Hi Gary,
With plastic BGAs it is more difficult to remove the trapped moisture because of ground planes in the substrate. If your parts have been sitting around for a long time they may be soaked with moisture and in some cases the 48 hours bake cycle is not sufficient to dry them below the safe level for reflow.
The best solution is to bake them for a longer period of time. The upcoming revision of J-STD-033 will propose an extended bake cycle of 96 hours at 125C for PBGAs. The real effect of vacuum on component drying is still a subject of controversy in the industry and it is not endorsed by the technical committee in charge of the IPC/JEDEC standard.
I hope this is helpful. Feel free to call or e-mail if you need more information.
Francois Monette Cogiscan Inc. Tel : 450-534-2644 fmonette@cogiscan.com www.cogiscan.com
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