Fowler Associates Labs

Abstract : This paper as printed in these proceedings is an abbreviated version. The full paper is available by mail, EMail or may be viewed on our web site: www.sfowler.com. Packaging of electronic products for shipment has developed rapidly over the past 30 years. Antistatic and other useful protective functions of the packages have progressed to a high technological state. This paper reviews that progress, describes present technologies and forecasts future trends. Antistatic technologies for loadings and coatings are covered both from a chemical as well as a practical point of view. Antistatic, dissipative, electrostatic and rf shielding properties are discussed. In this paper information is presented on new shrink film technologies which are just emerging. Automated packaging techniques as well as advances in moisture barrier packaging are discussed . Using air gaps as a method of electrostatic shielding has been around for a long time. New advances in thermoform and inflatable packages are covered.

History: One may assume that the Chinese had problems with fireworks explosions due to static discharge centuries ago. The munitions industry has always had to be careful of static discharges. The APOLLO command module fire in 1967 was thought to be static related. Several premature or unintentional rocket ignitions have been blamed on static discharges. Today we may hear of gasoline pumps exploding due to static. Workers in detonator fuse assembly areas know too well the problems with static. These problems are known and assumed to be real. In the electronics industry the problems of static damage are more recent and less visible. In the early days of electronics, items such as vacuum tube devices and other extremely robust products from an ESD point of view had little or no need for static control in the materials used for packaging. Then in 1947 with the invention of the transistor the course of packaging for electronics was beginning to change. The first transistors were very insensitive to static charges because of their size and sheer bulk. As the size of these components came down and speed became a factor in their operation, the idea of static sensitivity became known but not yet fully necessary. The progression of transistors to metal oxide versions such as MOSFET's on to integrated circuits with smaller and smaller sizes opened the industry's eyes to static failures which began to affect production yields. With the invention of the true microprocessor and the explosion of computer technology, static control became a very real issue which could not only affect the yields and profitability of circuit manufacturers but also keep latent failures from causing failures later in the product life with even safety consequences. During the late 1970's and 1980's many major companies developed well organized ESD control programs with major benefits to yields and profitability. Today the emphasis has shifted to one less concerned with preventative measures. Many companies have abandoned the gains realized in the early days of static control and believe the technology has developed to a point where concern is reduced to the lowest price per package. In the field of packaging, there are two types of "chips": IC chips and potato chips. A $500 IC chip may be packaged in a 3 cents bag while 1 cent worth of potato chips will be packaged in a 10 cents bag. The food industry does not normally underestimate the value of its packaging while the electronics industry allows its products to constantly be underpackaged. If food spoils, the consumer may not buy the product or may be affected by pathogens grown in poorly packaged products. The results are disastrous when botulism or salmonella affects someone or some group. In electronics the failure of high dollar products due to ESD may not necessarily cause sickness (pace makers, respirators, refrigeration controls....) however, they may ruin the day of an airplane crew and passengers when navigation equipment goes out ( Flight 007 KAL) . Automatic pilot systems could send planes into uncontrolled rolls. Premature firing of air-to-air missiles in helicopter #6 could make the flight leader miss dinner. Electronics is considered the highest technology industry. It is the most archaic in its packaging technology. This paper will help inform those involved in this industry in the advantages of value packaging of its products.

ESD Packages Overview: The ESD packaging industry has grown up to support the desires of the electronics industry. As stated above the industry has been made up of flat films, bags and boxes. Not too much innovation; hardly any automation. The bags began as black carbon loaded polyethylene until the advent of "pink poly" and other static dissipative films. Shielding bags were first foil laminates mainly used for moisture barrier. Now they are typically metallized polyester laminates. The driving forces of this industry has become costs, not function. Most electronic industry end users believe that the present technologies are OK and that cost is all important. This brings the need for more automation closer than ever to favor. Cross sections in Figures 1 through 10 describe the typical materials used today in ESD packaging.

