The Automotive EMC Network (www.autoemc.net) PO Box 3622, Newport Pagnell, MK16 0XT, England.
Abstract: The automotive ESD standard (ISO 10605) has seen very few changes since the first Technical Report (TR) issue of 1994, however, this year the first revision of the issued standard is due to incorporate some significant changes in the test methods. This paper uses the ISO discussion (DIS) document ISO/DIS 10605:2007 and looks at what these proposed changes are and how they might impact on test service providers, automotive equipment suppliers and designers and manufacturers of automotive electronic products. Background The automotive ESD standard ISO 10605 has been around since the early 1990’s, the first Technical Report issued in 1994 (ISO/TR 10605: 1994). Although not formally issued until 2001 (ISO 10605: 2001) as a first edition, the test methods were used widely from the TR version and available as SAE standard J1113/13 before this. It is almost universally adopted unaltered in the automotive industry (even in its initial TR form) and incorporated or referenced in most of the automotive vehicle manufacturer (VM) standards. ISO 10605: 2001 is based on similar models and premises as the commercial ESD standards derived from IEC 801-2:1991 (Electromagnetic compatibility for industrial process measurement and control equipment – Part 2: Electrostatic discharge requirements, now IEC 61000-4-2). The test simulates the effect of a human body discharge of static electricity, a relatively common occurrence experienced by most users, particularly when entering or exiting the vehicle. The changes between the 1994 TR and 2001 issue were mainly the inclusion of an unpowered ESD test that reflects handling of an electronic sub-assembly (ESA) while in transit or during assembly fit (i.e. not in-situ on the vehicle). The ISO 10605: 2001 standard includes testing on-vehicle as well as bench testing of individual ESA’s. ISO/DIS 10605:2007 was released for discussion in early 2007, and represents only the second iteration of this standard and the first revision since official release. The standard is due to be released in November 2007, but at the time of writing the author has no idea if this will be achieved or whether the formal release will lapse into 2008. Introduction At first glance the reader of the 2007 document may think there have been significant changes, if nothing else it is immediately noticeable that the standard has swelled from a modest 20 pages to a more substantial 48. However, much of the content is in fact similar and many of the additional pages are textural and contain additional terms and definitions, for example bringing terminology in line with IEC 61000-4-2, and several pages in the annexes is rationale and target construction for calibration of the discharge device (ESD gun). There is also more test application descriptive text on selecting air or contact discharge for example, text not found in the existing versions of this standard. Much of the additional text is certainly helpful and should make the application of the testing more consistent, however, in itself most of the essence of the Automotive ESD standard remains as before and the genuinely new changes can take some time to identify and realise what impact these may have on design and testing to meet the automotive ESD environment. Environmental Conditions The parameters limiting the environmental conditions under which the test is conducted have been changed, but mainly in that they have been relaxed for both operating temperature and relative humidity and now include a specification for barometric pressure. The relaxation of both temperature and humidity
parameters is probably more of a reflection of the reality of some test facilities than a response to any specific issues, although ESD testing at 15°C with 60% humidity must be a frightening experience!
ISO Edition: DIS: 2007 Released: 2001 Operating Temperature 25±10°C 23±5°C Relative Humidity 20-60% 30-60% Barometric Pressure 680-1060mBar Not Specified
Table 1: Environmental Test Parameters This innocuous change in operating temperature is not a problem for the vehicle or ESA being tested, but could present a safety risk to the test personnel. In cooling systems the general consensus on the use of water trap for condensation is with coolant temperatures below 16°C (where no dew point monitor is used). As a consequence allowing testing now to 15°C could lead to possible electrical shock accidents and I would urge test services to remain within the previous lower temperature limit of 18°C for their staff safety (dew could form on the test equipment, the device under test or other objects and form and unexpected discharge path). ESD Generator The ESD specification for the generator has changed, but only slightly. In all honestly I would be surprised if any existing ESD test equipment currently used for ISO 10605:2001 could not meet the new 2007 specification. The slight increase in the accuracy to <5% is undoubtedly within the capability of most ESD pulse generating equipment. The most obvious change is the reduction of the upper voltage requirement and is in fact inconsistent with some of the test levels listed later in the standard that still suggest 25kV pulses. Although there is a caveat in the table as written in the standard that states “or as required in the test plan”, this is not as explicit as the previous edition. Consequently the generator specification is now slightly misleading and at first glance anyone with a standard IEC 61000-4-2 specified generator would believe they could offer automotive ESD testing to ISO10605, but may find themselves in trouble if the client requires a 15kV contact and/or 25kV air discharge (as many VM standards do when referencing ISO 10605).
