COPING WITH ESD

by Ryne C. Allen, Desco Industries

From production to installation, today’s populated printed circuit boards (PCBs) are stored or transported in sealed ESD-protective bags. And with hundreds of thousands of PCBs used in everything from electronic games to pacemakers, that’s a lot of bags. To meet these packaging needs, many types of ESD bags with varying properties are on the market, ranging from pink polyester to metallized moisture vapor barrier (MVB) bag. So how do you know which bag is right for your job?

The evolution of the ESD-control bag and packaging-material design has been a compromise between various features. Ideally, you look for ESD bag materials that combine low cost with properties such as high ESD shielding effectiveness, permanent antistatic properties, humidity-independent performance, elimination of contamination and corrosion, MVB properties, heat-sealability, and transparency.

Traditionally, static-shielding bags are manufactured by depositing a thin metal coating such as aluminum over an antistatic polyester film substrate. The metallized layer protects the devices in the bag from electrostatic fields; the insulative layers prevent direct contact with potential ESD hazards. At the low end are amine-free, polyethylene film bags that are transparent or pink in color, referred to as antistatic or pink poly.

Over the past years, manufacturers have developed a number of static- dissipative materials that minimize tribocharging and improve puncture resistance and durability. Durability is a particularly attractive property in most ESD bags where sharp-edged PCBs are loaded into the bags.

Manufacturers also are developing more environmentally friendly materials than they had in the past. Historically, ESD protective packaging has environmental problems similar to 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.

Let’s review the current technology for ESD bags (Table 1). Essentially, there are three types or categories of bags: antistatic, dissipative, and metallized. Generally, the latter two categories are high-end bags that tend to have combined antistatic, dissipative, and shielding properties.

Antistatic Bags
Antistatic bags typically are coated with a topical antistat agent that helps minimize the generation of a static imbalance from triboelectric generation or contact and separation. Some antistatic bags are made with an antistat built into the film’s layers and tend to be more reliable and cleaner than the topically treated ones. A good bag has antistatic properties on both the inside and the outside of the bag’s film construction.

Dissipative Bags
Bags with a surface resistance in the dissipative range are preferred because charge dissipates across the surface at a controlled rate. Most dissipative bags also have antistatic properties. These are good bags for use in noncritical environments.

Metallized Bags
Metallized bags have either a metal film embedded into the bag construction or coated onto an existing layer. This metal film acts like an electrical shield against electrical discharges from the outside of the bag.

Depending on the energy and duration of the discharge and the thickness of the metal film, an ESD event typically is spread over the outer surface of the metal film. In a fully enclosed sealed bag a region void of electrostatic fields is provided to protect the contents. This is known as the Faraday cage effect and commonly used to control ESD via a metallized shielding bag, a conductive bag, or a conductive tote box with a cover.

Two common types of metallized shielding bags vary in construction: metal-in and metal-out. The metal-in or buried metal shielding bags are the most commonly used and recommended for packaging static-sensitive components. They also tend to be superior to the metal-out construction in durability and cost.

The metal-out shielding bags also protect against static-induced damage. The metal layer is closer to the outside surface so these bags have lower resistance readings than the metal-in bags, which can be important in some applications.
MVB shielding bags are a subset of metallized bags because they also provide EMI/RFI/ESD shielding, accomplished by using a metal layer about 10 times thicker than standard metallized bags. This layer inhibits the moisture vapor transmission rate by a factor of greater than 20 times when compared to ordinary shielding bags.

If you are storing ESD-sensitive devices for prolonged periods of time (six months or more) or if the devices are sensitive to corrosion, then package these materials, along with a desiccant pack to absorb moisture, in an MVB or a metal-in bag. The difference between an MVB film and a metal-in film is about one magnitude in the vapor transmission rate (grams of water/100 in.²/24 h @ 100°F). A device that is both moisture and ESD sensitive should only be packaged in an MVB bag. In both cases, the bag must be sealed to keep out moisture.
For all other ESD-sensitive devices that are not moisture sensitive, a metal-in, a metal-out, or a clear dissipative bag can be used. Heat sealing is an option; however, you can fold over the top of the bag and close it with an ESD-sensitive sticker for adequate protection.
Since they typically have a finite life, ESD bags should be inspected by an internal sampling plan before reuse.

Table 2 was derived from a set of experiments performed by Lucent Technology.¹ The voltages indicate the level at which the device was damaged by direct discharge to the bag. This shows that shielding bags are the most protective when new. But as they are used, they degrade to provide ESD protection similar to antistatic bubble-wrap bags. The bubble wrap uses the air gap as a means of protecting its contents from ESD, similar to clamshell packaging.

If an ESD-control program is properly implemented and well disciplined so the threat of ESD is almost squelched in the handling and transportation processes, then a good dissipative bag may be sufficient for the storage of most ESD-sensitive devices. Very few programs are implemented that effectively.

Most ESD programs use shielding bags because they increase the level of confidence and make the program more foolproof. Shielding bags prevent the ESD-sensitive components sealed within the bag from charging up via field induction and minimize the damage from a direct contact ESD event on the outer surface of the bag. Some high-quality ESD shielding bags can withstand up to a 30-kV discharge.

An antistatic shielding bag (type 3) can be considered unacceptable if it exhibits any of the following conditions:

Delaminates as defined in MIL-B-81705C Section 4.8.4.1.

Has a static decay rate exceeding 2.0 s.

Does not attenuate EMI at least 10 dB.

Does not shield an ESD event under 30 kV

In general, a metallized shielding bag still may shield if it is crinkled or has acquired small holes, but it is safer to discard these bags and err on the conservative side. It probably would cost more to test and qualify the bag than to replace it.  

MIL-B-81705C, Military Specification for Barrier Materials, Flexible Electrostatic Protective, Heat Sealable, states that there are three classifications of barrier materials that make up ESD-protective bags. See Table 3.
Table 4 presents most of the current ESD packaging standards. ESD-DS20.20, authored by the ESD Association, will include all the association’s ESD-control standards in one reference document.² Essentially, the nondraft version of this standard will replace MIL-STD-1686, possibly near the end of 1999.

ESD-sensitive devices should always be stored in an enclosed antistatic shielding bag, a conductive closed tote, or a bin when not being handled at an ESD-controlled workstation. Further precautions during transportation include using dissipative carts with conductive wheels or drag chains in conjunction with a conductive or dissipative floor.

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, and ANSI/EIA-541 and ANSI/EIA-583 for packaging materials (Table 4).

1. Dangelmayer, G.T., ESD Program Management, 2^nd Edition, 1999.