Chemistry of Dissipative Plastics: There are two dominate chemical technologies that achieve static dissipation as Type II electronics packaging films, as defined by EIA 541 or MIL-B-81705-C (the " Mil-Spec"), and as the static dissipative heat sealing layer of many EMI shielding Type I and static shielding Type III films. An amine is a long chain of carbon atoms surrounded by hydrogen atoms. This hydrocarbon chain is derived from a fatty acid and can vary from a few carbon atoms long to twenty or more carbon atoms. The last carbon is connected to a nitrogen atom which has two hydrogens linked on the sides. Because simple fatty amines need a higher affinity for water to perform well as antistatic additives, ethylene oxide is reacted with them to make ethoxylated fatty amines. This gives the molecules two polar end groups: OH or alcohol groups. Ethoxylated fatty amines are the additives used in dissipative polymer amine technology. Amide additives used in dissipative polymers are similarly based on a fatty acid attached to nitrogen which is also reacted with ethylene oxide. However, the carbon atom next to the nitrogen has a double bond attachment to oxygen instead of two single bonds to two hydrogens. In this case, the molecule is specifically an ethoxylated fatty amide. It should be noted that the commercial nomenclature "amine free" usually means that an amide additive is used instead of an amine. Other static dissipative additive systems for polyethylene exist with performance and properties very similar to amines and amides. Because these are not widely sold into the electronics industry, they are not mentioned in this paper. Either type of dissipative polymer starts as a homogeneous blend of additive, antiblock and polyolefin resin. However, two things must occur for static dissipation to take place. The amine or amide must first diffuse through the plastic volume to reach and wet the surface. This is commonly referred to as blooming. Good compatibility between the additive molecule and the resin results in an insufficient amount of amine or amide wetting the surface. Without enough additive on the surface, static dissipation will not occur. Too little additive and resin compatibility results in too much additive getting to the surface. In this case, the static dissipative property will work well but the surface will be excessively greasy with the additive contamination everything the film contacts. The additive must absorb at least a trace of atmospheric moisture with its two "claws." This hydrogen-bonded combination of additive and water is the surface that provides static dissipation. Essentially, the conductivity is provided by the layer of water attached to the bonds and helped by the ionic solution of the additive. It is an ionic soup. During certain seasons in some locations, it is possible that there is too little atmospheric moisture to be absorbed. It is not clearly understood the minimum relative humidity required for static dissipation. The best guide is probably the EIA's standard of 12.0 +/- 3.0% RH as a reasonable minimum limit. Electrical permanence: Some have referred to questions about the permanence of dissipative polymers as one of the dirty little secrets of ESD. Many films lose their electrical properties when exposed to the Mil-Spec accelerated aging test at 1600 F or after some period of time at storage conditions. The electrical performance lifetime of commercial dissipative polymers is often increased by increasing the film thickness and therefore the reservoir volume Polycarbonate compatibility: The term "compatibility" when used in conjunction with polycarbonates has a different meaning than that used to describe additive and resin compatibility. A more accurate term would probably be polycarbonate "coexistence." Some antistatic additives such as tertiary amines cause polycarbonate parts to crack or "craze". A general rule for polycarbonate compatibility is that there be no serious crazing up to a 2000 psi stress level at 1580 F.

Surface Resistivity and Static Decay : Federal Test Standard 101C, Method 4046.1 requires a static protective material to have a static decay time of less than 2 seconds. EIA 541 defines a static dissipative material as one which has a surface resistivity of between 1E5 and 1E12 Ohms per square. These two requirements are correlatable only for monolayer and homogeneous films. Multi-layer coextrusion technology as well as laminated materials separate these two parameters. The conception persists that a low surface resistivity is required to dissipate a static charge and that a good static decay is sufficient to characterize a dissipative material. The dissipation of static charges may be accomplished by several means: across the surface, by volumetric conduction through a relatively thin high resistivity skin, across a subcutaneous conductive or dissipative layer and out of the film again through the thin high resistivity skin. Metal laminates actually fool the static decay test. The reason for their seemingly instantaneous dissipation of charges is that the static decay time is dominated by voltage suppressed by the ground plane being capacitively coupled to the metal layer. For metal laminates with surface resistivities less than 1E12 Ohms/square, one can assume that the surface charges decay in less than 2 seconds. For those layers with resistivities above this level, no assumption can be made from the results of the static decay test as described in Method 4046.1 The application of laminations or coextrusion technology to ESD materials allows the separation of the parameters of surface resistivity and static decay for laminates that do not contain metal or metallized layers. Coextrusion is the process of forming multi-layer materials from several extruders directly out of a one special die. In a coextruded material, each layer can be made from different base polymers or a blend of polymers, each selected for design attributes such as moisture barrier, flame retardancy, sealability, stiffness or strength. Multi-ply materials allow an ESD protective packaging user to weigh the individual benefits of all material attributes without them being totally dependent on one another. Users of ESD protective materials need to weigh the cost/benefit parameters in selecting appropriate materials for specific applications. Probably no material can ever meet all the requirements of the ESD world. It would be nice to have a heat sealable ,transparent aluminum foil with a surface resistivity of 1E8 Ohms/square and a volume resistivity of 1E11 Ohms-cm and have an attenuation of at least 120 dB to all electromagnetic frequencies. However, this material is a dream. It is called: UNOBTAINIUM

