The Role of Static Dissipative Materials
Many antistatic materials also possess static dissipation capabilities when grounded or in contact with large planar conductors such as the floor. Static dissipative materials have similar volume resistivity or are covered with conductive materials, such as table mats for workbenches. When in contact with charged devices, dissipative materials limit the discharge current.
According to the definitions of EIA and ESDA, static dissipative materials are those with a surface resistivity of 10⁵ to 10¹² Ω/sq. Research by Bossard et al. shows that a lower limit of 10⁵ Ω/sq is appropriate for protecting ESD-sensitive devices, which are prone to failure due to thermal melting.
Besides surface resistivity, another important characteristic of static dissipative materials is their ability to dissipate static charge from objects; the technical indicator describing this characteristic is the static decay rate. According to the electrostatic decay model for an isolated conductor, the electrostatic decay period is exponentially related to the product of the resistance and capacitance (RC) of its discharge circuit:
V(t) = V0e⁻t/t
Where V(t) is the electrostatic voltage after decay, V0 is the electrostatic voltage before decay, t is time, and t = RC is the time constant.
A typical assumption in studying electrostatic discharge capability is that the electrostatic voltage will decay to a specific percentage, such as 1%, within a specific time period, such as 2 seconds. Furthermore, relative humidity is also an important factor for electrostatic dissipative materials and must be controlled and recorded during electrostatic decay testing.