ISO Edition: DIS: 2007 Released: 2001 Parameter Contact Air Contact Air Output Voltage 2-8kV 2-15kV 15kV 25kV Accuracy <5% Not Specified Rise Time 0.7-1.0ns 0.7-1.0ns <=5ns Holding Time 5 Not Specified Network Capacitance 150pF or 330pF 150pF or 330pF Network Resistance 330Ω/2000Ω 2000Ω
Table 2: ESD Generator Parameters The most significant change, as far as both testing and product design is concerned, is incongruously contained in the above table and is the additional use of the 330Ω resistor in the human body model for the discharge network. This is easily missed at first glance as 330Ω is the value used in the IEC 61000-4-2 standard and will be familiar to any EMC test service, but is the first time it has been introduced in the automotive ESD standard. Resistor Network This might seem a trivial issue, but there are several significant problems with this change, one being that it implies that existing testing is inadequate since this significantly increases the discharge energy, although shortening the applied pulse duration. Secondly it implies that there has always been something wrong with the premise of the original 2000Ω network (used since before 1994). Lastly I believe it has been introduced due to errors in other (VM) testing standards that instead of being corrected are in fact propagating into this standard due to peer pressure from parties with specific vested interests, but this is an unsubstantiated belief of mine and not necessarily fact (I am not a member of this ISO committee).
Why has the resistor network been changed? The standard itself lists the reason as 330Ω being representative of a discharge from a metal object, such as a tool, to the item being tested (ESA or vehicle). However, why then does IEC 61000-4-2 not use a 2000Ω for body-to-object as it is suggesting is used in the automotive environment? The reason is simple, the 2000Ω is used in the automotive environment because the environment itself is isolated from the earth reference, hence although the charge on a car and ESA is similar to the earth potential, the discharge path is not direct to earth but via vehicle ground (usually chassis and battery 0V), and as a consequence the resistor network is increased to compensate. The original 330Ω used in IEC 61000-4-2 was developed to model human body discharge to an earth referenced systems (e.g. domestic and commercial systems installed and primarily powered via an electrical mains network). The next issue then, accepting the “metal tool discharge” premise, is to question when this needs to be applied? The only reason to be approaching an ESA with a metal tool is to un-install the part, unless it is mechanically handled during installation and the assembly line is not earth bonded. Consequently the only time to apply this is in handling the product on the vehicle (in a workshop) and hence the discharge will be air-discharge only and to the case (it is extremely difficult to discharge to the pins via a wrench or some-such tool). Even if we consider the potential from a user holding the metal part of the keys, discharge will always be to ground (0V) referenced bodywork, rather than direct to an ESA (i.e. air-discharge). If the premise of the discharge from the tool is believed to be a true case, all indirect (air) discharge testing should be performed with the 330Ω network since this is the most severe test case and the 2000Ω network should be abandoned for air discharge testing. I have come to the personal conclusion that in reality there are almost no cases where this scenario is valid and as such this change to the resistor network is effectively redundant in the automotive environment (and that’s accepting the original premise for using 330Ω that I still believe is flawed for the vehicle environment). Functional Performance Status Classification Functional performance status classification (FPSC) is a new annex to the standard (an informative annex) and is a useful addition to the text. Effectively it incorporates most of what was implied in the 2001 edition that using references to ISO failure mode severity classifications, but more logically applies them. What this allows in the existing severity classifications implies a product could exceed its maximum parametric performance during exposure and be class B, but how would you detect this during test? As a consequence the class B classification is redundant for ESD test classification. Effectively though instead of just using the existing ISO classes A, C, D and E, these have been relabelled Status I, II, II and IV, but are effectively unaltered in their descriptive form. The critical difference is that the tests are being applied to functions on the ESA, not just the ESA as a whole. This might be difficult to conceive, but most ESA’s these days have multiple outputs with different functional implications, hence some simple on/off conditions of which some may be interior switch illumination lights and some exterior lights, some data bus communications and others maybe analogue. This allows the severity level and status of the functions to be specified, rather than of the ESA as a whole, hence maybe all on/off and data should be Status I under all conditions, but some analogue function (say RF reception) may be allowed to be compromised during a high severity event; status II. Anyone familiar with the Daimler-Chrysler (DC) standards will recognise some of this and as a designer or producer it makes sense, however, I expect it may cause some headaches to the test houses and it will be more difficult to give a clean “pass” as with the current standard. The wording of this annex could also be a little clearer as initially it looks like a specific number of fails can be tolerated and this is not the intention or the case. Additionally the examples show 3 categories of test level, but this is not introduced in the text and has no explanation of what the categories are. I believe these are intended to show various category of device and hence test severity, for example category 1 may be a non-safety device such as an in-car entertainment (ICE) system and category 3 being a safety critical function such as engine control module (ECM). The text does state users may group functions into categories, but it does not make suggestions of what these categories might be, how
