Doug,

see paragraph 9

Inge


Here is a list of what we consider to be 23 leading causes of ESD damage in
the industry. At least 30 companies have proven losses from each of these
causes. For some of the failure modes, more than 50 to 100 incidents were
documented.

These causes are the heavy hitters from our experience. The categories do
not appear in any set order of importance. They are all CDM failure modes.

   1. Hand Coverings

Ordinary plastic and even pink antistatic or black conductive finger cots
and gloves have a notorious history of charging the ESD item being handled,
especially the insulative portions of PCBs. Many assume that no materials
will become charged when touched with pink antistatic or black conductive
finger cots or gloves. In fact, most PCB materials charge dramatically
higher when contacted by hand coverings, even ESD-safe versions, vs. bare
skin. When product becomes charged, it is a CDM failure waiting to happen.

   1. Ungrounded Operators Walking With ESDS Items

Even if PCBs are held by the plastic edges, the operators tend to charge the
assemblies when transporting. This leads to CDM failures when the PCB
subsequently becomes discharged.

   1. Ungrounded Operators Walking With ESDS Items in Open Bags, Totes,
   Trays, and Containers

The Faraday-container principle really works. All sorts of damage occur when
ESDS items are transported in open static-shielding bags, conductive
containers without lids, and metal trays without covers.

   1. Transportation of ESDS Items on Ungrounded Carts

ESDS products, if placed and transported on ungrounded carts without proper
Faraday shielding, can become highly charged. CDM failures often are a
consequence.

   1. ESDS Items on NonESD Surfaces

Placing ESDS products on ESD protective mats and laminates is very
important. Products on nonESD-safe surfaces can become charged quite easily,
especially under low-humidity conditions inside a facility.

   1. Charge-Generating Containers

We still are surprised to walk into a facility that seems to have excellent
ESD controls only to find that ESDS products are routinely placed inside
high-charge-generating containers as part of a standard handling practice.
The major violators include quartz boats and Teflon containers.

   1. Aged Antistatic Packaging

Many antistatic materials, especially ones that derive their properties from
a chemical loading process, lose their noncharge-generating properties over
time. They revert to charge-generating materials. When the protection is
lost, ESDS products inside become charged, many times leading to damage when
operators discharge them by removing them from their packaging.

   1. Air Blow-Off Operations

All air blow-off operations should be routinely checked to determine if they
cause the ESDS item to become charged during the operation. Many times, a
simple field-meter check can alert the auditor so substantial damage is
avoided. We have witnessed significant damage due to this issue.

   1. Conformal Coating

Our files are loaded with product-damage accounts from conformal coating
applications. Most conformal coatings charge the ESDS item dramatically
during application. Discharging and subsequent damage are common.

   1. IC Handlers

We have spent considerable time detecting and eliminating CDM yield losses
due to the internal workings of modern IC handling equipment. For example,
ICs can become charged internally while moving and sliding through these
machines and then discharged and damaged as part of normal operation.
Substantial expertise and special measuring equipment are needed to detect
and eliminate these problems.

   1. IC Tubes in IC Handlers

This has been a very serious issue in the past; however, it is diminishing
as more and more IC handlers are using tape-and-reel instead of IC tubes.
Historically, the clear, antistatic straight IC tubes would lose their ESD
properties over time and cause the ICs inside to charge as they slid out.
The ICs would become discharged when contacting the metal input chutes on
the handling equipment.

   1. Tape-and-Reel Components

Problems continue for IC handlers using tape-and-reel components that are
charging ESDS parts on the reels.

   1. Semiconductor Wafer Containers

A major ESD failure mode is created when ESDS wafers are placed in
high-charge-generating containers as part of normal handling. The wafers
become inductively charged from resting in their plastic containers and then
can be discharged by a host of mechanisms including operators touching them
by hand or tweezers or by other wafer-grasping equipment, machine parts, or
conductive containers.

   1. Wafers Mounted on Sticky Plastic Films

Another contributor to ESD-induced failures is the standard procedure of
mounting wafers on sticky films prior to performing dicing and chip-picking
operations. The most commonly used films typically are very high in charge
generation and can inductively charge the wafers.

