Characteristics and ratings – i – Tutorial
M J Maytum Rev.0, May 2014
Characteristics and ratings – Definitions, test procedures and product specifications.
Abstract
This is a tutorial on characteristics and ratings as used in semiconductor engineering, but these are generally applicable to other technologies. Characteristics are a measureable property of a product. An individual product item characteristic value is measured with the product family specification test conditions applied. The product family specification value limits for that characteristic can be given either as a characteristic minimum or maximum or both or alternatively as a rated value. Product family ratings consist of rated values and (absolute) maximum ratings. Rated values are measureable during the rated value test. Absolute maximum ratings are not directly measureable and have to be verified. The absolute maximum rating test procedure is to first apply the product family specification absolute maximum rating conditions to the product. After that specified product family characteristic values are measured to detect any product degradation. These measured product characteristics should not be outside the degradation limits set by the product family specification.
The document content is of a general nature only and is not intended to address the specific circumstances of any particular individual or entity; nor be necessarily comprehensive, complete, accurate or up to date; nor represent professional or legal advice.
Contents
1. Introduction 1
2. Finding and crafting term definitions 1
3. Characteristic definition 2
4. Rating definition 3
5. Summary 5
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Copyright © M J Maytum 2014. All Rights Reserved.
Characteristics and ratings – 6 – SPDC Tutorial
M J Maytum Rev.0
Characteristics and ratings – Definitions, test procedures and product specifications
1. Introduction
A recent working group dialog revealed that there was some confusion over what a product characteristic is and what a product family rating is. One approach to resolve this matter is to consult the definition for these terms in a dictionary or glossary created by a standards development organisation (SDO). There are four sections to this document; finding and crafting term definitions, definition of “characteristic”, definition of “rating” and a summary.
2. Finding and crafting term definitions
My favourite reference work is the IEC Glossary. The Glossary is the collected terms and definitions taken from IEC standards published since 2002. The ISO/IEC Directives require that the definition formulation shall be able replace the term in a sentence. This means the definition shall be a phrase and any definition starting with, for example, “the” shows the formulation group didn’t understand the rules and the ISO/IEC editors were asleep at the wheel when the document was edited at central office. Being a phrase is limiting, meaning that usually there is only room for the essential elements of the term to appear. Informative information can be relegated to additions like notes or examples.
The IEEE doesn’t currently have the phrase constraint allowing some IEEE definitions to look more like a chapter of a book. May be the IEEE-SA should apply the Twitter tweet restriction of no more than 140 characters.
Because many standards use the same term, the glossary contains many entries for a given term. For example the term “routine test” occurs 81 times in the Glossary at the time of writing this document. This means routine test is defined in at least 81 IEC standards. In the perfect world all the definitions would be the same, but the world isn’t perfect and people inadequately formulate, tweak and customise definitions.
Here is an example of a (claimed) customisation of the “term” definition in IEC 60050-151 by IEC SC37A WG5
routine test: test made on each SPD or on parts and materials as required to ensure that the product meets the design specifications
Published in: IEC 61643-11, ed. 1.0 (2011-03)
Source: IEC 60050-151:2001, 151-16-17 (modified)
Customisation proliferates definition variances, restricts usage of what should be generic item into a specific one with the potential of causing confusion. As terms and definitions are shown without the rest of the standard they occur in, the acronym SPD should have been explained by writing “surge protective device” in full. Likewise references to figures and clauses in a standard should be avoided.
What does the referenced IEC 60050-151 source definition look like?
routine test: conformity test made on each individual item during or after manufacture
Quite how IEC SC37A WG5 justifies their definition is a modification of the IEC 60050-151 one is an interesting debate. The IEC 60050-151 definition is an example of daisy chaining as you now need to find out what a conformity test is.
conformity test (compliance test): test for conformity evaluation
OK, one last time
conformity evaluation: systematic examination of the extent to which a product, process or service fulfils specified requirements
Bearing in mind the term can be replaced by its definition we have:
routine test: test for systematic examination of the extent to which a product, process or service fulfils specified requirements made on each individual item during or after manufacture
The two substitutions don’t quite work and that’s the problem with daisy chaining definitions, you need to check that replacement works.
When using the IEC Glossary it is important to pick the most relevant definition for your purpose with an emphasis on a generic definition rather than a customised version.
3. Characteristic definition
The IEC Glossary returns seven definitions for the term “characteristic”. The most suitable definitions found were:
characteristic: inherent and measurable property of a device
Published in: IEC 61643-341, ed. 1.0 (2001-11)
characteristic: a physical, chemical, visual or any other measurable property of a product or material
Published in: IEC 62680-4, ed. 1.0 (2010-10)
The IEC 61643-341 definition restricts the item concerned to a “device” (81 IEC Glossary entries) a term that can interpreted in many ways. The most suitable definition for our use is:
device: combination of components having a given function, forming part of a piece of equipment, apparatus, or system
Published in: IEC 61000-5-6, ed. 1.0 (2002-06)
IEC 62680-4 uses the more generic “product” term (33 IEC Glossary entries) rather than “device”.
product: thing or substance produced by a natural or artificial process
Published in: IEC 61360-2, ed. 3.0 (2012-10)
Some other useful definitions are:
product family: group of products each of which contains the same substances or material at a similar concentration level
Published in: IEC 62474, ed. 1.0 (2012-03)
product specification: document which defines the parameters that are used for determining the expected performance of a product
Published in: IEC 62059-11, ed. 1.0 (2002-01)
specification: document that specifies, in a complete, precise, verifiable manner, the requirements, design, behaviour or other characteristics of a system or component and, often, the procedures for determining whether these provisions have been satisfied
Published in: IEC 61513, ed. 2.0 (2011-08)
The IEC 61513 definition could be made more generic by replacing “system or component” by “product”.
