A. 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.
A. Let us 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 are typically coated with a topical antistat agent that helps minimize the generation of a static imbalance from triboelectric generation or contact and separation (i.e., definition of antistatic). Some antistatic bags are made with an antistat built into the films layers and tend to be more reliable and cleaner then the topically treated ones. A good bag has antistatic properties on both the inside and outside of the bags film construction.
Bags with the films surface resistance in the dissipative range are preferred because charge dissipates across the surface at a controlled rate. Most dissipative bags also have the property of being antistatic. These are good general bags to be used in non-critical environments.
Metallized Shielding Bags
Metallized shielding bags have either a metal film embedded into the bag film construction or coated onto an existing layer. This metal film acts as 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 is typically spread out over the outer surface of the metal film and if fully enclosed, i.e. the bag is sealed, then the charges current from the ESD event is contained to the outside (outer surface) of the metal film, i.e., providing a region of no electrostatic fields within the bag, thereby protecting the contents within. This effect is known as the Faraday Cage Effect and is commonly used in controlling ESD via metallized shielding bags, conductive bags and the conductive tote box with a cover.
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.
Moisture Vapor Barrier (MVB) shielding bags are a special subset of metallized bags as they also have the property of EMI-RFI-ESD shielding. This is mainly accomplished by using a much thicker metal layer (about 10x thicker than standard metallized bags), which inhibits the moisture vapor transmission rate (MVTR) by a factor of over 20 times more compared to ordinary shielding bags.
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.
In both cases, the bags MUST be sealed to properly keep moisture out.
For all other ESDS (Electro Static Discharge Sensitive) devices [that are not moisture sensitive], a metal-in, metal-out or a clear dissipative bag can be used. Heat sealing is much preferred, however, you can often fold over the top of the bag and close with an ESDS sticker for adequate protection.
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.