Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.

PCB baking for rework


PCB baking for rework | 16 July, 2002

What is a typical time/temp baking cycle for an assembly prior to BGA rework? This bake is intended to prohibit PCB or nearby component damage as well as protect the component being removed for reball. A 24 hr bake at 125C would be great, but often there are mechanical items installed on the PCBA the prohibit this high of a temperature. A lower temperature will require a longer bake cycle and increase the rework cycle time. What time/temp are others using?

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PCB baking for rework | 16 July, 2002

We find that baking PCB assemblies at 90 degrees C in a Blue M forced air oven is safe for most all PCB's that we have encountered over the past 12 years. We remove any external plactic hardware that may be attached to the assembly and also like to remove any batteries. Refer to IPC for suggested bake times for BGA chips as these are usually the most moisture sensitive parts. I believe it is around 12 hours at 90 degrees C.

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PCB baking for rework | 17 July, 2002

The current IPC/JEDEC standard J-STD-033 for moisture-sensitive devices does not include a bake cycle at 90C (it includes cycles at 40C and 125C for non-assembled components in reels or trays).

However, the upcoming revision, which should be released before the end of the year, includes a bake cycle at 90C for populated boards, mainly for hot air rework or second side reflow.

The bake duration at 90C, as with other bake temperatures, varies based on the specific components that must be baked, according to their MS level, body thickness and their level of moisture content (i.e. saturated or at limit of floor life). At 90C it ranges from 11 hours to 10 days. Take note that the 11 days is only valid for the best case condition, for a component with MS level of 2a, body thickness less than 1.4 mm and at the limit of floor life. (I can send you a copy of the proposed revision if you are interested).

In other words, the 12 hour bake cycle at 90C is really out of line with the industry standard. I suspect that it will be hard to convince any informed customer that this is a safe approach. Don't forget that a partial bake is sometimes worse than no bake at all because initially the ingressed moisture is being pushed towards the critical interface at the center of the package.

Internal defects such as partial cracks and delaminations cannot be detected with standard test and inspection tools but they will reduce the MTBF and increase the risk of early life failures.

Francois Monette Cogiscan Inc. Tel : 450-534-2644

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