« The Nutcracker and Job Creation - Dick LaRosa | Main | The JFET and the VOM - John Dunn, Consultant, Ambertec, P.E., P.C. »

December 22, 2011



I would expect there to be some effect of the distributed thermal resistance inside the block as well. Especially in less conductive materials, this could dwarf the interface "R". I'd be interested to see how this is treated, it becomes even more of an interest when there's "something" on the other side of the main mass that is the real site of interest (like, say, a die on top of a Cu-W heat slug on top of that thermal plane).

Rick Collins

There are some assumptions missing in the above example. To get a temperature from a thermal resistance I believe there has to be a constant heat flow from the base plate through the mass. That means there has to be a thermal resistance from the mass into the environment, normally considered a type of "heat sink" at a constant temperature.

I suppose this is detailed in the previous article referred to here but not cited in a way I can find it.

John Dunn

It wasn't detailed in the previous item either. This is admittedly simplified in that sense, but temperature excursions do take time to happen. Thermal time constants are involved but often overlooked which is the point at hand.

Nick Name

In fact the problem is much more complex than described. The temperature rise time constant at the interface between source and heat sink is function of : thermal resistance at contact (as mentioned), diffusibility of heat in the sink material (not mentioned since conductivity is high) and thermal resistance at the sink-fluid surface. The assumption is made that the whole sink mass is at same temperature which is valid ONLY if the heat source has a quite low dynamics with respect to the heat diffusion, a fast changing source will lead to local higher temperatures since the heat cannot be diffused in the whole mass. From the other side the highest temperature depends on the possibility for the sink to transfer heat to environment. This last property is not only function of area and fluid velocity but also of the boundary layer thickness. This is the reason why profiles are designed so that a high turbulence occurs.

Verify your Comment

Previewing your Comment

This is only a preview. Your comment has not yet been posted.

Your comment could not be posted. Error type:
Your comment has been saved. Comments are moderated and will not appear until approved by the author. Post another comment

The letters and numbers you entered did not match the image. Please try again.

As a final step before posting your comment, enter the letters and numbers you see in the image below. This prevents automated programs from posting comments.

Having trouble reading this image? View an alternate.


Post a comment

Comments are moderated, and will not appear until the author has approved them.

Your Information

(Name and email address are required. Email address will not be displayed with the comment.)


  Learn about our  

 free engineering 

consulting referral 

      service at:


  IEEE Consultant's 

      Network of 

     Long Island


We have over forty 

electrical, electronic, 

mechanical and 

software engineers 

with expertise in more 

than sixty-five categories 

of technology and business. 

All are members of the 

IEEE and adhere to the 

IEEE professional codes 

of ethics. 

No fee is charged for the

referral service. 

Each member is an 

independent consultant 

and negotiates his/her 

own consulting agreement.


Editor: Jerry Brown


John Dunn

Marty Kanner

Murray Kleiner

Dick LaRosa

David Pinkowitz

Carl Schwab

Gerry Bodner

Larry Rachman


Unless otherwise noted, 

reprinting or republication 

of anarticle on this blog is 

authorized by crediting the 

author and prominently 

displaying the following 

sentence at the beginning 

or end of the article,

including the hyperlink to

IEEE Consultant's Network 

of Long Island

"This report is republished 

with permission of IEEE 

Consultant's Network of 

Long Island"



Blog powered by Typepad

Enter your email address:

Delivered by FeedBurner