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February 2006

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Subject:
From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Mon, 13 Feb 2006 16:02:30 -0600
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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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