Selecting Matedals For Protection Against ESD using an ESD shielding effectiveness meter

摘要

Until now, the characterization of antistatic shielding materials has been based on measurements of the surface resistivity of these materials. However, there are three problems with the use of surface resistivity as a measure of ESD shielding effectiveness: 1. for a new generation of filled conductive plastics surface resistivity measured is no measure of the overall conductivity of the material; 2. electronic design engineers are not interested in the surface resistivity as such, but in the ESD shielding behaviour of an enclosure in relation to the immunity standard IEC 801/2 [11; 3. the relation between surface resistivity and ESD shielding efficiency has been found to be questionable. This paper discusses a novel method, developed by DSM and Bekaert in coorporation with the KIH.WV, for the ESD shielding characterization of conductive enclosures. The method uses a reference electronic system as a model for printed circuit boards, allowing differentiation between materials concerning ESD protection for electronic circuits. The test is based on the IEC 80112 standard. Direct discharge measurements as well as indirect discharge measurements are carried out. The ESD shielding efficiency of several kinds of materials, such as non-conductive plastics, metal, metallized plastic, carbon fiber filled plastics, carbon black filled plastics and stainless steel fiber filled plastics, are discussed. When the mentioned materials are used in e.g. housings, other aspects, such as weight of the housing, colorability, moldability, system costs, etc., also have to be taken into account. The following conclusions can be drawn. 1. carbon black filled plastics, which are widely used to solve ESD related problems, offer an excellent ESD shielding efficiency in the case of a direct discharge, but with indirect discharges these materials fail; 2. both stainless steel fiber filled plastics and metals provide excellent ESD protection for both direct and indirect discharge; 3. grounding of the housing increases the ESD protection; 4. when practical aspects such as system price, colorability and moldability are also taken into account, stainless steel fiber filled plastics are to be preferred in most cases.