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Safeguarding Compliance of Creepage and Clearance by 3D Analysis

Printed Circuit Boards (PCBs) carrying higher voltages need to comply with requirements according to national or international standards for specifying creepage and clearance minimum distances. There are a significant number of such standards, that allow for specific applications different variants of demands and methods. Generic principles can be identified, though, that seem to rectify a fundamental concept for testing. The currently industrially available testing methods are purely heuristic and mostly manually based.

Here we’ll describe and analyze problems of identified requirements of DIN EN 60664 (VDE 0110), and the sub-standards based on it. By using a 3D printed circuit board CAD system, creepage and clearance can be checked automatically, and an electro-mechanical assembly can be analyzed according to given standards. A comparison concerning reliability and efficiency to available methods is shown by using an example from a product launched from a major automotive supplier.

Creepage and Clearance: Definitions
The goal of creepage and clearance analysis and measures based thereof, the so-called insulation coordination is the protection against hazards for humans and the failure of electrical systems by:
⦁ Flashover across clearance distance or
⦁ Creepage along insulator surfaces
Within an electronic equipment during the entire product lifetime operating in the specified environment.

Insulation coordination is specified nationally and internationally in various standards, e.g. IEC 60664-1 and its sub-standards, and in national standards mostly derived thereof. A third phenomenon, electrical breakdown through insulation material is not covered here.

Creepage is defined as the shortest distance along the surface of an insulator between two conductive energized elements (Fig 1). The determination of a minimum creepage distance requires the following parameters to be defined:
⦁ Permanent voltage stress (root mean square (rms)) of the circuit
⦁ Overvoltage category of the involved conductive elements
⦁ Pollution degree of the intended environment
A minimum allowable creepage distance can be identified.

The tendency of an insulation material to suffer building up a creepage path is noted in the so-called CTI (Comparative Tracking Index) value, that is identified in empiric investigations. In the mentioned tables, the CTI-value is honored for the selected material. The process of creepage path creation is a slowly proceeding process of material deterioration of the insulator by carbonization under voltage stress. Image 2 shows the building of dendrites of an insulator along a creepage path. (Source: Department of Engineering – University of Leicester)

Clearance
Clearance is defined as the shortest distance through air between two conductive energized elements.

Clearance between two conductors with differing field characteristics
To identify the minimum clearance distance the following parameters need to be characterized:
⦁ Permanent voltage stress (root mean square (rms)) of the circuit
⦁ Overvoltage category of the involved conductive elements
⦁ Pollution degree and other characteristics of the intended environment

The determination of the minimum clearance distances uses these values in the relevant standards’ tables. These empirically determined values additionally honor atmospheric pressure of the geographical application height and relative air humidity.

On the contrary to the creepage path generation, which proceeds over some significant time, spray discharging or breakdown happen immediately when the puncture resistance of the clearance distance is exceeded. Therefore, the peak values of surge voltages are taken for dimensioning the clearance distance.

Analysis of Creepage and Clearance Paths
Principal geometrical considerations reveal, that an allowable creepage distance between two conductors can never be smaller than the corresponding clearance distance between these. May the standards’ values for two conductors initially show smaller values for creepage than for clearance, these latter ones need to be replaced with the greater ones for the clearance.
The analysis of an electronic equipment for the compliance of minimum creepage and clearance distances is the task to find conductive, energized points within a design, whose relevant distance is smaller than the identified minimum value. Thereby the paths via insulators for creepage and paths through air for clearance are relevant.

Furthermore, a number of rules apply that complicate the identification of such paths significantly. One of these rules, e.g. is that all conductive elements short such paths, regardless if they are energized or not.
For analyzing a product, all conductive and energized elements of a circuit have to be checked against the elements of all other circuits. Beyond that, it is common to group the nets of a circuit with similar voltages into groups and handle each of these as one conductor. Can a path be found whose length underruns the allowable minimum distance, its length is known.

The state-of-the-art method of identifying violating creepage and clearance paths is to identify elements of significant voltage differences from the schematic and search the assumed shortest path between these in a physical prototype or CAD model and measure their length. On physical prototypes, the lengths of these paths are at best measured with rulers or even worse by measure of thumb.

