surface. The residual chemicals, gases or other pollutants during adsorption and corrosion will charge the semiconductor surface. The surface charge is passivated by the protective glue and adsorbs or repels the free carriers in the semiconductor. Surface space charge layers such as surface accumulation layer, depletion layer or inversion layer are formed at the edge of PN junction. Under the action of the applied voltage, the strong accumulation layer or strong inversion layer makes the electric field intensity at the edge of the PN junction greater than that at the edge of the PN junction in the body. At a voltage lower than the rated breakdown voltage, the critical electric field will reach and the carrier multiplication effect will occur, resulting in the concentration of the current at the edge of the PN junction, the excessive power density and the burning out of the PN junction due to the high temperature.

4. Chip crack

Chip crack is an important internal quality factor causing TVs tube short-circuit failure. It may be caused by residual stresses such as grinding, polishing, sandblasting and slicing and residual deformation after sintering. It may also be caused by thermal mismatch stress between protective glue and electrode system on chip when temperature changes.

5. Inhomogeneous impurity diffusion

The chip of TVs tube is usually formed by phosphorus diffusion and then boron diffusion on p-type or n-type silicon chip with a certain resistivity. If the resistivity of the silicon wafer is uneven in the axial or radial direction and the impurity concentration is uneven during the diffusion process, the breakdown voltage of the silicon wafer will be different, so that the core current distribution during the device breakdown will be uneven and the device will be burned locally after multiple surges.