Engineering for robustness

The last installment of Engineering on Purpose suggested that many products, particularly in, but not limited to, the consumer-electronics sector, have arrived at a state few, if any, industry participants predicted: A kind of product saturation in which typical models meet or exceed the performance and feature-set needs of the majority of customers. Though there remains an upper-market demographic that will continue to seek top-of-the-line models, the bulk of the market is approaching or entering this region in which meaningful product differentiation will become increasingly difficult to attain.

For products already there, the choices of values on which to compete appear quite limited. Only three come to mind: price, styling, and robustness. The first is the easiest to implement: It only takes a stroke of the pen to change a pricing schedule. Though that choice has the other advantage of being readily visible to customers, it invites a competitive scenario that harms all participating OEMs: a race to the bottom of the pricing curve limited only by the lower limit of customers’ quality tolerance at which point the third choice applies by default.The second, styling, is also known as industrial design—a discipline with a rich history of its own. Successful industrial design depends on designers’ abilities to capture customers’ interest through a product’s aesthetic appeal. Splendid examples of industrial design exist, some of which appear in the collections of modern-art museums. Some, such as the Apple iPod for example, amply demonstrate the influence this discipline can have on a product’s market acceptance—even, and perhaps especially, at a price premium. This area of design, however, lies largely outside of the electronics engineer’s domain.

The third strategy, by contrast, draws on the core of electrical- and mechanical-engineering skills. Companies that differentiate themselves by making robust products not only strengthen their pricing positions but also take advantage of so-called word-of-mouth advertising—the unstoppable tendency of networks of families, friends, co-workers, and social groups to share their experiences (both positive and negative) and, thereby, influence the buying decisions of a population that grows geometrically.

A consistent characteristic of such informal recommendations is that they tend to be persistent. It hardly matters whether the suggestions and warnings one receives refer to current models or not: only recent experiences matter. More important than price and features, customers want some assurance that the products they buy are going to operate as expected over a reasonable in-service lifetime. Equipment manufacturers who disappoint their customers also lose the goodwill their brands may enjoy with potential customers who learn of such experiences and re-establishing trust with those customers can take years.

Protect your periphery

Robustness takes many forms. It certainly includes the quality of your product’s core functions. Many of the most common failures in consumer electronics, however, trace to functions peripheral to the product’s core operation. It’s important to note that, although some engineering decisions that lead to greater robustness can incrementally add to the product’s manufacturing cost, many are implementable at cost parity with less robust alternatives. Those that do add to the manufacturing cost may still return net cost benefits by reducing product returns and their attendant damaging effects to the manufacturer’s reputation.

One of the larger classes of failure traces its origins to damage that ESD (electrostatic discharge) strikes induce. Although semiconductors can sustain ESD damage nearly anywhere in a product during its manufacturing process, once its case envelopes the product, its susceptibility substantially narrows. Correspondingly, OEM processes, practices, and materials guard against ESD damage during manufacturing and assembly but those controls end when the product leaves its shipping container and faces real-world operating environments. During their working lifetimes, products are most susceptible to strikes to nodes associated with signal and user interfaces.

Though ESD protection devices are important, they aren’t necessarily the first objects to consider when engineering your product for greater robustness to ESD events. Consider instead a systemic approach that minimizes ESD exposure, and then choose ESD protection strategies that match the electrical characteristics of each susceptible node in the context of the ESD source model.

Know who your friends are

In most electronic environments, insulators are benign materials. At worst, in circuits that process signals deriving from extremely high source impedances, the choice of insulating material and the insulator’s surface condition can determine leakage impedances and their resultant offsets and coupling coefficients for nearby interfering-signal sources.

For ESD susceptibility, however, insulators are potential sources of concern for three reasons. First, insulators form capacitors with adjacent conductors. Although these parasitic devices are volumetrically inefficient and exhibit small values, they can often accumulate large voltages compared to the absolute-maximum ratings of nearby semiconductors.

