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A Deceptive Thermal Question for PCB Layout? Answered

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Alas ... I was never strong in thermodynamics.
This is for you  Printed  Circuit  Wizards
Here is my problem in a White LED  layout,  See first picture
  • Trying to pass excess LED heat to back side copper through VIAs
  • I know the larger the VIA Dia the lower the thermal impedance.
  • Intuitively as the hole grows it will eventually make heat transfer much worse 

What or how do I find the OPTIMUM VIA Diameter for Heat TRANSFER ??

15 Replies

user
verenceBest Answer (author)2013-09-20

I'd think, as long as the hole in the via is filled with a metal (i.e lead, tin, whatever) it would benefit the thermal conductivity. Once the hole is big enough t be a hole (i.e. has air inside) thermal conductivity would get worse. The diameter would depend on the process(es) used. So, you should contact the manufacturer for detailed information.

Without further ado, I'd just put some 0.5mm holes/vias in a 1mm grid. - As long as you don't have to pay for every single hole separately,

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user
iceng (author)verence2013-09-20

The wire in hole is an excellent suggestion,
but would not survive the production expense requirement.
I particularly like the wire in the holes as then they could be placed in radially alignment.

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user
verence (author)iceng2013-09-21

I did not mean to put a wire into the hole. I meant to make the via small enough that it will be filled with copper during the plating process that normally only plates the sides of vias and through holes. Or let the via get filled during soldering. If you use a wave soldering process, just keep the vias free of solder stop lacquer so they can suck up the tin. If you use SMD make a separate stencil and fill the vias with soldering paste before the reflow process. If you have a lot of money to spend, look for a PCB manufacturer that can produce copper inlays. The perfect way to transport heat away from the LED (and money away from you :-)

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iceng (author)verence2013-09-21

You are quite correct about the general lack of money. 
My client is a capable scientist and good friend that may change my direction
at the rise and fall of a new factoid.

There are SMD LEDs on one side and through hole components with SMD 317s
on the other side with room for 2oz  copper cooling surfaces.

Low volumes and manual assembly only profitable in a military setting,
makes for a can of worms rules for production arrangements.

I greatly appreciate the most excellent proffered suggestions from this great
community but I have my exact answer in that Nichia application note PDF
that I shared with you..

A

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user
Bill WW (author)2013-09-21

I got a D in thermo.
I suppose you have visited this site: http://www.powerguru.org/thermal-impedance-and-thermal-resistance/

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iceng (author)Bill WW2013-09-21

Thanks Bill

Yes ..... but only hours ago  from a previous comment.

A

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user
iceng (author)2013-09-20

A client brought this super Nichia application note PDF to my attention today.

Optimal Thermal Design with Through Holes

I take it to mean, that the experimental data suggest a 1mm to 1.2mm hole as a best VIA LED  heat transfer construct.

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user
steveastrouk (author)iceng2013-09-21

Nice link. Saved for future reference.

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user
steveastrouk (author)2013-09-20

Use aluminium PCB material.....

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user
iceng (author)steveastrouk2013-09-20

Ill bring it up.......

However  this is a Re_Grade => Buyer wants Lamp with LED stock is no longer available.
A New LED start to discontinued product cycle is under three years !!
Which is faster then gov purchasing reaction cycle.

Improved efficiency causes consternation because the current is lower and DIFFERENT then the spec.

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user
steveastrouk (author)iceng2013-09-21

Put some 1W power resistors on the board ;-)

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user
iceng (author)steveastrouk2013-09-21

It was a serious option for a short one time meeting.

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user
iceng (author)iceng2013-09-20

PS running white LEDs cooler and below continuous design current significantly
extends Life and color of the phosphors being used.  

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user
mpilchfamily (author)2013-09-20

What does the data sheet for the LED say. That's the kind of information that will be on the data sheet. Especially if additional cooling is required.

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user
iceng (author)mpilchfamily2013-09-20

it's a white XL CREE LED... see the PDF
Most thermal management involves many mini VIAs directly under the LED.

My client supplies military lights and there are a mixture of conflicting product specifications
that were probably developed early in the LED white conception cycle,
Color being very high on the ranking order.

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