Method for evaluation of short-circuits caused by whiskers on PCBs and on electrical fuses

Status:

completed 01/2012

Aims:

The creation of safety-related machine controls generally involves the use of electronic assemblies which include printed circuit boards (PCBs) populated with electronic components. The effects of component faults and other factors are studied in order to ensure that the safety-related requirements are met. It is essential in this context that fault exclusions be assumed for the formation of short-circuits between adjacent tracks and between component terminations.
Following introduction of the RoHS Directive (EU Directive 2002/95/EC on the Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment), manufacturers of PCBs must now use lead-free solder for the soldering process instead of solder containing lead. On PCBs manufactured in this way, the growth of electrically conductive tin whiskers enables short-circuits to be created which rule out the assumption of fault exclusions. This would make economically viable development of PCBs for use in safety technology virtually impossible.
Within this project, a procedure was to be developed by means of which the risk of whisker formation on PCBs can be assessed and reduced, and which where appropriate permits the assumption of fault exclusions despite the use of lead-free soldering.
A further effect of the RoHS directive concerns low-voltage high-breaking-capacity fuse elements used for overload protection, for example in electrical systems in sewage plants. The chromate conversion coating of the contact material used in the past for protection against whisker growth is no longer permissible. Owing to the high voltage used on these components, arcing may occur. Accidents have already occurred as a result. A decisive parameter for whisker formation in this case is assumed to be the influence of ambient air with a high sulphur content.
A further objective of the project was the development of a procedure by which the risk can be reduced of silver whiskers forming on fuse elements in sulphurous environments.

Activities/Methods:

A literature survey was conducted of tin whiskers on PCBs, and manufacturers' experience gathered. Industrial PCBs were used to study the influence of the components and manufacturing processes used upon the creation of tin whiskers. The emphasis here lay upon the effects of different PCB coatings, the solder used, and the mechanical stress upon the PCB. Different types of PCB were considered by variation of these parameters. Whisker formation was then induced at increased ambient temperature. The test results were analysed and a fault model developed which must be applied when PCBs soldered lead-free are used in safety-related components.

No further studies were performed with regard to the LV HBC fuse elements, since no further problems have been reported since the IFA proposals (pole grease and gel mats) were implemented.

Results:

The project revealed that the fault exclusion provided for in EN ISO 13849-2, Table D.5, "Short-circuit between two adjacent tracks/pads" is now subject to limitations where lead-free soldering is performed on PCBs. In addition to D.5, it can be assumed tin whiskers of up to 1 mm in length are formed and that they cause short-circuits. Only short-circuits which connect redundant circuit groups with each other are critical, however. Short-circuits within one of the redundant circuit groups have the same effect as component faults, which must be assumed even in the absence of whiskers. In order for short-circuits between redundant circuit groups to be avoided, either intervals of > 1 mm must be observed between the circuit groups during development of the PCB, or a protective coating must be applied.

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