Technical_ESD_paper_revB Technical ESD Paper

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Technical Considerations

for Controlling ESD

in Electronics Manufacturing

Overview of ESD, Associated Risks and

Prevention Measures

Technology White Paper

Product Line Card

Technology White Paper

Technical Considerations for Controlling ESD

in Electronics Manufacturing

As device geometries get smaller and processing speeds grow faster, their ESD sensitivity increases. Designers face the challenge of fi tting

more active component features into smaller chip territory, often at the expense of on-chip protection devices. The trade off is greater risk

for ESD damage. This white paper gives an overview of ESD, the associated risks and recommended measures for ESD prevention.

- The Cost of ESD

- What Is ESD?

- Identifying ESD

- Preventing ESD Buildup

- ESD Measurement Tools

- Automated Tracking in an ESD Environment

- Evaluating ESD Handling Capabilities

Microscan Systems, Inc.

errors, testing, and calibration inaccuracies as well as mishan-

dling. All can cause signi cant physical component damage and

affect production yields. The affects of EMI tend to be random in

nature and can affect equipment across the room, but leave the

equipment where the ESD event occurred untouched. This can

make the location of the ESD event dif cult to locate.

What is ESD?

ESD, simply stated, is the rapid transfer of an electrostatic

charge between two objects. ESD happens when two objects

of different potentials come into direct contact with each other.

Charging results when one object’s surface gains electrons to

become negatively charged and another object loses electrons

from its surface to become positively charged. Triboelectric

charging occurs when an electron transfer results from two ob-

jects coming into contact with each other and then separating.

One of three events is usually the cause of ESD damage to

Mastering ESD control has always been critical to achieving high production yields, and it will become even more important

in the next few years. While the industry has a solid understanding of ESD safety in manual operations involving personnel,

there is room for improvement in automated applications. To be effective, ESD control programs must ensure that auto-

mated handling equipment is capable of handling tomorrow’s highly sensitive devices.

The Cost of ESD

ESD impacts productivity and product reliability in virtually every

aspect of electronic environment. Despite the effort made over

the past decade, ESD still costs the electronics industry billions

of dollars every year. Industry experts attribute an estimated

8 to 33% of all product losses to be caused by ESD.1 The

individual cost of these devices themselves range from a few

cents for a simple diode to several hundred dollars for complex

hybrids. However, ESD damage affects more than just the loss of

devices. It affects production yields, manufacturing costs, prod-

uct quality and reliability, customer relationships, and ultimately,

pro tability.

For today’s automated facilities, conventional methods of ESD

control must be re-examined and new methods applied. Auto-

mated assembly equipment is capable of processing 4,000 to

20,000 components an hour.2 At these speeds, poorly designed

equipment that is allowed to charge devices can damage large

amounts of components in a very short amount of time. Perhaps

even more importantly, an ESD event can in turn damage the

automated equipment.

ESD generates a signi cant amount of electromagnetic inter-

ference (EMI). The EMI resulting from an ESD event is often

powerful enough to interrupt the operation of the production

equipment. Equipment controlled by microprocessors is es-

pecially susceptible to damage as they operate in the same

frequency range as the EMI from ESD events. Often mistaken for

a software error or glitch in the system, EMI can cause a variety

of equipment operating problems, such as stoppages, software

Figure 1: EMI resulting from ESD is often mistaken for a software glitch.3

Equipment latch-up event

Software bug

“Extra” pulse

Induced electromagnetic

disturbance

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Technology White Paper

devices: direct electrostatic discharge to the device; electrostatic

discharge from the device; or  eld induced discharges. There are

several models used to characterize how devices are damaged

– the Human Body Model (HBM), the Machine Model (MM), the

Charged Device Model (CDM), and the effect of electric  elds on

devices. In an automated assembly facility, the last three models

or modes are the largest cause of concern.