Second, many common insulators that the electronics industry uses appear at the extreme ends of the triboelectric series (Table 1). When materials from different positions on the triboelectric series come into contact—separated by a distance less than several angstroms—they mutually and oppositely charge upon separation, positively for electron donors and negatively for electron acceptors. This process of triboelectrification does not require friction as is popularly believed although sliding surfaces do provide a high rate of surface contact and separation. That’s why, for example, moving air, which cannot generate much friction, can cause static charges to develop on the surfaces of insulating solids.

Third, although the voltage-breakdown characteristics of bulk materials tend to be consistent to a reasonable degree, the breakdown behaviors across surfaces exhibit substantial variations that can depend more on the compounds occupying the surface than the nature of the substrate material itself. Combinations of moisture and airborne oils that collect on an insulating surface trap airborne particulates such as dust and smoke particles that degrade the insulator’s surface characteristics both with respect to voltage breakdown and signal isolation.

Surface coatings, such as polyimides, that some manufacturers apply to printed circuit boards do not manifestly improve surface breakdown characteristics in any reliable fashion. As a result, standards that apply to conductor spacings in high-voltage applications do not relieve minimum conductor-spacing requirements for products in which manufacturers apply polyimide or similar coatings. While these standards tend to address high potentials in steady state, they can offer insights into good spacing practices for discharge events, although you’ll want to degrade the standoff voltages to accommodate the discharge event’s high dV/dt and dI/dt waveforms.Points of entry

As key points of entry, user and signal interfaces differ significantly in ways that differentiate effective ESD protection strategies. In both cases, the most effective and, conveniently, the most economic methods of mitigating ESD events depends on your product’s ability to shunt strikes to ground through the shortest path possible.

Human interfaces distribute over large areas compared to signal interfaces and are usually formed with insulating materials on their exteriors. Discharges through air gaps—commonly associated with mechanical spacings for keypads, switch matrices, and rotary controls—can terminate on either signal lines or ground depending on your circuit board layout. To the extent that you can protect signal lines from incoming strikes simply by means of layout geometry, at least a portion of your ESD-protection strategy adds nothing to the your product’s BOM or assembly costs.

Strikes to elements of a human-interface subsystem tend to derive from sources that follow a HBM (human-body model) source characterization. The IEC human-body model, for example, is a source with charge storage of 150 pF and a source resistance of 150 ohms. This model is among the more conservative of those in popular use. It has gained popularity because it better ensures product robustness than less demanding models.

Signal interfaces, by contrast, bring conductors right to a product’s external surfaces and as a result, are subject to strikes from either HBM or MM (machine model) sources. The exceptions are RF and optical links, both of which provide intrinsic ESD robustness by keeping conduction paths away from exterior surfaces. This is one of the benefits of inexpensive POF (plastic optical fiber), for example, which not only provides resist ESD damage but can provide very dense interconnect. For example, the popular TOSLINK electrical/POF interface can use a single plastic-fiber strand to carry eight channels of 24-bit digital audio at 48 ksps or four channels at 96 ksps over short distances for equipment-to-equipment or rack-to-rack runs replacing 48 and 24 electrical connection per strand, respectively.

For the majority of interfaces that depend on conductive links, ESD protection strategies must account for the electrical characteristics of the specific interface. The connector’s characteristics can play an important role in minimizing the incidence of strikes terminating on signal lines.

Compare, for example, common USB and RJ-45 connectors. Both are reasonably robust for their size in the face of mechanical stress. However, the USB connector provides a grounded metal shroud that projects beyond the plane of signal contacts whereas the RJ-45 shell uses a molded insulating compound. Though USB ports need ESD protection, the USB connector’s features reduce the likely incidence of strikes by shunting many harmlessly to ground.

Left ungrounded, however, the USB connector’s metallic shroud can a increase the ESD hazard by converting some events from HBM to MM source characteristics. Other influences include layout details, signal isolation methods (if present), and of course explicit protection devices, which will be the topic of the next installment of Engineering on Purpose.

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Table 1