MODERNIZATION OF

FACILITIES AND HANDLING

PROCEDURES FOR STATIC

SENSITIVE DEVICES (SSDs)

IN ELECTRONICS

INDUSTRIES

INTRODUCTION

WHAT IS ELECTROSTATIC DISCHARGE?

HUMAN BODY AS A DC TRANSFORMER

ELECTRICAL CHARACTERISTICS OF A MATERIAL

COMBATING ESD

CONCLUSION

Electrostatic damage of electronic equipment and

components by operating personnel is one of the most

significant problem, which directly effects the reliability

and operational maintainability of electronic equipment.

The electronic devices which can be easily damaged by

Electrostatic discharge are called Static Sensitive

Devices (SSDs).

All types of ICs and semiconductor devices come under

the category of SSDs.

These devices easily get damaged by voltages ranging

from 100V to 2000 Volts.

Human beings by normal movements like

walking, raising a hand or foot, getting up from

a chair, etc. can generate charges up to 20KV

In fact people can easily hold charges up to 2

KV without any feel of it and this 2 KV is

sufficient to damage SSDs either fully or

partially. The partial damage to SSDs under the

stress of continuous operation will result in

premature failure of the component / electronic

equipment affecting the reliability of the

electronic equipment.

REPORTED SUSCEPTIBILITY OF VARIOUS DEVICES

EXPOSED TO ELECTROSTATIC DISCHARGE

1000V – 2500V Schottky TTL 13.

680V – 1000V SCR 12.

500V – 1500V ECL 11.

380V – 7000V Bipolar Transistors 10.

300V – 3000V Film resistors (Thin, Thick) 9.

300V – 2500V Schottky Diodes 8.

250V – 3000V CMOS 7.

190V – 250V Op-Amp 6.

150V – 500V SAW (Surface Acoustic Wave) 5.

140V – 7000V JFET 4.

100V E PROM 3.

100V – 300V Ga As FET 2.

100V – 200V MOSFET 1.

Range of ESD Susceptibility

(in volts)

Device Type Sl No.

Table also shows that if the voltages in working environment can be maintained to

less than 100Volts then damage to Static Sensitive Devices (SSDs) should not

exist due to Electro Static Discharge(ESD).

WHAT IS ELECTRO STATIC DISCHARGE (ESD) ?

BASIC MECHANISM OF PHYSICS BEHIND ESD

• When two dissimilar objects are brought together in close contact and then pulled apart, they get charged.

• One object will exhibit a net positive charge and the other, a net

negative charge.

• This transfer of charge creates an electro static voltage

difference. This voltage difference between two objects is

typically of the order of few volts.

• When these two objects are drawn apart, the voltage difference

produced may be of the order of 10 KV to 100 KV. This effect of

charging through contact and separation is called ‘Tribo

Electric Effect’.

TYPICAL TRIBOELECTRIC SERIES

WHEN COTTON AND STEEL ARE

RUBBED TOGETHER,COTTON WILL BE

POSITIVELY CHARGED AND STEEL WILL

BE NEGATIVELY CHARGED. + _

0

THE CHARGE GENERATED

BY(COTTON,STEEL)PAIR IS LESSER

THAN THE CHARGE GENERATED

BY THE(PAPER,WOOD)PAIR

HUMAN BODY AS A DC TRANSFORMER

• Human body behaves like a capacitor and exhibits different capacitances, depending on the body posture, distance between the body and surrounding object.

• From the basic definition of capacitor Q = CV , if C decreases, V

increases.

• In a parallel plate capacitor C is inversely proportional to

separation between the objects (d) i.e. C 1/d

• Thus pulling the objects apart from an initial separation of 10

Angstrom unit to a separation of 1 millimetre we might expect a

magnification of the potential difference by a factor of 10 to the

power 6.

• Few actual measurements of human body capacitance changes

were taken by ‘Western Electric’ firm in USA which are illustrated

as follows:-

CAPACITANCE CHANGES IN PERSONNEL IN A WORK AREA

Description of Movement Change in capacitance due to

movement

% Change

Initial capacitance Final capacitance

Person seated raising

one foot

192 163 15% decrease

Person seated, picking

up both feet placing on

the foot rest

192 129 33% decrease

Person seated, leaning

forwards in chair (Desk

type chair with back)

192 184 4% decrease

Standing person

raising arms

167 141 16%

decrease

Seated person

standing up

192 167 13%

decrease

ELECTRO STATIC DISCHARGE (ESD) PHENOMENON

• When a charged human body comes in contact with an electronic component or electronic equipment it discharges through an available discharge path. This discharge is called Electro Static Discharge (ESD).

• Even if a charged person doesn’t touch the SSD, the electro

static field generated will induce static charges on the device

which can also result in damage to the SSD.

