Moisture Sensitivity Level: Design Rules and Process Limits

Moisture that a plastic package absorbs before reflow turns to steam at the peak of the profile, and the pressure it develops is what cracks the body or delaminates the die attach. The moisture sensitivity level printed on the bag says how long the part may sit open before that risk becomes real, and the floor life, bake and dry pack rules are how the risk is managed on the line.

What the MSL Number Actually Limits

The classification in J-STD-020 assigns a level from 1 to 6 according to how much moisture a package can absorb and still survive a 260 degree Celsius peak. Level 1 has unlimited floor life, level 3 allows 168 hours at 30 degrees and 60 percent relative humidity, and level 5 allows 48 hours at the same conditions.

The level is a property of the package body, the die attach and the lead frame, not of the part number, so two devices in the same body size from different suppliers can carry different levels. It is also tied to the reflow peak the part was qualified at, so a profile that runs hotter than that value invalidates the classification.

Reading the Humidity Indicator Card Correctly

A humidity indicator card carries spots that change colour at 5, 10 and 60 percent relative humidity. A card showing the 10 percent spot pink, or the 60 percent spot anything other than blue, means the desiccant is exhausted and the parts need a bake before use whatever the label says.

The card is read at the moment the bag is opened and the reading is recorded against the lot, because the spots fade back within minutes in dry air. A card read later in the shift is not evidence of the condition at opening, which is why the record carries a time stamp as well as a value.

Floor Life Accounting in Production

The floor life clock starts when the barrier bag is opened and stops when the part is reflowed, and it accumulates across every partial exposure. A reel taken out for two hours, resealed and reopened the next day has consumed two hours of the allowance, not a fresh 168.

In practice the clock is tracked on the reel label, with accumulated open time written on each time the bag is closed and a running balance kept in the feeder list. Where a shop keeps a moisture exposure log instead of a label, the log has to live at the machine rather than in an office, or it will be filled in at the end of the shift from memory.

Bake Schedules at 125 Degrees and at 40 Degrees

The standard bake for a 1.4 mm thick body at level 3 is 24 hours at 125 degrees in a tray, and the same body takes 192 hours at 40 degrees and 5 percent relative humidity when the carrier cannot survive the higher temperature. Tape and reel will not take 125 degrees, so the low temperature schedule is the one that applies to reels.

A bake resets the floor life clock but not the solderability clock, and each cycle consumes some of the finish’s ability to wet. Three bakes is a common limit for a tin-plated lead before a solderability test is required, and the count belongs in the lot record.

Dry Pack, Desiccant and Bag Integrity

Dry pack means a moisture barrier bag, a desiccant unit of the right size for the bag volume, and a vacuum seal made within an hour of the bake. A bag sealed while the parts are still warm traps the moisture it is supposed to exclude, so the parts are cooled below 40 degrees first.

Bag integrity is checked by feel: a soft bag indicates a leak, and a bag with a visible puncture is treated as an opened bag, with the clock started at that point rather than at the day the bag was first cut. Handling is part of the control, because leads puncture a bag from the inside when a reel is dropped.

Popcorning: Mechanism and Detection

During reflow the absorbed moisture vaporises and the internal pressure rises with temperature, reaching a maximum while the solder is liquid and the package is at its weakest. The crack usually starts near the die pad corner and runs to the package surface, and the same pressure causes delamination at the die attach and the lead frame interface.

Scanning acoustic microscopy, described in J-STD-035, images the internal interfaces without opening the package and shows delamination as a change in reflection. A cross-section confirms the extent, and both methods are used at first article and again after any change to the bake or the profile; our board failure notes place the same evidence in the failure analysis sequence.

Reflow Profile Limits for Sensitive Packages

A profile for a moisture sensitive part is written to keep the time at peak and the peak temperature at or below the qualification value, because the vapour pressure depends on both. A straight ramp to a 245 degree peak is gentler on the package than a long soak at 200 degrees followed by the same peak.

A soak stage does drive moisture out of the surface layers, but it cannot remove moisture from the die attach, so it is not a substitute for a bake. Where one profile has to cover the whole board, the sensitive device sets the ceiling and the rest of the profile is arranged underneath it; the measurement method is set out in our profile verification notes.

Storage, Humidity and Handling Between Operations

A dry cabinet held below 5 percent relative humidity keeps an opened reel at level 1 conditions, and its maintenance is a door seal check plus a desiccant change on the cabinet’s own schedule. Nitrogen cabinets reach a lower humidity but add a handling step, so they suit long-term storage rather than line-side holding.

Parts moved from a cold store to the line are left sealed until they reach room temperature, because condensation forms on a cold bag and the water ends up inside when it is opened. An hour is a normal equalisation time for a small reel and longer for a tray stack, and that wait belongs in the kitting sequence rather than in the operator’s judgement; the same principle governs storage shelf life for bare boards.

Records and Traceability of Exposure

The record for each lot should carry the bag open and close times, the accumulated floor life, the number of bakes and the assembly the part went into. Where a device fails electrically after reflow, that record is what separates a moisture problem from a placement or paste problem.

The bake count also feeds the solderability decision and should be visible at kitting rather than only in a quality file. A reel being opened for a fourth exposure after three bakes is a candidate for a solderability check before it is loaded, and that decision is easier at the machine than in the failure report.

FAQ

Does baking restore the floor life of a reel completely? It restores the moisture condition, so the clock starts again, but it does not restore the surface finish. Each bake oxidises the plating a little, which is why the bake count is tracked and a solderability test is called for after the third cycle.

Can a low temperature bake replace the 125 degree schedule? It can, but it takes 192 hours at 40 degrees and 5 percent relative humidity for the same body, and the cabinet has to hold that humidity throughout. For tape and reel there is no alternative, because the carrier and the cover tape will not survive the higher temperature.

What happens if a part exceeds its floor life and is reflowed anyway? It may pass, and that is what makes the practice dangerous. The damage is internal delamination that appears later as a field failure or as a shift in an electrical parameter, so the escape is not visible at the time and is not caught by electrical test.