Using the ground planes or even a metal core in the board to conduct heat
away from a hot component or part is done all the time... as long as there
is a low thermal resistance path for the heat to get to the ambient air (or
a black body for space vacuum applications) it should work... The heat
generating source will continue to rise in temperature until the heat
escaping it either equalizes with the temperature of the masses it is in
contact with it... or... until the source of the heat fails and stops making
heat altogether, in other words component failure.
There is a term called 'thermal runaway' that happens when the heat rises
faster than it can escape the part and it basically destroys itself... you
want to avoid that scenario. Just dumping heat into a thermal mass is not a
solution... The key to dissipating heat in the thermal layers is not
creating a thermal mass or 'heat pool' that has no path to the outside air.
It will only delay the inevitable... once it 'fills with heat' because there
is no path to the outside air, it will eventually rise in temperature to the
point where the component reaches critical temperature and fails.
'Clamping card guides' have been used for PCB's by the military for years to
get the heat out to the chassis and then to the air through the chassis. Any
hard mounted thermal path that has a low thermal resistance to the outside
ambient air would help. Just remember that each time you transition from one
surface or material to another (like from the heat tab on a TO-220 to the
Sil-pad under it and then from the sil-pad to the aluminum chassis) you are
creating a potential thermal resistance or 'barrier to thermal transfer' if
it is done wrong... and the more junctions you have the higher the temp of
the heat source or IC chip that is generating the heat will be from the
cooler ambient air.
Let's say you have this hot part soldered to a board with the leads of the
part conducting the heat into the conductors of the board and then through
them into the planes but fail to connect the planes to the chassis... what
will happen? When the unit is turned on... Heat generated in the IC will
initially travel into the planes through the thermal path because the planes
are cooler than the hot component and there is a low resistance path to the
plane. Let's say the resistance between the die and the chip it is mounted
to that is generating the heat in the circuit will allow it to be 30 degrees
above the temp at the solder joints due to the way it is mounted to the case
of the IC... As the temperature of the plane rises over time it will
eventually reach a point where the part reaches the failure point of the
chip at even though the soldered leads are 30 degrees cooler than the
silicon. That time to failure is dependent on the thermal mass of the
board... how much heat it takes to raise its temperature... so really the
planes if isolated only act like a delaying mechanism... they store
heat...and without a place for the heat to go, they will rise in
temperature.
You could almost think of it as a water tank that fills until there is no
more place for the water to flow so it backs up the system... If you are
still trying to fill it the water, the water will spill over at the
source... in this case because we are talking about heat, it causes the heat
spill over at the source causing it to overheat or exceed its rated
temperature. There must be an outlet for the heat to travel to... like water
to the ocean... or heat to the air, just providing a 10,000 gallon tank to
put the water into does you no good if after it's full if you still need to
get rid of more heat... you back up the system.
I think the design concern would be to make sure that you have a low thermal
resistance path to the air, and the temperature of the board material does
not exceed its thermal ratings in the process and the temperature of the
heat source does not suffer from thermal runaway because the path is
restricted somehow...
Hope that helps a little...
Bill Brooks - KG6VVP
PCB Design Engineer, C.I.D.+, C.I.I.
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
Datron World Communications, Inc.
_______________________________________
San Diego Chapter of the IPC Designers Council
Communications Officer, Web Manager
http://dcchapters.ipc.org/SanDiego/
http://pcbwizards.com
-----Original Message-----
From: Ofer Cohen [mailto:[log in to unmask]]
Sent: Sunday, February 12, 2006 7:26 AM
To: [log in to unmask]
Subject: [TN] Heat storage within PCB
Hi all,
One of the designers came with an idea: he has a high power component.
The space he has for a heat sink is small, so he wants to dissipate the
heat through the power layers. The component is a BGA, so the idea is to
put it on a metallic-capped via-in-pad by designing pad diameter of 24
mils, then define the solder area to 18 mils by solder mask.
Two questions:
1. Provided the temperature may rise to 120 degC, and the laminate
is high Tg low cost FR4 - are there any potential risks to the long term
reliability?
2. Are there any restrictions/hazards on deployment of defined
pads?
Regards
Ofer Cohen
Manager - Quality Assurance and Reliability
SIEMENS COM FN A SB
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