The heightened sensitivity of electronic products to ESD is a challenge for manufacturers, especially those that need the low-level particulate environment offered by clean rooms. To get the best packaging at a reasonable cost, they are balancing the need for static-control with the requirements dictated by an ultra-clean area.
Guidance for selecting the right ESD-control totes/bins/packaging to handle and transport products in a clean room, however, is sparse. There is information that covers specific handling issues, but nothing that addresses all the needs for controlling and handling static-sensitive products in a clean room. The Institute of Environmental Science (IES), for example, offers help with the particulate control and measurement of products in a clean room, but not with the ESD-control issues.
In an effort to remedy this lack of information, the ESD Association Standards Group is working on a clean-room advisory document related to all aspects of electrostatic control inside clean rooms. It may be released in draft form late this year.
The elements of this document cover all aspects from construction of a clean room through the use of different equipment inside it. The document will provide guidelines on the electrostatic considerations that must be accounted for before using materials in a clean room. It also will give information about how to merge electrostatic control and clean-room requirements.
Merging is not easy because the two concepts are very divergent. Clean-room requirements take precedence and you may have to give up dissipative or shielding characteristics because the material may be appropriate for a clean room but not for ESD-sensitive products.
Electrostatic Protected Area
If your clean room is classified as an electrostatic-protected area (EPA), the workstations, equipment, personnel, and most of the material must be connected to ground. This means there is little charge generation and sensitive parts can be moved within the EPA without electrostatic shielding. Materials used in an EPA must have low-charge generation characteristics or static- dissipative properties.
If carbon can be avoided because of problems with sloughing, many options for containers are available in an EPA clean room. However, carbon provides a higher degree of conductivity, and, if properly designed and manufactured for use in a clean room, totes and bins containing carbon can qualify as static shielding. Static- shielding containers, fortunately, are not necessary in a properly constructed EPA.
Problems are inevitable if the clean room is not an EPA, and ESD-sensitive parts are handled. Such a facility violates most of the acceptable handling practices and standards, including EIA 625 Requirements for Handling Electrostatic Discharge Sensitive Devices; EN 100015-1 Protection of Electrostatic-Sensitive Devices, Part 1: Minimum Requirements; IEC 47 (SEC) 1330 Standard to Protect Static Sensitive Devices, and the pending IEC 1340-5-1 Technical Report for Protection of Electronic Devices from Electrostatic Phenomena, Section 1: General Requirements.
A non-EPA facility requires the use of shielding containers to transport sensitive parts between stations that have electrostatic control. If adequate control is not present, it is impossible to handle the parts within the clean room and still meet the ESD-control practices, regardless of the packaging materials.
An Industry Solution
Some companies and industries devise their own requirements for handling and moving products in a clean room. For example, the disk-drive industry does much of the assembly and production in clean rooms, and they need safe handling procedures for their products. That includes personnel and their garments, static-sensitive devices, and the flooring.
The disk-drive manufacturers found that totes and bins used in production must be clean-room-compatible and ESD-safe. Because of the additives used in ESD-control products, this is not an easy task.
Packaging materials have chemicals added to the plastic, paper, or polymeric structure to impart the dissipative or antistatic property, which may not be appropriate for a clean room. Consequently, disk-drive manufacturers, concerned about outgassing characteristics or collected volatile condensable material levels, have borrowed test procedures from different industries and the military.
For example, ASTM E595 is the principal standard for volatile condensable materials. MIL-STD-1246C Product Cleanliness Levels and Contamination Control Programs also has helpful information about clean rooms.
Almost everything in the disk-drive industry is done on a conveyor system in a basic clean room. A cleaning tunnel sits over the top of the conveyor system and the workers sit along side the conveyor. This lets them do their work in a cleaner area than the rest of the room.
Basically, trays are the containers used in this industry. There are several different types of tray materials. For the bulk of the operation, the tray does not need static-control properties. It’s just a plastic molded tray with no requirement except to be the right shape and size to be stacked upon one another.
The trays used later in the assembly, when the parts are not sensitive to static, often are insulative. But, in the front end of the process, some companies use a conductive-type tray of the same shape and size as the product.
For example, conductive trays are used when attaching the read-write element to the head gimble assembly of the magnetoresistive and the giant magnetoresistive (MR/GMR) heads. The MR/GMR heads are some of the most electrostatically sensitive parts made today, and this is the most critical part of the process of building the read-write head.
The MR/GMR parts are sensitive to 10 nJ. According to the human-body model discharge, 10 nJ is equivalent to about 14 V. That does not mean that a person carrying a 14-V charge will damage the part. It means that the human- body model electronic equivalent test method shows that a 14-V level is needed to generate 10 nJ.
A person touching the part must have about 100 to 150 V to get that same kind of discharge because of resistance differences in a discharge. As a result, users must be grounded through their feet or wrist straps.
The choice of materials for reducing electric fields in the disk-drive environment is important, and manufacturers must prevent induced fields to grounded conductors from occurring. Ionization is useful for controlling these induced charges.
A number of the companies manufacturing disk-drive parts use conductive trays at least in the static-sensitive areas. Some drive manufacturers use the intrinsically conductive polymers based on polyanaline. These rigid plastic materials are quite expensive to process, but they are useful in static-sensitive areas. This is the first industry where intrinsically conductive polymers have been used commercially.
Clean-Room Packaging
The selection criteria for packaging is based on the type of activity in the clean room. Several companies use a classification system to approve products ahead of time:
Class 3: a material or item never can go into the clean room because it does not meet the requirements. If you are an electronics manufacturer, you don’t want your products ranked as Class 3.
Class 2: a product or item is acceptable for use in a clean room but must be cleaned by the user in compliance with the recommendations of the supplier. Cleaning is required before the user can put the packaging into a Class 1 environment. Many companies buy Class 2 packaging, particularly those in the disk-drive industry, because normally they wash and clean their products before they go into the clean room.
A common measurement technique used for packaging blows off particulate with ultra-pure compressed gasses in a hood and then measures the removed particles with a particle-counter technique. This process is continued until the particle count drops below a company-determined specification.
Particle counting also can be performed by washing the product and filtering the water to catch particles on a filter paper. Then the number of particles is determined with a liquid particle-counting technique.
Class 1: the manufacturer washes the product and provides the item in a double package. The outer trays, such as those used in the disk-drive industry, come in a sealed bag. The inside of the bag and the item itself are clean to the required level. The product is provided with an outer wrap that is removed at the staging area prior to entry into the clean room.
ESD-control techniques are not necessary when moving a tray in and out of the bag. They arrive in a cleaned package. The outer package is the same material as the inner package, but it does not have to be clean on the outside. The inside package as well as the item inside the inner package must be clean.
Today, more products are made in clean rooms because electronics industry requirements have become more stringent. At a recent conference, the IES conducted a panel discussion dealing with clean-room specifications. The panel members called it the tail-wagging-the-dog review because the technology for measuring particulate has progressed so far that users automatically specify the particulate levels lower and lower as the measurement technique improves. And that’s not necessarily appropriate.
What really is necessary is the question that must be answered for each situation. The bottom line is determining what constitutes clean enough.
Acknowledgment
Information for this article was gathered from conversations with Dave Swenson, manager of applications development and technical service at 3M, Electronic Handling and Protection Division.
Copyright 1998 Nelson Publishing Inc.
March 1998