What is the purpose of installing an antistatic floor? The most common answer to this question is: “We need an ESD floor to prevent static electricity from moving personnel when working on static sensitive components and systems.” wires and cord stops.
While this answer highlights a key attribute of a functioning ESD floor, it is of a very low standard. It also sells off many of the benefits that ESD floors actually offer. Like all other ESD protection components, ESD floors are only part of a larger integrated system that keeps all parts, machines, tools, packaging, work surfaces and people at the same potential.
When evaluating a floor, specifiers are guided by two main operational parameters: 1) the resistance of the floor system; 2) how much charge a person generates when walking on the floor in a particular shoe. But what about the details themselves? How do we protect them? When we transfer parts from one operation to another, we don’t put them in the palm of our hand. We use ziplock bags, wheeled pallet trucks and possibly automated vehicles to move parts and systems. In flexible manufacturing operations, ESD floors can even be used as the main base for wheeled workbenches.
ESD floors are designed to prevent ESD damage to electronic parts and assemblies in ESD protected areas (EPA). There are various reasons for installing them. An ideal floor protects against static electricity:
Some ESD floors meet all three needs. Others prevent the buildup of static electricity on people, but do little to protect equipment or ground mobile workstations, ESD carts and chairs.
In order to produce quality products, be ISO certified, and meet customer needs, electronic equipment must comply with ANSI/ESD S20.20. To meet ANSI 20.20 ESD flooring requirements, buyers and specifiers typically focus on the electrical resistance of the flooring/adhesive system. But resistance is just a performance parameter.
Finding a floor that meets the S20.20 requirements for point-to-point (RTT) and point-to-ground (RTG) resistance is a simple task. Compliance with all aspects of ANSI/ESD S20.20 requires the floor to perform multiple functions, and not just meet the resistance parameters. It is also important to determine the maximum stress that the floor will create on a person in combination with a particular shoe. Furniture, mobile workstations, and equipment must also be properly grounded through the floor, with resistance between the castors and ESD floor ground within the S20.20 acceptable range (< 1.0 x109). Furniture, mobile workstations, and equipment must also be properly grounded through the floor, with resistance between the castors and ESD floor ground within the S20.20 acceptable range (< 1.0 x109). Мебель, мобильные рабочие станции и оборудование также должны быть должным образом заземлены через пол с сопротивлением между роликами и заземлением пола в пределах допустимого диапазона S20.20 (< 1,0 x 109). Furniture, mobile workstations and equipment must also be properly grounded through the floor with resistance between casters and floor ground within the S20.20 allowable range (< 1.0 x 109).家具、移动工作站和设备也必须通过地板正确接地，脚轮和ESD 地板接地之间的电阻在S20.20 可接受范围内(< 1.0 x109)。家具 、 移动 工作站 和 设备 必须 通过 地板 正确 地 ， 脚轮 和 ESD 地板 之间 的 电阻 在 S20.20 可 接受 范围 内 (<1.0 x109)。。 Мебель, мобильные рабочие станции и оборудование также должны быть должным образом заземлены через пол, при этом сопротивление между роликами и заземлением пола должно находиться в пределах допустимого диапазона S20.20 (< 1,0 x 109). Furniture, mobile workstations, and equipment must also be properly grounded through the floor, with the resistance between casters and floor ground being within the allowable range of S20.20 (< 1.0 x 109).
The test floor was installed as part of an evaluation of anti-static boards by the equipment department of a medical device manufacturer. Various properties were evaluated, including flatness, sliding characteristics, resistance of the floor system, stress generation on the hull, ease of rolling of heavy equipment, maintenance, and complexity of installation and repair.
One of the flooring options meets all the criteria, including the possibility of using your own labor for installation without the use of glue. However, before ordering the floor, the production engineer placed several mobile carts on the test floor and measured the ground resistance from the surface of the cart through the conductive rollers to a ground point on the floor.
