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.
----- Original Message -----
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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