MM damage is what happens when a machine component

discharges through a device. Automated assembly equipment

uses a variety of methods such as conveyors to move and guide

devices through the assembly process. Poor equipment design

can cause the handling systems to accumulate signi cant

charges that will eventually discharge through the devices.

CDM damage occurs when the device discharges to another

material. When a charge builds up in a device, it will dissipate

through a conductor on the device when the device is placed in

contact with a surface with a lesser charge.

In uence of Electric Fields (E-Fields), or the space surrounding

an electrical charge, can cause a charged device to polarize.

Polarization creates a difference of potential, which may cause

the device to discharge to an opposite charge, causing two dis-

charges or equalization events.

Identifying ESD

While a great deal of attention is spent on preventing ESD

caused by the HBM, recent studies have indicated that less

than 0.10% of all documented damage actually resulted from

ungrounded personnel touching ESD-sensitive (ESDS) products.

The studies concluded that 99.9% of ESD damage originated

from the other models, speci cally CDM.4

ESD control embedded into machinery is essential but prob-

lematic. To effectively control static buildup, both MM and CDM

ESD events must be prevented. The  rst step in developing an

ESD control program is to identify exactly where ESD events

occur or are likely to occur. A good place to start is to ask two

primary questions:  rst, is the equipment properly grounded; and

second, does it handle devices in such a way that they do not

generate static charge above an acceptable level?

To be fully prepared for handling devices of the future, equip-

ment should be capable of handling components with an ESD

tolerance as little as 50 V. The following is a list of documented

areas known to charge devices, increasing the likelihood of a

CDM ESD event.

IC Handlers. ICs typically become highly charged as they pass

through the equipment and are subsequently discharged as a

part of normal operation. According to recent studies, IC han-

dlers have caused considerable yield losses due to CDM.5

Tape-and-Reel Components. Problems have been documented

with components charging while they are on the reels.

Gel Packs. If the proper ESD control methods are not in place,

IC chips can become highly charged as they are lifted off of the

sticky bottom liner and then immediately discharged by the col-

lets removing them.

PCBs Mounted in Plastic Panels. The plastic panels regularly

used for housing PCBs can routinely charge to very high levels

when handled, subsequently charging the PCBs themselves. The

assemblies are subsequently discharged during normal operator

handling.

Test Sockets. Normal operation can cause test sockets to

charge and then discharge into devices.

Plastic Covers Over Test Sockets. The  elds from the large

plastic covers required to shield operators during high voltage

tests often are strong enough to damage the devices under test.

Preventing ESD Buildup

In preventing or reducing MM damage, it is critical that equip-

ment is properly grounded while in motion. All equipment parts

that come into contact with the static-sensitive devices must

have a suf cient grounding path to dissipate accumulated

charge. Proper grounding of conductive and dissipative surfaces

prevents the buildup of static charge on machine components

and eliminates them as a source of charge-creating ESD events.

Grounding alone, however, will not prevent all CDM ESD events

from occurring. Component charging is a much more challenging

problem to solve, primarily because most electronic components

contain insulators as part of their design. Insulating materials

Board

Loading

Screen

Printer

Chip

Shooter

Pick

Place

Reflow

Oven

Through

Hole Assem-

bly

Wave

Solder

Rework

Inspector

ICT Function

Test

Board

Flipper

ESD risk via automated movement ESD risk via human interface

Figure 2: Common locations of ESD risk in automated assembly.

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naturally accumulate a charge and grounding the materials does

not remove or reduce the static charge. When the charge cannot

be removed or avoided, air ionization is often the most effective

method of neutralizing the charge on insulators or isolated con-

ductors. In the case of automated equipment, air ionizers can be

mounted inside the process chambers. Creating mini environ-

ments by enclosing speci c machines and mounting ionizers

inside is another option.

ESD Measurement Tools

Once ESD countermeasures are in place, it is important to verify

that they are working properly. Continuous process monitoring is

recommended over periodic audits of the ESD program because

ESD countermeasures will often fail. For this reason, if and when

failure does occur, it should be identi ed as soon as possible to

prevent ESD damage.

