Reliability testing is a critical part of evaluating the long-term stability of power devices. However, a failure observed during testing does not always originate inside the semiconductor die. In some cases, the actual failure mechanism is hidden in a small external feature of the package or at the PCB assembly level.
This case involves a MOSFET that exhibited a drain-to-source (DS) short circuit after reliability testing while the gate-to-source (GS) path remained normal. The failure could be reproduced consistently while the device was mounted on the PCB. However, the short circuit disappeared after the MOSFET was removed from the board. When the current limit was removed during a subsequent in-circuit test, a large instantaneous current also caused the short-circuit condition to disappear.
Failure Symptom: An Intermittent Short Circuit That Disappears
After reliability testing, a particular MOSFET was detected with a short circuit between the drain and source terminals. The gate-to-source path was not shorted. An in-circuit IV test confirmed the failure while the device remained soldered to the PCB.
Further testing revealed two unusual characteristics:
- Test after desoldering: After the device was removed from the PCB, the resistance between the drain and source returned to a normal value, and the drain leakage current, IDSS, met the applicable specification.
- In-circuit retest without current limiting: When another failed sample was retested on the PCB with the current limit removed, a high instantaneous current caused the short-circuit path to disappear and the device function was restored.
These results provide an important initial direction for the investigation. If the failure disappears after desoldering, the fault may not be located in the MOSFET die itself. It may instead exist on the package surface, between adjacent terminals, or across the PCB pads. The fact that a high current can remove the short circuit further suggests that the conductive path is physically fragile and has limited current-carrying capability.
Tracing the Evidence Behind the "Disappearing" Failure
The loss of the failure after physical handling indicates that the short-circuit path may be external to the semiconductor structure. In this case, the suspected mechanism was electrochemical migration (ECM). Under suitable electrical, thermal, and moisture conditions, conductive material can gradually grow between two biased conductors and eventually bridge the insulation gap.
The most important challenge in this type of analysis is preserving the evidence. Desoldering, heating, cleaning, or applying excessive test current can destroy the physical feature responsible for the failure. For that reason, nondestructive inspection should be performed before removing the device from the PCB.
High-Resolution X-Ray Inspection
A failed sample was first examined using high-resolution X-ray inspection while it was still assembled on the board. The inspection focused on the gap between the drain and source solder pads.
Figure 1 | High-resolution X-ray inspection of the drain-source pad gap.
The inspection revealed a branched or filament-like structure crossing the insulating gap between the DS pads. This structure connected the two conductors and provided a direct electrical path between the drain and source terminals.
The morphology was inconsistent with a stable, solid solder bridge formed during PCB assembly. Instead, the delicate, dendritic appearance was consistent with conductive material that had grown progressively across the gap. This observation supported the hypothesis that the short circuit was caused by migration rather than by an internal MOSFET failure.
Three-Dimensional X-Ray Analysis
Three-dimensional X-ray analysis was then used to examine the growth path and morphology of the conductive structure. The three-dimensional images showed that the material extended through the package or assembly region between the drain and source conductors.
Figure 2 | Three-dimensional X-ray view of the conductive migration path.
Figure 3 | Dendritic conductive material bridging the drain-source insulation gap.
Figure 4 | Three-dimensional X-ray evidence of tin migration between the drain and source pads.
The three-dimensional X-ray results clearly showed that tin migration had occurred between the drain and source pads. The observed structure connected the two terminals, matching the in-circuit DS short-circuit symptom.
Failure Mechanism: Tin Dendrite Growth
The branched structures identified in the X-ray images were tin dendrites. Under the conditions of the reliability test, flux residue on the PCBA surface can absorb a small amount of moisture at elevated temperature. The residue may then provide a thin electrolyte layer containing mobile ionic species.
The high voltage between the drain and source terminals creates an electric field across this contaminated surface. Under the influence of the electric field, tin at the anode, in this case the drain side, can oxidize and enter the electrolyte layer as ions. These tin ions migrate toward the cathode, the source side, through the flux residue. They can then be reduced and deposited on the opposing conductor.
As this process continues, the deposited material develops into a narrow, branched conductive structure. The dendrite eventually extends across the insulating gap and forms a metallic bridge between the drain and source terminals. The resulting path produces a DS short circuit even though the MOSFET die and the GS insulation structure may remain undamaged.
This mechanism also explains why the failure can be stable during in-circuit testing but difficult to reproduce after the device is removed. The conductive bridge is extremely small and mechanically fragile. Its electrical behavior depends on its shape, contact points, and connection to the surrounding residue and solder surfaces.
Why the Failure Appears to "Self-Heal"
Two observations require explanation: the short circuit disappears after desoldering, and a high-current retest can restore normal resistance.
- Physical disturbance: Desoldering applies mechanical stress and heat to the package and PCB pads. The process may also involve cleaning or handling that disturbs the surface residue. These effects can break the fragile dendrite or separate it from one of the conductors.
- Fusing by Joule heating: A tin dendrite has a very small cross-sectional area and therefore limited current-carrying capability. If the current limit is removed during testing, the high current produces substantial localized Joule heating in the narrow bridge. The dendrite can melt or vaporize at the weak point, opening the short-circuit path.
After the conductive bridge is interrupted, the measured drain-source resistance returns to a normal value. This does not mean that the original reliability problem has been eliminated. The surface contamination and the conditions that allowed migration may still be present, and another dendrite can form during a subsequent electrical or environmental stress.
The apparent "self-healing" behavior can therefore conceal the true root cause. If the device is only tested after removal, or if an unlimited-current test is used before physical analysis, the evidence may be destroyed. The failure may then be incorrectly classified as an intermittent event, a measurement error, or a false alarm.
Engineering Significance of the Analysis
This case demonstrates why failure analysis must distinguish between semiconductor-die damage and assembly-level conductive paths. A MOSFET that shows a DS short circuit on the board does not necessarily have an internally failed die. The location of the failure should be verified before destructive handling, especially when the fault disappears after desoldering.
A practical investigation sequence is to record the in-circuit electrical behavior, preserve the assembled sample, perform nondestructive X-ray inspection, and only then proceed with disassembly or cleaning. Comparing the in-circuit and off-board results is also valuable because a change in resistance or leakage current after removal is itself evidence of a mechanically or environmentally sensitive external path.
The analysis also emphasizes the importance of controlling conductive residue around high-voltage nodes. Flux residue, moisture, ionic contamination, and the electric field between adjacent conductors can work together to create electrochemical migration. When these conditions exist near the drain and source terminals, a dendritic bridge may develop without causing immediate damage to the MOSFET die.
Ultimately, the failure mechanism in this case was tin migration across the drain-source insulation gap. The resulting tin dendrite created an apparent MOSFET DS short circuit, while the GS path and the semiconductor structure remained normal. Desoldering mechanically disrupted the bridge, and high-current testing fused it open. Recognizing this behavior prevents misdiagnosis and helps direct corrective action toward the PCB assembly and contamination-control process rather than incorrectly replacing the MOSFET as the primary cause.
