The principal cause of static electricity is frequently cited as the movement of people and materials in the work environment. This routine movement, particularly the contact and separation of shoe from floor, generates charges as high as several thousand volts. Similarly, the movement of mobile carts or other equipment will generate electrostatic charge. This subclause will review the use of floor materials to dissipate electrostatic charge. It will cover floor coverings, floor finishes, topical anti-stats, floor mats, paints and coatings.
Static protective floor materials are used in the electronics industry for:
Grounding of personnel (floor materials can be used in combination with static control footwear as either a primary ground or a secondary, backup system, ground for wrist straps in critical ESD processes)
Grounding of ESD control items such as:
ergonomic stands used to raise the product to an employee’s working height
Floor coverings, mats, paints and coatings help control static charge by providing a path of moderate electrical conductivity from the human body or ESD control item to ground. Many flooring products use a conductive material, such as carbon, metal or other additives, that extends from the surface of the material to an underlying substrate such as conductive adhesive. The floor material is then connected directly to ground.
Floor finishes and topical anti-stats, on the other hand, function by two separate mechanisms.
First, they reduce the surface's tendency to generate a static charge. Second, they provide a path for the dissipation of charge. If the floor finish or topical anti-stat is used for primary grounding they shall be able to limit charging while dissipating it to ground.
Grounding through the floor is dependent on the type of footwear that is in contact with the floor. Typical street shoes or industrial footwear with rubber, crepe, or polyurethane soles insulate the wearer from the floor. Generated charges cannot readily flow from the body, through the insulated shoe sole to the floor material to ground.
Studies of ESD control floor materials indicate that charge generation and charge decay times as measured on a person's body vary with the type of footwear worn. The levels of performance depend upon the combination of floor material and footwear. The proper selection of footwear is critical to the performance of static protective floor materials.
The use of ESD floor (Semi Conductive) materials to control personnel or equipment generated static has a number of benefits. Floor materials tend to be passive. Employees who work in areas protected with floor materials simply need to wear and test the appropriate footwear. They do not need to implement any additional actions themselves to assure that the floor material is functioning properly.
ESD Floor materials, particularly floor coverings, finishes, and coatings, can be applied or installed throughout the work environment providing a broad area of control rather than isolated control at individual workstations. They improve the mobility of personnel who work in the environment. Finally, floor materials can help control static on trolleys or other mobile equipment if properly grounded.
ESD-Floor materials also have some limitations. When used to ground people, the person shall maintain contact with the ESD control floor. In order to do this the person must be standing.
This is the primary reason that IEC 61340-5-1 requires a wrist strap for seated operations.
The use of floor materials may be limited by installation considerations. For example, concrete floors may contain excessive amounts of moisture which may restrict the installation of resilient floor coverings. ESD control carpets might not be practical for processes where there is excessive water spillage.
Some static protective flooring materials cannot withstand the weight of heavy vehicles such as forklift trucks.
Some materials may be restricted from certain applications in the facility due to process considerations. An example could be a floor finish that contributes contaminants to the environment and would not be usable in a clean room.
Excessive amounts of dirt on a floor material can have a negative effect on the performance of the floor material. It is important that floor materials be cleaned on a regular basis. It is also important to ensure that the proper cleaning procedures and products are used to ensure that the performance of the floor is not compromised.
It is important to consider the entire process when selecting static protective flooring.
Floor material options can generally be classified into permanent and semi-permanent or non-permanent materials. Some of the major advantages and disadvantages of each material type are discussed below.
Permanent materials are broadly defined as floor coverings, including rubber or vinyl tile and sheet goods, epoxy coatings, high-pressure laminates, and carpet. As a group, these materials have an extended use life and provide protection over a broad physical area.
Resilient floor coverings are the most frequently used permanent floor materials. The material composition is usually rubber, vinyl, or vinyl composition. Material form can be either tile or sheet. Resistance ranges are typically from 2,5 × 104 Ù to 1,0 × 109 Ù. Various patterns, colours and sizes are available. Resilient floor coverings offer attractive appearance and resistance to many commonly used chemicals. Most materials can be welded and self-coved for seamless installation in clean rooms. They can be applied to raised floors.
Vinyl floor materials may require more maintenance than other permanent flooring alternatives; rubber flooring requires somewhat less maintenance than vinyl. Some resilient floors may be slippery, particularly when wet. They may be adversely affected by heavy vehicular traffic. The presence of carbon in some of these materials may restrict their use in some clean room applications, although the abrasion resistance of these materials is quite good. Additionally, vinyl flooring systems may outgas and may not be applicable in some clean rooms.
