Everyone knows that ESD bags are designed to protect sensitive electronic products. But not all bags are created equal.
To assess which ESD protective bag meets your needs, start by learning about its electrical, chemical and physical requirements. To begin, match the electrical properties, such as charge generation, surface and volume resistivity, shielding and charge retention, with the needs of your products.
The electrical and physical properties of the bag are the most important data needed to select the appropriate flexible ESD protective packaging, said Todd Somers of BayStat. Most significant is information on the surface resistivity of the inner and outer layers and the charge-retention characteristics of the shielding material used in the construction of the bag. For physical characteristics, seek out details on tensile strength, burst strength and resistance to puncture.
The bag should have a surface resistivity between 1 x 105 W /sq and 1 x 1012 W /sq to be considered antistatic per the EIA 541 specification Packaging Material Standards for ESD Sensitive Items. Or you can check the surface resistance using the ESD Association’s Standard 11.11, Surface Resistance Measurement of Static Dissipative Planar Materials. The bags should measure between 1.0 x 104 W to 1.0 x 1011 W .
Perhaps you want a shielding layer which prevents damage caused by direct discharge or exposure to changing electric fields. For these needs, look to the metal-in and metal-out bags.
“If you choose static-shielding bags for transporting and storing static-sensitive components, the shielding capability of the bag should be your primary concern,” advised Mary Fouts of Richmond Technology. “You can verify the shielding by using methods similar to the ESD Association’s Draft Standard 11.31.”
Typically, the test for shielding effectiveness consists of pulsing a 1,000-V human body model discharge through the bag film and measuring any voltage that penetrates it, said Dave Bermani of Charleswater. This procedure verifies the capability of the film to protect devices from ESD; however, it will not check the total integrity of the bag. For instance, if the bag is torn, the test will not necessarily indicate a shielding problem.
Moisture and Chemical Concerns
Electrical characteristics are not the only specifications to study. Some applications require moisture protection as well as shielding. Others need chemical compatibility between the contact layer and the product.
The water/moisture vapor transmission rate indicates how well moisture-sensitive parts are protected during transport. Guidelines for materials used to protect moisture-sensitive plastics and electronics are found in EIA-583 Packaging Material Standards for Moisture Sensitive Items.
Chemical compatibility includes the interactions of the bag and the electronic components. Solder-joint corrosion is one reaction attributed to residual acid compounds in the plastic bag. It occurs when the compounds bloom to the surface and attach to the solder, breaking it down. Check for compatibility with the current test method, MIL-B-81705C Military Specification, Barrier Materials, Flexible, Electrostatic Protective.
Polycarbonate compatibility also should be addressed. Incompatible materials cause polycarbonate components on the PCB to crack and craze at stress points, ultimately breaking. The procedures in EIA 564 Standard Test Method for Chemical Compatibility of Polycarbonate by Stress Crazing Evaluations evaluate material compatibility. The static-dissipative properties of an ESD protective bag vary with chemical makeup and are affected by time in service, humidity, temperature, film thickness, lamination and chemical additives.
The physical properties you should investigate include puncture resistance and tensile, tear and burst strengths. MIL-B-81705C and FTMS 101C Method 2065 check the integrity of the bags.
Table 1 lists the material characteristics and testing requirements for various types of bags.
Saving Money
No company wants to spend money unnecessarily, so it’s encouraging to know that many of today’s protective bags are reusable, said Mr. Somers. As long as the material retains its static-protective properties, a bag with a zipper closure or a resealable tape closure can be called into service many times.
No bag is without its limits, though. Tests by SECO found that bag performance usually is unacceptable after the sixth time, due in most part to rips or tears incurred during handling.
Using solely static-protective bags is another method that will help you realize a savings, added Ms. Fouts. Using one bag type gives you price breaks for volume buying and alleviates time-consuming decision-making on the manufacturing line where the question is often asked: “Do I put this item in a shielding bag or a dissipative bag?”
Typically, the cost of pink-poly bags are less expensive than metal-in, metal-out and moisture vapor barrier bags. But only the metal-type and moisture-barrier bags provide static shielding. Most bag purveyors now recommend these shielding bags because they can accommodate your electrical, physical and chemical requirements.
ESD Protective Bags
Moisture Barrier Bag
Provides ESD/EMI Protection
StratoGuard® 9900 Moisture Barrier Bag is made from Tyvek®, aluminum foil and polyethylene with static-dissipative inner and outer coatings. It is used for packaging moisture-sensitive components and provides puncture resistance of more than 25 lb. The bag also offers ESD and EMI shielding protection. It exceeds the electrical and physical requirements of EIA 541 and MIL-B-81705C. The moisture vapor transmission rate is <0.0006 gms. BayStat, (415) 364-3205.
Moisture-Barrier Shielding Bag
Meets MIL-STD Requirements
The #13806 Moisture-Vapor-Barrier Bag accommodates the needs of ESD, EMI static shielding and dry-pack applications. The material meets MIL-B-81705C requirements for ESD control and MIL-B-131 and EIA 583 for moisture-vapor transmission rate. The film is similar to a metal-in shielding material with the metal layer placed between two layers of film. The polyester portion provides pull strength and the polyethylene furnishes puncture resistance and a heat-seal capacity. The vacuum-deposited metal supplies a soft, flexible water-vapor barrier. Charleswater, (617) 821-8370.
Static-Dissipative Bag
Is Humidity Independent
Static-Shielding Bag
Has Amine-Free Inner Layer
The RCAS® 4150 Static-Shielding Bag provides a polyethylene inner layer that is amine-free, polycarbonate compatible and octanoic acid-free, and has a surface resistivity of <1012 W /sq. The metal layer is sandwiched between the inner and the polyester outer layer and has a surface resistivity of 102 W /sq. The 3-mil thick bag has a transmission transparency of 35% to 45%, a puncture resistance of >10 lb force and a moisture vapor transmission rate of <0.3 g/100 in.2/24 h. Richmond Technology, Inc., (800) 538-0750.
Reclosable Bag Meets EIA 541
And MIL-B-81705C Requirements
Zip-Top™ Shield Bags are reclosable and constructed from amine-free antistatic material. The static-shielding material meets the EIA 541, EIA 625 and MIL-B-81705C, Type III requirements. The bags are available in 19 sizes. Static Control Components, Inc., (800) 599-4380.
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. The transparent bag allows viewing of parts and labels. 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) 328-1368.
Moisture Vapor Bag Made From
Corrosion-Resistant Material
The new Moisture Vapor Barrier Bags are made from corrosion- and static-intercept material, and have 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 x 108 W /sq. The bags can be used to transport circuit packs, DIP tubes, and devices in a variety of sizes, including 10″ x 24″, 10″ x 30″, 15″ x 18″ and 17″ x 19″. The SIMCO Co., Inc., ESD Consumable Products Division., (215) 822-2171.
Table 1
Property
Material Characteristic
Value
Test
Standard
Static Shield
Moisture Barrier
Static Shield
(Metal In/Out)
Pink
Poly
Black Poly
Electrical
Static Shielding
EIA 541,
MIL-B-81705C
ü
ü
Surface Resistivity
<1012W /sq
EIA 541,
ASTM D 257
ü
ü
ü
ü
Chemical
Outgassing
<1.00% TML
<0.10% CVCM
ASTM E 595,
NASA
ü
ü
ü
Physical
Puncture Resistance
>6 lb
MIL-B-81705C,
ü
ü
Moisture Barrier
ASTM F1249
ü
Copyright 1995 Nelson Publishing Inc.
October 1995