Question: When creating a temperature profile on a 6 layer board, with fine 
pitch parts, in a convection reflow oven, with one thermocouple on the fine 
pitch leads, and the other on the board surface near the leads....

Is it possible that the PWB surface could be heating up and cooling down 
faster than the leads?

I thought the low mass leads would heat up and cool down much faster than 
the high mass PWB Pads, but I have a profile from a supplier that seems to 
indicate that the pads are leading the leads in temperature change.  I did 
not observe the setup and am not able to physically verify the location of 
the thermocouples and correlate them to the resulting profile curves.  The 
supplier, however, maintains that the board is reaching reflow temperature 
before the leads.  I would like to give them the benefit of the doubt, so 
I'm asking, is it possible?

I know with convection, heat is transferred to physical objects by contact 
with the hot air, and this would make temperature increase dependent on both 
mass and surface area.  Even though the PWB may be 100 times more massive 
than the fine pitch leads, could the PWB surface area to its mass ratio be 
enough higher than the lead mass to its surface area ratio to account for 
the statement that the PWB heats up faster than the leads?

Thanks for your comments.
Regards,
Scott Lolmaugh

Post or reply to [log in to unmask]
 ----------------------------------------------------------------------------  
 ------

***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To unsubscribe from this list at any time, send a message to:           *
* [log in to unmask] with <subject: unsubscribe> and no text.        *
***************************************************************************
* If you are having a problem with the IPC TechNet forum please contact   *
* Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask]      *
***************************************************************************