As an employee of a reflow manufacturer I would have to disagree with the comment
about attaching the thermocouple to the top of the BGA. You state that the delta T
will stabilize eventually but how long is eventually when a reflow profile is
typically 3 to 3 1/2 minutes? Also, the top surface will heat and cool faster -
the peak temperature may be the same but what about the time above liquidous? The
thermocouple on the top may show adequate time above liquidous while the solder
joints are not above reflow temperature long enough. As a fun little test you
might try attaching 2 thermocouples to the same BGA (one on top, the other at the
joint) to get a better idea of the delta here.

Brian

Jim Marsico 516-595-5879 wrote:

>   ------------------------------------------------------------------------
>
> Subject: RE: [TN] temperature profiling for BGAs
> Date: Thu, 26 Aug 1999 06:54:00 EST
> From: Jim Marsico 516-595-5879 <[log in to unmask]>
> To: TechNet <[log in to unmask]>
>
> The best way to develop your oven parameters is to install a thermocouple
> through the bottom of the board which protrudes the top directly into a solder
> joint.  You can attach it with a thermal epoxy or try soldering it with a high
> temp solder.  There is, however, at least one component manufacturer that I
> know of who recommends attaching the thermocouple onto the top of the
> component.  (I know, a lot of you will disagree...) They claim that the
> temperature differential will stabilize eventually.

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