Plastic package moisture absorption hazards in SMT board assembly

The package cracking hazard is discussed from an SMT (surface mount technology) board assembly manufacturer's point of view. On the basis of previous studies, the allowed maximum moisture levels after drying and after moisture absorption are defined. Drying curves and moisture absorption curves after drying are plotted for different component types, including quad-flat-packs, under specified conditions. As a result, recommendations are given on drying time, storage time, and temperatures and conditions to be applied. Because of the large number of large-size die memories in SOJ 20-26 packages currently purchased, a complete evaluation, including crack investigation for different moisture levels, has been performed. This moisture related reliability hazard drastically points out the need for new packaging materials, new packing procedures, and a mountability test in qualification files of SMT components. It is shown that a reduction of moisture-related hazards requires either that components be purchased in shipping bags which will guarantee a moisture level below 0.2% or that they be baked for 16 h at a temperature of 125 degrees C. Shelf-life is 40 h under uncontrolled conditions or 1 month if components are stored under 20 degrees C 50% RH.<
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