From the above one could simply state that a characteristic is an inherent product property that can be measured.
The procedure of checking or establishing parameter values is termed characterisation.
characterisation: process of testing a sample of components to determine the key electrical parameter values that can be expected of all produced components of the type tested
Published in: IEC 62396-1, ed. 1.0 (2012-05)
4. Rating definition
The IEC Glossary returns twenty-four definitions for the term “rating”. The most suitable definitions found were:
rating: value that establishes either a limiting capability or a limiting condition for a semiconductor device
Published in: IEC 62240-1, ed. 1.0 (2013-04)
rating: set of rated values and operating conditions
Published in: IEC 62850, ed. 1.0 (2013-02)
Source: IEC 60050-151:2001, 151-16-11
The IEC 62240-1 definition is specific but could be changed to generic by replacing “semiconductor device” with “product”.
A English language reference dictionary for IEC definitions is the Oxford English Dictionary (OED). There we have:
capability: the power or ability to do something
condition: the state of something with regard to its appearance, quality, or working order
My interpretation is that the IEC 62240-1 rating definition covers two attributes; firstly what the product characteristic limits are e.g. maximum limiting voltage and second what are the maximum stresses that can be imposed on the product e.g. maximum surge current.
The IEC 62850 definition is generic, two attributes again and we need to clarify the meaning of “rated values”and “operating conditions”.
For “rated values” one can find:
rated value: value of a quantity used for specification purpose, established for a specific set of operating conditions
Published in: IEC 62246-1, ed. 2.0 (2011-02)
rated value: value of a quantity used for specification purposes, established for a specified set of operating conditions of a component, device, equipment, or system
Published in: IEC 61236, ed. 2.0 (2010-10)
Source: IEC 60050-151:2001, 151-16-08
rated value: assigned value, generally by the manufacturer, for a specified operating condition of a component, device or equipment
Published in: IEC 60974-1, ed. 4.0 (2012-06)
For “operating conditions” one can find:
operating limit: stress level above which the product no longer functions nominally.
Published in: IEC 62500, ed. 1.0 (2008-07)
absolute maximum ratings: limiting values of operating and environmental conditions applicable to any semiconductor device of a specific type as defined by its published specification data, which should not be exceeded under the worst possible conditions
Published in: IEC 62240-1, ed. 1.0 (2013-04)
Source: IEC 60134:1961, Clause 4
The IEC 62240-1 and IEC 62850 definitions each have two attributes which can have equivalence if we say:
Attribute 1: product characteristic limits (IEC 62240-1) is the same as rated value (IEC 62850)
Attribute 2: maximum stresses that can be imposed (IEC 62240-1) is the same as stress level above which the product may no longer functions nominally (IEC 62850)
Attribute 1 is the one that causes confusion as it may be stated in the product specification either as maximum or minimum limits of the characteristics or as a rated value in the ratings. Both of these can be measured as a characteristic.
Here’s an example for cable current carrying capacity:
current carrying capacity: maximum current a cable circuit (one or several conductors) can support resulting in a specified increase of the surface temperature of the conductor beyond the ambient temperature, not exceeding the maximum allowed operating temperature of the cable
Published in: ISO/IEC 29125, ed. 1.0 (2010-09)
Source: IEC 61156-1:2007, 3.24
The test procedure here is to apply the specified maximum current to the cable and measure the temperature at thermal equilibrium at the specified ambient. The measured temperature value should not exceed the maximum cable operating temperature. This could be expressed in characteristics as a maximum cable temperature value for a specified ambient temperature and applied current. Alternatively it could be expressed as a rated current value for a specified ambient temperature and maximum temperature rise.
Attribute 2 is definitely only a rating as the test procedure is to apply the maximum stress given in product specification and then measure specified product characteristics to verify the product has withstood the stress without specified degradation.
Here’s an Absolute Maximum Rating example for a GDT:
Durability:
· 10 applications of 50 Hz, 1 s, 2 A
· 10 applications of 8/20, 1 kA
· 100 applications of 10/1000, 10 A
After a stress test the product characteristics shall be
· DC spark-over voltage 400 V ±30%, initial value was 400 V ±25%
· Impulse spark-over voltage at 100 V/µs < 900 V, initial value was < 800 V
· Impulse spark-over voltage at 1 kV/µs < 1050 V, initial value was < 950 V
· Insulation resistance at 100 V DC > 108 Ohm, initial value was > 109 Ohm
In this case, after the applied stress, the product must meet a relaxed set of specified product characteristic limits. There are other post test limits that can be used, such as meets product family specification characteristic limits or the product characteristic value change between pre and post stress must not be more than a certain amount.
5. Summary
Characteristics are measured. The procedure is to apply the product family specified conditions and measure the product characteristic value.
Depending on how the product specification is written the measured characteristic value would be compared with either the specification characteristic limits or the specification rated value limits.
The product manufacturer’s conundrum here is whether to specify a limit characteristic value as max. or min. or a rated value of the applied test condition, e.g. the current in the cable case above.
Personally I prefer the characteristics listing, giving the option of adding a typical value. This would then leave ratings as purely absolute maximum ratings.
Absolute Maximum Ratings are verified by looking for characteristic value changes after the stress test. The two step procedure is to first apply the product family specified absolute maximum rated stress conditions to the product. Following the stress test, specified product characteristics are measured to verify the product has withstood the stress without a characteristic value change that exceeds the specified degradation limits.
The following figure summarizes the situation. The top part of the figure deals with the two product test procedures, both of which produce measured characteristic values. The bottom part of the figure is the product family specification, whose entries can be; characteristic typical or limit values, rated values and (absolute) maximum ratings.
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Copyright © M J Maytum 2014. All Rights Reserved.