On printed circuit boards, another method is known to identify and avoid creepage paths: Circuits of highly different voltages are layout into different enclosed areas with minimum pre-defined distances between such areas.
For printed circuit boards measuring of creepage distances on inner layers isn’t possible at all on the real product. To bypass that problem, the production data (e.g. Gerber plots) are paper printed and paths measured with a ruler.
Computer-aided-design (CAD) systems for printed circuit design support pcb layout quite efficiently. Their most important task is to check for some clearances between conductive design elements on individual layers of the board, nevertheless no creepage analysis is supported by conventional pcb layout systems.

Analysis for violations of clearance (air gap) distances needs to be processed in 3D modelling systems like 3D mechanical CAD systems – or on physical prototypes. As MCAD systems do not have a conceptual knowledge about conductive elements and their imputing consequences, only simple manually governed analyses are possible: Direct point-to-point distances and rarely composite polygonal paths are likely to be measures which need to be governed by assumed shorted path segments given by experienced personal. This is not a systematic automated process which is needed for highly qualified products.

NEXTRA – a 3D PCB Layout System
For example, NEXTRA is a software product that unites the features of a 3D mechanical CAD system and a pcb layout system in one piece of software. While 3D mechanical CAD systems are able to characterize the 3D shape of a rigid body, they do not endorse the conductive and logical characteristic of electronic devices that are used on printed circuit boards.

The 2D CAD systems for pcb design however, work – due to the fact that the manufacturing technology for pcbs is a lithographic one – only with a 2D representation of their design elements. They feature however a conception of conductive and insulating elements, electronic and logical devices and their intended connecting elements. NEXTRA targets to overcome the inherent restrictions of such limited CAD conceptions and allow fundamental features of both such CAD systems within one system alone. Problems not being able to be addressed in conventional system should finally be solvable in NEXTRA.

NEXTRA features importers for 3D mechanical and 2D pcb design data of the most important design systems to make their input data to that of its own data model. This is true for 3D geometry from MCAD and also logical and electronical information and 2d geometry from pcb systems. With that input NEXTRA creates one model of the pcb and additionally the mechanical assembly surrounding it.

For the creepage and clearance analysis NEXTRA furthermore imports information about circuits, resp. net classes and minimum distance values that might have been defined in the pcb system’s constraint editing/information system (CES, CIS, …). NEXTRA also allows for manual definition of such rules.

Creepage and Clearance Analysis with NEXTRA
With imported information analyses regarding creepage and clearance compliance can be done fully automatically. NEXTRA performs a full analysis of all possible paths, which might entail a violating creepage or clearance path. To accomplish that efficiently, the algorithm uses any means to finish a partial search as soon as the length of the current path is longer than the minimum required distance.

NEXTRA needs information about the material characteristics, even more precisely the electrical conductivity, of design elements. For a subset of the electro-mechanical assembly this information is implicitly given: With the conductive and the insulating elements of the PCB everything is given. Anything beyond that needs to be assigned material characteristics, either in a MCAD system or within NEXTRA.

Creepage analysis is split in two different modules, where the part working on the PCB can profit from the inherent specifics of the PCB design using laminates of unique thickness, conductive elements of limited geometric complexity and so on, which the 3D creepage algorithm can not assume. Clearance analysis dos not distinguish between the PCB and other parts of the assembly.

Analysis Results
Analyses in NEXTRA eventually result in a list of identified violations of the given minimum distances. Using the GUI individual or a set of violations may be selected and analyzed further in more detail. The fundamental characteristics of the violation like identified distance, required distance, name and type of the involved elements, position and layer of the board are shown in the results list.

Due to the fact that the 3D creepage and the clearance analysis operate on finite elements, the result can not only identify paths that just underrun the minimum distance rather than can identify a range of paths, that underrun a certain distance and present areas of such paths with colors identifying decreasing path lengths from red (smaller than the minimum distance) via distances just over the minimum distance (orange) up to a path length that clearly surmounts the required distance (green) as can be seen in image 4. Such areas are grouped into so-called hot-spots and handled as one violation.
Image 4 shows the result of a clearance analysis of a product of an automotive supplier’s product. As can be seen NEXTRA could identify some clearance violations in an early design state that could be fixed before going into production.

Image 4 Display of the clearance violations identified by NEXTRA of an automotive supplier’s product

The features of NEXTRA for creepage and clearance of can be used to analyze an electro-mechanical assembly if it complies with standards. The identification of all possible paths and their discrimination in violating and not violating paths relieves designers from the residual risk of a limited statement of a manual supervision according to given standards.

 

By Dr. Thomas Krebs, Mecadtron {www.mecadtron.de}

For more information visit www.FlowCAD.com.

Components & Devices Technology

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