All sorts of discharge damage can take place at a variety of operations if
the proper controls are not in place. In addition, the diced chips that
eventually are lifted from these sticky films typically become charged to 10
kV to 12 kV during the lift-off process as verified through Faraday-cup
measurements. The chips are discharged by the conductive collet removing
them from the film or by the conductive waffle pack that they are being
placed into for storage.

   1. Gel Packs

Many times, IC chips are placed into gel packs that have a sticky bottom
liner to hold the chips stationary. These gel packs are quite useful from
the mechanical standpoint but can cause major ESD damage to the chips inside
when appropriate ESD controls are not in place. Similarly, the chips lifted
off the sticky bottom liner can become highly charged and immediately
discharged by the tweezers or collets removing them.

   1. Faulty Ionizers

The data underscores the importance of verifying that ionizers are indeed in
balance and operating properly. If an ionizer loses one polarity, it
actually can cause products to become charged. We have witnessed far too
many applications where old or improperly maintained ionizers were the
actual source of the product damage under investigation.

   1. Operator 12-Inch Rule

Most companies we interact with instruct production operators to keep
charge-generating materials such as common plastics or paperwork at least 6
inches to 12 inches away from exposed ESDS products to prevent them from
becoming inductively charged. We cannot over-emphasize the importance of
this discipline. Several companies have substantially improved their bottom
lines when this discipline was taken seriously.

   1. PCBs Mounted on Plastic Panels

Perhaps the largest total dollar amount of documented damage to ESDS items
has been caused by mounting PCBs on plastic panels. Especially under
low-humidity conditions, these plastic panels are routinely charged to very
high levels when handled by operators, 20-kV to 30-kV levels when measured
with a static field meter. As a consequence, the PCBs become inductively
charged. The operators then are constantly discharging the assemblies via
normal handling.

This problem is more widespread for manufacturers of copiers, fax machines,
computers, office equipment, and other products that incorporate regular
plastic housings. However, it also can occur in metal-encased systems that
contain large plastic parts.

   1. Test Sockets

Most test sockets can and do charge during routine handling (PROM
programmers, burn-in board fixtures, or automatic test fixtures) and
inductively couple into the devices under test, leading to CDM damage.

   1. Plastic Covers Over Test Sockets

Many applications exist where a plastic cover is placed over a test socket
before the test begins. High-voltage testing frequently dictates that an
insulative cover be implemented to ensure operator safety. Many times the
fields from these large plastic covers are powerful enough to cause CDM
damage to the parts under test.

   1. Flex Cables and Fiber-Optic Cables on PCBs

Another proven source of ESD damage comes from the fields generated by the
use of flex cables and fiber-optic cables on ESDS PCBs. Both of these types
of cables incorporate materials that generate huge static fields when
handled by operators. Over and over, sensitive components such as FETs in
close proximity to these cables have become damaged during manufacturing.

   1. Dry Box Storage

Another high-risk condition is created by the glass or regular plastic walls
and doors of the dry boxes routinely used in most facilities where
electronic devices are manufactured. Typically, nitrogen is circulated in
these boxes to keep the humidity levels very low. The result can be
tremendous charging of the ESDS items inside.

   1. Syringe-Needle Operations

Operations where syringe needles are used to apply epoxy or coatings to PCBs
and components are a major source of damage. Again, more than 30 cases of
documented damage have been reported.

This failure mode is a little different. The plastic body of the syringe
becomes highly charged when squeezed and handled by the operator performing
the application, which, in turn, polarizes the metal tip. When the tip
contacts something else, current will flow to the needle tip to charge it.
This current flow can be a source of damage if it comes from an ESDS item.
The charged tip then can discharge upon contact with another item. This is a
two-for-one ESD problem.

*Conclusion*

The majority of ESD damage in the electronics industry comes largely from
CDM sources, some of which are not widely recognized. The 23 most common and
most severe causes listed in this article should be investigated on a
routine basis as part of recurrent ESD auditing procedures.

*About the Author*

*Roger J. Peirce is the founder and president of ESD Technical Services.
Previously, he co-founded Voyager Technologies in 1983 and was involved in
engineering research at Bell Telephone Laboratories from 1970 to 1983. Mr.
Peirce received a B.S.E.E. from Fairleigh Dickinson University. ESD
Technical Services, 1732 Gibson Rd., Bensalem, PA 19020, 215-447-2506,
e-mail: *[log in to unmask]


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