Several test methods exist to validate the integrity of the ground

path to equipment parts and measure whether machines are

charging devices. When selecting the best measurement

instruments, consider the safe charge level to be measured and

select an instrument that can measure within that range. Note

the size of the area to be measured and whether the spacing is

 xed between the surface of the object to be measured and the

instrument.

Identifying and measuring static charge inside automated

equipment presents speci c challenges. The problem with most

conventional methods is that they are not particularly suited

to automated equipment. Most require direct contact with the

charged object or require the device to be removed from the ob-

ject, making it necessary to take the equipment of ine to do the

testing. To avoid lost production time, alternative solutions are

necessary for measuring charges inside the equipment.

To measure static charge without disrupting equipment operation,

assemblers can mount sensors or probes inside the equipment

or mount static event detectors (SED) on the devices themselves.

Two options for mounting instruments inside equipment include

static sensors and special electrostatic voltmeters and electro-

static  eldmeters with small probes. Static sensors incorporate

very high input impedance circuitry and can be mounted inside

automated equipment. This allows them to measure the  eld

generated by a charged part as it moves through the process.

Ideally, the sensor should be mounted as close to the part as

possible. Since it does not require the nulli cation of existing

 elds, it is ideal for measuring charges on parts moving through

high throughput machines.6

Electrostatic voltmeters and electrostatic  eldmeters with small

probes offer an alternative option for monitoring inside equip-

ment. The probes are small enough that they can be placed in

critical locations to measure the charge on components as they

pass by. However, care must be taken when mounting them to

ensure that they take accurate measurements and do not

interfere with the operation of the equipment. Several factors

can affect the accuracy of their measurements, including orienta-

tion of the charged surface with respect to the probe as well as

the size, speed and distance of the part from the probe.

SEDs are tiny sensors small enough to  t on a circuit board.

They are designed to measure the current pulse in an ESD event

and can be monitored optically as they pass through operating

equipment. SEDs are ideal for verifying whether the equipment is

generating dangerous static-charge levels. Several different types

are available, each with varying features. However, many must be

removed from the device and placed into separate instrumenta-

tion to ascertain whether an ESD event actually occurred.

Automated Tracking in an ESD

Environment

If an ESD event does occur, the data provided from a device

tracking system can help assemblers quickly identify damaged

components and contain the impact. In a device tracking system

model, a bar code reader is installed at various points through-

out the manufacturing process to read the bar codes (or 2D

codes) applied to the devices. Typically, bar code readers scan

the bar codes on the device before the device enters a station

and again after it exits. This documents the type of procedure

that was performed, the equipment that performed it and at-

taches a time/date stamp for when it occurred.

While ESD monitoring instruments output all types of data, the

bar code reader provides the only link between each device’s

serial number and the data supplied from the instrument. For ex-

ample, when equipment calibration is altered due to EMI from an

ESD event, the data generated from the device tracking system

can help identify speci cally which boards were damaged after

the equipment’s calibration was altered. It is no longer neces-

sary to pull, scrap, or rework entire lots because of insigni cant

data.

When selecting a bar code reader, careful consideration should

be made to ensure that it does not introduce additional risk

for ESD events. Printed circuit boards, integrated circuits, and

other electrically sensitive components typically use small, high-

density bar codes to conserve space, making it dif cult for some

readers to scan from a distance. When close-proximity scanning

is employed, the bar code reader may build up a static charge

depending on whether it is used on a non-conductive surface. If

the reader itself has built up a charge and is brought into close

proximity with a sensitive component, an ESD event could oc-

cur, potentially damaging the component. Some manufacturing

environments utilize a workaround by mounting the scanner after

applying a special anti-static spray, which is not without its own

risk.

First, the coating must completely cover the area for maximum

effectiveness; uncovered areas remain at risk. In addition, anti-

static sprays can wear off over time and require timely replacing.

