Electrostatic stress is the maximum voltage that can be applied across a PCB material, beyond this maximum, the PCB dielectric material breaks down.
Electrostatic stress is measured in volts per unit thickness (generally per mil) in PCB design.
High potential (HiPOT) testing is an electrostatic stress test that detects potential dielectric failures. It can be conducted as an AC voltage test or DC voltage test depending upon the type of power on which the PCB works.
Quality PCB design begins with the selection of appropriate PCB materials
In PCBs, current flows through traces, and the traces are separated from each other by insulating material. Any quality PCB design must start with the selection of insulating substrate or PCB material on which the circuit is laid out.
One of the hazards resulting from the improper selection of PCB material is the direct arc over. This is a phenomenon that occurs when the electrostatic stress experienced by a PCB material exceeds its withstanding capability. Electrostatic stress is the maximum voltage that can be applied across the PCB material; beyond this maximum, the PCB dielectric material breaks down.
Electrostatic stress, or dielectric strength, is measured in volts per unit thickness, generally per mil in PCB design. It is mandatory to perform electrostatic stress tests on your PCB design to determine whether the PCB material is good enough to withstand voltage higher than the working voltage for a given period of time.
Measure the Electrostatic Stress of PCB Material Using the HiPOT Test
The increasing complexity of electronic systems requires the stacking up of layers in PCBs while maintaining the same thickness of PCB materials. Layering in PCBs decreases the dielectric layer width, which is of great concern to the dielectric withstanding property of the PCB material.
Standards such as IPC-TM-650, IPC-6012, IPC-9252 A, and IPC-2221 provide guidance on PCB material thickness, minimum spacing requirements, dielectric breakdown, current carrying capacity, and insulation resistance. There are also electric field stress tests available to detect the insulation resistance and dielectric breakdown of a PCB. One such test is called the high potential (HiPOT) test, or dielectric withstanding voltage test.
The HiPOT test is an electrostatic stress test that detects potential dielectric failures. It can be conducted as an AC voltage test or a DC voltage test, depending upon the type of power on which the PCB works. HiPOT testing in PCBs can be classified as:
- Insulation resistance test-This test checks whether there is adequate insulation between the traces by measuring the insulation resistance.
- Dielectric withstanding test-This test detects the defects in PCB material and checks whether the PCB trace can handle a given high voltage for a specific amount of time.
- Dielectric breakdown test-This test is a destructive test that usually results in damaged PCBs. It is conducted to find out at what high voltage the PCB will fail due to high electrostatic stress.
HiPOT tests measure the dielectric strength of PCB material, which is an indication of the insulation capability of the PCB material between the power and ground layers. The test duration differs with each circuit, from a few seconds to a few minutes. The IEC 60950 recommends a HiPOT test duration of 1 minute.
HiPOT Test Parameters
According to IPC-TM-650, HiPOT testing is the test of insulation between the power and ground layers in a PCB. Typically, it is conducted using a HiPOT analyzer, which applies a high voltage across the power and ground nets of the PCB. The tests for fault conditions, vibration, and humidity are conducted before the HiPOT test to analyze whether any degradation has already taken place.
HiPOT testing is characterized by three main parameters:
Voltage-The potential to be applied to test the PCB can be either AC or DC.
Ramp time-The time taken in seconds to bring the test voltage to the required level.
Dwell time-The time taken in seconds to maintain the test voltage to the required level.
According to IPC-TM-650 specifications, there are two conditions, namely condition A and condition B, to test the insulation in a PCB, as given in the table below.
IPC-TM-650 conditions for HiPOT testing
Methods of Performing the HiPOT Test
There are three ways to conduct HiPOT testing:
Manual testing-In this method of HiPOT testing, an operator fixes the probes from the HiPOT analyzer between the potential layer pairs in a PCB.
Semi-automated testing-When multiple pair measurements are required, semi-automated testing is found to be advantageous. The operator is needed only to place the PCB on the fixture that is wired for the pairs requiring the test and to initiate the test. The pairs are automatically sequenced in this method.
Fully automated testing-In the fully automated test, a HiPOT analyzer and a flying probe are utilized so that all the pairs are tested automatically. With the proper programming interface to the HiPOT analyzer, this method is the fastest HiPOT test available.
The test procedure is to apply a voltage that is higher than the board’s working voltage to the trace pairs using probes. Generally, the high voltage applied in the HiPOT test equals twice the nominal input voltage+1000 V if it does not follow conditions A or B. During the application of test voltage, the leakage current in the PCB substrate is monitored.
A PCB passes the HiPOT test when the PCB insulating substrate resists the applied high voltage without breaking down and also prevents the flow of leakage current. If a PCB substrate gets punctured, decomposes, or allows the flow of leakage current, it fails the HiPOT test.
It is catastrophic to apply voltages to a PCB that produce electrostatic stress higher than the dielectric strength of the PCB material. As electrostatic stress failures adversely affect the reliability and integrity of a PCB, they should be mitigated in the early design cycle by using HiPOT testing.