A Recipe for the Best ESD-Safe Container

At first glance, picturing the right tote, bin or package to protect your ESD-sensitive components appears to be easy. Simply select the one that looks like it has the right shape, shows the right ESD control specs and has the right price. This may work if your are lucky—but there is a better recipe to zero in on the best tote, bin or package.

Start with an understanding of the needs of your product, including its failure mechanism. The basic failure mechanisms that damage an ESD-sensitive device are:

· The Human Body Model (HBM) discharge.

· The Machine Model (MM) discharge.

· The Field Induced Model (FIM).

· The Charged Device Model (CDM).

The HBM is used to test devices for susceptibility to damage from a statically charged human body. It is a commonly used parameter for classifying a product’s sensitivity to ESD.

The HBM test charges a 100-pF capacitor to a known voltage and discharges it through a 1,500-W resistor to one of the sensitive device leads while another product pin is connected to ground. The discharge voltage is increased until the product’s functional characteristics are out of the stated specification range. The EOS/ESD Standard 5.1-1993 RevisedElectrostatic Discharge Sensitivity Testing – Human Body Model provides the appropriate test procedure to evaluate the ESD sensitivity of components.

The MM is used when an ESD-sensitive device comes into contact with a charged metal object such as an ungrounded tool. Due to the low resistance of the metal surface, the ESD failure level could be significantly lower than that produced by the HBM wave form.

The equivalent circuit for the MM uses a 200-pF capacitor and a 500-nH inductor that discharges directly into a device with no series resistor. The test is performed by increasing the discharge voltage until the device no longer operates within its functional specifications. The EOS/ESD DS5.2-1996Electrostatic Discharge Sensitivity Testing – Machine Model provides the appropriate test procedure that simulates an ESD event occurring from a low resistance source.

The FIM occurs when a product is subjected to a static field and is subsequently grounded while the device remains in the static field. This results in a high-current discharge from the device to ground.

Some voltage-sensitive devices also can be damaged when subjected to a rapidly changing electrostatic field. This type of damage occurs if a high static field is moved quickly past devices that are not adequately packaged. For example, a device sensitive to fields that is placed next to a pallet wrapped with static-generating plastic could be damaged when the plastic wrap is cut away and removed quickly.

The CDM describes tribocharging of an ESD-sensitive device due to an interaction with the packaging media. When the device is removed from its packaging and comes into contact with a conductive object, such as a grounded work surface, the device can be rapidly discharged, causing an ESD event to occur. The recently approved draft standard ESD DS5.3-1996 Electrostatic Discharge Sensitivity TestingCharged Device Model establishes a test method for evaluating the ESD sensitivity of components and provides a system for classifying the sensitivity of these components.

Tribocharging occurs in some types of shipping tubes that use a chemical coating. If the coating is ineffective or wears away, the ESD-sensitive part can become charged during transport or from sliding down the tube.1

Know Your Environment

To select the proper packaging, you also must find out to what environment the device will be subjected. To find out what your packaging criteria are, said Michael Stejskal, marketing manager at Bradford, ask these questions:

· Are the containers handled within the facility or between plants?

· Are parts shipped one way or is a returnable program required?

· Are containers handled manually or by conveyor systems?

· Is individual product protection or protective foam required?

Other points to consider before selecting an ESD tote/bin/package include the final destination of the product and if it is an ESD-controlled facility that provides wrist straps, ESD workstations and flooring. If it is an uncontrolled facility you also need to know if there are experienced and grounded personnel or inexperienced and ungrounded workers.

Material Selection

Selection for packaging materials, regardless of type or style, depends primarily on the static-sensitivity level of the products and where they are going, said David Swenson, global applications development manager at 3M Electrical Specialties. You should use static-discharge shielding materials if the complete distribution system is not known or understood.

The packaging characteristics should meet your product’s requirements for shielding attenuation, grounding decay rate and charge generation, said Brad Ahlm of Conductive Containers. The tote, bin or package also must meet your needs for corrosivity and recyclability and be backed up by testing performed at an independent lab.

The manufacturers of totes, bins and packaging typically reference the Electronics Industries Association Standard 541 Packaging Material Standards for ESD Sensitive Items in their product-specification sheet. It defines the various material properties including antistatic, shielding, conductive, dissipative and insulative and describes the environmental conditions required for testing materials. Another portion of the document supplies a series of test methods.

EIA-541 is scheduled for a rewrite this year to reflect upgraded test methods from the ESD Association, said Mr. Swenson. Other new standards are expected this year from the IEC including IEC 1340-5-1 and -2, Handling Practices for ESD Sensitive Electronic Parts. They describe the level of packaging required for various circumstances encountered when shipping products.

Design of the Package

When you have gathered enough information on your product’s ESD failure mechanism, the environment and tested materials, you are ready to describe a product-specific package to your manufacturer. If your product is sensitive to electrostatic fields and you are shipping entirely within an ESD-controlled facility, you need an antistatic or static-dissipative packaging material.

For a product that is shipped from one controlled facility to another with no controls between, use antistatic or static-shielding material. Components shipped from a controlled facility to an uncontrolled environment also need antistatic or static-shielding packaging with discharge shielding attributes.

