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ESD BAGS: TO SHIELD OR NOT TO SHIELD:What type of bag should you use?by Ryne C. Allen
Reproduced with Permission, EE-Evaluation Engineering,August 1999
INTRODUCTION Populated printed circuit boards (PCBs) are typically protected from ESD when in storage or transportation by a sealed ESD-protective bag. "95% of ESD control programs use shielding bags", said Jim McKeon of Desco/Charleswater, "because it increases the programs confidence and makes a program more foolproof". There are several types of ESD bags on the market ranging from pink polyester to a metallized moisture vapor barrier (MVB) bag. Which bag is right for the job? What type of protection does your electronic board or device need? Why so many choices of ESD bags for packaging? I History of ESD Control Bags The packaging evolution of the ESD control bag and packaging material design has been a compromise between various desirable features. Ideally, users look for ESD bag materials that combine low cost with a myriad of properties such as: high ESD shielding effectiveness; permanent antistatic properties; humidity independent performance; eliminate contamination and corrositivity of packaged contents; excellent moisture vapor barrier properties; heat sealability and film transparency. Traditionally, static-shielding bags are manufactured by depositing a thin metal coating such as aluminum over an anti-static polyester film substrate. The metallized layer protects the devices in the bag from electrostatic fields, while the insulative layers prevent direct contact with potential ESD hazards. At the low end of ESD bad product offerings are amine-free, anti-static polyethylene film bags that are transparent or pink in color, referred to as Antistatic (which is their only property). These bags are commonly referred to as "pink poly" Over the past years, manufacturers have developed a number of new static dissipative materials that minimize of tribocharging while offering improved puncture resistance and durability. Durability is a particularly attractive property in most ESD bags where sharp-edged devices or printed circuit boards are loaded into the bags to minimize the threat of bag perforation. Manufacturers are also developing environmentally friendly materials much more than they have been in the past. Historically, ESD protective packaging has similar environmental problems as any plastic material; disposal and material decomposition can present an environmental hazard. Recently, manufacturers have conquered some of the disposal problems and we now see more "recyclable ESD bags" on the market, such as those made from polypropylene. II Types of ESD Bags Let’s review the current technology for ESD bags. There are essentially three types or categories: antistatic bags, dissipative bags and metallized bags. The latter two categories are typically the high-end of ESD bag product offerings and tend to have three combined properties of protection, (1) antistatic, (2) dissipative and (3) shielding. Antistatic Bags Dissipative Bags Metallized Shielding Bags There are two common types of metallized shielding bags
varying by construction, the metal-in and the metal-out shielding bags.
The metal-in (buried metal) shielding bags are the most common type
currently used and are recommended for packaging of ESD sensitive (ESDS)
components. They also tend to be superior to the metal-out construction
in durability and cost. The metal-out shielding bags are also designed
to protect against static induced damage. The metal layer is closer
to the outside surface resulting in these bags having lower resistance
readings than the metal-in which can be important in some applications. In general, if you are storing ESDS devices for prolonged
periods of time (6 months or more) or if the devices are sensitive to
corrosion, than you should package these materials in either an MVB
(Moisture Vapor Barrier) bag with a desiccant pack to absorb any moisture
that was sealed in or a metal-in shielding bag with a desiccant pack.
The difference between an MVB film and a metal-in film is about 1 magnitude
in the moisture vapor transmission rate (grams of water/100 in2/24
hours @ 100 oF). Moisture sensitive ESDS devices should only
be packaged in an MVB bag. ESD bags should be inspected before reuse by an internal statistical sampling plan as they typically have a finite reuse life. If an ESD program is laid out very well and is extremely disciplined so threat the threat of ESD events are almost squelched in both the handling and transportation processes, then a good dissipative bag may be sufficient for the storage of most ESD sensitive devices. There are very few actual programs out there that are implemented so effectively.
