ESD Journal - The Control of Risks from Static Electricity

THE CONTROL OF RISKS FROM STATIC ELECTRICITY

John Chubb

John Chubb Instrumentation, Unit 30, Lansdown Industrial Estate,

Gloucester Road, Cheltenham, GL51 8PL, UK

(Tel: +44 (0)1242 573347 Fax: +44 (0)1242 251388 email: jchubb@jci.co.uk)

Abstract:

A major concern in assessing risks from static electricity is the charge retained on materials. Materials cannot be properly and fairly assessed by traditional ‘resistivity’ measurements. Charge decay measurement are fine - but may be unfair to some newer cleanroom garment fabrics. A new approach has been developed and is reported for assessing materials. The approach is to measure the level of surface voltage created by known quantities of charge on the material surface. This gives a ‘capacitance loading’ factor. If the charge decay time is sufficiently short and/or if the surface voltage is sufficiently low for the largest quantities of charge likely to arise in practice then the material should be considered suitable.

1. INTRODUCTION

To avoid damage by static discharges or by the influence of static electricity it is necessary that the voltages on the surfaces of materials near microelectronic devices (conducting and insulating) remain low. For some microelectronic devices and systems (e.g. magnetoresistive disc drive heads) it is considered desirable to limit maximum voltages to no more than 10-20V in relation to a Human Body Model equivalent discharge [1]. Reliable control of surface voltage is often not easy, particularly in environments such as cleanrooms, because many materials need to be plastics for cleanliness and chemical resistance performance. A particular concern in cleanrooms is the garments worn by personnel.

Electrostatic charge is transferred between materials when these contact or rub against each other. The influence of surface charge on items nearby relates both to the voltage on the surface (in relation to the opportunity for static discharges) and to the local electric fields that arise (in relation to induced charge effects). The question to be addressed in thus how to assess the risk from charge retained on materials and how to judge if particular materials will, or will not, give rise to risks or problems. Where it is necessary to drain charge from a conductor in contact (such as a person standing on flooring), where there is risk of spark discharge ignition of flammable gases (incendivity) or where there is need to protect against nearby electric field transients (shielding) then other requirements and test methods need to be used.

Resistivity measurements can be appropriate for material assessment in a number of situations - e.g. for flooring and footwear, when the need is to drain charge from a conductor (such as a person) in contact. Where problems arise from static charge retained on a material itself then a measurement of resistivity is usually quite inappropriate. Resistivity indicates the fastest route for charge migration, whereas for charge retention it is the slowest route for migration that is relevant. Charge decay measurement is appropriate in such situations. However, it is important that a suitable method is used that is shown to give results that match to the decay of triboelectrically generated charge [2,3]. It is to be noted that Federal Test Standard 101C, Method 4046 does not achieve this [3]. For a material to be acceptable the charge decay time needs to be sufficiently short compared to the time of mechanical actions of charge separation so that no significant surface voltage can occur - even as a transient. Decay times below ½s have been suggested as suitable [4]. However, recent studies (see below) have shown that appreciable surface voltages can still occur transiently at rubbing of common materials, even when decay times are as short as 0.2s [5,6].

Fabrics for personal protective clothing and cleanroom garments are usually constructed to include conductive threads. The aim of these threads is to limit the influence of surface charges on nearby items by proximity to internal 'earthy' conductors. This capacitance coupling to internal conductors acts to limit the surface potential. A low fabric surface potential will avoid risks of damage by direct electrostatic discharge and by indirect induction effects. If the 'conductive threads' have a 'core conductivity' in an insulating sheath there is no opportunity for sensible assessment by resistivity measurement. For cleanroom garments the basic fabric is usually polyester, so charge decay times are likely to be very long - unless the fabric has been treated with an 'antistat' finish. With such materials neither resistivity nor charge decay measurement will give a fair assessment of performance and the ability to avoid risks from surface static charge.