Pink Poly Bags vs Shield Bags
Static electricity on the surface of a bag can cause the bag to be unsuitable for electronics packaging. Antistats and metallized PET are used to modify the bags in a way to either dissipate a static charge and / or prevent the charge from penetrating the bag. Static is a sheared off electron; it is unstable; charged (+ or -) and trying to get back in balance. That is why it ‘snaps’ when it finds a path. Static in certain packaging environments is bad news for several reasons:
- The first reason, related particularly to micro electronics is that printed circuit boards can be so tiny and delicate these days (photographically printed) that a static spark can burn a hole in the circuit and not be visible! It can happen at any point in manufacturing (after testing perhaps) particularly during packaging or removing the part from the bag for assembly. This can cause a computer NOT to work after assembly, an aircraft monitor or a missile guidance system to be inoperative. While the amperage is very low, the voltage can be very high (250,000 volts). House current is medium voltage; higher current; and lighting is very high voltage AND amperage produced by static electricity or shearing off electrons as billions of rain drops scrape against each other in the atmosphere. Another thing a customer considers when buying packaging is the cost of the micro chips, transistors, and the whole circuit! They are sometimes $500 per part in a $2 bag. The cost of the bag is worth it for parts that need to be protected.
- Antistat is also used to prevent fires! You might ask how a bag could cause a fire. Antistatic materials are used in hospital operating rooms and particularly where work environments are oxygen rich or where gases are being used. This might include cockpits of airplanes, the space shuttle, oxygen tents and many other applications. While the oxygen will not ignite, one spark in an oxygen rich environment could cause everything in the area to catch on fire in a split second. It happened on Apollo 13.
- The third reason to use antistatic poly is to resist dust. Dust in a manufacturing environment for pharmaceutical, aerospace or microelectronics can cause product failure. Dust in Pharmaceutical manufacturing may be in the form of mold spores. It may contain bacteria or viruses, allergens and other dreaded things. For aerospace dust may plug filters or contaminate switches or electrical contacts preventing them from working. In micro-electronics, it has been shown where dust falling onto a PC board has produced breaks or ‘disconnects’ in the photo printed circuit. This can be seen under a microscope.
In the case of an antistatic poly bag as seen with Pink Poly, the object here is to “dissipate” the charge. What this means, is that a charge that is already on the bag or gets onto the bag, will run off the bag and down a conductive wrist strap that employees wear when filling or removing parts. If the machine or table is grounded, (which is always the case) the static charge will leave the bag and become ‘neutralized’. Parts are then safe to insert or remove from the bag.
Pink Poly is used most commonly to ground a static charge and protect less sensitive parts. A Shield Bag is much more sophisticated and is typically used to package much more sophisticated parts that require greater protection. A Shield Bag will ground a charge and prevent penetration.
While pink poly contains a blooming tallow amine that grabs moisture from the air, thereby, providing an electrical path to ground*, a Static Shield bag is made with a metalized polyester which acts as a shield. The “shield” prevents a charge from going right through the plastic bag, ruining the part inside. That is the big difference between the two. The polyester is usually coated with antistat outside to acquire the same properties as an antistatic pink poly bag. The metallized polyester is always laminated to an antistatic polyethylene so that it can be heat sealed into a bag. The bag also uses an antistatic poly inside to prevent triboelectric charging! In other words, if the sensitive part were dropped inside the shield bag and shaken, the shaking action can break off little positive or negative electrons inside the bag. This would become static locked inside the bag that could ruin the part, even with the bag sealed. There is some valid argument from the experts that the charge has no way of getting out of the bag. True, but using an antistatic poly inside is the best way to prevent generating a charge to begin with.
Sometimes we see the metal side ‘out’ on a Shield Bag. This will make the outside of that bag, very conductive or antistatic. This is an unusual style, as the metal, (which is vacuum deposited aluminum – sometimes nickel) is thin (400 angstroms), like spray paint! It can be scratched very easily reducing the effectiveness of the shielding properties. The scratched off metal in any of the high tech applications can be a super nightmare, contaminating pharmaceuticals, shorting out electronics, etc. Metal to metal contact can create a spark, so this would not be a good structure to use in an oxygen or flammable gas rich atmosphere. A liquid coating, like a varnish, is often used over the metal to negate some of these problems. Some conductivity is maintained using this technique.
