Pascal: Responding to your questions:
1 Do we have to dry the PCB after cleaning (sic)?
Generally, nothing says you have dry the PCB after aqueous cleaning. Along this line, I received several thoughtful responses when I posted a similar thread. Look at:
Why Dry After DI? - Dave F 18:52:10 07/06/1998
Continuing, J-STD-001 states that boards must meet an established cleanliness requirement. It requires neither cleaning nor drying after cleaning. So, the method you use to meet the cleanliness requirement of the product shipped to your customer is up to you.
2 Which is the threshold humidity content to avoid any risk for FR4 material ?
Short [but not particularly useful] answer: It depends on how your process the board and on the design of the board.
The time of greatest risk posed to FR-4 boards by humidity content is during soldering cycles, as Chris mentioned earlier, because FR-4 is hydroscopic [ it wants to take-on moisture].
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