Wade,

   For back to back BGAs I use a profile that is
primarily top side heat only. There is little or no
bottom side heat in order to prevent Re-Reflow. The
profile ramps quickly to the >183C mark stays there
for as short a time as possible. Then quickly ramps
down. The curve looks something like an arrow point
straight up.

|        ^
|------/---\----------------------- 183C
|    /       \
|_/______\_____________

For bottom side components, I will cover them with
Kapton tape and instruct the operators to keep a close
eye out for any movement. I also work dilligently to
keep bottom side board temperature at something less
than 183 C. On some boards its is unavoidable due to
the layup of the various planes etc. but not
impossible. If the component and profile situation
becomes impossible to solve, I will have the parts
carefully removed by a high level tech and replaced
after the BGA work is completed.

Hope this helps.

Tony Di Mauro
Quality/BGA Process Engineer
Circuit Technology Center - Haverhill, MA

--- "Oberle, Wade" <[log in to unmask]> wrote:
> Dear Technetters,
>         How do you handle rework of BGAs when there
> are BGAs on both sides
> of the board and they are nearly directly on top of
> one another?
> Similarly, how do you rework BGAs when the bottom
> side of the board is
> loaded with passives and ICs inside of  the area
> outlined by the vias of the
> topside BGA?  Our typical boards are 12 to 14 layers
> and are 0.062" thick.
>
>
> Thanks in advance for your advice.
>
> Wade Oberle
>
>
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