Component Moisture Sensitivity Levels Explained

A moisture sensitivity level is a statement about how long a package may be exposed to ambient air before it must be baked, and it exists because plastic packages absorb water. The level is a property of the package, and the floor life that follows from it is what the factory has to manage.

The system works only when the level is visible on the packaging and the clock is recorded at the point of opening. Without those two items, the rule is a label with no effect.

Why Packages Absorb Moisture

The mould compound in a plastic package is hygroscopic, so it takes up water from the air until it reaches equilibrium with the environment. The amount depends on the material, the package thickness and the humidity.

When the package is heated to reflow temperature, the water turns to vapour and the pressure builds inside. The failure mode that follows is delamination, a crack from the die pad or a wire bond break.

The Levels

The levels run from one to six, with level one requiring no dry pack and level six requiring a bake before use. Each level carries a floor life, which is the time the part may be out of its dry pack in a defined environment.

The environment is part of the definition, so a floor life quoted at thirty degrees and sixty percent relative humidity does not apply in a different room. The level and the conditions should be read together.

Dry Packing

Parts are shipped in a moisture barrier bag with desiccant and a humidity indicator card. The bag stops the clock, and the card shows whether the clock has already been running.

The bag should be opened only when the parts are needed, and it should be resealed if it is opened for a partial quantity. A bag that is left open on a shelf has no protective value.

Floor Life and the Clock

The clock starts when the bag is opened and pauses when the parts are returned to a dry environment. It should be recorded at the station rather than in an office, because the decision to use or bake is made there.

Where a part exceeds its floor life, it is baked rather than rejected. The bake schedule is defined by the package thickness and the level.

Bake Schedules

The bake temperature and time are specified by the package supplier, and the limits account for the tape, the reel and the component body. A tray or a tube may have a lower temperature limit than the component itself.

The bake also has a count, since repeated baking degrades the solderability of the terminations. The number of bakes should be recorded and limited.

At Reflow and in the Oven

The failure caused by absorbed moisture appears at reflow, which is when the vapour pressure peaks. The symptoms include a visible bulge, a crack and an intermittent electrical failure that appears later.

The relationship between the moisture content and the profile is why a longer or hotter profile is not a fix. Reducing the moisture is the fix, and the profile only determines how much damage results.

Storage and Handling

The storage area should be controlled, and dry cabinets are used for parts that will be exposed repeatedly. A dry cabinet with a recorded humidity is a practical alternative to repeated baking.

Handling also matters, because a part that is removed from a bag and left on a bench accumulates exposure even when it is not being used.

Assemblies and Rework

A board with parts already mounted has its own exposure, and a second reflow pass adds another thermal cycle to the parts. The exposure accumulates across the passes.

Where rework is required on a moisture sensitive package, a local bake may be needed before heating. The decision should be made with the package data rather than with a general rule.

Records

The records should link the package lot to the exposure history, so that a failure can be traced to a part that was used outside its floor life. This is the same traceability that supports the rest of the build.

The practices sit alongside the laminate controls described for moisture bake, since the board and the components both absorb water and both are managed with a clock.

Process Control and Verification

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

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

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record.

FAQ

Can any part be used without dry packing? Level one parts can, because their floor life is unlimited at the defined conditions.

Does the level change if the part is stored a long time? The level does not change, while the accumulated exposure does, and the clock is what matters.

How many times can a part be baked? The supplier specifies a limit, and the count should be recorded because repeated baking affects solderability.

Is a humidity indicator card required? It is required for the system to work, since it shows whether the bag was compromised in transit or storage.