Today’s ESD-protective bags offer some very impressive design improvements over their predecessors. The early versions were good, but the current bags—such as metal-shield and moisture barrier—are much better at total product protection.
All the new bags can be used many times before losing their static-protection properties. The bags must be recycled or replaced before their static-control qualities break down.
Typically, a bag can be used about six times. After that, one recent study indicated that bag characteristics fell below the minimum quality standards due to rips and tears incurred during handling.1
The number of times a bag can be used is important, but it is not the only characteristic to contemplate. Also consider what qualities will give you the most protection for your product. For example, do you need only a static shield or do you also need cushion or moisture protection?
Cost is not the best indicator of a shielding bag’s performance, said Brent Beamer, Director of Technology at Static Control Components. Some of the most expensive bags are made by job shops with no quality control, for resellers that do not manufacture bags.
To be sure you get the right bag, review the manufacturer’s quality-control and testing capabilities. Avoid the product if the seller cannot produce proper quality-control documentation or the bag has no lot number. The best advice, he added, is to buy the cheapest bag with a quality pedigree.
Shielding Choices
For static shielding, there are three options—metal-in, metal-out and a metal fiber matrix, said Dixon Gleeson, Market Development Manager at 3M. The shielding-type bag costs more than nonshielding, but the risk of damage to your electronic products is too great to consider using anything less than one of these products.
The price of the bag is less important than the bag material and construction, agreed Dave Bermani, Corporate Marketing Coordinator at Desco. Your best defense is to become familiar with the ESD properties of metallized bags vs the lack of these properties in pink or clear polyethylene bags. Also, review the difference among the shielding-bag types.
For example, in a hierarchy based on testing, a foil-laminate bag was arbitrarily ranked as good, while a bag with a heavy opaque foil or a partially transparent metal screen was deemed excellent. Bags with see-through metallization were considered only fair. In separate tests, investigators at two major corporations concluded that foil provides very effective shielding but see-through metallization approximately 100 angstroms thick has a dubious shielding capability.2
Metal-out bags have a metal layer on the outside of a dissipative polyester layer with an antiabrasion layer on the outside of the metallized layer. Because the metal-out bag’s metal layer is closer to the outer surface, it provides a more rapid charge dissipation when the bag touches a grounded surface.
The metal layer is also more susceptible to wear, which affects the bag’s shielding capabilities. It is recommended for operations where faster charge decay is preferred.
Metal-in bags have the aluminum metallized layer placed between the polyester layer and the polyethylene layer. It has a slightly slower rate of dissipation than a metal-out bag.
A third metallized-bag construction is a metal-fiber matrix recently introduced by 3M. The shielding is a nonwoven matrix of fine stainless steel fibers captured in a 7-mil-thick, low-density polyethylene. The steel fibers are nominally 3 microns thick, allowing the bag to be reused many times without degrading its shielding properties.
However, some fiber bags have a problem with exposed metal fibers, said Mr. Beamer. The metal fibers may protrude from the edges of the bag.3 If the conductive fibers fall onto the circuit board, they could cause short circuits. And because the fibers are so small, they are extremely hard to see with an unaided eye.
Performance Tests
When you have decided which type bag meets your needs, perform tests to ensure that shielding qualities are acceptable. You can evaluate the performance of static- shielding bags by performing a controlled test, said Ed Weggeland, Vice President of Marketing at Richmond Technology. To determine static-shielding capabilities use ESD S11.31-1994 Evaluating the Performance of Electrostatic Discharge Shielding Bags.
The only test method that gives reproducible results for ESD shielding and that correlates to device failure is the ESD S11.31 standard, agreed Stephen Halperin, President of Stephen Halperin and Associates. You can use this method if you want to perform laboratory-level testing with sophisticated equipment or periodic testing with inexpensive instruments that only check buried or shielding layers.
While there is no proven instrument that will test the total integrity of a bag, some equipment like Desco’s A50030 Shield Check verifies the film for ESD shielding qualities. The instrument indicates if an ESD event can occur inside a bag that has a tear, a wrinkle or other compromising property anomaly.
Your packaging needs may also require certain resistance, moisture-vapor deterrence, chemical compatibility or thermal stability properties. Each characteristic should be verified by the appropriate test standard.
For example, you can check the surface resistance of the bag material using EOS/ESD S11.11-1993 Surface Resistance Measurement of Static Dissipative Planar Materials. Or you can use the guidelines for materials used to protect moisture-sensitive plastics and electronics found in EIA-583 Packaging Material Standards for Moisture-Sensitive Items. The test standard for the moisture transmission rate is ASTM-F-1249 Water Vapor Transmission Rate Through Plastic Film and Sheeting Using a Modulated Infrared Sensor.
Recycling vs ESD Protection
Now that you are using the right ESD-control bag and you perform the appropriate tests to verify the bags meet the required specifications, you move on to the issue of recycling. Yes, ESD-control bags are recyclable, according to Mr. Bermani, but no company wants to recycle if it costs more than the bag is worth.
According to recyclers, virtually any plastic object can be recycled, even those “contaminated” with metal, such as the static-shielding bags. Unfortunately, the applications for these recycled metal bags are more limited than those made from uncontaminated plastic.
Technology is available to produce recyclable shielding bags, said Mr. Beamer. However, test marketing has shown that most users are unwilling to pay even a small premium.
The most significant deterrent to recycling shielding bags is the lack of volume, said Mr. Gleeson. A very large volume is required before it is economically viable to recycle, and few end users could generate the quantity needed.
For a bag to be considered recyclable in the United States, a detailed end use must be identified, said Mr. Weggeland. Additionally, collection points must be established for the bags.
