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44 SMT Magazine • June 2017 5.3 Safe Storage: Safe storage means dry SMD packages held in a controlled humidity condition such that the floor-life clock remains at zero. Acceptable safe storage conditions for SMD packages classified as Level 2 through 5a are listed below. 5.3.1 Dry Pack: Dry-packed SMD packages in intact MBBs, stored per Clause 3.3, shall have a calculated shelf life of at least 12 months from the bag seal date shown on the caution or bar code label. 5.3.2 Shelf Life: The minimum calcula- ted shelf life is 12 months from bag seal date. If the actual shelf life has exceeded 12 months, but less than two years, from the bag seal date and the humidity indicator card (HIC) (Clau- se 5.5.1) indicates that baking is not required, then it is safe to reflow the components per the original MSL rating. Although unanticipated, factors other than moisture sensitivity could af- fect the total shelf life of components. Note: An HIC that has been continuous- ly sealed in the MBB is typically accurate for at least two years. 5.3.3 Dry Atmosphere Cabinet: A sto- rage cabinet which maintains low humidity by purging with dry air or nitrogen at 25 ± 5°C. The cabinet must be capable of recovering to its stated humidity rating within one hour from routine excursions such as door opening/ closing. Dry cabinet at 10% RH SMD pack- ages not sealed in a MBB may be placed in a dry atmosphere cabinet, maintained at not greater than 10% RH. A dry cabinet should not be considered a MBB. Storage of SMD packages in a dry cabinet should be limited to a maximum time per Table 7-1. If the time limit is exceeded the pack- ages should be baked according to Table 4-2 to restore the floor life. Dry cabinet at 5% RH SMD pack- ages not sealed in a MBB may be placed in a dry atmosphere cabinet, maintained at not greater than 5% RH. Storage in a dry cabinet may be considered equivalent to storage in a dry pack with unlimited shelf life. These guidelines address moisture within the component and mitigation of risks during reflow, but the solderabilty of components is also a significant consideration. Because of surface oxidation, components and PCBs can suffer from reduced solderablity, which often results in complete failure. Diffu- sion of vapor and noxious substances in the inner structure of the components or PCBs can result in long-term disintegration of con- ductor paths and insulation layers. Both ris- ks can be avoided by correct handling and dry storage. The Oxidation Process—Contact Corrosion In an ultra-dry atmosphere there is no cor- rosion. For corrosion to occur, two demands must be met: there must be a means of oxidati- on, and there must be a watery solution, which works as an electrolyte. The oxygen in the air forms the means of oxidation, the vapor (humi- dity) the electrolyte. The critical limit at which oxidation with oxygen takes place, depending upon the metal or alloy, at between 40 and 70% RH. This means that more than eight grams of vapor per m 3 must be present. As a side note, 0.5% RH, used commonly today, reduces water content to 0.05 grams per m 3 . The effects of long-term storage on the sol- derabilty of components was studied in some detail by DFR Solutions, including in one tit- led "Solderability After Long Term Storage." In this case study, the solderability was assessed for components from three different reels sto- red for up to five years to determine how much additional storage life was available. The com- ponents were either an ASIC in a SOIC package or a MOSFET in a TO-252 package. In both si- tuations, the lead frame plating was tin-based 1 . Both oxidation and intermetallic formation occurred, as would be expected for the reasons described previously. Oxidation can be preven- ted with the use of low humidity storage, or po- tentially mitigated with the implementation of more aggressive fluxes. LONG-TERM STORAGE OF ELECTRONIC COMPONENTS AND COMPOSITIONS • •

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