Moisture Sensitive Devices (MSD) are all non-hermetic solid state Surface Mount Devices (SMD) in packages, which, because of absorbed moisture, could be sensitive to damage during solder reflow. The term SMD as used here means plastic encapsulated surface mount packages and other packages made with moisture-permeable materials. The vapor pressure of moisture inside a nonhermetic package increases greatly when the package is exposed to the high temperature of solder reflow. Under certain conditions, this pressure can cause internal delamination of the packaging materials from the die and/or lead frame/substrate, internal cracks that do not extend to the outside of the package, bond damage, wire necking, bond lifting, die lifting, thin film cracking, or cratering beneath the bonds. In the most severe case, the stress can result in external package cracks. This is commonly referred to as the ‘‘popcorn’’ phenomenon because the internal stress causes the package to bulge and then crack with an audible ‘‘pop.’’ SMDs are more susceptible to this problem than through-hole parts because they are exposed to higher temperatures during reflow soldering. The reason for this is that the soldering operation must occur on the same side of the board as the SMD device. For through-hole devices, the soldering operation occurs under the board that shields the devices from the hot solder. (IPC JEDEC J-STD-020C) Cracks and/or delamination results in failures of the final product which brings loss of money and prestige. Thus, manufacturers should develop and apply a handling and storage program for MSD
AMBIENT STORAGE REFLOW HEATING
Furthermore storage of multi layer PCBs in humid conditions with one side assembled containing MSDs, before the other side is soldered results in same problems. It is better to handle and store these PCBs like MSDs of concern.
IPC/JEDEC J-STD-033B.1 (Oct 2005) and IPC/JEDEC J-STD-020C (July 2004) are two main standards published by IPC (Association Connecting Electronics Industries) and JEDEC (Joint Electron Device Engineering Council) which direct the industry with the definitions and regulations about Handling, Packing, Shipping and Use of Moisture/Reflow Sensitive Surface Mount Devices andMoisture/Reflow Sensitivity Classification for Nonhermetic Solid State Surface Mount Devicesrespectively.
According to J-STD-033B.1, SMD packages not sealed in a MBB (Moisture Barrier Bag) may be placed in a dry atmosphere cabinet, maintained at not greater than 5% RH. Storage in these dry cabinets may be considered equivalent to storage in a MBB with unlimited shelf life. In addition, storage of SMD packages not sealed in a MBB, in cabinets maintained at not greater than 10% RH should be limited to a maximum time. If the time limit is exceeded they should be baked to restore the floor life.
This means, storing MSDs in a dry cabinet maintained at not greater than 5% RH, offers unlimited shelf life without using MBB and removes possible re-baking costs which are much more than the storage costs.