Garments and electrostatic performance

Significant differences are found in the levels of risk presented by static electricity from garments used in microelectronics work.

With some garments, voltages of several hundred volts can arise – with others, only one or two. These differences are not shown by traditional "resistivity" measurements, or by charge decay measurements to IEC 61340. A new approach to assessing the electrostatic performance of garments for microelectronics work was described at the ESTEC Conference on Cleanroom Contamination Control, 18-21 May, in Phoenix, Arizona, USA. The paper, given by Dr John Chubb, was entitled "Predicting surface voltages on inhabited cleanroom garments in practical use". The work was carried out in conjunction with Paul Holdstock, of British Textile Technology Group in Manchester, and Mick Dyer, of William Barnet and Sons, South Carolina, USA. The work was in two parts: first, measuring the maximum surface voltages that arise on cleanroom garments of various fabrics while these are being worn (inhabited); second, examining whether these voltages could be predicted from measurements on sample areas of the fabrics. The work showed that meaningful measurements can be made on inhabited garments when these are charged by rubbing – as applies in practice. Significant differences were measured in relation to the risks presented by static electricity, ranging from several hundred volts to one or two volts. Measurements were made of charge decay times and of the capacitance experienced by electrostatic charge on the surfaces of the above fabrics. These measurements were made using the new JCI 155v5 Charge Decay Test Unit in conjunction with the JCI 176 Charge Measuring Sample Support. There were two main results: • Maximum peak voltages on inhabited garments relate to the capacitance experienced by surface charge. They do not relate to values of surface resistivity – or to charge decay times. • Peak voltages can be predicted from measurements of the surface voltage per unit charge on fabric samples. The new approach enables manufacturers of garments to choose the appropriate fabric for particular garment applications. The work also shows how to measure the practical electrostatic performance of garments with confidence.