8.1. Breakdown Factors during In-process Inspections

Electrostatic discharge (ESD) breakdown in a PCB assembly process is the failure factor that occurs most frequently in in-circuit testing (ICT). Meanwhile, electrical overstress (EOS) such as overvoltage is the breakdown factor that occurs most frequently in functional testing (FT) after PCB assembly process. In some rare cases, a semiconductor device deteriorated by ESD meets its standard in in-circuit testing and then breaks down in later functional testing.

8.2. In-circuit Testing

In-circuit testing is the test that electrically detects the failure modes (e.g., reverse insertion) of the individual components mounted on a PCB assembly . By applying a voltage of several volts to a specific terminal of the PCB assembly, you can measure leakage current to check an impedance value which determines whether a component is reversely inserted or not. A semiconductor device like an IC will have the impedance that greatly varies when the reverse voltage applied between each terminal and the ground terminal induces a parasitic operation of the internal circuits. For proper in-circuit testing, please fully understand the characteristics described above so that voltage with appropriate polarity will be applied to semiconductor devices.

8.3. Operational Testing and Equipment-level Testing

A common in-process inspection involves the following tests: operational testing to check operations by attaching a simple resistive load to a PCB assembly; equipment-level testing to check close-to-real device operations by simulating with an inductive load or the like. Please note that functional testing is included in the operational testing.
Here are the considerations you should take in conducting these tests.

1) Application of Overvoltage

● Overvoltage Applied during Testing
In operational or equipment-level testing, an overvoltage applied from inspection equipment may break down semiconductor devices. Before determining your test conditions, please thoroughly check and consider the waveforms of individual parts with an oscilloscope to protect semiconductor devices from any overvoltage breakdown.

● Overvoltage Applied via Antistatic Mat
An antistatic mat means a conductive mat. When performing an operational test, please do not put a PCB assembly directly on the antistatic mat. If any of the terminals of a semiconductor device makes contact with the antistatic mat used, a high voltage induced by testing can be applied to the low-voltage terminals via the antistatic mat used. This may cause the semiconductor device to have an overvoltage breakdown.

● Overvoltage Applied via Metal Discharging Plate etc.
Please do not put a PCB assembly on a metal plate or metal mesh because an electric charge stored in each capacitor of the PCB assembly is discharged after operational or equipment-level testing. High residual voltages of the capacitors can be applied to the low-voltage terminals of a semiconductor device via the metal plate or metal mesh used. This may cause the semiconductor device to have an overvoltage breakdown.

● Overvoltage Applied via Conductive Returnable Container
For a PCB assembly that underwent operational or equipment-level testing, please be sure to put it into a conductive returnable container after the electric charges stored in the capacitors inside the circuits are completely discharged. Residual voltages of the capacitors due to insufficient discharging may be applied to the low-voltage terminals of a semiconductor device, thus causing an overvoltage breakdown of the semiconductor device.

2) Application of Negative Voltage

Many of semiconductor devices like ICs are designed to have a reference point in their circuits, i.e., the ground terminal with a reference voltage of 0 V. A phenomenon in which a voltage lower than that of the ground terminal is applied to other terminals is called the “application of negative voltage.” Please note that such phenomenon may cause ICs to malfunction, deteriorate, or break down.

8.4. Burn-in Process

Burn-in testing is the test that screens for initial failures a PCB assembly or equipment may have, which is occasionally performed as part of an in-process inspection. In burn-in testing, overvoltage is the key consideration as well as other inspection tests. Especially when equipment under test is energized while being transferred by a belt conveyor, the energizing voltage may drop instantaneously. If a high dv/dt voltage is then applied to the device under test as it recovers from the voltage drop, the semiconductor device used may malfunction, deteriorate, or break down.