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PRABHAKARA VARNA

Protection from Electrostatic DischargeThe failure or degradation of electronics products due to ESD warrants attention and suitable preventive measures. General techniques for protection from ESD are explained here

When any two substances make contact and are sepa-rated, an electrostatic charge

is developed. This static charge results from an interaction of two materials in-volving the removal of electrons from the surface atoms of one of the materi-als. This transfer of electrons is related

the amount of surface electrification or charge transfer. The other mechanisms of voltage build-up due to charge gen-eration are induction and changes in capacitance.

The electrostatic charges may be brought into the work areas by people or may be generated by people during their normal movements. The pack-ing containers, processing machinery, work surfaces, tools, clothing and nu-merous other articles are prime sources of static charges.

A person can induce a charge into an integrated circuit as shown in Fig. 1.

ESD related damagesElectrostatic discharge (ESD) can cause intermittent as well as catastrophic failures of electronic equipment. There are a number of ESD related failure mechanisms. The major ones are given below.

Dielectric breakdown. W h e n a potential difference is applied across a dielectric region in excess of the region’s inherent breakdown charac-teristics, a puncture of the dielectric oc-curs. Depending on the pulse energy, this can result in either total or limited degradation of the part. The thinner the dielectric, the lower is its breakdown voltages. The typical failure mode for these part types is an electrical short between gate and drain, or gate and source. Parts utilising MOS structures are the most susceptible to dielectric breakdown. These include MOSFETs, MOS and CMOS microcircuits, and MOS capacitors.

Metallisation damage. Failure can also occur when ESD pulses in-crease part temperature sufficiently

to the amount of energy required to move the electrons over separation distance.

The major factors that govern static charge generation are intimacy of con-tact, rate of separation, conductivity of materials and their position on tribo-electric series. The cleanliness and the smoothness of the material surfaces in contact with one another also enhance

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ing capacity of a device is specified in the datasheet. Otherwise, it has to be determined using the human body test model. This is a critical parameter and considered while addressing the ESD control plan.

Workplaces are categorised into class A, B and C depending on the ESD sensitivity/susceptibility of the compo-nents being handled:

Class A: ESD susceptibility levels up to 100V

Class B: ESD susceptibility levels between 101V and 2000V

Class C: ESD susceptibility levels between 2001V and 4000V

Accordingly, the ESD control techniques to be adopted to achieve the desired protection vary. A typi-cal ESD-area warning sign is shown in Fig. 2.

Protective measuresAs part of the ESD control plan, the following protective measures are employed at various stages of storage,

to melt or fuse bond wires. This can occur, for example, in microcircuits where the metal strips have reduced cross-sections resulting in localised high current density with resultant hot spots in the metallisations. Open circuit of a metallisation strip is the normal failure mode for metallisation melt-type failures.

Intermittent failures. These failures may not be catastrophic, but may result in degradation of key electrical param-eters. These include increased leakage current, lower reverse breakdown voltages of p-n junction, and change of the V-I curve of p-n junctions in the forward direction.

Cumulative damage. A large vari-ety of part types from different tech-nological families are susceptible to cumulative ESD damages after numer-ous exposures even at levels below the damage threshold.

Device susceptibility to ESDDevice or equipment susceptibility to ESD is usually determined by a test utilising the simulated human body test model. This model is widely used. The human body test model is also an effective method for testing the input protection capability of a device. Generally, the ESD withstand-

Fig. 1: Typical charge induction scenario

Fig. 4: Typical external ESD protection circuit for MOS devices Fig. 2: Typical ESD-area warning signs

Additional protection can be obtained for

MOS devices by adding external series and

pull-down resistors to each input.

Fig. 3: Typical internal gate-protection network

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transport, design, fabrication and test-ing of ESD-sensitive items:

Design protection. The first consid-eration in the design of equipment is the selection of parts that are the least susceptible to ESD. The devices with inbuilt ESD protection circuit should be the next choice. Many protection networks have been developed to pro-tect the sensitive MOS devices. These protective networks designed into MOS devices reduce the susceptibility to ESD to a certain level. One of the internal gate-protection networks is shown in Fig. 3.

Additional protection can be ob-tained for MOS devices by adding external series and pull-down resistors to each input. A typical external ESD- protection circuit for MOS devices is shown in Fig. 4. Zener diodes too can be used to protect the MOS devices from ESD. Primary consideration is the performance of the device with the introduction of an additional protec-tion circuit.

Handling precautions. An effec-tive ESD control method will utilise a variety of protective materials and tools to prevent damage to ESD-sensi-tive items.

Wrist strap is the first line of defence in the battle against ESD. The purpose of the wrist strap is to provide a permanent path to ground for the individual operator to prevent unsafe static charge levels from be-ing generated during movements. A typical wrist strap is shown in Fig. 5. Personnel handling ESD-sensitive items need to wear grounded wrist straps.

After wrist straps, the ESD pro-tective work surface is the most important defence against ESD damage. There are different types of ESD-protective work surfaces. The ESD control materials depending on their surface resistivity are classi-fied into static conductive (surface resistivity: 104 to 105 ohms/sq.), static dissipative (surface resistivity: 106

to 109 ohms/sq.), antistatic (surface resistivity: 109 to 1012 ohms/sq.) and insulative (surface resistivity >1012

ohms/sq.). Static dissipative materi-als are optimal for a protective work

Fig. 9: Typical workstation with table top mat, wrist strap and ioniser

surface. A typical work area with ESD mat is shown in Fig. 6. ESD mat shall be grounded.

All ESD-sensitive devices should be handled with ESD-safe coats dur-ing their assembly. A typical ESD-safe finger coat is shown in Fig. 7.

As a preventive measure, all ESD-sensitive items have to be stored and transported in static shielded bags/containers (Faraday cage). The ESD-susceptible parts are protected or shielded from static charge damages by using ‘Faraday Cage.’ The static charges rest on the outer surface of conductors. Conductive containers provide a blockage of external fields and any charge accumulation rests on the outside surface only. Such containers are ideal for storage and transportation of susceptible parts. A typical ESD-safe bag is shown in Fig. 8.

ESD-safe workstation. Labora-tories handling ESD-sensitive items should have a separate static safe zone (SSZ) having a minimum of stat-ic safe apparels, static voltage-level indicator, grounded table and floor mats. Also, the chairs used should be of static safe type. There should be separate electrical ground for ESD and power.

Air ioniser is another effective ESD control tool. It is placed near the work area to eliminate electro-static charges. Humidity is continu-ously monitored and maintained at 55%±5%RH. The ESD discharge mechanism is provided from the body of the personnel at the entrance of the SSZ. Potential generators of ESD charges like plastics and paper are avoided in the work area. Fur-niture used is of grounded antistatic type. A typical workstation with table-top mat, wrist strap and ioniser is shown in Fig. 9.

Periodic calibration of ESD control items is essential. All the personnel involved in handling, transportation, storage, assembly and test must un-dergo ESD training depending on their sphere of activity.

The author is a T&E engineer at Systems Reliability Group, ISRO Satellite Centre, Airport road, Post Vimanapura, Bangalore

Fig. 5: Typical wrist strap

Fig. 8: Typical ESD-safe bag

Fig. 7: Typical ESD-safe finger coats

Fig. 6: Typical test set-up with ESD mat