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A TVS diode can meet a room-temperature leakage check and exceed the system limit at high temperature because reverse leakage depends on junction temperature and applied voltage. Board contamination, parallel branches, measurement settling, and previous surge stress can produce the same symptom. Diagnosis requires stressed and control samples measured at the same voltage, temperature, time, and instrument range.
Decide whether the measurement belongs to the TVS or the board
Current measured at a protected power input can include the TVS, bias resistors, capacitors, controllers, indicator circuits, contamination, and other protectors. That number should be called board current until the branches have been separated.
Preserve the failed condition before removing the part. Record the voltage at the TVS pads, chamber temperature, device surface temperature, settling time, board cleanliness, operating mode, and instrument range. Compare an unstressed board with the same layout and material lot when available.
The initial record should contain:
board and device identifiers;
exact reverse voltage at the component;
ambient and measured surface temperature;
soak and measurement time;
instrument range, resolution, and current limit;
board-cleaning and rework history;
previous surge waveform, polarity, count, and state.
Removing the diode too early can erase a solder, flux, or moisture path that caused the original current.
Reverse leakage is temperature dependent
Reverse-biased semiconductor junctions show higher leakage as junction temperature rises. The amount is product-specific and must be taken from the exact datasheet or measured characterization; there is no universal multiplier that can be applied to every TVS.
The chamber set point is not the junction temperature. Nearby converters, copper area, airflow, enclosure insulation, and self-heating change the device condition. A surface-temperature measurement on stressed and control boards can support comparison, but the final electrical result remains leakage at a defined reverse voltage and temperature.
Use the same ramp, soak, and read time for each sample. A quick reading on one device and a long stabilized reading on another can create a false lot difference.
A narrow VRWM margin becomes visible at high temperature
Reverse working voltage, VRWM, marks the intended continuous reverse-voltage boundary under stated conditions. A system labelled 24 V, 48 V, or another nominal value can reach a higher continuous value at the TVS because of supply tolerance, battery or charging state, remote regulation, startup, and repetitive overshoot.
Check the working window in a defined order:
Measure the TVS pad voltage across low, nominal, and high supply conditions.
Capture startup, shutdown, load steps, and repeated non-surge overshoot.
Repeat the leakage measurement at the highest operating temperature.
Compare the result with the datasheet voltage and temperature conditions.
If a higher VRWM is considered, recalculate clamping at the required surge current.
Confirm that downstream components still tolerate the new residual voltage.
Moving to a higher voltage class may reduce normal leakage while increasing VBR and VC. The normal-operation and transient boundaries must be solved together.
Surface contamination can look like junction leakage
Flux residue, moisture, dust, fingerprints, and rework contamination create parallel surface paths. High temperature and humidity can make those paths easier to observe, especially when the current of interest is in a small range.
Use a controlled A/B sequence. Measure the original board, isolate adjacent branches where practical, perform a documented cleaning and drying process, and repeat the same voltage-temperature sweep. Photograph each state. Do not clean, resolder, and replace the diode in one step because the effective change will be unknown.
A part removed from a contaminated board may test normally by itself. That result does not mean the original measurement was wrong; it means the current path may have been outside the silicon.
Parallel devices need to be separated carefully
Input networks can contain several TVS devices, common-mode components, capacitors with insulation resistance, sensing dividers, and IC protection structures. Disconnecting one branch changes the circuit and can alter applied voltage elsewhere.
Before lifting parts, review the schematic and mark every path that can carry DC current at the test voltage. Use current-limited equipment, preserve polarity, and record each disconnection. Measure a known-good board in the same sequence to distinguish normal branch current from the failure.
Useful comparisons include:
complete-board current versus temperature;
current after isolating one protection branch;
removed-device leakage on a controlled fixture;
breakdown behavior at the specified test current;
current after cleaning and drying;
stressed device versus an unstressed lot control.
Surge damage may remain parametric
A TVS that has absorbed a pulse beyond its validated boundary does not always become a hard short. Leakage may rise, breakdown behavior may shift, clamping may change, or a local thermal defect may emerge only at elevated temperature. A continuity tester cannot identify these states.
When prior surge exposure is suspected, compare leakage over temperature and reverse voltage, VBR at the specified test current, and powered thermal behavior. Apply another destructive pulse only under a written failure-analysis plan. Passing one more surge does not establish the original remaining life of the part.
The board around the TVS also requires inspection. A cracked joint, carbonized residue, damaged via, or overheated trace can create current and voltage behavior that resembles semiconductor degradation.
Use controls from the same production context
The strongest control is an unstressed device from the same ordering code and material lot, stored and handled with known conditions. When that is unavailable, use several known-good parts and state the limitation. A single unrelated sample from another voltage class or package is a weak reference.
Sample identity matters throughout the analysis. Retain reel labels, date or lot codes, board serial numbers, orientation photographs, and rework records. If devices are removed, place each one in a labelled position rather than a shared bag.
Define the production disposition separately
Failure analysis and production reuse are different decisions. Engineering may continue testing a stressed unit to learn the mechanism. Production should not release a TVS with uncertain remaining pulse life merely because room-temperature leakage is within a limit.
A disposition can classify a sample as:
failed against a documented electrical limit;
reserved for destructive analysis;
unchanged within the measurements performed;
inconclusive because identity or test evidence is missing.
The third result must retain its voltage, temperature, and method. It should not be shortened to “good as new.”
What to send ASIM for analysis
An effective analysis package includes the abnormal device, an unopened sample from the same lot, a known-good board, original labels, schematic location, photographs, leakage-temperature results, and the event history. For surge exposure, provide waveform, source impedance, polarity, event count, interval, and the product state at failure.
Those inputs define the electrical window and the next measurement. The ASIM TVS diode manufacturer hub lists related selection information. The conclusion must remain tied to the exact ordering code and observed evidence.
Record a result that another laboratory can repeat
State the reverse voltage, ambient and device temperature, soak time, instrument setup, sample identity, cleaning condition, stress history, IR curve, VBR test current, and control result. A conclusion such as “no difference was detected under these conditions” is more accurate than declaring a device healthy outside the measurements performed.
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