The term "thermal mass" is often used interchangeably with "thermal capacitance"
and "thermal inertia".  It is important during transient conditions, when 
something happens to make component temperatures change (change in external 
temperature or pressure, for example).  Components with more thermal mass will 
change less rapidly than those with very little thermal mass.  The thermal mass 
(usually thermal capacitance in textbooks) is used to determine the thermal time
constant of the component.  The formula

            time constant = thermal resistance x thermal capacitance

gives the time necessary to complete about 63% of the change from the initial 
temperature to the final temperature.

I hope this is the information you wanted.  You may E-mail me at 
[log in to unmask] if I can be of further assistance.

Bruce Smith
Smiths Industries


Hi,

How do people claim the existence of thermal mass issue?  

Say if I have a PWA with large and small components.  Do I measure the 
surface temperature of those components and relate the issue to the 
temperature discrepancy?  OR do I measure the temperature underneath the lead 
(or even the bottom of the components)?

Thank you very much.

Regards,
Victor




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