many might be specified and what test severity is appropriate (although the examples imply the later at least). Exclusion of Pyrotechnic Modules One classification of device that is specifically excluded from the 2007 standard is pyrotechnic devices. Personally I find this utterly bizarre, of all the products that you do not want to react to a user borne ESD event I would have expected air-bags to be number 1. This is not excluded in an annex, but in the scope at the start of the document, even though one of the pictures in the annexes for on-vehicle testing shows discharge to the steering wheel (I guess we wouldn’t want the horn sounding inadvertently while you are trying to disentangle the air-bag from your face). Calibration The method for calibrating the ESD generator is very well documented and a specific 2.1Ω target is suggested with construction details. There is also a 50Ω conical line adaptor target for current measurement illustrated in the 2007 standard. While both targets may be manufactured by a test service for their own use, given the details contained in the standard, I would expect some test equipment providers may wish to supply these additional calibration target for test laboratory use. The targets give a very quick and easy way to do routine calibration prior to testing (certainly the 2Ω target looks a more repeatable solution than some of the spark-gap targets I have seen being used). This calibration information represents 16 pages of the new edition, hence is a very significant addition in terms of the document size, despite its relatively low impact on the test itself. ESD Testing Stress Step Testing This was always implied in the previous standard, suggesting testing at all levels up to the maximum. In the new edition this is specified as testing at a minimum of 2 severity levels before the maximum is applied, a slightly more pragmatic approach. Time Between and Number of Discharges The minimum number of discharges remains the same at for direct (contact) discharge at 3 per test point of both polarities. The minimum time between direct discharges has reduced to 1s from 5s and this should speed up testing, however, a producer can specify a longer time between discharges if they feel this is required to allow charge to dissipate. The indirect (air) discharge minimum number is now specified as 50 whereas the current standard applied the minimum 3 to both discharge methods. This is for each test point and polarity again, but time between discharges is reduced to >50ms. The total test time is most likely going to be significantly increased, as it is unlikely many ESD generators can discharge at high voltage at these recovery rates, plus this high rate of discharge has no basis in simulating reality and is unfair to the product. The premise for this increase is to improve repeatability, however, surely a lower number (e.g. 10) would have been adequate and forcing fixed holding arrangement and discharge distance would be more consistent than simply increasing the statistical probablilty. If anything I would expect having 50 pulses will probably reduce repeatability. As a consequence of these changes I would expect total test time to the 2007 standard to double compared to testing to the current edition, and probably for no change in the end result. Test Cases There are still 3 test cases of handling (or unpowered test), powered ESA and vehicle testing. The most significant change here is in the unpowered test, where there is now an explicit requirement to prove that the ESD handling stress has not degraded components providing other EMC performance criteria, such
as capacitors that are ensuring emission or immunity performance. What this requires therefore is that the product that has the unpowered ESD test should be used for the rest of the EMC performance testing. While I can see the sense in this regime, ensuring that the EMC as well as the functional performance is not degraded by handling, this can be difficult to apply in a product test plan and it is not uncommon to submit several ESA’s simultaneously for testing so that all the different tests can be conducted at the same time, this will not be possible in the future. Furthermore, a failure on say a radiated emissions issue would have to be re-tested for handling ESD prior to emissions retesting, again to ensure the emissions improvement was still ESD immune. Although this is a sensible engineering change to the test regime, it will add significant test costs to the producer and increase the overall quantity of ESD testing being performed, since a handling ESD test should precede almost every other EMC test. The new standard now includes several photographs showing examples on testing on-vehicle, something that was absent in the current edition although a diagrammatic line image is provided. Optional Powered ESA Test Case There is a new test specified as an optional test in annex F of the update. The new test is quite a good idea in that is simulates discharges close to the ESA harness (i.e. to the chassis or body). The test places another 0V reference plane below the ESA and it’s harness and discharges to this, there are separate discharge levels for air and contact discharge up to ±25kV and only the 330Ω resistor network is specified.
