One common method of dealing with moisture contamination is to remove it before each manufacturing step. In the semiconductor industry, vacuum processing and bake-and-bag methods of IC drying accomplish this, but these operations slow down production, particularly if they must be repeated several times in the course of circuit manufacturing. Further, these baking and sealing processes themselves expose parts to thermal extremes that can cause damage.
Desiccant-based dry storage avoids some of these drawbacks, but introduces others. These systems remove moisture from an incoming supply line of air (or other process gas) and often feature dual-tower designs that perform on-line drying and off-line regeneration simultaneously for continuous operation. Such systems can be effective, but they require heating/drying components that may not be reliable. Further, they must be closely monitored to ensure that incoming gas flow remains below a critical humidity threshold. Their complexity and high operating costs makes them prohibitively expensive for long-term storage applications.
As an alternative to desiccant dryers, nitrogen-purged desiccator systems maintain dry conditions relatively cheaply and conveniently. Nitrogen is the standard medium for contamination-free storage because it is relatively inert – it neither reacts with stored materials nor carries moisture – and because it can be isolated and purified relatively inexpensively.
Desiccator cabinets must be set up so that an appropriate flow of nitrogen forces out all moisture- and contamination-laden air. Because nitrogen has a lower specific gravity than air, it is introduced into the upper section of the desiccator; the heavier air is then purged out of the bottom.
Failure to maintain the appropriate nitrogen flow into a desiccator, or to bleed the cabinet effectively, can be devastating. Once inside a desiccator, moisture can penetrate the molecular structure of stored components, requiring baking or vacuum processing.
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