Despite the fact that the floor by itself had measured in the conductive range (< 1.0 x 106) per ANSI/ESD S7.1 tests, the flooring failed the mobile workstation test, with the resistance to ground measurements from the cart surface ranging from 1.0 x 106 to 1.0 x 1012. Per ANSI/ESD S20.20, any measurement > 1.0 x 109 constitutes a failure. Despite the fact that the floor by itself had measured in the conductive range (< 1.0 x 106) per ANSI/ESD S7.1 tests, the flooring failed the mobile workstation test, with the resistance to ground measurements from the cart surface ranging from 1.0 x 106 to 1.0 x 1012. Per ANSI/ESD S20.20, any measurement > 1.0 x 109 constitutes a failure. Несмотря на то, что пол сам по себе был измерен в диапазоне проводимости (< 1,0 x 106) в соответствии с тестами ANSI/ESD S7.1, пол не прошел тест на мобильную рабочую станцию, а сопротивление поверхности тележки при измерении сопротивления грунту варьировалось от 1,0 x 106 до 1,0 x 1012. В соответствии с ANSI/ESD S20.20 любое измерение > 1,0 x 109 считается ошибкой. Although the floor itself was measured in the conductivity range (< 1.0 x 106) in accordance with ANSI/ESD S7.1 tests, the floor did not pass the mobile workstation test, and the surface resistance of the trolley in the ground resistance measurement ranged from 1.0 x 106 to 1.0 x 1012. According to ANSI/ESD S20.20, any measurement > 1.0 x 109 is considered an error.尽管根据ANSI/ESD S7.1 测试，地板本身已在导电范围(< 1.0 x 106) 内测量，但地板未能通过移动工作站测试，从推车表面测量的接地电阻范围为1.0 x 106 到1.0 x 1012。尽管 根据 ANSI/ESD S7.1 测试 地板 本身 已 在 导电 范围 范围 范围 (<1.0 x 106) 内 测量 但 地板 未 能 移动 工作站 测试 ， 从 表面 的 接地 电阻 为 为 为 1.0 x 106 到 1.0 X 1012。 Несмотря на то, что сам пол был измерен в пределах диапазона проводимости (< 1,0 x 106) в соответствии с тестами ANSI/ESD S7.1, пол не выдержал испытания мобильной рабочей станции с диапазоном сопротивления заземления от 1,0 x 106 до 1,0 x при измерении от тележки. Although the floor itself was measured within the conductivity range (< 1.0 x 106) in accordance with ANSI/ESD S7.1 tests, the floor failed the mobile workstation test with a ground resistance range of 1.0 x 106 to 1.0 x measured from cart. surface 1012. Any measurement greater than 1.0 x 109 is considered a failure according to ANSI/ESD S20.20. Seven of the first 40 test points measured values above the ANSI maximum (see Table 1).
More than 1000 measurements were made on this sample. The percentage of marriage is about 16%. Shopping cart problem? When placed on a metal plate, the cart’s ground resistance is well below 1.0 x 107. To rule out contamination as a variable, the floors and casters were thoroughly cleaned and retested. This is ineffective and measurements are still unacceptable. Just move the cart one inch and the resistance between the cart and the floor changes by four to six orders of magnitude. Given that the resistance of the floor and the resistance of the cart rollers seem to be constant, the only remaining variable is the random placement of the rollers (roller and floor surface) on the tile.
Figures 2 and 3 show photographs of pallet trucks commonly used in Electronic Manufacturing Services (EMS) facilities. The trolley is parked on a floor system that uses conductive chips. This floor will be classified as low density conductive chips (LD). This special floor system provides a conductive path from the black surface chip through its thickness to the carbon loaded ground layer below. Use 24″ copper tape as a grounding point. When tested with a 2.5″ (6.35 cm) and five lb (2.27 kg) NFPA sensor, floor resistance was well below 1.0 x 106.
In Figure 2, the cart to ground measurement exceeds the limits (< 1.0 X 109) of ANSI/ESD S20.20. In Figure 2, the cart to ground measurement exceeds the limits (< 1.0 X 109) of ANSI/ESD S20.20. On fig. 2 расстояние между тележкой и землей превышает пределы (< 1,0 X 109) стандарта ANSI/ESD S20.20. 2 The distance between cart and ground exceeds the limits (< 1.0 X 109) of ANSI/ESD S20.20.在图2 中，推车对地测量超出了ANSI/ESD S20.20 的限制(< 1.0 X 109)。 ANSI/ESD S20.20 的限制(< 1.0 X 109)。 On fig. 2 расстояние между тележкой и землей превышает пределы ANSI/ESD S20.20 (< 1,0 X 109). 2 Distance between cart and ground exceeds ANSI/ESD S20.20 limits (< 1.0 X 109). In Figure 3, the fit measurements are the result of small changes in the position of the same vehicle on the same tile. Like the results in Table 1, these resistance measurements confirm a high correlation between minor changes in the caster’s position and significant changes in resistance.
