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66 SMT Magazine • April 2017 er the components or PCB's absorb humidity and with this the permissible processing time decreases. All components classes 2a to 5a in accor- dance with the classification of IPC/JEDEC J- STD020D absorb no moisture with a vapor pres- sure of < 2.82. At this level, they can be stored and processed indefinitely. (IPC/JEDEC-STD033C table 7-1). Storage cabinets should maintain, over 24 hours, on average a vapor pressure of <0.95 mbar. In a humidity protection bag with a rest pressure of <6 mbar the vapor pressure is <0.15 mbar. Both systems, dry storage cabinet and hu - The oxygen in the air forms the means of oxidation, the vapor (humidity) the electrolyte. The critical limit at which oxidation with oxygen takes place lies in accordance with the metal or alloy at between 40–70% RH. This means that more than eight grams of vapor per cubic meter must be present. The Diffusion Process The vapor in the atmosphere diffuses into hygroscopic materials. The cause of this is the so-called vapor pressure—this means the par- tial pressure of the vapor which is present in the air. The higher the vapor pressure, the fast- SOLUTIONS FOR LONG-TERM STORAGE OF ELECTRONIC COMPONENTS AND COMPOSITIONS Figure 1: Component micro-cracking resulting from the absorption and rapid release of moisture. Figure 2: Component processing time.

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