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Date: | 03 Feb 1997 12:04:28 -0600 |
Content-Type: | text/plain |
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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
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