Moisture Absorption

Many electronic components, as well as solder paste, react sensitively to moisture and need therefore be stored in special, moisture protective packaging. If they are stored without this protection in normal atmosphere, they could absorb moisture from the environment. When these components then pass through the reflow system, the fast rise in temperature will cause the absorbed water to vaporize, building up immense pressures within the package. The formation of cracks or even the package popping can be the result. This bursting open of the package is designated as the popcorn effect. The maximum storage time in normal atmosphere is given by the MSL (Moisture sensitivity level), which normally is provided as part of the component label or on the datasheet. Solder paste are stored in refrigerators. The unopened containers should, as a rule, be removed from the refrigerator 6 – 12 hrs. before usage, so that the paste has enough time to warm up to room temperature. If the container is opened immediately after removal, moisture will condense on the paste and be absorbed by it. This would cause a change in the properties of the paste.