3 x BGA reflow or placing BGA with repair station?

Hi.

I have product in lead containing process that consist lead free BGAs and also big ceramic BGAs with high temperature bumbs. Pcb tends to bow in a reflow profile, since I need long preheating times and relatively high peak temperature to melt BGAs also, but not to burn chips. This product has 14 layers, so it contains also lot of thermal mass, and profiling is balancing on the edge. Product�s layout doesn�t stand bowing or twisting at all, so I must consider alternative methods. I try high Tg pcb�s without any mentionable success at twisting.

Now I have considered:

a.)Making standard reflow profile for chips and other stuff, and after reflow adding flux under the BGA�s and reheat them with repairing station to melt the Pb free and high temperature bumbs. Advantage: relatively fast process. Disadvantage: 3 x reflow for BGA�s, is this too much?

b.)Adding BGA�s with repair station. Advantage: safer for BGA�s. Disadvantages: Slow process.

c.)Using isotropic adhesive. Lot of question marks: self alignment, repairing, mechanical strength?

Suggestions anyone? I would prefer reheating BGA�s with repair station, but is 3 x reflow for BGA�s too much?

Thanks.

- Scotty

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