Most ESD programs use shielding bags because it increases its level of confidence and makes it more foolproof. Shielding bags serve two purposes, preventing the ESD sensitive components sealed within the bag from charging up via field induction and minimizing the damage from a direct contact (outer bag surface) with an ESD event. Some high quality films (ESD shielding bags) can withstand up to a 30 kV discharge. III Features & Benefits
of ESD Bags
Table I - Features of ESD Bags
III Shielding (Faraday) vs Non-Shielding bags
Table II - Device Damage Thresholds for Various ESD Bag Types [1]
The above table was derived by a set of experiments performed by Lucent Technologies [1]. The voltages indicate the level at which the device was damaged by direct discharge to the bag. This shows that metallized shielding bags are the most protective when first used, but as used they degrade to provide ESD protection similar to good as antistatic bubble wrap bags. Note: the bubble wrap uses the air gap (spacing), dielectric of air, as a protective means for withholding damaging ESD from it’s contents, similar to clam shell packaging. For an ESD from a 10 cm diameter electrode, the spark-gap length is about 0.12 inches for every 10 kilovolts of peak voltage at 760 mmHg and 25oC, so the air gap needs to be much larger than the spark gap to provide adequate ESD protection [7]. If an ESD program is laid out very well and is extremely disciplined so threat the threat of ESD events are almost squelched in both the handling and transportation processes, then a good dissipative bag may be sufficient for the storage of most ESD sensitive devices. There are very few actual programs out there that are implemented so effectively. Most ESD programs use shielding bags because it increases its level of confidence and makes it more foolproof. Shielding bags serve two purposes, preventing the ESD sensitive components sealed within the bag from charging up via field induction and minimizing the damage from a direct contact (outer bag surface) with an ESD event. Some high quality films (ESD shielding bags) can withstand up to a 30 kV discharge.
IV Packaging Standards The standard MIL-B-81705C, Military Specification
for Barrier Materials, Flexible Electrostatic Protective, Heat Sealable,
states that there are three classifications of barrier materials (bag
films) that make up ESD-protective bags, see Table III below. Table III - Classification of Barrier Materials – MIL-B-81705C
A table of most of the ESD packaging standards is provided in Table IV. The ESD Association is heavily involved in re-writing some of the most commonly used packaging standards as well as their current packaging standards. ESD-DS20.20 is assimilating all their ESD control standards into one reference document and the non-draft version of this standard will essentially replace the MIL-STD-1686 possibly near the end of 1999.
TABLE IV – Packaging Standards
V Packaging Precautions ESDS devices should always be kept in an enclosed antistatic shielding bag, conductive closed tote, or bin when not being handled at an ESD controlled workstation. This includes inventory storage, transportation, and work-in-process. Further precautions during transportation include using dissipative carts with conductive wheels or drag chains in conjunction with a conductive or dissipative floor when transporting ESDS devices in their shielded containers. The standards to help characterize and qualify packaging materials are ANSI/ESD S11.31 for shielding bags, ANSI/EOS/ESD S8.1 for proper use of package markings, ANSI/EIA-541 and ANSI/EIA-583 for packaging materials, refer to Table III. VI ESD Bag Questions and Answers [6] Q.
During the past few months I have been trying to change our old process
of transporting our circuit boards around our factory from the use of
'CONDUCTIVE BAGS' to the use of 'DISSAPATIVE BAGS'. I have been unsuccessful
due to the fact I cannot prove that this will benefit the reliability
of our products. A. You have a good
question. There is a white paper talking about discharge times that
may help. The more conductive an item is, the greater the energy density
in an ESD event. By slowing the charge transfer (ESD event) down with
a more resistive material (dissipative), you can minimize the risks
associated with conductive ESD events. With a dissipative material,
instead of an ESD event, you will have a current ‘bleeding’ or charge
balance that is better controlled.
