CTI refers to Comparative Tracking Index. It is a test to evaluate the 
     resistance of a material to form a conductive pattern when contaminates are 
     on the material. I believe the test requires a voltage be applied across a 
     material while an acid is dripped onto the surface. You should be able to 
     locate the CTI rating for raw materials on your suppliers yellow card, or 
     in their UL file. I see a CTI for some of Polyclads materials at 155. Both 
     UL and your laminate supplier should be able to help.
     
     John
______________________________ Reply Separator _________________________________
Subject: Re: laminate with CTI of 400+
Author:  [log in to unmask] at corp
Date:    8/14/95 12:59 PM


  On 10 August 1995, Dougal Stewart wrote:
     
    >> We have a customer who has been asked by his designers to      
    >> provide boards with a CTI of greater than 400 and a flammability 
    >> rating of 94V-1. They also ask if standard FR4 will comply - at 
    >> least I know the answer to that one.
     
     
    >> I am not sure what laminate will meet this requirement - FR3 
    >> has been suggested, but laminate suppliers always leave off  
    >> the CTI when quoting electrical properties.
     
     
  Dougal,
     
     I sent a response Friday to the second-half of your post, but then 
  came across this message that I hadn't read yet. I would like to add a 
  little more to the post I made Friday to help make that information a 
  little more useful. 
     
     When you speak of CTI, I'm not exactly sure what is meant by that...I 
  don't believe it's coefficient of thermal expansion (CTE) because the 
  values are expressed in parts-per-million...and I can't think of why 
  anyone would want a laminate that has a CTE of 400ppm. So I'm assuming 
  what it is that you're asking for is laminate that has a glass transition 
  temperature (Tg) of 400 degrees F. or above...that works out to 204.44 
  degrees celsius (400-32 x 5/9=degrees C.)...I'll list below the important 
  properties of the materials I refered to in my last email:
     
  (Note: All temperatures are in degrees C.)
     
     
     
                           Reinforcing Materials
     
                   Dielectric            CTE,ppm/degree C.
                    Constant
  Material           @ 1 MHz             Long.     Radial         
     
  E-Glass             5.5                 5.5       5.5