When it comes to determining the ESD effectiveness of packaging, you could use one of many different test methods. The choice depends on whether the package is rigid or flexible.
Melissa Siems, product manager at ORBIS, explained that the company tests materials for its totes for surface and volume resistivity according to the latest standards. But, she added, there are no ESD standards for finished totes. ORBIS does, however, test its products for mechanical strength.
The standard used most for testing the materials that go into ESD totes, bins, and packaging is EIA 541—Packaging Material Standards for ESD Sensitive Items. According to Dave Swenson, applications development manager at 3M Electronic Handling & Protection Division, this standard is 10 years old, and the test methods for surface resistivity, volume resistivity, and triboelectric testing need to be revised. Of these, triboelectric testing appears to be the most difficult to redefine.
A key issue in the battle against ESD is the prevention of charge, and the two primary generators are tribocharging and induction charging. Even though the triboelectric effect has been known for some time, the history of testing for this effect has revealed difficulties in getting reproducible, repeatable, and relevant results. To overcome some of these problems, testing techniques have been researched at many companies, including 3M.2
Triboelectric Testing Difficulties
In the triboelectric testing of bags, EIA-541 calls for a quartz or Teflon® disk to be placed in the bag, shaken, and removed. Then the charge on the disk is measured. Quartz and Teflon were chosen because they represent opposite ends in the triboelectric series classification. Various disk removal methods have been tried since some methods create additional contact with the disk which alters its charge.
The disk removal problem was overcome by using the inclined-plane method developed by James Huntsman of 3M. To measure the charge, a flat strip of the material to be tested is placed on an inclined plane and a small cylinder is rolled down it into a Faraday cup (Figure 1).
The results of this method were promising so it was included in EIA-541. However, 3M found that brass really was the most reliable in distinguishing between antistatic materials and conventional non-ESD film materials, and that neither quartz nor Teflon could reliably make the distinction between these materials.
A motorized version of the inclined-plane method gave repeatable and reproducible results using quartz and Teflon cylinders, but it was tedious.3 The cylinders must be heat-treated, then scrupulously cleaned and neutralized with ionized air between runs.
The revision of EIA-541 has been under consideration for more than two years. Mr. Swenson said that the EIA committee is working with the ESD Association to revise the test methods.
Movement also has been slow in developing new triboelectric test methods because of the difficulty in specifying the contact materials. Also, the correlation between triboelectric charging and the destruction of parts has not been clearly established. To date, testing at 3M shows that it takes more charge to destroy parts than previously thought, and the charged device model (CDM) specified today does not reflect actual handling conditions.
The inclined plane method works with flat strips of material. But it would not be practical to test finished products because they typically are not flat and are comprised of different shapes.
So with that drawback in mind, can you assume that testing raw material is sufficient for characterizing the product? That depends. During thermoforming of the finished product, hot spots can occur if carbon-loaded material congregates in one area. These hot spots could drain charge off too quickly.
If you need to know the actual triboelectric behavior of a packaging product, you may have to simulate the related operating and handling environments of the part being protected. For example, Mr. Swenson pointed out that years ago 3M researchers simulated the environment that a protectively packaged static-sensitive product was subjected to during the shipping process. This included driving a truck around a parking lot with the actual packages inside and then measuring the charge on the products. Today, he said, computers simulate the truck’s motion on a shaker table.
Due to the importance of charge prevention, better triboelectric test methods need to be developed. They should provide reliable results using actual materials involved in the contact process under investigation.
Induction Charging
Charging also can take place in the presence of an external field, such as another charged part. Should a positive field come near an unshielded item, the object will become polarized with its negative side facing the positive field.
The time it takes to polarize is related to the mobility of charges within the object. If the field is removed, the charges will migrate to an electrically neutral condition. However, the object will charge if a conductive path to ground exists while it is polarized. This is charging by induction.
With packaging, you also can prevent induction charging by completely enclosing the object in conductive material. This creates an electrostatic shield which can sustain the field on the outside surface but not affect the object inside. In packaging, a conductive layer usually is inserted between more resistive layers to protect the part from direct electrification or rapid discharge.