Like the carts shown in Figures 2 and 3, the carts used by medical device manufacturers consist of four conductive castors. Ground resistance between cart and ground point meets ANSI/ESD requirements 84% of the time. A penetration ratio of 84% means that 16% of the time none of the conductive rollers make sufficient contact with the conductive base plate of the chip.
Another way to look at this is to look at the data in terms of the probability that four consecutive events have the same outcome. In this case, the events will be simultaneous. For example, what is the probability that, in a coin toss experiment, heads will come up four times in a row? This equation will be
is the probability of one event multiplied by itself four times, or ½ x ½ x ½ x ½ = 1 in 16.
If we broadly apply this approach to our floor problem (for simplicity, we exclude the density of particles from the total area), we can say that after 100 attempts, we can randomly have all four rollers that do not contact conductive particles in one and the same time 16 times. So, how likely is it that one caster will not touch the conductive particles? At the very least, we question the possibility of four successive either-or events. Our simple equation might look like this. X times X times X = 16/100. So if we find X, the fourth power of 16 is 2, and the fourth power of 100 is 3.1. Basically, any single caster has a 66% chance of not touching the conductive element on the floor.
Firstly, this is a strong argument in favor of installing conductive rollers on each rack of the cart. But the real payoff is to get hold of that old stats book and do a valid experiment before assuming any ESD floor will be grounded based on test results from an ANSI/ESD 7.1 compliant mobile workstation.
This problem can be easily avoided when buying new floors. When assessing an ESD floor, the floor must be assessed as part of the facility and as a process within the facility. Floors must be tested for compatibility with all ESD protection components, including handling. A fully functional floor can act as an anchor for all mobile earthing requirements.
A key feature of many ESD floors is the ability to eliminate the cumbersome and redundant linking process within the EPA. ESD floors also eliminate the need to place components in covered carrying cases and protective bags. But to eliminate the use of cumbersome packing and securing protocols, the floor must provide an adequate ground path for the handling of rollers to move.
Some ESD floors cannot effectively ground conductive rollers due to poor contact between rollers or guides and low density of conductive dots or chips on the floor surface. In some cases, light layers of low-maintenance polyurethane or ceramic coatings, factory applied to the floor surface, can exacerbate the problem. These UV curable coatings reduce maintenance costs. Most tests have shown that the micro-thin coating increases floor resistance and reduces walker stress control.
The conductivity of some ESD vinyl tiles is due to randomly placed conductive chips like the tiles shown in Figure 4. The black shavings are the only conductive elements on the tile surface. The rest of the surface is plain vinyl, an insulating polymer that does not provide a ground connection.
As shown in Figure 4, we can evaluate this possibility by flipping the NFPA probe over to its edge and measuring the area of contact between the conductive chip and ground. The tile sample shown here measures less than 1.0 x 106 when the entire 31 cm2 sensor surface is used in the ANSI/ESD S7.1 test. However, the polymer between the chips is not conductive. The measurements differed by more than five orders of magnitude when the casters touched the non-conductive polymer between the chips rather than the conductive chips.
For portable workstations or chairs that comply with ANSI/ESD S20.20, the ground resistance must be less than 1.0 x 109.
To understand the problem, we looked at the dimensions of the conductive rollers and tried to determine how much surface area they actually touch the floor. We first put four sheets of paper under the rollers and moved the paper in four different directions until it stopped sliding (see Figure 5).
When we lift the paper, we expect the four sheets to not touch. The space or void will show us the approximate contact point of the rollers with the floor. Before moving the rollers, we taped the sheets of paper together to keep them in place. Then we rolled the chairs off the paper. Since we were able to fit quite a lot of paper under the rollers, we expected the contact area between the rollers and the floor tiles to be very small. We were surprised to find that it was larger than a silver bar. In fact, the actual contact area is less than a dime (see Figure 5).
Figure 6: The solid gray area between the 1/4 coin and the coin represents the caster’s contact area.
Think of a clearing on paper as a viewing window. We move the windows on the tiles. When we don’t see the black chip inside the viewing window, we’re looking at the part of the tile that doesn’t ground the caster. Although it provides some degree of conductivity, when most of the roller contact area is in the gap between the chips, the resistance can be higher than 1.0 x 109.
