Moisture Sensitivity Level Handling and Baking

A plastic package that has absorbed moisture will crack during reflow. The water flashes to steam, the internal pressure rises, and the package delaminates or bursts in a failure mode called popcorning. Manufacturers classify every component with a moisture sensitivity level that says how long it may be exposed to ambient air, and how it must be dried if that time is exceeded. Handling those levels correctly is one of the least glamorous and most effective assembly controls.

What the Levels Actually Define

The moisture sensitivity level is a number from one to six, and it corresponds to a floor life: the time a package may spend out of its dry pack at a defined temperature and humidity before it must be baked. Level one has no restriction, level three is typically 168 hours at 30 degrees and 60 percent relative humidity, and levels five and six allow only a few hours or require baking before use.

The level is determined by the package construction rather than by the die. A thin plastic ball grid array with a large body absorbs moisture quickly and releases it slowly, so it is assigned a stricter level than a small outline package of the same technology. The classification test is defined by industry standards that specify the bake, the exposure, and the reflow conditions used to provoke failure.

Floor Life and How It Is Counted

Floor life begins when the bag is opened and pauses when the parts are returned to a dry environment. A dry cabinet, a nitrogen cabinet, or a resealed bag with desiccant all stop the clock, which is why modern factories record the accumulated exposure rather than the elapsed time since opening. Labels on the reel, and increasingly a database, carry that accumulated time.

Humidity is the variable that is most often ignored. A part at level three in a workshop at 30 degrees and 60 percent humidity consumes its floor life far faster than the same part in an air conditioned area at 25 degrees and 40 percent. Where the ambient conditions differ from the standard, the manufacturer’s derating table should be used rather than the nominal hours.

Baking Correctly

Baking drives the moisture out, and the schedule depends on the package thickness, the body material, and the maximum temperature the part can tolerate. A typical low temperature bake runs for many hours at 125 degrees, while a higher temperature bake is shorter but is limited by the temperature rating of the packaging, including the reel and the carrier tape. The wrong choice either fails to dry the part or damages the tape.

Reels and tubes cannot always be baked at high temperature, so many processes remove the components or use a bake profile rated for the packaging. The bake also counts against the part’s total thermal exposure, and a part that has been baked several times may have consumed part of its reflow rating. Records should state the temperature, the duration, and the number of previous bakes, because a supplier asked to bake a part repeatedly is changing its reliability.

Dry Pack and Storage

Dry packing is more than a sealed bag. The components are dried before packing, a desiccant is sized for the bag volume and the intended storage period, and a humidity indicator card shows whether the desiccant has been exhausted. A bag that has been opened and resealed without fresh desiccant may show a satisfactory card reading while the components have already absorbed moisture.

Storage temperature also matters. A bag stored in a hot area can drive moisture out of the desiccant and into the parts, and condensation when the bag is moved to a cooler area can add humidity. Storing dry packs in a controlled area at a moderate temperature is a simple measure that prevents a whole class of problems.

Detecting Moisture Damage

Popcorning is visible on the surface of the package as a bulge or a crack, and on a cross section as a delamination between the mould compound and the die paddle. Internal delamination without an external sign is more common and more dangerous, because the part passes an electrical test and fails later. Scanning acoustic microscopy is the standard method for detecting it, and it is used for qualification rather than for every lot.

The reflow profile influences the outcome. A fast ramp to a high peak produces more internal pressure than a slow one, so a part that is marginal at the standard profile may pass a gentler one. Where a package is close to its limit, adjusting the profile is a legitimate measure, but it should be treated as a process change that requires verification rather than as a way to relax the handling rules.

Practical Floor Control

The practical controls are simple and depend on discipline. Label every bag with its opening date, keep a log of accumulated exposure for parts that are used in batches, and store partly used reels in a dry cabinet rather than on a shelf. Define who is allowed to bake parts and how the bake is recorded, so that the history is available when a reliability question arises.

Where a product uses several moisture sensitive parts, the strictest level among them sets the handling requirement for the assembly. Planning the build so that all the sensitive parts are placed in one pass, rather than being exposed repeatedly, reduces the total risk. The same planning applies to reflow process scheduling, because a second reflow for the bottom side exposes the top side parts again.

Audits and Documentation

An audit of moisture sensitive device handling looks for evidence rather than intent. It should be possible to show the level of a part, the accumulated exposure, the bake records, and the storage conditions for a specific production lot. Where a factory uses a database, the entry should be traceable to the physical reel, and stale entries should be cleared rather than allowed to accumulate. Where a package is close to its exposure limit, the quality records should show the decision that was made rather than the assumption that all was well.

Documentation also supports the customer. A build report that states the level of the critical packages, the floor time used, and any bakes performed answers the question before it is asked. Where a part must be baked before use in a particular process, the requirement should appear in the assembly drawing, because the operator at the line is not the right place to hold that knowledge.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Can a baked part be used indefinitely? No. Baking removes moisture but each cycle adds thermal exposure, and the number of bakes should be recorded and limited according to the component supplier guidance.

Does a dry cabinet reset the floor life? It pauses it. Time in a controlled dry environment does not count against the exposure allowance, but the accumulated time already used should be tracked and not reset to zero.

How do I know whether a part has absorbed moisture? The packaging label and the humidity indicator card give the first indication. For a definitive answer, the part must be evaluated by scanning acoustic microscopy or by a reflow simulation on a sample.