Safe Discharge
The other half of the battle against ESD is safe discharge, which is why static-dissipative material is used. It has higher resistance than conductive material and limits the discharge current to a safe level.
Most ESD totes, bins, and packaging use antistatic materials for preventing a charge, conductive materials for grounding or shielding, and static-dissipative materials for safe charge dissipation. The packaging we choose for static-sensitive devices should help us win both ends of the battle by preventing sensitive devices from charging or safely discharging them. Sometimes, antistatic compounds or coatings are added to static-dissipative materials to create an ideal combination.
Flexible packaging, like bags, may use inner and outer layers of static-dissipative material for safe discharge and a middle layer of conductive material as a shield against charging by induction. The inner layer also should be antistatic to limit charge generation by contact with the part.
Rigid packaging can provide air-gap separation from the static-sensitive component to the surface of the package to maintain a distance to external fields and reduce their effects. Bags with shielding or rigid packages with an air gap could protect components from a nearby high-voltage discharge, such as specified in IEC/EN61000-4-2—ESD Immunity Test, 1996.
Parts Handling
In all packaging processes, parts must be handled by people or machines. To avoid charging, either by contact or induction, you must handle static-sensitive devices properly. Here are some relevant standards for correct handling:
EIA 625—Requirements for Handling Electrostatic-Discharge-Sensitive Devices, 1994, (with a 1998 release due soon).
EN100015/1—Protection of Electrostatic-Sensitive Devices, 1992.
IEC 61340-5-1—Technical Report for the Protection of Devices From Electrostatic Phenomena.
References
1. Danglemayer, F., ESD Program Management: A Realistic Approach to Continuous Measurable Improvement in Static Control, 1990, p. 245.
2. Swenson, D. and Gibson, R., “Triboelectric Testing of Packaging Materials Practical Considerations,” EOS/ESD Symposium, 1992, pp. 209-217.
3. Börjesson, A., “A Method for Measurement of Triboelectric Charging,” EOS/ESD Symposium, 1995, pp. 253-261.
Totes/Bins/Packaging
Moisture Barrier Bag
The 3M 3300 Moisture Barrier Bag, featuring a multilayer 3.6-mil film, provides long-term moisture protection in a package which is puncture- and tear-resistant. The inner and outer surfaces are static-dissipative; the center layer is conductive. This combination shields against both ESD and EMI. The bag passes specifications for mechanical strength, water-vapor transfer, heat-seal strength, polycarbonate compatibility, resistivity, and shielding. 3M Electronic Handling & Protection Division, (512) 984-2146.
Injection-Molded Polymer
Stat-Rite™ M-200 Material is used in medical and semiconductor device packaging and electronic component handling. This opaque polypropylene alloy contains Stat-Rite® C-2300 static-dissipative polymer and provides humidity-independent, permanent ESD protection without particulate or outgas contaminants. It features high strength and flexibility and is easily injection molded into complex packaging containers. The material has surface and volume resistivities of 2 × 1011 W /sq and 6 × 1011 W -cm, respectively. BFGoodrich, (800) 331-1144.
Dissipative Stack Trays
TEK-TRAYS with a Steel Stacking Frame are used for handling and transporting static-sensitive assemblies and may be stacked or locked together for safety. Made from the company’s PROTEKTIVE PAK material, they have steel wire frames and nesting corners and permanently static-dissipative foam laminated to the bottom. The trays do not require assembly and are available in conductive corrugated plastic and two standard as well as custom sizes. Brick Container, (714) 994-4140.
Reusable Totes/Dividers
Heavy-duty, reusable CONTRIM® Tote Boxes prevent ESD during in-process material handling, storage, and shipping. A telescoping lid accommodates stacking, and removable dividers optimize partitioning. The permanently conductive polyethylene boxes are available in a range of sizes. They resist PCB cleaning solutions and have a maximum surface resistivity of 5 × 104 W /sq. The tensile strength is 2,750 psi with an impact resistance of 8.8 ft-lb/in. Crystal-X, (800) 255-1160.
