Have you ever asked yourself what is esd packaging in electronic world?
Now is time to reveal that information.
Shielding bags are arguably the most common form of flexible packaging. They are made of translucent metallised plastic film which shields against electrostatic fields and charges by creating a Faraday cage. The metallised layer attracts electrostatic charges which disperse harmlessly around the exterior or
drain to ground when in contact with a conductive work surface.
Metal out and metal in films are available which differ in construction but essentially provide the same shielding characteristics. Metal out is laminated polyethylene and polyester with the metallised layer applied to the external polyester surface. Metal in has the metallisation sandwiched between the two. Both have non tribocharging interior surfaces. Broadly speaking the trade off is between durability (metal in) and the ability to accept and disperse charges more readily (metal out). Surface resistivity reflect this, with metal out typically reading
10/5ohms/sq against 10/10ohms/sq for metal in film.
Manufacturing methods are vital to performance and reliability. Side seals must
be wide and strong. They are easy enough to test, try pulling a bag apart. The bottom fold should be pillowed, not creased under pressure: this can lead to the delicate metal layer fracturing and compromising Faraday cage integrity.
The non tribocharging inside surface of quality films is present in the bulk of the polyethylene, while lesser products rely on a washed application of a surface anti static treatment which is less durable.
Document series bag
Where documentation needs to accompany a board, traditional solutions can be hit and miss. Putting paperwork inside the bag is a potential risk and attempts to add a pouch defied mass production. Attaching papers with tape also presents a hazard as standard adhesive tapes create considerable charges when applied and removed.
As a solution, Twinbag recently appeared using a unique bag within a bag construction where the inner pocket is formed with conventional metal in film which is enclosed in an outer bag of dissipative polyethylene.
This system provides two outer compartments (the same size as the inner shielding bag) to take documentation. As the dissipative polyethylene has no metallic component, the outer pockets are clear and the contents can be seen. Twinbag is available with an open top or zip type closure.
Black conductive bags
The first shielding bags appeared in the 1970`s and were made of carbon loaded polyethylene film. Carbon loading, about 15 per cent carbon by volume, was also used to produce thicker materials for table and floor matting.
Being volume conductive, with a surface resistivity around 10/5ohms/sq, these bags are a genuine Faraday cage but opaque unlike their metallised film relatives.
The film is inexpensive to produce and can be rnanufactured as sheet or lay flat tubing (which facilitated bag manufacture). It
heat seals effectively and is, by and large, a widely available, reliable product.
The main disadvantage is not being able to see the contents, exposing boards and components to potential hazards during inspection. Another pro6lem arises in cases where the PCB has an on board battery as the bag`s conductive interior surface can create a short circuit and drain the battery cell.
Bags are available with open tops and closures: the double track zip systems are generally anti static rather than conductive but this does not represent a real hazard. These bags can be heat sealed by the user and new designs have a flap which is turned over and pressed onto a strip of low tack adhesive.
Anti static polyethylene bags and bubble film
Unfortunately, the word anti static has become a generic term for all things connected with ESD control. In fact, the definition relates to a precise specification and characteristics which determine a bag`s safe use.
Anti static polyethylene (commonly called pink poly) provides no electrostatic shielding: Faraday cage bags must contain a carbon or metal component. Anti static bags are made of translucent polyethylene treated with additives that inhibit the generation of electrostatic charges. The bulk material is filled with surfactants which leach to the film`s surfaces.
These additives are hygroscopic (water-loving) and attract airborne moisture. This creates an enhanced surface `sweat-layer` which reduces the coefficient of friction and lowers the propensity to charge. Poor humidity reduces the film`s effectiveness. Typically, modem surfactants do not contain the potentially harmful amines that, in the past, caused damaging reactions to PCB`s.
Anti static bags are non permanent since the surface layer can wear away through handling or evaporate at higher temperatures, leaving an untreated, hazardous product. Anti static film can be formed into bubble pack where mechanical protection is required.
