room standard of PCBA workshop,for example temperature,humidity| 10 June, 2003
Another important element to consider is the impact of ambient temperature and humidity on Moisture Sensitive Components.
The majority of plastic encapsulated ICs are classified as moisture-sensitive devices (MSD). This means that they are qualified based on their maximum exposure time (floor life) at conditions not exceeding 30C and 60%RH.
If your environment exceeds 60% RH, the impact on MSD is significant. For example, when the ambient conditions range between 60% and 70% the de-rating factor for a component with MS level 3 and thin body (Ex : SOICs <18 pins, All TQFPs, TSOPs or all BGAs <1 mm body thickness) is equivalent to a downgrade by 3 levels of moisture sensitivity, which means the floor life is reduced from 7 days down to only 1 day. (Ref: J-STD-033A, section 7, table 7-1)
At the other extreme, when ambient conditions are very dry, the floor life will significantly increase. In the above example, the same components will never exceed their critical level when conditions remain below 40% RH. Of course when it gets too dry, ESD issues come into consideration.