High-Density Storage System
The Rototower™ Conductive Storage System holds up to 16 of the company’s cabinets, each of which can contain as many as 60 drawers. The unit is 70″ tall and occupies just 3 sq. ft. of floor space. A rotating base provides access to all stored components. A conductive plastic or metal frame is available. Flambeau, (800) 344-5716.
SMD Chip Carrier System
A selection of static-dissipative carrier tapes, cover films, and plastic reels can be used to package MSOP, TSSOP, BGA, and TSOP ICs. Carrier tapes combine pocket-bottom pedestals and antireflective sidewalls with a class 6 trilayer styrene. Cover films are made of transparent polyester heat-sealable material which bonds to the carrier tape. High-impact polystyrene reels are offered in 7″ and 13″ diameters with widths of 8 mm to 72 mm. ITW Electronic Component Packaging, (817) 277-7977.
Clear, Dissipative Sheets
Pentatstat™ SC660/05C Clear, Static-Dissipative PVC Sheet is ULV 94 V-O rated and meets MIL-B-81705-C and EIA-541. This noncontaminating and nontoxic material complies with Bellcore GR-1421-CORE and exceeds the re-use requirements after 12 shipping cycles. The polycarbonate-compatible sheeting is available in 7.5- to 35-mil thickness and can be formed into ESD packaging, such as clamshells. Klöckner Pentaplast, (540) 832-3600.
Composite Material
FibreStat 2000 Composite Material provides permanent ESD properties to a variety of trays, boxes, and pallets used in conveyor applications during electronic assembly. Surface resistivities are available in the static-dissipative and conductive ranges. Products made from this material have dimensional stability and are resistant to solder and mild chemicals and unaffected by humidity. The continuous operating temperature is -60°F to +250°F without distortion, creep, or degradation. Trays and boxes may be cleaned in hot water or steam and common industrial detergents. Molded Fiber Glass Tray, (814) 683-4500.
Stackable Trays
A recyclable packaging system using PETG trays with interchangeable bases or lids is suitable for automated assembly. The transparent trays are reusable 18 to 25 times. A ring and button feature ensures stable stacking on pallets. Plastofilm, (630) 668-2838.
Moisture Barrier Bag
SCC Dri-Shield 2500™ Bags protect GMR/MR heads and disk drive assemblies from ESD, RF induced electrical transients, and moisture-caused corrosion. This metal-out, static-safe, moisture barrier bag has a semiconductive exterior that prevents charge retention. Multiple layers reduce charge penetration to less than 2 nJ, and interior tribocharging is limited by a humidity-independent polymer. Moisture is minimized by a 0.02gram/100in.2/24 h transmission rate. The bags are dated and lot coded for product traceability from raw materials to finished goods. Static Control Components, (800) 356-2728.
Conductive Foam
A conductive foam, available in high and low density, can fit the exact shape of a sensitive electronic component. Resilient enough for repeated use, it prevents mechanical or electrical damage to parts during handling or transporting. The foam comes in standard 6-mm to 25-mm formats and other sizes for ICs and small components. Other applications also can be accommodated. Teknis, (011) 441823 481248.
Clean-Room Storage
The Kiticcator™ is a transparent storage system for use in clean rooms. It features dual chrome-plated racks to accommodate slide-out bins and tote boxes. Made of permanently static-dissipative PVC, the surfaces are easy to clean, and there is no outgassing. In this air-tight environment, gas controls provide the desired relative humidity and variable nitrogen purging, and dessicators are not necessary. Materials are protected from contamination even with the access door open. Terra Universal, (714) 526-0100.
Protective Package
Static Intercept™ Protective Packaging allows you to ship static-sensitive parts without damaging delicate leads on Gullwing, J-lead, surface-mount, flat packs, and PGA ICs. The permanently dissipative plastic corrugate does not outgas, is unaffected by humidity, and tolerates 125°F. Packages are reuseable, recyclable, and biodegradable and come in single or multiple packs. The material’s anticorrosion property also makes it suitable for the long-term storage of electronic parts. L. Gordon Packaging, (410) 308-2202.
Copyright 1999 Nelson Publishing Inc.
March 1999