Fairchild Transient Voltage Suppressor SMBJ5VOCA

Gideon Analytical Laboratories received two Fairchild transient voltage suppressors (TVS) SMBJ5V0CA. Both diodes were identified as having failed.Transient Voltage Suppressors (TVS’s) are devices used to protect vulnerable circuits from electrical overstress such as that caused by electrostatic discharge, inductive load switching, and induced lightning. Within the TVS, damaging voltage spikes are limited by clamping or avalanche action of a rugged silicon pn junction which reduces the amplitude of the transient to a nondestructive level. All semiconductors have to be confirmed failure and in what sense they have failed. The diode gave an abnormal curve trace in which the surface is inverted.

On the curve tracer, we are looking at the breakdown in which a large reverse current flows.There are three types of breakdown: avalanche, Zener, and thermal. Avalanche breakdown occurs when, over a mean free path in the barrier region,acarrieracquiresenergythatissufficienttoionizethecrystallattice.We see this in MOSFETs all the time when the BVdss is measured. Zener breakdown occurs when carriers tunnel through the potential barrier (voltage regulators).Thermalbreakdownisassociatedwithinsufficientheatdissipationfromajunctionthatoperatesatlargecurrents. These TVS have the third type. Die surface has melted metal and no contamination. Inversion is simply the majority carries have been outnumbered by the minority carriers at the surface and thus we see the curve trace we saw in the report.Sometimes if an external path (but internal to the package) is present (going down the sides of the die, leakage path from anode to the cathode via the lead frame) you will get a curve trace that resembles the one shown in the report. To make sure the leakage is coming from the die, the die was cleaned and the lead frame eliminated and the probes were on each side of the die. Thus, it was confirmed that the die alone was causing the leakage path through the active area of the IC.

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Anode metallurgy

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Micrograph of the diode

Anode side

Anode side

Melted Metallurgy

Melted Metallurgy

High current diode

High current diode

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cathode side

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Diode between the lead frame