ESD Testing - Standard and Methodology

Introduction to ESD Testing

When a charged object contacts another object at a different potential, the charge is equalised between the two objects. A voltage v(t) and current i(t) transient as this occurs is as shown in the figure below. The amplitude and waveform of the transient depends on the voltage difference and the total impedance in the current path. Pulses will be induced and may create signals that may affect electronic circuits nearby.

A secondary discharge can occur within equipment if the discharge current through the product attempts to take a path which includes an air gap. The voltage across the gap increases until the gap breaks down. The breakdown occurs simultaneously with the applied primary discharge as shown in the figure below.

a) Select circuit configurations that are unresponsive to short transients b) Lay out the PCB pattern to minimise induced voltages at critical nodes c) Prevent unavoidable discharge transients from coupling into circuits and cables d) Design enclosures as far as possible to prevent discharges from occurring

ESD Testing According to IEC 61000-4-2

IEC 61000-4-2 is the standard for ESD testing for immunity. It applies a defined current waveform at a specified voltage level from a hand-held generator known as ESD Gun. It uses a capacitor supplied from a high voltage supply whose charge voltage is discharged via a series impedance through the point of contact to ground. Two methods of discharges are used. They are contact discharge and air discharge. The typical ESD generator circuit and waveform is as shown below. Further information on ESD Testing are given in the standard.

In the contact discharge method, the stress may be applied directly to the EUT or to a coupling plane adjacent to the EUT. Before each test pulse, the capacitor is charged to the desired level but its voltage is held off thegenerator's probe by a vacuum relay. The probe is applied to a suitably chosen point on the EUT or the coupling plane. The generator is thentriggered, so that the relay contacts close and the capacitor voltage is appliedthrough the probe to the EUT. This creates a pulse of current as the voltage discharges through the combinedseries impedance of the generator, the EUT and the ground plane. This actionis repeated the desired number of times, at each location, with the appropriatepolarities and levels.

ESD Testing - Air Discharge

The same generator is used for the air discharge method, but with a roundedrather than a pointed probe tip. The capacitor is charged to the desired levelas before, but the voltage is now continuously applied to the probe, which isheld away from the EUT. For each test pulse, the tip is brought up to thechosen point on the EUT, smartly, until it touches. Just before this, the air gapbetween the tip and the EUT will break down and a discharge current will flow,limited as before by the combined series impedance of the generator, the airgap, the EUT and the return path. Again, the action is repeated the desirednumber of times, at each location, with the appropriate polarities and levels.

ESD Testing Layout

The recommended test layout is shown in the figure below. The ESD pulse has asubnanosecond risetime and so radio frequency layout precautions are vital.The test must re-create the fast risetime found in reality, since this is animportant parameter in deciding both the path the discharge takes through theEUT and the response of the EUT itself. The ground reference plane (GRP) isan integral part of the setup and the generator’s return lead must be wellbonded to it, since this connection forms part of the current return path.

The indirect discharge part of the test uses two other planes, different from theGRP, known as the horizontal coupling plane (HCP) and the vertical couplingplane (VCP). Discharges to these planes simulate the stress caused by theradiated field from real-life discharges to nearby objects. Each coupling planeis connected to the GRP by a resistor lead, to ensure that any charge bleedsoff within a few microseconds. The construction of these leads is critical: thereshould be a resistor close to each end, so that the length of lead betweenthem is isolated from the connections and stray coupling to it is neutralised.Although power rating is unimportant, the resistors themselves shouldwithstand a high pulse dV/dt without breaking down, for which carboncomposition types are best suited.

For the few tens of nanoseconds of the ESD event, the plane carries the fullstress voltage, which is capacitively coupled to the EUT. Any stray capacitancefrom the plane to objects other than the EUT modifies the plane’s voltage andcurrent waveforms and hence the applied stress. Therefore, it is important tomaintain at least 1m clear space around the EUT, which implies someseparation of the tabletop setup from walls or other objects. Equally, theseparation from the VCP to the EUT is specified as 10cm; even small variationsin this distance can cause large changes in coupling to the EUT, so aconvenient means of controlling it, such as plastic 10cm spacers on thesurface of the plane, is helpful.

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