If the product is a conductor, it is driven to a voltage by a displacement current from the ionizer signal. Although the signal only lasts for a short time, it may be possible to damage the product during handling if a path to ground is supplied and the handling does not add capacitance to the product thereby “shorting out” the displacement current signal.
In order for damage to occur under the ionizer, a path to ground must be supplied to the product at exactly the same time as the ionizer switches on or off. Such a path to ground must occur with very little conducting material (operator or robot) closely approaching the product. Such an approach would increase the capacitance, Cproduct, and would lower the voltage induced on the product. See Figure 2. If the product is only handled after it leaves the ionizer, the effect of the displacement current is over and no damage can occur. This would be the case if a substrate is moved under or over a nearby ionizer over rollers. No path to ground is supplied in the vicinity of the ionizer and after the substrate leaves, there is no remaining charge on the substrate due to the displacement current.
One application where this could occur would be operations on an MR Head with leads attached. As the operator or robot approaches the leads, he or she remains many head “diameters” away from the head and thus not appreciably affecting the capacitance. This situation represents an ESD hazard.
In contrast, a large FPD substrate with circuitry in place is very insensitive to the displacement current in the manufacturing process as it is handled by large highly capacitive lifting robots. The huge grounded robots that handle the product represent a large increase in product capacitance. As the robot moves in, it provides a path to ground but also attenuates the voltage swing dramatically. The attenuation of the induced voltage is the result of the added capacitance. Although the CPM can show a large swing, the product remains safe. As stated above, for the case of such a substrate on rollers, no path to ground is supplied until the product moves on to the next process step so in that configuration, the product can be said to be immune to the displacement current.
When the product leaves the presence of the ionizer, it carries only the charge received from the last ionizer pulse and any voltage from imperfect ionizer balance with it. Thus, it can be damaged by the voltage swing from ion current if a ground is presented to the product but it cannot then be damaged by the displacement current. When the product leaves the presence of the ionizer, it also exits from the fields of the ionizer. Thus, the voltage from the displacement current cannot cause an ESD event, in this case, even when a ground is presented to the product.
Displacement current should be ignored for most automated processes unless the application involves manual handling with tools or very small robotics.
EOS/ESD 3.1-20004 does notconsider the difference between the two types of current sources which is a very conservative way to proceed which may result in unnecessary demands on ionizer performance. This issue will become more important as ionizers are made to cycle faster. A plan for evaluating this effect needs to be developed.
CPM Bandwidth
EOS/ESD 3.1-20004 defines the swing of an ionizer as the difference between the maximum plate excursion of the CPM to the positive and to the negative when the CPM is in the FLOAT mode. Many CPM’s have a bandwidth limited to a maximum response of only a few Hz for economic reasons.
A pulsed AC ionizer frequently is operated at 10-30 Hz. Typically such an ionizer has a much faster rise time (2-20 msec,) than conventional pulsed DC ionizers (50-100 msec) provide. When such a fast ionizer is placed under a low bandwidth CPM, the swing is underestimated.
The response curve measured for a typical CPM is shown in Figure 8. As can be seen, an ionizer being operated at 10 Hz or above cannot be adjusted with a CPM alone due to the reduced response of the CPM at 10 Hz.