Find out if the container manufacturer can provide the appropriate physical characteristics including size, strength, durability and shelf life, said Mr. Ahlm. The product may also need to be identified, so the packaging must accept print on the surface.

To increase the likelihood of success, work closely with your manufacturer and provide information about the size of your product and how static sensitive it is, said Marlene Friedman, ESD product manager at Crystal-X. How the product is moved around in the manufacturing process is also important.

Look for a manufacturer that has successfully introduced a returnable ESD container and can provide samples or drawings for a proper fit, said Paul Ramsay, vice president of marketing at Cymat. The packaging also must be compatible with your existing material handling systems and in-house storage systems.

Remember to Test

Before you purchase a tote, bin or package, get a sample to test to assure it meets quality standards such as surface resistivity, structural requirements and material appropriateness, said Mr. Stejskal. As part of incoming inspection, the purchased totes, bins or packaging also should be tested to ensure they meet the same standards.

Fortunately, test methods for packaging are becoming well defined, said Mr. Swenson. The new version of the EIA-541 will include ANSI ESD S11.11-1993 Surface Resistance Measurement of Static Dissipative Planar Materials and ESD DS11.12-1996 Volume Resistance Measurement of Static Dissipative Planar Material. Use these test procedures to make resistance measurements on dissipative materials.

Antistatic or low-charging materials need to be evaluated for limiting electrostatic charge generation in combination with your product, said Mr. Swenson. ESD ADV11.2-1995 Triboelectric Charge Accumulation Testing can provide assistance in devising appropriate test procedures to evaluate charge-generation characteristics of packaging materials.

Materials and packaging designed to provide static-discharge shielding should be tested according to ESD S11.31-1994 Evaluating the Performance of Static Discharge Shielding Bags, said Mr. Swenson. This test method is designed for bags, but modifications for boxes and other containers are developing. Equipment such as the 3M Static Event Detector may provide an indication of the static-discharge shielding afforded by containers and boxes.

It is also important to use visual checks to identify containers that need replacing, added Mr. Stejskal. Tape can damage the antistatic coatings of a box when it is removed from the package. Other checks that can be used include compression tests and shaker tests to determine the strength and durability of the totes and bins.

When the testing process indicates suitable results for your product, you can be confident you have the right packaging and material. Now you are ready to complete the recipe by finding the best price for the design and quantities you need.

Reference

1. Gibson, R., “Packaging Fundamentals for ESD Sensitive Products,” NEPCON West Proceedings, Anaheim, CA, 1994, pp. 1417-1427.

Totes/Bins/Packaging

Packaging Materials

Are 100% Recyclable

Strata-Shield ENV® is a 100% recyclable ESD packaging material that can be formed into partitions, totes and cartons. The material has an amine-free film featuring a permanently dissipative surface with a buried shielding layer. It is available in solid fiber with film on two sides or corrugated with film on one or both sides. Bradford, (616) 399-3000.

Corrugated Material Provides

Buried Carbon Shielding

Cortronic is a recyclable corrugated fiberboard which is either transparent or blue. A buried carbon shielding layer has a surface resistivity of <102 W /sq. Fiberboard surface resistivity is 106 W /sq to 108 W /sq. Conductive Containers, (612) 537-2090.

Conductive Tote Offers

Removable Dividers

The Heavy-Duty CONTRIM® Tote Box is made from a vacuum-formable and permanently conductive polyethylene material. It has a telescoping lid that allows stacking. The tote walls are fitted with custom slotting arrays so the box can be partitioned lengthwise with removable dividers. It has a tensile strength of 2,750 psi and an impact resistance of 8.8 ft-lb/in. Surface resistivity is 5 ´ 104 W /sq. Crystal-X, (800) 255-1160.

Carrier System Provides

Faraday Shield for Boards

The Circuit Board Transport System contains an aluminum carrier that provides physical protection for circuit boards. The static-dissipative cover provides a Faraday shield during shipment. The system has foam strips with a surface resistance of 104 W to prevent product scratches. Plastic inserts accommodate 10 circuit boards and provide a surface resistance from 105 to 109 W . The conductive chrome-plated transport system is equipped with clamping devices to secure the board carriers. Cymat Technologies, (905) 602-1100.

Containers Developed

For Custom Applications

A line of containers includes a hopper front, a straight-walled version, nestable or stackable types, trays, collapsible containers and custom styles. Customers may choose from ESD-safe materials such as fiberboard, plastic-coated fiberboard, high-density polyethylene, rigid PVC and molded plastic. Materials with conductive and antistatic properties are offered. Flexcon, (201) 467-3323.

Device Trays Meet

JEDEC Handling Requirements

Medium-temperature, multidevice packaging trays are offered for handling and shipping IC devices including PQFP, CQFP, PLCC, MQFP and PGA formats. The trays accommodate automatic handling and follow JEDEC dimension and tolerance outlines. Tray cells provide package containment to minimize device damage. The 140° C continuous-duty trays are made from a modified polysulfone with a conductive media of metalized glass or carbon fiber. 3M Electronic Products, (800) 328-0411.

Copyright 1997 Nelson Publishing Inc.

March 1997

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