Q. I have two questions about one material. Anti-stat bags. 1. How effective are anti-stat bags with a relative humidity of less than 20 % and temperatures as high as 110 degrees Fahrenheit. 2. How do you put a spec on an anti-stat (ohms/sq. etc.) bag to ensure maximum protection and re-use? A. The clear static dissipative
polyethylene (PE) that high quality vendors use in their bags is far
superior than the "topically treated washed" pink poly products.
Q. What is the recommendation or guidelines in determining when Anti Static shielded bags are no longer any good, i.e. crinkled or have small puncture holes, and put out of service?
A. All your answers are in the MIL-B-81705C Standard document. Specifically, an anti-static shielding bag (type III) can be considered no good if it meets any of the following conditions:
In general, a metallized shielding bag may still shield if it has crinkled or acquired small holes because of the Faraday affect, but it is safer to discard these bags and err on the conservative side. It would probably cost more to test and qualify the bag then to replace it.
Q. The static bags that electronic parts come, what are they made of and is the bag at a positive or a negative condition.
A. There are several types of static bags; shielding (metal-in, metal-out, moisture vapor barrier), antistatic and conductive. The most commonly used "static" or ESD bag is a shielding bag, which has all three properties listed above . A shielding bag has a layer of metal, usually aluminum (Al) (similar to Aluminum foil), that provides "shielding" producing a Faraday Cage affect. An ESD shielding bag has several properties:
A typical shielding bag is composed of three layers:
Q. In my company many rolls of bubble wrap, plastic bags, and PCB boxes (plastic cases) are listed as "Anti-static". While we all understand that these items will not produce serious static, many think that they will also protect the components inside from a static charge. My contention is that a significant amount of charge would pass through the bag/box and damage any ESDS item inside. Only a shielded bag would guarantee complete protection from ESD. The question really arises because a well-known and respected PCB manufacturer sends their PCBs’ to us in anti-static plastic cases. The clear plastic case only lists "Anti-Static and we can not see any conductor strands within the plastic. What are your thoughts? A. There are two ways to protect ESDS PCBs. One way is by shielding. The use of metallized shielding bags is a good way to protect the contents from external ESD. Another way to protect ESDS PCBs is to isolate them from external ESD with an air gap. There are "clam shell" packaging and other anti-static plastic packages that will give a spacing of about an inch of air between the outside plastic shell and the ESDS part inside. This "air gap" spacing can be an effective way to protect ESDS parts from external ESD as a 1" air gap acts as a dielectric to prevent discharges up to 30 kilovolts.
Q. If the ESD bag is of different thickness (total material thickness), will the electrical properties be different? A. Yes, the volume resistivity of a material may become higher with a thicker bag. The higher the volume resistivity, the higher a voltage the material will stand off. Most high quality metal-in, metal-out or MVB shielding bags can withstand over 30 kilovolts. A thicker bag (thicker dielectric or metal film) will be able to hold up to a greater energy ZAP. The spacing of the ESDS device relative to the inner bag surface can have a similar effect. The greater the "air gap" the greater the protection from an ESD event penetrating the shielding bag into the ESDS device. It is the metal film that helps transfer this energy into a surface current [Faraday Cage Effect] rather than penetrating through the bag to the sensitive device.
REFERENCES
About the Author Ryne C. Allen is the Technical Manager at ESD Systems, Marlboro, Massachusetts. ESD Systems is a division of Desco Industries, Inc. (DII). Previously, he was Chief Engineer and Laboratory Manager at the Plasma Science and Microelectronics Research Laboratory at Northeastern University. Mr. Allen is a NARTE-certified ESD Control Engineer and the author of twenty-seven published papers and articles. He is a member of the ESD Association and an active ADCOM member of the Northeast Chapter of the ESD Association. He graduated from Northeastern University with B.S.E.E, M.S.E.E., and MBA degrees. ESD Systems, 19 Brigham St., Unit 9, Marlboro, MA 01752-3170, (508) 485-7390, resume: http://ryne.hotresume.net/, e-mail: ryne@esdsystems.com, URL: http://www.esdsystems.com. |
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