This is a topic I've addressed in my book. The temperature rating
is really cumulative. I can't give a direct approximation because of the
difference between the way assembly houses assemble boards, but
the Tg really applies for board assembly. The Tg should be more than
the temperature of the solder flow...cumulatively. Every pass
diminishes the board.
for an example, if your board has a (unrealistic) Tg of 150F and the
bath was 100F, the material would be suitable, but it the board
must go through 2 passes for assembly you are in trouble.
Double sided assembly is similar. An assembly house will take 2 passes
of the board through the solder flow.
I haven't seen a table or chart showing the amount of degradation in the 
material for each pass. Therefore we must use common sense methods
and determine first the number of passes required for assembly, (getting
a quote on assembly before specifying the material) and then determine
the material.
I think most use experience and "hard lessons" to determine these values.

Chris Robertson
Author "PCB Designer's Reference"
[log in to unmask]


  ----- Original Message ----- 
  From: Robert Tarzwell 
  To: [log in to unmask] 
  Sent: Tuesday, July 08, 2003 12:48 PM
  Subject: Re: [TN] Operating temperature of PCB


  Hi Michel

  even though most people feel that as long as the temp is below the T/g then
  everything is fine in PCB land, however some long term studies I did showed
  that in fact everything is not fine in PCB land. When you run a FR4 pcb
  hotter a couple of problems arise, first the hotter you get it the more it
  starts to carbonize, the longer you keep it at a high temp the worse things
  get. one study showed with as little as three months at 100 deg C lowers the
  dialectric voltage rating as much as 30 percent. the temp your talking about
  is right on the border line for FR4,even high temp stuff a switch to BT
  epoxy or polyimide may be in order , polyimide is 2 to 3 times the price but
  BT epoxy gives you a higher operationg temp with only 1.2 X the cost. The
  damage to the base lam is based on time at temp and max temp the hotter and
  the longer the more damage is done, I have seen even 80 deg c type temp turn
  a fr4 board brown over the years. I have advocated for years that 160 deg
  FR4 has a max long term operating temp of 100 deg C.  i would suggest you
  talk with some of the laminate tech guys  ( not sales type) and see what
  they say.
  robert tarzwell   megadawn.com

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