INSULATORS – A major cause of ESD damage at the work station
Insulators are one of the major culprits in any search for static generating materials. Unlike conductors, insulators cannot be grounded and are generally found in wide proliferation throughout any plant, and on most workstations. A stringent control and removal of all insulators, from your work area, can lead to a decrease of up to 50% of all static related rejects and chip degradations. The process of removal should start at the receiving dept., and should begin with a mandate to all suppliers to package incoming electronic and electronic related parts, in appropriate static control packaging. All companies should strictly control all packaging materials, supplies, and products shipped into their plant. Good static control begins at the receiving dept. which must ensure that all insulating packaging materials are removed and discarded before any items are moved to other facility locations. It is important to eliminate all insulating packaging materials, from the beginning, since they are the hardest item to control in a repair or manufacturing environment. Even with the most stringent of measures, a fair amount of insulators will find there way to the work station. Insulators can be found throughout most any plant and are often in a variety of configurations. Any company preparing for an ISO 9001-2000 audit, or gearing up for per-certification,Should be concerned about the number and types of insulators within their manufacturing environment. Insulators, (plastics) constitute a major ESD potential hazard for four reasons:
- Insulators can charge to thousands of volts with just the touch of a finger.
- Insulators can charge to different potentials on the same surface.
- Insulators can store charges of different polarity on the same surface — and constitute “a tribo-electricCharge in waiting.”
- The resulting fields can induce a voltage on ungrounded conductors. A list of the most dangerous insulators could include the following: ABS totes and bins, ABS trays, bubble wrap, cellophane tape, computer monitors, curtains, circuit boards, data diskettes, equipment covers, light fixtures, masking tape, microscope covers, paper wipes, plastic carts, plastic folders, plastic lighters, plastic boxes, plastic trays, plastic staplers, safety glasses, stretch/shrink wrap, Styrofoam cups, plastic pens, poly bags, packaging peanuts, vinyl stools, vinyl binders, and vinyl holders.
For those insulators that are impossible to remove from the work station, and for insulators that are part of the work process, a viable solution is the installation of air ionizers, which will eliminate the static build up on any items within its air flow area. Ionizers, in conjunction with the use of anti-static topicants, will eliminate static build up on any Insulators. Although the removal and control of insulators will lead to a substantial decrease in ESD related problems, they are only one of the culprits to be considered in any program of ESD elimination.
Back in the early 1970′s amines were added to low density polyethylene pellets and extruded into films to provide antistatic properties. The original bags, developed by Richmond Corp contained a pink dye to differentiate between regular and anti-static polyethylene. Thus the term “pink poly” came into being. In the beginning, ethoxylated fatty amines were used, which after extrusion leeched to the surface of the bag which attracted and absorbed moisture on the surface of the bag. It was this moisture that dissipated static and made the bag antistatic. It was also this leeching material that gave the bag an Inherent greasy feel, caused excessive blocking, contributed to the contaminants that affected solderability, and resulted in a limited bag shelf life. It was also a product that only worked in the presence of humidity, but it did meet the first specification written by the department of the navy for static dissipation, Mil 81705 B. Type II Today anti-static poly bags can still be pink although they are more often found as clear printed bags to differentiate from the old leeching “pink poly”. Today, most anti-static bags are made from a permanent’ non leeching anti-stat additive that when added to the polyethylene becomes homogeneous with the film. This results in an indefinite shelf life, eliminates blocking, is humidity independent, printable, non corrosive, non contaminating, and will yield a resistivity range of (128th. to 1212th. ohms/ sq.).
In general, anti-static bags, because of their limited dissipative qualities should not be used for packaging very static sensitive electronic components. Components with static sensitivity of 50 to 100 volts should be put into an antistatic bag, if quality protection is desired (see our ESD technical bulletin on component sensitivity levels). More and more anti-static bags are being used to package items that have zero static sensitivity. Such items as bare boards, even nuts and bolts are quite often put into anti static bags because most companies want to ensure that regular poly bags, .which contain static, will not come in contact with static sensitive products. Since the 1970′s and the advent of miniaturization and an increase of static sensitivity, it has become obvious that no one material can be a panacea for all ESD needs. Because of this, anti-static bags are only one of four basic constructions available to the user seeking ESD Mil Spec. protection.
Conductive bags (104th to 107th ohms/sq.) are manufactured by adding carbon black to the polyethylene pellets, during the extrusion process. It is the carbon black that is conductive and conducts static electricity across the surface of the bag, protecting the components inside the bag from electrostatic damage. Carbon loaded bags act as a Faraday cage to protect against tribo electric charges, and offer the user adequate protection at nominal cost. The disadvantages are that the bag is opaque and hides the product, the cost is higher than an anti-static bag and some rub off of the carbon black may transfer to the packaged product. Degage Corp. through new manufacturing innovations has developed a carbon filled polyethylene bag that will not shed or transfer the carbon black and is therefore compatible with class 100 certification. A typical and very large use for black carbon bags is in the blasting powder, munitions and gun powder industries, where the packaged products must be kept away from tribo electric charges. Today, the greatest use of black carbon loaded bags is in the non electronics industries. It should be noted that not all black bags are conductive and not all conductive bags are black.
Because of the limited static protection offered by anti-static bags and the decided shortcomings of black bags, in 1978 3M introduced, to the electronics industry, the first static shielding bag, constructed with metalized materials. This was the beginning of the third evolution of bag technology which set new standards for electronic component static control packaging. This new bag consisted of three laminates rather than being formed from a single extruded film. A thin film of Mylar (DuPont) was used as a substrate on which to vacuum deposit aluminum or nickel, and still remain transparent. This was then laminated to a half mil of anti-static polyethylene, and the film made up on an FMC pouch machine into a bag. While the 3M metal shielding bag acted as a Faraday Cage, for protection from charged materials and the human body, its increased conductivity and construction also provided protection from direct discharge to the outer layer of the bag. There are now two commonly used types of static shielding bags in the market place: metal in (with the vacuum deposited metal on the inside of the bag, and metal out, with the vacuum deposited metal on the outside layer of the bag.) Both methods are effective, both have their faults and their advantages. Just recently a new standard has been set by the department of the Navy and we now have a new military specification for static shielding bags Mil 81705C Type III.
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