The Most Common Causes Of ESD DamageThe Most Common Causes Of ESD Damage
After studying hundreds of facilities, a veteran ESD consultant and auditor shares his top 23 causes of damage to ESD-sensitive parts.
Over the last eight years of investigating and eventually understand- ing the root causes of ESD, we have found that less than 0.10% of all the documented damage actually came from ungrounded personnel touching ESD-sensitive (ESDS) products, known as human body model (HBM) damage. That, of course, means that 99.9% of the damage originates from the charged-device model (CDM) failure mode.
In the CDM failure mode, the ESDS item, such as a wafer, chip, component, or printed circuit board (PCB), somehow becomes charged, most often by contact and separation. Subsequently, it is discharged by a large conductor, like a person or machine.
Aside from contact and separation, perhaps the simplest way to charge an ESDS product is through field induction. The susceptible item must be grounded or capacitively coupled with something larger while in the presence of an electric field. If an ESDS item is in the vicinity of an electric field, it will polarize to some degree; but unless the item is grounded, it cannot attain an actual charge.
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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- Tape-and-Reel Components
Problems continue for IC handlers using tape-and-reel components that are charging ESDS parts on the reels.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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: [email protected]
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November 2002