• This direct injection of charge in an SSD device can damage

the molecular structure of the device resulting in permanent

failure or partial damage to the device.

1

SHOWING THE WAY STATIC ELECTRICITY IS GENERATED AND

DISCHARGED

WALKING ACROSS A FLOOR RESULTS IN CONTACT AND SEPERATIONS OF SHOES WITH FLOOR RESULTS IN TRIBO ELECTRIC CHARGE i.e. BUILDUP OF STATIC ELECTRICITY

2

REDISTRIBUTION OF CHARGE RESULTS IN ELECTRICAL IMBALANCE. TO RESTORE BALANCE THE CHARGE MUST BE DISPERSED

3

IF A CHARGED PERSON TOUCHES A COMPUTER OR AN ELECTRONIC COMPONENT IT DISCHARGES THROUGH AN AVALIABLE DISCHARGE PATH.THE CHARGE THAT IS RELEASEAD CAN DESTROY OR DAMAGE THE SENSITIVE ELECTRONICS.IN MOST CASES THE PERSON WILL NEVER FEEL THAT DISHARGE EVENT HAS TAKEN PLACE.

4

DIFFERENT ELECTRIAL CHARACTERISTICS OF MATERIALS

There are four varying degree of electrical resistance :-

INSULATIVE : It does not allow flow of electrons across or

through its bulk. The material has high electrical resistance.

Surface resistivity is 10 to the power 13 Ohms per Square.

ANTISTATIC : Static electricity caused due to friction gets

dissipated within two seconds. Surface resistivity is less than

insulated material i.e. 10 to the power 9 and 10 to the power 13

Ohms per square.

STATIC DESSIPATIVE : Material allows transfer of electrons to

ground or through other conductive objects. Surface resistivity

10 to the power 6 and 10 to the power 9 Ohms per square.

CONDUCTIVE : It allows a charge to flow across or through its

volume easily. Surface resistivity is 10 to the power 6 Ohms per

square.

COMBATING ESD

• At present in Indian Industries anti-static precautions are hardly taken into consideration due to the lack of knowledge in the field of ESD phenomenon . Repair Depots, electronic repair labs are using anti-static measures to some extent but there is a requirement of complete modernisation of facilities and handling procedures for SSDs to combat ESD by operating personnel at Repair depots, Labs, and also by Logistics, who are directly dealing in storage, handling, packaging and shipping of SSDs.

• To combat ESD, it must be understood that the SSDs remain

Static Sensitive throughout their working life i.e., right from the

manufacturing, packaging, shipping, storage, and even when

mounted in a PCB or a Module. Therefore, all assemblies

containing SSDs are to be treated as if they are Static Sensitive

themselves.

VARIOUS METHODS TO COMBAT ESD

STATIC SAFE FACILITIES

THE PROTECTED AREA

MATERIAL : Use of materials, that are major static charge generators must be prohibited where SSDs are handled.

OBJECT OR PROCESS MATERIAL OR ACTIVITY

Work surfaces Waxes, painted or varnished surfaces

Common Vinyl or plastics

Floors

Sealed concrete

Waxes/ Finished wood

Common Vinyl tile or sheeting

Clothes

Synthetic garments

Non conductive shoes

Virgin cotton (below 30 % RH) clothes

Chairs

Finished wood

Vinyl

Fiber glass

Packaging, Handling and stores Common plastic bag, wraps, envelopes.

Common Bubble pack, foam

Common plastic trays, plastic Tote boxes

Assembling, cleaning, test and

repair area

Spray cleaners, common plastic solder suckers

Soldering irons with un-grounded tips

FLOORING For maximum protection, the floor of the work

area should be conductive. Conductive floor tiles and floor mats are commercially available.

ACCESS

Access to a work area where SSDs are stored or

handled must be restricted for the personnel not

trained in SSDs protection procedures.

SIGNS/ SYMBOLS

Warning signs can be displayed such as “ESD

PROTECTED AREAS AUTHORISED PERSONNEL

ONLY”.

LABEL FOR IDENTIFICATION OF STATIC SENSITIVE AREAS

ENVIRONMENT

HUMIDITY

The relative humidity where SSDs are handled or stored

should be controlled between 40% to 60%. Where this

cannot be achieved, the use of ionised air is required.

IONISED AIR

Ionisers generate positive and negative charge and

reduces the resistivity of the air, thus making the air more

conductive. When this ionised air comes in contact with a

charged surface, the charged surface attracts ions of

opposite polarity, as a result, the static electricity, that has

build up on SSDs and surface of the equipment, is

neutralised.

VARIOUS TYPES OF IONISERS

THE WORK STATION FOR SSDs

• Various types of static safe work stations are commercially available which include anti-static table, chairs and associated accessories.

