In terms of surface mount device (SMD) packages, any permeable packaging material, plastic or metal, absorbs moisture from the air. When the package is exposed to the sudden high temperature of solder reflow during the assembly process, moisture will expand rapidly and damage the component. Sometimes the damage is visible such as cracks and/or delamination in the package. Other times, it's internal and not visible. Either circumstance has seen product's susceptibility to failures with its quality and reliability threatened. MSDs, an acronym of Moisture Sensitive Devices, can be ICs, LEDs and even connectors. They demand particular storage and handling methods to end up guaranteeing the improvement of yield and reliability.
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