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.