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PCB007-Feb2021

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FEBRUARY 2021 I PCB007 MAGAZINE 59 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 lev- el 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 reduc- ing absorbed moisture to safe levels. Tradi- tional high temperature (125°C) baking re- duces moisture but induces oxidation and in- termetallic growth, increases wetting times, and compromises solderability. Lower baking temperatures (40–60°C) combined with ul- tra-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 nu- merous 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

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