Something to watch for regarding static Shield Bags, sometimes called ESD bags for Electro Static Decay, is that because the metal shield is fragile, it can be cracked very easily, particularly at the fold. If that happens, a charge on either side of the bag may sit there and not be grounded. This is very bad, as following all other procedures, a part could get zapped and ruined without detection. If you crease one of these bags by hand to pack or lay flat, you can be inadvertently ruining each one of them. It took Shield Bag manufacturers years to realize that high tension of film across the folding board can render the film and bags useless! This is a very simplified overview of PP vs a Shield Bag.
Testing of these products is very sophisticated. The procedures involved are:
- Mil-B-81705-C (regarding decay rate) Shield Bags (Must decay 5000v to 0 in 2 sec @ 15% Rh; 70F NFPA 99 (fire protection) Pink Poly (Must decay 5000v to 500 .5 sec @ 50% Rh; 70F
- EIA 541 (EMF Shielding) (ElectroMagnetic Field Capacitance probe tests)
- Federal Test Method 4046 standard 101c (Same as Mil-B @ 12% Rh) Surface Resistivity ASTM D 257-76 (Measures resistance: should be < 10″ ohms)
Antistatic Agents (How They Work)
What is static?
Static electricity is generated by friction between two surfaces. The friction causes stripping of electrons from one surface and adding electrons to the other. This is called tribocharging. It makes for a very unstable situation where the electrons want to return to their previous state of equilibrium. The donor that gave up electrons will have a positive charge and the surface that picked up the electrons will have a negative charge. The term static electricity describes a state where you have these groups of orphaned electrons just sitting there (or static), but very tense. Materials that have static have this excess or shortage of electrons. They are on an “island” in a way. There is no way for the electrons to get on or off the item. Once they get the chance to ‘jump’ back to their previous state, or “ground”; they will. This often causes a spark and for semiconductor packaging, or gaseous environments this is a real problem as the spark can destroy the microchips, circuits and parts or cause fires in places like the oxygen rich capsule of Apollo 13. Dust is attracted to films, because the dust has various electrical charges on it and it is a whole lot easier for electrons on an item to pull in these loose dust particles than to try to jump off onto the dust. Dust is a bad thing in Cleanroom packaging, and in industries using Cleanrooms as reviewed in the prior discussion.
Plastics, as in packaging, can be made antistatic or static dissapative by adding certain chemicals to the film. They can be added into the resin blend or topically applied after the film is made. These are often referred to as ‘internal’ or ‘external’ antistats. Generally, these things are tallow amines, “butter fat”, they also use coco butter, but there are other materials and forms including, some non blooming antistatic agents that can be compounded into the resin.
How antistats work:
Attempting to minimize the technical terms for Office Training purposes, but adding enough detail for a research paper, here is what happens in a “nut shell”.
A fatty amine, ‘grease’ is used as the main active ingredient for antistat. Unfortunately, it only has one little hydrogen tail on it (H). The manufactures expose the grease to EtO gas (Ethylene Oxide) and by ethoxylating it, an oxygen molecule (-O) is added to the hydrogen. Now they have a little polarized magnet going consisting of a negatively charged -OH. That -OH magnet is looking for something to grab on to, and what better than another H. It finds this H by grabbing it from moisture in the air. The -OH and extra H become H2O on the surface of the film. The two little hydrogen tails attract relative humidity. This is now very stable and water is conductive. Provided the bag, bag machine, work table or the people packing electronic parts are “grounded”, with a wrist strap or simple wire, any static that gets on the bag, will zip down the water that is sticking to the greasy antistatic coating and be gone! Parts are safe to package or remove from the bag. If staff handling product are not “grounded” through a conductive floor mat, or otherwise, the bag will hold the charge even with the antistat and moisture on it. That would be bad. People in the business take provisions to assure the workers are grounded and the environment is at a controlled humidity level.