Moisture sensitivity of printed circuit boards is getting increasing attention. In fact the SMTA’s Moisture Sensitivity Device Council is now focusing more of its attention on printed circuit boards, not on components, which had been the tradition.
Furthermore the IPC is proposing to establish some specifications on this issue. They have a council, an IPC D-35 Printed Board Storage and Handling subcommittee and they are preparing a draft spec, IPC 1601 Guidance Document for Storage and Handling Issues for PCBs and Assemblies.
As per recent meeting notes, the Moisture Sensitivity Council, Printed Circuit Boards (PCB) should not be left open for more than 72 hours. Most PCB suppliers guarantee a shelf life for six months for an unopened package.
So if you keep PCBs in their vacuum sealed bags, they should be good as long as you don’t expose them on the manufacturing floor for more than 72 hours. While this is a very general statement, the council also talks about different laminates have different sensitivities. For instance, newer high Tg laminates and those specifically developed for lead-free assemblies are more sensitive.
If you can guarantee that your boards aren’t exposed to humidity for more than 72 hours before their final reflow, you’ll probably be okay. Otherwise, pre-baking of PCBs is recommended prior to assembly.
You can refer to the IPC D-35 Printed Circuit Board Storage and Handling subcommittee for additional information and watch for their forthcoming specification.
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