A typical conductive roller is about 10 cm in diameter but has a contact area of only 1 cm². From this point of view, the contact area of the NFPA sensor used to measure the resistance from the ESD floor surface to the ground is 31 cm2. Distances between conductive particles used in low density chip technology (see Figure 9) ESD floors can be measured at distances of 0.5 cm to 10 cm, with an average of 2 to 5 cm. /ESD STM 7.1 cannot predict whether a particular floor will consistently provide electrical contact between the rollers and the floor.
The only way to make an accurate determination is to conduct a statistically valid sample of resistance measurements using carts, rollers and floors that the factory will purchase. This must be done prior to ordering any floors. Once the floor is installed, it is too late to fix the problem. Most flooring manufacturers do not provide data or guarantees regarding roller contact resistance.
If we place the same sheet of paper with a roller-contact-sized viewing window on an ESD vinyl tile made from a dense conductive texture matrix, we can move the window anywhere on the tile and still see the texture. Due to the close spacing between the cores, it is impossible to find non-conductive areas of the floor in this conductive matrix. This dense matrix of conductive texture increases the likelihood of contact between the tiny surface of the wheel and the conductive elements of the tile. Wherever we see veins, the conductivity of the tile will ground chairs and carts.
ESD vinyl tile made using conductive wire technology contains approximately 150 linear feet of conductive wires per square foot. Seen from this perspective, the veins on the thirty-six tiles represent a mile-long conductive point of contact. With such a large number of conductive points, even with contact with one roller, the measurement results are 100% compliant with the ANSI S20.20 standard. Can floors using conductive chip technology solve this problem?
On fig. 8 shows a visual comparison of a low density (LD) discrete conductive die backplane and a high density dispersed conductive (HD) backplane. The distance between chips on the LD floor can be 0.5 to 5 cm within one tile or sheet. Chip spacing rarely exceeds 0.5 cm on HD chip floors. Chip floors can be produced in sheets or rolls for seamless installation. Due to manufacturing process limitations, Vein Technical Flooring cannot be produced in rolls. Veins can only be used as tiles.
Figure 9: Note the large contact area of the NFPA sensor compared to a real object grounded through the ESD floor: D – contact area of the NFPA sensor = approx. 31 cm2E—Typical heel strap: > 13 cm2G—Caster contact area = 1 cm2F—Ground chain contact area = negligible 31 cm2E—Typical heel strap: > 13 cm2G—Caster contact area = 1 cm2F—Ground chain contact area = negligible 31 см2E — типичный пяточный ремень: > 13 см2G — площадь контакта с колесиком = 1 см2F — площадь контакта цепи с землей = незначительная 31cm2E – Typical heel strap: > 13cm2G – Wheel contact area = 1cm2F – Chain to ground contact area = negligible 31 cm2E—典型的鞋跟带：> 13 cm2G—脚轮接触面积= 1 cm2F—接地链接触面积= 可忽略31 cm2E—典型的鞋跟带：> 13 cm2G—脚轮接触面积= 1 cm2F—接地链接触面积= 可忽略31 см2E – типичный пяточный ремень: > 13 см2G – площадь контакта с роликом = 1 см2F – площадь контакта с заземлением = незначительна 31 cm2E – typical heel strap: > 13 cm2G – roller contact area = 1 cm2F – ground contact area = negligible
ESD floors must be fully evaluated for their many features, including compatibility with material handling equipment. There are two main technologies for the production of ESD floor tiles and sheets: conductive core technology and conductive chip technology. The technology used to produce ESD floors affects performance. In situations where the floor must be grounded for mobile workstations and carts, conductive floors are superior to low to medium density chip technology floors. This is due to the lack of conductive pins in typical LD and mid-range conductive chip boards. The new high-density chip technology solves this problem and provides the same level of performance as floors with conductive core technology.
Dave Long is the CEO and Founder of Staticworx, Inc., a leading supplier of static-free flooring. With over 30 years of industry experience, he combines his extensive technical knowledge of electrostatics and concrete substrate testing with a practical understanding of how materials behave in real-world conditions.
This is exactly what I found out after changing the specification of the ESD floor. I checked all the floors for ESD and it was obvious even by looking at them. In addition, debris seen on low/medium density floor surfaces does not always pass through the lower level, so there is no path to the ground. The floors were also untested and varied significantly (although passed the standard walking test). The higher density and textured floors we had previously were more resilient than the new specs.
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Post time: Oct-17-2022
