Well better late than never

I've been following the talk about large conductive areas the 
past few days and would like to add my 2 cents.

PER  UL 796
1.Under  PATTERN LIMITS;   MAXIMUM AREA DIAMETER " The maximum
diameter of a circle covering the greatest continuous unpierced
area of a circle."  Therfore it's the size of the circle that counts not 
the total area of the ground plane.  This circle thing has a very 
real functional bearing on how much heat for how long a particular 
material can withstand before delaming and becoming a fire hazard.

2.  TEMPERATURE RATING; " The maximum allowable temperature on the 
board in the end product."  ??? Specified by design engineers?????
This "continuous operating temperture" determines the temperature the 
material is cooked at during the DELAM & BLISTER TEST.
      120 C = 166 C for 10 days or 143 C for 56 days
      130 C = 176 C for 10 days or 153 C for 56 days
>From my past experience this test has been fairly repeatable 
(accurate) for any particular FR4 resin  system.

As an example of the relationship between "max area dia" and "max 
operating temp"  I recently completed a U.L. upgrade project where we 
now have;
   1.  4 materials qualified at 120 C with a 4" circle.
   2.  Three of the same materials qualfied at 130 C with a 1" circle 
        One material at 130 C with a 4" circle
This last one is a recently developed "thermally stable" resin system.
The pattern limits and process limits are listed on fabricators U.L.
 yellow cards.

hope this helps! 

Robert E. Welch
Process Specialist
Email: [log in to unmask]
Modem: 804-239-9120
Fax:   804-237-3048

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