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Protecting chips against electric floods

12 May 2014  | Dushyant Juneja

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The semiconductor industry typically classifies these "floods" into 4 types, depending upon their nature and the magnitude. A human touch to the chip may induce a static ESD discharge best emulated by the Human Body Model, or HBM. A machine touch, on the other hand, due to its metallic character, is best modelled by Charged Device Model (CDM) and Machine Model (MM), depending upon the requirements. A powered device touching an unpowered device would introduce the most aggressive charge flood, typically standardised as the IEC 61000-4-2 standard.

There are also other standards like Human Machine Model (HMM), but are less popular. Manufacturers typically qualify to one or more of these standards to rate their products' strength against ESD failures, depending upon application. For instance, a RS-485/422 compliant communication transceiver chip may be available with a stress qualification as high as ±15,000V HBM owing to the voltages it needs to forbear. Depending upon the other applications, these ESD qualifications vary among chips.

ESD mechanisms can be effective on chip, but the modern call for more and more features pushes the boundaries of chip design. This aggravates the problem for an ESD designer since more effective means more guarding must be employed. This is an area of active and ongoing research and innovation.

While the engineer can implant limited ESD guard devices in the chip, care must be taken at the assembly and consumer level to keep away from these failures. An ESD sensitive device would generally come with a warning on the label, manual or datasheet. An example is as shown in the figure below. Generally, such a device would come in an ESD protective package like conductive foam, an antistatic tube or a static dissipative bag. ESD warnings may be visible at the package to warn the end user over careful handling.

Similar care is taken during chip testing at the industry end, where specialised anti-static suits and specialised test-benches may be employed. It is best to inspect the required devices at grounded workstations and minimise manual handling. Any sub-assemblies made out of the device should also be handled with proper ESD cautions. Dedicated devices such as LVDS transceivers and TVS may be used for ESD protection on a system level.

Handle with care
Care should be taken at consumer level in handling the boards or the devices. For instance, it is advisable to not touch such sensitive equipment with bare hands as to avoid unintentional damage. Generally, one would use an anti-static suit and grounded gloves to assemble or operate on such equipment. A further precaution is to not expose them to direct or sudden high voltages, unless they classify to such an operation. Generally, placing a chip or a board in an already active supply would render a serious ESD spark, often called as "hot-plugging."

Only specially designed devices can sustain a long term stress of this intensity, and often the general devices will easily fail in first operation itself. A better option is in fact to gradually ramp up the supplies through the system using linear supplies or other equipment. If the board or system intends to an end user application, such equipment would be deemed mandatory for a reliable operation.

Another concern in this direction rises with using multiple interconnected devices on the board, not all of which may be operable at the same time. Care must be taken in such a scenario that parts that are active do not communicate with the unpowered parts, as to avoid unintentional electric current through them. Generally, one would need to use some kind of isolators in such a scenario. This can otherwise short circuit multiple devices, leading to significant inconvenience and losses.

Figure 4: Example ESD Caution Note in a datasheet.

For the end consumer, it is always better to not operate on an apparently not working equipment of this kind, and leave it to the specialised engineers to service the part. This way, additional damages due to touching the device can be avoided. Additional external spike guards like varactor-based plugs also act efficiently to improve the long term reliability and avoid ESD damage. If the equipment is known to be sensitive to such spikes, it is often a better choice to use an external stabiliser before connecting to the supplies.

About the author
Dushyant Juneja is with Analog Devices Inc. (India).

To download the PDF version of this article, click here.

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