Where shielding and life
4 Anti-static polyethylene bags and bubble fdm
Unfortunately, the word anti- static has become a generic term for all things connected with ESD control. In fact, the definition relates to a precise specification and characteris- tics which determine a bag`s safe use. .
Anti-static polyethylene (commonly called pink poly) provides no electrostatic shielding: Faraday cage bags must contain a carbon or metal component. Anti-static bags are made of translucent polyethylene treated with additives that inhibit the gen- eration of electrostatic charges. The bulk material is filled with surfactants which leach to the film`s surfaces.
These additives are hygroscopic (water-loving) and attract airborne moisture. This creates -an enhanced sur- face `sweat-layer` which reduces the coefficient of ftic- tion and lowers the propensity to charge. Poor humidity reduces the film`s effective- ness. Typically, modem sur- factants do not contain the potentially harmful amines that, in the past, caused dam- aging reactions to PCBS.
Anti-static bags are non- permanent since the surface layer can wear away through handling or evaporate at high- er temperatures, leaving an untreated, hazardous product. Anti-static film can be formed into bubble pack where mechanical protection is required.
Where shielding and life is not an issue, these materials have their place, typically storage of non static sensitive items in the proximity of ESD sensitive assembles or com- ponents. They are also inexpensive compared with shielding materials.
Clear dissipative bags
Dissipative polypropylene bags fall midway between anti static and shielding bags. Although this product does not offer Faraday cage shielding the non tribocharging properties are permanent. Visibility through the film is as good as anti static bags.
Moisture vapour barrier (MVB) or
dry packaging bags
Failure of some electronic devices has been linked to the rapid expansion of internally absorbed moisture during the soldering process. This can occur if the components are shipped or stored in non humidity controlled environments.
MVB film, used to make dry packaging bags, provides a higher degree of electrostatic shielding coupled to greatly enhanced moisture penetration protection. MVB film has a significantly thicker metallised layer which forms the vapour barrier but results in the film being opaque. Generally, these bags also use a desiccant to absorb trapped moisture when the bag is heat sealed shut, and a humidity indicator that shows the level of moisture that has penetrated after sealing.
This system suits the long term storage requirements of the military and products shipped to high humidity climates.
Many rigid packaging materials are available, from vacuum formed carbon loaded plastic to carbon coated corrugated cardboard. Enclosed, lidded containers made of conductive plastic provide the best Faraday cage protection, while anti static materials are not electrostatic shielding.
Clearly, the container`s application dictates which materials are used. Cost is also an issue. Broadly speaking, containers are either for storage, internal production or transit applications. Conductive, injection moulded boxes are the most durable but expensive, whereas coated
varie mechal ESD pi whi densi 1, al
sold that do not tribocharge and some are printed with ESD warning symbols.
Tapes for use with shield- ing bags, called grid tape, are non tribocharging and have a grid printed using conductive ink to provide a shielding effect whereas corrugated card is inexpensive but less robust.
Anti static and conductive tapes
Untreated adhesive tapes are great static generators. The plastic itself tribocharges and the action of separating two surfaces in intimate contact, as in unwinding a reel of tape, also ensures an electrostatic charge. Anti static tapes are sold that do not tribocharge and some are printed with ESD warning symbols. Tapes for use with shielding bags, called grid tape, are non tribocharging and have a grid printed using conductive ink to provide a shielding effect.
Conductive and anti static foam
Conductive carbon impregnated foam was one of the earliest ESD control products available and was easy to produce. The low density variety provides mechanical and ESD protection while higher density material, also known as pin insertion grade, accepts securely holds component pins.
Good anti static foam will not tribocharge but it does not provide the Faraday cage shielding of conductive foam.
This article was designed to illuminate the pros and cons of various materials and, in most cases, the differences are as clear cut as the applications. However, if one element of the story was to be flagged as the single most important fact it would be the difference between products that provide Faraday cage shielding and those that do not. This choice can be a matter of life or death for ESD sensitive components.