Although the multiple-layer bags can be recycled and reground, the metallized material can damage extruder equipment, said Mr. Weggeland. The polyethylene bag, however, is easily reground and recycled because it is similar to commonly used products such as plastic grocery sacks.
The plain-vanilla antistatic polyethylene bag is more readily recycled because it has no metal components, agreed Mr. Gleeson. The trade-off is the lack of shielding.
The only static-protective feature of the polyethylene bag is its resistance to frictional surface charging, or tribocharging, said Mr. Gleeson. The metallized bag offers good shielding, but is not readily recycled. It’s your call.
References
1. O’Shea, P., “Inspect/Reuse/Recycle Protective Bags and Save”, Evaluation Engineering, October 1993, pp. 80-87.
2. Kolyer, J. and Watson, D., ESD From A to Z Second Edition, Chapman & Hall, 1996, pp. 47-49.
3. Fowler Associates, Laboratory Report, July 1996.
ESD Protective Bags
Air-Cushion Packaging
Protects ICs from 20 kV Charge
The Static Discharge Shielding (SDS) AIR BOX® electrically shields and physically protects products by suspending them in a double-chambered bag filled with air. Independent tests performed on ICs susceptible to breakdown at 150 V were unaffected by 10 negative and positive charges of 20,000 V when placed in the bag. Results per ANSI ESD S11.31 show energy levels of <0.03 nJ at 12% RH and 0.13 nJ at 50% RH. The product can be incinerated without threat of toxic chemical release. Moisture-level cards are easily read through the bag without removing static-sensitive products. Air Packaging Technologies, (805) 294-2222.
Three-Layer Bag Prevents
Charge From Getting In or Out
The Series #108-5-X three-layer static-shielding bag consists of a nonsparking conductive outer layer, a buried nonshedding center with a metallic barrier and an antistatic polyethylene inner layer. It prevents static charge from entering or exiting the bag. The 3-mil-thick bags are transparent and are available with a resealable zipper or heat-seal closure. The bags meet static-decay requirements of MIL-B-81705C Type II and the performance demands of DoD-STD-1686. Associated Bag, (800) 926-6100.
Static-Shielding Bag Meets
EIA-541 Requirements
The #A13405 static-shielding bag has metal-in construction and meets the electrical and mechanical specifications of MIL-B-81705C as well as the tribocharging and static- field attenuation requirements of EIA-541. The outer and inner layers have a surface resistance <1012 W /sq and the metallic layer is 100 W /sq. The light transparency meets the requirements of ASTM-D1003 for incandescent, fluorescent, gloss and haze. The bag is available in standard sizes ranging from 3″´ 5″ to 18″ ´ 24″. Desco Industries, (909) 598-2753.
Shielding Bag Provides
Moisture Vapor Barrier
The DRYPACK® 3300 Electrostatic Shielding Moisture Vapor Barrier Package meets the critical requirements of moisture-sensitive devices and solder-reflow- process components. Applications include SMD tape and reel, Joint Electron Device Engineering Council (JEDEC) trays and shipping tubes, and vacuum packing or nitrogen flush packaging. Charge dissipation from 5 kV to <50 V is <1.0 s on the outer and inner layers and <50 m s on the foil layer. The bag meets surface-resistivity requirements of the EOS/ESD S11.11 standard. The moisture vapor transmission rate is <0.002 g/100 in.2/24 h. Richmond Technology, (800) 538-0750.Moisture Vapor Bag Made From
Corrosion-Resistant Material
The Moisture Vapor Barrier Bag is made from corrosion- and static-intercept-material, and has a vapor transmission rate of <0.004 g @ 100° F/100 in.2/24 h. Static decay is rated at <0.03 s and the surface resistivity of the inner layer is 1 ´ 108 W /sq. The bags can be used to transport circuit packs, DIP tubes and devices. They are available in a variety of sizes including 10″ ´ 24″, 10″ ´ 30″, 15″ ´ 18″ and 17″ ´ 19″. Simco, ESD Consumable Products Division, (215) 822-2171.
Static-Shielding Bag Protects
Electronics From Tribocharging
The SCC 1000 Static Shield Bag is a transparent, metallized, static-shielding bag that stops ESD, static fields and tribocharging from harming sensitive electronic devices. The bag is tested to meet or exceed the electrical or physical requirements of MIL-B-81705C Type III, EIA-541, EIA 625, MIL-HDBK-263, DoD-1686 and EOS/ESD standards. It is available in flat and zipper styles. Static Control Components, (800) 356-2728.
Static-Shielding Bag Is
Transparent and Reusable
The Model 2750 Static-Shielding Bag uses micron-sized stainless steel fibers suspended in 7 mils of antistatic low-density polyethylene. Parts and labels can be viewed through the transparent bag. The stainless steel fibers resist abrasion, creasing or cracking. The bag maintains its shielding effectiveness per EIA 541 specifications for more than 15 crumple tests. Light transparency is 70%; outgassing is <1% per ASTM-E-595. Resistance on the interior and exterior layers is <1011 W . 3M Electrical Specialties Division, (800) 665-7862 x74.
Moisture Barrier Bag Provides
EMI and ESD Shielding
StratoGuard™ 9950 bags are made from barrier materials for dry packaging moisture-sensitive electronic components. The bags are waterproof, vaporproof and greaseproof. They offer ESD and EMI shielding and meet the electrical and physical requirements of EIA 541 and MIL-B-81705C Type I. The moisture vapor transmission rate is <0.0006 g @100° F/100 in.2/24 h. The static decay rate is 25 dB from 1 to 10 GHz. It is available with zipper or tamper-evident closures. BayStat, (415) 363-8079.
Copyright 1996 Nelson Publishing Inc.
October 1996