Technologies > Immunity > Robotic ESD Gun Test
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The methodology for system level ESD testing is in accordance with the IEC 61000-4-2 standard. This standard sets minimal requirements and gives information on the test setup. The standard setting body had hand testing in mind, but did not include robotic testing. Because hand testing was in mind, many minimal parameters have been set such that repeatability problems result.
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• Number of dischargesThe number of discharges per test point is set to 10. This provides a very low number to capture windows of sensitivity. There are brief periods of time in which an EUT is much more sensitive to ESD. Those windows are usually caused by software activity. The influenced windows of opportunity and the required number of pulses to achieve a statistically stable result have been investigated by Bob Renninger and Habiger, and found its way into an ANSI ESD C63.16 standard. In brief the analysis says: one has to apply a much greater number of pulses to capture windows of opportunity. The exact number depends on the distribution of the sensitivity over time. The standard setting bodies knowingly had to ignore these facts, as the members did not want to force every test lab to perform e.g., 100 discharges at each test point due to strain placed on the operator, especially in air discharge mode. However, a large number of discharges is needed in order to achieve a statistically stable result.
• Approach speed, angle of approachAir discharge testing depends strongly on the length of the arc. The length of the arc and voltage can vary for the same test point strongly. Those variations are partially from statistical nature, but also influenced by the way the approach is performed. At higher approach speeds (this is the intention of the standard setting body) the rise time will be lower on average, and the peak values will be higher on average. The standard provides little insight into the expected speed of approach. Some test labs even drag the charged air discharge tip across the product (in contact with the plastic surface) to see if a discharge occurs. This is a very risky practice, as it can lead to highly over-voltage ESDs having unrealistically low rise times. It is important to control the way of approach (straight to the point of expected discharge) and to control the angle at which the ESD generator is held, as this will influence the discharge current (weakly) but the coupling to the product strongly.
If discharges with much faster rise occur, there is no way for the operator to know in manual system level testing.
• Voltage incrementsThe voltage is often set in large increments (such as 2, 4, 8 kV), and only pass or fail is reported. When comparing test labs, it is important to know the failure level. For example, if one lab passes up to and including 8kV but the product would fail at 8.001 kV, and if the same product is retested in another lab and fails at 8kV, then the uncertainty of the testing might only be 0.001 kV. Of course, the real uncertainties in ESD testing are much higher, but the example illustrates that we not only need a pass/fail but we need a failure level. This can only be found if voltages are increased in relatively small increments (e.g. 1000V). The standard-setting body did not want to require this as the strain placed on hand operators was considered to be too high.
• Repetition rateThe testing for air discharge is usually done at 1 pulse per second. In theory, one could test much faster if one can (1) Remove the charge between discharges, (2) Verify the function of the EUT, (3) and Move the ESD generator back to the discharge position. In a hand test this is difficult to achieve. However, a higher repetition rate can be achieved through robotic testing, even in air discharge testing.
Robotic ESD testing substitutes the hand operation with a robot. This has the following set of advantages:
The robotic system is able to approach the DUT at every discharge in a very precise way. Parameters, such as approach speed and angle of approach, can be finely controlled and repeated.
• Test Depth
Hand testing is limited in the number of ESDs applied, the levels tested at, and the repletion rate. Robotic scanning does not know fatigue. One can apply a much greater number of pulses to many more test points at much smaller increments to achieve a better, more statistically reliable test result. Robotic test results cover a wide range of possible test results that might be achieved in other labs.
The robotic test can capture the discharge current for each discharge, which enables the user to know if a very fast rise time event occurred or not and whether that discharge is correlated to an observed failure.
A robotic test is the gold standard for ESD testing and helps in training operators (e.g., demonstrating the effect of the speed of approach) or in settling any disagreements with other laboratories' findings.
• Extending to other functions
A robotic tester can perform other functions, such as pressing knobs to reset a product or for field matting and other immunity testing.
• Test speed
If a suitable number of ESDs is applied to a product, then robotic testing will offer a large testing speed advantage. This advantage grows further if the number of test samples is larger than one.