MSL Baking

(IPC/JEDEC J-STD-033)

Different packages have different levels of moisture sensitivity. The higher the amount of moisture inside a package, the higher the thermo-mechanical stresses inside the package during board mounting will be. These stresses are brought about by the rapid vaporization of the trapped moisture, and can be large enough to fracture the package during board mounting. Package cracking under such circumstances is known as 'popcorn cracking. IPC/JEDEC J-STD-020 defines the Moisture Sensitivity Levels (MSL) of surface mount devices.

Moisture-sensitive devices are baked and vacuum-sealed inside a moisture barrier bag prior to shipment in order to minimize their tendency to exhibit popcorn cracking during board mounting. Users of such devices are likewise required to board mount the units within a prescribed period after the bag has been opened. Failure to do so calls for a re-bake of the units prior to board mounting. Table 1 shows the bake conditions recommended by IPC/JEDEC J-STD-033 at the user's site if the out-of-bag time prescribed has expired prior to board mounting. Table 2 shows the bake conditions required by IPC/JEDEC J-STD-033 at the manufacturer's site, prior to dry-packing the parts.