![]() Information on the MSL rating is provided in the labeling on the device packing labels. Protocols for rating moisture absorption, as well as absorption abatement, are provided in J-STD-020, revisions E and later. MSL 3 is one-week shelf life in an open bag (at MSL 2 is one-year shelf life in an open bag (at MSL 1 is the most resistant to delamination, with a shelf life in an opened bag of unlimited time (at The peak reflow temperature and MSL level does need to be adhered to. Reflow profiles are not specified by Allegro it is up to the customer to produce the proper profile based upon their parts, board, and reflow oven type. Note that all Allegro devices are qualified for 3 passes of reflow. Thus, if a device package has a 260☌ MSL 3 rating but if the customer reflow temperature is 240☌, the MSL rating at that temperature may be MSL 2 (check with Allegro if there is data for that package). If a cus- tomer is using a reflow temperature below the maximum rating, such as 240☌, the MSL level will be better. If the MSL level states a different reflow temperature, such as MSL 3 at 250☌, the open bag shelf life is one week with a maximum reflow temperature of 250☌. In this context, floor life is linked to the rate at which the device absorbs atmospheric moisture, represented by the MSL rating (moisture sensitivity level) at the rated reflow temperature (if not stated, is assumed to be 260☌). To avoid delamination from vaporization effects, take care to avoid exceeding the maximum floor life of the devices. The transi- tion from Pb-based to the maximum limit of 260☌ (533 K) involves a temperature increase of only about 8%, but the vapor pressure increase is approximately 103%. From a Pb-based process peak of about 220☌ (493 K), to the Pb-free lower range in the vicinity of 240☌ (513 K), is an increase of approximately 4% in temperature, but 45% in vapor pressure. As shown in Figure 1, vapor pressure increases rapidly as temperature increases, resulting in a disproportionate increase in pressure for the comparatively small increments of process temperature between the different process types. Expo- sure to atmospheric moisture over even short periods of time can allow enough moisture to be absorbed to have serious effects when the moisture turns to steam during reflow oven heating. The package body is composed of plastic epoxy molding compounds that can absorb moisture and other contaminants. The device packages commonly used throughout the industry, including by Allegro, are nonhermetic. PROCESS TEMPERATURE AND MSLSĪ significant process concern is the absorption of atmospheric moisture by devices before exposure to elevated soldering temperatures such as reflow oven soldering. Parallel bodies of standards and information are available from the IEC (International Electrotechnical Commission), which is a particularly good source for international and European standards, and from JEITA (Japan). IPC-2221, Generic Standard on Printed Board Design, provides a notation for dimensioning joints. By becoming certified, customers will have the knowledge to evaluate the solder joints for acceptability to international standards. Training courses and certification for IPC-A-610 should be used by all customers performing soldering operations. Standards, with revision G and later including coverage of Pb-free processes. IPC-A-610, Acceptability of Electronic Assemblies, provides detailed workmanship A primary source is IPC J-STD-001, Requirements for Soldered Electrical and Electronic Assemblies, which is an industry-level consensus standard covering soldering materials and processes, with revision G and later including coverage of Pb-free soldering. The IPC has several publications and course materials that provide information on solder joint evaluation. The characterization of an acceptable solder joint can be somewhat different between Pb-based and Pb-free technologies. In addition, important information on determining optimum soldering process parameters can be found in J-STD-002, Solderability Tests for Component Leads, Terminations, Lugs, Terminals and Wires. It provides information on MSL (moisture sensitivity level) classifications for devices, and the corresponding protocols for device handling. AVAILABLE STANDARDSĪllegro recommends becoming familiar with IPC/JEDEC Joint Industry Standard J-STD-020, Moisture/Reflow Sensitivity Classification for Nonhermetic Solid State Surface Mount Devices. Both lead (Pb) free and traditional Pb-based technologies are examined. It provides information on SMD (surface mount devices) and through-hole packages. This document describes typical soldering methods that have been proven effective with Allegro products. ![]()
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