Moisture Sensitive Device Floor Life Tracking in Assembly

Moisture sensitivity is one of the few assembly risks that is entirely invisible until it becomes a crack. A package that has absorbed water from the air looks identical to a dry one, yet a single reflow pass can turn that moisture into steam and split the body from the inside. Floor life tracking is the discipline that keeps the risk visible, and it depends on labels, records and habits rather than on any measurement taken at the machine.

What Floor Life Means and Why It Is Limited

Floor life is the time a moisture sensitive device may spend out of its dry pack in an environment above the rated humidity. During that window the plastic body and the mould compound absorb moisture at a rate set by the material, the package thickness and the ambient conditions.

The limit exists because absorbed water becomes steam during reflow. At reflow temperatures the vapour pressure inside the package can exceed the strength of the interface between the die, the lead frame and the mould compound, and the package then cracks, delaminates or forms internal voids, a failure mode often called popcorn cracking.

The damage is often invisible from the outside. A cracked package may pass electrical test and fail months later in the field, which is why the industry treats floor life as a process control subject rather than a quality inspection subject. Inspection cannot detect the damage, because nothing has changed on the outside of the body.

Moisture Sensitivity Levels Explained

The moisture sensitivity level, usually quoted as MSL 1 through 6, describes how long a package may remain out of dry storage. MSL 1 has no floor life limit, while MSL 3 allows 168 hours and MSL 5 only 48 hours in a controlled environment at thirty degrees and sixty percent relative humidity.

Every level is defined against a specific environment, so a line that runs hotter or more humid than the reference shortens the usable time. Devices that must be baked or dried and cannot tolerate the heat may be rated as MSL 6, which effectively means they must be processed immediately after the dry pack is opened. Confirm the level on the reel label and on the drawing, since the package marking alone is often not enough.

Dry Pack, Desiccant and Humidity Indicators

Devices are shipped in a moisture barrier bag with desiccant and a humidity indicator card. The card is not decoration. Colour change beyond the marked threshold shows that the bag was compromised in transit or in storage, and the affected devices must be dried before use.

The bag should be opened only when the parts are needed, and it should be resealed if it is opened for partial use. Vacuum sealing is preferred where the bag will be reopened later, because it removes most of the air and therefore most of the moisture source.

Starting and Stopping the Clock

The floor life clock starts when the bag is opened and stops only when the parts are either consumed, returned to a dry environment, or baked. Time spent in a dry cabinet below five percent relative humidity does not count, which is why cabinets are the practical way to pause the clock through a weekend.

Partial reel consumption is the hardest case to control. A reel that is broken open, partly used and left on a feeder overnight has consumed floor life the whole time, and unless that reels are documented the next shift has no way to know how much time remains.

Baking Rules and Allowed Cycles

Baking removes absorbed moisture but it also consumes part of the device’s thermal budget. Standard practice allows a limited number of bakes, and high temperature bakes may be prohibited for parts with certain plating or packaging. Always check the manufacturer’s data rather than applying a general rule.

Low temperature bakes take far longer but are gentler, while high temperature bakes are quicker and risk oxidation of terminations. After baking, the clock restarts, and the parts must be returned to a dry pack or a dry cabinet rather than left on the bench. Record the bake temperature, duration and cycle count against the device lot.

Storage Cabinets and Humidity Control

Dry cabinets hold parts at a humidity low enough to stop moisture absorption, and they make floor life management practical for production volumes. The cabinet should be monitored continuously, because a door left open or a failed desiccant module produces the same result as no cabinet at all.

Logging is essential. Time stamped humidity and temperature data turns an assumption into evidence and shows whether the cabinet is actually holding its specification through the night and across weekends.

Rework and Partial Assembly Exposure

Rework adds thermal cycles to a device that has already consumed floor life in the first assembly pass. A package that was borderline during initial reflow may crack on the second or third exposure, and the failure often appears at a different corner of the body.

Where rework is expected, plan the floor life budget from the start and keep a record of every thermal cycle a device experiences. That total, not the last rework step, decides whether the unit is safe to return to service. That history is the only way to decide whether a marginal unit can be repaired or must be replaced.

What Popcorn Cracking Looks Like

Popcorn cracking appears as a bulge or a split on the package surface, usually after reflow. At the interface level it shows as delamination between the mould compound and the die pad, which can be imaged with scanning acoustic microscopy or revealed by cross-sectioning.

Delamination that does not crack the body still damages the bond wires and the die attach, and it changes the thermal path from the die to the board. An assembly can therefore be electrically good and thermally compromised at the same time.

Records, Labels and Audit Trails

Every dry pack should carry a label with the moisture sensitivity level, the date the bag was opened and the time remaining. At the machine, a simple log records when the reel was removed from the cabinet and when it was returned, and the arithmetic is done once per shift rather than trusted to memory.

gopcb recommends deciding who owns the clock before production starts, because floor life fails at the handover between shifts. Assign the responsibility, keep the records with the work order, and treat an expired reel as a controlled item rather than as material to be used up quickly.

FAQ

Does the floor life clock run while parts are in a dry cabinet? No. Time in a cabinet held below five percent relative humidity is generally excluded from floor life, which is why cabinets allow a reel to be paused between shifts.

Can I bake a device as many times as I need? No. Each bake consumes part of the device’s thermal budget and the number of allowed cycles is limited. Check the manufacturer’s specification, and never assume a high temperature bake is acceptable.

How do I know a moisture barrier bag is still good? Check the humidity indicator card and the vacuum seal. A card that has changed colour beyond its threshold means the devices must be dried before use, even if the bag looks intact.