Reliability of IEC 61000-4-2 ESD Testing on Components

Testing for immunity to electrostatic discharge (ESD), from cell phones to computers, is usually done with the IEC 61000-4-2 standard.1 But IEC61000-4-2 is purely a systems-level test spec, not a spec for individual components. Components manufacturers are thus forced to define test plans without the guidance of a standard, and test results coming from different component manufacturers may differ considerably. So what does components testing to this standard mean? Here are the main issues to consider, particularly with respect to contact-discharge versus air-discharge results.

Contact-Discharge Testing

In any case, when evaluating contact-discharge data for components, you need to know which pins have been stressed, which pins have been grounded during the stress, how the ground connection was made to the component being tested, and the number and polarities of the stresses to the component.

Air-Discharge Testing

All device-level ESD standards specify current waveforms for specific voltage stress levels and thus the standard tends to ensure some level of test reproducibility. The IEC standard does not, however, specify waveforms for air discharge. The air-discharge waveforms depend on both the voltage and the geometry of the target. The IEC standard specifies a current-sensing target for verifying contact-discharge waveform parameters. The target is mounted at the center of a ground plane and has a characteristic impedance of 2 ohms (to ground). When connected to an oscilloscope with an input impedance of 50 ohms, the target produces 1 volt for each ampere of current. The discharge point is a metal disk measuring 26 mm in diameter.

Figures 2 and 3 display contact-discharge and air-discharge waveforms directly to the IEC target at 2, 8, and 16 kV. At 2 and 8 kV, the contact- and air-discharge waveforms are very similar, but at 16 kV the initial current spike is absent from the air-discharge waveform. For air discharge at high voltage, a very long arc forms between the gun and the target. The long arc has higher resistance and inductance than direct contact and greatly reduces the initial current spike. The nature of this arc depends on the geometry in the region of the arc.

The gun tip used for air discharge is a rounded tip and the IEC specified target has a flat surface. Electric fields are geometry dependent. Pointed tips increase the local electric field and initiate electrical breakdown earlier than flat or blunt geometries. Electrical components in state of the art packages have very small contacts, very different from the flat surface of the IEC target. Modifications to the discharge surface of the IEC target have allowed study of geometry effects.

The center disk of the IEC target can be removed where it screws into the main body with a 3-mm threaded rod. A series of alternative target centers were made to study the voltage properties before an HBM event.3 These targets are ideal for understanding how current waveforms depend on geometry. Electrically the targets consist of zener diodes (breakdown is 10 volts) with the anodes connected to the IEC target and the cathodes connected to various geometry pins to simulate different integrated circuit packages. The diodes simulate the ESD protection circuit of an IC or ESD protection component. Contact-discharge and air-discharge measurements were made on a variety of pin geometries. A flat surface geometry produced very similar results as those shown in Figs. 2 and 3, although the zener had been added to the current path.

Figures 4 and 5 show the results for a pin that's 0.4-mm in diameter (similar is size to a modern BGA solder ball). The contact-discharge measurements still look very similar to those for a contact discharge directly to the IEC target as shown in Fig. 2. For air discharge to the small pin, the suppression of the initial current spike begins at a much lower voltage. That's because the electric field around the small pin is enhanced, and the result is an electrical arc that is longer and therefore has higher resistance and inductance than for a flat surface.

These measurements show there is no simple correlation between the stress level in air-discharge and contact-discharge conditions. The current waveform for contact discharge scales linearly with voltage, while air-discharge waveform shapes depend on the voltage level and the local geometry where the discharge occurs. For discharge to a flat surface at low voltages, air-discharge and contact-discharge currents are very similar.

At higher voltage, as the arc length for air discharge increases, the initial current peak diminishes and disappears while the rise time increases. For discharge to small geometries, such as pins on modern integrated circuit packages, the deviation between contact discharge and air discharge occurs at lower voltages. The deviation between contact and air discharge is not, however, so great as to make 8-kV Level 4 contact discharge equivalent to 15 kV Level 4 air discharge as shown in Fig. 6 for discharges to a 0.4-mm pin. Even though the peak currents for the two discharges are very similar, the total energy delivered by the 15-kV air discharge is considerably higher than from the 8-kV contact discharge.

Air-discharge testing is also affected by all of the features that change the properties of an arc, including humidity, atmospheric pressure, and the speed of approach. In the measurements presented here, several air-discharge pulses were taken at each voltage and a specific pulse was chosen that was considered representative for that voltage and geometry.

Manufacturers such as ON Semiconductor specify air-discharge ratings very conservatively. In many cases an air-discharge rating of Level 4, 15 kV, is specified based on a device's ability to pass contact discharge at a voltage level in excess of 15 kV. This approach is valid because in all cases contact discharge is a more severe stress than air discharge at the same voltage. For air-discharge testing on a component, the component is generally mounted on a board in which the pins under test are connected by short traces to flat pads to which the air discharge is applied. As shown above, air discharge to a flat surface gives a more severe stress than to a small pin. As in contact discharge, 10 stresses are performed with both positive and negative polarity. The test for contact-discharge stress can provide reproducible results, but only if the same pin combinations are used.

As mentioned previously, the standard gives no guidance for pin combinations because it is not intended for component-level testing. Test results for air discharge are more problematic because the nature of the stress is very sensitive to the geometry where the discharge occurs. In addition, a rating based on a contact-discharge Level of 1 to 4 will not usually be equal to the corresponding air-discharge level. An air-discharge rating based on the ability to survive an equal or greater voltage contact-discharge stress is a conservative and justifiable assumption.

By Robert Ashton, ON Semiconductor
Power Management DesignLine
(08/10/2008 7:30 PM EDT)