Without an accurate measure of a spray’s ef cacy period, compa-

nies either waste money by applying too much, or put their com-

ponents at risk by using them in an unprotected environment.

As an alternative solution, miniature bar code readers are now

available with a unique nickel coating and ESD resistant labels

for maximum ESD safety. These units are rated for discharges up

to 8kV and feature a surface resistivity of less than 10 * 10-9

Ω/inch2.

3 www.microscan.com

Technology White Paper

Conclusion

The consumer electronics industry has witnessed phenomenal

growth over the past few years. Industry analysts have attributed

this growth in part to the convergence of previously separated

markets of digital-based audio, video and information technology

to create state of the art electronic devices. As these devices

rapidly gain new capabilities, they are increasing their ESD

sensitivity almost as quickly. To be competitive in electronics

manufacturing tomorrow, facilities must work towards mastering

ESD control today.

Evaluating ESD Handling Capabilities

According to the ESD Association’s Technology Roadmap re-

leased in 2005, sensitivity levels to ESD in devices are expected

to drop so low, that assemblers must act quickly to ensure they

will be able to handle the new levels.7 Assemblers certi ed to

the ANSI/ESD S20.20, the ESD Association Standard for the

Development of an Electrostatic Discharge Program, already have

done much of the work in preparing for tomorrow’s sensitive

devices. For those manufacturers that are unsure of the voltage

capabilities of their automated equipment, the ESD roadmap

provides direction:

• Determine the ESD-control capabilities of the facility’s han-

dling processes.

• Ensure all conductive  xtures or tooling that contact sensi-

tive devices are grounded.

• Ensure that maximum voltage induced on devices is kept

below 50 V.

Following the requirements outlined in S20.20 will help man-

agers assess the sensitivity levels of the components being

assembled in their facility and identify ESD issues at each stage

in the process, from receiving and inventory through assembly,

test, rework and shipping. By using the appropriate ESD coun-

termeasures, managers will have the data available to them to

articulate their facility’s capabilities by voltage level.

1 ESD Association. “Basics of Electrostatic Discharge Part 1: An Introduc-

tion to ESD”, Compliance Engineering, January 2000.

2 Bellmore, Donn G. for Universal Instruments, “ESD Design Concerns in

Automated Assembly Equipment”. Retrieved from: http://www4.uic.com/

wcms/WCMS2.nsf/index/Resources_53.html.

3 Kraz, Vladimir for Credence Technologies, “EMI and Equipment Malfunc-

tion in Cleanroom Environment”. Retrieved from: http://credencetech.

com/products/more/EMI_and_Equipment%20_malfunction.pdf.

4, 5 Pierce, Roger J., “The Most Common Causes of ESD Damage”. Evalu-

ation Engineering, November 2002.

5 Pierce, Roger J., “The Most Common Causes of ESD Damage”. Evalua-

tion Engineering, November 2002.

6 Steinman, Arnie, Joseph C. Bernier, Donald Boehm, Thomas Albano,

Wayne Tan, and Donald L. Pritchard, “Detecting ESD Events in Automated

Processing Equipment” Compliance Engineering, Sept./Oct. 2000.

7 ESD Association, “Electrostatic Discharge (ESD) Technology Roadmap”,

ESD Association, 2005.

Additional Sources:

Steinman, Arnold and Lawrence B. Levit, Ph.D., “Coping with ESD:

Ionization for Production Equipment”. Evaluation Engineering, April

1997. Retrieved from: http://www.evaluationengineering.com/archive/

articles/0497cope.htm.

Maynes, Curtis, “Technology Roadmap Sees Higher Sensitivity to ESD”,

Evaluation Engineering, February 2006. Retrieved from: http://www.

evaluationengineering.com/archive/articles/0206/0206technology_

roadmap.asp.

Figure 3: Quadrus MINI ESD Safe is nickle coated to safely read codes on

ESD sensitive parts and components.

4

©2011 Microscan Systems, Inc. 03/11

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