ESD Systems’ ESD Technical Newsletter
HOT TIP of the MONTH (ESD Systems Web Site)
NEPCON EAST '99
A1: Yes, the volume resistivity of a material may become higher with a thicker bag. The higher the volume resistivity, the higher a voltage the material will stand off. Most high quality metal-in, metal-out or MVB shielding bags can withstand over 30KV. A thicker bag (thicker dielectric or metal film) will be able to hold up to a greater energy ZAP. The spacing of the ESDS device relative to the inner bag surface can have a similar effect. The greater the "air gap" the greater the protection from an ESD event penetrating the shielding bag into the ESDS device. It is the metal film that helps transfer this energy into a surface current [Faraday Cage Effect] rather than punching through the bag to the sensitive device.
Q2: In my company many rolls of bubble wrap, plastic bags, and PCB boxes (plastic cases) are listed as "Anti-static". While we all understand that these items will not produce serious static, many think that they will also protect the components inside from a static charge. My contention is that a significant amount of charge would pass through the bag/box and damage any ESDS item inside. Only a shielded bag would guarantee complete protection from ESD. The question really arises because a well-known and respected PCB manufacturer sends their PCBs’ to us in anti-static plastic cases. The clear plastic case only lists "Anti-Static and we can not see any conductor strands within the plastic. What are your thoughts? - Daniel, Wilmington, NC
A2: There are two ways to protect ESDS PCBs. One way is by shielding. Use of shielding bags is a good way to protect the contents from external ESD. Another way to protect ESDS PCBs is to isolate them from external ESD with an air gap. There are "clam shell" packaging and other anti-static plastic packages that will give a spacing of about an inch of air between the outside plastic shell and the ESDS part inside. This "air gap" spacing can be an effective way to protect ESDS parts from external ESD as the 1" air gap acts as a dielectric to prevent discharges up to 30kV.
A3: There are several types of static bags, shielding (metal-in, metal-out, moisture vapor barrier), antistatic and conductive. The most commonly used "static" or ESD bag is a shielding bag, which has all three properties listed above. A shielding bag has a layer of metal, usually aluminum (Al) (similar to Aluminum foil), that provides "shielding" of a Faraday cage affect. An ESD shieling bag has several properties:
A typical shielding bag is composed of:
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Topic: Shielding Bags
Information gleaned from white paper on Shielding Bags, Manufacturing a Quality Choice: http://www.esdsystems.com/whitepapers/wp_shieldingbags.asp
Shielding Bags, Manufacturing a Quality Choice
The static dissipative polyethylene ESD Systems uses in all of our bags is far superior than the "topically treated washed" products.
Our bags are made with volume loaded polyethylene which cannot be washed off and is inherent in the film. The PE resin that we use and all the additives are developed to minimize contamination.
We also have worked with our film supplier to permanently solve the greasy film issue, which leaves a sticky oily film on the users hands and product. In 1996 working with our film supplier we incorporated into the structure of the film an "easy open" capability which now makes it much easier to open the bags.
The polyethylene and polyester films that are used to make up the film that of our bags are FDA approved, are user friendly, and do not emit odors. All organic solvents are thoroughly evaporated in our vendors processing ovens.
I have clearly described why the film that we use in the manufacturing of our bags is far superior to the "topically" treated commodity films that are being used and converted into bags by other converters.
The 2nd reason is our converting process.
Our machines have been engineered and designed to manufacture and convert supported static shielding films specifically for the electronics market. Because our core business is ESD, the bags that we make and the film that we process are of the highest quality and performance of any ESD company in the world.
Our cutting edge technology for the film and the manufacturing of the bags are always being reviewed and improved upon.
We don't deal with just any raw material supplier. This allows us to be consistent and not substitute inferior commodity type films that are driven by price.
Our bag making machines are the best in the world for converting very specific high-end static shielding films. Our machines don't compromise the "FARADY®" layer. We achieve this by producing a "SOFT" fold to the bottom of the bag.
The bottom portion of the bag does not come in contact with the drive rolls of the machine that can in fact "crease" the film and fracture the aluminum layer that creates the "FARADY®" cage. I believe that we are the only ones in the world that do this and can guarantee the every bag from the 1st bag to the millionth bag maintains the Faraday® cage.
On our machines all of the rollers are covered with soft cork, foam and Teflon coverings so we do not scratch the interior or exterior of the film. Our machines have 2 sealing bars that in parallel with the our high performance polyethylene interior sealing layer produces one of the strongest bag seals in the market. It also allows us to run at significantly higher running speeds, which makes us the lowest cost producer of shielding bags in the market today.
We also have a "chiller" bar, which after the bag has been heat-sealed chills the polyester and polyethylene layers. This process allows us to cool the plastic layers and prevents the finished bag from distorting. It also allows our machines to run faster and produces a "clean" sealing edge.
Our machines run what we call "2 up". Our machines are actually 2 machines built into one. This allows us to be the lowest cost producers in the business.
We run 2 shifts a day and carry over 1.5 million bags in stock that are ready for immediate shipment. We have the capacity to produce over 3 million bags a week.
To summarize, not all shielding bags are the same on the surface. They may look the same but in fact they are not. The market place is also starting to ask serious questions about the interior of the bags. It is important to know and specify how the static dissipative properties are obtained. Because our products are volume loaded they are superior to the surface treated plain polyethylene films that still are being supplied to other converters.
Another concern is contamination. The question should be directed at the issue of what effect the antistatic generating technique has on your product. Seriously question the issue of amine versus amide because most suppliers in their effects to create an amine free product use amides. Also question the ability to regulate this property. How is the wash coat applied to the PE surface in the product? Too much antistatic grease ends up just where you don't want it.... on the component. It is also not unreasonable to ask for ion extraction data. These are serious sources of contamination.
In summary, all bags are not created alike. Ask the hard questions and demand to see the performance data of your product. It will be available lot by lot from reputable suppliers.
This is a free monthly newsletter, which specializes on issues in static control in the workplace.
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Copyright © Desco Industries, Inc. 1999