When storing moisture sensitive products, you need a container that keeps moisture out. While desiccants will remove excess moisture, and humidity indicator cards will let you know when to replace your desiccant, they are not much help if your bags do not provide a protected environment. By using moisture barrier bags, you can block new influxes of moisture and humidity, allowing desiccants to keep your products dry and safe.
There are many sizes of moisture barrier bags available, so you may be wondering how much desiccant you need to use. In order to figure that out you need to have some information about your application. First you need to know the standard that you want to use. There are three main standards: IPC/JEDEC J-STD-033 (dry packaging for SMD’s), EIA 583 (dry packaging for SMD’s and allows for environmental conditions), and MIL-P-116 (general dry packaging).
For IPC/JEDEC J-STD-033 the formula is:
Units= 0.304 x Storage Time in Months x Bag MVTR x Bag Area in inch2
For EIA 583:
Choose moisture capacity based off of Maximum Interior Humidity
10%=2g/unit 20%=4.8g/unit 30%=5.8g/unit 40%=6.2g/unit
Units= 0.231 x Storage Time in Months x Bag MVTR x Bag Area in inch2
Units= 0.011 x Bag Area in inch2
All of these formulas give you the “units” of desiccant needed. Typically desiccant pouches are measured in units, and each unit designates how much is needed at 77°F to absorb 3 grams of water vapor at 20% relative humidity or 6 grams of water vapor at 40% relative humidity. A unit of silica is smaller than a unit of clay, as silica is better at absorbing moisture.
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