Figure 1: New optional test to simulate the in-vehicle discharges close to the harness.
Although I think the test itself is a good representation of the in-vehicle situation, the implementation of the test is over complicated. Why have a secondary ground reference strip, why not just lay the harness on the GRP and use that? Secondly how is a 25kV contact discharge to the ground supposed to occur? If you are charged to 25kV and holding a metal tool you will discharge a long time before you come into contact with the vehicle (this is true even for charges as low as 3kV). The main problem with this test is that as an optional test few producers will specify it (who wants to pay extra for their testing) and will leave this to the VM themselves. And don’t get me started on that 330Ω resistor again! Minor Alterations The ground plane (or Ground Reference Plane, GRP and Horizontal Coupling Plane, HCP) can now be only 0.25mm thick rather than the 1mm thickness previously specified (although I’ve never come across anyone using such a thick metal sheet in practice. The minimum area is also better defined at 1.6m x 0.8m rather than the rather indeterminate 1m2, but these are cosmetic changes.
Summary To the test service industry and test equipment suppliers the changes to ISO/DIS 10605:2007 will have little impact, the equipment used today for automotive ESD testing is suitable. The equipment supplier may be able to offer additional calibration targets and fixtures, but essentially the ESD generator and discharge networks remain unaltered from ISO 10605:2001 and IEC 61000-4-2 (most ESD test equipment is supplied with the 330Ω network and the 2000Ω network has to be purchased separately). For test services the most likely immediate impact will be slightly more calibration might be required. The greatest impact occurs to the producer of ESA’s where the quantity of testing will be increased. The levels and/or severity of test will be the same (or increased if you wish to use the 330Ω network, but a technical argument for 2000Ω resistor only is easily obtained, most of it can be copied from this paper). The test cases and functional classification will be the same, although their reference names have changed, but the number of test impulses for air-discharge is significantly increased. Hence although the product design will probably remain the same and the same end result occurs as far as end product immunity to ESD is concerned, the test cost may go up by 50% to 100% due to sheer volume of discharges and additional handling ESD test required prior to other EMC performance testing. Conclusion I have great personal reservations about the use of the 330Ω network and think it is being introduced under a false premise and for an imaginary handling condition that will never be realised in practice. I also think the lowering of the operating temperature range for testing offers a small but real electrical shock risk to test personal, again a change I think has not been fully thought through. In hindsight I don’t think the changes as implemented make a massive difference to either product design or testing (other than the amount a producer will have to pay for the testing). In general the updates to the calibration are worthwhile and should improve repeatability, but overall the update has only a few benefits that are clearly out weighed by the increase in test cost for a producer, hence increase in product cost since these will be amortised and with no perceived improvement in ESD immunity (with the exception of the additional ESD handling test prior to EMC performance tests). Apart from the improvements in calibration and clarification of the FPSC, most of the other changes appear to be pointless and suggest changes are being made just because the standard is getting a bit old and the committee need to show signs of progress. Personally I think the current edition is a better standard that this revision and I’d urge any voting readers to reject this discussion document.
References ISO/TR 10605: Road vehicles – electrical disturbances from electrostatic discharges (1994), ISO 10605: Road vehicles – test methods for electrical disturbances from electrostatic discharges (2001), first edition. ISO/DIS 10605: Road vehicles – test methods for electrical disturbances from electrostatic discharges (2007), ISO/TC22/SC3/WG3 N 1563 SAE J1113-13: 1995-02, Electromagnetic Compatibility Measurement Procedure For Vehicle Components- Part 13 - Immunity to Electrostatic Discharge IEC 61000-4-2: Electromagnetic compatibility (EMC) - Part 4-2: Testing and measurement techniques - Electrostatic discharge immunity test (1995), incorporating amendments A1: 1998 and A2: 2001 Biographical Notes Martin O’Hara is the founder of the Automotive EMC Network (www.autoemc.net) and the annual Automotive EMC Conference organiser. Author of “EMC at the Component and PCB Level” and numerous papers on EMC and circuit design. Martin has previously designed satellite navigation systems, vehicle tracking products and traffic information devices for Trafficmaster PLC and prior to that was Electronic Technologist at Motorolas’ Automotive division in their European Design Centre. Now Technical Director for Danfoss Randall Ltd.
