Moisture Sensitivity Level: Design Rules and Process Limits

The moisture sensitivity level of a package is a number from one to six that describes how long it can be exposed to air before it must be baked. It exists because a plastic package absorbs moisture from the air, and at soldering temperature that moisture turns to steam and cracks the package from the inside. The damage is invisible on the outside and fatal to the device.

What the Level Means

The level describes the floor life, which is the time a package may be exposed to a controlled environment after the bag is opened and before it is reflowed.

Level one has an unlimited floor life, level three allows a hundred and sixty eight hours, level four allows seventy two, and the higher levels allow less. The exact times come from the standard and are printed on the bag.

The conditions matter as much as the time, since the floor life is quoted for a temperature below thirty degrees and a relative humidity below sixty per cent. A humid workshop shortens it.

A package that exceeds its floor life has to be baked before use, and the baking schedule is part of the component specification rather than a decision to be made on the line.

Why Moisture Causes Damage

The plastic absorbs water in the same way as the board laminate, by diffusion into the resin. The amount is small, often less than a tenth of a per cent by weight.

At reflow temperature the water turns to steam and the pressure inside the package rises quickly. The package has no route for the vapour to escape, so the stress is taken by the resin and by the die attach.

The result is a crack in the resin, a separation between the die and the lead frame, or a bond wire that is torn. The failure is called popcorning because the package surface bulges like a cooked grain.

The damage is often invisible from the outside, and the device may still measure correctly at room temperature. The failure appears later as a leakage path or an intermittent connection.

The Levels in Practice

Most surface mount packages are level three, which is the common default and allows a week of exposure. A large thin package is often level four or five.

A ball grid array with a large body and a thin substrate is more sensitive than a small package, because the body has more area to absorb moisture and less material to resist the stress.

A through hole package is usually not rated at all, because the body is not exposed to the full reflow temperature. The sensitivity is a property of the surface mount assembly.

The level is part of the purchasing decision, since a level five package that cannot be handled in a normal workshop may force a change of package or an additional process step.

The Floor Life Clock

The clock starts when the bag is opened and stops when the package is reflowed. Any time in a dry cabinet does not count, which is why the dry cabinet is the standard way to pause the clock.

The clock also accumulates across the assembly, so a package that is placed on the first side and reflowed twice has two exposures. The second reflow is the one that usually causes the damage.

A double sided assembly is therefore the critical case, because the parts on the first side see the full profile twice. The parts that are most sensitive should be placed on the second side where the process allows it.

The tracking is a paperwork exercise as much as a technical one, and it fails when the label is separated from the reel. The date and the time of the bag opening should be written on the reel label.

Baking and Storage

Baking drives the moisture out, and the schedule gives a temperature and a time for each package thickness. A thick package needs a longer bake because the moisture has further to travel.

The temperature is limited by the packaging, since a high temperature can damage a reel or a tray and can oxidise the leads. The low temperature bake takes longer and is the safer choice for a part on a reel.

The bake count is limited for some packages, because repeated heating ages the plastic and the die attach. The number of bakes that are allowed should be taken from the component specification.

A dry cabinet is the better answer for a part that will be used over several days. It keeps the humidity low, pauses the floor life clock and avoids the thermal cycling that a series of bakes would require.

Design and Process Rules

The package choice should consider the level, and a design that is otherwise equal should prefer the part with the lower sensitivity and the longer floor life.

The placement plan should be checked so that the most sensitive parts are not the ones that see two reflows. Where the plan cannot be changed, the second reflow profile has to be checked against the package limit.

The assembly line should have a dry cabinet and a record of the bag opening, because the rules only work if the time is actually tracked rather than estimated.

The process should also include a check after reflow for the visible signs of moisture damage, and the inspection should be part of the first article rather than a later audit.

Practical Rules

Read the level from the bag label and record the time the bag was opened, on the reel as well as in a log.

Keep parts in a dry cabinet rather than on a bench, and treat a part that has exceeded its floor life as needing a bake.

Plan the placement so that a sensitive package does not see two reflow profiles where the layout allows a choice.

Check the first article for moisture damage after reflow, and keep the storage record with the assembly documentation.

Process Control and Verification

On a design of this kind, floor life is the item that decides how the rest of the board is arranged. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

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

FAQ

What does MSL 3 mean? A floor life of about a hundred and sixty eight hours at a controlled temperature and humidity after the bag is opened, and a bake requirement if that time is exceeded.

Why does a package crack during reflow? Because the absorbed moisture turns to steam inside the plastic and the pressure has no route to escape. The crack is called popcorning.

Can a part be baked more than once? It depends on the package, and repeated bakes age the plastic and the die attach. The allowed number should come from the component specification.