Electrostatic Discharge

10.1 INTRODUCTION

This chapter reviews the problem of electrostatic discharge (ESD) as it affects wireless sensor network nodes, and the importance of proper electrical, mechanical, and product design in its control. Similar to electromagnetic compatibility (EMC) problems, ESD problems often require a multi-disciplinary approach for proper resolution. Unlike most EMC problems, however, ESD problems can be difficult to identify in the field; often, the first indication of an ESD weakness in a design are sporadic reports of "dead" nodes not attributable to other causes, or reports of very specific failure types, such as corruption of certain register contents in a microcontroller. ESD problems are difficult enough to detect in wireless sensor networks designed for consumer and home automation applications, where an ESD event may be instigated by a charged individual, but become even more troublesome in networks for industrial, agricultural, and military applications, many of which operate essentially autonomously, without direct human contact. Lack of human contact may lower the overall failure rate. but can result in a lack of information on the failures that do occur. It is important, therefore, to understand the ESD problem, so that it may be "designed out" of the finished product.

THE PROBLEM

10.2.1 Examples

Most people are familiar with static electric discharges on cold winter days or other especially dry conditions. These discharges, although only unpleasant to humans, may be fatal to modern electronic products; further, weak discharges that are undetectable to humans can still damage electronic products.^[1] What follows is a selection of ESD-related events associated with consumer electronic products, illustrating the importance of the problem and the variety of guises in which it may appear.

A new product was developed one spring, and volume shipments began the following summer. That winter, however, field failures, in the form of random access memory (RAM) errors, were noted in temperate regions. The problem was believed to be ESD-related, and became serious when the RAM errors could not be duplicated in the manufacturer's laboratories, because the problem could not be corrected until the product was made to fail under laboratory conditions. Although many attempted design improvements were tried in the field, the field failures did not stop until spring. It became apparent that the ESD test procedure as performed by the manufacturer at that time was inadequate, and a task force was organized to identify improvements. The recommendations of the task force, including an environmentally controlled ESD testing room and a new testing procedure based on internationally accepted standards, were adopted after the new test accurately duplicated not only the field failure of this product (which was cured by a bypass capacitor on a RAM integrated circuit [IC]), but also the warranty failures of several other products. (ESD had not even been considered as a possible root cause of the failures in the other products.) By the next winter, products of modified design were in the field, and there was no seasonal rise in ESD-related failures. The cost of the new ESD testing room and equipment was more than repaid by the improvement in the manufacturer's reputation among its customers.

During development of a product, several problems were uncovered during ESD testing. It was noted that the switching voltage converter IC on a microcontroller circuit board would go into a test mode when an ESD discharge reached a circuit board runner through a gap in the housing around the "5" key on the product's keypad. An added capacitor and diode to the runner cleared this problem. It was next noticed that the RAM memory locations storing trim values for analog circuits would become corrupted when a discharge occurred on the serial port contacts used to program the microcontroller, through another housing hole. Placing a diode between the two contacts and placing spark gaps (pointed circuit board runner shapes) between the runners to the contacts fixed the RAM problem; instead, a discharge to the serial port contacts would now cause the microcontroller to reset. Making the serial port runners thin to increase their inductance and modifying the ground metal beneath them, fixed this problem.

Now, after a discharge to the serial port contacts, the product's real time clock (RTC) appeared to stop — but the product would continue to work normally in every other way. It was found that the microcontroller had two programming bits that controlled the RTC increment rate. The RTC could be set to increment either once per second, once per minute, once per hour, or to not increment at all. The ESD event was somehow corrupting these bits so that the RTC would not increment, and, therefore, appear to stop. Working with the designers of the microcontroller, the product engineer assigned to this problem determined that the RTC was supplied power through a particular supply pin, the V_rr pin. A 150-Ω resistor in series with the V_rr pin (made possible due to the very low current drain of the RTC) fixed it. To speed testing, special product firmware was written during this part of the investigation that constantly checked the value of the sensitive bits. If a change were noted, a light-emitting diode (LED) on the product would turn on.

10.2.2 Failure Modes

Product failures resulting from ESD events may take many forms. They can, however, be placed into two general categories, long-term and short-term failures, depending on the time scale of the failure. Short-term failures are generally found immediately after the ESD event. Long-term failures take time, perhaps even months or years, to become apparent.

Long-term failures and latent defects. Long-term failures, by definition, do not become apparent until some time after the ESD event.^[2] The most common long-term failure involves CMOS ICs, which may have a shorter operating life after an ESD event, due to gate oxide degradation. The noise figure of radio frequency (RF) transistors may also increase over time after ESD exposure. It has also been demonstrated that ESD events during manufacturing processes produce latent defects in Schottky diodes used in microwave transceivers, leading to a loss of sensitivity after a few months of operation.^[3]
Short-term failures. Short-term failures become apparent immediately following the ESD event. They can have a confusing, application-or market-specific taxonomy; in particular, the distinction between catastrophic ("hard") versus noncatastrophic ("soft") failures can be vague. Although one may debate the classification of a particular product failure, the point to remember is that all failures are failures, from a quality and customer satisfaction point of view (the only point of view that matters). It is rare to find a user interested in entering into a lengthy discussion with a manufacturer on the precise categorization of a product failure due to ESD.
- Catastrophic failures. A catastrophic failure is, simply put, any failure from which the network node cannot be made to recover by action taken over the air or at the user interface. This includes transceiver parametric failures, such as loss of receiver sensitivity, and failure of any internal sensors and actuators. Typically, catastrophic failures involve physical component damage, and require the replacement of hardware (e.g., integrated circuits) to return the network node to its proper operating condition. These types of failures are those most commonly encountered when reading the ESD literature.

^[1]Steven H. Voldman, Lightning rods for nanoelectronics, Scientific American, v. 287, n. 4, October 2002, pp. 90–97.

^[2]Owen J. McAteer, Electrostatic Discharge Control. Upper Falls, MD: MAC Services Incorporated. Available from the Electrostatic Discharge Association, Rome, NY. 1990. Chapter 11.

^[3]Yogi Anand and Dana Crowe, Latent ESD failures in Schottky barrier diodes, Proc. Electrical Overstress/Electrostatic Discharge Symp., 1999, pp. 160–167.