Hi Richard,
I see I failed to mention that the thermal model I was using pre-supposed
for illustrative purposes that convection (or transfer of heat directly to
moving air) was not an option... Moving air over the surface of a pcb
assembly can greatly reduce the need for conducting heat through the metal
core or planes to a 'thermal conduction' solution as long as it is free air
and not trapped inside an enclosure and as long as there aren't any 'heat
traps' or obstructions to the flow of air. True... the solutions are more
complex than solving for a single component and the 'reservoir' gets filled
more rapidly when there are more 'tributaries' into it... ambient temp rises
inside an enclosure due to the heat transfer to the local air and affects
the ambient adjustment to the Tjc of every component in the enclosure.
Thermal vias are a part of many component foot print solutions now days and
there are good articles on the implementation of them. Check any component
with a belly pad...you will most likely see notes about thermal vias.
But when using convection or air flow to cool... this does little to solve
your heat problems without a thermal conduction solution as well. Each
design must take into consideration the environment into which the board
will be exposed as well as the entire assembly.
Judy , I probably wasn't too clear there... my apologies. I was looking for
a way to describe a radiant thermal transfer from the exterior of an
assembly into space as a thermal path rather than a transfer to air through
convection.
"'Black body' refers to a surface that absorbs all radiation incident upon
it and it must radiate in the same manner if it is in thermal
equilibrium..."
http://www.egglescliffe.org.uk/physics/astronomy/blackbody/bbody.html
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: Stadem, Richard D. [mailto:[log in to unmask]]
Sent: Monday, February 13, 2006 2:03 PM
To: TechNet E-Mail Forum; Brooks,Bill
Subject: RE: [TN] Heat storage within PCB
A Carnot cycle can indeed be created in a PWB, specifically one that has
several sources.
In this case the coolant is the PWB itself. Unfortunately this includes the
other nearby components connected to the PWB.
The efficiency ç of the heat or energy transfer is defined to be the amount
of work divided by the amount of heat transferred between the cooling system
and the hot reservoir, the component and/or PWB.
The heat sources can be external to the pwb, such as a power supply placed
nearby, or in conjunction with several sources internal to the pwb,
typically dc converters, transformers, high wattage resistors, high-speed
processors (BGA or otherwise), Zener diodes, lamps, high-Z inductors, and
other power dissipators.
Bill is correct in that the cooling system must be designed to be able to
radiate more heat than the component can generate. But the cooling system
for a single BGA or component must have sufficient capacity to wick away the
heat faster than both the component AND the pwb and other components
immediately surrounding the component can generate, or the problem feeds on
itself.
Attempting to use the PWB as the refrigerant is not usually a good idea, as
trying to wick away heat through a thin piece of copper insulated inside of
fiberglass is certainly not a very efficient engine.
Although a common practice, sinking heat into the pwb is never a good idea,
just for the reasons spelled out in Bill's email below. The heat from one
device on the assembly can then act on others, and as they heat up they also
in turn generate more heat.
More heat stresses placed on the components doesn't do anybody any good. It
ultimately leads to some reduced time-to-failure somewhere. In most cases,
if a metal-clad PWB or 2 ounce copper power/ground planes is enough to
transfer the heat to the outside world, fine.
But a component generating 120 deg. C. is extremely hot, and in this case
will cause problems if not cooled properly. Cooling properly is defined as
placing a heatsink directly onto the component to dissipate the heat into
air, or in the case of space product, into the chassis frame or a liquid
nitrogen blanket. These methods should always be used whenever possible,
rather than hoping the pwb and the other components nearby can absorb the
heat. They may be able to absorb it but reduced reliability, like death and
taxes, is inevitable if you choose to do it this way. And as Bill pointed
out, thermal runaway can eventually take place, and the "flameout" that can
occur when one component succumbs to overheating will quite often cause a
chain reaction where several components will overload in a single
catastrophic failure.
Cool it correctly. Take the heat out of the component and the pwb, and
dissipate it away from the assembly.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brooks,Bill
Sent: Monday, February 13, 2006 10:35 AM
To: [log in to unmask]
Subject: Re: [TN] Heat storage within PCB
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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