• The work surface (table top) and the floor mats are grounded

through a 1 Mega Ohm resistor. This ensures operator safety,

and limits the discharge current, which protects a charged

device.

• The conductive table top should provide a surface free of

static charge and must be capable of removing static charges.

The maximum allowable resistance to ground of the work

station should be such that, the elapsed time of discharge

from higher voltage to 100V must not exceed one second.

ANTISTATIC TABLES

WRIST STRAP ANTISTATIC TABLE/ FLOOR MATS

ESD SAFE CHAIRS

COMPLETE ANTISTATIC WORK STATION

WORKER PROVISIONS

CONDUCTIVE WRIST STRAP

A conductive wrist strap in both the hands must be provided to ensure

that the worker is at the ground potential. 1 Mega Ohm resistor is

required between the strap and the anti-static earth to limit the discharge

current and for operator safety.

HEEL GROUNDERS

Conductive foot ware (or a conductive strap on conventional foot ware)

offers additional protection.

CONDUCTIVE GARMENTS

Some fabrics can generate static electricity. Workers should wear over

garments that dissipates charges. Few general fabrics which are

commercially available are as follows :

– Cotton

– Polyester

– Cotton Polyester blends.

– Cotton Polyester blend with additional stainless steel threads.

ANTI STATIC TEST EQUIPMENT

Following Test Equipment are mandatory specially for electronic repair labs :-

Surface resistivity meter : It measures both surface resistance and

resistance to ground of an Anti Static work station.

Wrist Strap and Footwear Tester : It checks wrist strap resistance maintained

between 1 Mega Ohm to 10 Mega Ohm and Footwear resistance which should not

be more than 100 Mega Ohm.

Work Station Monitor : It continuously monitors the serviceability of ESD earth

of Wrist Strap & Work Stations. There is no need for periodic checks.

Static Charge Meter : It provides on the spot measurement of Static Charge. It

checks the presence of Static Charge on any surface, material or people. It also

detects the value of static charge and its polarity.

Human Body Voltage Checker : It detects the value of Static charge a person

is carrying. It can be wall mounted outside the ESD protected area. It allows the

operator to discharge to a safe handling limit before entering the ESD protected

area.

ESD Portable Kit for field repair : It is used by the technicians/ engineers for on

site repair of electronic equipment where Anti Static work station is not available.

The only requirement is of an Anti Static earth at the site.

ANTISTATIC TEST EQUIPMENTS

STATIC CHARGE METER SURFACE RESISTIVITY METER

WRIST STRAP&

FOOTWEAR TESTER WORK STATION MONITOR HUMAN BODY VOLTAGE

CHECKER

ESD SAFE MAGNIFIER

ESD PORTABLE KIT FOR FIELD REPAIR

OF ELECTRONIC EQUIPMENT

STORAGE AND TRANSPORT

CABINETS FOR STORAGE OF SSDs

Should be conductive (e.g. Steel) and should be grounded through 1 Mega Ohm resistor.

Containers used to store or transport SSDs must be made out of conductive material. Containers holding SSDs should be stored only on conductive, grounded surface.

PACKAGING OPTIONS

Products to be shipped must be packed in one of the following :

Anti Static/ Conductive bags.

Conductive tubes (for ICs)

Anti Static/ Conductive sheets/ cushions, etc.

Packages should have and ESD warning, stamp or label, affixed on each package.

In case of any doubt regarding the exact type of packaging to be used, an Anti-Static/ Conductive bag should be used.

CONDUCTIVE CURTAIN WALL FOR ELECTRONIC

EQUPIMENTS STORAGE WIRE SHELVES FOR STORAGE

STORAGE OF SSDs

CONDUCTIVE STORAGE

CONDUCTIVE COMPONENT ORGANIZER

ANTISTATIC AIR BUBBLE BAG

PACKAGING/STORAGE OPTIONS

ANTISTATIC CUSHION CONDUCTIVE SHEET

PACKAGING OPTIONS

ANTISTATIC BENCH FOR PACKGING & SHIPPING OF SSDs TO BE USED

BY LOGISTICS

CONDUCTIVE TAPE WARNING LABEL ON PACKAGING

CONCLUSION Practically in today's world all aircrafts, space vehicles

missiles, and communications systems , Radars and

all type of electronics systems have ICs and other

semi conductor devices as the electronic system. To

improve the reliability of the electronic systems, one

of the major areas of protection is ESD protection for

all electronic equipment/ component. This will also

result in high maintainability of state of the art

electronic equipment which in turn will fulfil

requirement of mission critical applications and also

the users requirements. It will also result in savings of

foreign exchange since spares requirement will be

comparatively less.