ESD monitoring and control
14 March 2014
ESD is a spark, (a micro lightning bolt) that passes from one charged surface to another. This incredibly rapid transfer of static (unmoving) charge can cause fires, explosions, EMI, RFI, heat, light and even sounds. This unseen, unfelt and unheard micro lightning spark must be controlled. Humans are a major source of static charge, hence the need for ESD control.
For the electronics industry, ICs, such as EPROMs can be damaged by voltages as low as 100V, while Schottky diodes have a threshold susceptibility of 300 to 2,500V, or bipolar transistors have a threshold of 300 to 7,000V.
Meeting the demands of industry
ESD testing has been dominated by low performance, low price pass/fail testers that rely on users signing a sheet of paper for test records. Thus, there is no effective means of monitoring their use. In response, Sentry-esd launched its Sentry static control and monitoring system.
A differentiating feature of the system is the fact that it is the system (and not the user) that applies the test and records the result. Test results shall include user ID, measured values, time and date of test and a pass/fail flag (not just a tick or initials on a sheet of paper). Another difference is that the system, rather than the user, determines what test to apply i.e. to wrist or feet or both.
The ESD co-ordinator sets the test limits to be applied and the system measures resistance and not test to preset pass/fail thresholds with test results held online and available on demand for managers and supervisors. The results are also available in real-time and on demand for compliance inspection. The system is versatile, catering for small non-networked, to large companywide network configurations.
Important considerations are that the user is not asked to decide what test to apply, or whether a test passes or fails, and is not asked to log the result.
ESD testing has been dominated by low performance, low price pass/ fail testers that rely on users signing a sheet of paper for test records.
Sentry-esd also assists its users to achieve compliance with industry standards IEC 61340-5 and ANSI/ESD S20.20. This involves regular testing of wrist and foot straps: daily, per shift or on entry and re-entry to an area.
Compliance verification records shall be established and maintained to provide evidence of conformity to the technical requirements (ANSI/ESDS20.20-2007 section 7). Finally, conductive footwear should be tested to an upper limit of 100KO for hazardous areas or handling sensitive explosives (IEC/TS60069-3-1).
In creating the static control and monitoring system, the design requirement was for a system to measure and test accurately from below 100kO and up to 1GO and to log the result within two to three seconds.
Since, there is no lower limit in the standard to test that the resistance of conductive footwear is less than 100KO, it is left to the discretion of the equipment manufacturer. However, not to have a specified lower limit allows a pass to be given for “dead short”. Investigation led to the conclusion that a lower limit of 2kO would allow the standard to be met without ignoring the lower limit.
Measuring across 2kO to 1GO presented a different challenge: to measure with accuracy better than 10% within the time limit. An innovative analogue design and a fast 12bit ADC satisfied these conflicting requirements.
The next step was to meet the requirement “Verification records shall be established and maintained to provide evidence of conformity to the technical requirements”.
The solution was the integration of multiple functions, namely measurement circuit, firmware for instrument control and results logging, USB port for engineering maintenance and calibration, Ethernet interface for software communication and control, non-network and network configurations, database driven software, numeric keypad and LCD for communication and an optional barcode, magnetic stripe or proximity card reader to input user ID.
The Sentry firmware controls the user interface, the application of the ESD test and storing the test results in local memory. The test results include the user ID, resistance values, time and date of test and a pass/fail flag.
Observer software collects this data and assembles it into a database. It includes the Sentryservice and database, installed on the ESD co-ordinator’s client PC or on a server PC, which runs 24/7. As the units are used, it reads the test results then adds them to the database. There is also Observer Administrator, installed on the ESD co-ordinator’s client PC, provides system set-up facilities and full read-write control of the database.
Finally, Observer Monitor can be installed on multiple client PCs to provide read-only access to the database, for managers and supervisors to view and analyse the test results and prepare reports.
It is a startling fact that throughout many industries there are conflicting attitudes to the need for ESD protective measures. For example, manufacturers and repairers of disk drives and mobile phones apply stringent procedures yet there remain many manufacturers that ignore or only pay lip-service to ESD protection.
When no ESD testing is in place, the decision is influenced by the question: what is the cost of not testing? It is not a simple question to answer. The decision to invest will be based on generalised figures from industry and data on the cause and occurrence of ESD- related failures.
Where compliance with the international standards is required then the whole life cost for a manual test solution should be compared with that for automated test, which saves administration costs and the cost of logging data.
The only way manual testing can match the whole-life cost of automated testing is to ignore the compliance requirement:
“Verification records shall be established and maintained to provide evidence of conformity to the technical requirements”.
For more information , please contact Sentry-esd at firstname.lastname@example.org
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