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Selecting the proper Electronics Packaging Bag
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Electronic Packaging Products Information - Most people are aware of the effects of static electricity in general. Dragging shoes on carpet and touching a person or something metal, makes a brief shock. Drying synthetic textiles in a cloths dryer will often produce "static cling". The static electricity behind these common occurrences can damage modern electronic devices and circuits. As electronic circuits and their connecting pathways continue to shrink in size, their vulnerability to damage from static electricity has swelled. A factor of 10 shrinkage in feature size can make gate oxides 10 times more sensitive. Protective handling and packaging techniques have been adopted by all segments of the electronics industry from the chip foundry to the production floor to the field service arena. One of the most common static anticipatory items is a bag. Use of protective bags began in the 60's with the introduction of "pink poly" bags. Static shielding bags were introduced in the late 70's. While the military has long used moisture barrier bags, the increase of Surface Mount Technology has greatly increased usage. A device needs to be protected from three primary static threats:
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| Direct Discharge (ESD): A discharge directly to a bag can subject the device inside to very high current, melting or fusing the circuit. |
| Static Fields: Fields can induce destructive currents in circuit conductors. Field differentials can break down the circuit dielectric. |
| Tribocharging: Friction between the bag and device can produce damaging static voltage and fields. |
| The Static Shielding test, applies an undeviating discharge to a bag. An oscilloscope connected to a feeler inside the bag measures the amount of voltage that goes through the bag. This test also addresses the field threat. Fields are generated by the release to the outside of the bag. Fields that penetrate the bag are represented in the voltage amount. Tribocharging tests are difficult to perform and are not very repeatable. Information from bag specs are only a yardstick, and do not tell how materials will perform in use. Bag users are left to compare these standards or perform their own tests. Surface Resistivity is a sign of a material's ability to allow static to move around (dissipate). It does not necessarily suggest low tribocharging. The bag has the capacity to dissipate a static charge to ground. This keeps static electricity from building up on the package or the mechanism. The material is also antistatic, suggesting that it will not charge up when rubbed against other materials. The resistivity is in the dissipative range of about 109 - 1011. These bags have no shielding ability. A static field or discharge happening outside the bag will penetrate the bag and harm electronics inside. Pink Poly consists of plastic that has been loaded or surface coated with a chemical antistat. The pink color is only a colorant that was added to tell the difference between static control materials from normal packaging. A small number of users are now requesting clear transparent dissipative poly bags. In the past, a reactive form of amine was used in the chemical antistat. This antistat caused oxidation of some metals and stress cracking in some plastics. The pink color is sometimes incorrectly associated with this amine type antistat. Some manufacturers converted to amide-based antistats and removed the pink color in response to customer demand. Pink Poly bags are useful for packaging items that have no static vulnerability. Their primary use is to package support or processing material that will be in close proximity to static sensitive devices. This keeps static generating packaging materials away from sensitive areas. Black Poly is very conductive, usually about 103 - 4, and will dissipate a charge very fast. Regretfully this fast dissipation also means that a charged person or object can 'spark' (ESD) to its surface. The basic idea in static control is to swap charges at a slow enough rate to avoid sparks. However, not so slow as to allow a static build up. Due to the material being conductive it does provide some minute amount of shielding (<30%). Unfortunately, there is no plastic layer to isolate a device inside a bag. A charge may be transferred through the volume of the material to the item inside instead of around the material to be grounded. Black Poly bags are a poly plastic that is volume loaded with a conductive form of carbon. The material is black and solid in appearance. In the past, black poly was used as a connection between pink poly and shield bags due to slightly lower cost while offering some shielding as opposed to nil with pink poly. As the price of shield bags continues to drop the usage of black poly will likely drop as well. Since black poly bags are opaque, the bag’s contents must be removed for identification. This creates a additional opportunity for static damage. Shield bags provide the dissipative and antistatic attributes of the poly bag all the while adding a metal shield and polyester dielectric to stop static from passing through the bag. The test for shielding demonstrates the difference between the various bags. Shield bags will for the most part, stop 97% of a 1000 volt static pulse applied to the outside of the bag from reaching the inside, black poly about 30% and pink poly will stop only about 10%. Static Shielding bags consist of several layers. The layers are: dissipative poly, glued to metalized polyester. The outside polyester has an antistat covering. The metal is vapor deposited in a vacuum chamber. Aluminum is the dominent material used in this process, along with copper and nickel also being used. This arrangement of a shield bag, with the metal sandwiched between two layers of plastic, is called "buried metal" or "metal-in". In a "surface metal" or "metal-out" formation, the poly is plastic-coated to the polyester with the metal above. There is an abrasion coating on the metal. The metal generally used is nickel and it is sputtered and not vapor deposited. The metal-in or buried layer bag makes available better protection of the translucent metal shield by placing it between two plastic layers. The surface metal bag has a conductive outside surface, which provides fast charge dissipation. Like the black poly bag, it can be sparked to. Applications: Static shield bags should be used for all electronic components, boards and assemblies. Moisture barrier bags provide dissipation, antistatic make up, static shielding, and add a moisture vapor barrier. The moisture divider protects moisture sensitive items and improves long-term storage. This bag is stronger than a shield bag. Moisture barrier bags are similar in construction to the shield bags. The two types are "foil and tyvek" and “heavy metallization". The "foil and tyvek" structure from the interior to the exterior is dissipative poly laminated to thin aluminum foil and tyvek. Tyvek is Du Pont spun bonded polyethylene material. The "heavy metallization" structure is at its root that of a shield bag but with opaque, broad aluminum metallization or many layers of metallization. Nylon may be used in place of Tyvek or polyester. Allowing the needed strength at a lower cost than Tyvek. Both foil and metalized moisture barriers provide good service. The moisture barrier bag is issued when barrier protection is needed or when maximum shielding protection is required but when transparency is not an issue. Surface mount devices are positioned in moisture barrier bags to avoid moisture assimilation by the case material. During reflow soldering, immersed moisture will expand and crack the case. Static protective bags are an important part of a static maintenance program. Selecting the correct bag can help reduce static damage and save money on costly repairs and rework. The dollar amount of static protective packaging is insignificant when compared to the protection it affords the costly items placed in the package. Use static shielding or moisture barrier bags for all electronic circuits. Use pink poly for non-electronic parts and production goods that must be near electronics. Bags alone cannot provide complete static protection. Implementing a program using grounding technique and static handling procedures is a must. |
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