ESDA Threshold Vol 25 No.1 The Use and Abuse of the term “class 0”
Q1: A customer has asked if our facilities are certified to class 0 CDM. We are accustomed to class 0 for HBM, so is this the same for CDM?
A1: In November 2007, an Open Forum article in Conformity Magazine clarified that a “class 0” designation does not exist for CDM(Charged Device Model), and would not be added to the latest revision of the CDM standard which will be published in 2009. This is because there was not enough data and analysis at the time of the new release. The increasing use of the term “class 0” to refer to CDM by many users in the industry is being misused and needs proper clarification. The application of the “class 0” term to situations, places and things where it does not apply seems to be “out of control” based on the number of questions being fielded by the ESDA. The very first acknowledged and accepted use of the term Class X type designation can be traced to the HBM (Human Body Model) standards of the 1980s and the 1990s. The classes all refer to a range of pass/failure voltages for each “class X”. The highest and least susceptible class X for the HBM standard is class 3B ( > 8000V) and the lowest and most susceptible class X for HBM is class 0 (< 250 V). There are 5 more classes between these 2, all published in the standards. These classes are device testing (DT) classifications and the users who attempt to use these as a means to classify/certify their facility/factory are simply not in line with the proper use of the terms/designations.
It is well established that the ANSI/ESD S20.20 document is the standard which is to be used to certify a factory as an (Electro Static Discharge ) ESD controlled facility. More specifically, the scope of the S2020 standard states that the facility/factory is in compliance when its ESD control program can protect electrical or electronic parts, assemblies and equipment susceptible to ESD damage from Human Body Model (HBM) discharges greater than or equal to 100 volts. S2020 therefore addresses HBM directly, but not CDM (as yet) . Any stated facility/factory certification voltage number must always have a proper reference to HBM or CDM or MM whichever is relevant. Such voltages are simply those measured from the human in the factory or from an existing field regardless of the size of the existing capacitances, resistances or inductances. The DT classification number in the standard for HBM, CDM and MM are based on specific human, device or equipment related capacitance, resistance and inductance, but the existing factory number in S2020 is related to HBM only. There is no real exact correlation between the factory certification numbers and the DT classification numbers. The design engineers know the HBM failure voltage threshold of the device based on testing and Failure Analysis in the ESD and FA Labs. to accepted and established capacitance (100pF) and resistance (1500 ohms) from humans. They also know the failure voltages from devices for CDM and MM, but the factory only knows the voltage/in or voltage/cm based on the existing field where the measurements are made. If we use the term Q=CV or V=Q/C, we see that the different fields (from the different charges) in the factory could produce the same or a different measured voltage depending on how the capacitances change, but this idea, though not new, needs more data and analyses before it can be applied to the factory and related to a DT CDM class X. In recent years, many users in the IC industry have attempted to make the “jump” from HBM class 0 (< 250V) to a fictitious CDM class 0 (which does not exist) without the benefit of solid and relevant data from the factory. Users are now applying the term class 0 to CDM, and that is just “wrong”. Let’s be clear, the CDM standard has No class 0; repeat, No Class 0. The lowest and most susceptible classification for CDM is class 1 (< 125 V). Any attempt by users to relate any CDM class to ESD control in the factory must first be justified using data which is relevant. The industry council is working on this, and is expected to publish a CDM white paper II in 2009. A factory paper presented at the 2008 symposium in Tucson, presented data and a procedure for measuring fields and voltages in the factory, but the complete “bridge” to DT failure voltages was not made. It must be remembered that CDM Device testing (DT) relies on the capacitance of the individual device and the capacitance of the testing environment in which the packaged device is being tested.
Q2) We know the ranges for the capacitance of the devices. Is the capacitance of the factory environment known through which device has to travel? How and what can be accurately measured in the factory? Q? V? Are we stuck with Q and V only? What are the ranges within the factory from factory to factory?
A2) In summary then, it is not established by any standard that a class 0 exists when referring to CDM. Further, it is not yet established that any of the classes in the DT CDM document can be tied directly to the same or any specific voltages in an ESD control factory. ANSI/ESD S20.20 covers HBM class 0, but does not address the CDM issue. There is no correlation established as yet between factory ESD control voltages and DT voltages in the ESD test lab. Class 0 for CDM is fictitious, and should NOT be used in reference to CDM until it is established by a recognized
standard body. As of publication, the lowest and most susceptible classification for CDM is class 1 (<125V).