RE: [SI-LIST] : FR-4 and PCB Tolerances

Dave Hoover ([email protected])
Mon, 30 Aug 1999 08:52:02 -0700

Here in the USA we have UL that additionally sets and maintains
criteria on this.

CTI Range (Tracking index <in volts> Assigned PLC
600 and Greater 0
400 and up to 600 1
250 and up to 400 2
175 and up to 250 3
100 and up to 175 4
Less than 100 5

PLC=Performance Level Category

Most FR-4 lands in PLC #3


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In reviewing the Low Voltage Directive now required for CE, there is a table
(6)


of the relevent standard EN 60950 which calls out minimum creepage
distances.  
In this table they have classified three materiials I, II, IIIa & IIIb.  The


material classification depends on its Comparative Tracking Index (CTI)
where:
Material group I  600 <= CTI
Material group II 400 <= CTI
Material group IIIa 175 <= CTI
Material group IIIb 100 <= CTI


CTI is obtained in accordance with method A of IEC 112.
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