Surface Resistivity and Triboelectrification: Triboelectric charge generation by plastic packaging materials is widely believed to be dependent on the surface resistivity of the materials in question. If a material has a low resistivity it is sometimes regarded as having a low propensity for charge generation. Surface resistivity and charge generation can not be correlated. However, the belief of a relation of these two parameters persists. For a material to be "antistatic" it must have a low propensity to generate triboelectric charges. As the following charts show, earlier surface resistivity scales listed an antistatic category. Presently the EIA, ESD Association and Military specifications have dropped any reference to such a relationship. Current standards recognize only three basic resistivities for nonnshielding materials: CONDUCTIVE, DISSIPATIVE, INSULATIVE Triboelectricity is "a positive or negative charge which is generated by friction. " Triboelectricity is from the Greek, Tribein , which means: "to rub." On the other hand, "contact charge" is the positive or negative charge generated by first the contact and then separation of two materials. Typically, in ESD work, these two mechanisms are lumped together in the term triboelectrification or just tribo. Early electrostatic work placed a great deal of emphasis on the relative position of materials in a tribo series. The relative polarity of charge acquired on contact between any material in the series with another was predicted by its location. There is little correlation between the series developed by different researchers due to the very complex nature of the triboelectrification process. The question of whether or not materials at the positive end will always charge positive when rubbed with or contacted by materials lower in the series is not clear. If electron transfer was the only mechanism for charging, at least for certain material combinations, then such a series would certainly exist. However, instead of a uniform series of materials, some "rings" have been shown to exist. Silk charges glass negatively and glass charges zinc negatively, but zinc charges silk negatively. This is the case even though glass is higher than silk and silk is higher than zinc in most tribo series. One may not rely totally on a tribo series to determine the polarity of the charge for the contacting or rubbing together of two materials. No tribo series may be used to determine the actual quantity of charge resulting from the contacting or rubbing together of two materials. The mechanisms for determining the quantity of charge transfer are extremely complex. Surface resistivity does not play a role in the tribelectrification process. It does however, contribute to the material's ability to bleed off any charge which has been transferred. Materials with surface resistivities in the static dissipative range will not retain static charges accumulated by tribocharging if those materials are grounded. A total packaging system must be designed, one should not just chose a material which is "anti- static". All the requirements of the application must be taken into account before a material can be chosen. The user and the manufacturer must work together to design an appropriate static dissipative and low tribo generating packaging system.

New Packaging Systems

Shrink Packaging: Shrink films are usually thin biaxially oriented films which shrink back to their original size when exposed to relatively high temperatures. They can provide 40% - 50% shrink characteristics. They are used in consumer packaging to bundle items or to provide tamper evident viewable packages. Despite several unique advantages for packaging electronics parts, subassemblies and finished goods that include overall package cost effectiveness, tamper evidence and immobilizing parts, antistatic shrink film has been generally considered novel in this industry. One reason for the novelty is that, while general consumer shrink films are well known, a genuine antistatic shrink film is a relatively recent development. In the late 1980's Cryovac pioneered the first anti-static (dissipative?) shrink film. The Cryovac shrink film was a three layer coextruded film with clean surfaces made of unloaded and uncoated EPC (ethylene Propylene Copolymer). This film used a dissipative core to provide a film with excellent surface cleanliness and shrink properties combined with suitable ESD properties. Even though its surface resistivity was on the order of 1E13 Ohms/square, it had good static decay and tribo properties. Several companies began to use the film using L-bar sealers and shrink tunnels to automate their packaging. Cryovac withdrew from the ESD market taking its EP films out of production. As of the timing of this symposium, two companies have entered the market with good replacement films and in some cases with better properties. The shrink film provides the ESD event wall against which any static discharges will occur when the product is handled. It is by its function separated from the product by some distance providing the air gap. In laboratory tests, an air gap of 1/8th inch or greater provides very good "shielding" attenuation to ESD events. With good package design, the use of shrink films can show effective costs savings in an automated packaging system. The use of air gaps in boxes and thermoformed packages is also effective as an attenuation technique or shield. Anytime the effective distance to a packaged product from the point of an ESD event can be increased, the protection goes up essentially by the square of the distance. This concept can be designed into any package whether it is retail or industrial. The idea of not always using metal shields may be uncomfortable with some end users. However, it is similar to one wearing a bullet proof vest all the time because he or she enjoys being in a hail of bullets. It is much better to put a little distance from the shooter and the body. No matter how good the vest, it always hurts. ESD shrink film has been shown to be an effective packaging system suitable for electronics components, subassemblies and finished products. For chips tested under lab and field conditions, the antistatic shrink films' static protection compares favorably with other well known materials. The costs are very favorable compared to conventional static protection films and bags. It has been shown that a few cents of shrink film automatically or semi-automatically applied can replace several tens of cents of shielding bags hand packaged.

Air Cushioning/ Static Shielding/ Moisture Barrier /Desiccating Package: A recent development led to a protective packaging system which uses air channels surrounding the product to cushion and protect it, provide electrostatic shielding and moisture barrier both with and without desiccants. The system as it is conceived does away with the individual components used in the normal combination packaging of integrated circuits. Physical cushioning is provided by the air channels surrounding the product. Moisture barrier is provided by the multilayer materials in conjunction with the dry air in the channels. Since the air gap provides the majority of the electrostatic shielding, the reuse of the system does not degrade its efficiency.