Issue link: https://iconnect007.uberflip.com/i/1335241
FEBRUARY 2021 I SMT007 MAGAZINE 77 When component moisture levels become critical, encapsulant damage can occur during reflow. Plastic/epoxy resin packaging material is per- meable to moisture (as are PCBs). Compo- nents should be delivered in properly prepared moisture barrier bags. Once the bag is opened, components absorb moisture from the atmo- sphere. If moisture levels become critical (0.1% water weight), damage occurs during reflow as the moisture attempts to escape too quickly, exceeding the elastic limit of the encapsulant. Components are rated with a moisture sensitivity level (MSL) which dictates available floor life. e moisture sensitivity level (MSL) of com- ponents is identified by the manufacturer in one of six levels as defined in J-STD-020, dis- played in J-STD-033D. is identifies the avail- able safe floor life of components (time out of MBB). For instance, MSL 3 components have a floor life of 168 hours. Tracking the exposure time is critical to preventing defects. Oxidation will occur when com- ponents are improperly stored, compromising solderability. Oxidation will also occur on components stored in ambient RH. is negatively affects solderability. e same safe storage conditions (<5%RH) that will stop moisture absorption by encapsulants will also stop oxidation. A level of <5% RH provides unlimited safe storage time, thus "stopping the clock" on the MSL floor life. is is particularly significant for low-volume high-mix operations. If the floor life is exceeded, it is possible to restore it under carefully controlled conditions. Expired floor life can be restored by reducing absorbed moisture to safe levels. Traditional high temperature (125°C) baking reduces moisture but induces oxidation and interme- tallic growth, increases wetting times, and compromises solderability. Lower baking temperatures (40-60°C) combined with ultra- low RH (1%) will rapidly restore floor life without reducing solderability, and unlike high temperature, this process can be safely repeated. Dry air atmospheres stop oxidation better than nitrogen. Nitrogen was a traditional method for safe stor- age. However, dry air is much less expensive and provides lower RH%. X-ray data of numer- ous alloys proves low %RH air stops oxida- tion better than N 2 . is is because water is the more aggressive bearer of oxygen than tightly bonded O2 molecules. Removing the moisture removes the catalyst and prevents the corro- sion process. Super Dry Totech EU® www.superdry-totech.com is a moisture management specialist, providing hardware and process control software for safe storage, floor life reset and automated tracking of moisture sensitive components and materials. 1 2 4 5 3