Generally a poured permanent flooring material, these products are frequently 3mm or more in thickness, but also can be installed in a thinner gauge (see 126.96.36.199.2.4). They have good chemical, solder, and abrasion resistance and will withstand heavy vehicle traffic. They are easier to maintain in comparison to other materials. They are seamless and can be used in many clean room environments. However, they cannot be used on raised floor panels.
Because epoxies are virtually manufactured on-site, proper installation techniques by experienced installers are critical to the successful performance of this type of material.
Similar to the popular work surface materials, these products are normally limited to raised floors or as floor mats. Laminates tend to be moisture sensitive and should not be used in areas of high chemical or water spillage or directly on concrete sub-floors that may be subject to high levels of moisture. Changes in humidity may change the resistance characteristics of these materials. High-pressure laminates also lack the physical flexibility to be installed on most standard sub-floors.
Carpeting is available in resistance ranges of 2,5 × 104 Ù to 1,0 × 109 Ù. It has aesthetic and acoustical benefits and is a morale booster to employees. Maintenance costs tend to be lower on carpeting than on resilient floor materials. In the form of carpet tiles, it can be used on raised floor panels. Carpet, however, is not well suited for use in area's subject to excessive soiling, water and chemical spills, exposure to significant amounts of hot solder, heavy vehicular traffic or in clean rooms.
The second group of floor materials is described as semi-permanent or non-permanent and includes mats, floor finishes, topical anti-stats, and paints and coatings. Their life expectancy is less than that for permanent materials and they require periodic re-treatment or replacement. The single most striking characteristic of these materials is their flexibility and ease of use.
Floor mats are available in a variety of types and styles ranging from soft dissipative mats to hard conductive mats. Their portability and ease of use provide for customization and flexibility of work space design, particularly when ESD control requires protection in limited areas. Easy replacement of mats allows their use around wave solder machines or other equipment where major chemical spillage can damage many floor materials. However, mats tend to curl, create tripping hazards and complicate floor maintenance. They are also expensive if static control flooring is required throughout a complete area. Their applicability in clean rooms is limited due to contamination of the clean environment. Additional attention is required to assure a mat is continuously connected to ground.
Static-limiting floor finishes may be applied to a variety of non-ESD control floors: standard vinyl, rubber, or vinyl composition tile, or existing static control flooring, to reduce the generation of electrostatic charges. As a floor finish, they protect and they improve the appearance of the floor, and make floor maintenance easier. Floor finishes have flexibility of use, can be applied throughout a facility, and can provide whole area protection. These materials also have some disadvantages:
Some surfactant type finishes may be too slippery and pose a hazard to employees.
Some finishes are subject to being washed off with ordinary water. Some finishes can be worn away readily and they may require additional, frequent monitoring to make sure that the finish is still working.
Improper application and maintenance may cause inconsistent performance of the finish.
Some floor finishes may be incompatible with clean room requirements.
Topical anti-stats are similar in function to floor finishes, but do not provide physical protection to the floor material itself. They can also be used on carpet. Topical anti-stats are relatively easy to apply; however, they lack permanency and durability.
Paints and epoxy coatings are applied to concrete floors in thin coats. The primary advantages of these materials are their ease of application and coverage over a wide area. They have a longer usable life than do floor finishes, but less than permanent floor materials. Paints and coatings tend to wear off in time and shall be reapplied on a continuing basis. Some materials are not applicable for clean rooms because they abrade or chip away or are highly loaded with carbon.
The standard for testing the electrical resistance of floor materials is IEC 61340-4-1. The test method is designed to operate in the range of 1,0 x 103 Ù to 1,0 x 1010 Ù. IEC 61340-5-1 requires that the maximum test voltage used for flooring systems used as part of an ESD control program shall not exceed 100 V.
When first installed, the floor materials should be tested to make certain that they are functional and meet specifications. Typically, the resistance from the top of the floor material to ground is measured to ensure that the floor material has been properly installed. Verifying that the floor material is connected to ground should be done before ESD sensitive devices are handled.
Periodic testing of floor materials is necessary to ensure that they continue to meet specifications. Resistance-to-ground measurements are typically used to verify that the path to ground is intact. In cases where the resistance to ground measurement exceeds the established resistance limits, the following steps can be taken to identify the cause of the high resistance readings:
Verify visually that the floor material is connected to the ground reference.
Clean the surface of the floor material. Sometimes a dirty surface can cause the resistance to exceed acceptable limits. Once the surface has been cleaned (note: clean the bottom of the resistance measuring electrode as well) repeat the resistance to ground measurement. If the second measurement is within specification this might lead to a further investigation concerning the cleaning practices used by the organization. The frequency of periodic testing is normally specified in corporate operating procedures.
However, a common guide would be to conduct these measurements at least once every three months.